No voltages

lowell · 01-08-2008, 07:19 PM

Hey all,
got a gibson class A tube amp here... filaments light up, no sound... NOTHING. measured voltages and there is NONE. disconnected p transformer secondary and the black reads 81VAC and red reads 177VAC... filaments are reading 14VAC. My feeling is that this is far from right. Can someone tell me how to test the p transfomer? It isn't blowing fuses so most info i've found regarding testing PT is of no help.

thanks1

52 Bill · 01-08-2008, 08:18 PM

Right voltages or not, if you are reading ac output from the transformer, then it is working enough to not be the problem that you describe.

I'd look at the rectifier and power supply parts first.

tboy · 01-08-2008, 08:22 PM

What model is the amp? Have you checked the rectifier?

lowell · 01-08-2008, 08:46 PM

thanks for help guys... but how can it be the rectifier if I have disconnected the secondary? amp is a Gibson Super Goldtone

52 Bill · 01-08-2008, 10:54 PM

I'm sorry, but what are you asking here?

The rectifier converts the ac output from the transformer to a dc voltage to run the entire amp on. If you measured no voltage, but the filaments did light up, then the amp has no DC voltage, which comes from the rectifier (be it tube of solid state diode).

You measured ac output from the power transformer (that's why the filaments light up), so it appears to be working.

Hook the secondary windings back up and check the rectifier section of the power supply.

lowell · 01-08-2008, 11:08 PM

i guess my question is... why is the black secondary wire ony 81VAC and the red secondary so different at 177VAC and is this normal to have such a huge difference? I will try testing the rectifier. Also, shouldn't the secondaries of the PT be in the order of at least 200+VAC? There are 4 EL84 in this amp.

52 Bill · 01-08-2008, 11:49 PM

According to the schematic there should be 340 vac from the red wire to the black. How are you reading the voltages?

lowell · 01-09-2008, 01:59 AM

ok... now the amp works... hmmm... don't know what happened there, but I reconnected the PT to check rectifiers and all was fine...??? I also figured out the initial problem which was that the "series" fx loop was engaged w/ no effect plugged in, thus killing the entire signal of the amp... a dumb design in my opinion. they should've used a shorting jack in there.

thanks for help!
oh and any ideas on why the PT measured that way?

01-08-2008, 07:19 PM	#1
lowell Senior Member Join Date: Jun 2007 Posts: 1,125	No voltages - p transformer? Hey all, got a gibson class A tube amp here... filaments light up, no sound... NOTHING. measured voltages and there is NONE. disconnected p transformer secondary and the black reads 81VAC and red reads 177VAC... filaments are reading 14VAC. My feeling is that this is far from right. Can someone tell me how to test the p transfomer? It isn't blowing fuses so most info i've found regarding testing PT is of no help. thanks1

01-08-2008, 08:22 PM	#3
tboy Site Janitor Join Date: May 2006 Location: Behind The Zion Curtain Posts: 309 Member's Picture Albums	What model is the amp? Have you checked the rectifier? __________________ -tb

01-08-2008, 08:46 PM	#4
lowell Senior Member Join Date: Jun 2007 Posts: 1,125	thanks for help guys... but how can it be the rectifier if I have disconnected the secondary? amp is a Gibson Super Goldtone Last edited by lowell; 01-08-2008 at 09:51 PM.

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01-08-2008, 08:18 PM	#2
52 Bill Old Timer Join Date: Jun 2007 Posts: 1,244	Right voltages or not, if you are reading ac output from the transformer, then it is working enough to not be the problem that you describe. I'd look at the rectifier and power supply parts first.

01-08-2008, 10:54 PM	#5
52 Bill Old Timer Join Date: Jun 2007 Posts: 1,244	I'm sorry, but what are you asking here? The rectifier converts the ac output from the transformer to a dc voltage to run the entire amp on. If you measured no voltage, but the filaments did light up, then the amp has no DC voltage, which comes from the rectifier (be it tube of solid state diode). You measured ac output from the power transformer (that's why the filaments light up), so it appears to be working. Hook the secondary windings back up and check the rectifier section of the power supply.

01-08-2008, 11:08 PM	#6
lowell Senior Member Join Date: Jun 2007 Posts: 1,125	i guess my question is... why is the black secondary wire ony 81VAC and the red secondary so different at 177VAC and is this normal to have such a huge difference? I will try testing the rectifier. Also, shouldn't the secondaries of the PT be in the order of at least 200+VAC? There are 4 EL84 in this amp.

01-08-2008, 11:49 PM	#7
52 Bill Old Timer Join Date: Jun 2007 Posts: 1,244	According to the schematic there should be 340 vac from the red wire to the black. How are you reading the voltages?

01-09-2008, 01:59 AM	#8
lowell Senior Member Join Date: Jun 2007 Posts: 1,125	ok... now the amp works... hmmm... don't know what happened there, but I reconnected the PT to check rectifiers and all was fine...??? I also figured out the initial problem which was that the "series" fx loop was engaged w/ no effect plugged in, thus killing the entire signal of the amp... a dumb design in my opinion. they should've used a shorting jack in there. thanks for help! oh and any ideas on why the PT measured that way?

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