How hard is it to rewire my amp from 6av6/6cg7 to just 12ax7 types?

[jimytheassassin]

Hi, I posted a long time ago here calculating b+ voltage drop, decoupling?

Basically I want to use the transformers from an old Phono Amplifier and preserve the output stage, but re-wire the Pre-amp section to better suit Guitar. I have 2 pre-amp tubes currently, 6av6 and 6cg7. The draw is thus .3A and .6A . Is it incorrect to assume that I can redesign the circuit with up to three 12ax7 type tubes running on 6v at .3A each? The 5y3 rectifier puts out around 300volts, but I suppose I need to know how much current the signal will desire? That's where I sorta get lost now. I picked up morgan Jones Building Valve Amplifiers and it's still not clear to me.

[m6tt]

first off, 6cg7 is a much better tube for a guitar amp than 12ax7, especially as a phase inverter or 2nd/3rd stage. Check out old magnatone schematics, and then go see what a big magnatone amp goes for on fleabay. The 6av6 (if I recall correctly) is similar to the 12av6, single high-mu triode dual diode. First off, it's a 7-pin tube. No 12ax7 has any fewer or any more than 9 pins. However, you could try a 6j5 (dual high-mu triode, kinda like 12ax7 and cheaper and 7-pin). Or you could try a 6au6, a single 7-pin pentode. Some of these tubes are much better than others, but a lot of amps from about 63-64 on had pentode input tubes, gives a lot more overdrive.

[jimytheassassin]

Ya, I'll be redoing the whole amp, soup to nuts, so I'll buy new 9 pin sockets for the project. You're right, the 6cg7 is a great tube, but the gain factor of 20, and the linear quality seems to be a bit of a negative IMO.. but I am new at this. Guitar amps don't need to be linear. My goal ultimately is to achieve more output volume, and maximize the gain in the pre-amp section. This amp was designed to run lean since it's a Phono amp, and musical quality was more of a concern. Anyhow.. I'll check out these amps you speak of.

So, If I redo the amp here's where I'm getting stuck
1. If the heater values are equal ( 6av6 .3A + 6cg7 .6A = .9 VS 3 - 12ax7 .3A each = .9)
am I safe relatively speaking?

2. how do I figure out the current required of the signal after they're biased.. I'm still fuzzy on this. My schematic for the RCA shf-7 says something like 72ma on the 300v feed. But I'm not clear on whether that's the actual usage..or the maximum rating.

3. Should I just do 2 valves instead of 3?

[Tom Bavis]

12AX7s and the like draw about 1 mA typically, so they won't affect the total current much. Why not leave the 6CG7 alone and just add two stages of gain? A 12AX7 should give plenty (I assume you'll want some guitar style tone controls, which will eat up a few dB). Ditch the feedback, though, (just delete the R and C from the output transformer) unless you LIKE an abrupt jump into clipping...

[jimytheassassin]

I would be fine with leaving the 6cg7 for the PI, except I have no idea what my transformers are. I figured to be safe I should first work within the current design. Since the 6cg7 has a Ih of .6, I figured the only way to safely add a couple valves is to remove it and substitute the 12ax7 types in it's place. I will probably use a 12ay7 or 5751 in it's stead for the PI.

Thanks for the info on the ma requirement.

BTW is it complicated to ascertain the true capabilities of an unknown transformer, if all I have is a multimeter?

[m6tt]

PI isn't dependent on the transformer, and I would make sure the 6cg7 isn't already the phase inverter. In any case, you could use the 6cg7 as a paraphrase style PI (bad for hifi, great for guitar) and get more gain than you would from an LTP. I would imagine the old circuit went 6av6 -> 6cg7 (both sides phase invert) -> output tubes? Also, what kind of output tubes does it have? You might not need much gain in the phase inverter if they're el84s or 7868s.

[tubewade]

What you are wanting to do can be done. Do an Internet search for "Fender Deluxe Schematic" to see several examples. The 6CG7/6FQ7 is much better suited to inverter/driver duty in a hi-fi environment, but in a guitar amp using less than optimal valves often proves to provide the best overall tone. The Fender Deluxe 6G3 ought to be very similar to what you are looking for.

Best, Wade

[jimytheassassin]

Hi m6tt .. ya it's 6av6 ->6cg7, but 1/2 of the 6cg7 is gain make up.. and the 2/2 of the 6cg7 is the PI via cathode/anode , connected to two - 6v6gt in Push/pull. Thanks for the help. Anyhow the reason I'm removing the 6cg7 is for the Ih amps. I don't know what my transformers are capable of, and I don't want to overburden them by adding more current draw on the 5v than it was originally designed for. By removing the 6cg7, I can add two 12ax7 in it's place and redesign the circuit to my liking, with out changing the supply in any way.

Tubewade: Thanks for the suggestion. I was already scanning the 5E3 design.. which has 6L6, but exactly the preamp design I'm thinking of. I don't totally understand how inputs can be cross patched to loop both triods in the first 12ax7, but.. I'll figure it out.

Thanks again everyone, this really boosts my confidence

[jimytheassassin]

Originally posted by tubewade
What you are wanting to do can be done. Do an Internet search for "Fender Deluxe Schematic" to see several examples. The 6CG7/6FQ7 is much better suited to inverter/driver duty in a hi-fi environment, but in a guitar amp using less than optimal valves often proves to provide the best overall tone. The Fender Deluxe 6G3 ought to be very similar to what you are looking for.

Best, Wade

So I looked at the 6g3 schematic or "brown face deluxe". That's a great idea. I've never seen a PI structure like that where V2 is the PI, and v3 is just for gain after the fact. I'm still looking around to see what options I can work with. I'd really like something with 1 input, and a tone stack consisting of treble and bass, with a single volume. If I can work more into the equation that would be cool too. I'll probably add a switch for a bright channel as opposed to individual inputs. And I'll top it off with a matchless combo, 8" and 10" speaker most likely.

If anyone else has some good ideas, thanks in advance. And again, thanks for the help.