I had some more time yesterday for initial startup. I knew everything wiring wise was good as I built the amp in deliberately in sections double checking as I went but I went over it again prior to firing things up and it all looked good. I did use my current limiter with a 200Watt bulb. Amp was plugged into my current limiter and the current limiter was plugged into my 12 amp Staco Variac. I brought the amp up on 10 volts intervals over an hour and half from 10 ACV to 80 ACV to allow any electrolyte to form in any of the filter caps. I know it's not necessary with new caps but it's just something I do that alot of techs recommend so I personally do it. I then installed 3 random good preamp tubes for startup, a Sovtek 12ax7C and two good JJ's ECC83'ss and set the amp to exactly 120ACV on the variac and checked all the voltages along the rail and everything checked out 100% good so I then installed the power tubes.
The amp with no power tubes installed showed a B+ of 498DCV. I had a bias range of -30 to -55 DCV on pin 5 with my 50K bias pot and my elevated heater circuit was providing 50DCV to the center tap of the tranformer tap for the AC heaters,exactly what I had calculated. For a 50 watt Marshall with 6550's a bias range of -38 to -50 usually is sufficient for most in spec 6550 tubes and a bias range of -34 to -40 is usually sufficient for most in spec EL34's according to my resources. So it looks like I have enough bias range for both EL34 and 6550's.
I used the old Svetlana 6550B power tubes that had some age on them but I figured they were still good, but I had not tested them.... B+ was 455 DCV which is what I remembered this amp to be with these tubes when I checked it prior to disassembly... but at a cold bias of-55DCV the tubes were pulling 58 ma and 60 ma which is just too much. I wondered if those tubes maybe were on their way out pulling too much current so I then installed the set of GE6550A's that I bought for this amp and my B+ went to 470 DCV and the current draw on the power tubes dropped significantly to 37ma and 33 ma at -55 DCV bias voltage which is closer to 50% of 35 watt dissipation( I know GE 6550A are 42 watts but I used 35 watts for my calcs). So I finally then had some room in the bias and set the bias to 40ma and 36ma about where I like 6550's . So it seems those old Svetlana 6550's were just pulling the B+ current down hard so I concluded the 6550's were just pulling way to much current or may be going bad, either way I'm not using them. I will install some Chinese foil getter 12ax7's for V1 and V2 and an RFT for V3 once I get the tweaking done.
So after all of that checked out I then plugged the amp in to my 2x12 Mesa Boogie roadster cab and fired that amp up for the first time and see if it passed signal and I am happy to report it does and it sounds really good and with the elevated heaters the amp is totally dead quiet, I'm not getting any crosstalk hiss or any 60 cycle hums anywhere.
I also may try out some .039 uf or .047uf coupling cap in the PI to fatten things up a tad, I dunno we'll see.
Amp sounds glorious with the 20V/20V zeners and 2N5400 is way better than I remember. I also picked up some 2N5401's that 4T6and2 recommended.
The only thing I don't care for is the taper on the 1 Meg Audio when I engage the 20 V zeners or the 2N5400 transitors the amount I have to turn up the master is too much for me. So I am going to have to change the master pot so the taper comes in sooner, I have some 1 Meg Reverse audio taper pots that I bought from Suckerfree gear that supposedly Mark Cameron uses for this very reason but they are mini pots. I don't know if a 1 Meg linear taper would be better than the reverse log pots or maybe a 500K audio or even a 50K linear Master that the Randall RGT100 uses after 4V/4V zeners in that Jose master setup in the RGT100 amp. I am running the master at 5 with the 20V zeners and 8 with the 2N5400's to get loud volume and I 'd like to get the taper similar to that of a PRE-PI JCM800 1 Meg Master which will work better for my solo control function as well.
I hate having to deconstruct anything but looks like I am going to have to. Any suggestions would be appreciated regarding the master volume taper.
All in all I couldn't be happier with how things have turned out functionally and audibly, I love how the amp sounds and I just have to enter tweaking mode to what I want the amp to be.
