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Peavey ValveKing 100 Overhaul
#1
Picked up a Peavey VK100 recently for cheap 'cause it powers on but doesn't make sound. Starting this thread now for me to log my progress fixing and hopefully later improving it. I checked before buying, and it looks like there is plenty of room inside the chassis for future mods, the board isn't too crowded to work comfortably on, and it's not a huge pain to get out. It is single-sided though. Maybe I'll install some eyelets in the through-holes to remedy that. Some spots on the board may be a bit tight for that though. Anyhow...

First issue: no sound. Tried different inputs, tried different channels, tried a patch cable in the fx loop, tried going direct into the effects loop return, nothing. Next I tried checking for signal coming out of the FX send, but at that moment my scope which I was hooking up to the other end of the cable decided to go for a smoke break, so I'm going to fix that first then resume troubleshooting the VK100 in between rebuilding a Bogen PA for a friend.

First planned upgrade: 1k 5W screen resistors. Stock it comes with 330R 2W resistors. I wonder if the screen resistors have gone out to lunch, and that's why the amp is silent.

Second planned upgrade: threaded inserts for the front and back panels. A previous owner thought self-drilling screws were a bright idea when they replaced the stock front panel with diamond plate, and my efforts to secure all the screws on it, not just the bottom two, did several of the holes in Angry . Something to deal with after the electrical issues are sorted. Maybe before, depending how annoying it gets it deal with.
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#2
Hi Guys

Everyone has their own style and approach for repairing amps. personally, I hate repairs and take a bit of a paranoid approach to it to reduce damaging anything further than what is already broken.

I ALWAYS REMOVE OUTPUT TUBES FIRST

For the VK100, since the symptom is no sound, I would do a resistance check of the OT primary and secondary. Make sure the output jack is open.

You should see steady DC resistance readings for all windings. Any ambiguity likely means there is an open winding and the OT is defective.

You can measure the screen-stops if you suspect them, but they are likely okay.

I do an incremental power-up through the Power Limiting Safety Socket. Connect the negative meter probe to circuit ground using an alligator test lead. This frees one hand for voltage measurements and you can have the other on the power switch for a quick turn-off if required.

Since you already know this amp does not blow fuses, did you check to make sure none are already blown? Do the tubes light up? Assuming those things are okay, measure the voltages on the power tube sockets to see that all the voltages are present, of the correct polarity, and of the correct magnitude. With limiting from the PLSS, all the voltages will be low but proportional. If all the 9-pin tubes are in place, limiting will be modest as the heaters need something close to their rated voltage.

If you have a scope and sine generator, feed a signal into the effects return and see if signal passes to the output sockets pin-5 (grid). You could similarly feed a signal into the guitar input and scope through to the PA, adjusting EQ for a good sine wave rather than using "instrument" settings. Seeing the sine wave retain its shape especially through the PA tells you a lot about the functionality of the circuit.

If everything is satisfactory up to this point, you can power down, install the power tubes, preset the bias for maximum negative voltage on pin-5 (this amp lacks bias controls), then power up with a bit less limiting. With the load on the output open, you should be able to see a good sine wave there, and it should clip evenly positive and negative as the Volume is turned up and down.

It is dreadful that there is no bias adjustment! Poor design. Gives no opportunity to hum-balance the output stage for best tone.

You can attach a speaker and try out the amp. Usually I connect a bench load before that and just sweep the volume up and down to see that the amp can drive the load.

With an older amp like this, solder connections are the main culprit for most symptoms. Solder pads for large leads, say for power resistors, will definitely need to be resoldered NOT simply "touched up". The latter is a sure way to degrade the connection further and guarantee a cold joint sooner than later. For PVs, the insulation-displacement connectors are a weak point, especially for heater power connections. I would solder those terminals to improve reliability.
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#3
Thanks for the suggestions Kevin! I'll keep them in mind as I work things out.
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#4
Quick update: Got the amp open today to have a look inside, and found that the fuse on the power transformer high-voltage secondary is blown. My hypothesis is that a power tube failed. It's a through-hole type, so I will have to pull the board to replace it (single-sided PCB). I think I'll replace it with a PCB mounted fuse holder or maybe run wires to a chassis mount fuse holder to make things easier if the fuse blows again in the future.

Luckily, it looks like there are no insulation-displacement connectors in this amp, unless I missed 'em.
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#5
Happy Holidays

When working on tube guitar amplifiers I believe it is a common practice to have the rear of the amplifier facing you, as this gives you the best view of the tubes. Once you are at the point of testing with power tubes, you should preset the bias to the lowest-current end of the bias control or controls and tie one meter probe to ground, as Mr. O'Connor suggested above. Then power up and see hat the tubes are at a low or close-to-zero idle current.

Watch for any suggestion of red-plating. The metal of the anode (plate) is a dull grey and should remain so; If it begins to change colour turn off the power immediately. Pull the tube that changed colour and check that the bias voltage is still present at the tube socket. If all the tubes red-plated then something common to them all has failed. If just one tube failed it could be the tube or the circuit. In a 4-tube output stage, two tubes on the same side red-plating suggests a circuit error where a single tube red-plating suggests something wrong with the tube or the circuitry for that socket alone.

Always take your time when doing repairs or testing of high-voltage circuits, and especially when dealing with power tubes. I believe there are several cautions throughout The Ultimate Tone books.

Cheerio
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#6
(12-24-2023, 07:14 PM)Sherlok Ohms Wrote: Happy Holidays

When working on tube guitar amplifiers I believe it is a common practice to have the rear of the amplifier facing you, as this gives you the best view of the tubes. Once you are at the point of testing with power tubes, you should preset the bias to the lowest-current end of the bias control or controls and tie one meter probe to ground, as Mr. O'Connor suggested above. Then power up and see hat the tubes are at a low or close-to-zero idle current.

Watch for any suggestion of red-plating. The metal of the anode (plate) is a dull grey and should remain so; If it begins to change colour turn off the power immediately. Pull the tube that changed colour and check that the bias voltage is still present at the tube socket. If all the tubes red-plated then something common to them all has failed. If just one tube failed it could be the tube or the circuit. In a 4-tube output stage, two tubes on the same side red-plating suggests a circuit error where a single tube red-plating suggests something wrong with the tube or the circuitry for that socket alone.

Always take your time when doing repairs or testing of high-voltage circuits, and especially when dealing with power tubes. I believe there are several cautions throughout The Ultimate Tone books.

Cheerio
Thanks for the tips! Yeah, definitely watching my step around the circuits, trying to follow a defined safe procedure each time, implement engineering controls so I take the human factor out, etc. Can't get confortable...
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#7
Small update:

Checked the output transformer, the windings seem good. Primary side leads shows 25 and 30 ohms to the center tap, secondary side shows less than an ohm for each of the impedance settings.

Stopped by the electronics shop today and picked up the needed fuses, plus a PCB mount fuseholder that I'm going to install so that I don't need to pull the board in the future just to replace the fuse. While I was there I ran the quad of 6L6's from the amp through the sylvania #2500 tester, and they all came out as having "good merit" (~100 on the scale) and "no shorts". However, based on what I've been reading in Tomer's book Getting the Most Out of Vacuum Tubes, these readings likely don't count for much, particularly with regards to shorts. Observation of the tubes shows thin brown rings around the edge of the getters, and one of the tubes' plastic base has come unglued. Once I have the fuses installed I'll power up with my lightbulb limiter and variac, watch for funny stuff, and if that goes well pop the tubes in and keep an eye on things as I bring the amp up again. If it's /not/ the tubes... Hmmm...
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#8
Desoldered the old fuse. Found another busted part nearby, C201.

   
   

Tests as a dead short. Maybe that is what blew the fuse? Not certain why it would up a die by itself. The mains socket has a bit of filtering on it, so a nasty spike coming through the power transformer doesn't seem too likely, but I don't know enough about this area of electronics (mains inputs) to say for sure. Alternative explanation is that the cap was collateral damage of the fuse-blowing event, which from my limited knowledge seems less likely as no plausible over-voltage events are coming to mind quickly.

   

I'm gonna dig through my parts stash for a suitable replacement cap. To be continued...
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#9
Hi Guys

That cap might have died and taken out the fuse, which would be great as it means everything else is likely fine.


A cap across AC should have safety ratings and/or be polypropylene, which is self-healing. The circuit will work fine without the cap. Overall reliability would be improved with a bleeder resistor across the first filter cap C202. You may need two Rs in series to have sufficient voltage rating for the resistors. They can be 330k or 470k metal-oxide flame-proof 1W each. This will drain the cap at turn-off and it will protect the bridge when you switch to stand-by.

Of course, we all know that the safest stand-by switch is the one disconnected or never switched off.
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#10
(12-30-2023, 08:05 PM)K O'Connor Wrote: Hi Guys

That cap might have died and taken out the fuse, which would be great as it means everything else is likely fine.


A cap across AC should have safety ratings and/or be polypropylene, which is self-healing. The circuit will work fine without the cap. Overall reliability would be improved with a bleeder resistor across the first filter cap C202. You may need two Rs in series to have sufficient voltage rating for the resistors. They can be 330k or 470k metal-oxide flame-proof 1W each. This will drain the cap at turn-off and it will protect the bridge when you switch to stand-by.

Of course, we all know that the safest stand-by switch is the one disconnected or never switched off.

I did wonder about the cap ratings, this one doesn't have any fancy approval markings like an X or Y film cap usually does. It's on the secondary side of the power transformer, if that makes a difference. What function does the cap play in the circuit? My best guess so far after some internet searches is that it's there to help manage oscillations due to the HV secondary winding inductance and bridge diodes recovery capacitance interacting.

Thanks for the reliability tips, bleeder resistors are definitely on the list of things to do. First I want to get this thing working though, and then I'll go from there. Using the standby switch/hole for something more useful is another possibility. Quick question on that actually: if I recall, one of the TUTs says that until we get up to kilovolts on the plates, standby switches are not needed. From reading elsewhere (Getting the Most Out of Vacuum Tubes by Tomer), the cause of cathode damage when there is no standby is that it gets pelted with ions until the space charge builds up. A quick websearch indicates that it takes about 3kV to ionize 1mm of dry air, if I read correctly. Can it be said then, that the reason we don't need to worry about standby switches is that the plate voltage is not high enough to start ionizing the gas in the tube, meaning the cathode isn't in danger of an ionic hailstorm?
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#11
Hi Guys

There is no interaction between the plate winding of the PT and the circuit in the manner described. The cap across the winding is too-low a value to have any such interplay; rather, it is there to help suppress noise, as described in TUT3 and elsewhere in the TUT-series.

The noise suppression function is bilateral in that noise coming through the PT to this winding MAY be suppressed slightly by this cap, while at the same time rectification noise can also be very slightly suppressed. Limiting factiors to effectiveness are the lack of known resistances or impedances for the cap to work against.

In any case, the cap you pulled out is a multi-layer ceramic cap (MLCC) which has quite a range of characteristics good and bad. I would not bother installing anything similar, and broadly would not use a ceramic in that position. "PP or go home".

Considering that the filter caps in the plate supply are rated for 550Vdc, the AC from the winding could be as high as 385Vac, and may be higher with no load. A safety cap rated at 440Vac will work here. Choose a polypropylene types, which in such applications is a box-style format. Yes, there are ceramic safety caps but why go there? As mentioned, you do not really need this cap - the resistors across the first filter cap are far more important for safety.

There is a Technical Article on my site that discusses stand-by switches. Such switches are illegal in some countries.
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#12
(12-31-2023, 03:43 PM)K O'Connor Wrote: Hi Guys

There is no interaction between the plate winding of the PT and the circuit in the manner described. The cap across the winding is too-low a value to have any such interplay; rather, it is there to help suppress noise, as described in TUT3 and elsewhere in the TUT-series.

I see, thanks.

(12-31-2023, 03:43 PM)K O'Connor Wrote: The noise suppression function is bilateral in that noise coming through the PT to this winding MAY be suppressed slightly by this cap, while at the same time rectification noise can also be very slightly suppressed. Limiting factiors to effectiveness are the lack of known resistances or impedances for the cap to work against.


Yeah, the lack of any explicit AC voltage dividers seemed weird.

(12-31-2023, 03:43 PM)K O'Connor Wrote: In any case, the cap you pulled out is a multi-layer ceramic cap (MLCC) which has quite a range of characteristics good and bad. I would not bother installing anything similar, and broadly would not use a ceramic in that position. "PP or go home".


I see. Given the apparent tendency for ceramics to fail short, I think I'm starting to develop a bias against them too. In any case, based on your input and other research I don't plan to put one back in right now, especially not one of the ancient bulk ceramics I was initially considering using as a temporary replacement.

(12-31-2023, 03:43 PM)K O'Connor Wrote: Considering that the filter caps in the plate supply are rated for 550Vdc, the AC from the winding could be  as high as 385Vac, and may be higher with no load. A safety cap rated at 440Vac will work here. Choose a polypropylene types, which in such applications is a box-style format. Yes, there are ceramic safety caps but why go there? As mentioned, you do not really need this cap - the resistors across the first filter cap are far more important for safety.

Noted, thanks.


(12-31-2023, 03:43 PM)K O'Connor Wrote: There is a Technical Article on my site that discusses stand-by switches. Such switches are illegal in some countries.


I forgot to check the website, thanks. It was an informative article, but I'm still left wondering about the physics behind cathode stripping/bombardment, and why it's not a problem at our plate voltages. Can you or anyone else recommend good books/papers on the physics of vacuum tubes? It's okay if they're math and physics heavy. Right now I'm reading through Theory and Applications of Electron Tubes by Herbert Reich, which came highly recommended from tubebooks.org. Anyone have any opinions on the book, or additional sources I should look into?

Anyhow, I added a fuse holder to the board for the HV fuse, left out the cap, and soldered a jumper wire in place of the standby switch. While I'm still uncertain about cathode stripping, between what KOC said and everything else I've found so far I haven't found a convincingly thorough argument for it being a problem at our plate voltages, plus my shoddy napkin physics seems at first glance to suggest that it's not a problem until around 3kV, give or take a bit due to electrode spacing. That, and the whole safety thing.

Next mods in the pipeline would be bleeder resistors and replacing the screen resistors with 1k 5W flameproofs. For the bleeder resistors, I'm figuring on installing roll-swaged eyelets in the relevant traces and then installing the resistor(s) in the eyelets. Anyone got any better ideas? Eyelets seem the cleanest, most repair friendly solution to me so far, though ideally they'd have flange swages if I read the NASA soldering standard correctly. I'll probably install eyelets in the old screen-resistor holes too, so that it's easier to service should they fail in the future.

Thanks KOC for all the info and input you've provided!
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#13
Separate post since the last one was getting long.

I popped the proper fuses in, and brought it up on my variac and lightbulb limiter. Seemed fine, and voltages in the power amp seemed reasonable. Popped the pre-amp tubes in, and they didn't light. Stupid series heaters... Popped the power tubes in too and brought it up again, and all the tubes lit and everything seemed fine. Plugged a cable in, hooked up a speaker, and tapping the instrument cable with my thumb showed that it was output sound. Hooked up my guitar for some tests, and this amp is loud! Had to be careful with volume since it was late at night. Took it off the current limiter and variac, and played it that way for a bit, and it was fine. The tone is nothing to write home about, but it's a starting point for mods.

Restrung my guitar and plugged in today when I could open it up a bit more, and received further confirmation of it's loudness. Things in my house shake and I could feel the pressure wave when I accidentally hit the strings too hard with the volume up. It'd be fun to mod the amp to have the level of punch certain boutique amps like Fryettes and Wizards are fauned over for. Something else to look into. Overall seems like a promising mod platform, though a master volume or power scaling might be needed. There is also a weird thing happening where turning the clean volume all the way down doesn't actually mute the clean channel. Something to figure out later. If I had the boost on and didn't know it, then the clean volume's wiper wasn't grounded even though it was turned fully off, so that could be it.

For now, I have an amp project for a friend that needs attention, so the Peavey will probably get shelved for a bit aside from maybe the screen resistors so I can play without much worry for tube failures. To finish for now, here's a picture of pretty glowing power tubes loaded up in it:
   
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#14
Quick updates:

Last week I removed the self-drilling sheet metal screws holding on the front diamond plate panel, installed t-nuts, drilled bigger holes in the diamond plate, and re-installed with machine screws.

Saturday I replaced the 2W 330R metal oxide screen resistors with 5W 1k flameproof wire-wounds.

Today I placed an order for the bias mod kit so I can properly bias the amp and run whatever tubes I can get cheap within the confines of the applicable ratings, form factors, pin-outs, etc.

The intent is to use this amp regularly over the coming school semester for rehearsals and gigs, so I'm trying to get reliability mods done sooner rather than later. I think I have the resistors on hand to do the HV bleeder mod as well, so those will likely being going in soon too. Gonna clip out the other two ceramic discs to avoid having them short out and cause issues. One of them is strapped across the bias winding, and I'd really prefer that didn't short. I think I read somewhere that some rectifier diodes are under-spec'd too, will have to get to those as well.

Edit: Oh, and while the amp is fairly comfy to work inside, the board quality isn't great, to no-ones' surprise. I had at least one trace flake off some solder mask during rework despite my efforts, so the motivation to spin my own circuit card is growing.
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#15
Quick update: Just installed net 200kΩ 4W flameproof resistors across the first filter caps to bleed off charge when the amp is off. I was looking for 220kΩ or 330kΩ but to get a high enough voltage rating I had to to put two 100kΩ in series. Some quick math shows that 220kΩ is close enough to 200kΩ with respect to power dissipation, time constants, and bleed current that I don't think it will matter, so in they went.

Also, it looks like someone modded the tone stack of the lead channel before I got it. R4 was replaced by a 68kΩ unit. The person didn't want to pull the board, so they just clipped the last one out and tack soldered in the new one. I have parts on hand to almost fully convert the tone stack to the london power standard's values (mid pot is the only one I don't have on hand) so at some point that will get done. I've also identified several places in the circuit I suspect are contributing to what I don't like about the sound, we'll see if I'm correct. In short, there are two interstage attenuators that roll off everything below 329 Hz and and 720 Hz respectively, which seem to suspiciously correspond to my dislike of certain mid characteristics of the amp. That 720 Hz roll-off in particular... it reminds me of tube screamers, and I'm not a fan of tube-screamers. The lead tone kinda reminds me of one too, so that seems like a good candidate for the snippers.

C103/R108 and C106/R112 below are the two filters mentioned. The first one I'm thinking I'll just remove R108 and short C103. The second I think I'll just remove C106 from so that there is no high frequency emphasis.

   
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#16
Hey Physics

You said R4 in the tone stack but the schematic part you show does not have R4?? If it is like R113 and used to be a bigger value going smaller makes it suck mids - which sucks man!

I see lots of really tiny plate caps in high gain preamps. TUT says to make these small to have a tight low end, but how small can you go and not lose bass?

keep on rockin
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#17
(02-18-2024, 02:09 PM)nauta Wrote: Hey Physics

You said R4 in the tone stack but the schematic part you show does not have R4?? If it is like R113 and used to be a bigger value going smaller makes it suck mids - which sucks man!

I see lots of really tiny plate caps in high gain preamps. TUT says to make these small to have a tight low end, but how small can you go and not lose bass?

keep on rockin

Hi! The snippet I attached was to show the two attenuators I'm eyeing. R4 is elsewhere in the lead channel tone stack, what's shown is the clean tonestack. The pre-amp is stacked, with the lead channel adding two stages and a tonestack after the clean's two stages. I haven't checked all the plate caps yet, but the ones I have appear to roll-off below around 3Hz when combined with the next resistor to ground so I'm not too worried about those yet.
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#18
Greetings

You may wish to refer to the Tube Tone chapter in TU5 as the plate time constants are effected by the internal resistance of the valve. The low-frequency roll-off is actually lower than the visible components suggest, although, as KOC states, one can use the circuit values around the valve to gain insight to the relative filter effects as you are doing.

Cheerio
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#19
(02-18-2024, 05:10 PM)Sherlok Ohms Wrote: Greetings

You may wish to refer to the Tube Tone chapter in TU5 as the plate time constants are effected by the internal resistance of the valve. The low-frequency roll-off is actually lower than the visible components suggest, although, as KOC states, one can use the circuit values around the valve to gain insight to the relative filter effects as you are doing.

Cheerio

Thanks for mentioning that. I am aware of that and will be double checking things later to make sure I correctly understand and calculate what's happening, but for now the attenuators are what stick out to me as plausible causes for stuff I don't like. Plus at the moment I'm trying to figure out what's thinning out the sound (harmonically and otherwise) so if the roll-off is lower than I calculated than that's not really a problem for me yet. The amp is a bit too bassy/loose for my taste though, so I'll probably come back and revisit the coupling caps later. Another suspect on my list is what looks like a feedback loop around stages 2 and 3. I'm wondering if it might currently be linearizing things too much for my taste. Of course I haven't thoroughly evaluated it yet to see if it's doing something important, right now it's just a hunch that I should examine it a bit closer at some point.

Thanks for the input!
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#20
Currently working out how to make the bias adjustable. I have the bias mod kit so all the parts are there, it's just pot and jack placement that's taking a while. The peavey isn't layed out with the thought that you'd want to nestle some pots near the tube sockets, so clearances are tight and inconsistent. The reverb tank is pretty inconveniently placed. That, and the traces off the cathodes are small enough that I think my options for wiring up meter jacks are to either install an eyelet there or to solder my sense wire to the sense resistor lead itself, PTP style.
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