Tybe radio conversion to guitar amp

[gilmo789]

For posterity and for anyone else looking to embark on a similar project I thought I would add a conclusion this thread ...1.5 years later
I enjoyed this and learned a fair bit, but basically I gave up because the quality of the wires, resistors, caps and valve sockets made too difficult. I never managed to get the pentode to work as a second preamp stage, partly because the old tube sockets disintegrated.

The radios are definitely a good source of parts like transformers and valves.
There is no issue with using a double diode triode as a first pre-amp stage - you can find similar parts in some old fender amps.
The chassis of this amp was enormous and never really suited to a 5W guitar amp
I was very impressed with how the existing amp dealt with line level signal from my computer as you can see in the video in a previous post - sounded great!
Ultimately not enough gain from a single stage pre-amp

thanks to all who tried to help

[gilmo789]

thanks for all the suggestions guys, I'll have a play about and see what I can make work.
the pencil diagram should now be correct btw

[Helmholtz]

R 18 in the original schematic (470K) is a grid leak resistor.

[gilmo789]

Enzo, g1, that’s exactly what I meant. Grid not input. New to tubes! Don’t quite have the terminology down

[Enzo]

His 6Q7 is no pentode, but the previous tube in his original circuit, V2, is a pentode and would be worth giving a try. Wire it like the 5C1 pentode.

g1, when he said 47k to ground from the "input" I think he meant the grid of the power tube. Y'know the power tube "input". he said that along with telling us to move the cathode bypass parts from that grid to where they belonged.

[Helmholtz]

Are the fixed resistors at the input on fender amps just for the voltage divider when 2 inputs present or should there always be a fixed resistor somewhere?

The input resistors serve voltage divider and grid stopper purposes. With a single input you don't need the voltage divider but you might add a 33k grid stopper at the tube socket.

(The 6Q7G is no pentode. A pentode would not amplify without screen supply. When wired as triode, the pentode's screen needs to be tied to the plate. But generally pentodes have more gain than triodes.)

[g1]

Here is the 5C1 again. You can see it has a 500 ohm and a 25K instead of the two 10W resistors you asked about. Not really a good idea to bypass those.
If there really is a 47K to ground from the input, that should not be. It will decrease the signal too much.

[Enzo]

Look at any basic Fender amp with two-jack input. It is not intended for plugging in two instruments. The resistors only form a voltage divider when you use jack 2. So the two jacks are basically higher and lower sensitivity. In many amps these days they label them Active and Passive (pickups) inputs.

Volume control wasn't on the input in the original circuit, it was halfway through the system. Yes, the volume control should be after the preamp tube somewhere. Look at those various Champ versions.

[Enzo]

Wiring the pentode as a triode? WHy not try it, you have it there. Or replce the triode with a pentode.

The extra filter stage is there to remove hum in this single ended amp. Push pull amps cancel power supply hum, single ended do not.

Look at Champ 5C1. It has a single pentode preamp and of course the 6V6 output. Look at Champ 5E1, They replaced your resistor with a choke. And now the pentode is two triodes. And Champ 5F1, they added a B+ node - your resistors again - to lower the 6V6 screen voltage.

So to get Champish, you can either use one pentode or two triodes as a preamp.

[gilmo789]

Tbh I was just lazy drawing the input. It does go to ground and the volume works. It’s a bit hard to decipher what’s going on in the original schematic. To get signal in I just crocodile-clipped to the gramophone input.
Are the fixed resistors at the input on fender amps just for the voltage divider when 2 inputs present or should there always be a fixed resistor somewhere?
also volume pot on input seems wierd. 5e1 champ has it between the amp tubes does it matter?

[Helmholtz]

As shown, the vol control won't work as it's wired as a variable resistor and not as a potentiometer = variable voltage divider. You need to ground the other end of the pot. This will also make sure that grid voltage stays at zero VDC, which is essential.

[gilmo789]

The above circuit is a pain to read so I redrew it in the Fender style. Seems fairly similar to a champ. I have a few questions of anyone can answer:

1. What's the point of the 2 x 10W resistors between the filter caps (R19 and 21); these don't exist in any champ circuit I've seen? They are also shot in that amp so I've bypassed them and the amp works fine
2. How bright should the bulb be in the LBL? It's fairly dim without the 6v6 but noticeably brighter when the 6v6 is on.
3. What's the point of c24 before the output transformer

My plan:

1. To get more gain I was thinking of using the other pentode (V2) as a triode and running V2 & V3 together as a something like a 12ax7. Is that daft?
2. Stick in an NFB loop

Any other thoughts or comments welcome!



fixed

[gilmo789]

Well it works!.
Enzo was right though, there isn't enough gain for the guitar. Works great as a Hifi amp for the computer though!

[gilmo789]

on the issue of gain:
Valve V2 of the radio circuit is a 6K7G. It's a screned pentode like the 6SJ7 (maybe thats here the similarities stop). Is there a reason I couldn't work that into the circuit to give decent gain at the preamp?
I'm new to this tube stuff so apologies for any egregious ignorance.

Cheers

edit
I realise now that's What enzo meant, I hadn't quite caught the meaning as I hadn't looked very much at the radio part of the circuit

[gilmo789]

It didn't actually come with a speaker but I have one of the correct vintage and impedance. I know that woks because I wired it to the out of a wee practice amp i have and it made noise.