Ampeg B 15S heater issue

[gbono]

I have a B15 S which was running just fine after cleaning pots and resoldering some bad connections on the PCB. Buttoned up the chassis and the amp sat for a few weeks - just getting ready to send the amp on its way and I powered it up with an 8 ohm restive load and 1KHz input. The red pilot light came on and I took the amp out of standby but the output signal was very low/distorted. The pilot light for power flickered and went dead. The standby switch would still turn its pilot light on/off AND the tubes were not lighting up.

Took the amp apart Pulled all the tubes and checked for open heaters but no tube issues found. Checked all traces on the PCB without issues. Un soldered heater wires from PCB and using a variac brought up the mains voltage. Plate voltage present but no heater voltage I have a winding tester but it doesn't seen to work on heater windings even using a large 12V battery. So I'm not completely sure what's up with the heater windings on this amp's PT other than it just decided to die.

I have a V4 that was given to me years ago. Is the PT close to the one used in the B 15S? Any ideas on testing the heater windings on the B 15S PT or what?

[Enzo]

Did you measure for continuity through that winding while it was disconnected? With power applied, what voltage comes out those wires? And if you get 6-8v, put a load on it, say the dummy load you use for speakers. Does the voltage hold up or drop to very little?

If you made a transformer tester like RG has over at Geofex, then it tests all windings at once. Windings are all linked magnetically, so a shorted turn on any winding will affect them all.

[Jazz P Bass]

"No heater voltage' implies an open winding, does it not?

What was the reason for the amp being worked on?
The winding may have been on it's way out.

[gbono]

Heater voltage is now present after some resoldering but now the output is weak and distorted. I chased the signal all the way to V5/6 without issue (even replaced the output tubes). There is proper plate voltage at pin 3 on V6/5 - what should I be reading for grid voltage. Checked the primary secondary windings of the OT with the RG winding tester and the winding appear good.

[Jazz P Bass]

The output tube grids should have -50 some Vdc on them.

With a 100mv input signal & the controls on full, I would expect a 50Vac signal on each of the grids.

[g1]

Also check that your screen voltage is good at pin4 of both power tubes.

[gbono]

-49 volts at pin 5 (V5/6) and the screen voltage is good too. With 125mV rms at the input of the amp the output into a resistive load will quickly distort and he amp starts drawing excessive current before you can get past 3-4 on channel one and two. I bet if there was a speaker attached to the output it would sound like DC or arcing. I had a similar issue with a Peavey Valveking 100 and that was due to an arcing secondary of the OT. Looks like I should pull out the Sunn 200s OT and switch out this OT for a test -

[Jazz P Bass]

Yep.

Time to sub in a known good OT.

[gbono]

post mortem - the issue turns out that the combination of a new resistive load, the length of cable and scope probe would make this amp oscillate. Placing the scope probe on the output jack (ext speaker) stops the oscillation I'm getting 23 Vrms into an 8 ohm load just before clipping.

[Rob's Radio-Active]

post mortem - the issue turns out that the combination of a new resistive load, the length of cable and scope probe would make this amp oscillate. Placing the scope probe on the output jack (ext speaker) stops the oscillation I'm getting 23 Vrms into an 8 ohm load just before clipping.

I always use the 10X probe setting on hi z circuits regardless of whether it is an audio frequency or radio frequency device.
Rob