Hello everyone,
I have recently purchased an "as is" st-70. The visual revealed the diode mod for the selenium rectifier. And a suspect looking can cap. when attempting to bring it up with my variac, with no input or speakers connected, the amp began to hum at about 30% power, then at about 70% the hum was quite loud and subsequently popped the fuse.
I pulled all of the tubes, replace the can cap and found no joy as a repeat of the power up made no difference. The problem still exists. Where would you suggest that I start the process of ruling out components?
thanks in advance for any and all help.
I have recently purchased an "as is" st-70. The visual revealed the diode mod for the selenium rectifier. And a suspect looking can cap. when attempting to bring it up with my variac, with no input or speakers connected, the amp began to hum at about 30% power, then at about 70% the hum was quite loud and subsequently popped the fuse.
I pulled all of the tubes, replace the can cap and found no joy as a repeat of the power up made no difference. The problem still exists. Where would you suggest that I start the process of ruling out components?
thanks in advance for any and all help.
First : NEVER power a tubeamp without speakers or suitable load .
Second: If the only mod is replace se-diode with a si, then if correctly done
should removal of all tubes cut out B+ as problem. This as the GZ34
supplies B+
The only thing that the transformer supplies is filament and bias ( via the
bias Si diode). Look for shorts, and if none found get a new mains transformer.
If you pulled all the tubes and the amp blows the fuse then you may have a bad power transformer.
Bob Latino
Yes, being that +all+ of the tubes includes the GZ34 / 5AR4 rectifier - so that takes the entire HT PSU out of the equation. With no tubes in the chassis, the only current draw should be the (few mA) magnetization current of the pwr xfmr, and any current (very tiny) required for the bias supply.
I suppose it's possible that a shorted part in the filament or bias supplies might be doing this, but more than likely it's going to be a failed pwr xfmr or some other accidental short in the circuits. You need to isolate the pwr xfmr to be sure - once you know the status, you can proceed.
Look for shorts at the ends of all the power transformer secondary leads, including on the pcb.
Sometimes the leads, especially if fabric instead of pvc, can crack open and short.
As a last resort, disconnect and tape off all the ends of every secondary of the power transformer.
If the problem then persists, the transformer is bad. New ones are available here: PA060 POWER XFMR
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thanks everyone,
My worst fear is most likely to be confirmed per 5 out of 6 replies. I believe i will just go ahead and get a new pa 060 and the use the old one as a test subject due to the fact that i need to acquire a ammeter to do the rest of the testing. so, i will kick that can down the road a bit, so to speak.
I appreciate all the weigh in!
My worst fear is most likely to be confirmed per 5 out of 6 replies. I believe i will just go ahead and get a new pa 060 and the use the old one as a test subject due to the fact that i need to acquire a ammeter to do the rest of the testing. so, i will kick that can down the road a bit, so to speak.
I appreciate all the weigh in!
One by one, start disconnecting the pwr xfmr secondaries from the chassis.. start with the HV, then the 6.3V, then the 5.0V, and finally the bias tap (if it exists, don't recall).
If all of the pwr xfmr secondaries are open-circuit, and the amp is still wanting to blow a fuse, then disconnect the primary of the pwr xfmr.. that's about the last option. Point here, is I (and perhaps I speak for many of us) don't want to see you spend the $$ to drop-in a new xfmr that just blows the next fuse..
You're going to have to disconnect all of those leads in any case, so why not keep testing the unit as the work progresses. You might discover that your pwr xfmr is OK after all.
If you're lucky.
And build a current limiter so you don't keep wasting fuses.
Its informative to watch the bulb on the dim bulb tester continue to glow brightly after all secondaries are disconnected and taped off. It is a great tool with two functions, a visual indicator of current draw and it is also a protection device because the bulb filament will go open as a fuse will You can easily switch off the power when witnessing the Bulb is increasingly glowing brighter.
Its informative to watch the bulb on the dim bulb tester continue to glow brightly after all secondaries are disconnected and taped off. It is a great tool with two functions, a visual indicator of current draw and it is also a protection device because the bulb filament will go open as a fuse will You can easily switch off the power when witnessing the Bulb is increasingly glowing brighter.
Err, +will not go open+ as a fuse will.. A dim-bulb tester is a controlled current source, limited to the operating current of the bulb in the circuit. Using a 120V 60W bulb on 120V mains will limit current to ~500mA. Likewise, a 120W bulb on 120V mains will permit a short-circuit current of ~1A.
You can dead-short the 'output' of the dim-bulb device, and all you'll get is a light bulb at full brightness. That's the whole point of it, the bulb acts as a moderate-power current limiter.
The OEM ST-70 power trafo is marginal, for the job it has to do. The replacement Triode Electronics and, perhaps, other sources offer have a larger lamination stack and is a definite improvement over the original.
Triode Electronics Replacement ST-70 Power Transformer
Triode Electronics Replacement ST-70 Power Transformer
thanks to all,
Solid advice from everyone. As it turns out, I decided to run a check on my variac output. Very odd results on the vom. the best it would put out was 65 volts ac. This is a very nice Staco and never gave me an indication of failure. But after trying an alternate, this old ST came up with no tripping on the fuse. So, I popped in the tubes and grooved to some Joe Bonamassa for the afternoon.
S.O.B.! has anyone ever had their variac crap out? Maybe I had a lighting hit or something?
Great suggestions and much appreciated weigh in from all of you...... thanks again!
Solid advice from everyone. As it turns out, I decided to run a check on my variac output. Very odd results on the vom. the best it would put out was 65 volts ac. This is a very nice Staco and never gave me an indication of failure. But after trying an alternate, this old ST came up with no tripping on the fuse. So, I popped in the tubes and grooved to some Joe Bonamassa for the afternoon.
S.O.B.! has anyone ever had their variac crap out? Maybe I had a lighting hit or something?
Great suggestions and much appreciated weigh in from all of you...... thanks again!
New wrinkle...
after this gem is up and running.... I am doing some mods starting with rca inputs. while plugged in to AC with power off, I ran some new rca cables to my preamp. while leaning on a grounded source metal frame and plugging in the RCA cables, I experienced a nice tingle... So I checked the dynaco rca ins with my vom and found a low voltage coming out of those bad boys. THe Dynaco has a 3 prong power cord mod. I suspect this is not a good thing.... any thoughts?
after this gem is up and running.... I am doing some mods starting with rca inputs. while plugged in to AC with power off, I ran some new rca cables to my preamp. while leaning on a grounded source metal frame and plugging in the RCA cables, I experienced a nice tingle... So I checked the dynaco rca ins with my vom and found a low voltage coming out of those bad boys. THe Dynaco has a 3 prong power cord mod. I suspect this is not a good thing.... any thoughts?
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