Hot Fender Showman Transformer!

Doug · 08-22-2006, 10:05 PM

My Dual Showman Reverb, TFL5000D Power transformer is getting almost too hot to touch, I can't hold my hand against it for more than a quick second. Also my 6L6 plate voltages are around 400V, if I remember right it seems I've lost around 15V somewhere. What are correct voltages for the silverfaces? Is this transformer headed south? Any tips on diagnosing a transformer? The amp is still sounding good, I'm running it past 7 (no master volume), is this heat normal? Output transformer is comfortably warm. Filter caps are new btw.
Thanks
Doug

Don Symes · 08-22-2006, 10:10 PM

Quote:

Originally Posted by Doug

My Dual Showman Reverb, TFL5000D Power transformer is getting almost too hot to touch, I can't hold my hand against it for more than a quick second. Filter caps are new btw.
Doug

They'd actually be my first suspect - rather their ground connections - as the last thing changed. Unless one or more of your 6L6s (or its bias voltage) is going out.

Double-check the filter connections.

Then, with power tubes out, make sure their grid pins are negative - could be the bias network, but more likely the PI coupling caps.

Then, there are the screen connections and resistors to look at.

Good luck!

Doug · 08-23-2006, 12:42 PM

I took a look at the filter cap grounds, they seem good. But when I cranked it up and really listened, I could notice ghost notes way off in the background. The caps are new but I wonder if I got one too hot when I replaced the power resistors between them. Are they very heat sensitive? How do I troubleshoot the caps?
Thanks
Doug

MWJB · 08-23-2006, 01:09 PM

They're pretty sturdy as a rule. Any unusual hum, dirty distortion? (You did replace main filters exactly how they were originally - physical polarity, rather than electrical polarity, differed from amp to amp).

It might be useful for us to know plate current &/or bias voltage and heater voltages. Shorted primaries in the PT can make a PT run hot, but then your B+ would be up, not down.

Some Showmans are reknowned for getting hot. Did you used to routinely feel the PT before you changed the caps?

Rob Mercure · 08-23-2006, 01:14 PM

Doug,

Don hit it in passing but one of the first things I would do is determine the current draw of the output stage - that is, measure the cathode currents in the output tubes. Other than the heaters for all practical purposes this is where most of the "power" the power tranny puts out is dissipated and sometimes the output tubes can dissipate a lot of power without "redplating." And it wouldn't hurt to measure your heater voltages across the output tube pins and also to ground.
If the filters are shorted then the output tubes shouldn't make much difference - in fact since the B+ supply should rise w/o the outputs shorted filters should draw more current. So pull the outputs and just let the thing sit for a while - if the PT gets warm/hot then either the filters, or a shorted rectifier, or something in the heater string is funky.

Rob

Doug · 08-23-2006, 03:42 PM

Amp is drawing about 42mA across 1 ohm cathode resistors. Plate voltage is 405V, that's dead on according to the schematic. I'll take some measurements on the heaters tonight. I did replace the caps just like the originals, the brown mallorys. I measured voltages on the filter 20 uF caps at 400v, 380 and 327.

MWJB · 08-23-2006, 03:52 PM

If you drop the plate current by 10mA, or so, you'll reduce the heat generated by the tubes. I'd be surprised if your amp ran at 42mA when it was built.

This will also bump up your B+ a little....it may just be me, but I tend to get suspicious when a Fender actually runs the voltage quoted on a schematic :-)

08-22-2006, 10:05 PM	#1
Doug Member Join Date: Aug 2006 Posts: 46	Hot Fender Showman Transformer! My Dual Showman Reverb, TFL5000D Power transformer is getting almost too hot to touch, I can't hold my hand against it for more than a quick second. Also my 6L6 plate voltages are around 400V, if I remember right it seems I've lost around 15V somewhere. What are correct voltages for the silverfaces? Is this transformer headed south? Any tips on diagnosing a transformer? The amp is still sounding good, I'm running it past 7 (no master volume), is this heat normal? Output transformer is comfortably warm. Filter caps are new btw. Thanks Doug

08-23-2006, 12:42 PM	#3
Doug Member Join Date: Aug 2006 Posts: 46	Suspecting the filtering I took a look at the filter cap grounds, they seem good. But when I cranked it up and really listened, I could notice ghost notes way off in the background. The caps are new but I wonder if I got one too hot when I replaced the power resistors between them. Are they very heat sensitive? How do I troubleshoot the caps? Thanks Doug

08-23-2006, 03:42 PM	#6
Doug Member Join Date: Aug 2006 Posts: 46	A few more measurements... Amp is drawing about 42mA across 1 ohm cathode resistors. Plate voltage is 405V, that's dead on according to the schematic. I'll take some measurements on the heaters tonight. I did replace the caps just like the originals, the brown mallorys. I measured voltages on the filter 20 uF caps at 400v, 380 and 327.

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08-23-2006, 01:09 PM	#4
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	They're pretty sturdy as a rule. Any unusual hum, dirty distortion? (You did replace main filters exactly how they were originally - physical polarity, rather than electrical polarity, differed from amp to amp). It might be useful for us to know plate current &/or bias voltage and heater voltages. Shorted primaries in the PT can make a PT run hot, but then your B+ would be up, not down. Some Showmans are reknowned for getting hot. Did you used to routinely feel the PT before you changed the caps?

08-23-2006, 01:14 PM	#5
Rob Mercure Lifetime Member Join Date: May 2006 Location: Wise, Virginia Posts: 624	Doug, Don hit it in passing but one of the first things I would do is determine the current draw of the output stage - that is, measure the cathode currents in the output tubes. Other than the heaters for all practical purposes this is where most of the "power" the power tranny puts out is dissipated and sometimes the output tubes can dissipate a lot of power without "redplating." And it wouldn't hurt to measure your heater voltages across the output tube pins and also to ground. If the filters are shorted then the output tubes shouldn't make much difference - in fact since the B+ supply should rise w/o the outputs shorted filters should draw more current. So pull the outputs and just let the thing sit for a while - if the PT gets warm/hot then either the filters, or a shorted rectifier, or something in the heater string is funky. Rob

08-23-2006, 03:52 PM	#7
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	If you drop the plate current by 10mA, or so, you'll reduce the heat generated by the tubes. I'd be surprised if your amp ran at 42mA when it was built. This will also bump up your B+ a little....it may just be me, but I tend to get suspicious when a Fender actually runs the voltage quoted on a schematic :-)

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