Hiwatt DR103 Fuse Blowing

[Pryde]

Hey guys bumping this one up with a question:

So I replaced the power transformer with an identical stock Hiwatt one. Everything fires up fine with no blown fuses or excess current/bright bulb using the limiter. Plate voltage is ~460vDC and bias is set at 37m/a (~70% MPD for EL34s).

Originally per the owner the amp was blowing fuses when cranked very loud. I am running it into dummy load and injecting 1Khz signal in. Master volume on full. When preamp volume is 0-5, bias and plate voltage stay as above but when cranking it to 6-10 the tube bias climes up to ~80m/a and the plate voltage drops to 440vDC under these "cranked" conditions. Also when it is fully cranked I can hear the signal from the generator through the dummy load/circuit.

Any concerns over these findings? Thanks for the help.

[Enzo]

"Bias" is an idle current measurement. You measure tube current at idle. What you measure while the unit is amplifying signal is simply tube current, not bias. This is like setting the idle speed in your car engine. You do it while the car sits still. You cannot measure engine speed while driving around and call it idle speed.

The more power the amp produces, the more current flows through the tubes. High voltage is not regulated, so it naturally drops the harder you pull on it.

The output transformer windings have a little wiggle room, so when an amp is cranked, the windings and laminations can "sing."

All of this is normal.

[Pryde]

"Bias" is an idle current measurement. You measure tube current at idle. What you measure while the unit is amplifying signal is simply tube current, not bias. This is like setting the idle speed in your car engine. You do it while the car sits still. You cannot measure engine speed while driving around and call it idle speed.

The more power the amp produces, the more current flows through the tubes. High voltage is not regulated, so it naturally drops the harder you pull on it.

The output transformer windings have a little wiggle room, so when an amp is cranked, the windings and laminations can "sing."

All of this is normal.

Thank you for the informative response Enzo. I always appreciate the knowledge you (and many others here) share.