Champ 25 SE tube replacement
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tomd
Oct 22, 2010, 9:45:33 AM10/22/10
to The Amp Guide
Does anyone know of this amp requires biasing when replacing the power
tubes? I have one and think it would benefit from a fresh set and am
wondering if I can just buy a set and drop them in. Also, if anyone
can recommend a good, moderately priced 6L6, I'd appreciate it.
tubes? I have one and think it would benefit from a fresh set and am
wondering if I can just buy a set and drop them in. Also, if anyone
can recommend a good, moderately priced 6L6, I'd appreciate it.
mi...@nycap.rr.com
Oct 22, 2010, 11:34:38 AM10/22/10
to the-am...@googlegroups.com, tomd, mi...@nycap.rr.com
A stock single ended champ is cathode biased and no adjustment is needed when replacing the stock 6V6 power tube because the bias is developed across the 250 ohm resister on the cathode (pin 8) of the power tube. If you are useing a 6L6 it will work for awhile, maybe indefinately, but it opens up a whole new can of worms regarding plate current and OT primary impedence.
Mike
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Stratopastor
Oct 24, 2010, 4:49:30 PM10/24/10
to The Amp Guide
specifically on the Champ 25 SE, (which is not single-ended but class
AB with two 6L6s; I think Fender were crazy to give it the Champ name,
but that's just me) and looking at this schematic,
http://www.ampwares.com/schematics/champ_25_se.pdf
the answer is, yes, strictly speaking this amp calls for rebiasing
when fitting new power tubes. See how there's a bias voltage applied
at pin 5 of the power tubes - it's minus 41V applied a couple of
resistors away. Over on the left-hand side of the schematic you can
see where the -41V comes from, being -59V and a potential divider
created by the 33K and 75K resistors. Adjusting one of those resistors
would be the way to go if some bias adjustment were needed.
However if you fit new power tubes, they sound OK to you, and the
tube's plates don't appear to glow red when you're playing loudly in a
darkened room, then you're probably OK. Note (1) that's the plates
(flat/boxy parts) glowing or not glowing - the tube's heater runs up
the middle of the tube like a spine and that will always glow (2) this
visual check will not satisfy the tube supplier in the event that you
try to make a warranty claim later!
I hope this helps - sorry I have no experience of 6L6s; for current-
production brands, others speak well of JJ and Tung-Sol.
best wishes....
On Oct 22, 4:34 pm, <mi...@nycap.rr.com> wrote:
> A stock single ended champ is cathode biased and no adjustment is needed when replacing the stock 6V6 power tube because the bias is developed across the 250 ohm resister on the cathode (pin 8) of the power tube. If you are useing a 6L6 it will work for awhile, maybe indefinately, but it opens up a whole new can of worms regarding plate current and OT primary impedence.
>
> Mike
>
AB with two 6L6s; I think Fender were crazy to give it the Champ name,
but that's just me) and looking at this schematic,
http://www.ampwares.com/schematics/champ_25_se.pdf
the answer is, yes, strictly speaking this amp calls for rebiasing
when fitting new power tubes. See how there's a bias voltage applied
at pin 5 of the power tubes - it's minus 41V applied a couple of
resistors away. Over on the left-hand side of the schematic you can
see where the -41V comes from, being -59V and a potential divider
created by the 33K and 75K resistors. Adjusting one of those resistors
would be the way to go if some bias adjustment were needed.
However if you fit new power tubes, they sound OK to you, and the
tube's plates don't appear to glow red when you're playing loudly in a
darkened room, then you're probably OK. Note (1) that's the plates
(flat/boxy parts) glowing or not glowing - the tube's heater runs up
the middle of the tube like a spine and that will always glow (2) this
visual check will not satisfy the tube supplier in the event that you
try to make a warranty claim later!
I hope this helps - sorry I have no experience of 6L6s; for current-
production brands, others speak well of JJ and Tung-Sol.
best wishes....
On Oct 22, 4:34 pm, <mi...@nycap.rr.com> wrote:
> A stock single ended champ is cathode biased and no adjustment is needed when replacing the stock 6V6 power tube because the bias is developed across the 250 ohm resister on the cathode (pin 8) of the power tube. If you are useing a 6L6 it will work for awhile, maybe indefinately, but it opens up a whole new can of worms regarding plate current and OT primary impedence.
>
> Mike
>
tomd
Oct 24, 2010, 5:56:21 PM10/24/10
to The Amp Guide
Thanks for the replies! I actually have ordered a set of JJ 6l6's for
it and will watch (and listen) closely after installing. The older
Sovtek tubes that are in it now are in dire need of replacement so
hopefully this is an improvement!
it and will watch (and listen) closely after installing. The older
Sovtek tubes that are in it now are in dire need of replacement so
hopefully this is an improvement!
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