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In Reply to: RE: 6883B as a audio output tube posted by Paul Joppa on April 09, 2015 at 21:47:35
The tube was re-designed for higher power in the B version.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Follow Ups:
the CCS rating is 27 Watts...and it is not a large plate; I'd think more like 25 watts is more like it.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
For audio, ICAS seems a reasonable rating for the 6883B. I'm pretty happy with the audio presentation from them. Getting 90 watts from a pair is pretty good performance in my estimation.
Pretty happy with the build quality, and very happy with the price. Just need to run them in pentode to not worry about the screens.
All in all, it's a good tube, IMHO.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Per RCA, "...ICAS covers applications in which high tube output is a more important consideration than long tube life. The term "Intermittent Commercial" in this title applies to types of service in which the operating or "on" periods do not exceed 5 minutes each, and are followed by "off" or standby periods of the same or greater duration." When tubes were in their prime, consumer equipment rarely if ever used the ICAS system except maybe for CB linears.
While one of the benefits of one-off, non commercial design is that you can choose your own set of tradeoffs and compromises, operation under ICAS conditions puts a very high premium on output at the expense of short tube life. Compounding the ICAS tradeoff is the fact that the 35W rating you reference is under the "Absolute Maximum" system and not the more conservative "Design Center" system. For audio, the 27W design center rating would seem more applicable.
There is a link on the Mastero amp, which sports a pair of 6146 tubes that makes 90 watts a side (circa 1952).The Acrosound To-350 schematic uses a pair of 6146's for 100 watts a channel.
This tube can provide some clean high wattage.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Edits: 04/10/15
I'm quite familiar with the "Maestro": it was part of the Sarser & Sprinkle trilogy of amps beginning with the "Musician's Amplifier", the U.S. version of the original Williamson and ending with the Maestro. They were featured in the "Audio Anthology" series published by Audio Engineering (Audio mag).
You'll note that the operating point chosen for the Maestro's finals is firmly within the ICAS region of operating conditions @ 750V on the anodes and ~ 22W dissipation (25W is max under the "absolute" rating system). Also note that the reader was not informed of this detail nor was there any discussion of the potential consequences this operating point might have on finals life. Thei primary objectives were to achieve higher power than the "Musician's Amplifier Senior" more efficiently and in a more compact package.
It's also interesting to look at RCA's use of the 6146 in audio apps. One relatively common product was the MI-12182 power amp. This one ran the 6146 finals at ~ 570V and 28 mA (16W) and 210 reg V on g2 for short term output of 70W or 35W continuous. Clearly RCA chose to stay within CCS ratings for their commercial offerings. Legend has it that even under these relatively conservative operating conditions there were reliability issues such that conversion to 6550s was a common mod not unlike what happened to the Ampeg SVT, another 6146 amp.
The bottom line here from my perspective is that tubes are extraordinarily forgiving wrt how they're used but with tradeoffs. The tube manuals had operating points for EL34, 6550 and 8417 where 100W could be obtained if one wished to push ratings to the limit. Most commercial products avoided the extremes to gain reliability. In this case the tradeoff is probably power vs life. If you have a large stock of finals, the possibly shorter life may be a good trade for the resulting power and compact packaging. You be the judge.
Thanks, Steve. Some good insight provided.
The purpose of this project was to come up with an amp that would provide between 75 to 90 watts that had excellent sonic character, used quality tubes that don't break the bank and are readily available, and good reliability. Also was looking for a slightly different variation from the 1625 amp sound.
The choice of the 6883B was made because the "B" version is the most rugged version of the 6146 family, and it's 12 volts. The 12 volt tubes are generally less to purchase, draw only half the current of the 6 volt types, and in this case, plentiful, as it was used in FM gear extensively.
The amps succeed on all counts. I got tired of shoddy performance and questionable longevity from the current production Chinese/Russian 6500/KT88 types (not to mention cost).
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
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 04/10/15
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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