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Actually........ [message #31268 is a reply to message #31258] Tue, 16 August 2005 18:51 Go to previous messageGo to next message
colinhester is currently offline  colinhester
Messages: 1349
Registered: May 2009
Location: NE Arkansas
Illuminati (3rd Degree)
My Guinevere bit the dust. Moved it from downstairs to upstaris for the WSotT fest, hooked it up, and got a real loud hum. Let me tell you, 3.5 watts through the VOTTs made the walls rattle. Got to get under the chassis and figure it out......Colin

heh-heh-heh... [message #31269 is a reply to message #31267] Tue, 16 August 2005 19:19 Go to previous messageGo to next message
Thrint is currently offline  Thrint
Messages: 43
Registered: May 2009
Baron
I think I'll stick to best tool for the job.

It remains to be demonstrated that a direct gate input will perform worse than the closed loop version as applied in Guinevere's plate load circuit.

We're looking at another CCS for the cathode load, but that one is a drain-end rig instead of the source end as applied in plate load apps. Actually, if it could be easily arranged to deliver + and - voltages at the same time, a pentode would make an acceptable CCS. I like the heaterless action of the MOSFET's for this level of experimentation.

Low voltages do have their attraction.
regards,
Douglas

Re: heh-heh-heh... [message #31270 is a reply to message #31269] Tue, 16 August 2005 22:16 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18783
Registered: January 2001
Illuminati (33rd Degree)

One of the best sounding amps at the Great Plains Audiofest was the Selectronic ProFet. There were some mighty fine tube amps, nice to look at and nice to listen to. But the plain black ProFet amp was reeeeaaally smoooooth.


any news yet? [message #31271 is a reply to message #31268] Sun, 21 August 2005 04:55 Go to previous message
Thrint is currently offline  Thrint
Messages: 43
Registered: May 2009
Baron
Have you been under the hood?

The circuit is simple enough that there can't be too many troubles.

On to the FMEA...
Hum can be from two sources, the plate supply or the input. THe cathode counted as an input.

If it is the plate supply, I suggest MOSFET failure. It is not likely that a working CCS lets enough hum through. Check plate voltage, and if possible measure out-of-circuit gate-source resistance. it should be infinite, excepting some capaciatance( but we're talking DC...)

If it is the input, there are more troublesome things to check. grounding is too wide a net, look at individual ground points, and check physical integrity. Of lesser likelyhood, a filament-cathode short would cause hum of some, given that we're not running pure DC.
regards,
Douglas

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