Marshall clone, what to do?

[Zozobra]

Not much to add other than for the most part I prefer my bulbs biased colder than the magic 70% idle value.

Another point to consider; has the user pulled and replaced/switched the preamp tubes at any point? I know that some of my more high gain amps are incredibly fussy in that regard and a great sounding compliment can sound like refried ass by simply swapping two preamp tubes around.

[bob p]

I've noticed that too. And to keep it in the perspective of what work I think techs should to and which work should be designated to users -- I don't like to do tube rolling to optimize tone for other people. Once the amp is working to spec, I let the customer do the tube rolling.

[Chuck H]

I know that some of my more high gain amps are incredibly fussy in that regard and a great sounding compliment can sound like refried ass by simply swapping two preamp tubes around.

Funny. In my higher gain combo I'm just happy to find a V1 that doesn't whistle. The last of my stash of Mullard, Phillips, RCA, etc. (only 10 left ) 12ax7's are all V2 or later at this point because I've used any totally non microphonic tubes already. So I have an old Mesa Shuguang tube in V1 for now. Sovtek 12ax7wa and wb tubes are usually well behaved, but often noisy and a little lower gain than many others. The Sovtek 12ax7wc is supposed to be a full gain version of the same tube. The two I tried hummed like they didn't know the words. Such is tube rolling.

[g1]

Ok. Usually most types of 6L6 variants are considered 30W tubes, so 18W puts you at 60%.
Is that where is was when he brought it to you?

It is how it was brought to me. I always considered EL34s 25 watt tubes. Am I wrong there?

Sorry Randall, my mistake. You are correct, EL34 is a 25W tube. I saw a reference to a JTM45 at the top of this page, and forgot that the actual amp being discussed is a plexi clone. JTM45 uses 6L6, plexi and most other marshalls are of course EL34.

[Randall]

Finally got the builder to the point of saying the amp is a clone of a 1966 100 watt Marshall, and that in line resistor was there from the get go to lower screen voltages. He says feel free to remove it, but I don't think that is the source of the problem. Spoke with the owner who bought it new and says no work has been done. We agreed to try a new matched quad and see. If that doesn't give him his mojo back, I think I will admit defeat and move on.

[nosaj]

Finally got the builder to the point of saying the amp is a clone of a 1966 100 watt Marshall, and that in line resistor was there from the get go to lower screen voltages. He says feel free to remove it, but I don't think that is the source of the problem. Spoke with the owner who bought it new and says no work has been done. We agreed to try a new matched quad and see. If that doesn't give him his mojo back, I think I will admit defeat and move on.

Still think you should adjust the bias while he's there just keep it in a safe range. Tell him it's in acceptable ranges, should he want to hot rod it (fiddle with the bias like adjusting a carb) that he can pay xx amount of dollars . But then your showing him what to do and he blows it all to hell. Guess neither solution is good. Having a players ear would help me a whole heck of a lot but I can't play.

nosaj

[Randall]

Just to put closure to this thread, I put in a new set of JJ EL34s, biased them at 70%, and with this set the plate voltage rose several volts. Anyway, the customer now loves the amp again. 20 minutes after he left here I got a text, "HOLY SH*T!" Then called me to thank me, so whatever it was, it's gone.

I left the extra screen resistor in there, as the builder finally remembered it was original.

[1ampman]

Block that phone number!