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previous: Bruce Stephen,I wonder what you sense... -- 2/12/1998 11:09 AM

Re: Screen Grid Voltage

2/15/1998 12:12 AM
stephen	Re: Screen Grid Voltage Hi Bruce, Hmm...I didn't think I'd get away with that one ..without some further explanation I thought I could hear some difference at low output levels. If I had to put it in one phrase, - - a little less fat and slightly clearer and more sparkling. I can suggest various things - some or all are interrelated. At lower screen volts...the curves are a little different; the optimum load may be different - so if you don't change the OT taps, there's a mismatch (or a different degree or direction of mismatch from what existed before). At the same B+ and same Iq, the required -ve bias volts will be different; AC signal required at o/p grids will be different; so for same output, the driver and/or phase splitter will be operating at a different point. You can probably think of some more. Some mixture of these could well account for a small percieved difference in the overall sound from the amp (and speaker) Sorry, I don't know which are the most significant factors If I had to make just one guess, then I'd look first at changed plate resistance, versus OT ratio and speaker impedance curve. One day maybe there will be time to look more closely.... . Whether dropping the screen volts might clean up an existing amp depends much on the present operating conditions in the amp. Not much I'd expect if it;s a well-designed amp already. .I think you probably need more power for this job, - or more efficient speakers - like some of the VST's maybe. His ceramic ones aren't as expensive as the Alnico models, but they still have similar high efficiency. A clean studio-type compressor set maybe 3 to 10 dB compression might help to get a bit more clean headroom from existing amps. Worth a try... Otherwise, I think you are looking at a bunch of 6550's and some big lumps of iron Sorry I can't be more specific. - Stephen

2/15/1998 12:12 AM

stephen

Re: Screen Grid Voltage

Hi Bruce,
Hmm...I didn't think I'd get away with that one
..without some further explanation :-)

I thought I could hear some difference at low output levels. If I had to put it in one phrase, - - a little less fat and slightly clearer and more sparkling. I can suggest various things - some or all are interrelated.
At lower screen volts...the curves are a little different; the optimum load may be different - so if you don't change the OT taps, there's a mismatch (or a different degree or direction of mismatch from what existed before).
At the same B+ and same Iq, the required -ve bias volts will be different; AC signal required at o/p grids will be different; so for same output, the driver and/or phase splitter will be operating at a different point.
You can probably think of some more.
Some mixture of these could well account for a small percieved difference in the overall sound from the amp (and speaker) Sorry, I don't know which are the most significant factors If I had to make just one guess, then I'd look first at changed plate resistance, versus OT ratio and speaker impedance curve. One day maybe there will be time to look more closely....
.
Whether dropping the screen volts might clean up an existing amp depends much on the present operating conditions in the amp. Not much I'd expect if it;s a well-designed amp already. .I think you probably need more power for this job, - or more efficient speakers - like some of the VST's maybe. His ceramic ones aren't as expensive as the Alnico models, but they still have similar high efficiency.
A clean studio-type compressor set maybe 3 to 10 dB compression might help to get a bit more clean headroom from existing amps. Worth a try...
Otherwise, I think you are looking at a bunch of 6550's and some big lumps of iron :-)

Sorry I can't be more specific.
- Stephen