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41

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

« Last post by jbrrrrr on April 20, 2026, 05:11:32 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.  I tend to prefer the way cathode bias amps feel, but I always get that urge to put an option in to stiffen up the output section, or to change that as part of the voicing structure.

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 wanted to have this as a switchable option, you could just shunt the zeners to ground, and switch in the appropriate cathode resistor value for 100% cathode bias.  I think.  I'll have to prototype this on the amp I'm currently working on.

AX84CH - thanks for keeping the archive accessible regardless!  I'm relatively new to the amp building forum world, and I'm always trying to scrounge up forgotten little tidbits that got left by the wayside or weren't as relevant then as they are now.

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

« Last post by AX84CH on April 20, 2026, 03:19:59 pm »

I have no memory of this.  That's how asleep I was on my own forum.  A neat idea.  You still lose plate voltage due to the cathode being non-zero but sometimes that might be helpful.

Could this also be used in situations where you wanted to run a particular tube but your plate voltage was higher than the tube needed?   A 30v Zener would give back 30 volts to the plate voltage limit and then you could adjust the grid bias to land the biasing where you wanted it?

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

« Last post by AX84CH on April 20, 2026, 03:07:01 pm »

This is the one I'd zero in on... 30bux. 300V @ 300mA; enough for 20-25W amp without working it too hard.

Still, I prefer linear PS for tube amps. JMO.

https://www.amazon.com/Vacuum-Preamplifier-Switching-Transformer-100-265V/dp/B0DZGR1HPR/ref=pd_sbs_d_sccl_2_1/141-4689532-8312021?pd_rd_w=SY8Ej&content-id=amzn1.sym.aa738fbd-ad05-4d11-aae2-04b598db6305&pf_rd_p=aa738fbd-ad05-4d11-aae2-04b598db6305&pf_rd_r=PXC57AFWBQH84PVYSA2Y&pd_rd_wg=cdqpX&pd_rd_r=cf9d8605-54ab-40e5-9375-719858e5fb22&pd_rd_i=B0DZGR1HPR&psc=1

--Pete

This time I have ordered one of those exact units to try.  The other one I got is considerably larger.  I could use the 300V without the doubling in size.

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

« Last post by HotBluePlates on April 20, 2026, 02:54:41 pm »

To me it seems an alternative way to obtain a Mixed Bias

Stop thinking in terms of a "Cathode Bias" "Fixed Bias" dichotomy, and think only in terms of grid-to-cathode voltage.

The Left diagram has an adjustable voltage applied grid-to-cathode; it just keeps the cathode tacked to 0v and allows the grid voltage to be changed.

The Right diagram has Zeners to clamp the Cathode to +22v, and tacks the grid to 0v.  But this is non-adjustable.

The middle diagram uses Zeners to keep exactly 22v across the cathode resistor.  However, the ground-side of the cathode resistor is not tacked to 0v but to +15v at the top of a zener to ground.  The grid is not tacked to 0v as with Cathode Bias, but to some point between +15v and something more positive than 0v (due to the 10kΩ resistor to ground).  That allows for adjustment not unlike the biasing arrangement in the Williamson Amplifier.


I wouldn't call the middle diagram "Mixed Bias"... it is 100% "Fixed Bias" but just doesn't have a bias supply derived from a transformer tapping; the fixed bias voltage is created by zener diodes after the tube draws sufficient current to drop volts across the cathode resistor.

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

« Last post by HotBluePlates on April 20, 2026, 02:46:23 pm »

What does Fixed Bias do?

With a grid nailed to a single unvarying voltage, the grid-to-cathode voltage stays absolutely steady.  There is no counteracting change of grid-to-cathode voltage in the face of a large driving signal.

At some point, the Peak Input Signal exceeds the Bias Voltage (pretend this Bias Voltage is -22v on the grid).  One side has already long since been driven to cutoff.  However, the remaining side is getting say, +24 volts peak applied to a grid with -22v grid-to-cathode.  +24v + -22v = +2v momentarily on the grid of that output tube.

When the grid goes positive of the cathode, its impedance falls from ∞Ω to some lower impedance (something like 1kΩ seems typical for a 6V6).  The output tube grid tries to pull current from the phase inverter or driver tube (which usually is not designed to supply it), and the sudden low input impedance clamps the drive signal from the driver/phase inverter.  We get an abrupt onset of distortion as the positive-going side of the Grid Input Signal is lopped off.


You know how Negative Feedback is supposed to keep the associated circuit clean?

A Super Reverb has negative feedback around its power section, yet we know a Super Reverb will distort.  What happens is the feedback counteracts the signal applied to the power section (really, all the stuff inside the feedback loop), and causes the amp to behave as if the signal were smaller by a fixed %.  It does this until the drive signal exceeds the amount of gain thrown away as feedback, at which point the feedback stop counteracting drive signal and preventing distortion.

The result is an abrupt transition into distortion for the amp with feedback, as compared to the searlier and more gradual increase of distortion that would happen in the same amp without feedback.


The zener diode across the cathode resistor clamps the Cathode Resistor Voltage (and so Bias Voltage) to a fixed amount.  As the amp transitions into Class AB, the bias cannot rise with increased Total Tube Current as it would have.  If the Peak Input Signal exceeds this fixed value of grid-to-cathode voltage set by the zener, the output tube will attempt to draw grid current, and will clamp the input signal in the same way as happens with Fixed Bias.

We don't have "compression" but we also get an abrupt transition from "clean" to "dirt" the way we would expect with an amp with negative feedback around the power section.

Maybe that sounds like a good option, but we do not get a different benefit of negative feedback:  the bass resonant frequency of the speaker is not damped as it would be with negative feedback.  The low end can sound woofy and uncontrolled, though normally the amp with feedback has lost this control when it is distorting for the same reason it stops keeping the power section clean:  the feedback has run out of gain to use, and the power section is essentially running with no negative feedback (until drive signal is reduced).

46

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

« Last post by HotBluePlates on April 20, 2026, 02:31:17 pm »

To me it seems an alternative way to obtain a Mixed Bias

Better yet, say it's a way to make Cathode Bias act like Fixed Bias.  Or maybe a little bit like "Applying Negative Feedback" to an amp that has no negative feedback around the power section.


How does Cathode Bias create compression?  Only when driven to distortion (or at least much into Class AB).


The diagrams assume something like a 5E3 tweed Deluxe.  There are no voltages marked on the schematic, but anecdotally they were higher than the 5C3 tweed Deluxe and might have had a voltage across the 6V6 cathode resistor of 21-22v (compared to the 5C3 Deluxe's 18v of bias).

If the amp stays perfectly in Class A, the current draw of one side increases with a positive-going grid signal exactly as much as the current draw of the other side decreases with its negative-going grid signal (except for rise of screen current and/or imperfect matching of tube currents when driven).

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.

The side that is Off cannot flow "negative current" to match the other side's current increase, so the Net Current through the Cathode Resistor increases.
The Cathode Resistor has a voltage-drop that is Total Tube Current x Cathode Resistance.
Higher Net Current results in greater voltage drop across the Cathode Resistor, so the Bias Voltage at the cathode increases.

We might usually think simply "this turns down plate current" but think about "bias" in terms of "volts from grid-to-cathode."  A large incoming signal tries to pull higher total tube current, and when only one side is on the voltage across the cathode resistor rises.  For that side, the grid signal is positive-going, but now the cathode is also positive-going by some smaller voltage.  This has the same effect as applying a slightly smaller grid-input voltage because the tube does not feel as-large a grid-to-cathode voltage-change as it would have if the cathode were nailed to a single, steady voltage.

The end result is after the output tubes run into Class AB, some amount of rise of Total Tube Current happens, and it tends to counteract the driving signal that pushed the output tubes into Class AB.  The intensity of the compression effect depends on how large a current-rise happens, and also the resistance of the cathode resistor (and the tube's transconductance).  So we're not necessarily at extreme-squish the moment the Total Tube Current rises above Idle Tube Current.

47

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

« Last post by DummyLoad on April 20, 2026, 12:23:51 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

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Other Topics / Re: Meme of the day

« Last post by shooter on April 20, 2026, 10:36:01 am »

...

49

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

« Last post by johnnyreece on April 20, 2026, 09:11:24 am »

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!

50

Tube Amp Building - Tweaks - Repairs / Re: Grid Bias Versus Cathode Question

« Last post by HotBluePlates on April 20, 2026, 05:42:51 am »

So what amps do compress the signal more than others? 

How do we design an amp that will respond with more tube and power supply compression for sustain?

I have an Ampeg Reverberocket from the early 60s that compresses more than any amp I've ever heard.

Yes, it is cathode-biased, but it seems more than a 5E3 Deluxe copy, despite having substantially the same power section and slightly beefier filter caps.

I haven't analyzed the circuit to figure out why it compresses so much, but I suspect it is a result of how the reverb circuit passes the Dry signal from the cathode of one section of V2 to the cathode of the second section of V2.  That's because the apparent compression happens even at low volume, and without the power section being driven hard.  However, I would need some kind of pulse generator to spot peak signals being compressed.