Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: aabbs20 on June 28, 2018, 01:42:04 pm
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Hey all,
I've nearly completed a Gibson EH185 build and I'm having an issue with the power section. The thread below has been super helpful along the way. I used the later EH185 schematic (attached) with 6SQ7 preamp tubes. The amp is more or less stock save for a few tone stack changes on the hi gain channel.
http://el34world.com/Forum/index.php?topic=19723.0
The amp has a Mojo 762 PT from Doug and an Allen TO35 OT.
I used a 20 watt 750 ohm resistor between the first two filter caps instead of the field coil speaker, which Sluckey suggested in the above thread. Everything works fine but that resistor is dropping a lot of voltage before the OT and the plate voltage on the power tubes is only at 288. I tried moving the OT primary up to the first filter cap instead and the plate voltage came up to 369 and the amp sounds great. The tubes are biased at 85%. The issue is that the PT is now getting much too hot. After 5 minutes it's too hot to touch for more than a second or two and it stays really hot for a long time after shut down. I've built enough amps to know that this amount of heat and stress on the PT is not normal. I'd love to keep the plate voltage up but obviously don't want to be stressing the PT this much. The OT is also getting a little warmer than I'm used to but not nearly as hot as the PT.
Any suggestions would be much appreciated!
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It's OK for that 750Ω resistor to drop a lot of voltage That's exactly what the field coil did in the original. It's also OK for a PT to get too hot to leave your hand on it for more than a couple seconds. Here are a couple easy things to try that should cool the PT a bit and also raise the B+.
Remove the 20K resistor that's across the second filter cap and move the OT back to the right side of your 750Ω resistor.
Replace the 200Ω cathode resistor with a 500Ω.
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> tubes are biased at 85%.
85% of what??
If you are looking at the 30W number on 6L6GC (all modern "6L6"es) you are at 25.5W.
Original-recipe 6L6 was rated 19W. All the old amps worked near this zone. And VERY frequently ran B+ at 250V plus bias and loss or 275V. I think your 288V was right, 369V is over the top for this vintage amp.
Yes, it won't get as loud, but you did not build a EH185 to fill a World Cup soccer stadium.
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What? A veritable quandry:
Right: And now this:
Dude, it's all about the stadium.
Fox.
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I build an EH150 using Hammond iron, 158M choke 10h 100mA; PT 274Bx 750vct 175mA; and OT 1645 30w. Same schematic as you provided.
The only thing to play with is the voltage divider because the OD side really takes off and can oscillate.
Mark
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Thanks for the suggestions guys, I love the knowledge base here. I've moved the OT back to the right side of the 750ohm resistor and I've tried a few different cathode resistors and the original 200 ohm sounds the best to my ears. I didn't realize the B+ of the original amp was so low, I just assumed I was shooting for around 400 volts for 6L6s.
Thanks again for the help!
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> I didn't realize the B+ of the original amp was so low, I just assumed I was shooting for around 400 volts for 6L6s.
At the time, 6L6 was rated 360V MAX.
1954 6L6 data (http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6L6.pdf), page 2
In a simple self-bias stage you didn't need this much to make "big" (for the 1940s) output.
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How much actual current are the tubes drawing?
You burn (hopefully under) 20W in that resistor so if you bias the tubes based on dissipation, the whole amp can soak up 20W more than a Bassman, which the PT was meant for. Actually, you burn more in the cathode resistor and then bias the tubes hotter which you get away with because of cathode bias. So your idle power consumption is higher than a Bassman and closer to full-volume power consumption than most PP amps.
Directly figuring current draw on the PT is likely the easiest way to check if you’re exceeding the PT specs. Current isn’t listed on that model but it’s easy enough to guesstimate what a Bassman would have been drawing... less, thanks to the tubes seeing the full B+ and being boxed more conservatively.
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Doug does list the specs for that Mojo 762 PT. 354-0-354 @ 250mA. Probably a lot bigger than what was on that original EH185.