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In Reply to: RE: DB110G (Bogen) 1950s. posted by ivan_terrible on December 26, 2023 at 11:20:29
"I won't use self bias (cathode bias) in any push-pull amplifier".
Well that was what came with the amp, and for that matter, a large proportion of the 60s amps.
They do say, beam tetrodes only sound good when they are about to melt.
I reckon that's good advice.
I have run quite a lot of the old 6L6oids ie. 807/KT8
They do give best performance/lowest distortion when the anodes are glowing red in bits, or well over 85% of max Pa at idle.
The 6V6 (7408) Westinghouse valves are clearly better.
They look almost identical to the 7591, a far higher gain more linear component than the ancient 6V6 or the miniaturised 6BW6 and their clones.
Talking of this...I bought a load of 6BW6 cheap cos nobody wants them.
In the space for ONE octal (or 7C5 Loctal) you can fit a pair of 6BW6 so as to run PPP.
Actually quite a nice idea if that floats your boat.
I'm a big fan of the 6V6, but being a cheapskate, I've been collecting sleeves of NOS 12V6 for a long time. Recently replaced the well-worn 7408s in my small dual-mono SEP with 12V Sylvania chrome domes. They're really superb. I also have a large stock of 6P3S-E and a couple dozen NOS 5881, so my mid-power DIY plans revolve mostly around 6L6 pentode designs. Those 6BW6s look interesting. I bought a large quantity of little 5686 8.5W pentodes some time ago. I'm using them in triode in a preamp project now, and hoping to use them in pentode push-pull in a future guitar amplifier. So many tubes, so little time!
Of course you can carry on collecting stuff for ever, but it's of little use.
Currently the loctal 7C5 go for peanuts.
I bought a large qty of Mcmurdo loctals to put them in if needed.
The 6L6 types can also be had for peanuts in loctal using the 254/255M series from STC - UK mil spec made.
I managed to start up a pile of them making an adaptor from UX to Loctal, and found out NOS is not so smart.
Production spread is huge, and some just arced on startup.
I did manage to make a matched quad like that and a few pairs, but then what to do with them? 807s/6L6 are known for hideous amounts of 3rd harmonic distortion.
The much vaunted KT66 was a distortion mush maker, but few seem to worry.
As for the Bogen I will convert it to EBL71, another superb piece of kit with much lower distortion than any 6V6 and 2-3x the gain. Those also can be had for peanuts, as they made zillions of them in E Europe for lousy noisy old AM radio sets.
They are supposed to run the same A-A load as the 6V6.
Look how low the anode voltage can drop without losing linearity!
It's going down to 20-40V and still conducting!
Now compare it with the dirty old 6V6oids..
Pentodes can reduce IMD, now you see why.
That particular one (with the 2 redundant diodes) was scarcely ever imagined in push-pull and has no info on fixed bias at all.....ie. probably works brilliantly without all that beam tetrode IMD, and heavy THD.
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