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Transformer wiring questions

Intro: Stephe's original BOM called for the EDCOR trafo (as our EU friends call them, and it's easier to type) which developed a short.

Note where the 50+ volt bias is coming off the 6.3 VAC winding.

Enter the Hammond 273BX which was wired differently (from Mouser.com's page)

Red/Yellow "High Voltage" is reference ground for Red/Red, NOT 50 VAC. 50 VAC comes off the GRN/YEL for Filament #2 and does go to ground in the bias circuit.

HERE is my question:

From the Hammond 374BX data sheet (https://www.mouser.com/datasheet/2/177/300-951115.pdf):

First: IGNORE Filament #1 (refer to PDF if any questions), there are only two LV outs on this model.

Second, the 50 VAC comes from between the two RED HV lines, Red/Yel goes to ground.

Third: YEL/YEL goes to rectifier heater, CT Yel/Blk not used.

On the 274BX the Grn/Yel line between the two GRN 6.3 VAC is for 50 VAC.

Does it (Grn/Yel) go to ground for the 374BX as reference ground for the GRN/GRN leads to the 6SN7 heaters??

If the answer is obvious, and yes, please affirm.

I've looked at several dozen different schematics for tube amps from Aiken, Robinette, Valvewizard to Stephe and many others and see a YUGE variation in how different amps/trafos get wired so this has me confused enough to post before I finish wiring what I've started.

Thanks in advance.

Norm

8 answers0 replies

8 Comments

NormB

Jan 31

Question; when is Ohm's law just a suggestion?

Power supply is wired, powered it up and I'm getting 478 +VDC at B+ on either feed.

Same voltage out of the rectifier tube + out.

In SOME schematics I've seen a 100 Ohm 10 watt resistor there (prior to any capacitors), so I ran some numbers and came up with a dropping resistor value of 240 ohms (478 to 430) at a nominal 0.201 Amps (what Hammond advertises the output as at the HV red leads). Took a pair of 470 ohm resistors (close enough) in parallel and clipped one side to B+ for one channel to compare.

Nothing. No difference, same voltage output.

It's been 40 years since physics, algebra, trig, calculus but this is simple arithmetic, not complicated derivitives or something, but NOTHING?

Should I put the resistor UPstream (at the filament B+ side of the rectifier) FIRST?

How/why should that make any difference?

Thanks in advance.

Skunkie

Jan 31

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Replying to

Unloaded, that's what you will see. Get the tubes drawing current and it will fall into place.

Skunkie

Jan 19

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There is no bias circuit, this is a cathode biased amp. To use these hammond transformers, you need two seperate 6.3V transformer windings for the 300B heaters to go to the rectifiers/voltage regulators. You can't run these off the same 6.3 winding, each tube needs their own.

"Note where the 50+ volt bias is coming off the 6.3 VAC winding." This is not what is happening. We are applying 50VDC to the center tap (green yellow) of the 6.3V winding. This is elevating the heaters for the 6.3VAC winding to the 6SN7 heaters by applying 50VDC on the center tap, created by a votage divider off the B+ DC power rail using the 357K/47K and the 22uf cap. The green yellow center tap wire is for applying 50VDC not AC coming out. Again there is no bias circuit on this type amp.

Ignore the violet 50VAC wire on the 374 transformer, you aren't using it. Red yellow to ground, the other two red go to the rectifier tube. The yellow wires go to reactifier tube heaters, ignore the yellow black.

NormB

Jan 19

Replying to

I have this from this website. It's the 374bx wiring which is just different enough that it's confused me.

Skunkie

Jan 19

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Replying to

This PSU schematic shows the changes with this different transformer. The only thing different between a 274BX and the 374BX is the extra primary wiring and the violet 50vac wire we aren't using.

NormB

Jan 19

Replying to

And THAT made all the difference.

That "extra" 50 VAC violet line and the arrow pointing TOWARDS B+ (not towards the CT at 6.3 lines for the heaters) really mixed me up.

I'm obviously less than a novice (been troubleshooting, fixing SS and tube amps - and a handful of old Heathkit HF ham radio linear amps - by reading manuals and instinct mostly since the early 70's but I lack a fundamental grounding in circuit design and function. I can read schematics to a degree but it's not second nature figuring out the what and the whys).

There's another person here asking a question I still have in mind and figured I'd glean from your videos and that is exactly from what point in the circuit is the "COnnect to B+" connected? He drew a picture showing takeoff before the 10H choke, at the takeoff of the two 1.5H chokes, or one of the B+ lines into one of the 300b tubes. Pulling down 50VDC from ONE tube will offset power output, won't it?