How much of a EL34 TW's tone...

[iknowjohnny]

If at all would you say is due to having low plate voltages? from what I've seen they are probably 100 volts lower than the typical el34 marshall, no? (granted, I haven't spent much time looking at them but don't they hover in the mid to high 300V range?

[Blackburn]

That's definitely a big part of the Express tone. One of the reasons they clip so early and are very 'brown' sounding.

[rooster]

395 VDC, and this is not too untypical of the 50 watt Marshall MarkII metal panels being made in the 70s and 80s.

I used to think this was too low, but now I think it's all about the tubes. True, you can increase the voltage here by 35 VDC and the amp will be very different. I've done this and it became tighter overall, brighter, and I ended up biasing it much cooler, 28 mas, and it was a good sounding Express using JJ KT77s.

But back to the 395 VDC, with the right tubes, vintage, the Express can sound remarkable, of course. Which I think can also be said about some of those 70s and 80s Marshalls, although having a much thinner tone.

[Jana]

380 to 395 is the tone zone for EL34's.

784-139 OT and 380 B+ -- Mmmm, tasty tone!

[iknowjohnny]

I've done this and it became tighter overall, brighter, and I ended up biasing it much cooler, 28 mas, and it was a good sounding Express using JJ KT77s.

Are you saying that biasing cooler makes a PA section with higher voltages on the plate sound more like one biased normally with lower voltage plates?

[JoeCon]

...and how does the OT primary effect this tone? Marshall 3.4k vs. TW 6.6k.
A lot of this is personal too as we all want little different tone. I was just reading here another builder swears by a 4.2k OT for his express.

Thanks.

[rooster]

Good questions.

In regards to the higher plate voltage version I built, it was noticeably louder, with a tighter feel. Which translated to adding a PPIMV and then biasing on the 'cleaner' side of things for the player - who, BTW, prefers HBs. This and, owing to his small club gigs, also uses a modified TS for his lead tones. All of which seems like a formula for the misuse of a great amp but turns out to not be the case. The touch, feel, and sustain are all there...and it also helps that he is a very good player.

But this did not answer your question very well. If you were to skip the pedals and turn this amp up, I would have to report that it breaks up differently than one with the typical Express voltages. Running this amp vs. an identical amp using the same tubes, same OT (Heyboer/Toneslut) but with 395 plate volts and biased at 40 mas, I think the higher voltage amp sounded better - right up until you push the volume knob past 12-1 o'clock and start compressing things. Here, the amp sounds more 'edgy/ratty' than the typical version. I attribute this to the higher plate voltage, but also the slightly higher voltage on the preamp tubes. One is 95 VDC, the other, 125 VDC. To my ear and thinking, it's as if the higher voltage version needs either a much bigger OT at higher volumes - or at least a smaller choke resistor. If you have pursued a higher voltage Express, I would easily suggest a smaller choke as an experiment. And actually, in the higher voltage Express, I installed a dual switchable choke circuit with the option for either the 1K stock version or something in the 200 ohm range (which makes for a louder amp with a 'cleaner' grind, IMO). Since the amp has PPIMV, it's not a problem using either choke. But again, my simple point here is that the higher voltage Express behaves differently than the typical version.

So there's my answer.

In a nutshell, IMO, the Express is a complete package. If you mess with it, there are compromises to make. If the compromises work for you, great. Me, I have rethought the circuit to the point of now using even the tubes that Kenny preferred. I also switched to the Pacific OT in my 395 plate voltage Express and currently bias the power tubes, Mullard xf2 single getters at 28 mas (And, BTW, I have been told by a friend of Ken's that 28 mas may be too high with these particular tubes. I went as low as 24 mas and and went back to 28, BTW). In my case, FWIW, finally, I just had to rethink the Express and do everything I could to create a good version of the original circuit that Ken designed. This has been a good decision for me, YMMV.

I hope this helps.

[rooster]

Oh, as to the tone of the lower primary OT vs. the higher primary, I have read that the higher primary OT attached to a tube that is seeking a lower primary tends to lose some of the some of the upper frequencies (of the tubes? of the transformer?) at the speaker. I have never read anything scientific about this so take it with a grain of caution.

In the scenario I describe, the question might be: Is it possible to create a 6.6K primary OT mated to EL34s that sounds overburdened with high frequencies? I don't have a scientific answer. However, many people building clones of Express's say their amps are 'very bright' so, since I have used the same OT at one time and my builds were/are not overly bright, I think there is more to it than the primary impedance.

So no solid comment from me on this subject, sorry.