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31

Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by shooter on April 21, 2026, 05:35:05 pm »

the math

it goes way back, can't recall who came up with inductance, math, but it wasn't any fun cranking out just so I could pass a class

32

Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by jbrrrrr on April 21, 2026, 04:31:38 pm »

Is that where the math for using a composite curve of 1/4 the primary and 1/2 the primary is derived from in calculating a push-pull loadline?  i.e. maps to the 1/4 Za-a until the bias point for class B, and then continues on through the 1/2 Za-a loadline for class A?  I just took it for granted that it was written and it must be so - I didn't think about the trafo turns ratio being what was dictating why we use 1/4 and 1/2 specifically.

https://www.valvewizard.co.uk/pp.html

33

Solid State / Re: Peavey CS800 Blowing Fuse

« Last post by passaloutre on April 21, 2026, 03:13:09 pm »

haha I'm certain that is the best advice I will receive about this thing!

I did find a good long thread here that seems worth mining for some troubleshooting steps: https://www.diyaudio.com/community/threads/peavey-cs-800-issues.216627/

At least good enough to know if this thing only needs a quick fix or if it needs $200 worth of transistors

34

Solid State / Re: Peavey CS800 Blowing Fuse

« Last post by shooter on April 21, 2026, 03:06:18 pm »

What do I need to know before opening this thing up?

you have hair?  if so;
you'll have it all pulled out by the time you give up and place amp in recycle bin 


blowing mains is typically indictive of the PA section, the internet makes lots of "Blackbox" SS pa's
take the section i've copied, toss in trash, install the black box


i've T.S. many a SS circuit, this one is very complex, old, n broke, none of which is good for learn'n as you go.

35

Tube Amp Building - Tweaks - Repairs / A Question about system design

« Last post by pbman1953 on April 21, 2026, 02:41:22 pm »

You all have helped me with tech support so I know you can chime in here on my question. The bass control on the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound.


My Sunn 2000s single volume is right after the input. Looking at the Traynor YBA-3,same thing. The volume is after the input but before the v1 input. The master volume is way down the chain right before the power tubes



Shouldn't the volume be the only item in the middle of the tube and have the bass control after the tube output? I wonder if anyone moded that?

36

Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by pdf64 on April 21, 2026, 01:19:18 pm »

The impedance ratio is the square of the turns ratio.
So halving the turns ratio quarters the impedance ratio.
eg if the anode to anode turns ratio is 10:1, when one anode is in cut off, the CT to other anode turns ratio is 5:1.
So the anode to anode impedance ratio is 100:1 and CT to anode is 25:1.

37

Solid State / Peavey CS800 Blowing Fuse

« Last post by passaloutre on April 21, 2026, 12:38:01 pm »

I don't have much familiarity with solid state stuff, and if you've followed my posts around here you know I'm only starting to get the hang of the tube stuff. As fas as solid state, I've made plenty of fuzz pedals, but nothing remotely approaching this type of power.

I was gifted an ancient Peavey CS800 stereo power amp. Though I'm unlikely to ever need it's full rated power, I'm inclined to put it to use for sound reinforcement. I don't think it's completely fried, as it will pass small signals, e.g. I can plug a guitar straight into it, crank it up, and hear the guitar come out of the speaker, louder than I started.

But it pops the main fuse when I feed it a larger input, e.g. plugging that same guitar into a small mixer and plugging the mixer into the power amp. It doesn't pop until it gets a big transient, but when it does it dims the lights lol.

I haven't cracked it open, or even done anymore extensive testing than the above. I was hoping to get some old time wisdom before I do. What do I need to know before opening this thing up? Any caveats to 40 year old transistor devices that I should be aware of? The biggest things I've worked on are 70s era hundred-watt Fenders. I've attached the schematic below...what's a good plan of attack? Replace electrolytics, test transistors, look for anything burnt up? How do I test those big power transistors?

I have a scope, a signal generator, at least 2000W worth of dummy load, I could probably even get my hands on one of those cheap amazon transistor testers if I need to. What else do I need

38

Tube Amp Building - Tweaks - Repairs / Re: A small friedman-inspired amp using an inexpensive power supply

« Last post by passaloutre on April 21, 2026, 12:00:47 pm »

Yeah, I saw that same pattern PS on Ebay as well, seems like several accounts to same vendor, USA stock BS, etc., bulk ordered and shipped from China; why they state the 4-7 days ship transit time. 


On the unit I have, adding the 6.3V load did not change things. Test results in link.  3x 10Ω 5W resistors in parallel, for 3.3Ω total load, or about 1.92A current load. Some of the resistors were cooking at ≈130°C. 


--Pete

It looks like your jumper is set at 280.  Thanks for sharing the testing results!

It was! Thanks for pointing that out. I was completely oblivious to the existence of that jumper   Seeing 304VDC now with the 1200Ω load bank.

Link to test results.

--Pete

Living up to your username lol!

How clean is the output? Good enough for preamp work, or would it need some extra filtering?

39

Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by passaloutre on April 21, 2026, 11:53:42 am »

Once the Peak Grid Voltage driving the output tubes exceeds the bias voltage across the cathode resistor, the negative-going grid voltage shuts off 6V6 (or close, because it often takes a bit more negative-voltage to shut off the tube).  The tube receiving the positive-going grid voltage now has 1/4 the load impedance at the output transformer primary, so its Rate of Change of Current is 4x what it was when both sides were on.

I've seen this come up in discussion before, but it always seems to trip me up... Where does the 1/4 load impedance figure come from? Why is it not half, since half the transformer is not doing anything? Or why is the impedance reduced at all?

40

Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by HotBluePlates on April 20, 2026, 11:17:31 pm »

This seems useful as a way to allow for more variations of cathode-bias-that-feels-less-cathode-bias-y without having to set up a bias voltage tap.  ...

Obviously if you want a fixed bias amp then this is a silly substitute, but it's the range in-between fixed and full cathode bias that I think this could have some use for ...

If you focus on the self-adjusting nature of cathode bias, then I think you will agree that using the zener creates a fixed-bias supply.  Assuming the tubes pull sufficient current through the cathode resistor, the bias voltage is locked at a single voltage value.  So it behaves as non-adjustable fixed bias.