How can I get my 12ax7's to distort as good as transistors?

[Chuck H]

I have to say that an exact schematic of your amp would be very helpful now. Even if you hand draw it. To say it's very much like a MKI MB only goes so far. I could say that a Maserati is very much like a VW Bug. Four tires, two headlamps, steering wheel, rear mount engine, gas and brake pedals, manual clutch, runs on gasoline and roughly the same weight. Considering the similarities, should we be surprised that one is so much faster and better handling than the other? Anything untold about the circuit could be the lynch pin in your efforts that no one here can see.

And then there's the limitations of your circuit board and layout. I'm not going to much trouble attempting to trace it from the photo. But it does look pretty screwy. Attempting to rearrange the triodes for more stable conditions seems fine. But there are other problems with that. Like using a triode further up the chain will be more susceptible to B+ noise and possibly on an inappropriate ground. Or the added lead length needed and crossing the circuit back and forth across differing signal levels and phases. This is a tricky amp to get the most out of considering your method and expectations.

[defaced]

I'm just surprised that moving identical stages did so much change to the sound. Maybe the diifferences in the tubes are to blame...I'll swap them and see.

Yes, they're identical, but that only applies to the schematic. That doesn't take into account what signal they're being presented. While we're on using cars as an analogy, it's like my '90 Mazda Protege which handles great at 65 mph, but things get a little hairy up around 90 mph (if it can even get there ). Same car, different conditions.

I'm thinking that I can try to tone shape the #2 inputs(boost stage) by using a different bypass cap in series with a resistor, and putting a parallel cap across the plate resistor. Like a fixed tone control so I'm driving the right frequencies.

It's certainly worth a try. Keep us posted. I also echo Chucks calls for a current and correct schematic.

[guitician]

I have to say that an exact schematic of your amp would be very helpful now. But it does look pretty screwy.

I changed the few differing values and removed the extra tubes and GEQ. I left that one bias resistor as "Factory Select" because it just sets the bias to whatever voltage is needed.

The control panel is just like this Pro-100 except the "middle" is "reverb"
This is the inside of the pro-100 that I attached by mistake.

[guitician]

I'm thinking now, after looking at the MKII schematic, that using a DPDT switch, as they have a relay, switch the "Boost" stage in/out of the circuit. there is a hole in the chassis for a toggle switch and It never had one in there, so maybe that's what it had? But mine did have that custom input jack that "inserts" a stage inline. Maybe one switch replacing two relays wont work because of the close proximity of the contacts.

[guitician]

I was looking at how I thought I had it wired and noticed that I forgot to add a capacitor after the last stage, the one the master volume control is feed by. I used a 100k pot instead of a 1M too. Well, adding a .01uf cap made a difference. The current through the 270k + 100k must have caused a shift in the operating point of that last stage. I remember messing with how the master volume was wired when I fixed the reverb. I also had a ceramic .001 coupling cap for the boost stage and changed it to a film/foil type. It's sounding better now. I think I'll just get this cable routing figured out nice and neat and call it done. Thanks guys for all your advice, even though my lack of attentiveness was the real problem. At least now there is some info here on one of these rarities.

[defaced]

Sounds good! Glad you got it sorted out. Enjoy your amp!

[guitician]

There's supposed to be an image here of the amp in this thread......?

[loudthud]

I got an Invalid Attachment error when I clicked on it.

[J M Fahey]

Well, attachments have been dissappearing regularly, some kind of server error or over zealous anti virus protection.

[Chuck H]

The Chinese are taking any infrequently used images, scrapping them for parts and making new, sellable images out of them