Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on March 10, 2015, 09:02:58 pm
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I'm mocking up a semi Plagiarized sluckey "stuffed vox" and a matchless chieftan. I have the plate dissipation at 17.95W/tube. The data I've found says the EL35 is a 25w tube. If I do 25 X .7 That's 17.5, which I'm calling close enough. the amp though is coming in at 23W(15.2vac-rms into a 10ohmR), Is that close, I was guessing closer to 30W, but it did rattle paintings off my wall in a 12X14 room!!!
thanks
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I have a Cathode bias EL34 switchable to fixed. Plate voltage is 450. In fixed bias mode it will Pout at very close to 60 watts, but in cathode it will drop to around 50. Waaaaaaaaaay too loud for a home amp, but great tone.
What is your voltage? When I first built mine I had about 330vdc on the plates and could not get much clean headroom. I am curious if you are getting a nice clean tone and at what voltage. I am considering taking mine down a notch or 2. I mean it is just stupid loud. Reminds me of a Sound City amp.
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I'm running 430 on the plate, EDIT:470R 390Rat the cathode, -19.4v bias, and 1k at G2. I read that more often than not in cathode bias the tubes are running closer to 90% max plate, but I also read the hotter you are the harsher the sound, which in my SE builds seems to hold true. It's not a bedroom amp, it just to cold in the barn, but you're right, even 23W hurts!
I'll probably drop the EDIT:470 to 430 390 to 330 as an experiment but I'm still tuning the pre, it's not driving the ELs into compression so most of my distortion is pre-amp and I'm looking for most to be PA.
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Cathode-biased amps can run at 99.9% of the rated 25 Watts.
The "70% rule" is for FIX-biased amps.
Note also that current measured from cathode resistor and voltage is not just Plate. It is Plate and G2 combined. For 6L6/6V6 the difference is minor, but on EL84 and EL34 types the G2 current may be 20% of the total.
> I have the plate dissipation at 17.95W/tube. .... the amp though is coming in at 23W(15.2vac-rms into a 10ohmR), Is that close, I was guessing closer to 30W
A cathode-biased amp can't get out of class A, or not much. 50% efficient. So if you are dissipating 2*17.95W= 36W, the expected output is 18 Watts. We can often let bias-shift skew this up 25%, which is about your 23W number.
With fixed bias, the idle does not have to compromise with the maximum output. You "can" bias super-cold and load the tubes to huge current at maximum output. With 430V and a 4K load you should get near 40 Watts.
I liked 400V, 6.6K load, 250r common cathode, a several-K common G2 resistor. Measured power output was like 19 Watts, current was 100mA, and you could put "any" big tube in the holes and they would work yet run conservatively (<20W Pdiss).
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A cathode-biased amp can't get out of class A, or not much
I proved that to myself last night, even tho you and others here have been showing me in words for a year now!!! :think1: As I increased drive, monitoring cathode voltage, the cathode volts rose to the occasion every time! Also scoped the AC signal and except for subtle changes around the cross-over point she stayed quite uniform.
So it's on to lowering Kr to about 90% Pd and cleaning up anomalies in the preamp
Thanks all!!
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I liked 400V, 6.6K load, 250r common cathode, a several-K common G2 resistor. Measured power output was like 19 Watts, current was 100mA, and you could put "any" big tube in the holes and they would work yet run conservatively (<20W Pdiss).
i like 450V, 6K6 load with 250-300R
kick 'em the 'nads i say...they howl better.
--pete