Understanding Output transformers

[Toxophilite]

Hi
I'm starting another thread on just transformers because I find it interesting and would like to expand my knowledge if the kind folks here will oblige me.
I'm new so f i'm starting too many threads and offending forum etiquette please let me know

I've been trying the AC on the primaries, then measure it on the secondaries test I've read about here and elsewhere

I have a variac and I hooked up 10VAC (actually 9.96)to the primaries of a couple of my OTs
Right now I'm trying the ones I've been swapping around on my AB763lite build but I want to test others I have/acquire too

On the original transformer I get .438 on the secondaries, when I do the division I get a ratio of 22.74
If i square this and multiply it by an 8 ohm load(which my test speaker is) I get :
22.74 x 22.74 = 517.1 X 8 =4136.8 ohms

On the other(lowrey) transformer I get .33 when I do the division I get a ration of 30.18
30.18 x 30.18 = 910.8 x 8= 7286.6


I was trying to research 6V6 OT primary impedances in push pull and they seen to have a variety depending on the wattage of the OT
Am I doing the math right? and I'm curious what this tells me about these two transformers?

I'm going to go test a couple more for kicks...as you can see it's a slow eve..I just finished my folk coffee house gig and the girlfriend is off at a housewarming!

My hammond L-100 organ dual 6BQ5 OT measures about .33 for on the secondaries and my Lowrey organ Dual 7491 OT measures about .37 on the secondaries

[jazbo8]

Your math is correct, you have established the winding ratios of the OPTs, but it does not tell you the power ratings, which may be estimated by their sizes, weight and the tubes they were used with. As for the PP 6V6, the required Ra-a (anode to anode) depends on the operating condition that you use, at a minimum, you need to know the B+ and screen voltages that you intend to use, then the Ra-a could be calculated.

[Toxophilite]

In this instance I was measuring about 410 on the screens and 407vdc on the plates of the 6V6s

[jazbo8]

Mama mia! The poor 6V6s, those voltages greatly exceed the maximum ratings of the tube. Do you mean those are the existing voltages in the amp or what you are planning to use in your build?

[Toxophilite]

It's a deluxe reverb AB763 I believe 415 is the amount on the schematic

[jazbo8]

When you say the "original transformer", do you mean the one from the DR made by Fender? If so, then something is not quite right with the impedance, which should be 6.6k to 8k. Another thing to bear in mind - nothing to do with OPT - are you sure that your 6V6's can handle the high voltage? Most current production tubes can not.

[Toxophilite]

Hi
I appreciate the insights
I wanted to talk about transformers in general, not so much what 6V6s can handle and what type I'm using. I'm referencing the transformers I'm using in my AB763 lite build and also referenced in my Cooked transformer thread
Perhaps I just should have kept all these questions in my build thread to save the confusion
The fault is mine
But so you know
My test pair (which I use when I'm not sure)are a couple of almost done 1960 GEs, they still work fine but they're running out of getter flash
I also have been using a pair of late 50s proabably smoked glass RCAs that test strong with lots of life left.
This is not a already existing fender deluxe reverb
It is a conversion based upon an electrohome Hifi amp
So there is no roginal fender transformer in use. I am using existing vintage transfomers that came with the Hifi
I also have a pushpull 6V6 transformer from a lowrey organ

[alerich]

Those voltages really aren't all that high for a 6V6. Fender has been routinely running 6V6 tubes at higher voltages than those in their 6V6 equipped amps for decades. I usually like to seem the screens at least a few volts lower than the plates but that is probably splitting hairs as long as appropriately sized screen resistors are installed.

Am I doing the math right? and I'm curious what this tells me about these two transformers?

Yes, your math is correct. It tells you the turns ratio and little else, but that is the purpose of the procedure. Knowing what type of amp they came out of if they are donors will tell you more about the iron. If they are new they will have the specs.

Usually when I use an AC voltage to calculate OT turns ratios I inject the small AC voltage into the secondary and read the primary. Since this is stepping the voltage up you will be reading volts on both sides. I don't like reading fractions of a volt to do those calculations. Either way is probably fine.

[Toxophilite]

THanks!
As the fellow was saying
THe original fender deluxe OTs were designed for around 6 k on the primary side
Does the approx 4.1 k on one transformer
and the 7.2 k on the other tell me anything about how they'll work in the deluxe reverb circuit?

Is it just about what sort of load they want to see on the secondaries?
I do have a schematic of the original amp that is being converted (opr at least the power amp section)

[jazbo8]

Right, sorry for going OT on the 6V6s, and I also didn't see your other threads so excuse the rambling... Anyway, just based on the OPT specs alone, the Lowry would seem like a better choice, if you use the 4k hi-fi OPT, the Ra-a would be too low and may result in too high a plate current when cracked, and no one wants to see another pair old 6V6's go up in smoke... Of course, the 4k could also be used if you put a 16 Ohms speaker on the "8 Ohms" tab (which I assume is the standard match for the hi-fi OPT).

[HotBluePlates]

THe original fender deluxe OTs were designed for around 6 k on the primary side
Does the approx 4.1 k on one transformer
and the 7.2 k on the other tell me anything about how they'll work in the deluxe reverb circuit?

The original Fender Deluxe Reverb OT was around 6.6kΩ. The earlier tweed Deluxe was more like 8kΩ.

Quick, bottom-line answer: for stock power output, you should use the stock OT primary impedance as the load for your tubes. Generally, any other loading will result in less power, more distortion, or both.

This stock, ideal impedance is valid for the original supply voltage, bias, class of operation, etc. If you change any of these, a different load impedance might give maximum clean power.

Does the approx 4.1 k on one transformer
and the 7.2 k on the other tell me anything about how they'll work ...

It helps if you know most 6V6 OT's present an 8kΩ primary impedance for 2x 6V6's.

If the 4kΩ OT is bigger than the 8kΩ OT, you might guess it can pass more power. Then maybe it is presenting a 4kΩ load for 4x 6V6's (double the output devices, half the primary impedance).

Or if it is the same size, maybe it is passing the same power and presenting an 8kΩ primary impedance; in that case, you need double the speaker load impedance to reflect 8kΩ. So you'd attach 16Ω to the secondary winding you measured, 16Ω * 517.1 = ~8kΩ.

I was trying to research 6V6 OT primary impedances in push pull ... I'm curious what this tells me about these two transformers?

Ahem... You pulled them from their original environment, so you should be able to tell us what that proper use was (secondary impedances, #/type of output tube, supply voltage, bias).

Assume the way they were used originally is the proper way to use them. That's easier & more accurate than having us divine their properties from the (possibly incomplete) supplied information.

I've been trying the AC on the primaries, then measure it on the secondaries test I've read about here and elsewhere

I have a variac and I hooked up 10VAC (actually 9.96)to the primaries of a couple of my OTs

You might want to think about whether your meter is accurate at that range of measured voltage. If it is a small percentage of full scale on that range of your meter, accuracy might be poor. Note that resolution ("bunch of digits") is not the same as accuracy ("indicated value corresponds with reality").

Some meters are more accurate on their higher voltage ranges. You could apply a very small a.c. voltage to the secondary (say 1-3vac) and measure the resulting primary voltage. This should also make the math easier. The drawback is your variac may not easily output such a small voltage (though you could get is from other sources), and you have the risk of accidentally creating a very high voltage on the primary (like applying 10vac to your 22.74:1 OT, getting 227.4vac on the primary).

[jazbo8]

You might want to think about whether your meter is accurate at that range of measured voltage. If it is a small percentage of full scale on that range of your meter, accuracy might be poor. Note that resolution ("bunch of digits") is not the same as accuracy ("indicated value corresponds with reality").

That's a good point, to get a more accurate reading, you can connect the variac to the secondary of the OPT and take the readings on the primary taps, just be careful with the voltage that you apply, perhaps just 5Vac would be sufficient.

[alerich]

Is it just about what sort of load they want to see on the secondaries?

I think it's the other way around. As I understand it, a certain tube type will require a certain load impedance (primary Z) to operate at their highest efficiency. An OT has no impedance of it's own - only DC resistance.  Assuming an 8 ohm or 4 ohm or 16 ohm speaker load (secondary Z) the transformer is wound with a turns ratio that will reflect the proper Primary Z back to the tubes with that specified speaker load impedance connected to the secondary.

This is an overly simplified explanation and only relates to turns ratio and reflected impedance. It doesn't account for power handling capability or a dozen other aspects of transformer manufacture that I lack the experience to explain.

[Toxophilite]

Lot's of excellent knowledge to assimilate here
Thanks all for the time taken

The 4.1k transformer is the original one from the electrohome hifi AB763 lite conversion
It is physically the bigger of the two
In it's orginal format it had 2 6V6s and 5Y3
I screwed up on determining the speaker load because the speakers themselves had "8 ohms for series operation with matching speaker" written on them. I thought this was 8 ohms together, however it could be interpretted as 16ohms as it seems likely now. I made the mistake of not measuring it, prior to gutting.
The OT is a single tap with speakers leads hardwired on , no hookup point with impedance info.

Also I do not have the schematic, only one from a very similiar electrohome PA300 which upon examination uses the exact same power amp. I believe I posted it, however the PA300 uses a multi-tap OT so it doesn't tell me much about this OT

My apologies for this mishmash of incomplete possibly inaccurate information

I am not using it in it's exact original configuration as the circuit is now an  ab763lite circuit


The 7.2  OT is from a 2 6V6 lowrey organ that used a 5U4GB rectifier
I don't have the schematic for it either, and the power amp section has gone on to a more fulfilling existance betond this mortal coil.

Both these were acquired a while ago when my knowledge was even more incomplete (can you believe it??)resulting in my lack of acquiring all the proper information other than tube compliment.. I thought they were both 8ohm secondaries, but I could've been in error.

Perhaps, as suggested, I shouldn't be asking people to supply information they can't possibly know
I wasn't aware of that , but now am

My meter is Greenlee Dm-210 digital multimeter
I think it's a reasonably decent one for what it is

My variac seems to be okay, I usually meaure the AC voltage coming off it with the above mentioned meter rather than trusting to the variac's guage, which is in 10's of volts


Thanks again
I think I've achieved all I can with the limited  and incomplete information I have.

Time to do some reading and acquire more information in general before I confuse others

[HotBluePlates]

The 4.1k transformer is the original one from the electrohome hifi ...
In it's orginal format it had 2 6V6s and 5Y3
... the speakers themselves had "8 ohms for series operation with matching speaker" written on them. ...
The OT is a single tap with speakers leads hardwired on , no hookup point with impedance info.

You mention "speakers", which tells me there were 2 or more, right? So that sounds like the original pair of speakers were in series for a total load of 16Ω.

4.1kΩ:8Ω = 8.2kΩ:16Ω

Both these were acquired a while ago when my knowledge was even more incomplete (can you believe it??)resulting in my lack of acquiring all the proper information other than tube compliment.

There was a time when I thought all I needed to build an amp was a donor chassis with some "tube amp transformers" on it, cause they're all the same, right? 

I have since come to learn that the output tubes, output transformer, power supply (including power transformer, rectifier and filtering) and phase inverter (if present) all work as a cohesive output system. They are all exactly the right size to work with each other the way the original output stage operated.

That means it's usually more fruitful to keep all these bits exactly as they were in the donor amp, and change only the preamp. Or at least recognize they may never be 100% right for a different circuit.

[shooter]

I was thinkin about OT specs last nite n, well, I guess it's a good tread to jump in with a ?
My quad build is basically a 20w class A, but it requires about 40watts to make that happen.  The OT i'm using is for single ended class A rated 25watts or less, which watts are the manufactures specing?

thanks

[HotBluePlates]

... My quad build is basically a 20w class A, but it requires about 40watts to make that happen. ...

Very likely more than 40w to make that happen.

This is about efficiency, and theoretically, class A could be 50% efficient at best. That's power output / power input (to the tubes), and is the tubes' average plate current times the (possibly sagged) supply voltage.

Real class A amps never hit 50% efficiency, and many are closer to 25%.

... The OT i'm using is for single ended class A rated 25watts or less, which watts are the manufactures specing? ...

The watts that end up in the speaker.

Because while current may flow through the tubes (and through the OT primary), if power is not transferred to the speaker then it doesn't "go through" the OT.

Think about your amp at idle. The output tubes draw current, plate voltage is present, and the tubes dissipate power as heat. But the speake isn't moving, so there's 0w through the OT.

[PRR]

> The OT i'm using is for single ended class A rated 25watts or less

Hammond ESE?

[shooter]

Thanks, that was my dilemma, knew the primary was my load, and the math of summing v1 tru v4 current * plate volts comes in at about 42w, knew there was no sound out the speaker (a great thing!) then I remembered 25w spec.   The tranny is an edcor  CXSE25-4-1.25K.  N I think it's just sitting there thinking, what, that's it, yawn!

[tubeswell]

Usually when I use an AC voltage to calculate OT turns ratios I inject the small AC voltage into the secondary and read the primary.

Me too. Much more accurate method of getting a meter reading (than trying to measure the secondary with a small VAC on the primary).

[HotBluePlates]

... math of summing v1 tru v4 current * plate volts comes in at about 42w ...

The math there gives you idle dissipation, which has little to do with output power.

[PRR]

> edcor  CXSE25-4-1.25K

1.25K??

> v1 tru v4

Oh. So four tubes suitable for 5K each.

http://www.edcorusa.com/p/464/cxse25-4-1_25k
Wattage 25W
Max. Current 250mA
Primary (input) Impedance 1.25K Ohms

For *pentodes* (not triodes), in single-ended, you want the V/I to be similar to the load impedance.

And we have a Max. Current spec.

So 250mA times 1.25K is 312 Volts assumed B+.

And 250mA=0.25A times 312V is 78 Watts dissipation.

Efficiency is 40% at-best. 78*0.4= 31 Watts out.

The "25W" core rating actually depends on the THD at the lowest note. I doubt it is good for 25W at 20Hz, in SE that would be huge. It is however 9 pounds, not chintzy. And guitar only goes to 82Hz. And if that bottom string has high THD, we just say "flavor". Also an impression of BASS with a speaker too small to deliver real bass. I don't think the 31W/25W discrepancy matters.

Four tubes suitable for 5K load each and 78/4= 20 Watts Pdiss each... that's too hot for 6V6 or EL84, quite reasonable for 6L6 or EL34.

First look at what the Old Guys said. Tung-Sol 1952 6L6 data page 2 shows 4.5K load (near 5K) at 300V sucking 48-55mA and delivering 6.5 Watts. Times four is 1.1K, 300V (plus 13V bias), 200-220mA, 26 Watts. A very reasonable condition IF you can source a 313V 250mA power supply. Uh, ignore the 200V G2 supply, take G2 to 300V and jack-up the G1 bias the same amount (about 19V). That avoids an odd-voltage G2 and gives you a little headroom between plate bottoming and grid blocking. The 1.5X grid drive and gain are minor details.

EL34 is a *cheap* tube and the specs suggest low supply voltage for lower overall cost. However Mullard 1961 shows 300V 83mA 3.5K load making 11 Watts out; you could do the deed with three tubes at 249mA for 33 Watts out, or split to four tubes with less heat/stress in each tube. Bias and drive is again about 18V.

Ass-uming 40% efficiency (that's at-best), the 25W spec suggests at-least 63W Pdiss. In a Quad that's 16W per plate. Which nominally rules-out 6V6 and friends. (Yes, I know Fender sells amps with 6V6 cooking 17W Pdiss, and it works for them, but they just sell them. We have to live with them.)

[shooter]

Thanks, that's the kind of info i'm trying to get wired - in my mind!  My specs are 240v @ plates, 234 @G2, 43-50mA at cathode, neg 4-5v @G1.  I found a power curve that showed max pwr @ 5ishTHD for a quad set of el84s at 1250ohm impedance.  And your right, finding the PT that can run without screaming for help. I have 200mA but after 2hrs play it starts popin n scrachin like an 'ol well worn record.

thanks

[PRR]

> after 2hrs play it starts popin n scrachin

Sounds like a bad joint or an over-stressed tube, more than PT strain.

[shooter]

I didn't want to drag it back to the scope, so I started swapping input signals in cuz at idle seemed ok, after 4 or 5 audio sources, I've come to the conclusion my mass storage device is the culprit.  hopefully it's not a worm eating through 60gig of stored music!!!