Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: isaac_teller on February 03, 2021, 09:43:34 am
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Hello. I'm building this amp with cathode bias for a friend who wants lower output. The power transformer is 315-0-315 with diode rectification which should give a B+ in the neighborhood of 375VDC. What would be a ballpark value for the cathode resistor? I'm a little apprehensive about turning it on for the first time and causing damage if the value is too low. Thank you.
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Shooter had some helpful insight on this old thread. Maybe find a schematic of a similar amp as a starting point and then plug in your exact numbers in the calculator below. I think Marshall’s newer studio vintage 20 could be a valid schematic to look into. It’s a cathode biased Plexi.
https://el34world.com/Forum/index.php?topic=18533.0
bias calculator here to help you reverse engineer the system.
https://robrobinette.com/Tube_Bias_Calculator.htm
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Another thread here
https://el34world.com/Forum/index.php?topic=15267.0
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Much thanx!
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I experimented with cathode biasing two EL34's in a 1987 model 6V6 Plexi. I tried several resistors and did the math. My PT, 325-0-325 with a 5AR4, is around 400v B+. The best I got from each EL34 was 21watts dissipation w/250ohm Resistor and the 5AR4, SS plugin 23 watts, (I was temped to use a 5Y3 but first filter was 50uf). I tried a 150 ohm got 28 watts dissipation in cathode stage (too hot, IMO). I have a fixed and cathode biased switch. From my experience with this amp and your B+, I'd try 220 to 250 at first, do the math, if you're only going to used EL34's, I'd shoot for about 23 to 25 watts dissipation, I believe EL34 are rated at 25 watts each. I could have settled on a 200/220 R for about 24 watts but wanted to use other octal tubes, I left the 250 in at 21 watts. I can't get the 6V6 tubes biased cool enough in cathode mode with the 250ohm but using a bucking transformer and a 5U4GB, still a little hot but doable.
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Thank you.
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Hello. I'm building this amp with cathode bias for a friend who wants lower output. ... B+ in the neighborhood of 375VDC. What would be a ballpark value for the cathode resistor? ...
"How much lower" does your friend want the output to be?
Philips' EL34 data sheet (https://frank.pocnet.net/sheets/010/e/EL34.pdf) shows a condition on the top of Page 3 with 375v plate (and presumably screen) with a 130Ω cathode resistor, making 35w. That's right in the ballpark of a JTM45 or Bluesbreaker amp.
With the same speakers, dialing back from 50w to 35w is only a ~1.5dB reduction of volume (which is "almost nothing").
Truly-lower volume would need to substantially drop the supply voltage (and change the OT loading), though arguably it might take the amp away from sounding like a Superlead.
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I used a 150 ohm cathode resistor on my newest amp (https://el34world.com/Forum/index.php?topic=27051.0) with 2xEL34s. Sounds perfect, but it does get hot enough I'm thinking of adding fan. I may try 180 or 200 ohm and hope that cooler doesn't equal "lame".
Not trying to muddy your water - just my (non-techy) observation.
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I know that 35 watts isn't a significant reduction but that's what he wanted so that's what I built. I also added a VVR at his request.
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I look at the operating conditions HBP posted pg 3, running in AB push/pull. I see the supply voltage at 375v, "plate" voltage at 350 and a 130 ohm Rk for 35 watts, plate dissipation is 25 W (and 27.5 ??). When I figured the dissipation in my above post I didn't substract the screens, the grid dissipation is 8 Watts, the bias is 150 mA, I assume that's 75mA's per tube. Seems like that would be biased very hot. What am I missing here? Just don't want to give incorrect info.
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Adjusting for screen current, I get 26.4 watts dissipation per tube at idle. Mullard has the same operating point reflecting in their data, but split the cathodes and assigned separate 260 ohm cathode resistors to each tube. And the plate dissipation at idle is 26.4 watts. I guess that 25 watt rating was on the conservative side. Vintage Mullards were built to take it. I doubt that new production tubes are built to the same standards.