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transform simple low voltage OTL tube amp into lower impedance by paralleling tubes
transform simple low voltage OTL tube amp into lower impedance by paralleling tubes
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Old 20th June 2018, 11:20 AM   #11
Demonkleaner is offline Demonkleaner  United States
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Don't forget the Circlotron. I enjoy mine every day. Tubes are reasonably priced and readily available.


What tubes for a OTL tube amp?
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Old 20th June 2018, 01:21 PM   #12
kodabmx is offline kodabmx  Canada
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I dunno. I tend to design my amplifiers to perform reliably for years at full output 24/7.
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Old 20th June 2018, 07:24 PM   #13
Demonkleaner is offline Demonkleaner  United States
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Please elaborate. Other than normal tube replacement, what in my amps do you expect to fail?
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Old 20th June 2018, 07:31 PM   #14
kodabmx is offline kodabmx  Canada
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Define "Normal". I expect my amps to be able to pass a sine wave at full power for more than 2 years before the tubes need changing. If you passed a 25W sine wave out of that 6C33C amp, how long do you think it would last? It's well known that, like a light bulb, a tube can work at over it's max design at the expense of reduced service life, right? I can use 100W bulbs for 400W and the light is much brighter and purer, but it'll only work for a few minutes. Even the 6C33C data has ratings for 750h and 3000h

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Old 20th June 2018, 07:47 PM   #15
Demonkleaner is offline Demonkleaner  United States
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They're not 6C33's.
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Old 21st June 2018, 06:09 AM   #16
tubetvr is offline tubetvr
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Quote:
I tend to design my amplifiers to perform reliably for years at full output 24/7
Why? Do you never sleep?
Quote:
I expect my amps to be able to pass a sine wave at full power for more than 2 years before the tubes need changing.
Wow! This is much better than ordinary tube amplifiers, what tubes are you using?, this would be 17000 hours at full blast with a continous sine wave. Normally power tubes in ordinary power amplifiers often need changing after less than 5000 hours with normal operation, (and that is with average power 10 to 100 times less than max).

Quote:
Even the 6C33C data has ratings for 750h and 3000h
Yes but have you looked into what the conditions are for that spec? It is while fulfilling all specs of anode dissipation, cathode current, acceleration, shock etc. The 6C33C designed as it is for millitary use in tanks, fighter jets and missiles is specified much more strict than most power tubes. I think it is safe to say that 6C33C have better quality than most ordinary power tubes.

My estimate of 4 hours daily average use, (meaning average, i.e some days maybe 8 or 24 hours, some days 0) for 4 years is what I say to my customers as many have heard that OTLs burn tubes very quickly, In reality the tubes usually last much longer, my first prototype finished in 2000 still use the original power tubes even though I use it almost every day.
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Old 21st June 2018, 09:28 AM   #17
cnpope is offline cnpope  United States
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Originally Posted by tubetvr View Post
Why? Do you never sleep?
Well, with that full power sinewave playing 24 hours a day, it is probably pretty hard to sleep!

But seriously, I suppose one could, if one wished, double up the output tubes and then the ratings would not be exceeded even at full 25W power. But there is presumably little point in doing this, since the tubes seem perfectly capable of lasting a long time in the present configuration.

In fact, if one did double up the tubes one could presumably in practice push the power output to much higher levels, given a suitable driver. Roughly speaking, if the limitation is the current the tubes can pass, I suppose the maximum output power should grow as the square of the number of tubes, and so something like 100W for four output tubes instead of two.

Did you ever try experimenting with paralleled 6C33C output tubes, I wonder? Presumably the driver stage would need modification to handle the required larger voltage swings?
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Old 21st June 2018, 10:17 AM   #18
tubetvr is offline tubetvr
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Did you ever try experimenting with paralleled 6C33C output tubes
Yes, in one of my commercial amplifiers I use 4 tubes and this give 80W in 8ohm, I use equalising resistors in the anodes to even out differences between tubes and avoid thermal runaway if one tube goes bad. Driver is similar but I use cathode followers as buffers between the split load inverter and power tubes, in that way I can optimise the splitter for high output voltage and still drive the power tubes to 0V on the grids. See here for a description on my commercial web page Eabout | Sverige | Tillverkare rorforstarkare
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Old 21st June 2018, 11:51 AM   #19
Koonw is offline Koonw
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Presumably the driver stage would need modification to handle the required larger voltage swings?
The existing driver should be able to drive extra pairs, and only need about 100V-p to drive 3 pairs of 6c33c to full output @-50V bias, see attached sim.
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File Type: png Drive Level 6c33c.png (84.1 KB, 125 views)
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Old 21st June 2018, 02:27 PM   #20
kodabmx is offline kodabmx  Canada
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6P3S and 6P1P both work for years at high power levels in my set up.

I also have a commercial tube receiver that was never turned off and the ECL82 tubes lasted 5 years before I changed them to 6F3P. They were weak but still functional. They were the original tubes from the 70s, so they lasted 25 years before I got the amp in the first place. 30 year tube life

What is the class A limit of your 25W design? How much power would it make into 64 ohms?

Last edited by kodabmx; 21st June 2018 at 02:29 PM.
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