Express B+ voltage?

[Rick]

I've read that the "zone" B+ voltage is around 395V. I'm using the Hammond 272JX which is 300-0-300 250 mA DC which I thought would be plenty adequate, with 6V6s anyway. I get 450V on the plate which seems quite high. I have just added 250 ohms to the 25W 1000ohm position right before the 9.1K dropping resistors making it now 1250 ohm drop instead of the typical 1K 25W drop. It didn't seem to drop the B+ very much at all, but amazingly, the amp has cleaned up tremendously! It can go from clean to dirt like a typical tube amp. It no longer seems to be "dirt only". It this what the amp is supposed to be like? What have I done?

BTW, is there a secret download area where voltage charts, bias levels, and all the necessary info is kept???

[joe6v6]

Very intresting, did you add the 250ohm resistor befor or after the first filter cap & Power tube screens? I do know that Hammond transformers are made to operate at 115V, Where I live wall voltage is never below 120V so a 300-0-300 transformer is now 315-0-315 even higher if your wall voltage is higher. I think putting that resistor there lowerd the preamp voltages slightly, that coupled with your higher than normal B+ equaled less preamp gain.

JOE

[Rick]

The added 250 ohms is before filter, just as an extension to the usual 1k power resistor (25W). My wall voltage just read at 121VAC! No wonder my B+ is so high! I was getting some weak scratchy, intermittent sounding signal when I would turn my guitar way down. That has drastically improved. I'm thinking I should probaby lower the B+ rail even further. I still have 350V on the V1 plates yet! Shouldn't this be more like around 300V?

Do you know where there is a voltage chart for the Express? It would help to know what the correct voltages are, thanks, Rick

[mlp-mx6]

The 1K resistor is from the plates to the screens, the plates are still at full xfmr voltage, unless you wired it different from the schematics. You've lowered everything EXCEPT the plate voltage, if I understand you correctly. Your screens, PI, and preamp tubes are all at a somewhat lower voltage. (probably not a lot, but apparently enough to hear a difference!)

[Rick]

Ahhh, I see now, correcto. I need to drop the plate voltage between the rectifier output and the OT center tap in order to reduce plate voltage, correct? Full rectified voltage typically goes to the plate off the OT primary legs via the center tap? Am I getting this right?

OK, the 6V6 is shown to have a max plate rating of 315V. I'm waaaaay over that right now at 450v. It looks like to use 6V6s, this transformer makes way to much plate voltage to run direct to the plate off the rectifier without dropping first. I should be using EL34s only maybe with this tranny. ???

This gets curiouser and curiouser, schem A1a shows B+1 with 410V, but claims to use either 6V6 or EL34s. Schem A3 shows B+1 of only 325vdc with either 6V6 or EL34s. Is it OK to run 6V6s at -450V is where I'm at right now. My common sense says no, the amp is working OK and says maybe...

[dartanion]

Look at the plate voltages used in some of the BF and SF Fenders withj 6V6s. 410 - 420V right on the schematics. Most 6V6s are bomb proof as they were also used as a vertical deflection amplifier in TVs and can handle 1200V peak voltages.

[Rick]

That is so I guess, but I just witnessed one of my 6P6S (russian 6V6) go into meltdown. I fused the B+ at 250 mA and it blew the fuse as it went. I guess lowering the screen voltage and B+2...B+5 slightly and leaving the plate voltage up there wasn't so smart.

Still my voltages are so high, I wonder what others are getting on the B+ rail with a 300-0-300 PT. My bias was a bit high also at around 40 mA. It was really cookin' good there for a little while, hehe. Time to back things off and see if I can sustain this thing.

Lucky I have several of the 6P6Ss to fool with, some EL34s are on the way. The 6P6S is supposed to be a Russian miliatary 6V6 sub I was led to believe, maybe they are not nearly as robust as I thought.

[Tube_S_Cream]

6V6 Tubes from JJ are rated for plate voltages up to 450V. Depending on the desired sound i am using a pair of JJ 6V6 or a pair of Svetlana EL34.
Both tube sets work fine with the same bias adjustment ( ~35V) so i can easily swap them.
The only thing to recognize: Using the 6V6 doubles the output impedance, so i have to use the 4 Ohm tap for my 8 Ohm speakers.

With 6V6 it is not really quieter, but it sounds different (more mellow). It saturates earlier and smoother. (clean output approx. 20 Watts)

Greetz

Tube_S_Cream

[Fischerman]

I built a smallbox Marshall using a 272JX and not only was the B+ voltage higher than I wanted...the heaters were running at something like 7.4vac. I put some low value resistors in series with each side but if you're going to swap between EL34s and 6V6s that won't work...I think you can use zeners though.

That PT also put off a huge magnetic field which the p'ups would 'pick up' when standing anywhere near the amp so I just recently swapped it out for...ironically...a Heyboer Express PT. The Express PT got the voltages right about where I wanted them all along (plates are at 402vdc; screens at 395vdc @ 122vac wall voltage) and the amp sounds better than ever and isn't as loud...and that magnetic field thing isn't nearly as bad.

I think there are Francesca voltages somewhere around here...I know I found them when I was building my Express. It's something like 395vdc plates, 380vdc screens and the PI node is at 299vdc...or something very close to that (I'm not 100% sure about that exact screen voltage but I'm pretty sure the other two are right).

[Rick]

I built a smallbox Marshall using a 272JX and not only was the B+ voltage higher than I wanted...the heaters were running at something like 7.4vac. I put some low value resistors in series with each side but if you're going to swap between EL34s and 6V6s that won't work...I think you can use zeners though.

That PT also put off a huge magnetic field which the p'ups would 'pick up' when standing anywhere near the amp so I just recently swapped it out for...ironically...a Heyboer Express PT. The Express PT got the voltages right about where I wanted them all along (plates are at 402vdc; screens at 395vdc @ 122vac wall voltage) and the amp sounds better than ever and isn't as loud...and that magnetic field thing isn't nearly as bad.

I think there are Francesca voltages somewhere around here...I know I found them when I was building my Express. It's something like 395vdc plates, 380vdc screens and the PI node is at 299vdc...or something very close to that (I'm not 100% sure about that exact screen voltage but I'm pretty sure the other two are right).

I noticed the exact same effect about the magnetic field/pup interference. I used some dropping resistors in my 1250 ohm 25 watt pad to put 394V on the plates and that kept the light show in the 6V6s much less dramatic. Evidently for the Russian 6p6s, 450V IS TOO MUCH. If you are looking at getting a PT, in light of our experiences, it would be wise to AVOID THE 272JX .

I will get it working right, but I will surely remember not to get this type of PT again. It is not exactly cheap either, so it really looks as though some other source of PT is in order.

[mlp-mx6]

FYI - the Hammond 300 series (like the 372JX, 300-0-300 @250ma & 6.3V @ 8A) has 120V taps on the primary side.

[bueller]

I've used the 372JX version with no problems at all. In fact my voltages are 360-390 VDC typical with 2 EL34s depending on the design, etc. and when wired for 120 VAC.

The 272JX only has the one primary wiring input and that's 115 VAC. The 372JX can be wired for 100, 110, 120, 200, 210, and 240 AC. The downside is it'e heavier and more expensive. And Mouser usually only stocks the 300 series.

I've used the 372JX for probably six builds in a Marshall circuit. But I have a set of Heyboers coming for another TW Express. It will be interesting to hear the difference.

[Rick]

All fixed, here's how to do it, wire for octal rectifier tube. Put parallel and opposing pair of 1N4007s in one line of each filament supply. This drops the filament voltage a little over a volt for each which is almost exactly what I needed. Then, using different rectifier tubes B+ can be varied dropping from 17v (GZ34, 5AR4) to 30V roughly with a 5U4. It's a little ironic, I had to use silicon so I could use a vacuum tube

It looks like the 5Y3 is the magic bullet for 6V6s in this thing, who'd a thunk it?

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