Transformers - What do I have here?

[lpresnall]

Got these transformers from a Peavey Classic 30. Buddy wants me to try a Marshall type circuit in the gutted chassis. Tried Googling for Peavey specs and the best I could find was Mercury Magnetics specs on their replacement transformers. Their colors are different so I'm asking the gurus here - what do I have here and can I acheive my goal?

[Phil_S]

It would help to know something about the donor amp. Was it a tube amp? If so, what was the tube line up? I don't know much about Peavey but I hear they are generally responsive to inquiries. Getting an answer will get you there mo' better faster.

I'll guess it was a 30W 4x EL84.

For the PT, it looks like you have two secondaries, neither of which is center tapped. I'll assume the red are the high voltage secondary and the yellow are the filament secondary. Since there is no center tap, you will need to use a full wave bridge with silicon diodes. For starters, I'd rig up a fuse to one primary leg and plug it in to the wall supply (adding the appropriate line cord). Then your can meter the VAC output on the secondaries, which will help assess what you have. Post results. Include the wall voltage. The unloaded voltages will be higher than in-circuit.

For the OT, it looks like a push pull type transformer with 2 secondaries. I am going to guess 8 and 16 ohms. Again, you'll need to test if you can't/don't get specs from Peavey. I use this method for finding unknown impedance. http://www.radioremembered.org/outimp.htm Since you have the PT, you can source low voltage from the filament supply.

[martin manning]

Since the Classic 30 is a 4x EL84 amp, I'm guessing the OT impedance is ~4k, and the plate voltage is ~300. The OT would work for a pair of EL34's, but if the plate voltage is only 300V, it will not be a typical Marshall. It could certainly be made to work at 300V, and I think the PT current required running 2x EL34 will be a pretty close match to the 4x EL84 in the Classic 30. Measure the PT secondary voltage as Phil suggested, and the OT for impedance.

[Phil_S]

Thanks for the tubes, Martin. On that basis, I'd go out on a limb and say those transformers can be used for the traditional 18W, TMB18W, or 36W (4x EL84 versions). You can also use 6V6's. These will give you the plexi vibe. I would not try for a pair of EL34 because lower plate voltage will not give that sound. Schematics and layouts are available at 18watt.com.

[Cygnus X1]

Since the Classic 30 is a 4x EL84 amp, I'm guessing the OT impedance is ~4k, and the plate voltage is ~300.

Hard to get past the B+ issue but wouldn't a 6L6GC be better suited for a 4k primary?

[martin manning]

Looking at the photo of the PT I see what looks like 278 written next to the secondary leads... that would say a rectified, filtered B+ of somewhere around 330-350 is available. If the OT is 4K, then it would have to be a 4x 6L6 or EL84, or 2x EL34.

lpresnall, if you can get the DC resistance of the PT's HT secondary, and confirm the unloaded VAC, we could make a good guess at it's current capability.

If the OT secondaries are 8 and 16, it could be used at 8K with the taps relabeled 4 and 8, of if they are 4 and 8, they can become 2 and 4.

[martin manning]

Since the Classic 30 is a 4x EL84 amp, I'm guessing the OT impedance is ~4k, and the plate voltage is ~300.

Hard to get past the B+ issue but wouldn't a 6L6GC be better suited for a 4k primary?

2x 6L6 would be a good match for the impedance, but the original 4x EL84 had combined max plate dissipation of 50W, where 2x 6L6 would be 60W. 2x EL34 would be the same as the 4x EL84 at 50W. I'd like to have the PT VAC and DCR, as well as the OT impedance data.

[lpresnall]

Arrrrrggghhhh! You guys are gettin' waaay over this guy's head! My trade is flyin' jets which is far less complex than all this 'lectrical talk! If you can direct my steps I'll get this info and post it! Thanks, you folks are awesome!

[Reeltarded]

hahahaha

Dood, just get the VAC and the DCR and the NAACP and NRA while you are at it you might want to check the RCMP and the NAMBLA and stop messing around. rofl



Get the unloaded power, meaning hook the PT up to 120 and check the pair that go to the rectifier.. THEN TURN OFF THE POWER then check resistance across that pair.

Simple. You fly jets? Awesome!

[martin manning]

OK, pretty simple really:

For the PT

First measure the DC resistance across the red wires on the PT.

Then rig up a power cord to the PT's primary, plug it in, and measure the VAC across the secondaries, red wires and yellow wires. I like to separate the secondary leads by an inch or so by putting them between two strips of masking tape, which you can then tape down to your workbench so they don't accidentally touch anything. Be careful, the red wires are high voltage! The yellow wires should have about 6.5-7VAC on them.

Post the DCR and the unloaded VAC for the red wires. There is a correlation that I'll dig up to estimate the PT secondary current capability.

For the OT

Strip the ends of the red, blue, and brown primary leads and separate them with tape, if necessary.

Connect the yellow wires from the PT to the secondary of the OT (black, green, and white). I suspect that the black is the common lead, so connect one yellow wire to the black lead and the other yellow wire to one of the others. Tape off the red leads on the PT while you're doing this; they aren't needed.

Power up the PT and measure the VAC across the secondary connections (yellow PT wires), and also the VAC across the blue and brown wires on the primary. Be careful here too, there will be around 150V or so on the OT primary, and a lot of current available on the yellow leads. Also measure the VAC from the red to blue, and red to brown on the OT primary, to confirm that they are equal and about half of the blue-brown voltage.

Power down and move the yellow PT wire from the not-black OT secondary wire to the other not-black wire. Power up and measure the VAC across the secondary connections and on the red and brown primary wires.

Calculate the ratios of the OT primary VAC (blue and brown wires) to secondary VAC (yellow PT wires) for both configurations, and square them. Those will be the available impedance ratios, speaker impedance (ohms) to primary impedance.

[Phil_S]

Are you looking for this one, Martin?
For a 120 volt AC supply the VA rating and primary resistance is as follows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms

Really, though, I expect a phone call or email to Peavey will get accurate info.

[martin manning]

Hi Phil, Sometime back there was an article posted here "Rating Unknown Power Transformers." It's on TAG somewhere, but I located it on my computer.

[lpresnall]

Ok, I think I can follow all this when I hit town again Thursday. I will email Peavey as well. But since I need to know how to do all this for future reference I'll still take the readings. Are you guys saying to plug a power cord into the wall, and strip and stuff the blue 120? into the hot and the black 0? into the neutral to do the powered up test?

[Cygnus X1]

I use a variac for testing.
Scary to apply full power all at once.

[Phil_S]

Ok, I think I can follow all this when I hit town again Thursday. I will email Peavey as well. But since I need to know how to do all this for future reference I'll still take the readings. Are you guys saying to plug a power cord into the wall, and strip and stuff the blue 120? into the hot and the black 0? into the neutral to do the powered up test?

Basically, yes. Use wire nuts to secure your power cord pigtails to the PT primary leads. As Martin says, tap the leads to your bench so they don't move around. I use a surge protector between the wall socket and the PT -- not as good as having a real fuse, but it does offer an on/off switch and it should blow if there is a short.

If you don't have an old lamp cord with a plug on it, you can buy a cheap extension cord and slice off the plug. I'm assuming you are in the US...Target sells really cheap extension cords of very nice quality. I"m thinking $2.

Tape down the secondary leads, too, to keep them from flying about and doing unintended harm.

The whole thing should take you a minute or so once you screw up your courage. Beer is reward for after you do it, not to help you screw up your courage!