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Old 5th March 2005, 05:30 AM   #1
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Question questions about power supply for 6l6 pp

I've just recently cobbled my 6l6 pp amp back together enough to get it playing music again. some of the voltages are a little higher than the schematic notes. especially on the primary for OT.
483v instead of 450. I've done some reading here and there and have gotten the feeling that even 450v is hard on the output tubes but may be necessary for the OT to be right.
I've read that the 6l6 has a nicer sound in the 350v range and would like to experiment with this but need some advice on the proper way to drop the voltage in this circuit. I am not yet very acomplished in the language of electrical mathmatics but I have been trying to figure it out myself so as not to post yet another series of dumb newbie questions.

V=IxR Im not sure how to measure to find the current here.
I'm guessing if I knew the current of that section of the curcuit I could adjust R to get the desired voltage.

Ok question 2
I hear distortion when heavy bass lines erupt or with loud quick sounds . It sounds like the power supply is bottoming out. I've replaced all the power supply capswith larger values, about double. So I'm hoping thats it's the old diodes that aren't keeping up. Oh forgot to mention this thing is 45 years old.
I'm thinking maybe bridge rectification here, but would also like some guidence to find some good PS circuits to study.

I guess thats all for now
thanks for reading
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Old 5th March 2005, 05:47 AM   #2
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Default Re: questions about power supply for 6l6 pp

Quote:
Originally posted by dionisio
I've just recently cobbled my 6l6 pp amp back together enough to get it playing music again. some of the voltages are a little higher than the schematic notes. especially on the primary for OT.
483v instead of 450.
Pffbbt, 6.8%. Could be line voltage variation (especially since the old days, 117V vs. 120-125V today).

Quote:
I've done some reading here and there and have gotten the feeling that even 450v is hard on the output tubes
Pffbt, 6L6GC can handle upwards of 600V no problem. Arcing is a problem (with peaks up to 1200V) so you need a reliable brand to do it, but they'll do it just fine.

If you have anything less than a GC, limit is around 315V. Russian surplus (6P3C...er... 6P3S right?) may fit between these American specs.

Quote:
I've read that the 6l6 has a nicer sound in the 350v range and would like to experiment with this but need some advice on the proper way to drop the voltage in this circuit. I am not yet very acomplished in the language of electrical mathmatics but I have been trying to figure it out myself so as not to post yet another series of dumb newbie questions.

V=IxR Im not sure how to measure to find the current here.
I'm guessing if I knew the current of that section of the curcuit I could adjust R to get the desired voltage.
Measure it with a small series resistor, or one already in place. If the output is self-biased, it will have a cathode resistor. I = V/R.

Using a dropping resistor has several really bad consequences:
1. It gets hot. Uh, very hot. Ya.
2. Did someone say regulation? No? Nevermind...

#1 doesn't matter if you don't need to drop much, but then again you do. #2 doesn't matter much if the amplifier is completely class A (if self-biased, it might be), but still isn't nice.

Quote:
Ok question 2
I hear distortion when heavy bass lines erupt or with loud quick sounds . It sounds like the power supply is bottoming out.
High output impedance or lack of power (resulting in overdrive) can do that too. If it's a UL or pentode mode ZNFB amp, I can promise your bass will be really poopy on modern speakers. If your speakers are very insensitive (less than say, 85dB/W/m), you may not have enough power to reach high volume, resulting in clipping distortion, particularly on bass, which since the ear is less sensitive to it, requires more power. Not to mention the complex load the speaker represents at this frequency.

Quote:
I've replaced all the power supply capswith larger values, about double. So I'm hoping thats it's the old diodes that aren't keeping up. Oh forgot to mention this thing is 45 years old.
Doubling capacitance is a very bad thing then. If those are selenium, you're lucky your room hasn't filled with poisonous vapors yet...

It certainly wouldn't hurt to replace with a pair of modern five cent 1N4007's.

Tim
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Old 6th March 2005, 05:16 AM   #3
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Thanks for the reply Tim.

Quote:
Pffbt, 6L6GC can handle upwards of 600V no problem
I have sovtek 6l6gcs so its good here.

Quote:
#1 doesn't matter if you don't need to drop much, but then again you do. #2 doesn't matter much if the amplifier is completely class A (if self-biased, it might be), but still isn't nice
how do you know when it's in class a. I've read that an amp is in class a when it reproduces the entire wave form and AB reproduces slightly less than the total . Is that why the purists dislike AB?
does "self biased mean" no adjustment?

Quote:
High output impedance or lack of power (resulting in overdrive) can do that too. If it's a UL or pentode mode ZNFB amp, I can promise your bass will be really poopy on modern speakers
Ihave voight pipes, with the RS 1354, should be a fairly easy load.

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It certainly wouldn't hurt to replace with a pair of modern five cent 1N4007's
The're on order
Thanks again Tim
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Old 6th March 2005, 07:45 AM   #4
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Hi,

I recently went through a Sherwood S8000 amplifier after using it nearly 30 years. The bass was as you described, kind of muddy at high levels. Now it can rock the house and cleanly too. I went a little overboard with the power supply, but the following link describes what I did. The audio stages are still stock except for current sense resistors on each output tube cathode.

If the hyperlink below gets cut off, and the link is dead, just type in the rest manually.

http://pages.prodigy.net/jcandela/Sh.../regulator.htm

Regards,
Jim
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Old 6th March 2005, 09:54 AM   #5
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Quote:
Originally posted by dionisio
[B]how do you know when it's in class a. I've read that an amp is in class a when it reproduces the entire wave form and AB reproduces slightly less than the total . Is that why the purists dislike AB?
does "self biased mean" no adjustment?
Usually it does. You can have class AB with a self-bias cathode resistor, but it doesn't work well. I'll get to this below.
Which reminds me, is this a guitar or hi-fi amp?

Purists hate AB because it's too nice: high power, efficiency and low distortion. SS has the same characteristics, so they sometimes sound alike.
On operation: a tube does cut off during a portion of the waveform, however since it is push-pull the opposing tube takes the full load and completes the wave. Class AB single-ended would be horrible indeed, because with no opposing tube to fill in, you literally end up shaving off some of your waveform.

The problem with self-biasing class AB is power supply current varies, so cathode current and thus bias voltage varies. This causes compression, solved by applying _fixed_ grid bias with a seperate voltage source (often adjustable to account for tube mismatch and drift).

If your amp has a cathode bias resistor, it is probably class A, unless it is a guitar amp -- if compression was desired, it may be either.

Tim
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Old 6th March 2005, 02:34 PM   #6
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I'll chime in on the lower B+ rail voltage front.

If there is enough space on the chassis for the choke(s) needed, consider switching to a choke I/P filter in the PSU. Ignoring losses, a cap. I/P filter yields about 1.4X the AC RMS voltage, while a choke I/P filter yields about 0.9X the AC RMS voltage. When cap. I/P filtration is employed, the available DC current is about 1/2 of the AC RMS current. When choke I/P filtration is employed, the available DC current is a tad greater than the AC RMS current. Cap. I/P filters are voltage rich and current poor. Choke I/P filters are current rich and voltage poor. Choke I/P filters are quite well regulated, while cap. I/P filters droop badly under load. Yet another advantage of choke I/P filtration is the fact that the power trafo runs cooler and lasts longer due to reduced I^2R heating. If the decision to use choke I/P filtration is made, follow the ARRL recommendation of LCLC topology. FWIW, I like a 20 H. 1st inductor, as it allows the use of a 20 KOhm bleeder resistor, with its reduced losses, across the 1st filter cap. When choke I/P filtration is employed with SS rectification, a 10 nF. CERAMIC cap. of the highest possible WVDC should be placed from B+ to ground in front of the 1st inductor. The 10 nF. part protects the SS diodes from inductive kickback spikes that could destroy them.

Oh yeah, while only slightly more expensive than 1N4007s, UF4007s produce a lot less switching noise. Less switching noise = better sound.
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Old 6th March 2005, 03:01 PM   #7
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Sorry,

I should have looked before I leaped. That PSU is a voltage doubler. You can't use choke I/P filtration with a voltage doubler. :~(

BTW, the schematic is incorrect. The "bottom" diode should be connected to the trafo at the same point the "top" diode is.

Something to keep in mind is the fact that I^2R heating in the trafo will increase with increasing values of filter capacitance.
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Old 7th March 2005, 04:00 AM   #8
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Jim,

Thanks for sharing your voltage reg design! I'm going to have a good time wrapping my head around that one.
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Old 7th March 2005, 04:17 AM   #9
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Tim,

Quote:
You can have class AB with a self-bias cathode resistor, but it doesn't work well. I'll get to this below.
I whipped up a schem for you to look at, is the hum balance pot an adjustable cathode resistor?

Quote:
Which reminds me, is this a guitar or hi-fi amp?
HiFi Stromberg&Carlson model asr 444, circa 1960. built by general dynamics co. I think they built these in their spare time when they weren't building military hardware.

Quote:
Purists hate AB because it's too nice:
LOL!!

Thanks for the help
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Old 7th March 2005, 04:38 AM   #10
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Eli

Thanks for heads up on the capacitance/ transf loading problem.
The original caps were getting pretty bad and the trans was getting pretty warm and actually making an audible hum. I used what Ihad on hand to replace them and the hum disapeared and the tranny only runs luke warm now. Which will probably drop even more once circumstances allow me to replace these with original values.

Also thank you for the tip on the UF4007s Iwill definately try those

Lastly thanks for spotting my goof on the PS schematic
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