Vintage Power Transformers

[HotBluePlates]

... they ain't the same as modern ones.


We often hear gripes that a clone's supply voltages are higher than the schematic says.  Lots of reasons for that (including schematics that aren't 100% accurate on every detail).  But one reason seems to be higher winding resistance of the vintage power transformers.

Below are details of 4 Deluxe-style amps, two of which I own(ed).  In each case, the wall voltage was adjusted by variac to get approximately 6.3vac on the heater wiring.  This showed that by early 1964 Fender was using power transformers expecting 120vac (the blackface Champ/Vibro Champ is an exception that proves the rule).


6G3 Deluxe
Year: 1962
Part #: 125P17A
EIA Code: 606146
Wall Voltage:  117v AC
High Voltage DCR (Red to Red): 209.2Ω
AC Output Voltage: 325-0-325v AC
DC Volts (6V6 plates): 405v DC

Deluxe Reverb
Year: 1964
Part #: 125P23B
EIA Code: 606-413
Wall Voltage:  120v AC
High Voltage DCR (Red to Red): 220.4Ω
AC Output Voltage: 334-0-334v AC
DC Volts (6V6 plates): 394v DC

Deluxe Reverb
Year: 1965
Part #: 125P23C
EIA Code: 606450
Wall Voltage:  120v AC
High Voltage DCR (Red to Red): 122.3Ω
AC Output Voltage: 348-0-348v AC
DC Volts (1st filter cap): 442v DC

Deluxe Reverb
Year: 1979
Part #: A 014956
EIA Code: 606-908
Wall Voltage:  120v AC
High Voltage DCR (Red to Red): 125Ω
AC Output Voltage: 360-0-360v AC
DC Volts (1st filter cap): 432v DC


I don't know if the 1979 Deluxe Reverb has fading filter caps, a weak rectifier tube, or high plate current for the 6V6s.  I would have expected higher B+ voltage, but that is what the owner measured very recently.


6G3 Deluxe:
The first of these amps used the 125P2A power transformer noted on the schematic.  125P2A with 17th week 1961 date code.  However, by at least late 1961 these were replaced by the 125P17A (46th week 1961).  Quite a few 125P17A transformers carried a 35th week 1962 date code.  By at least early 1963, these were replaced with the 125P23B transformer that would be used into the blackface Deluxe & Deluxe Reverb amps.

The blackface Deluxe Reverb used at least the 125P23B into 1964, 125P23C by early 1965, and 125P23D by late 1965, and then switched to the 025130 power transformer by early 1966 as noted on the AB868 schematic.  Whew!!

IMO, Fender wouldn't keep having transformers made with different part numbers each year just to get a different number on the end-bell.  I had always figured there was some kind of difference among the models, though even now I don't know what all the changes might have been.

Hammond is the only replacement company I know of that provides DCR figures for its transformers.  But unfortunately, their "Deluxe Power Transformer" (290BX) references every chassis model from tweed 5A3 up to blackface AB763, and every Schumacher PT from the 1960 125P2A to the late-60s 025130.  The spec sheet shows 330-0-330v, but only 112.5Ω d.c. resistance for the high voltage winding.  That's almost half of my 1962 Deluxe or 1964 Deluxe Reverb amps, and well below even a 1970s silverface.


So when guys gripe that their clone delivers more B+ voltage than the schematic, winding resistance may be a factor.  Ditto when they say their clone (or another modern take) "sounds stiff" compared to a vintage amp.

[dwinstonwood]

Thank you for all that research HotBluePlates.

I'm not suggesting anyone would want to do this, but how might one increase the DCR of the B+ winding of a current production transformer?
Adding something like a small Triad 50 Ohm inductor between each transformer red wire and the rectifier?
https://www.newark.com/triad-magnetics/c-24x/inductor240ma-50-20/dp/91B0323
Or, just standard wirewound resistors?

Thanks.

[HotBluePlates]

I didn't state it above, but the transformer-winder might go for fatter wire & lower DCR to reduce copper-loss (self-heating) in the transformer.  In the parlance used then, this would improve the transformer's "regulation" (not "electronic regulation" but just less voltage-change with varying current).

I'm not suggesting anyone would want to do this, but how might one increase the DCR of the B+ winding of a current production transformer?

Say you have that Hammond 290BX and want to emulate my 1964 Deluxe Reverb.  You need to make 112.5Ω look like 220.4Ω.  So 220.4 - 112.5 = ~108Ω total, and 108Ω / 2 = 54Ω.

Hammond's spec sheet notes a rating of 138mA for the B+ winding, and 138mA² x 54Ω = 1.02w.

Use a 55Ω 3w wirewound between each PT Red wire and rectifier plate, and the Hammond PT will have the effective resistance of the vintage transformer.  B+ will be reduced slightly, though I suspect a softer attack or slightly greater sag at high volume will be the more noticeable result.

[dwinstonwood]

Thank you!

And, BTW, I really like Hammond iron. Not just the lower cost, but also for the wider range of choices. Their specs sheets also make it easier to model in Duncan's PSUD2.

In fact, they do offer a couple of PT's with dual secondary windings, one of which I'm using:

https://www.hammfg.com/files/parts/pdf/290D2X.pdf
https://www.hammfg.com/files/parts/pdf/290AX.pdf

[tdvt]

Never thought about this before, nor would I, if not pointed out, but this makes a lot of sense to me.

I am surprised it isn't already a more common topic. But definitely something worth fooling with.

Thanks for putting that out there.



 

[Jalmeida]

Awesome info. With British clone amps, you will also have variances due to US versus metric wire sizes for the windings. So getting DCR and winding ratios in US wire to match British specs is difficult.

[trobbins]

Perhaps Fender shopped around when ordering batches of PT's, so some subtle differences crept in with what winding wire was in stock or purchased in for the batch.

There may still have been a designer mindset of using thinner wire to allow alternate use of valve rectifiers (without the need for external resistance to keep within valve peak current specs), and back then they could 'manage' using thinner wire better, whereas now Hammond may get less production hassle with using thicker wire.