Quote from: g1 on Today at 01:54:20 PM

Quote from: Frank75 on Today at 08:18:36 AMAs soon as I moved the volume pot I lost, say, 80% power.

Have you cleaned the pots?  If not, do so. 
If you already have cleaned them, it sounds like the volume pot is defective or needs to be re-soldered.

A dirty pot was my first thought too so I did give it quite a few twists and turns. No crackles, no pops, no jump scares.
Potfluff doesn't seem consistent with losing volume over time. Strange though that moving the pot seemed to trigger the defect, so not excluding any options at this time. Including coincidence 

Quote from: Frank75 on Today at 08:18:36 AMAs soon as I moved the volume pot I lost, say, 80% power.

Have you cleaned the pots?  If not, do so. 
If you already have cleaned them, it sounds like the volume pot is defective or needs to be re-soldered.

Received the new tubes set today. They seemed to do the trick. After half an hour or so, I still had full volume...until I adjusted it. As soon as I moved the volume pot I lost, say, 80% power.

I was reading back in this thread to see if there's something I should know about biasing (is there?) and came across this comment from Mprall00:

Quote from: Mprall00 on January 29, 2019, 12:40:17 AMReplaced r17 with the 50k pot for gain.  It sounds good now.  Incredibly loud.  It was oddly quiet stock for 30 watts

That sounds a lot like the reverse of what I am experiencing. At start up it is by al means incredibly loud.

Anyway, while replacing the tubes I noticed that the sockets were not screwed to the chassis so somebody's already been messing around with this amp. I'll add this one to my project list.

Quote from: DrGonz78 on May 15, 2024, 11:01:19 PMList voltage reading for Q8 & Q9 at  all pin's connection points. Measure all power supplies. What are your -/+40v rails? -/+12v rails? Can you scope a signal through the unit and figure out where the signal goes bezerk?

Edit: If you scope the output is the fuzz actually crossover distortion?

Thanks for the reply, and sorry for my belated response.  I had someone at my company walk out, which pretty much threw my last two weeks into a tailspin. I checked the power rails, and both have -/+40v and -/+12v I expect.  As for the long tail pair, here are the readings I got from them with a 1kHz signal at 5v:

Q8
C -0.704
B -0.127
E 38.83

Q9
C -0.713
B -0.137
E 39.35

I'll admit my oscilloscope skills are lacking in terms to how to best use it to figure out  where the signal goes off of the rails.  I did put a scope at the end of the chain as suggested, and I can see that the bottom part of the sine wave appears to be clipping as it appears to be pretty flat. I'm currently looking into how to measure crossover distortion via watching some youtube videos, but any suggestions you can provide to an amp noob would be greatly appreciated.

EDIT: the link posted previously was strange. To say the least. Now it should be fine
https://m.soundcloud.com/user-996907905/joe_davisson_vulcan_preamp_bc3

Ok, here it is.
Guitar: LTD MH 50 with pickups from Dean Vendetta 2
Amp: single-ended EL91 Fender Champ-a-like
Speakers: some 1980s 12" marked Celetion (sic!)
Settings change as I go. I do not touch the guitar's volume knob, I just play with the knobs on the preamp.
The values of I used for the tone stack, presence and level are different than those on Mr. Davisson's schematic because I only had pots for tube amps. All caps have been scaled accordingly.
Enjoy.
Oh, and please forgive my sloppy playing 😅 I don't play as much as I used to...

Ok, so here's an update.

I've put it together on my breadboard and performed a sound test. It is VERY promising. I might tweak the tone stack a tad, but, man, does this thing sound powerful!! Perhaps not an uber-extreme-downtuned-modern-metal kind of powerful but great nonetheless. Great job, Mr. Davisson!
I've made a quick recording at a baby-sleeping-in-the-next-room volume. I'll upload it later, on SoundCloud perhaps, and post the link here.

There are amps out there like Mesa boogie simulclass where they have a pair of each EL34's & 6l6's. But these are pairs of tubes and the amp is designed to allow this arrangement. No amp that I know of uses one of each different tube as a pair. It just doesn't work in a balanced way. People spend all day trying to create a matched pair of tubes for their amp circuits. EL34 tubes require a power transformer that can handle the current demand which is more than a 6L6 demands on a circuit.

Thanks Jazz.   

Quote from: jbarrie88@sbcglobal.net on May 23, 2024, 05:44:09 PMPardon my inexperience. How would I do that without providing a URL?

When you are in the 'quick reply' area click on the 'preview' button.  There will be an option to directly attach your files here.

I recently purchased a TSA30H head for a small price. According to the seller the volume goes way down after a little while. After reading rave reviews on this amp and figuring the problem sounds like just some bad tubes I went ahead with the deal.
Then looking for a schematic I stumbled on this thread and started thinking that my small price wasn't small enough.
Anyway, plugged it in today, hooked to a V30 cab, and I'm quite happy with the sound. Yes, there's a slight hum on ridiculous volume, but not more than on many an old Fender. After a few minutes I got the opposite of what I was expecting. The volume shot way up giving me a slight heart attack and some angry looks from the missus. Played for another 10 minutes or so without further issues. Put the guitar down and left the amp to warm up for half an hour. Sure enough, after that the volume was extremely low.
So I pulled the tubes for visual inspection and much to my surprise it had one 6L6 and one EL34 inside. Now I understand that you should not put EL34's in an amp set up for 6L6, but could that be the whole reason for the soft-loud-soft sequence?
I'm trying to get my head around it, but I just don't understand tube behavior well enough.
I also understand that asking this on a solid state forum is sort of blasphemous, but this seems to be the only thread on this amp in the whole world wide web.
One way or the other, I ordered a pair of 6L6's for proof and pudding but that will take a few days.
I was also wondering if using an EL34 can do damage to the rest of the circuit and if so, what I should be looking for?

Here are the files.