Choosing an output transformer...

[alchemy]

I have an old knight km-15 mono hi-fi amp that i decided to turn into a guitar amp. Been playing with it for a bit, different crappy sounding circuits etc, and realized the OT is on it's last legs. AT this point it's an 18 watt Marshall.

So the chassis is all wired up and I just need to replace the OT. I have another OT out of an organ that I think will work, but wanted to double check with folks more knowlegdable as I am still learning this stuff. I am still a bit unsure as to how to calculate the needed impedance.

The original Knight tranny had a 10,000 ohm primary (the spec sheet didn't say at what load). The tranny I have (if my math is correct) is about 6,000 ohms with an 8 ohm load, and 12,000 with a 16 ohm load.

I got these numbers by putting 7.23 volts across the primary (have a 7+v tap on my variac) and measured .263 volts on secondary. I divided 7.23 by .263 and squared it and came up with approx 756 which i then multiplied by 8 and 16, to come up with approx 6,000 and 12,000.

First off, do my numbers seem right ?
If so, is it safe to use this tranny if I run it at 16 ohms (is 12,000 close enough to 10,000 to not cause problems)?

And lastly, on the replacement OT, one of the secondary wires (NOT the center tap) is slightly smaller gauge than the CT and the other leg. Anyone ever seen this? What gives?

As always, thanks for the help....This is a project to test my skills and my resourcefulness as I am trying to build it all from scrap parts (though I did put 3 new coupling caps in). Otherwise, I would just buy the correct OT. Just trying to see what I can come up with in spare time with spare parts and what I can learn in the process.....Cheers!

[octal]

Seems like your math is OK. I think you're going to be better off running with the 6K reflected load (8 ohm load.) The normal impedance for el84s is 8K (I'm assuming since it's an 18W circuit, you're running EL84s?) if you go much higher, it can be hard on the screen grids and might not sound too great.

Nathan

[alchemy]

Thanks for the reply...Yes, EL-84's in PP.

The normal impedance for el84s is 8K

Is this number (8K) for EL-84's from the tube manual (i.e. effective load resistance plate to plate)? When trying to figure OT impedance for any given tube, is this the parameter I should be looking at? If so, does it change depending on whether or not there are 4 tubes or 2 or 1?

And lastly, if the above is all true, then how much is it safe to mismatch?

Appreciate the help as always. Just now getting around to trying to learn how transformers are spec'd to mate with tubes, and it 's a bit confusing to say the least. Yet, I can't help but feel that it is inherently simple and that I may just be over complicating it. Anyhow, thanks and cheers!!

[BiBi]

First off how do you know your original output transformer is on its last legs? I can't say that I've ever heard a transformer "going" bad. Sometimes people claim that hifi transformers sound bad with guitar and adjust with conjunctive filters and the like, but that's something I've never run into either. I use old hifi transformers all the time.
Secondly, I think your math is wrong. You want one side to equal 1 to know your ratio. So 1/.263= 3.8 times that by 7 is 26.6, squared is 707. So your ratio is 700 to 1.
My other guess is that your replacement txfmr may be actually single ended. Sometimes on these there is a third wire for either negative feedback or ground. Are you taking the readings from the two heavier wires or from the one smaller, one heavier?

[octal]

Thanks for the reply...Yes, EL-84's in PP.

Is this number (8K) for EL-84's from the tube manual (i.e. effective load resistance plate to plate)? When trying to figure OT impedance for any given tube, is this the parameter I should be looking at? If so, does it change depending on whether or not there are 4 tubes or 2 or 1?

Yes, 8K is the plate to plate load resistance for EL84s in a tube manual.
Note that in many cases guitar amps use lower than textbook load resistances. You can get down to the nitty gritty and calculate using load lines, but if your voltages and impedance is within a reasonable distance of an 'off the shelf' design, you'll be fine.

If you have four tubes, your load impedance should be halved (4K in this case.)

Single tubes are a completely different case and require a different load impedance and a single ended transformer.

And lastly, if the above is all true, then how much is it safe to mismatch?

Always better to go lower than higher. (the converse of solid state amps!)
I think you'll be fine in the 6K region with EL84s.


"Bibi" is mistaken about the impedance calculation. Although turns and impedance ratios are commonly expressed in the format 1:X , there's no need to do so. The voltage ratio is the same as the turns ratio. Divide your input by your output voltage to find the turns ratio, square to find the impedance ratio and multiply by your load impedance to find what's going to be reflected back to the tubes.
I do agree with him that it's odd for an output transformer to be "going out." Generally they either work or don't. You can have low or distorted output due to shorted turns or something, but it's most likely not going to be intermittent.

Cheers
Nathan

[alchemy]

I guess "going out" was the wrong choice of words. I previously had this wired up as a Vox Berkeley type circuit, sounded OK, but not great. I checked the OT on the variac and both sides measured within 10%. But the resistance was really high on one side, like 180k or something (it hadn't gone completely open yet). After I decided to change it to a Marshall circuit and I had it all rewired, I kept suspecting the OT and measured the resistance. CT to one side is like 180 ohms, the other is open. The tip off was when I was checking the bias (across 1 ohm resistors) and could not get both sides to be close at all. Voltages were good, but the screens on one side were glowing brightly so I assumed it was pulling way to much current. So that is why I said "going" bad. It seems to have progressively got worse.

As for the replacement OT, I measured resistance across all the wires and from the red wire to either wire it is 1/2 of the resistance between brown and blue.. I had thought of the possibility that is was SE, but googling the serial # brought only 1 hit and it was this:

Vintage Output Transformer for Tube Amp 6BQ5 EL84 x 2 - eBay (item 250635447819 end time Jun-17-10 14:27:16 PDT)

And also, I have another organ amp that runs 4xel84, and it is wired the same way (the CT is one of the big wires while the two sides going to the tubes are one big and one a bit smaller.)

It is seeming like I will be OK to run with this replacement OT as long as I hook it up for 8 ohms, correct? Any other considerations I need to take into account?

[alchemy]

Well I went ahead and stuck the replacement OT in, and all seems to be fine. It is now pulling the same (30ma) current through both sides, and the screens are not glowing.
A couple questions:
What should I be checking to be absolutely sure that all is well? Anything to look for other than abnormal voltages, high or uneven plate/screen/cathode currents?
How do I determine which way the wires hook up on the new OT (colors are different)? Secondary seems easy as I'm assuming that black is the common (and there is only one other tap for 8 ohms), But on the primary how does one know if the wires from the output tubes are reversed?
Thanks a lot for the help. Now it's just down to tweaking this thing to sound good....

[octal]

If your circuit has negative feedback and it's not oscillating, you've got the plate leads attached in the correct phase. (It will be an audible oscillation- nothing you need a 'scope to view.)

If your circuit does not have negative feedback, it really doesn't matter which plate lead goes to which "end" of the transformer.

Sounds like you're on the right track. Good luck with your project.

Nathan

[alchemy]

Seems to be working pretty well, but one last question. When I initially measured the voltage on the plates of the power tubes, one of them was about 10 volts higher then the other. I figured it had to be the windings of the replacement OT, but I went back today to swap the tubes and double check that they were not the culprit, and low and behold the voltages are dead on now. Could this maybe have had to do with the tubes being brand new and needing to be burned in for a few minutes, or does it indicate a problem with the transformer?
Thanks alot for the help guys...

[octal]

I don't really know. I've always assumed that there's a burn in time as part of the tube matching process, but who knows if it's long enough? You'd also expect that two similar tubes would drift about equally over such a short time...dunno. Perhaps others can weigh in on this.

With the amp off, you could check the resistance from the CT to each leg of the OPT and make sure it's close. You could monitor the idle current in each tube... I wouldn't worry too much about what it was doing yesterday, just keep tabs on it and see what happens.

Can you load test the amp with a signal generator, a dummy load and a 'scope? If you're getting fairly symmetrical clipping and the power output is about right, it should be a healthy output stage.

Nathan

[alchemy]

I would love to put it on the scope, however I am only just learning how to use one. And I mean "just"! If you would be interested in PM'ing me and walking me through it, I'll give it a shot. I have a Tektronics 465 scope and a signal generator. I don't have a dummy load (by the way, where is a good source to get a few?). I'm assuming I cannot use a speaker since the impedance changes under load, correct?

[clyde1]

Octal, What is it about higher impedances that causes screen over-dissipation? Would larger screen resistors help, possibly at the expense of tone though?

[octal]

Octal, What is it about higher impedances that causes screen over-dissipation? Would larger screen resistors help, possibly at the expense of tone though?

Yes, larger screen resistors will help limit screen dissipation... you'll have more screen sag and somewhat lower output as well.

I truthfully don't know the theory behind why lower load impedances help hold screen dissipation down, but I have observed it and I also recall reading the same thing elsewhere. I suppose that a lower load impedance will equate to smaller voltage swings on the plates at the same output level, so I imagine it must have something to do with that. No doubt some experts on load lines and such can clarify this for you....

Nathan