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Have a pair of 8552 power amps. They output around 90 watts/channel via a pair of 8552 tubes, configured in pentode, with fixed bias. The 8552 actually makes for a good output tube, provided the bias is kept at 20ma or so, and the screen voltage is regulated.
The amp initially was designed to use the 12SN7. After some listening, it was decided to change the input driver and phase splitter to the 6BL7.
The results of the change was quite positive. The performance of the amp improved across the board. The bass improved, the midrange sounded more open, and treble was more extended.
I'm not sure why the 6BL7 works better than the 12SN7 in this application. A buddy of mine who knows a lot about tube amps suggested the change.
Any ideas?
" Don't look back. Something may be gaining on you"
Satchel Paige
Follow Ups:
That must have taken some redesign of the circuit.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yes, it did some. My buddy got the circuit from Gary Dodd.
My buddy was the one who suggested the change. We had also changed out the driver/[phase splitter tubes on a quad 1625 power amp as well, with more or less the same perceived improvement.
" Don't look back. Something may be gaining on you"
Satchel Paige
The 6BL7 isn't as linear as the 'SN7, but it probably doesn't matter much in an amp with significant NFB.
"The performance of the amp improved across the board. The bass improved, the midrange sounded more open, and treble was more extended."
Are these claims from actual measurements, or just an emotional response?
Primarily from aural listening. The change was so noticeable, did not bother to make many measurements.My buddy had obtained the front end circuit from Gary Dodd.
" Don't look back. Something may be gaining on you"
Satchel Paige
Edits: 08/14/23
I've heard this sort of thing way too many times, only to see it shot down later. Differences like this are almost always the result of the builder hearing what he wants to. Would be interested in seeing the two schematics.
Sadly, no longer with us.
Perhaps some of the answers are in the listed thread?
" Don't look back. Something may be gaining on you"
Satchel Paige
Hmmm, I was part of that discussion you linked. Sorry to hear about the designer. You can't post the two schematics?
6BL7 looks very linear to me to -20 volts bias. 12SN7 linearity craps out after -10 volts, but very good to -10 volts. Some 6SN7 tubes do not sound that good.
How much idle current? At what angle is your load line?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Assuming constant current (high load impedance, a horizontal load line) it looks pretty linear to me well beyond -10.
It looks like a 16 volt peak to peak grid signal will drive the plate voltage 162 volt below idle and 162 volts above idle. You can't get any more linear than that.
Sorry, I can't draw a straight line.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre, on first plate curve what tube is it? No tube is shown on that plate curve like your second plate curve post.
6sn7/12sn7
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Is the 6BL7 running on 12V heaters or did you modify the amp?
Modified the amp to run the 6BL7 with 6 volt heater voltage.
" Don't look back. Something may be gaining on you"
Satchel Paige
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