Need Some Help With Tube Circuit!

[Coral Head]

Hi all,

I have decided to learn more about how amps work and have been reading some heavy material on the subject with mixed results. I decided to buy a simple amp to learn from......an old mono record player amp made by Voice of Music.

I now have the amp and the circuit diagrams which are not too far removed from a Champ (12AX7, 6V6 tubes). The amp works fine and sounds pretty good in a clear, dry sort of way. It does not break up much although it has a little hair if you pick hard. I have it connected to a 12" speaker in a Peavey amp.

Would anyone care to look at the circuit and comment on what is good or bad about it and what mods would help it be a better guitar amp? It is all original with original USA tubes as far as I can tell.

Thanks,

Colin

http://i233.photobucket.com/albums/e...Amplifier1.jpg

[SatelliteAmps]

How far do you want to go? If it was me, I would rewire pretty much the whole thing. If you want to just try a few things to see what they would do, I would probably add a cathode bypass cap (pin 3 and 8 on the first preamp tube). Put a 25uf @ 50v cap across R9 and R12. Should add a bit more bight. Might also change C7 to a .047uf.

[Coral Head]

I am not ready to rewire the whole thing but will definitely try your suggestions and see how they affect the tone......Thanks!

C

[VacuumVoodoo]

I don't mean to discourage you but this circuit perhaps is not the best learning platform for beginners. On the other hand, this circuit can really teach you a lot of stuff.

Here's why:

There are several things going on there not immediately obvious.
Firstly we have two major negative feedback loops, a global one going from loudspeaker output to the first input triodes cathode. This loop contains bass and treble tone controls.
The second loop is a local feedback around output 6V6 tube. This loop is formed by the tubes cathode being connected to ground via transformer secondary winding.

Now, when you implement Adams suggestions ( good ones for an amp without global feedback) you will notice two things:

An electrolytic cap parallel with R12 will not do much as it will only increase gain of that stage by some 4db, global feedback loop will reduce this gain increase down to perhaps 1-2db. It will also be affected by tone control settings.

A 22uF cap parallel with R9 will virtually render global feedback inactive, the amp input to output gain will increase quite a lot. THis comes with penalty: tone controls will become disabled.

Observe that when you plug a mono plug into microphone input it will put C4 0.47uF in parallel with R9 having in very similar effect to the above.

another interesting detail is how -15V for output tube bias is derived. What? No rectifier diode? don't worry, it's not needed in this circuit.

But by all means go ahead and experiment, it's the best way to learn.

Have fun

[donnyjaguar]

The first thing I'd do is add a fuse!

Its an interesting circuit. However, it was designed with economy (and phono records) in mind and there is much room for improvement. You can bump the HT up to 350Vdc for more drive and re-work the rest of the circuit for more gain. The treble control will need to be replaced.

[Coral Head]

Thanks for the input. I have already added a fuse and a grounded power cord.

Keep it coming!

[hasserl]

I'd move the Volume control after the input stage rather than in front of it, then I'd give the input a 1M reference to ground. Interesting tone controls, being in the nfb loop, but I'd either rip it out, or return it instead to the 2nd stage cathode as in the 5E1 Champ. Then as noted above, bypass R9 & R12, I'd use 4.7uf caps but I like the reduced bottom end. The classic Fender design would use 25uf caps there. IMO they tend to get too farty when over driven, the 4.7 helps reduce this tendency. If it's still too boomy I'd drop the values of C6 & C7 to reduce bottom end. Hard to say what it will sound like though with those tone controls in the NFB. You'll have to experiment to find out.

[donnyjaguar]

Because this is a closed loop design, changing values and adding components within the loop will not have the same effect as the classic modifications. I have to admit I don't like the thought of the higher DC voltage riding on the speaker output, as this design will have.

[VacuumVoodoo]

Originally Posted by donnyjaguar

I have to admit I don't like the thought of the higher DC voltage riding on the speaker output, as this design will have.

It's only some 30mV, nothing to be concerned with. All data needed to calculate this are in the schematic.

[doctorx]

I have a couple of these amps. I basically gutted one and rebuilt it into a super charged Champ like circuit that kicks ass. After redoing the power supply the voltages were too high for a 6V6 so I wired it to used a 6L6 or EL-34. I don't care for 6V6's anyway. It's pretty loud, it puts out about 10 watts. I also installed a Pentode/Triode switch, but it hums in Triode mode.

It gets three kinds of tone: Overdrive, Overdrive, and Overdrive. That is to say, very little clean tone. That's ok, I like it like that, it's very Marshally for a single ended amp and scorches with an overdrive pedal.

I tried a few mods to get the stock circuit to sound good but there is too much weirdness all over the circuit to overcome, that's why I gutted it. After a couple more tweaks I'm going to rebuild the other one I have.

[donnyjaguar]

Noted, VacuumVoodoo. See what DrX says as I'd take this route too with the 6L6. I *think* you could reduce the gain enough to get some clean tone out of it though, no? Simply replacing the plate resistors with 100k should bring things down to normalcy. I would plonk the volume control between the triodes myself. I think another has suggested this. It looks like a cool project amp to be sure!

[Coral Head]

I am learning a lot from all your comments so keep 'em coming. I will have a chance to work on it next week I think.

C

[doctorx]

Here is a schematic of what I turned my VOM into, and a spec chart I drew up. Also an original schematic of my VOM, which is slightly different from the 168.

As I said, it basically is a Tweed Champ, with higher voltages. It can take either a 6L6 or EL34, but I have to unscrew the top of the chassis to get the EL34 in. If I could shoehorn a 6550 in it that would work too.

It is voiced for overdrive, with no negative feedback and cathode caps to suit. Not shown in the schem are these additions: A tone control after the preamp tube, a pentode/triode switch, and a Fender style high/low input jack setup. I was trying to make it into a poor mans Univalve.

The PT in this amp has higher voltages than the 168, but otherwise the stock setup is essentially the same. I've seen these go for over a hundred bucks on ebay, and to me it's not worth it because stock they sound like ass for guitar. On the other hand if you pay a C note then put another 50 bucks into it you have a killer amp.

[donnyjaguar]

It looks like DrX has the prescription for this amp!

I betcha if you put a tone circuit in there the added loss would allow some clean tone. You could always put a switch to override it.

[JJman]

http://i233.photobucket.com/albums/e...Amplifier1.jpg

Where's the diode feeding the -15v bias supply? Actually I now see that this is not a "tap" of the HV winding but rather tapped off a portion of the CT's ground. I've never seen that for a -bias supply.