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  High voltage Super Reverb
Posted by: Tomislaw - 10-22-2021, 05:59 AM - Forum: Power Supplies - Replies (2)


I've got an interesting case of excess voltage in power supply on the bench right now - it's my pal's mistreated 1972 Super Reverb AA270. It's got a whole bunch of problems but the most interesting one is that it the voltage is way too high. When I first turned it on, the Chinese 6L6WGS were dissipating 26 watts with 540V on the plates! I had to modify the bias supply in the first place to get enough negative voltage to stop them from everheating, but they should probably be replaced with proper 30W 6L6GC's as well. Rectifier tube was GZ34 even though the tube chart calls for 5U4GB. I put one in and we are now at 500V which is probably still too much for those cheap tubes. A correct rectifer does help the voltage problem a bit, but there's 7,2V AC on heaters under full load and the voltage selector is already in the 240V position. I assume this gives away that the PT could have developed a small short in the primary winding and needs to be replaced. The owner is hesitant, though, as he's worried that the unique - as he say - sound of his amp could change.

Are silverface Fenders notorious for the voltage creep up problems? I know the network mains voltage is now higher than 50 years ago, but in case of this amp the difference is too big anyway, and it's not even running on the 230V selector setting either. It's obvious that voltages in old amps are usually higher than specified in the schematic, but I'm sure we're not talking about 540V on 6L6s' plates. 

On the side note, only the filter caps were replaced in this amp at some point in the past and probably the tech was aware of the higher voltage as they are now all doubled in series, not only the first one. No resistors across the converted ones, unfortunately. It's definitely the AA270 model judging by the bias supply arrangement and a few other details, but values of the resistors in the supply doesn't match the schematic which calls for 1K and 4,7K after the choke and I have 2,2K and 10K instead.
All other caps including filter caps in the bias supply are still original from 1972. I could observe the bias voltage go rapidly up or even disappear as I was lightly tapping the bias board - frightening, so I had to replace the associated caps immediately. The 12AT7 driver tube turns off and on by itself when it feels like it, which is always fun.


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  Random Access Footswitch
Posted by: Strelok - 10-13-2021, 12:28 PM - Forum: Switching - Replies (3)

Hi guys,

In TUT2 there is a random access footswitch, built around a 74175.

It works perfectly with alternate switches set up as momentarily.

Very cleverly done!

I'm trying to use normally open momentaries but I can't get it to work.
I tried moving the 10K pull-ups to become pulldowns by connecting them to the ground rail, but no go.
Can this only be done with normally closed momentaries?

Or do the diodes need to change direction?

Talk to you later,


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  Hey everyone, long been a fan of the forward thinking of LP & Mr O’Conner
Posted by: TopBoostChimer - 10-03-2021, 05:38 AM - Forum: New Member Introductions - Replies (1)

Ive gotta be honest.. The lack of posts, as well as no visual signs of regular user traffic/dialogue concerns me. At first glance I thought it was maybe that this forum is new. But then noticed unanswered treads dated back to 2017 so  Undecided

**I truly hope I’m wrong about this. As I I briefly mentioned in my title I’m a huge fan of everything Kevin O’Conner and LP have been doing for many years!! I would’ve gotten involved here as well as utilizing the available circuit PCB’s sooner’. But unfortunately it’s taken a bit for my personal experience and level of knowledge to catch up with my enthusiasm! Anyway, I actually have a couple projects that I’m working on currently and it would be so huge to be able to reach out to the members here for potential help/advice!! 


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  Kits needed to build an LP-PRE?
Posted by: galejt - 09-25-2021, 01:33 PM - Forum: Personal Projects - Replies (2)


Greetings. I have a question for you and/or the Community.

I would like to build a complete LP-PRE in a 1U or 2U rack unit.

As I see it I need the following kits:

And a chassis kit.

Am I missing anything to complete this?

Can you tell me whereabouts I can secure the pieces/parts to mount cards in vertical orientation in the Hammond chassis?

Many thanks,
Jim Gale
Roanoke,VA USA

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  On/Off switching for preamp tubes
Posted by: Kai - 09-22-2021, 03:54 PM - Forum: Switching - Replies (10)

Hi guys,

I just posted another thread concerning power tubes. In this thread I would like to raise a similar problem.

I built all LP preamps during the last years. I have a standard layout with one PSU-PRE and two LP preamp boards in one rack unit. For example there is my "British preamp" rack unit with M-PRE and V-PRE. With a simple SPDT I switch between outputs of M-PRE and V-PRE. The SPDT is connected to the main preamp output jack. 

Most of the time I only use either Marshall or Vox sounds while I'm playing. This means, one preamp board including heat dissipating preamp tubes is permanently turned on but not used.

Is there any mechanical switching that would turn off the B+ plate supply and the heater feed at a time between PSU-PRE and for ex. M-PRE respectively V-PRE? Maybe anything better?

Unfortunately, most DPDT switches can only handle DC voltage up to 250V. B+ plate supply usually is much higher. The heater feed can easily be broken. It is 12 V AC from the power supply.

Can anybody help me?

Best wishes 

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  On/Off switching for pairs of power tubes
Posted by: Kai - 09-22-2021, 03:30 PM - Forum: Switching - Replies (3)

Hi guys,

to cut along story short, I have a PA-10 with two boards, each  with a pair of tubes. A PA-66 with two EL34s and a PX-66 (actually a cut PA-66) that carries two 6L6s. I regularly use either EL34s or 6L6s. Since there is a lot of heat under the hood, I would like to turn off the pair I don’t use while playing through the other one. I'm not talking about muting, rather about powering on/off. 

In TUT, p. 6-57 there is a paragraph on Cathode switching as a stand-by for power tubes. Is this the best way to reduce heat? I am thinking about a solution that completely turns off either the EL34 or the 6L6 pair if technically possible.  

Is there any mechanical switching that would turn off the B+ plate supply and the heater feed at a time between PSU-PA and PA-66 respectively PX-66? Maybe anything better?

Unfortunately, most DPDT switches can only handle DC voltage up to 250V. B+ plate supply usually is much higher. The heater feed can easily be broken. It is 12 V from the power supply.

Can anybody help me with this issue?

Best wishes 

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  TUT5 SuperScaler question
Posted by: stephenl - 09-17-2021, 10:14 AM - Forum: TUT Q&A - Replies (1)


I recently built this amp - it's claimed to have about .5W sine output and I'm guessing maybe a watt at full output. 

I was looking at the Super Scaler project in TUT 5 and though that would be a cool next project.

From the TUT5 text I think it has a gain of 3 to 6? Boosts a 20W amp to 60W?

I'd like to scale my .5W amp up to say 30W, so I'd need a gain of more like 60. Can I get there by changing the input and output transformer impedence ratios?



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  Upgrading Eagle version
Posted by: K O'Connor - 08-20-2021, 12:04 PM - Forum: PCB Design - No Replies

Hi Guys

If you were lucky enough to buy Eagle back when you could actually buy it for a one-time outlay of $$, you may be in a good position to upgrade to a better version. You are given a "key" on a floppy or other means and this is your licence to use Eagle and must be loaded when you load Eagle.

When CADsoft was around, they had various version of Eagle available for download. Your licence would open them once they were on your computer - it's been ages since I did that, so I'm probably not describing it correctly - and for most of my Eagle experience I've been using 4.16r2. The computer that lived on began recently to randomly shut itself off. This was a nuisance until one time I had just saved my Eagle work and the computer went off. I restarted it but then could not reopen that file - it was corrupted - lots of hours of work gone in a flash (more of a black-out).

Fortunately, I have another computer that a friend built for me and had loaded three Eagle versions onto: 6.3, 7.7 and 8.2. Also fortunate that the first computer behaved itself long enough for me to load my Eagle files and some other things onto a key so I could transfer them to the second computer. I placed the files into 6.3 and 7.7, both of which I had used minimally  in the past. I figured I would use 7.7 to rebuild the lost design.

The lost design was a rework of a previous design, so I began with that file. The board for this project is actually a panel of six boards and they happened to be positioned all around the origin instead of all in the upper right quadrant. This allowed me to place things accurately on the specific board I was working on, which was referenced to the origin. I tried to move  component but was blocked and a "board limit" window popped up. It turned out the 7.7 version was a crippled form, but my friend who loaded it would not have known that. Fortunately, the 6.3 was not crippled, so I am using it now.

The first computer with version-4 on it had a fatal flaw and is scrapped. The newer computer is faster and quieter - it is the quietest computer that has a fan that I've ever heard. It was designed specifically to be quiet so it could be in a living space. Frankly, having two potential work stations did not work for me; my brain can barely keep track of one computer, so the new paradigm is like the way-back paradigm and it's much easier. When my friend built this computer he had my Eagle key to be able to load the different versions. I believe 6.6 is still available on the web.

Like every Eagle version, 6.3 has its quirks. I believe they were trying to integrate Eagle with spice and 3D softwares, so some of the menus and file management is oriented towards that. For example, in older Eagle a project simply was saved directly into the Project folder. In 6.3, there are two subfolders in Projects, Eagle and Examples. Your projects go into the Eagle folder BUT not directly. Maybe there is a bug? but when you go to "ave" or "save as..." the Project folder only shows the Examples subfolder, so you have to save your project into there, then open both subfolders and rag and drop the project into Eagle.

Another quirk of 6.3 is that when you select the "Change" tool, the menu that appears has six completely useless-to-me options listed above "Layer", which used to be the top choice.

Another quirk is when you use the Group function. Old Eagle drew a box around the grouped elements, where 6.3 makes a shaded box. You used to be able to select the next function, say Move, then right click anywhere in the box to move the group. Then right click again to make a smaller move adjustment. In 6.3, when you right-click on the group a menu pops up of things you can do with the group. Move is way down on the list, so you cursor down and click and the the group has jumped down to your cursor. if you need to make a fine adjustment to the move, you have to right-click, cursor, have the group jump again and try positioning it more accurately. between these moves, it is best to change the scale so the adjustment can be made more precisely. of course, once the group is tied to the cursor the first time, you could cursor up to the Zoom-in function and try to move the group in one go.

A problem with group moves is that the cursor may not be in a convenient place within the group to accurately place the group where you want it. At least in old Eagle the group did not bounce around with each new attempt to move it.  You could make the first move and drop the group. Then zoom in and grab the group by the corner and move that corner to exactly where you want it.

Eagle is always throwing you little curves Smile

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  Fusing for Bias Supply? Fusing for the OT?
Posted by: makinrose - 08-19-2021, 01:02 PM - Forum: Power Supplies - Replies (4)

I have a couple of questions on fusing. 

A.  I recently started using some transformers with a separate bias winding that I plan to use with a full wave bridge rectifier.  Is it good idea to protect it with a fuse?  I'm assuming if I did fuse it I'd use a slow blow 2 to 3 times the current rating of the winding?

B. While I fuse the windings on the power transformer is advisable to use fuses for the OT too? Or would that be redundant?   If I should so what's the most effective way to do that?  

Thanks everyone for the help!

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  FFT Software Spectrum Analyzers
Posted by: ZeusMC - 08-05-2021, 03:01 PM - Forum: Test & Measurement - Replies (2)

Are any suitable for tube circuit development?


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Come in where it's warm!
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
Please remember these three principles: respect, sharing, community.
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