Express wiring v1 in parallel

[mumford]

I'm thinking of rewiring my express with v1 in parallel. I've seen several posts saying to use separate 47k plate resistors, and 820 (or thereabout) cathode resistors.

The only amp I can think of with a parallel v1 is the matchless lightning, and it uses a shared plate resistor. My question is, how to do I connect the outputs from both plates going to the tone stack?

It would seem that if I just connect them to one another, that eliminates the point of the separate plate resistors, as both plates would be directly connected to both of them. Do I need to add coupling capacitors or at least small resistors to them?

[mumford]

...or did I have it all wrong, and I am supposed to use a single shared 47k plate resistor (meaning my question is moot?)

[Blackburn]

The Songwriter 30 has a single 12AX7 in parallel for it's only preamp stage with a 220K plate load resistor.

[mumford]

Seems like I need to use a single half value plate resistor, not two separate ones. My question is moot. Thanks!

[pablogt]

If you keep shared plate and cathode circuits, you're supposed to half cathode and plate resistors and double the cathode bypass cap. If you have a shared 50k plate resistor (double the wattage!) and independent cathode circuits that will give you a lot of room for tweaking. Or having a two inputs, one for "traditional" Express, another for parallel input.

Pablo

[mumford]

Thanks. Last night I wired it up in parallel with a 47k plate resistor, 820 cathode resistor and I think a 44uF cathode cap. I only got a chance to verify that it was working, so I'm not sure if it helped much. There is still hiss and a small amount of hum (I redid the heater wiring, too), but it might not be bothersome at the volumes I play.

I'm probably going to need to starting looking again at that v1 tube. The one I have in there (an old telefunken 12ax7) is possibly part of the problem. The other tubes I tried before (modern ones) were much worse.

[Zippy]

If part of your reason for running V1 in parallel is noise reduction, you will get best results from a tube with well-matched halves - time for some tube swapping.

[mumford]

I spent some time playing through it last night. Definitely saw some reduction in noise, but not the magic bullet I was hoping for.

That said, I really like the difference in sound. It feels a bit more 3D somehow, and although it gets dirty lower on the dial, it has more of a range of gains than it did before. Before, it was "up to 11 o'clock, semi clean, above that full bore." Now it's more tweakable.

Yeah, I need to try some tubes. I am not excited about investing in a bunch of expensive tubes to find "the one," though.

I recently did some power tube rolling, and that made a huge difference in the feel of the gain. Old RCA 6V6s were smooth, almost mushy, while JJs were cleaner and super punchy. Neither set would bias to anywhere near the same point, though.

[gingertube]

For your possible interest.

I used common 47K anode load but separate cathode bias resistors and bypass caps for dissimilar operating points - aim was to emphasize some harmonics which I think it did.

Note also the split grid stops, not required but convenient to wire. You could just as easy use separate grid stops for each triode section.

Cheers,
Ian

[Miket]

Thanks for sharing that ginger ninja. Very nicely drawn might I add.

[rooster]

I agree, nice drawing. I think I see Merlin all over that amp but I could be wrong. The 22K buffer resistors, the parallel cathode caps on V2A, the shielded wire to the PI, the 300pf bridge.... I also see lineal bass and treble controls. Hm. How does your amp sound?

[gingertube]

The Amp is big and bold, copes nicely with turning it up loud, how much of that is this preamp variation and how much is the cathode biased 6CA7 power section I don't know. What I like about it is the harmonic "ring", "overhang" (whatever the term used is) as the volume dies away after a chord or even a single note. The 3rd seems to fade in as the note fades out.

Some of those 22K grid stops are a bit of a legacy from setting it to work. Passed about 12 o'clock on the volume knob it used to burst into ultrasonic oscillation. This was cured by grid stops on the phase splitter but grid stops already fitted elsewhere in an attempt to cure that problem were left. I would have had grid stops there in any case but not necessarily as high as 22K.

Mid control doesn't seem to do a lot but then it rarely does in a FVM tone stack.

Cheers,
Ian