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Old 10th February 2010, 10:57 AM   #1
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Default Are these ALL really EL34 subs?

I've recently acquired an EL34 based push-pull amp and as such have been doing some reading here, on AudioAsylum, AudioCirlce, etc about EL34 options. It seems that every time I turn around I read "If you like the EL34, you'll love____". I have compiled quite the list of tubes people claim are EL34 subs and I want to know what everyone thinks.

The list so far:

EL34
6CA7
KT-77
KT-66
7581
6L6GC
6P3S-E (or 6N3C-E depending on your Russian skills)

I've read in various spots that these can all be used in place of an EL34; so what do the experts think? Do they really all work? Am I missing any? I know some bias adjustments will be necessary for some of them, but assuming the amp has adjustable bias...
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Old 10th February 2010, 12:32 PM   #2
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The 6ca7 and kt77 may be used as el34 substitutes. The other tubes listed are not substitutes but may or may not work in a particular amplifier.

E.G. IIRC the el34 has a 800Va rating, look up the other tubes listed and compare. Compare filament ratings, etc.

Often times someone installs a tube in an amplifer and it makes sound so they say it is a substitute.
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Old 10th February 2010, 12:48 PM   #3
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Yes and no. Here's an interesting thread for you to read:
Which EL34 tubes for Hi Fi

If you intend to start substituting, watch carefully for pin differences. The EL34 has the suppressor brought out on pin 1, which must be connected in the amplifier. The 6L6GC has the suppressor internally tied to cathode, so no amp connection is required.

Different tubes have different drive requirements. Generally speaking, the pentodes will require less drive to reach full output. This may affect the way a given tube performs in your amp.

Another important consideration is the grid leak requirements, as specified on the tube's data sheets. Per spec, the 6L6GC will tolerate no more than 100K from grid to ground when operated fixed bias. The EL34 should handle up to 470K. Take a look at your amplifier's schematic to make sure you don't have too large a Rg1 before you start swapping.

There are other factors as well. Maybe the harmonic distortion spectrum of the driver stages complement the power tube in a particular way, which might not occur with a different tube type. Perhaps the load presented by the output transformer is better suited for a specific type of power tube.

By all means, feel free to roll away. Many times it will work just fine. Pay attention to the tube datasheets and understand the schematic of your amplifier. Keep in mind the engineer who designed your circuit may have been talented, and perhaps he carefully optimized the circuit for a particular type of tube - or maybe he didn't. Have fun!
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Old 10th February 2010, 03:11 PM   #4
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Don't forget about heater requirements. The 6550/KT88 and friends require an additional 100mA each.
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Old 10th February 2010, 05:16 PM   #5
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Its also good to know what your el34 amp is doing with the suppressor.

I have a single ended el34 in which I ground the suppressor, it really does sound good.

It would not be a good idea to put a substitute in which has suppressor tied to cathode.....

Ed
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Old 10th February 2010, 09:10 PM   #6
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Thanks for all the advice/info. I'm still waiting on the schematic (and the amp for that matter!) but once I get both hopefully some of the more "exotic" tube rolling options will be sorted out.
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