I never thought this would happen to me! (PT failure)

[dstrat]

this is a perfect example of everything matters design wise.

Edit: I would say 200ma is overkill for a pair of 6v6 tubes.

[eschertron]

this perfect example of everything matters design wise.

I couldn't agree more. And, as a budding 'designer', I'm getting my head around how B+, current consumption, and total power (delivered + dissipated) relate. My too-simple equation of Voltage(B+) x rated HT current = power dissipated + audio output left me holding a PT with shorted turns. Will my equation be more correct if I can measure B+ under full load conditions? Did the old PT simply sag too much and die of a brown-out? Alas, my wayback machine is broken, too, and I can't measure the B+ at full power any more.

We've talked about fusing the B+, but again this is a scheme for protecting PTs in properly-designed amps. I'm still left wanting the magic equation (cue angel's "ahhhh") that predicts the right size PT for the application.

[dstrat]

you can get a general idea from this:

Interactive Valve Data Sheets

[pdf64]

A bit more sag at the g2 node will reduce max continuous plate current and order may be restored, without the need for big iron.

[eschertron]

you can get a general idea from this:

Interactive Valve Data Sheets

Thanks! That was my primary resource.
What I don't see is any calculation of what average current would be at any given signal level. At any point in class A operation, I understand that as one tube is drawing less, the PP complement tube is drawing more, so total current remains constant. As one tube goes into cutoff when we enter class AB1, the "other" tube continues to draw more, increasing total current from idle. I'd really like to be able to model that with some kind of equation.

edit: ... without integrating the area under the curve, I mean...

[dstrat]

I agree , with data measured from my 48 deluxe the graphs do seem off, but mabe I am the one thats off..

drawing a load line is not super hard , reading the data gets me.

[eschertron]

I emailed Hammond with a query “Can I ask how a designer might best spec the needed current rating of a certain tube complement and topology?” and got an answer of sorts:

I really wish I knew the answer to that question but unfortunately we “Hammond” have little experience in amp design or theory. Those engineers are long gone or retired by now. We make great transformers but the required information needs to be spoon fed to us. If I had a schematic of a Fender or similar amp that was similar to what you are making I could see what transformer we have designed for it.

Which confirms the rest of the sage advice I've received in this thread, basically "find the schem with the closest matching operating conditions and get the replacement PT for that amp". Which is all well and good, but I'm not buying the magical mojo black arts of amp design

On a whim, I calculated the dissipation on the cathode resistor network and the PSU resistors. Came up with a few VA, but nothing near what I expect to need in a real-world fix for this amp. I fully agree that 100mA or more should be supplied to the power amp to make it work as designed, but still a bit hesitant to accept the fact that I can't get a simple '1st law' energy balance equation to work. I just can't find all the missing power. A few W here or there from power resistors, some heat from the trannys, it doesn't all add up. Unless the amp was generating WAY more audio power than I'd expect from the models. sigh.

[nosaj]

I emailed Hammond with a query “Can I ask how a designer might best spec the needed current rating of a certain tube complement and topology?” and got an answer of sorts:



Which confirms the rest of the sage advice I've received in this thread, basically "find the schem with the closest matching operating conditions and get the replacement PT for that amp". Which is all well and good, but I'm not buying the magical mojo black arts of amp design

On a whim, I calculated the dissipation on the cathode resistor network and the PSU resistors. Came up with a few VA, but nothing near what I expect to need in a real-world fix for this amp. I fully agree that 100mA or more should be supplied to the power amp to make it work as designed, but still a bit hesitant to accept the fact that I can't get a simple '1st law' energy balance equation to work. I just can't find all the missing power. A few W here or there from power resistors, some heat from the trannys, it doesn't all add up. Unless the amp was generating WAY more audio power than I'd expect from the models. sigh.

You might try the Antique Radio Forums ? View new posts
There are a lot of older older guys there, every year there are fewer. Maybe you can tap one that has the formula or answer you are looking for.

nosaj

[pdf64]

I...On a whim, I calculated the dissipation on the cathode resistor network and the PSU resistors...

But at max loading, do you know the current through the zener?

[eschertron]

But at max loading, do you know the current through the zener?

Well, I don't know the actual current through the power tubes, which is how I got here in the first place. But I did calculate the total power lost in the cathode network assuming 100mA current. And what fraction in the resistor, the remaining in the diode.
27v * 100mA = 2.7W total
27v / 470R = 55mA dissipated in the resistor
27v^2 / 470R = 1.5W in the resistor

so the rest, 45mA and 1.3W dissipated in the zener. And yes, if the current was higher, the dissipation in the zener goes up quickly.

[Dave H]

“Can I ask how a designer might best spec the needed current rating of a certain tube complement and topology?”

You can get an idea of the DC required by the amp at full output from the tube's data sheet (as in post #15) but that isn't the current value needed to specify the PT. What is really needed is the PT RMS current and using a solid state rectifier the RMS can be 2 x the DC current for a bridge rectifier and possibly up to x1.5 for a full wave CT rectifier so to be on the safe side I'd be thinking of using a 150mA rated PT.

[eschertron]

I ordered a Hammond 270DX (104mA at 550VCT up from 75mA, about a 38% bigger current rating). That plus larger screen grid stops may keep me from buying another transformer. I will measure voltage and current under load this time. I have some 1R/1W resistors, I'll put one on the tail of the cathode network.

I popped the end bells of the PT last night, took a visual. Looked OK. I measured the dc resistance, seemed to get the one winding's value to fluctuate from 170R (good) to 43R (bad) as I wiggled the leads around a bit. I am convinced that the winding's turm-to-turn insulation was compromised. I am not convinced that excess current was the sole factor.

[pdf64]

Maybe see if Hammond are willing to check it for a manufacturing defect?

[eschertron]

Maybe see if Hammond are willing to check it for a manufacturing defect?

The thought had crossed my mind, but I didn't ask and they didn't offer. Some companies are eager to do failure analysis, others not so much. I held on to it for a few days, then pulled it apart myself. After taking the resistance readings, I started to peel back the tape, to see what I could see. Not much. With the tranny potted in epoxy (as it appears to my untrained eye) I'm guessing a quick visual inspection is the best this will get.

[mikepukmel]

The thought had crossed my mind, but I didn't ask and they didn't offer. Some companies are eager to do failure analysis, others not so much. I held on to it for a few days, then pulled it apart myself. After taking the resistance readings, I started to peel back the tape, to see what I could see. Not much. With the tranny potted in epoxy (as it appears to my untrained eye) I'm guessing a quick visual inspection is the best this will get.

definitely ask and more than once if y ou need to. When I bought the transformer set from Mouser (Hammond) and got everything running, the PT madea nice hum. Enough to be annoying at low playing volumes. I asked a few times, got them to take the transformer to their tech center in USA for testing, and they did replace it. Their tech support office seems quite busy, but I kept after and they did replace it. Could be a bad PT. Hmm, so you jiggle the windings and the mreasured resistance fluctuates? If it was a burn in the middle of a winding, less change you'd see that I think. Maybe the connection where the lead wire meets the magnet wire is compromised. Definitely ask about warranty testing.