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Hi fellas, some advice appreciated.

The situation. While running a dr103 through a THD hot plate with channels jumped I heard a cackle then dead, no sound. Fuses all good. But since I was in the middle of a session with no time I just put another dr103 on and within 20min, the same thing happened. So today I opened one of them up to suss out the problem. First I checked the OP tubes by putting the amp into 8 ohms and testing the inside pair of tubes - still no sound, but one of them started cackling making a noise through the speakers similar to when they were in session but with that "electrical smell". Switched pairs, no cackling but no sound. Looked inside and found a burnt resistor (see pic). I suspect if I open the other head I'll find the same thing. Clearly I'll have to replace it, but I'm really concerned about how this could happen in the first place, what could cause this sort of thing? The hot plate? Bad speaker cable? Bad tubes coincidentally going south on two separate amps in the same day? Any advice would be appreciated!



Thanks, Tony

Is the "spuzz" on the filter cap part of the burnt resistor or has the cap itself failed?

Attenuators put a great load on the amp's power section. Any weak point will be revealed faster in an attenuated amp. How much were you cutting the signal?

I would suspect the attenuator first, the speaker cables second, maybe the line power third and an evil spell fourth .

Good luck in the repairs and let us know if you find anything definitive.

Dave

EDIT: P.S. Check the wiring of your speaker cab, too. And verify the impedance to the best one can using a Multimeter.

Thanks for the reply Dave. I just noticed the spuzz on the filter cap - I don't know if the cap itself failed, but it's as old as the amp. The other head that blew has new filter caps so I'll be able to tell if this failure is doing something to the cap as well. The burn resistor (the yellowish one) is actually completely severed in half and too far away from the filter cap (IMO) to have jizzed on it.

PS, I was only cutting the signal 4dB on the attenuator and had the master volume about half way - so not crazy crazy...

kats1 wrote: PS, I was only cutting the signal 4dB on the attenuator and had the master volume about half way - so not crazy crazy...

You're right, that doesn't sound excessive.

Were you using known-good cables and cabinet?

Dave

That cap *definitely* needs to be replaced.

That resistor is downstream of the OT/plates, I'd suspect screen failure in one of the output tubes. Unless you have an intermittent speaker/cable problem, it seems likely a coincidence that they both failed. I'd be surprised if the second amp had the same exact problem.

I've used a HotPlate for years, and I've never had any problem.

--mark

The cabinet is good. I've been running a JTM 45 on without issue. The cables I'm not sure. I knew we had a bad one but I tied it in a knot and put it away... I'm wondering if it somehow got mixed in by someone else without me knowing. I'll open up the second one tomorrow to see what I find.

Again, thanks for the help guys!

Tony

Well, bad luck.

I re-tubed the EL34's, recapped the head and replaced the burnt resistor. Still no audio. All tubes are glowing, just dead silence. I'm worried about the OP xformer. How can I test for that?

kats1 wrote: I'm worried about the OP xformer. How can I test for that?

After ensuring that the filter caps are drained and the amp is safe to work on, I'd check both sides (primary and secondary) of the output transformer with a multi-meter. If it's not open circuit or hard-shorted then check each half of the output transformer primary to ensure there's no partial short. They should each roughly measure half of the total value measured. On the primary that'll be from pin 3 of an output tube socket on one half of the amp to pin 3 of a socket on the other half to measure the total DC resistance and then from each of those points to the centre-tap to give you the values for each half. To measure the secondary, take the reading from the 16 ohm tap to ground.

I don't recall what the DC resistance values should be (and don't have a 103 to check on) but I'm sure the more experienced members of the board will chime in with more info soon!

good luck, hope it turns out to be nothing serious.
-Matt.

Dumb question, but did you check the HT fuse?

--mark

Yeah first thing. Thanks for the xformer tip, I'll check it today.

Each half of the secondary should measure around 20 ohms IIRC.

Measure the two large power resistors and the resistors on the output tube sockets, on rare occasions under severe load resistors will fail open with little or no external evidence.

--mark

Okie doke...

Measuring the primary I get about 43 ohms, measuring on each half o the primary I get about 21 ohms, Sounds okay. I'm a little confused about the secondary. If I measure from the 16 Ohm tap to ground (chassis) I get something like .8 Ohms. If I measure from pin 3 of the OP tube to the 16 ohm tap I get almost 1M. IE, I think I'm measuring the wrong points for the secondary and misunderstood how to do it. While I wait for clarification I'll check those power resistors. Thanks again for the help fellas, I'm praying that my xformer is okay...

Odd, the 100R 5 watt cement resistor is measuring 700K. Could that be the culprit?

kats1 wrote:Odd, the 100R 5 watt cement resistor is measuring 700K. Could that be the culprit?

I would say yes

Try replacing it, of course.

Good News!

Replacing the resistor worked. Thanks for all the help. Now one more head to fix. If it's the same issue, I'll really be wondering what the heck happened. I'm afraid to use the hot plate now...