Voice of Music Power Transformer Question

[Platefire]

I am using a VM PT that powered a 12ax7/6L6/5Y3 tube power section in a build for a AA764/6l6 Champ.

I can't find the correct VM schematic for this PT. I have located my heater voltage on a green and green/yellow secondary taps. There is another yellow/blue wire that I'm wondering rather it is a center tap ground for the heater wires? I was planning on using two 100 Ohms to chassis ground to ground the heater wires but not knowing if it has a heater center tap leaves me not sure on how to proceed. My questionsis as follows:

1-I have a schematic to another model VM 730(not mine) and it has a heater center tap. If the yellow/blue is a center tap on my PT---how would I determine if it is???? Should have "0" voltage when PT is live right??

2-Will it work OK if I just put the 100 Ohm to ground on the two heater wires if I can't determine if yellow/blue is a center tap?


Second Issue: On biasing the 6L6 power tube. Instead of just sticking a 470 Ohm cathode resistor in like is used on a 6V6/AA764, I'm thinking I should probably see what the final plate voltage is loaded and do a current calc and adjust the cathod resistor based on the finding up or down. I think you subtract cathode voltage from the plate voltage to do calc. About what mA should Idle be?

Plate

[HotBluePlates]

If the yellow/blue is a center tap on my PT---how would I determine if it is???? Should have "0" voltage when PT is live right??

Without applying power, why not do a continuity/ohms check to see which wires are on the same winding? Some lowish resistance will indicate that a group of wires are on the same winding. If you guess right about a wire being a center-tap, you should see similar resistance to each of the ends of that winding; if you guess wrong about your reference wire and it is an end instead of center, you should see a resistance to one other wire and double/half the resistance to the other wire.

The resistance check should be your first plan of attack, to see what is connected to what.

If/when you apply power, you will not see "0 volts" on anything necessarily. A center-tap is only at 0v because we attach it to our  reference voltage. In other words, but both the red and black lead on the same wire (any wire) and you'll see 0v between the meter leads.

For a 6.3vac winding with center-tap, if you hook your black lead to the actual center-tap and measure to the other 2 wires, you will get 3.15vac at each of those wires (higher obviously due to no load). If you guess wrong about the center, you'll measure 3.15vac at one wire and 6.3vac at the other; that will tell you that you were actually using an end of the winding as your reference.

Will it work OK if I just put the 100 Ohm to ground on the two heater wires if I can't determine if yellow/blue is a center tap?

Sure it will. But I'm confident that if there is a center-tap that you'll be able to locate it.

Instead of just sticking a 470 Ohm cathode resistor in like is used on a 6V6/AA764, I'm thinking I should probably see what the final plate voltage is loaded and do a current calc and adjust the cathod resistor based on the finding up or down.

Sounds reasonable to me.

I think you subtract cathode voltage from the plate voltage to do calc.

Very true. The important thing is plate to cathode voltage, as that is what the tube feels.

About what mA should Idle be?

No clue without knowing the supply voltage. From there, you'd decide on an idle dissipation and estimate a bias voltage. Then you could figure the bias resistor using ohms law by knowing bias voltage wanted and idle current. You could refine your guess with idle screen current, etc, but you'll probably have to round the calculated bias resistor anyway to be an actual standard value.

[Platefire]

Got ya, HBP

Yeah you never think about these things when you install a PT where everything is known--you just hook it up and move on. I didn't pull this PT out of the original equipment, or I would have noted everything while it was still connected. Everything else was standardard wiring color codes except the heater wiring. Thanks! I haven't got the power cord installed yet, so I'll go give the continuity check a try.  Plate

[Platefire]

On a 200 Ohm's range setting, I get a consistant .3 Ohm reading between the green and green/yellow.  Also using my continuity beeper on my meter, I get a continuity "beep".

When I do a Ohms check on same range, I get no reading between the green and yellow/blue or between the green/yellow and green/blue.

I will check actual voltage when I get a power cord hooked up and connected. From these findings it appears I need to proceed with my plan to hook the 100 Ohm resistors to ground on the heater wires. Platefire

[HotBluePlates]

What are all the wires on the PT? Meaning what color codes and how many?

It sounds like the green and green/yellow are on the same winding (possibly 2 ends of the same winding) given no other information. Is there a blue wire along with you green/blue? I wonder about that being another heater winding.

And old, old color codes were different than simply "old color codes". My thinking is that they may have had a solid color and a that color with a tracer to denote 2 ends of a single winding.

[Platefire]

There is a Green, yellow/green, yellow/blue and a gray or faded blue. Using the 200 Ohm range I get a consistant .3 Ohms reading between the green and yellow/green and also get the same reading between the yellow/blue and the blue. These are old cloth covered wires and the colors are faded pretty bad.

When I try to cross the Ohm test between the Green & yellow/Green and Blue & yellow/blue I get no readings that there is any connection there in any combination of hookup.

As far as the total wiring:

primary=two yellows, red and black.

Secondary=red/yellow, Green, yellow/green, blue, yellow/blue, Red and Red.

This is odd because normal PT have the yellows to rectifier on the secondary side,  however I did hook this PT up previously to 120 VAC wall voltage and a 5Y3 and a 5U4 rectifier just to test it befofore I started the project. I got an unloaded 400 VDC at pin 8 on both rectifiers. I also discovered the 6.3 to 7 volts accross the green and yellow/green. I hooked it up to a pilot light and let it run a while unloaded.

So do you think the yellow/blue and blue might be a serperate heater taps? I was also thinking that the unloaded ouput B+ should be higher for a 6L6---because it will come down substantually when loaded. Plate

[dynaman1]

You listed the yellow wires as being "primary". Every PT I've came across uses yellow for 5V.

[Platefire]

Yes, me to. That's why I said it was odd. But they are coming out the same hole in the PT as AC wall voltage taps (Red & Black) and are suppling 5V to the rectifier. Plate

This will be my last time to accept transformers from somebody else to do a project but at this point I just want to get it hooked up and working--finished and shipped off! 

[sluckey]

But they are coming out the same hole in the PT as AC wall voltage taps (Red & Black)

That was probably just a convenience and doesn't mean anything.

Based on what you've said so far, primary is probably red and black but you need to verify continuity It won't be zero ohms. Two yellows are probably the 5VAC winding. Red , red, and red/yel are probably the HT winding with center tap. Green and green/yellow are probably 6.3VAC (but need to be confirmed). Blue and yel/blue are another separate winding, probably another low voltage winding, but need to be verified.

I would group the wires into separate bundles, ie, red, red, red/yel as one group, yel, yel as another group, etc. Strip about 1/4" insulation from each wire and separate so no wires touch any other wires. Connect the VERIFIED primary wires to the wall outlet and measure voltages for each group.

[PRR]

> they are coming out the same hole in the PT as AC wall voltage

A tube-amp PT has 2 primary leads and 5 or 6 secondary leads.

It might be logical, even safer, to segregate pri and sec in separate holes.... but that gets crowded in one hole while the other is spacious. So they put the leads where they fit nice, not by "logic".

[RicharD]

If you are uncertain of the primary leads, pick the pair you think are primaries and bring it up slow and unloaded on a variac with meters on the suspected secondaries.  If you don't have a variac, see if you have a 12V transformer and put that between the wall and the suspected primary leads.  Your secondaries should read .1x the off load voltage.  This should protect you in case you are mistaken.  I once hooked 120V into 5V windings.  It was unloaded but still made one helluva racket.

[Platefire]

I'm in the proces of wiring/soldering up all the intital power supply up to standby switch. After all, I have done this before to make sure the PT was working OK. Plate

[Platefire]

Hooked up the power cord, fuse/fuse holder, pilot light, power switch, standby switch, PT, rectifirer and heater wiring to a terminal strip with two 100 Ohms to ground---run wires to pilot light.

I did a unloaded check:

Purple pilot light lit up!

5Y3 rectifier pin 8=380VDC (quite a bit lower than the 400VDC I got first time I tested it)

Heater= 6.9 VAC (is it suppose to be a little high unloaded?)

The other Yellow/blue and blue wiring read 7 VAC. I capped these off and moved them out of the way.

Guess I'm good to proceed with the rest of the amp! Platefire 

[Shrapnel]

Heater= 6.9 VAC (is it suppose to be a little high unloaded?)

The other Yellow/blue and blue wiring read 7 VAC. I capped these off and moved them out of the way.

Guess I'm good to proceed with the rest of the amp! Platefire 

Unloaded, the 6.3 just may be a little higher. It sounds like you may have a second 6v heater tap there. If your heater voltages take too much of a dive just using the one, I'd fall back on that one to run some of the heaters.

[Platefire]

Well I finished putting this one together today and fired it up. Sound OK, not quite as loud as I hoped but when your in the pocket of using someone elses tape recorder transformers, you do the best you can. I won't agree to do that again! Supposably the Voice of Music transformers powed a 6L6, I was told. So I got a 6L6 in there and here are the readings:

Heater=6.33
from 5Y3=380 VDC
At 6L6 Plate=361 VDC
Cathode=29.5 VDC
To Screen 470 R=373 VDC To Screen=372 VDC
to preamp plate resistors=350 VDC
to Preamp stages=232 VDC

Do I need to install a one Ohm resistor to pin 8 or is there another way to determine correct bias? Cathode resistor in 470. At this point I have a AA764 circuit that was designed for a 6V6 using transformers from a VOM reel to reel that the audio used a 6L6. The preamp voltages are a little high, schematic says 200VDC. Request help/suggestions regarding preamp voltatge and bias resistor value. I'VE BEEN LOOKING AT THE FORMULAS BUT NOT SURE HOW TO GET THERE?


Cathode Current=Cathode Voltage/Cathode Resistor Value(29.5/470=.068)
Screen Current=Voltage drop across Sreen resistor/Screen R Value(1./470=.00212)
Plate current=Cathode Voltage/Cathode Resistor Value-Screen Current=Plate Curent in Ma(29.5/470=.068-.00212=.065mA
Plate dissipation in Watts=(plate voltage-cathode voltage) x Plate Voltage(361-29.5=331.5)

Plate

[RicharD]

Plate dissipation in Watts=(plate voltage-cathode voltage) x Plate Current
331 * .065 = 21.6W 

[HotBluePlates]

331 * .065 = 21.6W

If that's a metal 6L6, or a 6L6GA you're a little high but not bad. If it's any other kind of 6L6 variant, you're golden. You could bias hotter, but why? You probably are already getting 90% of the power output you're gonna get.

[Platefire]

Great! and thanks. It's a SoVtek 6L6WXT. I've attached some photos. It's to be mounted in a Dr Z 1-12 cab. I worked out the mounting with the customer and am shipping the amp with mounting bolts and he will install it in cab. It will be mounted in cab with tubes down, so the view shown is upside down. Notice in one picture I have the end of a toothpick stuck in the turrets for the 6L6 cathode resistor to hold them in place until I determined if it was right. Guess I will go ahead and solder it up. Platefire

BTW-This was a 100 watt chassis for another amp that I cut down and patched the remaining holes---thus the plastic plugs and metal cover patch.   

[Platefire]

Some more shots

[HotBluePlates]

Pretty cool little amp!

It looks like a blackface Champ with a 6L6 for the output tube. If you're getting 7-8 watts out of it, you're already ahead of the game.

It oughta be a fun platform to play with. And if it pushes some efficient speakers, it'll make plenty of racket!

[Platefire]

I've been jamming on it a bit on several different speaker cabs. It sounds pretty good but it just don't have the volume and clean head room it should. My 6L6 5F1 is louder. I suspect the OT. It has three secondary wires (Green, Black, yellow) and I wasn't sure about hookup, so I looked up a Voice of music schematic on the net for I think a "710' reel to reel. On that schematic the green went back to the driver tube grid and yellow and black to speaker. So I capped off the green wire and sent yellow to + and black to - on speaker jack. Not sure if that green hookup was like an ultra-Liner or what? I double and triple checked all the component values and hookup and I pretty sure the AA764circuit is right! The PT seems to be doing its job--I suspect the OT. Plate