Hotrod deluxe PT

[lowell]

This amp has very low HT voltage. I disconnected the HT secondaries to check em unloaded. I have about 340vac.

I indeed have 120vac on primary.

So just double checking that I'm right in that the PT is bad. I THINK I should have about 500vac on that secondary unloaded. About 10% more than the rectified DC with a non-CT PT and FWBR.

I have 50ohms between HT secondary end to end.

Gonna order one and replace but just roughly checking my theory.

Thanks!

[nickb]

340VAC sounds about right. 340VAC == 476Vpk == 430Vpk assuming 10% regulation. I'd say your problem is elsewhere. Measure the secondary volts under load and at the same time measure the HT voltage.

[lowell]

Wait so FWBR gives PEAK DCv x.9? Glad I asked. I'll investigate.

[lowell]

Hammond says it's 90% of secondary ACv. And that all AC voltages are RMS. So I'm a bit confused.

https://robrobinette.com/images/Guit...fier_Guide.png

[nickb]

Hammond says it's 90% of secondary ACv. And that all AC voltages are RMS. So I'm a bit confused.

https://robrobinette.com/images/Guit...fier_Guide.png

If their formula were correct then why do Hammond spec a PT with 319VAC model 290UX for a Hot Rod Deluxe?

Ignore that formula. It's a solution for just one case. Reality is more complicated.

Consider a limiting case of no load. The cap will charge to Vpk = VAC * 1.4. Now draw some current and you'll get ripple and so the average voltage will drop. The size of the cap and the ability of the transformer to re-charge it will affect the final Vdc. So you can see it depends...

PS: For a FWB with NO capacitor the, Vdc is Vac * 1.41 * 2/pi = 0.89. So the formula they gave applies to that case, more or less.

[The Dude]

I agree with nickb. What led you to the PT? What are the symptoms and what are you trying to fix?

[lowell]

Ok so Hammond is full of c!@p haha. It's peak.

It was super easy to pull the secondary lead clips and check it. So I did. And thought it was low...but alas that's clearly not the problem. Now onto the usual suspects.

Thanks for clarifying!

I do know that a center tapped PT and half wave rectifier will yield the sum both ends of secondary in series. So with this thread and that info I have a better idea of AC to DC conversion.

[lowell]

Hey waddya know. First time my Blue ESR meter worked as a useful tool. Wasnt getting readings on B+ or screen caps but DID get readings on others down the line. I also noticed the voltage rail was less than 380v if I disconnected the choke. So it clued me in on the first filter or two being the problem. Sure nuff new caps fixed it. +422v at CT node.

Thanks again guys.

Rectified voltage is Vac rms * 1.4 *.9 - got it