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Biasing a bogen srb-40

Ok so here are the actual measurements for the 4 tubes. Including the out of spec resistors. If I get back into it I’ll replace the resistors. I’m thinking about just mounting the potentiometer permanently to the chassis to adjust the bias. IMG_2963.jpeg
 
TriodeLuvr said:
Mullard lists the "limiting values" of the EL84 as 12W/2W, anode and screen. Sylvania shows the maximum for the 7189 as 13.2W/2.2W. That's 10% greater dissipation, for whatever it's worth.

Sylvania 7189

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Jack
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That's just because they used a different testing method. For instance, the data sheet you posted says Design Maximum Method which results in dissipation ratings that are 10% higher than the same tube tested under the Design Center Method. Eli Duttman used to post about this a lot on both AA and DIYAudio.

So a plate dissipation of 12w Design Center = 13.2w Design Maximum.

American data sheets will typically tell you which method they used. The European data sheets don't specify the testing method but Mullard obviously used the Design Center Method as their specs are the same as the 6BQ5 tested with Design Center method.

I usually stick to the Design Center specs and try not to run at a total dissipation (plate + screen) that exceeds the spec for plate dissipation only.
 
Carbsandcaps said:
The plate voltage is 370v on the button. The resistors all measure out to about 16.7 ohms. So about 10% off. I ended up putting the matched el84’s back in (actually Soviet 6p14p tubes). There’s about -11v going to the grids. The voltage though the resistors is about .55v which come out to about .33ma@ 370v which is about 12.2 watts dissipation through the tube.

My math may be off please correct me if it is. It’s on and playing now. No red plating as of now.
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You're running at 87% of total dissipation under the more conservative Design Center method. The Soviet tubes should be fine at those levels.
 
TriodeLuvr said:
Mullard lists the "limiting values" of the EL84 as 12W/2W, anode and screen. Sylvania shows the maximum for the 7189 as 13.2W/2.2W. That's 10% greater dissipation, for whatever it's worth.
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design max v design center though. If you look at the Tung-Sol 6bq5 and 7189 sheets I posted earlier they have the same dissipation ratings under design center. the Sylvania one is a design max sheet and 10% higher. I don't quite know what systems Euro designers used but fair guess if Mullard says the EL84 ratings are the same as the US design center 6bq5 sheets, its design center.

the really early pre-war stuff were "absolute max", which is another 10% beyond design max. Don't think there is much that hung around long enough to find sheets for both sets of parameters though.


one of my RCA books explains all that in better detail but basically design center is where you can run it and expect very good life. Design max is as hard as you can run it and still expect reasonable life, absolute max is what you think it means.
 
gadget73 said:
design max v design center though.
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Sorry, no. Both sheets - the Mullard EL84 and the Syulvania 7189 - refer to the specs as a maximum limit. It's possible they didn't make the measurements the same way or with the same lifespan in mind, but nothing in the sheets defines that. I seem to recall that a number of hi-fi amplifiers using 7189 are also rated at 15-20% more output power than any EL84/6BQ5 amps. Not referring to musical instrument amps here, their designers generally care less about tube life.

Jack
 
The Tung-Sol 7189 sheet also uses the word "maximum" but again, different rating system. I wouldn't read too much into that word in this context. Without knowing what rating system we're talking about, "max" alone doesn't mean all that much. its simply the maximum allowed by that specific system. its not the point at which it just melts down and everything turns to smoke and sadness.

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Bogen could engineer an amplifier. They could also over-engineer one in some cases but its nice stuff. Some does have questionable visual design though. You can see their industrial design heritage in a lot of it. How many people were painting the entire chassis on stuff like this and making the effort to silkscreen the tuner alignment points?
 
gadget73 said:
Bogen could engineer an amplifier. They could also over-engineer one in some cases but its nice stuff. Some does have questionable visual design though. You can see their industrial design heritage in a lot of it. How many people were painting the entire chassis on stuff like this and making the effort to silkscreen the tuner alignment points?
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Ya it’s definitely an “in the eye of the beholder” thing. And the painted chassis is as well.
 
I'd love to come across a PR2 preamp to pair with my DS225. would also like to at least fool with a DS265 and a DB230A.

I really need to do the switcheroo with my 225s. I have a crusty one with good iron and a nice one with a bad output transformer. Plan is to repaint the good but ugly iron, move it to the nice chassis, and use some commonly available transformer to build a 6L6 amp on the crusty chassis. The voltages are nearly perfect for it.
 
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