6J5 and 6F6 SE - enough gain for guitar?

[PRNDL]

I have an old tube amp that has a 6J5 Triode and a 6F6 Power Pentode.

It has very low volume with guitar, which I believe might be because there's only one preamp gain stage.

I calculated the gain of the 6J5 to be 20.
6J5: Av = (mu * Rp)/(Rp + ra) = (20 * 220K)/(220K + 7K) = 20

The 6F6 output tube has an old Champ transformer connected to the plate.
It has a 390 ohms in parallel with a 25µF cap connected to the cathode.
I believe it has unity gain.

Is this enough gain for guitar?

I could replace the 6J5 with a dual triode. I have a 12SN7GT on hand.
Usually amps with two preamp gain stages also have negative feedback. This one does not.

Thanks!!

 

[Tom Bavis]

6SL7 would be better - especially if you want any tone controls. Without tone controls, 6SN7 or 6SJ7 pentode might be enough. Negative feedback is not usually used with instrument amps.

 

[gerryc]

Take a look at the schematics for the original Fender Champ amps. They used either a 6SJ7 (old version) into a 6V6 or a 12AX7 into a 6V6. There are slight variations over the years but they are very similar. If you want to stick to octals, you can also use a 6SL7 instead of the 12AX7 with little real-world difference. Also, a 6F6 would work fine in place of the 6V6 if that's what you already have. Take a look here:

http://tdsl.duncanamps.com/schematics2.php?make=Fender

Old Champ:

http://www.ampwares.com/ffg/schem/champ_5c1_schem.gif

Newer Champ:

http://www.schematicheaven.com/fenderamps/champ_5e1_schem.pdf

check some of the others there too.

 

[PRNDL]

Thanks!!

I have a 12SN7GT on hand and will most likely use that initially, and order a 6SL7. I believe the pin outs are the same.

Is negative feedback necessary?

I also have a single knob tone stack, which will drop the gain a bit.

 

[gerryc]

Thanks!!

I have a 12SN7GT on hand and will most likely use that initially, and order a 6SL7. I believe the pin outs are the same.

Is negative feedback necessary?

I also have a single knob tone stack, which will drop the gain a bit.

Pinouts on the 12SN7 and 6SL7 are the same but the heater voltage is 12.6 instead of 6.3 on the 6SL7. You'll have to accomodate the higher heater voltage for the 12'. The 12AX7 allows you to series or parallel the two heater halves to use with 6.3v (as is normally done) or 12.6v respectively, the 12SN7 does not.

Negative feedback is not absolutely necessary but it would not sound the same without it. The distortion would be higher, and the sound would be less "tight" (higher output impedance to the speaker) although the gain would be higher as well (not a bad thing if you use a lower gain preamp tube). If the tone stack is just a passive tone control (treble cut) it shouldn't affect the overall gain that much. I would experiment with the feedback (none, or various amounts). Just be careful of the polarity of the output transformer when you add feedback. If it's backwards, you'll have positive feedback and it will oscillate, in which case you would need to swap the secondary leads to the feedback resistor.

 

[boergars]

By the way, I suppose 6J5 still is half a 6SN7...

 

[PRNDL]

Thanks Gerry.

I ordered a 6SN7 today.

 

[Miles Prower]

By the way, I suppose 6J5 still is half a 6SN7...

A 6J5 is pretty much half a 6SN7GT. However, the 6SN7GTA and B are different: higher Pd, hardened control grid and characteristics extended into grid current territory. This being done for improved operation as a TV vertical deflection amp.

 

[SHiFTY]

6SN7 will prolly not have enough gain- you need a 6SL7 or get a 12V transformer and run that 12SL7 you have; or run a 12AX7 with a socket change. ((If you really want to get creative you could run a 6J7 pentode, replace the rectifier with a couple of diodes and run a 6L6... )

Base the input stage off something like a fender princeton 5E2 but using the 6SL7 and you will get a nice sounding unit.

Negative feedback is not necessary in guitar amps, but good earthing/grounding schemes are, you could get some hum otherwise...

 

[engels]

no

I would not put the 6SN7 - it's heater rating is 0.6A which might be crucial in such a small amp - twice more than 6J5. The power transformer may overheat or you'll be getting lower voltage. Those old 6F6-period amps never had overrated transformers, everything was minimal to save costs.

I'd be looking for 6SL7 (newer champ with 12AX7 circuit) or 6SJ7 (the first champ) which has the same 0.3A heater rating as 6J5. 12AX7 is not a good idea - you already have an octal socket there, so why?

 

[engels]

nfb

and no feedback please! It will reduce too much gain and probably improve nothing in such a small amp.

And by the way, one more thing:

6F6 is very sensitive and overloads earlier than 6V6, so you don't want too much gain. Even 6SL7 may be too much, you may need a grid-stropper resistor on the 6F6. 6SJ7 should work okay. Or use a 100K or 250K volume pot (instead of champish 1M) before the 6F6 - I'm afraid with 1M you might get some oscillations at maximum volume.