40W Texas Tone

[bluescaster]

Hi, i just finish my amp.

It's a mix beetween blackface, Silverface, Bassman and Dumble with Weber trannies. I post the schematic.

Can you put an eye on it and say to me what do you think about the voltages i found on it please ? The 327V on PI seems curious no ?

It's a really good amp. You can get Little wing SRV kind of tone or Jimi Hendrix Voodoo chile blues.

kind regards.

[VacuumVoodoo]

The 327V on PI seems curious no ?

Why? Do you think it's too high or too low?

[Structo]

It depends how it sounds.

12at7 have a max plate voltage of 300v per data sheet.

You can experiment by increasing the R40 1K resistor.

This will lower the PI and preamp voltages.

Try a 2K

[bluescaster]

Hi, yes i think it's too high. I never seen this voltage on fender schematic. SuperReverb for example has 230V.

[tictac]

Blackface Fenders usually have 450-460V on the plate supply; you've got 480V, so yes your PI voltage is going to be higher....

Looks fine to me but if it will ease your mind...change that 1k dropping resistor to 2K2 or 2K7


TT

[Phil_S]

Blackface Fenders usually have 450-460V on the plate supply; you've got 480V, so yes your PI voltage is going to be higher....

Looks fine to me but if it will ease your mind...change that 1k dropping resistor to 2K2 or 2K7

TT

Or place a pair of 5K (or 4.7K) in parallel in place of the 1K, so you can more easily experiment with 2.5K or 5K to see what you like. If you like 5K better than 2.5K, just snip out one of them.

[seedlings]

That's really similar to my latest build, but I used a 12AU7 for PI and cathode-biased the power tubes.

Would there be any way for you to post an audio clip?

CHAD

[bluescaster]

Hi, i'll get my SM57 back in two week, but ok, i'll put something here.

[Luthierwnc]

If you are going to make it Dumbley, add a poor-man's PAB. Use a 50k push/pull mid pot with a 47k resistor across the outside lugs (25k's are hard to find) to get it back into range. Solder a little trimmer to the switch lugs and dial in the tail resistance to control how much you bottle-up the tone circuit bleed. Set you back 6 bucks unless you are using knobs that don't push/pull well. I'd imagine about 120k is the sweet spot. You could try resistors instead of the trimmer and season to taste.

FWIW, Skip

[bluescaster]

I switch 1K/2w resistor for a 4k7. Now i have:

B=474v
C=437V
D=380V

250V on V1 plates.
305V on PI plates.

[VacuumVoodoo]

Hi, yes i think it's too high. I never seen this voltage on fender schematic. SuperReverb for example has 230V.

Re: PI plate voltage
What matters is voltage between plate and cathode, as long as it's lower than 300V and cathode relative to heater is less than 100V it's fine.
You have ca 200V between plate and cathode. Cathode is ca 130V above ground. THIS is too high.
You might want to put heater on DC offset at ca 70V

Or you can increase plate resistors to 68-82k and increase shared cathode resistor to 0.82-1,2k if you really feel uncomfortable with 320V at the plates. I wouldn't.

[bluescaster]

OK, i understand that 300v max is beetween plate and cathode. So it's good.

Uk is 120v and 12AT7 datasheet have UKFmax=90v. What i must to do for this ? How to get an offset ?

[martin manning]

Put a resistive divider (two resistors in series) from the first filter to ground to get about 70V at the junction, and add an electrolytic cap from the junction to ground to filter the ripple voltage. Connect the center tap from the filament winding to that junction. I think for your situation a 220k - 39k or 270k - 47k divider with a 10uF cap will be good, and use less than 2 mA. Use a 2W for the larger resistor.

[bluescaster]

Like this ?

[martin manning]

Yes, exactly.