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In Reply to: Bogen DB110 etc AGAIN posted by ivan_terrible on January 27, 2016 at 23:38:25:
Having discussed most of the finer points of rebuilding the old Bogen DB to a modern standard, I felt it was time to start selecting bits for it.
Most of this, are being sourced from Eastern Europe, partly because of the huge price advantages, that proximity, good stock and high quality can give.
I was also NOT going to buy in Asia! It costs more and who knows what is inside?
This could only be possible in the modern internet age, recreating an old US classic amp, and redesigning it propely with USSR made bits!
Most people exposed to the ex-USSR are going to be using websites translated into English, where those people doing their level best to multiply prices by up to 100x or more.
What most people don't realise Russia, Ukraine & others, are jammed full of old stocks of hi quality valve technology and the prices are rock bottom. On top of this, there are electronics shops in most major cities, sometimes loads of them, with an enthusiastic valve amp base with loads of good design ideas.
I started with the most important thing which is to start replacing the 110V yankee power transformers.
After months of web site trawling I came up with an EXCELLENT company in Poland,-
250V - 0.18A
20V - 0.18A
6.3V - 3.5A
The transfo costs all of 23 EURO, but it's a tiny bit on the limit to run 2 PP 6V6 amps as it's only 45W.
This would give me 350V HT once silicon diode FW rectified and another supply of an extra -/+ 28V, so 378V if needed.
This would suit the 6V6 amplifier rather well bearing in mind the maximum anode rating is only 315V officially, (Plus the 20V cathode bias).
Bogen had no worries running them at 365V on the anode with 325V on G2. That's already 30V over max ratings, but presumably the whole lot sagged under full load, which leads to a good bit of extra distortion.
I don't want any sag and I want absolute minimum distortion.
To get this effect they used a 340-0-340 secondary with a 5Y3 to drop the HT down to about 375V to start with.
The Polski company also do another model which is basically an 80W transformer and costs 38 EURO.
450V would put me well outside the tolerance of my old 6V6GT even with 30V drop through the transformers and cathode bias. or would it????
This was the point at which I had already been rumbling on about the possibility of doing away with 6V6 altogether and moving to a much more modern valve,- the 7591.
The 7591, most people don't realise, is HALF a 8417.
I came across a nice quote which goes something like this, summing it all up pretty well:-
"7591 is a royal huge pain to make,
basically to make a 7591, you are taking the filament current
and plate dissipation of a 6L6, the sensitivity of an EL34, and stuffing it in a 6V6-GT bulb"
The modern replica look like this.
I don't think it's really a proper 7591/A at all, it looks like a repackaged 6L6 doesn't it:-
The original Westinghouse 7591 was also put into lots of other Bogen/Scott amps while tiny stuff like the near identical 7868 was shoved into the MO industrial chassis M030A.
The Scott amps like the 299/LK/steromaster series of the period filled with 7591A are highly sought after because like the Bogen DB110 they sound BRILLIANT!
The same recipe is there, - Superb output transformers-good output valves.
The 6V6 was basically a 6L6 with a smaller 450mA heater and cathode for mobile battery use.
They then uprated it to 0.6A.
The valve uses high impedance transformers, not much current, and the damping must be quite poor.
This is what we grew up with as kids in the UK, the good old bottles of pop...!
The modern version "tung sol" which we all know is made in the same old Saratov Reflektor plant looks like this with 0.5A heater:-
Or this!
The new "6V6S" isn't really a 6V6 at all, it's some sort of "6V6 on steroids" 500V anode rated 6L6 which can run G2 at 450V.
6L6 used to run at only 350V too with G2 max at only 270V.
The 0.9A heater is of course the one like in the 807.....so no suprise they conduct more current and use lower impedance transformers.
Here's the latest generation from Saratov again..."you paysyour money and...you aren't at all sure of anything any more"!
So this is still a 6V6???
The old round compact anode has given way to a boxy thing reminding us more of a KT66, which had an outsized 1.27A heater, boxy shorter fatter anode and
Remember how they looked?
It brings us back to where?
The 7591 of course, at the beginning of this post.
It has a heater closer to the 807 at 0.8A & high gain.
Everyone says it's great for audio, even ultralinear, sounding somewhere between the output of the 6L6 and the sound of the 6V6.
It's built for high currents and audio like the 8417.
The 8417 has exactly DOUBLE the heater current of the 7591, which is exactly what I said. It's HALF a 8417!
When is a 7591 NOT a 7591?
When is a 6L6 not a 6L6 or a 6V6 not a 6V6?
When it's made by the Russians and called something completely else!
Here are the 2, USSR versions of the 6L6, which is in fact is an uprated 6V6 - like the "downrated 6L6" the 6V6 is.
In reflector Saratov they carried on making something unlike the 6L6 which used to melt in US amps.
Guess which one.
Confused??
BELOW appears to be the proper valve to be looking for:- made in a factory called "photon" in far off Tashkent in the 1960s.
Note the boxy anode, with the "coin base".
The absence of the pinch construction and the "coin base" means much shorter leads, and a less fragile construction with lower internal capacitance.
This is the way modern, better more compact valves were made like the STC ones on Loctal bases.
Sure as anything, this looks the one that's going to get used here.
They cost 200-250 roubles, (about 3EURO) so 4 is a steal at under 10 UK pounds.
Here it is again, before I lurch off into the subject of changing the entire preamp design to a proper PENTODE input instead of those horrible cheapy 12AX7 stages.
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Follow Ups
- RE: Bogen DB110 etc AGAIN - ivan_terrible 11:09:20 02/09/16 (1)
- RE: Bogen DB110 etc AGAIN - ivan_terrible 13:43:17 02/09/16 (0)
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