Adding screen resistor to a 5f2a fender

[Mikante]

Hello,
Since i had some trouble fixing the sound of my champ 600 i decided to convert it into a princeton 5f2a. I just had to remove some component and replace some of the values. I finally got rid of some problem this amp had but something else came out.
The overdriven sound is awful, hard to describe it, just awful. So i went reading and find out that could be the lack of a screen resistor on the 6v6. I then dropped the 470ohm back in the pin 4 of the power tube and wow. Much better overdrive sound but guess what, the amp is back to sounding harsh and unpleasant in the high frequencies spectrum, not as before, but almost.
I don t understand. I read that adding a screen resistor would have tamed the high frequencies a little bit and not increase them.
Any thought?
Should i go for a bigger value? 1k maybe?

[Reeltarded]

Tweed designs can have a nasty pinched whine treble when driven fully. Did you follow the schematic exactly?

[Mikante]

Hello,
There are a few differences, it is SS rectified, all filter caps are fairly larger, 22uf, i added a choke and the screen resistor just now.
Bias is within spec, hot as hell like all champs. the power transformer is a 300/300, a bit higher in terms of current but nothing huge.
In the signal flow there are no differences, i followed the 5f2a schematich.

[martin manning]

What are your plate, screen, and cathode voltages now?

[Mikante]

Cathode is 18,5v
Screen 307v
Plate to cathode is 343v
Plate is 362

[martin manning]

That looks pretty good. Is your cathode resistor 470 ohm? Did you determine what the output transformer’s primary impedance is?

[Mikante]

Yes right now there is a 470ohm as a screen resistror. It is a common value for champs.
About the impedance, no I didn t. Should I check?

[martin manning]

I was asking about the cathode resistor, to see what the idle current and plate dissipation are now. Yes, it would be nice to know what the primary impedance is. It's likely to be 8k ohms, but it would be nice to know for sure. Recall you have to apply some AC voltage to the primary or secondary and determine the voltage ratio Vprimary/Vsecondary, square that, and multiply by the secondary impedance (4 ohms).

[Mikante]

I have never done that, any voltage will do?

[martin manning]

You can use any available low voltage AC source such as a filament transformer or an AC-AC power adapter. The voltage ratio is expected to be (8000/4)^0.5 = 44.7. Usually some relatively low voltage, say 6-12 VAC, is applied to the primary, and both the primary and secondary voltages are measured. With 12 VAC on the primary, I'd expect to see about 268 mV on the secondary. You can apply AC to the secondary, but note the voltage on the primary will be quite high.

What is the value of your cathode resistor now?

[Stevem]

If when maxed out your signal drive voltage into the cathode biased 6v6 is greater than the the D.C. Voltage on the tubes cathode by too much then the overdrive can sound like crap.

[Mikante]

You can use any available low voltage AC source such as a filament transformer or an AC-AC power adapter. The voltage ratio is expected to be (8000/4)^0.5 = 44.7. Usually some relatively low voltage, say 6-12 VAC, is applied to the primary, and both the primary and secondary voltages are measured. With 12 VAC on the primary, I'd expect to see about 268 mV on the secondary. You can apply AC to the secondary, but note the voltage on the primary will be quite high.

What is the value of your cathode resistor now?

470ohm.

If when maxed out your signal drive voltage into the cathode biased 6v6 is greater than the the D.C. Voltage on the tubes cathode by too much then the overdrive can sound like crap.

Now with the screen resistor the overdrive sounds much better, but only when the amp is just starting to crunch. Everything after this sounds crappy. I how can i see if what you are saying is happening to my amp?

[martin manning]

I think you are in a good place with respect to the operating point. Reasonably well centered, and approximately 100% plate dissipation, as long as your primary impedance is 8-10k ohms. What about your feedback? Can you get any improvement in the overdrive tones playing with that?

[Mikante]

I don t know something must be wrong.
Everything looks in the right place, voltages and bias are right, valves are new and tested, it should not sound this bad when i crank it.
Converting it to 5f2a specification was actually an excuse to go over the circuit once again and look for mistakes.
Might be the OT?
I need to keep investigating and find out or i will not sleep

[Phil_S]

I have never done that, any voltage will do?

If you have no other source, you can tap the filament supply from the power transformer. Pull all the tubes. Use alligator clips on the appropriate tube pins. You'll want to lift the center tap on the OT from it's B+ connection.