Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: munkeyboy on February 04, 2021, 06:15:03 pm
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Working on a build and would like to compare voltage numbers on the 5b2 Princeton's. I'm using 6SL7, 6V6. Can't seem to find any info on expected voltage.
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Is there a Champ of similar lineup?
But really: either it is way-wrong or it will work. It's not a rocket to Mars.
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You will not find any voltages on these old Fender amps.
Use the right parts and voltages will follow.
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Working on a build and would like to compare voltage numbers on the 5b2 Princeton's. I'm using 6SL7, 6V6. Can't seem to find any info on expected voltage.
Look at the 5C1 Champ (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5c1.pdf). 340vdc from rectifier to first filter cap. The Princeton almost certainly used the same power transformer, and we should expect the voltages to the 6V6 to be similar if not the same.
The 6SL7 voltage is not critical: 100-200v at the plate, negative 1-2v at the grid, 0v cathode for the Input Stage. 100-200v plate, 1-2v at the cathode, 0v at the grid for the 2nd Stage.
If the voltage at the first filter cap is correct, and you use the specified parts-values, everything else will fall in line.
I once owned a 5B2 Princeton, maybe 24 years ago. I'm sad to say it didn't occur to me to measure & annotate the voltages.
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Thanks. I was wondering about that 6v6 dissipation %. Looking at the 5c1, 300v at the plate and 16v at the k seems to confirm it is biased cold quite cold at 70%. Do I have that right? This is about what I get on my build too, except I have a 6v6gt. So that's 60% dissipation.
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Why do you think it is too cold? Moving from 9W Pdiss to 16W Pdiss would not move you from the garage to the stadium.
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All 6V6 tubes, regardless of envelope, are rated as either 12 watts under the Design Center rating system or 14 watts under the Design Maximum rating system. The difference is in the rating systems, not the tubes. Notice in the attached page from the RC 30 Receiving Tube manual that he lowly metal can 6V6 has the same 14 watt plate dissipation rating as the prized 6V6 GTA. The differences are in a, the envelope, and b. the controlled heater warm-up feature or the A series tube.
IMO it is a mistake to bias as a portion of maximum dissipation rating. I believe that it came about from a miss interpretation of Class A Service. In the attached page from RC 30, " A Class A amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows at all times." I have seen other definitions that in Class A operation, plate current flows 100% of the time.
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Why do you think it is too cold? Moving from 9W Pdiss to 16W Pdiss would not move you from the garage to the stadium.
I didn't say "too cold", I said quite cold. :icon_biggrin: Just comparing it to the various bias calcs out there saying 85% is cold for cathode bias and others saying 100% is ideal for class a.
I'm not looking to make more watts, just confirming my numbers. This way I can swap in 6k6 for 100% pdiss or swap back to 6v6 for 60/70% pdiss. Roughly same output @ 4w.
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> Class A Service.
Yes, class A the current never stops.
The dissipation is entirely different. We run 12AX7 preamp tubes in class A at maybe 10% of the rated plate dissipation. There is no great sin running 6V6 at a few Watts. When we want the MOST for our tube dollar, then we "need" to work as close to rating as we dare. 140% and keep spares on hand. 100% and expect to go out for a spare every few years. 50% if you have to send a dog-sled over the frozen Arctic to get spares. Or if you can be happy with 2-3 Watts output rather than 5-6W.
1941 Tung-Sol data (https://frank.pocnet.net/sheets/127/6/6V6.pdf) shows a condition for 5.2W Pdss (43% design center) and 2.0W out. You can find a smaller tube for 2 Watts, but it may not be cheaper. In 1942 and certainly today.
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Awesome, thanks PRR. I was always under the impression that the target should always be between 85-100%. Anything less and it sounded bad, anything more and your tube exploded (jk).
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... I was always under the impression that ... Anything less and it sounded bad ...
The guitar electronics "information" world is full of disinformation, or made up stuff.
On another forum a guy mentioned biasing his Deluxe Reverb by ear until it sounded its best. He said the result was 45% of rated dissipation.
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On another forum a guy mentioned biasing his Deluxe Reverb by ear until it sounded its best. He said the result was 45% of rated dissipation.
It's a big mistake, amps always sound good when overbiased with very short tubes life........
Most Fender amps I fixed ( many since 20 years) work fine with 50% to 60% power dissipation and for all amps. I test by ears too, but I never go over 60% on AB class amp
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...Most Fender amps I fixed ( many since 20 years) work fine with 50% to 60% power dissipation and for all amps. I test by ears too, but I never go over 60% on AB class amp
60% of a design max anode limit is equivalent to 70% of a design centre limit.
So perhaps a useful guideline upper limit (NOT target!) for idle anode dissipation in typical AB fixed bias, eg so 8.4W for 6V6, 18W for 6L6GC.
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For an SE amp, I'd just go for 100% (Pmax). If your Zout is in the ballpark of Vp/(Pmax/Vp) - and here 8k is as good as 9k or 7k, you will rarely do any harm. If Zout = 340/(12/340) = 10k, then 8k will be perfect for keeping Vg=0 at or below the load line, but if you're concerned about Pg2 under big signal conditions, relax knowing that those old fender SE amps had a 10k* screen node dropper, which keeps Vg2 about 40V to 50V below Vp ( which keeps the knee of the Vg=0 grid curve at or below the load line anyway).
*5B2 has 50k
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Thanks. I was wondering about that 6v6 dissipation %. Looking at the 5c1, 300v at the plate and 16v at the k seems to confirm it is biased cold quite cold at 70%. ...
Keep looking at tweed Princeton schematics: 5D2, 5E2. They inch upward on supply voltage & especially screen voltage. 10.4 watts, then 12.2 watts.
Flip over to the blackface, silverface Champs and supply voltage continues inching upwards. Many folks have a blackface or silverface Champ/Vibro Champ/Bronco with 15-17w of plate heat in the 6V6.
So it could be worse. Low is fine.