Screen Voltages 5E3/6G3 Build

[Colossal]

Really nice looking build! Very clean layout.

[syscokid]

Really nice looking build! Very clean layout.

Thanks.

And now, I wish I would have added an adjustable fixed bias circuit with the option to switch between the two. It’s still possible though!

[maxkracht]

Very tidy, the clear heatshrink on the filaments is a nice touch.

[Colossal]

B-nodes dropped slightly less than one percent, except for B+2 which dropped from 410v to 403v.

Very subtle change in sound. Yes, it's a bit smoother and I don't seem to hear the instantaneous mild crunchy fizz when slamming full chords when the amp is cranked anymore. It was pretty loud in this 11 x 11 converted Mancave room. DB meter was displaying 115 dB's... Ouch!

Glad you got the voltages sorted. I just finished a Tweedle Dee and it peels the finish off a 10x10 wood paneled room. Stunningly loud amp for 12-14W.

[B Ingram]

... screens are fed from B+2. Using a variac to take measurements at a consistent 120 VAC, here's a more comprehensive voltage report just recorded:

The 6v drop from B+1 to B+2 implies 6mA through the 1kΩ resistor.

If you want the 6V6s to be sensitive & easy to drive, it is better to lower the screen volts & drop the cathode resistor to 250Ω. If I were building this amp I'd shoot for ~300v at the filter cap feeding the screens. That would require:

406v - 300v = 106v drop ---> 106v / 6mA = ~18kΩ

Change the dropping resistor (that your image calls "R1") to 18kΩ, and add a filter cap across R2.

You will find plate current has fallen, and you won't need 400Ω at the cathode to tame plate current. Now when you move back down to 250Ω, there will be less voltage across the cathode resistor.

Lower Volts across the cathode resistor = Less Bias Volts = Lower Peak AC Volts fully drives the 6V6s = Easier to drive/distort.

[syscokid]

Change the dropping resistor (that your image calls "R1") to 18kΩ, and add a filter cap across R2.

Another interesting contribution, thanks! At what values for the filter cap across the 220K? What does this cap add to the recipe?

[B Ingram]

... add a filter cap across R2.

... At what values for the filter cap across the 220K? What does this cap add to the recipe?

The way R1 and R4 are connected now, they create More Volts Dropped when Screen Current is high. That will be "sag" but only when you play very loudly (screen current has momentary high peaks when the amp is pushed to/beyond max clean output power).

- These are behaving at though they are just a "series screen grid resistor" (which is intended to only reduce screen volts when screen current is high, in order to momentarily reduce screen dissipation).


What I'm suggesting is a solid DC Volts node just before R4.

- The 18kΩ is not intended to add "sag" but to drop 406v down to 300v, which mostly stays as a steady 300v DC.

- Lowered screen volts reduces plate current; it just affects plate current less than grid-to-cathode voltage.

- R4 can be a larger/smaller depending on the amount of sag imparted at the loudness you typically use. Fender would have picked 470Ω to limit screen dissipation at high power output (or distortion), but give the least sag feasible before that point. Some folks would use a 3.3kΩ resistor there to make the amp sag noticeably well before it reaches max power output or distortion.

... Plate voltage is 407v ... Screen voltage is what has encouraged me to post this thread… It’s at 410v. ...

Your voltages would have been fine had you stayed with fixed bias (and deeper Class AB).

That the screen volts were a little higher than plate volts doesn't matter. Until the plate voltage is <80v, this has no impact for 6V6s.

But you tried to cathode bias this power supply, without realizing Fender moved from 5E3 to 6G3 by shooting for higher power output, by using higher supply volts, a higher screen voltage, but then needing to work deeper into Class AB, and using fixed bias to help shut a 6V6 off part of the signal cycle (so that it can cool off from the high peak current it will deliver).

- I'm trying to guide you back towards how a 1950s Amp Designer would deal with this.
- Ex: 1950s Ampeg has 440v on the plates of the cathode biased output tubes.
- But Point B of the power supply inserts an 0C3 regulator tube that drops the screen voltage to be 105v less than the plate voltage.

Cathode biased amps can get a little way into Class AB, but not far. Lowering the screen volts is a way to keep the tubes from overheating while maintaining reasonable power sensitivity.

[syscokid]

... Plate voltage is 407v ... Screen voltage is what has encouraged me to post this thread… It’s at 410v. ...

Your voltages would have been fine had you stayed with fixed bias (and deeper Class AB).

That the screen volts were a little higher than plate volts doesn't matter. Until the plate voltage is <80v, this has no impact for 6V6s.

But you tried to cathode bias this power supply, without realizing Fender moved from 5E3 to 6G3 by shooting for higher power output, by using higher supply volts, a higher screen voltage, but then needing to work deeper into Class AB, and using fixed bias to help shut a 6V6 off part of the signal cycle (so that it can cool off from the high peak current it will deliver).

At first, my intention was to reimagine my 5E3 clone with a LTP inverter and the front end of the 6G3. While I was planning the conversion on paper, I continued to fine tune to my ears a more refined low end output out of my 5E3 clone. IMO, it sounded so good that I decided to leave it as is and just build another amp. But now, you have reminded me that I’ve never personally heard or played through a real 6G3. And currently I’m still hearing a lot of similarities with both of these builds. I believe that I just talked myself into replacing the cathode bias circuit with a fixed bias circuit.