Cleaning up an Express' act

[FUCHSAUDIO]

The AU7 and AT7 tubes are lowering ALL the rail voltages in the preamp substantially. The 12AT7 is about twice the current of a 12AX7 in the same circuit (with no changes) and a 12AU7 is DOUBLE a 12AT7 (basically each half is two 12AT7 halves in parallel) so things can get loaded down pretty far with each tube sub fwiw. Nothing wrong with any of this as long as the plate load resistors and dropping resistors can handle it....

[rooster]

Fuchsaudio makes a point but this is a non-consideration in the Express with a 7 amp filament supply. Example, a SR has a 4-5 amp filament supply and runs 6 pres and two 6L6s, with two of the pres being 12AT7s.

[FUCHSAUDIO]

Sorry if I was unclear Rooster, but I meant the preamp B+ voltages (by rail voltages). Filament currents are insignificant and not affected .

Plate currents and their overall impact on all the voltages in the preamp are not what I might call "insignificant"

[rooster]

Um, OK. But moving on, Fuchsaudio, the current of the SR PT is rated at 230 mA. The Heyboer, Hammond (AC30), and Pacific are all rated at over 250 mA.

So, let me ask you, are you saying two EL34s and three 12AT7s are going to max out the power supply current of the Express PT?

[vibratoking]

Um, OK. But moving on, Fuchsaudio, the current of the SR PT is rated at 230 mA. The Heyboer, Hammond (AC30), and Pacific are all rated at over 250 mA.

So, let me ask you, are you saying two EL34s and three 12AT7s are going to max out the power supply current of the Express PT?

Andy didn't say that at all. He just mentioned that the power across the plate loads and B+ dropping string resistors will increase as the current increases.

[FUCHSAUDIO]

Thanks Vibrato King, that's exactly my point. The overall amp current from the PT is plenty good. That dropping string is where the affect of higher current tubes will drag things down. I've seen V-1's at 100 or less on the plates if all the tubes are AT or AU's...Part of the tonal change is the different tube but likely a bigger part is the much lower plate voltages.

[RockinRocket]

So if im understanding this right putting a 12at7 in a PI spot(or others) will lower voltages to the rest of the preamp?

[FUCHSAUDIO]

The PI has the least effect since it's early in the string. Some effect, but not much. The input tubes swing the voltages down the greatest, being lower on the dropping string.

[monkeymagic]

Just to chip back in on this thread, I've continued to mess around with tubes since I originally posted on this and currently I'm most happy with X7s in 1 & 2 a black plate T7 in the PI - retains all the harmonic richness,slightly better balanced to my pick attack (personal pref) and cleans up even when heavily attenuated.

That said, it sounds pretty good regardless of what I try!

[Tone Lover]

Just to touch bases on this. Not many people use this as a tweaking point but differant voltages across the preamp tubes have a big differance on sound . Adjusting those are an under utilized sound changer to your preamp.
Bill

[FUCHSAUDIO]

Just to touch bases on this. Not many people use this as a tweaking point but differant voltages across the preamp tubes have a big differance on sound . Adjusting those are an under utilized sound changer to your preamp.
Bill

Agreed. I just tamed a nasty early boogie that had 250 volts on the preamp plates...Added a series resistor and brought them to 175 and it's a whole different amp. Went from screaming to singing....In the Express (or most any other amp) adding a 2-W 100-K, 150-K etc at the lowest point (the Dumble guys call it the Fet-simulator) pulls the preamp tubes down while not messing with the PI...This, or change the series dropper post PI that feeds the rest of the amp. Tune to taste.