Power Transformer Specs

[TomCarlos]

Ok... time to understand PT Specs...

I have a PT for a Fender Amp, part 036958. This is a common PT and there are several replacement options.

The thing I cannot understand are the "specs" vs actual voltages you would measure with no load on the secondary windings.

Take a look at this schematic for the "Classic Tone' PT:

http://www.classictone.net/40-18015.pdf

On the secondary, it shows Brown: 54vdc, Red: 388 vdc, Green: 6.3v (and we know that is volts in AC).

Take a look at the Hammond PT:

http://www.hammondmfg.com/jpeg2/290UX_B.jpg

This is supposed to be a direct replacement. Yet, the transformer says Brown 39v, Red 440vdc, Green 6.6v.

I just purchased P-TF36958 from Antique Radio and Supply.

Antique Electronic Supply

The ARS transformer has similar specs to the Hammond.

QUESTION: Why is it that the transformer DOES NOT list the actual AC voltage that will be read across the secondary windings with no load?

I am reading: Brown: 41.5vac, Red: 341vac, Green: 7.2vac

Can anyone explain this?

Thanks in advance,

Tom

[R.G.]

This is a point that always irritated me, too.

What's really going on is that the process of converting AC to DC and filtering carries some subtleties, and it's easy to get wrong. Further, if a tube rectifier is involved, the losses and final DC voltage are even less clear. The power capability of the transformer, the internal resistance, the rectifier internal resistance and cap values all enter into it.

Since most of the time a tube style power transformer is wanted for a circuit where the details of the rectification and filtering are the same, the makers skip a step and tell you what you'll get out - they think - not the steps along the way.

For instance, 39Vac gives a peak voltage of 39*1.414 - 0.7 = 54.446V peak on a filter cap for the bias supply, which would sag very little under the light load of a bias. The heater supply is heavily loaded, and sags a lot, so 7.2Vac unloaded will sag down into the 6.3V +/-10% range under a "typical" load, which the maker assumes.

This style of specification is good for the only semi-technical repair guy who just wants a transformer to go in an "X" box, but hard on people who do a modest amount of calculation.

[TomCarlos]

You got it RG....

I figured that someone was converting ac to dc BUT not including the exact method of rectification (Half, Full, etc). It is annoying!!

Tom

[Enzo]

Half wave or full shouldn't matter. A bias supply is usually half wave, because it has to provide very little current, so it is easy to filter it clean. Higher current supplies use full wave because the filter caps then only have to hold up 1/120 of a second rather than 1/60 of a second. But either one will charge up to the peak voltage of the AC...more or less.

[Bruce / Mission Amps]

Not that it matters much but if you look at the spec sheet from MagComp... Mark clearly states that it is used with a FWB rectifier and a 44uF main filter cap... and it looks like the transformer is rated at the 388vdc under the stated 200ma load.
With respect to the Hammond trannys... there was a time when you had to use 1.04 to 1.07 times their rated AC output to get the real voltages because they were wound with 115vac primarys in mind.
I have not used any new Hammond PTs in over 6-8 years so I don't know that they still do that, but I wouldn't be surprised.

[MWJB]

Tom you are never going to use the PT in an amp with no load on the secondaries, therefore the only voltages that really make any sense are the "loaded" voltages. The reason that the PTs from differing manufacturers differ is because...well, they are different products, wound to the mfr's preference. The "036958" only referes to a PT suitable for the amps listed (~40W, 2x6L6/5881, 3 to 6 x12AX7), Fender transformer part numbers have never really correllated to any specific voltages, as during many amps prouction runs they kept the same part number, but operating voltages & even manufacturers changed. Any of these specs are nominal in any case, not accurate to +/- 1volt.

If yours is the Fender part then running a pair of 6L6/5881, moderately biased & 3-6 preamp tubes you might expect to see 425-445vdc on the power tube plates, with a FW bridge rectifier, at idle.

[TomCarlos]

MWJB...

Exactly. The transformer in question came from a Fender Deluxe Hot Rod. It has 2 - 6L6 and 3 - 12AX7 tubes.

My background and experience is primarily with solid state circuits. So much of what I learned and encountered while working on this amp (my first experience with fixing a tube amp) was new. Yes, I was able to fix the amp (thanks to the friends on this site and their previous postings).

I understand your point about measuring test points under a loaded condition. That makes perfect sense. But I simply wanted to make sure the PT was an equivalent replacement BEFORE connecting to the amp's circuitry. And reading the label on the PT made no sense at all.

Tom