Need a little help converting a PA amp to a guitar amp

[JUSS10]

A while back I had picked up a little PA amp that had a 6sl7 preamp and 6l6 power amp section with a 5y3 rectifier. Its a cool little amp. I recapped it, couldn't get it to work right and set in on the shelf. a few days back I grabbed it, found a few issues and got it work pretty well. hooked it to an old 12" speaker and it sounds pretty good. That said I have a few questions. For a 6L6 amp it seems to have pretty low gain. The amp had a mic input and I swapped it for a 1/4" to plug in a guitar. Looking at the tubes I figured it would be similar to a fender champ with that used the 12AX7 preamp and then using a 6L6 instead of a 6V6. I went ahead and drew the schematic for the amp. I went over it a few times so I'm pretty sure its right. There were a few of those molded caps with the dots that I couldn't figure out the value and I didn't get the values of the potentiometers but the rest should be accurate. There are a few things that seemed weird to me. The tone circuit is right before the power amp and seems rather simple, when the amp is on, it doesn't seem to do much. There is also a cap from pin 3 on the 6L6 to ground before it goes to the OT which I have never seen before. Finally the feedback loop seems a little weird as to where it goes back into the preamp, as well as the cap that is inline with it. Like i said, the amp seemed quieter than I expected and when it would overdrive, it kind of just fizzed out. Also, its rather bassy compared to any other amp I have. Could a few of you tube genius' take a look at the values and schematic and make some suggestions?

I attached a schematic

Thanks!

Justin

 

[JUSS10]

Just noticed there is an instrument and amp section to the forum. If a moderator feels its better to have this there, please go ahead and move it.

Thank you,

Justin

 

[Mooly]

Consider it done 🙂

 

[FranciscoPerez]

Hey!!
Not a tube genius here, but maybe I'd do something like this:

An externally hosted image should be here but it was not working when we last tested it.

It would be great to have a schematic of the PSU though or some voltage readings

Hope it helps a little.
Cheers,
Fran

 

[FranciscoPerez]

I drew it wrong, the 68k resistor next to the 50k pot goes to the cathode of the 2nd half of the 6sl7, and not to the 1M pot.

Sorry!!!
Regards,
Fran

 

[tinitus]

I drew it wrong, the 68k resistor next to the 50k pot goes to the cathode of the 2nd half of the 6sl7, and not to the 1M pot.

Sorry!!!
Regards,
Fran

you mean like this ?

 

[JUSS10]

Thanks for the tweaks to the drawing. That's about what I was thinking as far as circuit paths go. I can get some voltages tonight and draw in the power supply.

Justin

 

[FranciscoPerez]

Right on Tinitus, thanks!!!
Great Justin, and sorry for the rough drawings 🙂

 

[JUSS10]

whats the 50K pot for?

 

[JUSS10]

Well i went ahead and made a few changes via the schematic you changed. I ditched the tone setup, pulled the cap that was tied to the output transformer, fixed the feedback loop (i didn't have a 50K pot so i just bypassed it for now, still not sure what thats there for) changed the caps in the signal chain and the input resistors. I didn't add the resistors right before the output tube or the resistors on the cathodes yet. However, just doing those few things made a huge difference, way more gain from the amp! That said, its also quite noisy, mind you, I am using a telecaster with single coil pickups. Is there anything I can do to reduce the hum? it sounds like typical tube amp hum to me, nothing out of the ordinary, just curious if there is a way to bring it down.

Thanks everyone! I may just strip the whole amp and rewire it from scratch as there are a lot of left overs from its original state.

Justin

 

[ArcticBreaze]

Hum?

hum is usually caused by lead dress, get in there with a chop stick or other NON_CONDUCTIVE thingy (iE not your fingers) and move the wires round abit at the valves, see if that makes any diferance most hum comes from heater wires, but and SE amp is always noisier than a push pull. here is a schematic its for a harmonica amp, but its been my experience that it should sound good for guitar as well. you might wana raise the plate voltages abit thou.

Good luck and happy new year😀

PS lowering R16 raises plate voltage it would depend on your power tranny tho

 

[JUSS10]

Thanks arcticbreeze. I was messing with it yesterday and thought I'd turn up the gain a little. It distorted but not in a good way. You'd hit a chord and it would have a nice distortion then it would degrade quickly and drop in volume. I turned the gain down and now its still there. Tubes? Speaker? I think I may just re wire it with a known schematic like the one posted above.

Thanks

Justin

 

[FranciscoPerez]

Hi and happy new year!

I placed the 50K pot in the NFB loop to change a little bit the sound texture. It may be quite subtle, though. You'll need to tweak the values in the loop to taste.

Is the hum present without guitar plugged, and increases when you turn up the volume knob?
As ArticBreaze says, one possible cause may be lead dress. Make sure that heater wires are tightly twisted and kept away from signal wires.
I'd say a bad grounding scheme could be the cause here though. What is your grounding arrangement?

Hope it helps!!
Cheers,
Fran

 

[nigelwright7557]

The output transformer is drawn wrong.
The 8r tap should be where the 4r tap is and vice versa.

 

[bparnell57]

The hum may be the shielding in your guitar or cable, or just the nature of single coils, but check for lead dress in the amp, such as tightly coiled filament wires and shielded input cable. A new low noise 6SL7 may be in order, or perhaps the amp was never intended for this kind of gain. Good luck with your mods.

 

[ArcticBreaze]

just a thought as this is an old amp hav you replaced your filter caps? electrolytics degrade overtime and it maybe ac hum comin thro, that maybe why it's getting worse.