Bogen CHB-20A Rebuild

[physics]

Stub thread for me to add details to later. I just want to get something done for this before bed. Quick teaser: I wasn't the one who started the rebuild, I obtained it after it could make sound, and it's gonna need some work to be brought up to snuff. Until then I have a nice noise source though.

[physics]

Ok, some more details.

Picked up a CHB-20 cheaply to mod/rebuild into a guitar amp. Once I examined it, it became apparent that someone else had the same idea before me and got pretty far, even installed a hammond choke and output transformer. Looks like they were going for the 18W EL84 Marshall Plexi clone topology. Some stuff was done almost-well like 5W ceramic screen resistors (but only 1k) or the installation of a three-wire power cord (but the safety ground is anchored with one of the transformer bolts). Other stuff wasn't, like numerous grounds placed throughout the chassis, some questionable solder joints, un-decoupled gain stages, the tubes running at around 12.6W each, missing wire grommets on chassis through-holes, and several exposed high-voltage terminals on the top of the amp chassis (four for big beefy dropping resistors that feed the plate node capacitor, and three more for the output transformer primary connections). The layout of signal paths might be questionable too, I'm still tracing things.

Anyhow, it runs. Makes a boatload of noise, but it runs and sounds fairly good doing so. I bumped up the cathode resistor by 10 Ω to reduce the plate dissipation a bit, the tubes now idle at 12.16W each. I'd like to change that eventually to be more in line with recommended dissipation levels, but that depends on the trade-offs my friend wants to make w.r.t. power and reliability. My current puzzle with it is figuring out why it's not putting out 17W like I expect it to. It has two cathode-biased 6BQ5/EL84 tubes in push-pull with a shared cathode resistor of 130Ω and a 220 μF bypass cap across that.

Quiescent power supply voltages are as follows:

Voltage Doubler output node:        412  V
Plate node:                                    316.3 V
Choke-fed screen node:                314.6 V
Phase-Inverter node:                    284    V
Stage 2 & Cathode follower node: 261.7 V
Parallel Input stages node:            254    V
Heater winding:                              6.31  Vrms

The shared 130 Ω cathode resistor has 10.11V across it and 79.44 mA through it at the Q-point.

To test power output, I've fed an 100mVpk, 1kHz sine into the input of the amp with my oscilloscope showing the waveform across an 8 Ω resistive load hooked up to the speaker output, which given the Hammond 125E transformer installed will present a 8kΩ plate-to-plate impedance to the tubes. I turn down everything on the amp, then turn up the normal volume until I see the wave peaks start to squash on my scope, which I assume indicates the start of clipping and the end of "clean" power. I then measure the RMS voltage across the load resistors using my Brymen 789 DMM and calculate the power from that.

So far, it appears to put out about 11.75W-12W, which is about 5W less than I expected. Per the tung-sol 6BQ5 data sheet I'd expect about 17W. Below are the voltages I measured while the test signal was being fed into the amp:

Voltage Doubler output node:        406.3   V
Plate node:                                    301      V
Choke-fed screen node:                298.6   V
Phase-Inverter node:                     270.8   V
Stage 2 & Cathode follower node: 250.35 V
Parallel Input stages node:            242.7   V
Heater winding:                              6.3  Vrms

The shared 130 Ω cathode resistor has 11.3VDC across it and 88.95 mA through it.

So while there is some sag, plates and screens are still about 290V, which is still well above the 250V Tung-Sol says would give 11W output.
   

Any ideas as to what may be the problem or what I should check next? I already removed the incandescent pilot light to reduce the load on the transformer, and the above numbers were measured like that. I checked the shared cathode bypass cap, and it measures 260 μF on my DMM. Removing it brings my output down to around 6.9W before the wave peaks squash, again about 5W less than I'd expect.

To see how much output power I could get at max distortion, I turned the normal volume all the way up and with the cap in the circuit I got 14.34W, without the cap I got 13.7W. Better, but still low and with way more than 3% or 4% distortion as suggested by the data sheet.

I also tried upping the shared cathode resistor to 230Ω per the Class-A₂ design procedure in TUT2 which suggests Rk of (√300 V)/80mA, but that seemed to reduce output power and resulted in a lower quiescent current than 80mA and a lower bias voltage than √300 V = 17.32 V, it was around 11V if I recall. I might try it again though because I think I may have made some measurement errors since I was still reading Vpk-pk from my scope screen then using that to calculate RMS, which won't account for distortion if I understand correctly. I think I took RMS measurements with my DMM and/or USB scope though and it read a bit lower too, so maybe I really screwed something up in the measurement.

I have not drawn out a schematic yet, but that is the next thing I'm doing. So far I've gotten caught up with poking around and seeing how it works, hoping things will be a quick fix, but I think to make further progress I'm going to need to actually see and understand the rest of the circuit. I'll post it here once I'm done.

Thanks to anyone who decides to chime in.

[K O'Connor]

Hi Guys

Why waste valuable plate voltage as heat?

I would eliminate the dropping resistors and go with the circa 400V plate voltage.

1k-5W per tube screen is good in a guitar amp, as TUTs state, because guitar amps get clipped a lot.

Is the goal actually to have maximum output? If so, install a fixed-bias supply and go from there. A pair of EL-84 / 6BQ5A can produce 30W easily given the right voltage environment. There is absolutely no advantage in running the tubes at their maximum heat - just because you can and they will do it, does not mean you should or that they like it. Set the idle current to a proper value.

If this needs some warming up tonally, add a Body control so warmth can be dialed in at will. And use a concertina splitter instead of the tone-sucking Schmitt.

The numbers you have are about right for a pair of 12W tubes, ie no more than 12W output class-A. Cathode-bias without a bypass cap is as close to class-A as most hobbyists get. There is often an economic reasoning proffered for cathode-bias , but to me it is an admission of fear of dealing with negative voltages - who can understand them? Personally I find cathode biased amps to be a waste of an almost good circuit. Is that too harsh?

If you think that Va is limited to 300V for EL-84 / 6BQ5A then you are mistaken. TUTs explain why. The data sheet may say so, but it is wrong. Do not confuse applications with tube limits.

Should we assume the Bogen 20 would normally produce 20W? Do you have any idea of the PT specs? The label on the rear panel should state the power inut as VA - the fuse rating tells you a limit of protection, leaving you to guess how much lower the true maximum is.

Have fun

[physics]

I was wondering about the dropping resistors myself, I'd guess that the person before me wanted to bring the plate voltages closer to EL84 datasheets. I'd read in the TUTs that the actual allowable plate voltages are higher than the data sheets, if I recall correctly it was because the rating system used was often not "max values before destruction" but instead "headroom for application variations", in particular the design center system which was eventually update to the design maximum system. I tried looking in the TUTs for what EL84's can actually take on their plates when I first started work on this amp, however I didn't find anything for those particular tubes. Maybe I missed it. I didn't find a method for determining actual maximum voltages for tubes either, is this something you find by digging around for data sheets that give "absolute maximum" ratings or can it be calculated from the design center/maximum values?
Edit: Actually, I think something was mentioned in passing on this forum in another thread, about Seymour Duncan knowing how to read datasheets and protect EL84's while running them at 450V on the plate using high screen resistors (I assume 1k at least?).

Regarding the screen resistors, I had thought I'd seen you state either on this forum on in one of the TUTs that for smaller tubes like the EL84 that 1k is not enough and it should be 2k2 or something like that instead. If 1k is fine though then that is less parts to replace.

As to the overall goal, it is to get maximum power on a budget. There are actually two amps my friend has the choice of being rebuilt. One, this CHB-20 which is already well on it's way to being a guitar amp, and two, a CHB-50 that is still in it's PA form. Which one he takes depends on which one is cheaper to bring up to snuff, which includes being in the ballpark of at least 20W. The CHB-50 should be loud enough without a doubt, though it has all the original carbon comp resistors, old electrolytics, wacky input/output jacks, etc. The CHB-20, since it's already been rebuilt with new 'lytics, resistors, choke, etc. could end up being cheaper if it doesn't take many components to get it pushing around 20W, hence why I've been screwing with the cathode bias arrangement I found it with before switching to fixed bias. I was hoping a resistor swap or two would bring it into Class A2, with about 17W output, but that hasn't been the case so far.

In the name of reliability and max output, I think switching to fixed bias would be a good choice, as you mentioned. However, I'm largely bound by what my friend wants here unless I start investing my own money into the project. I'm mostly just doing my best to present him the options and risks and provide suggestions on what I think should be done for reliability, repairability, etc., but if he wants to run the tubes at 12W each after being made aware of all the risks in the name of not blowing the budget now and just paying for it later when tubes die prematurely, transformers need replacing, etc, then that's likely what will be done unfortunately. I think I can convince him to go fixed bias though.

I'll have to keep the Body control in mind, though I think he'll choose a colder tone over more parts. We'll see though, thanks for suggesting it.

With regards to splitters, I didn't know you felt so strongly about the Schmitt. From reading TUT, I got the impression that you favored the Schmitt for your designs.

As for the numbers, I agree that they look an awful lot like class-A numbers, however my issue is that those particular numbers were obtained /with/ a bypass cap. Removing the bypass cap gives me even less, as shown in the second post. Per TUT2 and the tung sol data sheet I'd expected somewhere around 12*1.414 = 16.9W with the cap, though admittedly the bias voltage isn't quite right per TUT2's recommendation.

Regarding the CHB 20's output, yes my understanding is that it normally would output around 20W, though I have not gone through all the details to verify that based on the original schematics, I'm only going off the word of other techs who have worked on them before.

I don't have much of an idea of the power transformer specs unfortunately. I can say that without any tubes plugged in, the voltage doubler outputs about 480V, and that the original tube complement plus pilot light for the amp drew about 1.85A from the heater winding compared to the 2.5A of the current tube set (with pilot light removed). Back of the amp is labeled for 90W and a 1A Slo-Blo, though the previous owner installed a 2A Slo-Blo, presumable because of the extra heater current but I have not verified if that is too large yet.

I'm thinking that later today I'll swing by the electronics shop and pick up parts to convert to fixed bias, worst case is I uninstall it at my friend's request and get to use the parts in my own builds.

[K O'Connor]

Hi Guys

Arc voltage is the limit for plate voltage in a tube and this is rarely listed. Many tubes have other notes in their spec sheets that suggest the true capability. For the EL-84 / 6BQ5A, note that the pins to either side of A are left empty to provide isolation for the voltages that may be present on the anode.

Get rid of the dropping resistors and go fixed-biased as the original schematic shows and you will have 20W.

EL-84s are heater current hogs for sure, pulling 760mA each compared to a 6V6 at 450mA and 6L6 at 900mA., and a pair of them is already near the stock tube complement's full heater load (1850mA; 2x700 + 300 +150)). A cathode-biased amp tends to be garbled at and near clip, where fixed-biased amps tend to have some definition left.

You could save current by switching to 6V6s, or just change one tube to 6V6. The mix of the '84 brightness and the V's cream is quite nice. This mix would require separate Rk+Ck or fixed-bias with individual controls.

Every tech has his own style and deals with customers differently, accepting or rejecting certain customers and/or their projects. As I mentioned elsewhere, I would ask a player what the goal was tonally and/or functionally, and then tell him the cost of different options. It is up to me to interpret his goal into tech. I would choose components and circuits based on what I know to be reliable and that suit my beliefs and experience. Once the player starts demanding the use of yellow caps and green resistors and red transformers we are done. There are other techs willing to cater to those compromises.

Your friend is obviously reading forums or blogs and assuming what he reads is accurate. He may not even know what his own goal actually is? In electronics there are many ways to achieve the same goal. The 50W amp is a better choice if he wants something that will stay clean to use pedals with. The 20W may be closer to the loudness he wants with inefficient cabinets and a cranked sound. He is getting a bargain having you do the work for him. You are doing it for the experience and to help your friend and he is just paying for parts. You will end up going around in circles although in the end if he is not happy with the result it is only because he dictated what you should do rather than properly defining the goal and let you loose. It will be "your fault" either way.

I suppose the original tubes are history? You could easily make a nice amp using those with the output stage and PSU left in their stock form, either rewire the splitter and PA front-end or leave as is, and revoice the preamp.This applies to either amp. The stock power tubes are a similar construction to the '84 and will sound about the same when driven hard. The key PA point is to not have a feedback loop to the secondary - which most old PAs did not have anyway.

Did the PA work at the beginning? Did your friend assess the sound? Open the NFB and the tone would likely be how he wants. 6EU7 is like a 12AX7 with a different pinout, so it should get the same tones. The 6GW8 triode is identical to 12AX7 and the pentode is remarkably similar to the '84.

Yes, the 2A slow-blow mains fuse is not protecting the PT at all. TUT3 explains how to select fuse values.

You see Schmitt splitters in most of the TUT3,5 projects simply because that is what the iconic circuit had and what hobbyists expect. I haven't used them since 1985, and then only for a hifi amp. Concertina is king for versatility, stability and tone.

[makinrose]

Nice pick for an amp to mod!  I've worked on and actually used to buy Bogen's to mod and they are nice platforms. I often used the octal pre-amp ones but did a bunch of 60's ones too.  The transformers generally sound pretty good.  It's a shame they got so expensive to acquire.

I have to agree that the Contertina is probably the best sounding PI too.  I do have a fondness for various iterations Paraphase inverters too but in the stability and versatility department they are inferior.  They sound great though.  To my ear the Concertina are warmer and have more character than the Schmitt.  It is hard to explain that to customers that believe that Fender/Marshall/Vox's PI must be the gold standard but most players I know seem to agree after they've tried both types.

[physics]

Hi Guys

Arc voltage is the limit for plate voltage in a tube and this is rarely listed. Many tubes have other notes in their spec sheets that suggest the true capability. For the EL-84 / 6BQ5A, note that the pins to either side of A are left empty to provide isolation for the voltages that may be present on the anode.

Get rid of the dropping resistors and go fixed-biased as the original schematic shows and you will have 20W.

EL-84s are heater current hogs for sure, pulling 760mA each compared to a 6V6 at 450mA and 6L6 at 900mA., and a pair of them is already near the stock tube complement's full heater load (1850mA; 2x700 + 300 +150)). A cathode-biased amp tends to be garbled at and near clip, where fixed-biased amps tend to have some definition left.

You could save current by switching to 6V6s, or just change one tube to 6V6. The mix of the '84 brightness and the V's cream is quite nice. This mix would require separate Rk+Ck or fixed-bias with individual controls.

Every tech has his own style and deals with customers differently, accepting or rejecting certain customers and/or their projects. As I mentioned elsewhere, I would ask a player what the goal was tonally and/or functionally, and then tell him the cost of different options. It is up to me to interpret his goal into tech. I would choose components and circuits based on what I know to be reliable and that suit my beliefs and experience. Once the player starts demanding the use of yellow caps and green resistors and red transformers we are done. There are other techs willing to cater to those compromises.

Your friend is obviously reading forums or blogs and assuming what he reads is accurate. He may not even know what his own goal actually is? In electronics there are many ways to achieve the same goal. The 50W amp is a better choice if he wants something that will stay clean to use pedals with. The 20W may be closer to the loudness he wants with inefficient cabinets and a cranked sound. He is getting a bargain having you do the work for him. You are doing it for the experience and to help your friend and he is just paying for parts. You will end up going around in circles although in the end if he is not happy with the result it is only because he dictated what you should do rather than properly defining the goal and let you loose. It will be "your fault" either way.

I suppose the original tubes are history? You could easily make a nice amp using those with the output stage and PSU left in their stock form, either rewire the splitter and PA front-end or leave as is, and revoice the preamp.This applies to either amp. The stock power tubes are a similar construction to the '84 and will sound about the same when driven hard.  The key PA point is to not have a feedback loop to the secondary - which most old PAs did not have anyway.

Did the PA work at the beginning? Did your friend assess the sound? Open the NFB and the tone would likely be how he wants. 6EU7 is like a 12AX7 with a different pinout, so it should get the same tones. The 6GW8 triode is identical to 12AX7 and the pentode is remarkably similar to the '84.

Yes, the 2A slow-blow mains fuse is not protecting the PT at all. TUT3 explains how to select fuse values.

You see Schmitt splitters in most of the TUT3,5 projects simply because that is what the iconic circuit had and what hobbyists expect. I haven't used them since 1985, and then only for a hifi amp. Concertina is king for versatility, stability and tone.

I see, ok. So it's mostly an empirical process guided by educated guesses to find the max voltage, assuming it's not published?

That's a good point about the original schematic, I had not consulted with that since seeing that a previous owner had (mostly) obviated it.

I like the tube swapping idea, installing all 6V6's was one of the things I proposed as an option initially both because of the heater current and due to the plate voltage question being up in the air at the time.

I may have given the wrong idea about my friend, pretty much the only constraint I have is to not blow the budget. When I mentioned convincing him to go with fixed bias, I meant purely from the standpoint of convincing him that it's worth the extra cost in parts vs running the tubes at full tilt in cathode bias and trying to resolve the issue of the missing 5W. With just about everything so far he's pretty much said he'll leave it up to my discretion, with cost being the only limiting factor. I agree that the 50W would be a better choice, and after further discussion yesterday and some rough ballpark figures in what it will cost to make either the 20W or 50W useful now that I have a better idea of what's needed, he decided to go with the 50W instead. So the 20W is now my personal playground.

Yes, the original tubes are gone as far as I know. The same guy I bought the amps from though also gave me a pile of vintage tubes he didn't feel like selling, and I think I saw a full complement of the original tubes among them. Not original (some are labeled hammond), but the same types and mostly USA or Britain in origin. The 50W does have original tubes though, at least the twin 6L6 tubes are Bogen-labeled, and it is pretty much stock (down to the two-prong power cord ) including the 6C4 concertina inverter, so I'm glad to hear the ringing endorsement of that particular topology.

The 20W PA was already gutted and rebuilt as marshall clone when I got it, the 50W PA appears to still be a PA, I've yet to turn it on since he and I agreed it would be worth appraising the 20W first since it looked really close to being useful.

I'll have to re-read TUT3 then, thanks for the pointer to it.

Regarding schmitts and TUTs, I suppose that makes sense. I'll have to do a deep dive into the behaviors of both at some point, and do some comparisons. While this 20W will probably go on the shelf for a bit while I work on the 50W and my Peavey VK100, I think it would be interesting to change the 20W to a concertina and do some before/after recordings and measurements for a more quantifiable description of the differences. When I have time I guess.

Thanks for the informative reply!

[physics]

Nice pick for an amp to mod!  I've worked on and actually used to buy Bogen's to mod and they are nice platforms. I often used the octal pre-amp ones but did a bunch of 60's ones too.  The transformer generally sound pretty good.  It's a shame they got some expensive to acquire.

I have to agree that the Contertina is probably the best sounding PI too.  I do have a fondness for various iterations Paraphase inverters too but in the stability and versatility department they are inferior.  They sound great though.  To my ear the Concertina are warmer and have more character than the Schmitt.  It is hard to explain that to customers that believe that Fender/Marshall/Vox's PI must be the gold standard but most players I know seem to agree after they've tried both types.

Yeah, prices are a bit stupid now. I got lucky and found a guy who wanted space, so was selling them cheap. Thanks for another endorsement of the concertina! My Peavey VK100 has a concertina in it, and it sounds like it'd be worth seeing what I can do with that instead of trying to rework as a LTP. To no-ones surprise, the PCB doesn't hold up the best to rework.... A shame since space-wise it's fairly comfy working inside the chassis.

[physics]

Thanks for all the replies and info everyone! I'm putting this project on pause for now as my friend and I have determined it's best to work on the 50W instead, but I plan to come back to the 20W at a later date and complete its transformation into a great amp. There may be a thread for that 50W project popping up at some point too if anyone is interested in following along with that.

[physics]

Just got the 2 tube bias mod kit in the mail (didn't feel like taking the time to source parts myself), and I should have the components on hand to rig up a bias supply, so hopefully I can get to that soon. I also removed the large dropper resistors so I should be getting full plate voltage now. Might need to tweak the series resistors in the proportional supply to adjust for that. I'm thinking I'll use my FR4 sheet and eyelets I've been sitting on to rework this amp into something I can be proud of, so I guess I need to add metal stand-offs to my shopping list.

[physics]

Already, did the first bit of "real" work on the amp in a while. I replaced the screws holding down the power transformer with some longer ones so that I can build a power supply circuit card on them like in TUT5's LP Standard project.

   

This particular transformer is a lay-down unit however, unlike in the TUT5 project. Is there any issue with resting the circuit card directly against the transformer bell, given that there are no eyelets that get shorted this way? I'm concerned about whether space for airflow under the board is required to keep the transformer happy. Additionally, if I do place eyelets over the transformer bell, how much vertical space should I have between the eyelets and the bell for good electrical insulation? The chassis isn't that tall, and I was planning to put electrolytic caps on this board, so I'm playing a balancing act between having the caps touch the bottom of the chassis and eyelets shorting against the bell.

Any input is appreciated.

Thanks,
         physics

P.S. I know the safety ground needs improving, I got the amp that way and until I replaced the bolts I didn't realize how bad the connection was. I'll be drilling a new hole dedicated just to the safety ground and replacing the termination of the wire with a locking solder lug, possibly topping things off with a nylock nut.

[K O'Connor]

Hi Guys

Note that some PTs have insulated bolts holding the laminations together - although, once the unit is vacuum varnished, the varnish is what holds the laminations together and the bolts are just for mounting. If there was tape over the threads through the lams and/or nylon bushings and/or fiber washers at either end, these should all be incorporated with the new longer bolts.

An alternative to the long bolts is to use long threaded standoffs screwed onto the stock bolts. Then use short bolts to secure the card. A quick look at the Keystone catalog shows threads up to 8-32 and lengths up to 2". PT bolts are often #10, so this may not be an option. Only short threaded spacers are threaded the entire length; from 1" up the thread at each end are about 3/8" deep.

The PSU card in The Standard carries the fuses and bias supply and is more or less full. How is your card empty?

Regarding the toothed lug for use as a chassis ground: In my amps and kits I use a flat washer over the lug, then the nylock nut. This way, the teeth on the lug does not interfere with tightening the nut.

In many of the amps I've built where a board sits over an OT, say, I add "captive bolts" to the card. These are comprised of a 3/8" long bolt with a nylock on the underside of the card facing the standoff. I tighten the nylock then release it very slightly so the bolt can turn in the hole in the board - not wildly, but with a screwdriver. This also works nicely when there is a stack of cards.

[physics]

Hi Guys

Note that some PTs have insulated bolts holding the laminations together - although, once the unit is vacuum varnished, the varnish is what holds the laminations together and the bolts are just for mounting. If there was tape over the threads through the lams and/or nylon bushings and/or fiber washers at either end, these should all be incorporated with the new longer bolts.

An alternative to the long bolts is to use long threaded standoffs screwed onto the stock bolts. Then use short bolts to secure the card. A quick look at the Keystone catalog shows threads up to 8-32 and lengths up to 2". PT bolts are often #10, so this may not be an option. Only short threaded spacers are threaded the entire length; from 1" up the thread at each end are about 3/8" deep.

The PSU card in The Standard carries the fuses and bias supply and is more or less full. How is your card empty?

Regarding the toothed lug for use as a chassis ground: In my amps and kits I use a flat washer over the lug, then the nylock nut. This way, the teeth on the lug does not interfere with tightening the nut.

In many of the amps I've built where a board sits over an OT, say, I add "captive bolts" to the card. These are comprised of a 3/8" long bolt with a nylock on the underside of the card facing the standoff. I tighten the nylock then release it very slightly so the bolt can turn in the hole in the board - not wildly, but with a screwdriver. This also works nicely when there is a stack of cards.

Yes, this transformer had fiber washers under the old bolt heads, which I made sure to put under the new bolt heads as well.

I considered using stand-offs to extend the stock bolts like you mentioned, however it ended up being cheaper to just buy new bolts.

The card is currently mostly empty since the only thing I've decided to move to it so far is the voltage doubler portion of the supply, maybe a cap for the screen, and probably the bias supply. There is enough space around the edges of the card that overhangs empty space instead of the transformer bell that I might be able to build everything on those edges if I didn't want to have eyelets over the bell, but that might end up requiring some contortions. The caps I have to work with are axial so they're amenable to that, but aside from that it might get messy. Hence inquiring about required clearances between eyelets and the transformer bell.

Good to know about the ground lug, I was wondering whether a washer would be necessary.

That's a nifty idea with the "captive bolts", I'll keep that in mind.

Thank for the info!

[Sherlok Ohms]

Hello Mr Physics (sounds like a nickname for Einstein)

Since you plan on having the board touch the end bell, is there room for a second blank card to be squeezed in there? My experience with the typical card stock used for eyelets is that it is fairly flexible. This would give you security against any arcing and allow fuller use of the card.

Another notion comes to mind, where you turn a locking nut down each of the long bolts to use as a stop to apply a short spacer above, into which you can thread a short bolt once the board is in place.

We like to podge things together here - jolly fun! - saves money some time - saves time some time - but some times it looks quite odd after a while if you go back into the chassis to do something later. Then you wish you had done it cleaner, or more in a typical fashion that would not incur your own regret let alone comments from others. I recall Mr O'Connor saying in one of the books that it takes the same time to install a good quality component as to install a poor quality one. I believe that was with regard to quarter-inch jacks. In this case, it would be a comparison of improvisations; the long bolts could look a bit crude with so much exposed thread, where covering those threads with any kind of tubing may look nicer. The outer sleeve of coax cable comes to mind.

This is an aesthetics comment, I must admit, and every engineering solution has its own beauty.

Cheerio

[physics]

Hi Sherlok!

I like the idea of a second card. I think there is room, but I need to double check. I was considering using fish paper for that purpose, but a card seems like a more robust solution that will also say more loudly to other techs who may work inside the amp: "DON'T REMOVE ME!"

I can see where you're coming from with respect to aesthetics. Personally I think I'm fine with the exposed threads, we'll see how things wear on me. I'm figuring on using a regular nut and washer to hold the top card down against the bottom card against the nylocks beneath that. Given I'm not using nylock screws/standoffs for the other cards I assume this is fine, but should I actually use locking nuts (or maybe threadlock?) to secure the top circuit card?

Thanks!

[K O'Connor]

Hi Guys

It seems there will already be some flexure of the card and some pressure from it against the top nuts. This suggests that those nuts should be locking types.

NEVER use threadlock grease in electronic assemblies.

[physics]

That's a good point about board flex Kevin, locking nuts it'll be then.

@Sherlok, I just did some measurements and with the second board, assuming about 3mm of space between the two boards, I'll have about 2mm of clearance between the capacitors and the chassis. Assuming the chassis itself doesn't flex a bit. I think that might be a bit too close for comfort. I guess options now are to get smaller capacitors, insulate the capacitor cans with something (kapton tape?) other than their sleeves, figure out how to layout the board so I can rest it against the bell without shorting, perhaps insulate the bell (fishpaper?), or just stop fussing and mount the board next to the transformer since there is space.

Edit: Another option would be to add a few nuts as spacers between the transformer and the chassis, so that the bell doesn't sit as deep in the cutout. There is already one nut of space stock, two more nuts would give me an extra 6mm of space. Hmm...

[physics]

I think I found the related info to Kevin's comments about threadlock, for those who are interested: https://www.emcstandards.co.uk/never-use...-electrica

[physics]

Update: I got 3" 8-32 screws to replace the stock transformer screws, and the raised the transformer out of the chassis recess by 4 additional nut heights (about 12mm). I must have done the math wrong originally, I have way more space than I thought I would doing this method. Currently it's using screws from Lowe's labeled as "zinc" screws, but down the road I'll be replacing those with stainless steel to match the rest of the hardware. Just wanted something cheap for now until I can place an order with McMaster-Carr. Not looking forward to undoing and redoing the nylock nuts though...


In doing this, the leads from the transformer are now in a place to chafe against the chassis edge. To fix that I installed some grommet edging. Down the road I plan to get some with good adhesive on the inside to make sure it doesn't come off when it doesn't have the wires blocking it. It sticks for a while currently with the expired adhesive mine has, but eventually over an hour or so it will release and straighten out. Another option would be to use something like Loctite 422 to stick the polyethylene grommet down for good. You might be able to see in the pictures where the cloth insulation has already chafed/been cut through. Luckily there is plastic insulation underneath. Another concern I have for the leads is bend radius. I think I pushed it too far here, and should make the cutout in the chassis a little bigger.

   


   


After installing the nylock nuts on the transformer bolts I started working on the power supply board that's supposed to mount to the bolts. Took some measurements, carefully marked out dimensions on my FR4 sheet, cut, drilled, and.... nothing fit. Nice. Manually made some holes bigger, and now things fit. This will be the insulating card to keep the power circuit card from shorting to the transformer bell.


   

[physics]

Oh, I also put some grommet around the cutout that the choke wires come through. It's the old hole that the original multi-section cap mounted over, and the prior builder didn't do anything to the edges. Now that's fixed.
   
For bonus points, the choke wires were not twisted and had a wire for the plate supply, I think going between the transformer and the voltage doubler, passing through their loop. I've also found several sadness inducing solder joints. : (
Hopefully my work won't be sadness inducing.