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OTL ECC88 amp
OTL ECC88 amp
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Old 14th February 2005, 09:33 PM   #11
PRR is offline PRR  United States
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> is it possible to modify-repair this circuit to working. Do you know something very simple OTL to try? ca: 4-6 W on 16 Ohm?

If there were a good way to drive speakers without costly transformers, the wise old men of the Golden Era would have done it.

Fact is: electrons do NOT want to cross a vacuum. And a speaker voice coil can't be wound with super-thin wire. Philips used thin wire for 800 ohm OTL amps, and even with the improved shellac of the late 1950s they had problems. The vacuum has to be much-much fatter than the wire. With more common speaker impedances, you need a BIG FAT VERY HOT vacuum.

The plan below will not work well as-shown but indicates the practical problems. You can do push-pull, a Futterman, but it is not simple nor easy to get happy. For "Simple" you end up with resistance-coupled Class A. Idle current has to exceed peak load current, about 1 Amp for your 6W in 16 ohm spec. No single tube will pass 1A continuously while negative-grid (and positive-grid adds complication). We need several big fat tubes. 6c33 is actually your best bet: ironic that you can't get them in former USSR-market areas. Type 6080 is a smaller US standard beast-triode.

Note that you will be sucking over 300 Watts from the wall to get up to 6W peak in the load. It is hardly transformerless: you have replaced 10VA-25VA of audio-iron with 50VA of power-iron. You also have an electrolytic cap in series with the output: different faults from a transformer, but hardly faultless.

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Old 15th February 2005, 07:42 AM   #12
tubetvr is offline tubetvr
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Although OTL's are not simple it is not that difficult to get reasonable power with relatively uncomplicated circuits. 2 6C33C in single ended push-pull in class AB give 25W in 8ohm with high reliability, I have pushed out 25W in 8ohm from my own OTL during 24hours continous without any problem except that the amplifier got a bit hotter than usual. I agree that it is not a very efficient way to build an amplifier, (my 2 x 25W amp idle at ~400W dissipation) but it is not impossible to make it work well.

However to build an OTL in class A is something else, using 4 6c33c the idle current can maybe safely be up to 2A, (one 6C33C can idle at 550mA but this must be derated somewhat to allow for unequal curent sharing) but 2A idle can probably give up to 10W in 8 ohm at reasonable distortion but each 6c33C will then dissipate ~40W from the anode and the same from the heater so total dissipation will be at least 320W for one channel for the output tubes only.

BTW Both tubes.ru and GS tubes sell 6C33C for ~15 USD each

Regards Hans
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Old 15th February 2005, 08:11 AM   #13
richwalters is offline richwalters
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Hi there......I
use a similiar circuit in my preamp with parallel sectioned ECC88 but into 3:1 stepdown to balanced 600 ohm o/p......the feedback from o/p into the input (in diag shown)will lower the input impedance.....In my case I don't use feedback so that that the preceeding audio stage can supply enough signal swing headroom without the stage loading introducing distortion. (there is no gain with cathode follower.)

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Old 16th February 2005, 04:40 PM   #14
Tyimo is offline Tyimo  Hungary
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Thanks for everybody!


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