Q for you amp gurus

[straycat]

I friend of mine had a Q and I wasn't sure so I'll ask you.
He has a low wattage amp 15 watt home built but looks very nice with clean wiring inside.
A fantastic amp tone wise it has 6V6's (two) and the usual 12AX7's.
I'm guessing based loosley on a Fender design.
He runs a pedal board thru the front and cascades 3 OD pedals along with delay and chorus He says if he runs a 3 OD's he gets a volume drop but back off one OD pedal it seems OK.
I think he is slamming the front too hard and the pre amp tubes just can't take the push.
Could I be right or is he hurting the transformer and looking to melt down his amp?
I know jack about tube amps so not much help.
P.S. no FX loop on this amp.

[Twitch]

I dont think hell hurt it doing that. Im surprised hes happy with the tone doing that, but to each his own. Id rule out his 3rd pedal isnt busted first. If hes always adding and removing the same pedal, it may be that pedal. If thats the case, have him remove a different pedal and if the problem persists, then that one pedal is busted, if the volume comes back like usual, its either compressing or hes running beyond the capability of the power brick if hes using one.

At some point based on the sensitivity of the input, you either clip or compress your input signal if the voltage exceeds the sensitivity range of the input on the amp. My moneys on a broken pedal or too many pedals on one DC brick.

[straycat]

OK thanks I'll get him to take that one out and try my OCD and see if that has any affect.

[DonP]

There are different ways to bias a preamp tube. If the amp has a funky bias it is possible for a large signal to drive the grid positive, which at that point it flatlines and sometimes farts out.

[straycat]

Thanks Don we tried his pedal board on my Jet City 20 watt head and it did the same thing thru the front no effects loop on it either.
We tried it thru my Ibanez TSA15h combo thru the loop and it sounded killer.No problems at all.
So its pushing the pre amp tubes too hard?
The offending pedal seems to be the OCD IMO, would putting a battery in it help that issue he runs a voodoo lab power block.

[DonP]

You'd need a schematic of the preamp to dig in deeper. Some things I'd check is what voltages the tube is being operated in (is there enough headroom?), the type of tube being used (you already said 12AX7), how much gain this tube is being asked to do. Everyone quotes 12AX7's as having a gain of 100 but in reality that is the maximum gain possible. Guitar amps rarely push them that hard. A good indicator of how hard it's being push is the plate resistor. Most 12AX7's use a 100K ohm. But hotter amps may use 220K, or even 400K. If the first tube has a 400K plate resistor, I could easily see it being overloaded fairly easy.

There are other types of bias too. I can't remember but I think there are three. Cathode bias, grid leak bias, and fixed bias. Grid leak doesn't like to be overloaded.

Maybe try a 12AY7 or a 12AT7 in the first tube position to see if it helps.

[Cygnus X1]

Grid leak doesn't like to be overloaded.

Maybe try a 12AY7 or a 12AT7 in the first tube position to see if it helps.

This.
Good call Don.
A few really detailed gut shots would help too.

[Twitch]

Hmm, I very rarely think about the amp being at fault. Very interesting. Id say a good amp will take anything you throw at it, but I guess cascading 3 OD pedals in front of one isnt the typical thing. I would think you can beat this by adding an EQ after the OD pedals and leaving it set flat and just using its level control to bring the overall output voltage back down to what the amp can handle leaving the saturation from the OD pedals intact, just at a lower volume. You could try swapping that tube like Don suggested, but your issue is youre hitting the amp to hard. The tube wont help that as much as reducing the amount of signal hitting the amp will. Maybe a mixture of both options?

[DonP]

Twitch,

One thing you gotta remember is that amps weren't originally designed to be distortion generators. In the old days they were designed to amplify weak guitar signals, not the output of several boost pedals. I get the feeling from the way Straycat talked about this amp that it might be a classic design.

[Twitch]

......but I guess cascading 3 OD pedals in front of one isnt the typical thing.

Twitch,

One thing you gotta remember is that amps weren't originally designed to be distortion generators. In the old days they were designed to amplify weak guitar signals, not the output of several boost pedals. I get the feeling from the way Straycat talked about this amp that it might be a classic design.

Read ya loud and clear Don.

[DonP]

Sorry Twitch, I blurbed on one of your posts again.