Help on wiring my new power transformer (Weber)

[slidincharlie (Carlo P)]

I have ordered a Weber power transformer for my '77 SF Deluxe Reverb: it's 025130EU export model, with primary taps for 120/220/240 VAC. Please find the schematic below.

I took VAC readings from the original (US version) tranny, with all the wires disconnected (except the red/yellow ground). Each wire read like this (with reference to chassis ground):

- yellow wires to rectifier filaments: 112 VAC each;
- red/blue wires to rectifier plates: 356 VAC each;
- green wires to amp heaters: 190 VAC each;
- blue/red wire to bias resistor: 49 VAC.

The stock tranny had only one red/yellow wire connected to ground.

Now I have installed the new tranny using the 240 VAC primary (black wire to power switch, gray wire to white wire from the mains supply).
I connected only the red/yellow wire to ground and took VAC readings from each wire before connection, but most readings don't match the original readings at all :-( , as follows:

- yellow wires to rectifier filaments: 112 VAC each (this is ok);
- green wires to amp heaters: 80 VAC each;
- blue/red wire to bias resistor: 22 VAC each;
- the remaining two couple of wires from the tranny read 165 VAC each (red) and 130 VAC each (red/white).

Several questions arise:
1) why do I have such different readings (except for the yellow wires)?
2) which one of the red and red/white pairs should I connect to the rectifier plates?
3) do I have to connect the new tranny's green/yellow wire to ground? (and why isn't there such a g/y wire in the stock tranny)?

Hope to read your precious reply soon. I don't intend to connect any wire before I know if I am doing something wrong... and I need the amp working before my next Thursday gig!

Carlo

[Rob Mercure]

Hey Carlo,

Whatever are you measuring these voltages with? While the unloaded voltages will be a bit higher than the loaded (hmm, is this the 1.414 peak?) the rectifier and other heater/filament voltages should read close enough to the loaded voltages that you can recognize them. 112 and 190 volts where you should have 5 and 6.3 is really strange. I would check the final results with another meter - you're probably picking up some stray electrostatic field in the air and until you solve this I would be suspicious of any other measurements the meter might produce.

But as to your original question the two red/white leads should be attached to the rectifier tube plates. But your bias voltages are also contrastingly strange - while your original was reading of the heater voltages is much too high the 49VAC bias voltage looks about right while the 22 VAC your now getting is way too low!

As to the G/Y wire this is used when you don't have any sort of hum balancing resistors to ground or an "elevated" positive voltage - if you've got a hum balance pot just tape off the lead.

Why did you decide to replace your PT? You don't describe any defect - is the Weber an "upgrade" of some kind?

Anyhoo you've got my e-mail if you need more help - I'm still putting together a tube package for you.

Rob

[Amp Kat]

Where are you getting those readings from ? Are you reading across each winding with your meter set to AC. You should be getting 5 VAC across the two yellow wires. 6.3 across the two greens with the CT grounded. You have two options for HV with the red and red or the red/white red/white and you can use the red/yellow and blue/red as 45 volts for bias or take it off the plates and ground either red and yellow if you use red and red or ground red/blue if you use red/white/redwhite. Where are you getting 112 VAC 80 and 22 ?

[Rob Mercure]

KB,

While you and I agree about the funny readings I'm not sure about your "either/or" C.T. grounding suggestion. While the attached diagram would suggest that both the R/Y and the R/B are equally "distant" from the R-R/W "ends" I suspect that this is due to a simplified drawing. If the tranny manfacturer subscribes to 60+ years of electronic coding practice the red and yellow are the center tap and the red/blue is "offset" to one side by about 50 volts. To put this another way, if you used the red/blue as the centertap one rectifier plate would see 50 more than the other. Not a "kiss of death" tragedy but a situation that might add more confusing readings to Carlo's situation once he gets him meter issue straightened out.

It's easy to see how you came to view this as the diagram is "symmetrical" in respect to these two leads - the illustration assumes too much. But a resistance reading from either of the "ends" to the R/Y and R/B would, hopefully, quickly sort this out.

Rob

[Steve Conner]

It makes no sense to measure voltages with reference to ground, when the transformer windings are floating. The resulting readings are determined by stray capacitance which depends a lot on the transformer construction. They don't bear any relation to the voltages that will be delivered to the amp.

If you want results that mean anything, you should connect the voltmeter across a transformer winding.

Rob, I think Carlo lives in a 220V country and imported an amp with a 110V only PT. It seems to be fairly common to change the PT in that case, to save lugging a 220 to 110 transformer to gigs.

FWIW, the transformer wire colour codes are an American thing. European gear never had them, AFAIK, instead we had random coloured wires or rows of little tags (sometimes labelled with voltages if you're lucky)

[slidincharlie (Carlo P)]

1) Yes, I measured AC voltages with reference to ground, with the tranny hooked to the wall supply (240VAC) and all wires floating except the Y/R tap: is this a mistake? Should I read from end to end of the same winding, if I understand Steve's words?

2) I swapped tranny because I'm tired to carry 3+ kilos stepdown transformer up and down my home stairs! One of the reasons I bought a Deluxe Reverb was its weight lighter than my beloved Vibrolux

Thanks a lot guys for explaining things to an inexperiencd guy like me. I always learn a lot on these forums!

[Rob Mercure]

Carlo,

Steve got it right and I should have noticed it myself - only those windings that would normally have a lead grounded could be measured to ground once you connected the centertap there - the heaters not being grounded need be measured by connecting directly to the leads. And I was unclear about the primary voltages but I did suspect that this was why you were changing the tranny - that's why I asked.


But as far as Steve's comments about lead coding - while this may only apply to the USA/Canada products both the tranny Carlo is removing and the one he's replacing it with are either produced in the the USA (original unit) or produced in/sourced for the USA so I feel my color coding comments still stand. But I have had this issue with overseas units - have a little Japanese tube preamp that I'm recapping this morning and the 120 VAC - FWB rectified B+ supply - is fed from a red and blue lead (at least they coded the heaters green).

Now I admit that codes can be arbitrary and silly and it's hard to defend these from a nation that so stubbornly resists the metric system but still, when we do come up with something that generally works well like tranny coding, I feel it's superior to paper tags that don't last over the years (while maintaining the accepted color codes USA trannies also used to come with the paper tags on the lead - as best I can tell every one has dropped off my stock of 1960s Stancor trannies).

Carlo, what model Vibrolux do you have? I use a 1967 one as my main stage amp - yeah it's heavy but at one time I used to use a Twin!

Rob

[Steve Conner]

Should I read from end to end of the same winding, if I understand Steve's words?

Yes.

BTW, Rob, I'm not saying there's anything wrong with the US system, just that they don't use it in Europe. We have to figure out what all the taps on our "vintage" transformers are for by ourselves.

[Bruce / Mission Amps]

I'm don't know why you aren't asking for an explaination from Weber, since it is theirs. I'd think they know everything there is to know about their own products.

The higher voltage on the filament taps is probably leakage voltage and I bet it will not appear when under filament loads.

I think this PT is a Chinese made, modified version of a reissue Deluxe Reverb PT.
Possibly with suggestions I made to Weber a few years ago regarding my Amercian made old TMI power transformer.
In my custom wound power trannys, the red/white taps (275vac) are for my 20 watt EL84 amps which required a lower plate voltage and the red ones (340vac) are for my 20 watt 6V6 Crusader amps.
If you wired the primary correctly, you should be measuring similar voltages from the red/yellow center tap to the outside legs, red and red/white.

Use the high voltage taps accordingly.

Many older Fender amps did not have the green/yellow center tap for the 6.3v filaments and neither does my custom PT. It appears the Weber does so just ground it.
Or, use a pair of 100 ohm resistors wired across the pilot lamp lugs or the power tube lugs, 2 and 7 with the common junction of the two 100 ohm resistors tied to ground forming a virtual center tap.
Check the layouts for a silver face and black face Deluxe Reverb amps...
or like kind for examples.

[slidincharlie (Carlo P)]

Boys, gimme some rest!
I'm re-doing all measurements with the old as well as the new tranny, and will post my results by one hour or so (gonna get my children at school right now!)

I asked help to Ted Weber. He's generally a kind guy ready to help his customers, but in this case he only reminded me that I had the schematic...
I posted my question on the Weber transformers forum, but got no reply so far.

Later,

[Tom Phillips]

I asked help to Ted Weber. He's generally a kind guy ready to help his customers, but in this case he only reminded me that I had the schematic...

Hmm. You do have the “schematic” but it’s drawn a little goofy and I’ll bet you didn’t get any other information with it explaining the winding configuration or you would not be asking these questions.

My take on the high voltage secondary windings is this:

1) The RED/YEL is the center tap. This, as you know, will be grounded.

2) The BLU/RED is the bias tap

3) The RED/WHT leads are an extra high voltage winding that you will not use for your Deluxe Reverb. With respect to the center tap they provide
270V – 0V – 270V. Don't connect them to anything. Just heat shrink the ends.

4) The red wires are the HV lines you will connect to your rectifier tube. With respect to the center tap they provide 340V – 0V – 340V.

I think all your other questions have been answered by the other guys.

Regards,
Tom

[Tom Phillips]

Carlo,
Attached is a diagram showing an alternate way to draw the schematic of the Weber transformer high voltage secondary winding.
I think this gives a much clearer perspective of the beast.
Tom

[slidincharlie (Carlo P)]

I got it clear now! (almost...)

[I apologize for posting late... I sent this post last night, but for some reason it never reached the forum!]

Fellow Ampagers,
I have found what I was doing wrong thanks to your advice and to a webpage that I do suggest all newbies to read.
I do apologize for not reading that page before.
Reading AC voltage from end to end of each winding yields now the expected values in both the old and new tranny.

I still have one doubt:
since I already have the two 100 ohm resistors on each side of the pilot lamp in this amp, shall I leave the green/yellow wire (CT for the 6.3V heater winding) disconnnected?

Thanks again,

[Rob Mercure]

Yes - if the resistor balance is good. If not a potentiometer gives you better options for adjustment. As the heater center tap might not quite be in the electrical "center" of the windings, or as one side of the centertap might have more hum influence than the other two precise resistors will "center" the winding while an adjustable "center" would allow you to offset as needed.

Rob

[slidincharlie (Carlo P)]

Ok. I'll keep the resistors and tape the CT off.
Looking forward to hook everything up tonight...