In a previous post, I asked about the advisability of using a directly-heated rectifier tube in the VTA ST-120. While I lusted for an indirectly-heated Mullard GZ37, it would have just been too pricey. So, I had ordered some Shuguang 5Z3PAT rectifiers that were very inexpensive to try instead of my JJ GZ34 rectifier. Based on the previous thread, it looked like the Svetlana =C= 5C3S (= 5U4GB) would be another option. I found one of these stateside at a very reasonable price and decided to get that while I awaited the Chinese tubes to make it over the Pacific. So I got it in the mail and put into the rectifier tube socket. The amp sits in a relatively dark are and I thought I had the tube key properly aligned with the socket. The tube seemed rather hard to seat but it went into the socket. I dialed down the bias adjustment resistors just a tad to be on the safe side and turned on the amp with the preamp volume turned down to 0. I anxiously awaited the tube filaments to start to glow but nothing! My first thought was a bum tube, but as all the other tubes were alight, I turned up the volume and lo and behold, the amp was putting out a signal. Strange, I thought. I checked the bias and it was reasonable at about 500 mV. I turned off the amp and pulled the rectifier to see I had miskeyed it and the pins were rotated about 2 pins off design! The only thing I can think of is that as I had already done the diode modification, a couple of the pins must have allowed some current to reach the diodes which passed DC current to the caps. I reinserted the tube properly and got the filaments to heat. I had to readjust the bias resistors quite a lot due to what I guess is a bigger voltage drop, but everything is now working great. If anyone can tell me how my amp played with an improperly inserted rectifier and how no damage was done, I'd appreciate your explanation! I do like the big coke-bottle style but it was a pretty strange adventure.
Bill
Bill
