ST 120 - Blown Fuse

Hi Bob,
My 'pride & joy' has been working beautifully for several months.
Switched it on today, the Ruby 5ar4 tube arced and the fuse blew.

I am a novice - what the standard procedure?
Many thanks in advance.
R.

(FYI setup: Revox b77 to Pioneer SA9900 (switched to pre only) to ST120 to AR98ls)

I am sure Bob will chime in soon as he knows his amps better than me but I would use a Weber Copper Cap Rectifier model WZ68 to handle the load of 4 KT88's in your amp. Its possible that your Ruby 5AR4 tube just died or a KT-88 can be bad. 3rd guess would be the quad capacitor but it is unlikely.

Regards,
Sal

Agreed that Bob will be chiming in soon. Usually in the early morning hours.

My guess is that your JJ rectifier tube took a dump. Do youself a HUGE favor and order one of the Weber WZ68s and be done with it.

In spite of what all of the "tube gurus" might say there is absolutely no difference in sound between a tube rectifier and a solid state rectifier in the ST-120.

Yes I know I probably just ignighted a war but I've used both and I MUCH prefer the reliability of the Weber over any tube rectifier that I've tried.

My amp did the same thing so dropped in the GE 5AR4 from my Heathkit AA-100 (Probably an 50 year old tube) and the system came back up. I did find that the bias changed some so I dialed it back in but where all the pots used to be at the 12:00 position now the pots for V7 and V3 were a little past the 12:00 position. The amp ran fine for about another week but then it blew the fuse again a few days ago after about 10 minutes of listening at a pretty low level. I replaced the fuse and powered it back up but the fuse popped as soon as the TDR kicked over and applied power the high voltage to the rectifier (the rectifier also looked to have some arcing inside as it happened). I placed an order for a WZ68 as well as a couple new Ruby 5AR4's then today, on a whim, I put the GE rectifier back into the Heathkit and the amp played fine. I then put the Ruby 5AR4 in the Heathkit and it also played fine. I guess my question is does the VTA-120 push the limits of the 5AR4 and is that why the WZ68 is the recommended solid state replacement of might I have other issues (bad output tube possibly?)

I plan on powering it up with the Ruby and the driver tubes sometime today and seeing if it stays on, then adding the left channel output tubes or the right channel output tubes just to see what happens but I was curious if others had seen this issue using the 5AR4.

Dave

On an ST120 you are using about 50ma per output tube, plus about 17ma for the driver board, so 217ma total.
The 5AR4 tube is rated at 250ma output. The Heathkit probably only needs about 3/4 as much (about the same as an ST70).

I am hoping that is the issue...I was able to get the amp back up on the old RCA rectifier and I juggled the output tubes so each sides bias pots are close to the same position (V2 and V3 at around 1:00, V6 and V7 are at around 12:00). Currently everything is biased at 50mA (.5 Volts).

I am curious about what grid voltage I am needing per tube to get each tube to my bias point. Does anyone know if it is ok to to run a wire from pin 5 (or 6) of each output tube to the front take off sockets where we also check the bias voltage so I can match both current and grid voltage? Since I don't have a tube tester it may be a quick way of seeing if I have an output tube that is drifting away from the others.

you might pick up some noise running a wire, but you can check bias voltage on pin 6 (the 1K to pin 5 gives you some isolation)
it's usually around -34vdc on EL34s (40ma) and closer to -40vdc on KT88s (50ma)

Thanks Roy, I hadn't thought about picking up noise. I opted to measure the bias voltage on the VTA board, at the 150K resistor. I was more interested in how closely matched the bias voltages are on each channel when flowing the same amount of bias current. The left channel tubes are within .1 V. The right channel tubes are within .4 V but there are few volts difference between the left and right channel. Left was at around -56.5 Volts and right was around -59.2 Volts. I had noticed this on the positions of the bias pots previously which is why I juggled the tubes around until I could at least get each channel's tubes pretty close in bias current and voltage.

Thanks,

Dave