Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bnwitt on June 30, 2010, 07:59:31 pm
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Ok so I cranked the amp up after fixing the bias circuit and I've got 394VDC on the plates and 34ma. So I've got to change the bias circuit the other way to get more current flow. The amp sounds pretty good but I'm surprised by the 394 plate voltages. The VAC of the transformer measures 304-0-304 with the DC output of the diode rectifier 395. Is that right for a 70's era Marshall PT?? It matches 50 watt early era Marshalls but I expected this amp to have higher output on the Pt. :huh:
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After redoing the bias again (back to 180 kohms and keeping the 50kl pot) I now have 388vdc plate and 43ma. Seems good to me. I don't hear much difference in the output with the 2.2uf bypass cap on V2a in or out. It seems to increase the bass and low mids a little. Maybe with the stock 0.68uf it would do more. The switchable bright cap on the output of V1b does increase the treble response of the amp as expected. I didn't use any shielded wire from the inputs to the V1 grids nor an artificial center tap for the filaments but the amp is dead quiet. There is some scratchyness on the presence pot and a little on the Volume I pot.
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Yeah, I always thought the supply voltages crept up over the years in Marshalls. Maybe sometime around the mid-late 70's they dropped back down. I had a '73 50w, but it wasn't a JMP50; it was a MKII Superbass. I don't recall the supply voltage, but it was at least 450v, maybe a bit more.
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Some of the "Small Box" JMP's were brutes (1971-72ish) with about 425-435v on the plates. I had two of those and they were fantastic - think Foghat, plug and play right out of the box screamers. I've also heard that some of the Small Box's were down in the MKII range (maybe late?). The MKII were around 390v and the Master Volume around 380v. But you could probably say these were averages, as we all know things were pretty "fluid" at Marshall over the years..... I think to, by the time the 70's rolled around they were more interested in establishing a long-life-reliable reputation on their products. The Marshall "tone" had been set, lets dial it back and mass produce.
Jim
ps Forgot to mention that I bought a used MKII chassis in 1978 that I never could get to sound right. Everything checked out good. 6550 or EL34, did not matter - sounded like crap. I did not have the knowledge I have now (thanks to this site), and even took it to a tech. Could not find a thing wrong. I'll bet the next one off the line sounded fine, that's Marshall for ya.
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Well the owner of the newly rebuilt 1987 head picked it up today. I always enjoy hearing someone who can actually play the guitar play one as opposed to my awful noodling when testing an amplifier. Boy does that 1987 circuit kick rock and roll booty! It's mornings like this I wish I lived on 40 acres instead of in suburbia. I could have enjoyed the tones for much longer than my neighbors could have standed it. :laugh: