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In Reply to: RE: 6883B plate dissipation IS 35 watts, not 20 posted by Steve O on April 10, 2015 at 15:54:52
Well, I'll have to see how long they last in the application I'm using them in. My primary goal was to get between 80 to 90 watts per channel, which the amps achieve. If I get 2-3 years out of a quad under moderate use, I'll be happy with that. I have lots of them on hand.The amp has 600V on the plates, and the bias is set at 50ma.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Edits: 04/10/15 04/10/15 04/10/15Follow Ups:
You can expect very short lifetime at those operating parameters. The plates will turn red, the heat will have nowehere to go, and the internals will go belly up. If you're unlucky, this can take out your output transformer in the process.
There is a CCS operating point for 82 Watts. That's about 1dB less than 100 Watts.
For reference, there's a Class A1(!) operating point for a pair of 6550's or KT-88's that makes 100W. It takes similar plate and screen voltages to the beefy 6883B operating points, but at higher current.
In the end, I say go for it! But don't build something that may melt down when you're not looking (at least don't do that intentionally).
Good feedback.
If I run the tubes at 600V (560 loaded) and 50 ma, are you saying that will be a short tube life? That's what they are running at now, and it sounds great!
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Yeah, lifetime will be really low. ICAS ratings for tubes are provided for purposes like public address, where there are long resting periods. I don't think I'd run them any harder than 35mA.
I've been using this amp for several months now, with the following results:
1) Dropped the bias to 40 ma, and the results have been fine from a reliability standpoint. I had a couple of tubes go early in this project. The failed tubes all had a mid 80's date code from the same time frame. Swapped in tubes with 70's date codes, dropped the bias to 40 ma, and it's been great since.
2) Swapped out the coupling caps to Jupiter Copper foils, and that improved overall performance significantly. The mid range is much more open and life like sounding, and the treble sounds clearer as well.
3) The 6BL7 is a great driver tube. MUCH quieter than the 12SN7. In fact, it is so much better, I'm going to change out the driver tubes in my 1625 DIY amp to run 6BL7's.
All in all, this has been a worth while project. The sound from this amp is outstanding. Very happy with the results.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
I have a bunch of them and have not seen a scheme that I like using them. Specifically, I wanted to try them as a driver for an 813 SE project but don't have the skill to make a design. Maybe your schematic could be adopted to the 813? thank you, Dak
Interesting. I've been running the 1625 amps at 40ma for well over a year now under heavy use, and the tubes still measure very near NOS. I would have thought the 6883B could run at 50ma all day long.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
A pair of 811As can output 400W plus, and Hammond can provide OPTs for this service at reasonable cost. No little 35W pentode can even come close to the sound quality of 811A triodes running at 1.5KV.
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Buy Chinese. Bury freedom.
...significant drive power necessary. Not a typical audiophile mode...for tubes at least.
"Not a typical audiophile mode..."
It's less common because it's more difficult and costly to do right. The reward is that a pair of 811As will simply squash lesser tubes. I do rather like the idea of trying a 6146 as a grounded-anode preamp though.
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Buy Chinese. Bury freedom.
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