Triode Power Amps

[cartoonweirdo]

I have been tinkering with glowing glass bottles for some years now, and happened upon a fellow noise nerd at an unlikely happy hour. He was tanked and wanted to talk about his glory days running live sound in the early days of the industry.

After sitting through about an hour of stories (some of them quite amusing too), he came around with the information (perhaps accurate, perhaps not) that he had been the FOH engineer at the loudest recorded rock show in history. "The Who?" I asked, "No, Iron Butterfly..." ,he said, "...and it was all tubes too." I expressed befudlement and he said that everyone was moding Altec "voice of the theatre" pwr amps with 811A triodes for use as PA power amps. I practicaly peed in my pants... High wattages AND triodes? Hmmmm.....

I have looked for information on the 811(A) and found plenty of radio information (a can of worms I'm not ready to open at this point), and a new Svetlana "811A" that isn't realy an 811A but promises high wattage triode output. My questions are:

Does anyone have actual experiance with 811's (old or new) as audio power tubes they care to share?

Does anyone have information about the same? A loadline? caveats?

And finaly, given that I want to wind them up as loud as they can get without sounding terrible (or perhaps put them into a bass rig in which they are supposed to sound terrible);

Where does one find parts for these?

I'm getting pretty hefty numbers like Heater amperage (4 amps a tube), HT voltage (1500ish +/- some room for filtering and biasing), and the Iron Weight involved in the OPT (a sextet would put out around 800 watts, talk about boat anchor)

Furthermore all the old literature I encounter talks about 811's as operating in class B (you can do that with an audio amp?), and with a funny bias structure I have never encountered (Is the input signal biasing the tube?)

According to legend 811's have been used in this capacity and sounded good too, ideas anyone?

[kheper]

Here are the app data for the 811A:

http://www.nj7p.org/Tube4.php?tube=811A


A schematic for the Altec 1570b
170W amp:

http://www.triodeel.com/al1570b.gif


The 811A does not seem to be a very
easy tube to drive. Some on this board,
and someday me, are building SE 813
amps, ~45W per block.

Do you really need a 248W PP 811A
amp?

[Shannon Parks]

Small Iron Butterfly-related anecdote:

My aunt had just turned 18 and was visiting my folks in Hayward, California in the summer of 1969. Dad gave her a tour around San Francisco: Fisherman's Wharf, Lombard Street, Haight-Ashbury, etc. Being a kid from the Midwest, it kinda 'blew her mind'. So one evening, dad dropped her and a neighbor girl off at the Fillmore West to catch some live music.

My aunt until recently almost forgot about the event and couldn't remember who she saw that night (umm, how 60's). After a little research and knowing the dates she was there and the clue "they had a big hit", she remembered it was Iron Butterfly, with In-Da-Gadda-Da-Vida being the huge hit of course.

The irony is for as loud as Iron Butterfly might have been, my uncle and I call her the 'Stereo Police'. Can't rock out with her around. A few more evenings at the Fillmore would've done her good.

[kheper]

An even smaller Iron Butterfly-related
anecdote:

The progressive rock group YES bought
Iron Butterfly's PA system in 1970. I think
Iron Butterfly retired that year. The rest they
say is history...

Hold on. I cannot remember where I was
in the summer of '69 either. But, I was
some 15 years younger than your aunt,
so the cause could not be pharmacological,
I hope.

[cartoonweirdo]

The 811A does not seem to be a very
easy tube to drive. Some on this board,
and someday me, are building SE 813
amps, ~45W per block.

Do you really need a 248W PP 811A
amp?[/quote]

First of all thanks for the info!!
As to 248W monoblocks I don't supose need exactly enters into it, but in terms of being a mad scientist why not be the craziest one you can be? In all truth even 45 or 135 watts is more than plenty. I shall start from the altec. :parp:

[Shannon Parks]

Cartoon, have you built any push-pull KT88 amps? This is the gravy way to go.

[EWBrown]

My only real experience with 811s is in RF "linear" power amp use, in the Collins 30L1. I fixed LOTS of these in SE Asia back around 1971, usually a matter of replacing tubes because the operator screwed the poooch and didn't check the VSWR, and just tuned up for "maximum smoke" :o Yellow_Light_Colorz_PDT_21 .

The earlier Russian made "real" 811s could not be used in the 30L1s because the tubes were horizontally mounted, and the Russian internal design was rotated 90 degrees, so the filaments could sag and hit the grid, not a good situation...

The easy ham radio cure for this was to rotate the sockets 90 degrees, or some lazy folks just stood the amp up on its end, so the tubes were vertical. Yellow_Light_Colorz_PDT_07 Yellow_Light_Colorz_PDT_11 :o

Another alternative to the 811s is teh 572 and SV572 power triodes.



There are Russian 811 "knockoffs" from Svetlana, the SV188-3 and SV-811-10, for audio use, available at Parts Express, these are not the traditional 811s, as they don't have the plate caps on top, it is brought out through a pin on the base, so it's not capable of handling the full HV and power. These come in gain factors of 3 and 10. They're more than a 300B, but less than "real" 811s.

Some circuits for these tubes here:

http://www.bonavolta.ch/hobby/en/audio/sv811.htm

These use 6EM7 or 6BM8 to drive the SV811.


/ed B in NH