Transformer quandry

[Randall]

A studio customer who torments me by bringing me all manner of things not guitar or amp has dropped a new unwanted challenge on me. He has two vintage tube limiter devices, one is a expensive Federal AM-864, and the other is supposedly a clone that someone built in a chassis for who knows what. The clone works, but the PT is noisy and can be heard by the mics in the room. He wants me to replace it, and pointed me towards this schematic.

http://www.preservationsound.com/wp-...-864_Schem.jpg

My problem is, the schematic doesn't match the readings I am getting from the one that is in it, which is I had to guess, I would say is original to the chassis. As you can see the schematic calls for 175V at 70mA. First dumb question, that means each red wire measured to CT, correct? The one that is in there now measures 677vac from red to red, or pins 4 and 6 on the rectifier tube. I measures 336vac from each red wire to the CT. This is with the 5Y3 in socket.

So, what gives? At first I was trying to find a tranny that would be close enough to the schematic, but was having trouble finding one with secondary voltage as low as that, that also had both filament windings. Here is the Hammond page I am working from:

https://www.hammfg.com/electronics/t...00?referer=787

So I am confused as to whether I should follow the schematic, or copy what is in there now. It really just makes things difficult when you have to try to figure what has been done with no accurate map.

What would you do?

[Enzo]

OK, so you have a working clone? ANd a working original? What voltages are in it?

The clone is the noisy one? It works, but his only complaint is that the transformer is noisy. SO it seems to me we should stick to the voltages in there now, since that is what it has been working with.

Are we talking mechanical noise, rather than something coming out the speakers? Perhaps can be fixed? If the bobbin is loose, a shim can secure it, if the laminations are loose, perhaps snug up the corner bolts. Got a copper band, could it be buzzing?

[Randall]

I don't have the original, it is in use at his studio. The noise is mechanical. I tried everything to stop it to no effect. It is actually loudest at the hole at the underside that the wires come out of. It is too close to the rectifier tube to wrap it, which I tried. I don't want to burn his studio down. It is situated close to the chassis, so pulling that bell cover would mean pulling tranny and reinstalling, then repeat. Hoping not to have to do that.

[Enzo]

Fair enough, but it remains that the thing works fine as it is other than the transformer making noise. SO that tells me regardless of the schematic, whatever voltages it has now seem to meet his approval.

[Randall]

Enzo so given that logic, which transformer from the Hammond page I posted would you go with?

[Enzo]

Offhand I'd be attracted to the 620v smaller one 272X.

Of course that assumes it will physically fit.

[Randall]

Please clarify something that I always forget. On the schematic it shows 175vac on each of the secondary red wires. This is to CT, correct? And the two combine for 350vac from red to red, correct?

[Enzo]

Yes, that is why it is there twice, once between the CT and top wire and once between the CT and bottom wire.

[pdf64]

It would measure 350VAC between pins 4 & 6 of the 5Y3. The 350V would still only need to have a 30mA rating, because current will never flow through the whole winding, only between CT and one leg, then the other, with each half cycle.

[Randall]

Well now, as I get deeper into this hole. I got the Hammond 272X PT and as I was installing it, a bit late into the game, I realized the original has an extra wire that the new one and the schematic does not. It has the 5v winding, the 6.3v winding, primary winding, and secondary winding with CT. But it also has a ground wire that was grounded to chassis ground. I ran my new CT to ground as per the scematic, but did not yet power it up for fear that this extra wire was a potential problem.

It now appears the old CT was run to three 10 watt power resistors in series on strips. They are 150R, 62R, and 175R wired in series to ground. Is this why this larger PT was working in this circuit? Should follow and run my CT there as well?

https://www.hammfg.com/files/parts/pdf/272X.pdf

[pdf64]

Is it just a string of resistors that nothing else connects to? If so, they could just be dropping unwanted voltage.

[J M Fahey]

My crystal ball tells me a couple things :

1) the original schematic , a FACTORY drawn one, from a SERIOUS Company is right.

2) I BET it matches the original AM864 to a T, the one which is successfully working in the studio-
Why am I not surprised?

3) the clone was built/repaired by a NO CLUE Boutique/Guru type builder.
Why am I not surprised? [2]

He probably saw/measured a 350V secondary (2 x 175) and shopped for a "350V transformer"
He got one ... a "full wave" 350+350VAC one that is.

Mr No Clue must have been quite surprised (the understatement of the year) at the HUGE voltages found, and he desperately tried to pad those down, hence the 3 x 10W!!! resistors in series with center tap.

I bet everything inside that poor chassis must be suffering under the brutal voltage onslaught, including the PT itself.

Obvious solution is ordering the proper 175+175 VAC one.

Not available off the shelf?
Order a properly wound one.

EDIT: from that list,closest is 269EX , 380V CT
https://www.hammfg.com/part/269EX?referer=1052

YouŽll need an extra 5V filament transformer.

Or, again,have a custom wound one, matching original (schematic) spec.

Not your typical Tube amp, but if accepting Studio equipment jobs, deal with it.

[Randall]

Well that's all well and good, but what I am really asking is should I hook this PT center tap to these 3 10 watt resistors to ground? The original transformer I believe is original to this chassis, judging from the footprint it left when I removed it. SO I think the builder just made it work. Customer says it sounds great, the problem was that the PT made enough mechanical noise to interfere with studio mics. This new PT is lower secondary voltage than the original, so I am wondering if connecting the CT to the three 10 watt resistors will provide a lesser secondary voltage, which I suppose it will. It also has two primary taps, 125v and 115v. It is now connected to the 125v tap. If after connecting the CT to the 3 resistors to ground and the voltage at pins 4 and 6 of the rectifier are low with regard to the schematic, can I use the 115v tap to raise the voltage if I need to?

[g1]

I don't think you should change the primary tap if your HV is lower, change the number of resistors there as required instead. Changing the primary tap will mess with heater voltages, possibly stress the transformer, etc.

[Randall]

OK, so I connected the CT of my new Hammond PT with the lower secondary voltages from the original to the three 10 watt resistors to ground. I measure 661vac across the rectifier pins 4 and 6, and I get a B+ of 358Vdc. This is with all tubes installed. I wired up the original PT on the bench, and with only AC input and secondary hooked up, I get 695vac. This is with no CT connected. So it appears the new PT is running a bit lower voltages, but still seems very high to me. Is there something else I should check?