XPost: sci.electronics.design
On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio
wrote:
>On 28.12.18 10:45, upsidedown@downunder.com wrote:
>> On Thu, 27 Dec 2018 21:11:35 -0600, tubeguy@myshop.com wrote:
>>
>>> I'm not intending on building this, but it's interesting and makes me
>>> wonder if someone could use EIGHT 6L6 tubes, or TEN?
>>> (Of course adding more tubes mean bigger audio output transformers and
>>> heftier power supplies.
>>
>> Tubes may age at different rates, so the characteristics might be
>> quite different after a long time. It might not make sense to replace
>> a single (failed) tube and you may have to replace the whole set of
>> tubes with new tubes preferably from the same manufacturing batch.
>> This can be quite expensive :-)
>
>
>In the 1960's, I made a guitar amplifier with six EL500's. The tubes
>are not characterized for linear use, and they were not matched in
>any way. Each tube had an own cathode resistor, and there was no
>evidence of any overload from unbalanced operation.
Using individual cathode resistors to make the whole grid bias helps a
lot to equalize the current in each tube.
In general one tries to avoid cathode resistor bias in power stages,
since quite a lot of the output voltage swing is lost in the cathode
resistor(s) and use fixed low power negative grid bias supply instead,
but this may require separate bias adjustment for each tube.
I am not familiar with 6L6, but the similar EL34 is used in pairs for
30 W for HiFi, using 4 or 6 for "100 W" guitar amplifier with 450 V
anode voltage. One bass qui tar amplifier is rated at 100 W using only
two EL34 tubes, but runs at 800 V, but only Telefunken "Special
quality" EL34s seemed to survive more than a few gigs.
>
>The tubes had a tendency of parasitic oscillation, so I added a
>ferrite bead on the control grids and an inductor-resistor parallel
>combination on each plate (for constructions, see nearest ham handbook).
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