Tybe radio conversion to guitar amp

[gilmo789]

I got myself as old 5 tube radio which I reckon I can convert into a 5ish watt guitar amp. It's just the chassis so I'm not destroying anything. The valves light up when I plug it in (LBL used).
This is the exat thing: http://kbmuseum.org.uk/kb_images/ar30/ar30.htm

I reckon I can get rid of most of the radio kit but I'm wondering if there's any pitfalls ya'll can foresee before I get going.

I've got the circuit diagram and I've greyed out everything that's not the amp, It's got a line i and a speaker out. Most of the parts are in good nick but I'll certainly need to recap it.

One thing I'm not sure of is the preamp tube is a double diode triode (V3). Can I just use this as a triode or do the diodes need to be dioding?

Thoughts?

V3: 6Q7G (double diode triode)
V4: 6V6G (pentode amp)
V5: 5Z4G (rectifier)

[nosaj]

Tear it all out and make a fender champ amp.
nosaj

[gilmo789]

could I do that with the existing tubes?

[nosaj]

could I do that with the existing tubes?

6SJ7 6v6 5y3
Fender champ 5c1

nosaj

[g1]

6SJ7 6v6 5y3
Fender champ 5c1

nosaj

I think his 5Z4 rectifier should work ok in there.

[Enzo]

ANy reason to ditch the probably working existing power amp?

Probably needs more gain than the single triode, but the tube just before is a pentode. WHy not try to get that to sound good?

[gilmo789]

ANy reason to ditch the probably working existing power amp?

Probably needs more gain than the single triode, but the tube just before is a pentode. WHy not try to get that to sound good?

That's the spirit!
I thought triodes made decent preamp tubes, is this not the case?
I haven't got the thing to make any noise yet, The caps are all measuring in the pF range but the rectifier works and the tubes light up. I tested the existing tubes for shorts and they seem OK in that regard.

I think I can recap it for a small amount of money, There's only 10 in the amp circuit.

I'll see what happens!

[nosaj]

before you spend any money on it check the transformers first.

nosaj

[gilmo789]

In the instruction manual, which is found in the link above, there's a list of voltages taken at various points of o the circuit. I measured the rectifier AC input and DC out and it's in the right ballpark. A bit lower but I put that down to the LBL. Is there a test I can do other than verifying that it transforms?

[nosaj]

You could inject signal at the grid of v4, but first I guess you should test the speaker with a AA bather for clicks.

nosaj

[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.

[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]

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]

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

[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.