Upping 5F1 6V6 Plate Dissipation… raise Voltage or lower Resistance?

James Knox

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I have noticed that using the 470R Cathode Resistor on a 5F1 usually yields a ”cool” (70-80%) plate dissipation. I’ve been experimenting with dropping the Cathode Resistor down into the 360R, 330R and even the 320R range in order to get to ~100% Dissipation.

Using the venerable Hammond 290AX Power transformer, for most of my 5W 5F1 builds I utilize the 275V HT taps, and for 10W (GZ34/6L6) builds I use the alternate 325V HT Taps. For kicks, I put a 5Y3/6V6 tube combo in a 10W build (with a 470R Cathode Resistor) and got a nice hot Dissipation of ~105% (sounded awesome, btw)

Now, this is the question I have…

What are the Pro and Cons of achieving ~100% Plate Dissipation by dropping the Cathode Resistor values or alternately just using the Higher Voltage PT taps?
 

D'tar

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Dropping the the cathode resistor/increasing the bias will reduce plate voltage. Why not use the 325-0-325 taps?

Edit... what is your plate voltage?
 

King Fan

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What he said. :) Sadly, I'd already typed my typical long-winded wandering:

Why *wouldn't* you use the higher HT? I'm the opposite of an expert, but I thought dropping the cathode resistor would increase 6V6 plate voltage by stealing voltage from the preamp -- rob Peter to pay Paul kind of deal. (FWIW that trade-off is my simpleminded idea about the downside of adjusting bias resistance too much).

What B+ are you getting in those 5Y3/6V6 Champs running at 70-80% dissipation? What's your cathode voltage there with the stock bias resistor? I've seldom seen anyone target 70-80% dissipation in cathode bias, though I have seen one or two folks who claim they liked less than 100. But we've also heard from a few who'd mistakenly targeted 70% and wondered why their amps sounded anemic...
 

D'tar

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Take a champ running 110%+ and plug into your bucking transformer, IMO its like going from exA to exB

A
ggyt8xbqiq731.jpg


B

f25ede8863b1d56dd65abfa5abd169d9.jpg
 

2L man

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What are the Pro and Cons of achieving ~100% Plate Dissipation by dropping the Cathode Resistor values or alternately just using the Higher Voltage PT taps?
James, you should play more with Universal Loadline Calculator!!! :)

It take less than minute to input data and you immediately see how that stage behave. Obviously you have built many different SE amps and if you know how they sound so you get very good idea what effect placing operating point has. For cathode bias Anode voltage window setting is actual amp Anode voltage against ground where Cathode (bias) voltage is deducted. Notice how Screen voltage effect to g1 voltage!

Basically there is linearity in low power (headroom setting window) what effect to mostly "warm" 2nd harmonic distirtion and then when more power is taken comes clipping and it can happen other end. Either high voltage/low anode current or high current/low anode voltage end which produce 2nd harmonic. Or when operating point is placed to the middle of the loadline, both ends clip same time and distortion turns more 3dr harmonic more sudden.

Using loadline simulation you can do very drastic changes because it does not break ;)
 
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2L man

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Physical tubes have tolerances but ULLC bases to datasheets which obviously were done measuring lots of tubes some very long time ago so "curves" can not trust nowhere 100%. Current tubes are different and when a new tube is installed it can drift and sometimes require even 100 hours to settle but cathode biasing is nice there and concerns mostly fixed biasing :)

But when you know what carhode resistance procuce certain cathode current you can change Screen voltage window to match what g1 voltage you measure over cathode resistor. Then change Ic (quiescent Bias current window) what you are after and you get "new" g1 voltage and when you use it to calculate new cathode resistor the bias current comes very close what you want.

If actual amp cathode current is changed a lot it can change B+1and B+2 voltages and voltage loss over OT primary and it again has effect to operating point but soon you will see the pattern :)
 

James Knox

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Why not use the 325-0-325 taps?

I guess I just read that the 275-275 taps put 5F1’s in the perfect zone. Then I just started building them that way. Probably a perfect example of not fully understanding what I was doing, so taking internet advise.

To use your Lion photo metaphor, I got used to building cute little Lions. But now, with a little more experience, I am realizing I could be building some roaring lions!

The advice commonly given is, “does it sound OK, the just leave it”. Sound is so subjective. They always sounded fine, I thought. Then recently I built a GA5 that Dissipated at 102% and I realized I had been missing something. I had been be operating under the misconception that 70% Dissipation “saves your tubes”, etc. Recently re-reading Aiken on Biasing I discover that 70% is a standard for push/pull NOT for Single Ended!

Slow learner here….
what is your plate voltage?

Not Sure. I HAVE written down every Plate To Cathode reading for every build. Should I be targeting Plate Voltage instead of Plate Current?
 

D'tar

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I HAVE written down every Plate To Cathode reading for every build. Should I be targeting Plate Voltage instead of Plate Current?

Not at all. I was just curious. If your plate was say 420vdc, then one may pursue the increase in bias and lower plate vdc at the same time. If 275 tap yields 350vdc and 70%, well its all up to you if you like that then carry on. Just don't get too set in your ways without at least trying all your options.
 

James Knox

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Why *wouldn't* you use the higher HT? I'm the opposite of an expert, but I thought dropping the cathode resistor would increase 6V6 plate voltage by stealing voltage from the preamp -- rob Peter to pay Paul kind of deal. (FWIW that trade-off is my simpleminded idea about the downside of adjusting bias resistance too much).

Good point. I had not thought of that.
What B+ are you getting in those 5Y3/6V6 Champs running at 70-80% dissipation?

Usually in the ~350V range. While compiling a Spreadsheet for my builds over the past couple years, I realized that my 10W 5F1 builds (in which I utilized the 325-0-325 taps gave me ~400+ on the Power Rail. My happy accident to pulling the GZ34/6L6 and subbing a 5Y3/6V6 put me in the 114% range. My thought then was that it was too high and I would blow through tubes. I had not realized at the time how the B+Voltage affects the bias.

What's your cathode voltage there with the stock bias resistor?
Not sure. I only kept notes for Plate To Cathode readings.
But we've also heard from a few who'd mistakenly targeted 70% and wondered why their amps sounded anemic...

I am sheepish raising my hand …. I am realizing that my Champs sounded great (to my ears) up to ”point of breakup”. I was happy to use pedals for more gain if I wanted more. Now I’m beginning to see why I did not care for the sound of a ”dimed” Champ!

Its actually kind of embarrassing to admit my ignorance, lol. Good thing I’m among friends…
 

James Knox

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James, you should play more with Universal Loadline Calculator!!!
You have been telling me this for at least a year, my Friend! I am so sorry to report that every time I have tried to use it, I just did not understand what I was seeing. This morning I pulled it up and am determined to “go back in”. I even started reading his book!
But when you know what carhode resistance procuce certain cathode current you can change Screen voltage window to match what g1 voltage you measure over cathode resistor. Then change Ic (quiescent Bias current window) what you are after and you get "new" g1 voltage and when you use it to calculate new cathode resistor the bias current comes very close what you want.
Exactly what I am trying to wrap my head around right now. It has been way over my head, but now a “little bit o light” is beginning to break through. I hoping for a breakthrough!
soon you will see the pattern
Yes, just putting all my past builds in a spreadsheet revealed some patterns I had not seen before!
 

James Knox

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Not at all. I was just curious. If your plate was say 420vdc, then one may pursue the increase in bias and lower plate vdc at the same time. If 275 tap yields 350vdc and 70%, well its all up to you if you like that then carry on. Just don't get too set in your ways without at least trying all your options.
Good advice. I guess I became set in my ways not realizing it.
 

King Fan

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James, your curiosity and willingness to learn are a model for all of us; we all get 'set in our ways.' :)

One of the trickiest bits of building an amp is choosing the *actually* right PT. Not a science, exactly, but that fact makes it even harder. If you have B+ over 400, you're right, that's pretty hot for a tweed Champ (tho typical for a BF/SF Champ).

And for you, the 275-325 choice seems to be missing Mama Bear's porridge. I don't always believe Fender '50s voltages, and they were on old low wall power anyway, but I like B+ 370-390. Weirdly, my Champ PT put out about 390. But hmmm. IIRC, your 'venerable' 290AX is a BF/SF 'Bronco'-class PT, at 100mA? For a 5F1 I like the old 70mA tweed size, which'll get loaded down more.

But but but, don't tear out those PTs! @D-tar is so right about the bucking transformer. Rob's -7% / -12% version (plus a standard outlet nearby) should get you into several sweet spots starting with Bronco B+ and dropping well into 1950s B+. Did you see the nice example here recently built into a Hammond alu box?

In my voltage logs I try (tho often fail) to note wall, bucked voltage (if any), B+, plate voltage, *and* cathode voltage. But if you just note plate voltage and cathode voltage, you get P-C voltages, and you know your measured cathode resistance; then Rob's 'Tube Dissipation Using Cathode Resistor Voltage Drop' spits out the % MPD with no other calculation or measurement.

Final annoying question: What kind of 5Y3s are you running in these? As you'll know, "the kind that drop the most voltage" is the best kind. :cool:
 
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2L man

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One more ULLC interpret tip for James: Entered Headroom box voltage is linear anode voltage sweep from red operating point to left which appear green. To have it the Control Grid drive voltage must change and it is assumed being linear both up and down from g1 idle voltage because simulation is good to use same kind variables. You can read grid line voltage values pointing them. Usually on guitar amps SE A-class loadlines the green anode sweep to the right gets "squished" and it produce 2nd harmonic distortion.
 

peteb

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What are the Pro and Cons of achieving ~100% Plate Dissipation by dropping the Cathode Resistor values or alternately just using the Higher Voltage PT taps?
I am under the impression that the 470R value was chosen to center bias the single ended signal half way between cut off and saturation, making a single ended amp a true class A single ended amp. changing the cathode resistor value is going to move you away from center bias, moving the amp away from class A towards class AB. There is nothing wrong with class AB but I think in this case of you move the operating point towards cut off or saturation, you could lose headroom and lose max clean power output.
 

dan40

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Why *wouldn't* you use the higher HT? I'm the opposite of an expert, but I thought dropping the cathode resistor would increase 6V6 plate voltage by stealing voltage from the preamp -- rob Peter to pay Paul kind of deal.
It's actually the opposite KF...dropping the cathode resistor value will cause the plate voltage to drop as the power tube begins to draw more current from the power supply. The increased demand on the power supply will cause voltages throughout the entire amp to drop.
 

King Fan

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Aha, thanks, Dan. I think @D'tar was maybe saying the same thing above. As will be obvious, I don’t recognize Herr Prof. Dr. Ohm half the time we pass on the street. I had understood, perhaps incorrectly, that the more typical scenario, where somebody increases their output bias resistor to cool output bias, increased voltages in the preamp, so I extrapolated, no doubt incorrectly, that dropping the resistor to heat up bias would have the opposite effect, and lower voltages in the preamp.

Heh, seriously, KF, buy a vowel. Am I right in thinking that increasing *or* decreasing the cathode bias resistor too much has the downsides of “messing with“ preamp voltages?
 

James Knox

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But hmmm. IIRC, your 'venerable' 290AX is a BF/SF 'Bronco'-class PT, at 100mA?

Yes, 100mA …. Been using the 290AX exclusively. Seems pretty robust!
Did you see the nice example here recently built into a Hammond alu box?

Yes, really nice. Hope to build one soon.
What kind of 5Y3s are you running in these?
I have a few NOS RCA and Sylvanias for my personal amps, but have been building with JJs for consistency.
 

James Knox

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James, chnaging the power t7be can immediately change the current draw /plate dissipation in that amp. I once tested about 10 NOS RCA 6V6GTAs in a Champ. I got everything from 82% of MPD to 150%.
That’s amazing Wally - good to know!
 




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