Tube PT Troubleshoot

[bassicstu]

Im working on a 1969 Sunn 200S. Original problem was it blew power fuses when switched on from standby; it didnt smoke and stink. I pulled all the fuses and stored it about 5 yrs. Recently I reinstalled the fuses, turned on, powered up, and played bass thru it for more than an hour - no problem - it sounded Great! Then I took it out to play, set it on standby for about half hour and the fuse went. Since then, the fuse blows when switched from standby to play. We measured 9000 ohms at one side of the B+ voltage with the PT all hooked up. Suspecting a shorted PT, we then removed it. We checked for open circuits and measured resistance from main center tap to B+ volts and the heater center tap to heater outs. Ohms were about the same everywhere, but the primary (power input) circuit showed 1.5 ohms. When we sent 9volt dc battery current into this circuit, dc volts came out center taps, BUT the 9volt batt got to hot to hold and would have destructed. Does this indicate a problem with the PT plates or something else? Im considering (carefully) powering the disconnected PT with a fuse. If the fuse blows, replace the PT. If not, measure voltage outputs. Of course, I dont want to spend $100+ on a new PT to find out I still have an unknown problem. Does anyone have any ideas or observations?

[52 Bill]

Putting a 9vDC battery across the primary would cause the battery to get hot, as it would see basically a dead short. Transformers are AC devices try putting a small AC voltage on the primary.

Do you mean that with the 9vDC on the primary to get some sort of DC reading on the secondaries?

Do this, take resistance readings across each winding, primary and secondary. Check for any continuity between windings and between the windings and the case. If all resistance tests are ok, then apply voltage to the primary (full or reduced) and check secondary voltages.

You also could test with a light bulb limiter as well.

[Wilder Amplification]

Usually when a PT blows it takes the mains fuse out no matter what position the standby switch is in.

If this thing has all original filter caps I would replace those absolute first thing before going any further.

[Enzo]

Wilder is right, the PT would be blowing fuses regardless of the standby switch.

Pull the power tubes. Still blow fuses? Disconect the center tap wire of the OUTPUT transformer. ANy help?

[Gtr_tech]

The filter caps in those Sunns were run on the ragged edge right off the git. With todays increased line voltages, it won't take long for them to go tits up. I have a 100s that I measured way over the rated voltage on the first cap section over 20 years ago. I rebuilt the pwr supply with series stacked lytics for the plate supply and its been happy since. SAme with my "new" old Traynor custom reverb head. Overvoltaged plate/screen caps. Rebuilt with series stacked 220u/400v snap ins and new 47/450s for the PI/front end (bias supply and K bypass caps replaced as well). Kicks like a pissed off mule now....and really quiet noise-wise. GOtta love those electronics surplus stores....cost me peanuts to recap the whole damn thing...and I doubled the values of the plate/screen caps.

Just a side note on these old Traynors. The ones that came fitted with big bottle 6CA7s and no grid stoppers on G1....you may run into motorboating using current production EL34s with signal present...and it won't take much to get it going. I used 4.7k's on G1 (was even spare eyelets right where they needed to be!) and that was enough to make it behave.

Gonna gig it this weekend....should be fun.

[bassicstu]

the PT is all disconnected now so Im going to do the resistance checks 52 Bill suggested. when PT was still in the amp, we tried the no tube test and the fuse blew when switched off standby. since Wilder and Enzo have confirmed that a bad PT will blow fuses regardless of standby switch position and, this PT never smoked and stunk, Im starting to think it may be ok. I still wonder why this amp worked after 5 yrs storage (original post) then started blowing fuses when switched to full power. After I do the resistance tests I will post results. Thanks very much to all - with your help I hope to use this great sounding amp again.

[52 Bill]

There are no 9vDC tests that I would recommend, other than the resistance tests of the windings.

Applying any continuous DC voltage to a transformer winding will not create a continuous DC voltage on a secondary winding. It will cause a short spike voltage on the secondaries upon the application of the DC voltage, as well as when the DC voltage is removed but that's it.

[Wilder Amplification]

the PT is all disconnected now so Im going to do the resistance checks 52 Bill suggested. when PT was still in the amp, we tried the no tube test and the fuse blew when switched off standby. since Wilder and Enzo have confirmed that a bad PT will blow fuses regardless of standby switch position and, this PT never smoked and stunk, Im starting to think it may be ok. I still wonder why this amp worked after 5 yrs storage (original post) then started blowing fuses when switched to full power. After I do the resistance tests I will post results. Thanks very much to all - with your help I hope to use this great sounding amp again.

Well for starters...the amp was stored for 5 years. The dielectric goo in electrolytic caps goes from a liquid state to a crystalized state when they haven't been powered up in that period of time. After that long of a storage period...and I'm not sure what the environmental conditions were at the place it was stored but...you're supposed to reform the filter caps PRIOR TO running it at full voltage, which is basically just slow charging the caps for a period of hours to slow heat the dielectric goo inside the cap to convert it back from its crystalized state into a liquid state again, not just throw the voltage to them right out the gate.

It was also mentioned that these caps in these amps were being ran at the ragged edge of their working voltage rating as it is...so throwing that kinda voltage to them while the dielectric goo is not in its normal liquid state can wreak havoc on filter caps.

Again...if those filter caps are original I'm making the call that they are your most likely culprit. The fact that the thing blows fuses upon taking it out of standby without the tubes in definitely points to this..they're the only thing pulling current from the power supply at that point.

Another possible suspect as well would be the rectifier diodes. Seen plenty of those short and they'll easily take out a mains fuse.

[Gtr_tech]

If the filter caps were punching through then the recto tubes (IIRC the 2000s uses 2 GZ34s) are trashed along with the caps. Pull the recto tubes and pwr up. Got milk?

[Wilder Amplification]

OK I wasn't aware that this amp used tube recs. So for all we know this could just be a bad tube rectifier. But honestly if the filter caps are original, regardless of if they're good or not I'd still replace them due to age.

[bassicstu]

PT resistance tests

PT resistance test results seemed OK to me, but if someone sees a problem please tell me! Results were: main center tap to the B+ and bias outputs was .5 ohms ( B+ to B+ was .1 ohm). Heater center tap to heater outs was 0 ohms (heater out to heater out was also 0 ohms). Bias to both B+ was .75 ohms. No windings were shorted to the case. IF confirmed as the way to go here, my new plan is to reinstall the PT, replace the filter caps and rectifier tube, form the new filter caps, and see whats up. Sound right? Seems the goodwill of folks in this forum saved me spending over $100 to replace a good PT. THANK YOU Enzo, Wilder, 52 Bill, and Gtr_tech!

[52 Bill]

I would reinstall it, power it up on Standby and see if it still holds the fuse.

If it does, then check all of the voltages coming out of the transformer to see if they seem right.

If it passes this test, then add the rectifier tube and see what the DC voltages are, etc.

Just take it one stage at a time until you find the short.

Have you tried taking a resistance reading across the main filter caps?

[Gtr_tech]

I wouldn't worry about trying to read the filters. They're old, stressed, and way overdue for replacement. Besides....trouble may not show itself until you get them under working conditions and they start drawing current.

To the OP: did you ever pwr it up without the recto tubes in? Or you can disconnect the plate supply leads from the recto socket....same thing either way.