ST35 Help Please

[Richard]

My ST35 has been up and running with no problems for about 2 months. A few minutes after I turned it on today, the PT started to make a high pitch hum and then the fuse blew. I disconnected everything and the same thing happens. I checked the wiring and everything looks intact. The only thing I have done lately was to adjust the bias yesterday, it wasn't off by much maybe a point or two. Anyhow I have several hours on the amp since doing this with no problems. Can anyone offer any advise or suggestions?
Thanks

[Shannon Parks]

Hi Richard,

When in doubt, ohm it out. First unplug the unit and 'ohm it out' according to the resistance chart in the manual. Anything look out of range?

Shannon

[Richard]

Hi Shannon,
My manual lists the reading for the PA774, I have the Hammond 270HX. Would the reading be the same? Also is best to ground to the chassis or the pcb when taking the reading. I seem to get a wide variance when I ground to one or the other.
Thanks

[Shannon Parks]

Hi Shannon,
My manual lists the reading for the PA774, I have the Hammond 270HX. Would the reading be the same? Also is best to ground to the chassis or the pcb when taking the reading. I seem to get a wide variance when I ground to one or the other.
Thanks

Take the ground readings off the PCB. If you have a Rev D, the PCB ground is totally separate from the chassis ground. 270HX should be similar. Post if anything looks amiss.

Shannon

[Richard]

Well I went back to check my reading, but decided to give it one more try and turned on the amp. Now the tubes don't light up, and no hum. The fuse doesn't blow but the led on the switch lights. Most of the reading were very strange. J1 pin1-30.4, pin2-28.8, pin7 & 8 52. All the rest flashed a number then went to infinity. The only other reading I got were J6 &J7 pin 52. Thanks again Shannon for taking time out of your weekend.

[Shannon Parks]

Well I went back to check my reading, but decided to give it one more try and turned on the amp. Now the tubes don't light up, and no hum. The fuse doesn't blow but the led on the switch lights. Most of the reading were very strange. J1 pin1-30.4, pin2-28.8, pin7 & 8 52. All the rest flashed a number then went to infinity.

This is all good. Pin 1 and 2 are showing the secondaries internal resistance. Pin 7 and 8 show the filament 100 ohm resistors. It shows 52 ohms because the internal resistance is just a couple ohms, so it looks like two 100 ohm resistors in parallel (plus a couple ohms).

The only other reading I got were J6 &J7 pin 52. Thanks again Shannon for taking time out of your weekend.

Not sure what reading you have here. Is the input jack shorted on something? On J6 & J7, pin 1 should be high Z, pin 2 ground.

Shannon

[Richard]

I re-checked the reading on the inputs, I'm ok there. Not sure what I did the first time. Anyhow I'm still getting a loud hum then the fuse blows. What do you suggest next?

[Richard]

Well I did a little more troubleshooting. After disconnecting the secondaries one by one I determined it’s the right output giving me problems. With nothing connected to J4 and J5 the amp fires up and runs fine. I connected the right OPT to the left side and everything was ok and when I connected the left OPT to J4 and J5 the PT was humming again and the fuse blew. So I guess I must have a bad component somewhere.

To recap… my amp has been running fine for almost two months. The fuse doesn’t blow immediately it takes a few seconds and first there’s a loud hum, then the fuse blows.

Any suggestions on how I should proceed?

[Shannon Parks]

Well I did a little more troubleshooting. After disconnecting the secondaries one by one I determined it’s the right output giving me problems. With nothing connected to J4 and J5 the amp fires up and runs fine. I connected the right OPT to the left side and everything was ok and when I connected the left OPT to J4 and J5 the PT was humming again and the fuse blew. So I guess I must have a bad component somewhere.

To recap… my amp has been running fine for almost two months. The fuse doesn’t blow immediately it takes a few seconds and first there’s a loud hum, then the fuse blows.

Any suggestions on how I should proceed?

Sounds like your right channel may be unstable and going into oscillation meltdown. You mentioned that you had re-biased. My hunch is that the may have accidently adjusted the right channel NFB some.

Disconnect the feedback taps from both sides and re-check where you have the negative feedback set at. After you check those, leave the NFB taps off, and retest with the inputs grounded. Turn it off at the first appearance of the loud humm to save yourself a trip to the hardware store for more fuses.

Shannon

[Richard]

Ok. Gave that a try. My reading was 20.4, so I ajusted it to 20.3 , same results. I'm not sure what you mean "with the inputs grounded". Is this something different then they way they are mormally connected?

[Shannon Parks]

Ok. Gave that a try. My reading was 20.4, so I ajusted it to 20.3 , same results. I'm not sure what you mean "with the inputs grounded". Is this something different then they way they are mormally connected?

Rich,

I assume that's 20.3K, correct?

Re: the inputs, I meant shorted to ground - making it impossible for noise (or anything) to couple onto them. Go ahead and leave the feedback disconnected on both sides and give it a test. Open loop should be much more stable.

Shannon

[Richard]

Hi Shannon,
Yes that's 20.3K. Well I shorted the inputs and nothing changed. Same results.

[erichayes]

Hi All,

When I hear "loud hum after a few seconds" and 'fuse blows" my first thought is of an output tube(s) drawing excessive cathode current. Have the tubes been swapped out? Two months is long enough for a good tube to go bad.

[Richard]

I only have one set of tubes. What I did thought was to swap the right tubes with the left but did not install the tubes on the left side(originally the right op tubes) and still got the hum. If I put these tubes in and disconnect the right opt , no hum.

[EWBrown]

Check the OPT red (B+) to ground (disconnect the terminal from the terminal strip first, and EL84s removed from their sockets) and check the resistance. It should be "open" or infinity, as there are no other direct circuits to ground to the remaining plate and SG taps. If you get a resistance from red to ground, then there is an internal short from the primary to ground, probably through the secondary. I've seen this happen once in an expensive VTL amp, the insulation between windings "punched through" and the primary arced over to the secondary and caused an internal short. This is not a common occurance, but it does happen on rare occasions.

HTH

/ed B in NH