Power transformer voltage question

[bmacrae]

Newbie here, bear with me...

I'm rebuilding an old Nivico (JVC) tube amp from the 1960's. I've never been able to get much output from it. Right now I get nothing. Nothing is getting hot, tubes all glow (all are new), there's no redplating, just no output. Output transformer appears fine (tested against a new one).

The primary leads from the transformer to the 6CA4 rectifier socket are reading 216VAC at pin 7 and 124VAC at pin 1 (powered on with the tube removed). These two voltages should be the same, correct? I'm flying blind without a schematic but the amp is similar to an 18W Marshall (2xEl84's, 6CA4, and a 12AT7).

The secondary heater voltage is right at 6.4VAC.

Thanks ahead of time - Brendan

[olddawg]

Newbie here, bear with me...

I'm rebuilding an old Nivico (JVC) tube amp from the 1960's. I've never been able to get much output from it. Right now I get nothing. Nothing is getting hot, tubes all glow (all are new), there's no redplating, just no output. Output transformer appears fine (tested against a new one).
The primary leads from the transformer to the 6CA4 rectifier socket are reading 216VAC at pin 7 and 124VAC at pin 1 (powered on with the tube removed). These two voltages should be the same, correct? I'm flying blind without a schematic but the amp is similar to an 18W Marshall (2xEl84's, 6CA4, and a 12AT7).
The secondary heater voltage is right at 6.4VAC.
Thanks ahead of time - Brendan

The secondary AC is pretty low for an 18 watt Marshall. They are around 290-300. Here is what I would do if you do not want to find a new PT. Convert it to an SS rectifier with a sag resistor. That will bring your B+ up significantly. Refer to the Lite IIb schematic at 18watt.com. Use 12AX7s for the PI and preamp. The Lite is a very simple single channel 18 watt Marshall variant that is perfect for this conversion. It has plenty of gain. 2 X 12AX7 and 2 X EL84. Using the SS rectifier will also give you more heater current available and I doubt that you will be able to tell the difference between a tube rectifier and an SS with a sag resistor in an 18 watt. Oh, use new caps and meter out any old resistors. Personally I would get new ones. They're cheap.

[Gtr_tech]

I'd check for DC on any gain stage grids. Leaky coupling caps are common on that real old stuff....'specially if they're the paper type.

[bmacrae]

Thanks for the replies...

Actually, I've already swapped out all of the caps and pretty much everything else at this point. I'd like to just get something out of it and judge the sound before I decide to change anything significantly. It already uses a 12AX7 PI, a 12AT7 preamp, and even a 12AU7 for the tremolo circuit.

In your opinion is my PT toast?

[tekhed7]

Well... If you put a Known good 6CA4 rectifier tube in, fire it up & it's filament is glowing O.K., what kind of DC voltages are you getting throughout the amp? At the O.T. red lead? At the output screens? At the preamp tubes' plates? (Be CAREful w/ those measurements, 'Charlie', esp. if you're a "newbie"
If there's hardly ANY voltge anywhere, you might suspect an open power resistor or shorted filter cap in the power supply somewhere.
Going w/ silicion diodes for the H.V. rectiying in such an old amp can stress the already tired filters, not to mention a turn-on surge that can possibly pop a fuse. Stick w/ the 6CA4 until you've got it running. Then you can replace the filter caps later if the thing "sounds" like it's worth restoring.

[pontiacpete]

Have you tried disconnecting and testing the PT?

[guitician]

With all tubes in what is at pin #3 of the 6CA4? That mis-matched secoundary voltage may mean a fryed winding. But with a DC output voltage you should hear something if the pre-amp tubes are getting enough voltage.

[bmacrae]

Well... If you put a Known good 6CA4 rectifier tube in, fire it up & it's filament is glowing O.K., what kind of DC voltages are you getting throughout the amp? At the O.T. red lead? At the output screens? At the preamp tubes' plates? (Be CAREful w/ those measurements, 'Charlie', esp. if you're a "newbie"
If there's hardly ANY voltge anywhere, you might suspect an open power resistor or shorted filter cap in the power supply somewhere.
Going w/ silicion diodes for the H.V. rectiying in such an old amp can stress the already tired filters, not to mention a turn-on surge that can possibly pop a fuse. Stick w/ the 6CA4 until you've got it running. Then you can replace the filter caps later if the thing "sounds" like it's worth restoring.

Well, I'm a newbie to this list but I have taken classes in ac/dc circuits (many years ago). I also worked for an electrical contractor for a time so I have a healthy respect for the voltages involved.

After poking around, the voltages appear good. Through the filter filter caps, to the power tubes, out of the power tubes, b+, etc. I'm starting to think it's not a power trans issue but a signal path issue. There may be something something shorting to ground there that's killing the output. I went through checking continuity to ground but haven't found it yet. I'll just have to keep looking.