Dead PT...autopsy help requested

[johnnyreece]

johnnyreece, did you progress the repair?

Sorry for the lack of replies; was on vacation with my in-laws. I have not decided yet whether I'm going to replace the PT with the same one that failed or a different one. So, short answer is, I don't have any updates. I wasn't able to find where the short was, but none of the secondary taps gave any voltage whatsoever.

Can you confirm you just tested the rectifier tube on a typical tube tester, as that wouldn't have applied anything like rated PIV or been able to measure leakage current at a reasonable PIV? Were you going to add ss diode protection to the rectifier tube anodes, as a way of avoiding rectifier tube arcing from being a possible fault scenario?

Yes, it was just an Eico 667 I tested with. I suppose I've got a million diodes hanging around, might as well put those in for protection on the next go-round.

There is no bias adjustment - did you measure the bias voltage with the 6K6's and check if idle dissipation was ok and balanced?

The amp uses a shared cathode resistor. Honestly, I don't think about matching too hard. I've never witnessed any red plating, if that helps.

Do you have a way to measure mains primary current with a meter? You may want to check what current can be reached, and then check if a lower valued fuse rating is reasonable.

It's very possible. The issue becomes, if I use different power tubes, I would likely need to swap fuse sizes when I swap power tubes. I'm sure it's not that big of a deal to do that, though. What I do to measure primary current is remove the fuse and clip my leads to the two terminals on the fuse holder.

You say this amp is "made to run a variety of power tubes". Something tells me that you fail to consider the heater current differences (and I doubt any Fender style transformer is going to be wound for the heater current of many of them).

The PT I used (link in above text) is suitable for 200mA HV, 6.3V @ 6A, 5V @ 3A. So, let's round up to 1A for all three preamp (2x12AX7 and 1x6SL7), that leaves 5A for the pair of power tubes. As the amp failed while running the most power-sipping filament tubes I have, I would hesitate to say that was the reason for the failure, but hey, I'm sure stranger things have happened.

What output transformer are you using? These tubes also have very different load expectations.

Weber 6M45 (their version of a JTM45) OT: https://www.tedweber.com/wot45/. Long story short, it's a 6k8 Primary. So, sometimes I look at it as half or double, depending on what tube I'm using. For KT66 and 6V6, I set the output as if it's 6k8. For 6L6 and EL34, it's 3k4. For 6K6, 13k6. For example, on the green tap, that would be a 4 ohm load for a 3k4 primary (6L6, EL34). However, the same setting would be a 8 ohm load for 6k8 (KT66, 6V6) or a 16 ohm load for 13k6 (close enough for 6K6). I've attached a picture showing the data sheet for the OT and how my selector switch label is laid out on my faceplate.