Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on August 30, 2013, 10:46:41 am
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Some time ago in an italian forum we were discussing about lowering power in a fixed bias amp
at that time I was thinking that it can be done disconnecting the cathodes of a pair of tubes (4 or more final tubes in PP) as is possible to do in a cathode biased amp
someone, that is much more experienced that I'm, told that disconnecting the cathodes will be a way to damage the final tubes and maybe the OT
so I convinced myself that to do that wasn't a good idea
Few days ago I noticed that Engl in some of their amp uses a switch to disconnect the cathodes from ground with standby purpose, thing that to me was a bit odd
http://www.el34world.com/charts/Schematics/files/engl/engle_e330_ii.pdf (http://www.el34world.com/charts/Schematics/files/engl/engle_e330_ii.pdf)
In that amps there are also diodes connected in reverse between the plates and ground as I've seen in some other amp (with protection purpose), but this are disconnected from ground at the time the cathodes are disconnected (by the same switch)
Is the presence of this diodes that prevents problems coming from the disconnection of the cathodes from ground ?
Do you think it will be possible to use this kind of switch also for lowering power in fixed bias amps ?
Many Thanks
K
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Ok for Standby.
Not great for power drop. There will be can impedance mismatch; and the sudden jolts of power mismatch ask for trouble. Better to have a vol control to dial out signal to one pair of power tubes. This will preserve impedance matching to the OT primary. Still 1/2 power yields only a tiny difference in perceived vol.î
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> told that disconnecting the cathodes will be a way to damage the final tubes and maybe the OT
Why? How?
Or: switch to a 5K cathode resistor. Current and output will be very small, but the cathodes can't go to "dangerous voltages".
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Thanks for answers
@ JJasilly
if I understand well
you are saying that disconnecting the cathodes of a pair of tubes in a fixed bias amp
will change the reflected impedance
thing that as far as I can know didn't happens in a cathode bias amp
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I agree with you that 1/2 power isn't an 1/2 perceived sound, however it is less
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the idea was to build a mix of KT88 and EL34 tubes, something like in the Mesa Boogie Blue Angel where there is a mix of EL84 and 6V6
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@ PRR
Why? How?
You are right, I can't find a reasonable reason, I believed in what was told me
K