Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: eurekaiv on October 03, 2020, 08:12:02 pm
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Hello everyone... I'm about to dive into a new rebuild project using an old Admiral PA amp I picked up for a reasonable price. The amp is 5Y3, 2 EL84 and 2 12AX7 so I thought I'd go with a LiteIIb style build with volume, tone, master and maybe a mod or two after the fact. According to the schematic for the Admiral, the PT will give me 270V on the plates for the EL84s which is quite a bit lower than the specs for the LiteIIb so I'm trying to figure out what modifications I should make to the circuit to accommodate the lower plate voltages. I've drawn up some load lines for the first stage and decided 1k should get me close to the 820 spec'd on the schematic I've found on mhuss's site. Beyond that, I haven't been able to get my head 'round pentode load lines yet and I'm not sure what, if anything, I should do with the phase inverter so any suggestions are welcome. Should I bring my power supply voltages up some by adjusting values in the filtering/voltage divider network as well or roll with the default LiteIIb values? Schematic attached...
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Replace the 5Y3 with a ss rectifier to bring the B+ up to close enough.
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It's not like I hadn't thought of it, but I also thought it might be a good learning exercise to use it and make adjustments. And I kinda like using everything that's there. But let's say I did decide to do that, any reason I wouldn't just mount the diodes on the socket for the 5Y3?
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Any reason you wouldn't just buy a ss octal plugin and be able to use tube or ss?
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If the point of switching to a SS rectifier is to get the voltages up because they're too low, I tend not to care much about the option of using the tube. I can always make myself a plugin rectifier as well. I'd definitely prefer not to buy one if I don't have to. I'm $90 into this project and hoping to not break $100.
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Using a ss rectifier will bring the B+ up. You're not likely to hit the same voltages shown on the mhuss schematic, but the voltages will be fine for the Lite IIB. My Lite IIB has 325V directly out of the rectifier and it sounds fine. I could have used a SS recto to hit the same voltages as Huss shows but I actually felt 325V was more appropriate. This amp will sound fine with a wide range of B+ voltages. It's fine to mount the diodes directly on the tube socket.
BTW, I noticed an error on your schematic with the VOL/TONE controls. See attachment...