Tube output section and Magic Eye.

Anyone want to have a look at this and let me know if you spot any major blunders.
I'm converting an old webcor tube tape deck into a guitar amp. The original output section had some wierd things going on. 1 6v6 was labelled bias osc. output, and one was output, but it was still push pull as far as I could tell.
I ditched all the crazy switching and the bias oscilattor coil, and now it's more or less the same as a Gibson GA6, that was easy and required only 2 new resistors :grin:
The output transformer is pretty cool, with an 8 ohm and a 500 ohm output, so I'm looking to make a record line out but I'm not so sure if It's going to work. I don't want to blow the output transformer.

I've drawn it as it was in the webcor. Maybe I should move the bias resistor in between the two plates and parallel the two primaries :?:


There's a magic eye tube 6e5 circuit as well, I've left this as is, the hookup poionts are marked X and Y. Do you think this will need some compensation somewhere so as not to throw off the phase inverter balance too much, or does it look OK as is.

Thanks
Kelly

been reading a bit more and going over the original switching layout on the webcore... I'm for sure going to have to move the 500ohm primaries to use that for a line out and put the bias resistor in between the 2 6v6's.

Can I safely parallel both OT primaries? seems like it should be OK.

Kelly

Kelly,

You can parallel the two primarys only if they are of equal turns and the center tap is really centered. Otherwise the two windings will fight each other and the magic smoke is very likely to escape rendering the transformer useless. The turns being equal is very unlikely.

The second "primary" looks like a feedback secondary in the cathode circuit. What are you trying to gain by paralleling these windings? I would think that the 8 ohm and 500 ohm secondaries would satisfy your stated needs in themselves.

Confused in P-Burg .... Irv

Hi Kai-(Irv?)

First, thanks for the smoke advice... That's really what I was after.
I haven't seen many examples of amps where the primaries are like this so I'm askin cause I don't quite understand it.


the 8 ohm and 500 ohm secondaries do seem to satisfy my needs in themselves... the webcore managed to get a line out off the 500 ohm configured AS IS..

To clarify.
AS DRAWN, both primaries of the OPT are in the original configuration.

I have redesigned the phase splitter AS DRAWN but haven't tested it yet. (It should work fine because it's a straight lift from a Gibson GA6)

I'll test it this way for now.

.

The push pull cathode circuit balances DC right? I guess I didn't realize you could pick off some audio signal there as well.

I've been working on a bunch of tube projects right now and I still don't fully understand how the cathode works sometimes there's a negative voltage on it, sometimes positive, some amps have extra resistors in there to balance out the push and the pull, I've even seen some hi-fi amps that have trimmers in there for tweaking the balance which they say pulls distortion way way down.


Kelly

Hey Kelly,

Irv is me in the real world and Kai is my doppleganger in the internet world.

I think the root of your confusion is in the determination of "primary" and "secondary" windings of a transformer. First all windings are equal until connected into a circuit. Usually but not allways, only one is termed as the primary. This winding is the one that energy is input to the magnetic circuit from the electrical circuit. Generally all other windings convert magnetic energy to electrical energy that is extracted from the transformer. I keep saying "usually" because there are exceptions such as mixing transformers that may have multiple primarys and a single or dual secondary. The transformer in the circuit you provided has one primary (the plate-plate) circuit and three secondarys; ( the 8 ohm output ), ( 500 ohm output ), and ( lower impedance tertiary feedback winding). Of course I'm guessing on the impedance of the cathode (tertiary) winding. You can test this by taking resistance measurements of one side of the primary (plate) winding .vs. one side of the feedback (cathode) winding.
Taking the feedback from the output transformer is pretty cool in that distortion correction for the output stage includes any non-linearitys of the iron.

If you want to not use the feedback windings, just dont connect them or use for a third output, probably of some rediculusly low impedance. If you do not use the feedback winding, you will need to check the bias current of the output toobs and adjust the common resistor or add separate cathode resistors to trim.

Hope this helps,

Peace - Out, Irv

Thanks Irv,
I'm getting it-slowly but surely...
I figured I'd make all the tube stuff I've lusted after at once so I might learn a thing or two.
My bench is littered with glass right now.

Doing some final tweaking on the following-
complete rebuild of a wurlitzer 145 electric piano, finally tamed the hum... that's one tweaky mofo :grin:
My bass preamp, basically a fender twin/alembic F2B with a 6n7 line stage is ready to button up.
The LA2A is fully assembled but not tested or calibrated yet.
This webcor conversion, which is nearly there.

Got a varimu in early stages, and my R*E*D*D 47 just needs to have the front panel and output wiring finished up.
Phewww

Thanks again for the tips
Kelly