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does lower B+ cause scratchy distortion?
does lower B+ cause scratchy distortion?
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Old 20th June 2008, 11:13 PM   #1
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006
Default does lower B+ cause scratchy distortion?

I have a PA amp conversion from a small Dynavox amp that used a pair of 6AQ5's, which I switched to 6AK6 or 6AN5 to lower the power and volume to around one watt.

The design is based on the Matchless Spitfire (one gain stage with a LTP PI), which has a B+ of 350, which is significantly higher than this amp (289).

The amp now has a scratchy distortion when the volume is at about 8-9 that occurs when notes are played. It's very apparent with low E notes. This is high enough volume to cause the amp to distort (for rock and roll). At lower volumes, it's nice and clean without issues.

I've gone bananas changing and testing components and the scratchy distortion remains. I've swapped tubes and put in a different output transformer.

I'm wondering if there's a design flaw in the amp, especially with the phase inverter.

Here are the voltages:

V1-6 = 160
V1-8 = 1.1

V2-1 = 177
V2-6 = 173
LTP center = 48

V3/4-7 =
16 w/280
24 w/400

The 400 ohm cathode resistor was used to lower the bias on the 6AN5's. At the time I thought the output tubes were way too hot. That wasn't it.

The PI has about 80 volts to the B+ (250 - 170), which seems to be plenty.

I'm pretty ragged now ... this little amp is a tough nut to crack!
Any help is greatly appreciated!!!!

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