How to measure primairy impedance on OT with secondary centre-tap??

[BosAmps]

Hai there fellow amp-nerds!!!

I got a PT, choke and OT out of an old Webcor tapemachine and they are all working fine after 50+ years!! YEEHAA!!
I had plans to use them in a small PP guitaramp (HT ≈ 325V with 2x 6V6 as used in the original tapemachine), but then I found out that the OT has a bunch of wires hanging out of it to use in a cathode biased poweramp with cathode-feedback (speakersignal is fed back to the cathodes of the 6V6's and the biasresistor and bypass-cap are located on the centretap of the secondary winding of the OT.. So far a simple explanation of the CathodeFeedback-circuit..
My question:
I do not have any info about the used speaker(s) in the original tapemachine (probably a WebCor 2110 or 2010), so I do not have a clue about the primary impedance of the OT (the 6V6's makes me guess it's something like 8k??). The speakers used were probably 4 or 8 Ω, so I can always connenct it to an 8Ω speaker and see what will happen, but in the learning proces I want to figure out how to measure the primary Z...
I normally put 1kHz @ 1V on the secondary and maesure the voltage on the primary to get the winding-ratio, squaring the outcome and by multiplying it with 4 or 8, you get the primary Z, but how does the secondary centre-tap 'interfere' with this approach???

Any tips/help anyone??

Thanx and all the best,

Jeroen

[pdf64]

Just ignore it, measure the voltage across the full p-p primary, see The Valve Wizard -Push-Pull

[BosAmps]

If everything in live would/could be so simple!!! Thanx!!

[BosAmps]

I measured the OT in the normal way, ignoring the secondary winding centre-tap and it turned out to have a primary Z of ≈7,5k when you load it with a 4Ω speaker.. So that's quite normal for 2 6V6's running at ≈325 Vdc.. 325 squared and divided by 14 (watts = Pmax for a 6V6) = ≈7,5k

But there is also a second set of taps, in the original schematic used for the cathode-feedback, and when I measure those up I calculate a different primaryZ of 5k @ 8Ω load.. (pair of 6L6s?? Or perhaps a pair of EL34's)
So perhaps a lot of options, but my first issue still remains: How do things 'change' when I connect the secondary centre-tap??

[pdf64]

Is the cathode feedback winding beefy enough to drive a speaker? It may just be thin gauge, only sufficient to provide a NFB signal. Is its winding resistance similar to the 'proper' speaker winding?

What are you thinking of connecting the secondary centre-tap to?

The Leslie 147 has an interesting NFB to power tube cathodes design, which may be along similar lines as the donor system http://www.captain-foldback.com/Lesl...matics/147.GIF

[BosAmps]

All the wires coming out of the OT are of equal gauge (≈20AWG) and I found the schematic of the donor-amp. They made two versions of the tapemachine, one with a single speaker and a 'deluxe version' with two speakers (just mono, nothing fancy LOL).. They connected the speakers to axact the same windings as I am planning to use, but there is nothing to be found about why the impedances do not logically correspond.. We'll see...
About the secondary centre-tap: I'll leave it disconnected for now, because the 'new amp' has fixed bias instead of cathode-bias, but in the layout there's some space left for future alterations!!
The Leslie 147 uses exact the same cathode-feedback-trick as is being used in the tapemachine. And FWIW I emailed with Menno van der Veen, who is a very well-known tubeamp- and tubeamp-transformer-designer and he told me to seriously experiment with the cathode-feedback-circuit for guitar purposes.. He developed an outputtransformer for guitarpurposes, which can be wired in 20 ways including a couple of them with cathode-feedback..
I have just finished the layout of the amp I'm building and it has enough space to fiddle around with the feedback-circuits afterwards.

[loudthud]

The cathode feedback windings are probably the same gauge wire as the primary. You don't need to use cathode biasing to use the windings, just ground the center tap and use fixed bias. The hard part will be getting the polarity right for all the feedback paths.

[J M Fahey]

The cathode feedback windings are probably the same gauge wire as the primary. You don't need to use cathode biasing to use the windings, just ground the center tap and use fixed bias. The hard part will be getting the polarity right for all the feedback paths.

If it's used only for cathode feedback, yes.

Cathode current is basically the same as plate current.

But, if that same winding was also used as speaker output (the fact that it's a balanced out does not hurt or affect the speaker at all) , in that case it should need to be of higher diameter.

[BosAmps]

So if I understand correctly:
-The secondary centre-tap MUST be used when applying CathodeFeedback, but does not relate to the used bias-methode.
-The polarity of all the feedback-related wiring (2x anode/HT from the primary, 2 pairs of 'speakeroutputs' out of the secondary) might give some troubles, but I have allready figured out the wiring of the secondary, so as long as I don't mess up the colours while putting it all together, it is exactly the same as wiring up an ordinairy OT

What did give me some headaches, was how to ground the speakeroutputs, because of the strange wiring scheme. Permanently grounding both taps would end up in shorting out parts of the secondary, so I ended up using isolated stereo jacks for both outputs, connecting the signal + to the 'tip', signal - to 'sleeve' and ground to 'ring' . (Changing 'ring' and 'sleeve' actually makes more sense of course, but this is how it's wired up right now LOL) This way, the actual ground reference is only being made in the outputjack that is being used and leaving the un-used output 'floating'..

BTW Thanx for all your reactions and thoughts!!!

[J M Fahey]

Don't understand your explanation very well, but the main point is: if the cathode feedback winding is also the speaker out, you can not ground the speaker, period.

No matter where you ground it, even if "just at the speaker jack" which is what I think you are trying to do, you'll end up shorting an OT winding ... which means shorting äll" of the OT, since magnetically all windings are in parallel.

Please draw and post the schematic of what you mention above.

[BosAmps]

Sorry for being somewhat confusing!! I'll try to be as clear as possible..

- When using cathode-feedback, you can't ground any speakersignal exept for the secondary centre tap.. Clear!!

- But when not using cathode-feedback, I always ground the OT-secondary. Just for being safe, like if the insulation fails and HT gets on the secondary/speaker-output. Randall Aiken and Merlin Blencowe both explain very clearly how and why to ground the OT-secondary winding..

Are there any easy to use circuit-drawing programs for Mac?? I only use pencils and paper for my own records/archive.. LOL!! If I can find a working program for my use, I'll post the schematic ASAP!!

[jazbo8]

Are there any easy to use circuit-drawing programs for Mac?

SchemeIt...

[BosAmps]

Cool!! Thanx!!

[Silvertone Jockey]

see The Valve Wizard -Push-Pull[/url]

Thanks for posting that