6BM8 Question

[Peter Menting]

Found a schematic for a PP 6BM8 amp called "WaveDT" ( can't remember the URL) ----but it raised a question. The screens are wired directly to the B+ and the plates are fed through the center tap of the OPT. As a result the screen voltage is 10 V higher than the plate. Never saw that before---can someone comment as to the how and why of this configuration.

Pete

 

[SY]

Actually, that's pretty common, especially in the old days when we didn't use regulation too much. IIRC, this was also true fro some of the older Audio Research amps.

In any case, once the screen voltage is set, varying the plate voltage doesn't have a first order effect. So as a matter of convenience, they're often run off the same supply. The screens, not being fed through the OPT DC resistance, naturally end up at a bit higher voltage.

 

[Peter Menting]

Sy; As always, thanks for your response. How would this impact the output power level? Would the tube resemble a triode and yield less? A pp 6BM8 amp should be good for approx 8 Watts, would a circuit like this yield much less?

Pete

 

[SY]

Well, that's a difficult question to answer- except for one part: it will not act in any way more "triode-like". What you'll get out of the circuit you're asking about power-wise depends principally on plate voltage and load resistance. Do you know what they are?

 

[ohenry]

That schematic (Wave DT) is for the last iteration of the Wave 8 monoblock amp by Divergent Technologies (ASL).

You've probably heard about those being a great buy and very tweakable. I have them and enjoy them even more after substituting some resistors and capacitors. They are rated to deliver 8 watts.

I hope that helps a little. DT stopped making those in 2003 and have replaced that model with a 20 watt version.

 

[Peter Menting]

Sy; Plate is 212V , screen is 222. Cathode biased @ 15v.

Pete

 

[Peter Menting]

ohenry; I suspected that it was related to the "Wave 8" hence my guess at the 8 Watt output. I was thinking of building a couple and wondered if there was anything special about the OPT's which I believe are manufactured by Divergent Technology. Do you have any info on them?

Pete

 

[ohenry]

Pete,
I don't know anything special about the output transformer.

I collected a post from AA where a guy was considering upgrading to a Hammond output transformer, either a Hammond 125H (8w, 10k ohms prim imp) or a Hammond 125J (8w, 2.5K/4K ohm prim imp). He was uncertain which of those would be appropriate as am I.

 

[fdegrove]

Hi,

I was thinking of building a couple and wondered if there was anything special about the OPT's

As far as the the outputstage goes, there's no need for the OPT to be anything "special".
Any decent OPT appropriate for a PP 6BM8 should do....
If that's what you're wondering about.

Cheers,

 

[Circlotron]

Actually, that's pretty common, especially in the old days when we didn't use regulation too much. *snip* In any case, once the screen voltage is set, varying the plate voltage doesn't have a first order effect.

It follows then, that if you are going to regulate anything, regulate the low current screen supply first. The plates pull whatever current the control & screen grids tell them to. Any instantaneous variation in plate voltage, i.e. hum, does not affect the plate current very much.

 

[Peter Menting]

ohenry;
InSvetlana Tech Bulletin#8, (using the 6BM8 in audio appl.) Eric Barbour uses a Hammond 1620 (6600 0hm CT) for his 10 watt amp, and indicates that any comparable OPT from4300-10K would work with only adjustments to the feed back circuit.

I would assume that the 125C, D, H or J would work. As Frank has indicated.

My only concern was the drop across the windings which sets the plate voltage, however, the information provided by Sy and Circlotron would suggest that this is not an issue. Think I'll try a pair of monoblocs and see what the result is. Thanks to all for the help.

Pete