Question about possible B+ switch...

[mort]

So I'm looking at a PT That has separate taps for 280-0-280, 312.5-0-321.5, and 345-0-345 supply levels. Would placing all these taps on a selector switch throw off the bias too much to be practical? It's gonna be an EL34 50W Plexi style circuit. Any other concerns come to mind?

[big_teee]

Those power supplies run a lot more money if you get multiple taps like that.
345-0-345 in the US you can count of right around 500v rectified.
The last one I built I used a 325-0-325, and it ran around 450-465v after filtering.
To me that is about right.
I like the lay down type, and I used this one.
Fender Power Transformer, Vibrolux, Vibrolux Reverb, Tremolux, 022723 & 125P26A, 120V
I used the separate bias voltage for bias, and used the 5v tap to run the pilot light.
It gives a dimmer bulb, (about right), and reduces the load on the 6v tap.
I used it for 6L6s, but it worked fine for EL34s to, and it is not such a big transformer.
Another option is this one. It is a little larger, with bigger specs.
Fender Tweed Bassman & Super Style PT, 120V, 8087
GL,
T

[mort]

here's the one I'm looking at. It's only 20 something bucks more.

http://www.classictone.net/40-18095.pdf

I might do away with the filter cans and use regular caps right on the board, so using the upright would give more room in the box too. And when I saw all the taps I thought it might make for a useful tonal option. Perhaps being a little more organic/vintagey with the lower voltage and edgier/aggressive with the higher voltage.

[jmaf]

I don't see a bias tap on that transformer. Where do you plan on getting your fixed bias AC from?

[Mike Sulzer]

I don't see a bias tap on that transformer. Where do you plan on getting your fixed bias AC from?

Complete the bridge to make a high voltage negative supply, and drop it down with a resistor divider. This takes very little current

[jmaf]

Complete the bridge to make a high voltage negative supply, and drop it down with a resistor divider. This takes very little current

Sounds like a Marshall alright

[mort]

Plexi doesn't usually have a bias tap anyway.

http://www.drtube.com/schematics/marshall/1986u.gif

Was also considering using a dual 250k pot as an MV in place of the 220k resistors. Did this recently with an AC15 but it doesn't use a bias supply like that, the 220k's just go to ground. Is adding the MV going to cause me problems in the bias?

[jmaf]

Was also considering using a dual 250k pot as an MV in place of the 220k resistors. Did this recently with an AC15 but it doesn't use a bias supply like that, the 220k's just go to ground. Is adding the MV going to cause me problems in the bias?

I wouldn't use that kind of MV on a fixed bias amp. First you don't want DC on your pots, second because a pot failure means bias failure and melting EL34's and possibly a OT replacement.

[mort]

doing some google reference and coming up with a Ceriatone drawing where the 220K's are directly replaced by the pot.

http://ceriatone.com/images/mods/Ple...rontCeriat.jpg

Also saw a drawing that put a 500k dual pot in series with the signal path and leaves the 220k's in place.

http://cfile3.uf.tistory.com/image/1...4E1D241905CA16

[jmaf]

Yeah, placing the MV on the power tube grids is not uncommon, other amps do it in various ways. E.g. Orange Tiny Terror also does it, but it's self biased.

IMO it's not a good idea for fixed bias amps.

The Rich Mod works by cancelling the inverted signals through common mode rejection. At zero volume it shorts the two grids, since the tubes are in push pull they cancel out. This kind of MV never fully zeroes the volume unless the inverter sections and power tubes are fully matched. This rarely ever happens in guitar amps so you end up with some audio leakage.

The risk of shorting the bias to ground is lesser in that configuration, but again you know Murphy's Law I wouldn't put any movable parts near the bias supply unless necessary.

Are you absolutely against a Mv placed before the phase inverter ?

[mort]

Are you absolutely against a Mv placed before the phase inverter ?

Not at all. I just have very little experience with them [yet]. I'm the kind of hobbyist that first learned how to assemble a circuit, and now I'm trying to learn how they work.

[big_teee]

I use both pre and post MV, on a 50 watt build.
Works great and makes the amp very controllable.
My 6V6 build in progress is only going to have the pre before the PI.
But, it is only 20 watts and not quite as loud.
T
**This is the PPIMV I always used.
So far no problems with it.
http://www.maxless.nl/PPIMV.jpg

[pdf64]

I think that the LarMar provides a useful improvement over the Type 2 master in regard of bias loss mitigation, see http://www.maxless.nl/PPIMV.jpg or robrob's https://robrobinette.com/images/Guit..._MV_Type-2.pdf

[mort]

To jump back to the B+ options, is it probably not going to change the bias enough to either sound bad or be too hard on the output tubes? Will I likely be able to find a happy medium by adjusting the bias so that the lower B+ setting would run the tubes slightly hot on current, the mid B+ setting hit right in the middle, and then the hi B+ setting run a little lean on current? Am I thinking about this in the right way... (?)

[jmaf]

Not at all. I just have very little experience with them [yet]. I'm the kind of hobbyist that first learned how to assemble a circuit, and now I'm trying to learn how they work.

Every master volume architecture has drawbacks. And every kind of master volume kills some tone in some way. So much so some purists never use master volumes in their boutique amps.

So the first question is do you really need a master volume? What kind of music do you plan on playing? A lot of the distortion is fed into the preamp from effects pedals these days, so if you plan on doing a lot of the tone and distortion shaping via pedals then you don't really need a master volume. I rarely see a musician using the master volume as it was designed to be used anyway.

If you do want a MV, the simplest and most fail-safe one is this: http://www.drtube.com/schematics/marshall/2204u.gif

A 1M log pot working as a voltage divider right before the coupling cap going into the phase inverter. That's the Marshall recipe right there and it never fails.