Power transformer wiring question

[Erik Aa]

Hi. I have a tube amp power transformer that I plan to use in a Bassman build. The power transformer is currently in a diode rectified amp with 4 el34s in it.

My question regards the secondary wirings. The PT has 3 HT wires (blue and white in photo), what I suspect are filament wires (white in photo), and then it has some fat red wires that go to a standby switch (red wires in photo). Since it is solid state rectification, I don’t understand the existence of both the red wires to the standby and the white filament wires. It can’t have two sets of filament wires? There is no tube rectifier so no need for a 5v winding…How do I deal with the red wires if I am to use this PT in a Bassman 100 build? Thanks!

[shooter]

the 2nd image has the schematic but it's way too small to read, maybe post the schematic for the donor amp, then I might be able to help, some

[Erik Aa]

Schematic does not exist to my knowledge I’m afraid.

[BrassElephant]

There's 2 things I would do.

Source a schematic for your specific amp. It should dictate what the secondaries are.

Then I would disconnect all secondaries and (carefully) measure each one while energized to confirm what the schematic says.

[HotBluePlates]

... The power transformer is currently in a diode rectified amp with 4 el34s in it. ...

What's the name of the manufacturer & model?

... The PT has 3 ... wires (blue and white in photo), what I suspect are filament wires (white in photo), and then it has some fat red wires ...

One Possibility:
   Red = High Voltage (to diodes on board; other red wires to/from Standby & power supply)
   White = 6.3v filament (all the tubes in the amp)
   Blue/White/Something-Else = Low Voltage supply for the various solid-state bits in the photos

[Erik Aa]

The amp is a Morgan GVH-100 solid tube amp. 100w.

[mresistor]

Maybe a long shot but have you tried contacting Morgan?    https://www.morganamps.com/contact/

[BrassElephant]

If you plan on removing it anyway, just disconnect all secondaries, gator clip meter leads and insulate/isolate/etc, then energize and take readings. Great care should be taken to ensure no bare leads touch anything but the meter leads. Very carefully set this up. Again, insulate/isolate/etc.

But that's what I would do. 5 minutes and you'll have the info you need.

[HotBluePlates]

Maybe a long shot but have you tried contacting Morgan?    https://www.morganamps.com/contact/

It's not that Morgan.

It's a very inexpensive (probably Chinese-made) amp with a fake western-sounding brand.

[sluckey]

Don't disconnect anything! Just measure the AC volts on the PT leads. Measure between the red wires probably high voltage). Measure between the big white wires (probably filaments). Measure between the small blue and white wires (probably low voltage for bias and/or any ss circuits).

[Erik Aa]

Ok, thanks guys. Yes, this is indeed not that Morgan. If I measure the leads and find that the white are filaments and the red are HT, can I just insulate the blue/white and disregard them in a Bassman build? Thanks!

[BrassElephant]

Don't disconnect anything! Just measure the AC volts on the PT leads. Measure between the red wires probably high voltage). Measure between the big white wires (probably filaments). Measure between the small blue and white wires (probably low voltage for bias and/or any ss circuits).

Fair enough. I just figured if they're gonna take it out anyways, might as well disconnect leads so the entire amp isn't energized/capacitors being charged. I tend to overthink things, even testing voltages.

Ok, thanks guys. Yes, this is indeed not that Morgan. If I measure the leads and find that the white are filaments and the red are HT, can I just insulate the blue/white and disregard them in a Bassman build? Thanks!

Yes, or possibly use them for negative bias if they're in the right voltage range

[Erik Aa]

I measured AC between the big red wires: 334v. Between the two small blue wires: 38,5v. The whites are the filament wires then. What du you guys think? Best, Erik

[sluckey]

Between the two small blue wires: 38,5v.

There appears to be a small white wire associated with those two small blue wires. Possibly a center tap. Measure AC volts between the small white wire and each of the small blue wires. What are the two measurements?

I'm counting wires on that side of the PT. I see two whites, two reds, two small blues, and one small white. Are there any more wires on that side of the PT?

[Erik Aa]

I pulled the PT off the variac, so the voltage readings increased a little. Between the two blue wires: 40,3v approx. Between either blue and white wire: 19,8v approx.

No more secondary wires out of the PT than the ones you describe and I point at in the pictures. Thanks!

[sluckey]

OK, here's what you have...

Red wires for B+. You must use a FWR rectifier. Unloaded B+ will be close to 500V. OK for 6L6s.

White wires for filaments. You never said what the actual voltage is but I suspect it will be fine for a 2x6L6 bassman.

Blue wires for bias. 40VAC is fine for EL34s but probably is not enough for 6L6s. However, you can use a simple voltage doubler circuit and have plenty of bias voltage for 6L6s.

I'd use it for a 2x6L6 amp.

[Erik Aa]

Thanks Sluckey! I will check the filament voltages too. Will it be ok for a Bassman 100 as well (4 6l6 tubes)?

Another newbie question: is it ok to power up the amp and take quick PT measurements with the tubes pulled but with no speaker load?

Is the danger to the OT with no load also present when the tubes are pulled? I read that pulling tubes may stress caps, but I will throw these caps away so I don’t worry about them, but I was worrying about the OT…Thanks again. 

[sluckey]

Should be fine for 4x6L6s.

It's OK to power up with no tubes. No stress to the OT or speakers.

I use caps with sufficient voltage rating to withstand the increase in voltage when tubes are pulled. Don't know what the Chinese do. And I have no idea what the normal B+ is in that amp. Do you?

[Erik Aa]

I did not measure B+ before I gutted the amp. Horrible pcb stuff. I will rebuild from scratch. Look at the crap in the pictures. The transformers are pretty huge though. Both PT and OT. Only amp I have with bigger transformers is an old Traynor yba-1a amp with those massive hammonds.

[Erik Aa]

I measured the filament voltage in the white wires now. It is only 5,9v.

Is this too low for 12ax7, 12at7 and 6l6 tubes? Should it be 6,3v?

Thanks!

[sluckey]

Should be OK.

[Erik Aa]

Thanks Sluckey. I took new white wire filament measurements with a different multimeter and now I got 6,4v readings. Apparently, I need to upgrade to more accurate multimeters. The ones I have are taking me for a ride.

[PRR]

> more accurate multimeters.

Batteries.

1) A meter with a weak battery will read off-kilter before it actually quits

2) A fresh carbon or alkaline "1.5V" cell reads 1.56V

[Erik Aa]

Thanks PRR. I will change batteries as well.