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Old 8th February 2008, 04:23 PM   #1
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Default output tubes for stereo 70

Can I use 6BG6G's for outputs? If so could I bias them at 40mA ea?
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Old 8th February 2008, 11:20 PM   #2
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A little more info? Is this a Dyna ST-70? If so, my recommendation would be "no".

The 6BG6 is a fine valve, but requires more drive and different bias than an EL34. The Dyna output transformer primary impedance is a little lower than optimum for 6BG6 and a concertina splitter is not well suited to drive 6BG6 valves without a buffer/driver stage between. G2 should never be taken over 400 volts so that will probably be exceeded considering a UL setup. If B+ is 400 volts and cathode current is 40ma then you are dissipating a total of 16 watts per valve. Maximum anode dissipation is rated 20 watts and maximum G2 dissipation is rated 3.2 watts so you should be OK on dissipation but there are a number of other things that would need to be addressed in order for the amplifier to work satisfactorily.

Are you scratch building or modifying an existing set?

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Old 8th February 2008, 11:29 PM   #3
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you could get away running the later production 6BG6 - the one based on the Philips 6L6GC/7581. It may not be optimum. There is an adapter kit available for the plate cap connection.

But really the SED EL34 and JJ KT77 are cheap enough - so why bother?
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Old 9th February 2008, 12:03 AM   #4
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Sorry...Its a Dynaco Stereo 70.

I found out after I posted that the grid rating I believe is only 350V on the 6BG6G. I liked the ST shape.

I put in some 6GB6GA from vacuumtubes along with the adapters.
They are biased at 50mA and are running quite well. They seem to put the 6CA7's to shame.
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Old 9th February 2008, 12:31 AM   #5
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Well, the ones you bought should be able to handle this amp with no problems. And the price! If you think it sounds great then that's the part that really matters. I'm glad to know it works!
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Old 9th February 2008, 02:39 AM   #6
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As I've posted before, the internals of the "super" 6BG6 are identical to 7027 (and presumably 6L6GC, though I didn't have any on hand for comparison). The screen voltage is no problem- I've run 7027s routinely in UL and triode at 500V. I've used the SND 6BG6 in a couple of amps here and am still amazed at the bargain they represent.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 9th February 2008, 04:46 AM   #7
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Nice when one finds an unexpected value like these today. I have some 7027As and some RCA 6L6GCs and they are identical internally. They both have the very large plates, presumably 30 watts. Also of interest, I have some RCA valves that are labeled as 6L6GB but they have the large plates and I have one RCA marked "5881" but it is obviously a large plate 6L6GC. It looks nothing like a Tung Sol 5881. One never knows just what you have until you have them in your hand. Labels can be deceiving.
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Old 9th February 2008, 12:13 PM   #8
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I can't tell the difference between the 6BG6GA and a 6L6GC in my collection. I certainly like the performance but I do wish they were ST body.
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