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D.I.Y. Amp Help Needed

6/12/1999 2:05 AM
Aaron Flynt
D.I.Y. Amp Help Needed
I just got finished putting together a homebrew single channel non-tremolo Super Reverb clone, and am running into some problems with it blowing fuses. I used a Hammond 373BX PT, with a 50 volt bias tap, and a modified star grounding scheme. I'm getting around 360 volts AC at the GZ34 , and about 500VDC coming out of it at pin 8. I'm getting .6 Amps of currrent draw at the fuse holder at idle out of the amp. The problem is that when I try to check the bias w/the OT shunt method the fuse blows. I also noticed that when all the tubes are removed except the rectifier I'm getting 500VDC after all the filter caps and on the preamp plates, too. I checked and rechecked my wiring, and everything seems right.  
This is my first attempt at building an amp from scratch, and any help would be appreciated.  
6/12/1999 12:16 PM

The reason your plate voltage climbs when the tubes are gone is that there is little/no current flowing through the various resistors in your power supply circuit, thus there is no voltage drop. (Ohm's law, remember?) In order for the voltage to drop across a resistance, there must be current flowing. With no tubes to conduct current, no voltage drop. Don't worry about that - as long as your PS caps can take the voltage. As to your shunt measurements, I don't have enough info. I prefer the cathode resistor method, personally. Safer, and I'm measuring the entire tube current (plate + screen). In fact, for my DIY amps, I put a 1/8" jack in there for each tube so I can measure the idle current without removing the chassis.
6/13/1999 0:44 AM

I've had a problem with chinese made GZ34 tubes blowing fuses and not being able to handel the .75 amps per plate rating. I've heard of people using IN4007 diodes with the tube but i'm not sure how.
6/13/1999 2:06 PM
I'm not trying to insult you, but do you have your meter set to DC milliamps while trying to check your bias? If you are trying to measure the bias with a DC voltage setting instead of DC will blow the fuse on the meter and on your amp. Are you measuring from the center tap of your OT to the plate(pin 3) of your 6L6-GC? This is the correct way to measure bias current.  
Good luck.  
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