Harvey Wells Modulation transformer 500 ohm second winding

[wa4fty]

I posted a question earlier about the Harvey Wells modulation transformer 500 ohm secondary and was informed it could be used to drive a input transformer to a pair of 811 tubes. I have been looking for a 500 ohm to 811 push pull grids transformer and I found information on a multi-tap CG-59AX that would work. I have not found the transformer!  Since I have another Harvey Wells modulation transformer, I want to know if it were turned around so that the 500 ohm to 500 ohms were connected. could the plate secondary be used to drive the grids of the 811? Would the impedance be acceptable?

Thanks in advance for any input,  Wayne  /  Wa4fty

[W3GMS]

Wayne,

If you use the HW modulation transformer secondary as the grid to grid transformer on your P.P 811A modulators, you would not have the center tap connection to provide the bias voltage for the 811A's.   Even if you use zero bias the CT connection would have to be grounded.  

Ton's of ways to go but I would be likely to look for either a single ended plate to push pull grid transformer or a push pull plate to push pull grid transformer.   If you use a 500 ohm input transformer you will likely use another transformer to drive that one.  I always try to minimize the amount of transformers in an audio chain.  

Joe - GMS        

[w4bfs]

......   If you use a 500 ohm input transformer you will likely use another transformer to drive that one.  I always try to minimize the amount of transformers in an audio chain. 

Joe - GMS         

I agree, all other things being equal , with Joe .... transformers add distortion

You might consider letting your 500 Ohm output from the HW just as an audio source rather than the driving power for your pp 811's .... take a look at the parallel thread started by Patrick J. on how he is driving 3-500's without a transformer ... I especially liked the approach quoted from the Clegg Zeus where they dc coupled drove 811's with a 6bl7 / 6bx7 as a cathode follower .... I have heard the audio from a Zeus (a long time ago) and it was potent

[wa4fty]

Thanks for the input fellows. Perhaps I failed to present my question properly. I have a Harvey wells  modulation transformer removed from a parts unit. I would like to know if it is turned around with the P TO P center tap connected to the grids of a pair of 811 tubes would this work? I do not know what impedance the Harvey transformer has. The original 500 ohm secondary will now be used as the primary audio input. I will also take a look at Clegg's approach. 

Thanks, Wayne / Wa4fty

[KA2DZT]

Problem with what you're trying to do is that the primary has the CT, which could work, but the impedance (or turns ratio) would be a step-up.  You need a 500-600 ohm to class B grids driver xfmr.  A 600ohm to grid input xfmr is a different type xfmr.  They are usually a big step-up ratio, no good.

Driver xfmrs are usually rated primary to 1/2 secondary ratios.  A 1-1 ratio would mean the primary has the same turns as 1/2 the secondary. So the same xfmr is a 1-2 step-up if you say it is the primary to the entire secondary.

You need the right type xfmr to have it work.

To better answer your question,  I would use the class C winding (of the spare HW mod xfmr) connected to the 500 ohm winding and use the primary (with the CT) to the 811A grids.  This would give you a turns ratio close to what's commonly used to drive class B grids.  This might work well, you'll have to try it.

Remember, it's the turns ratio that is important with these type audio xfmrs.  The impedance on the primary is a function of the load placed on the secondary.  The impedance of the primary may be 10K CT with a certain load on the secondary placed there by the class C load (we'll say 5K).

Now, if you turn the xfmr around, connect the class C winding to the 500 ohm winding and then load the primary down with the low impedance 811 grids, the impedance on the class C winding will drop to a low value, very close to the 500 ohm range.

Try it, may work.

Having said all of this, I'm assuming the HW mod xfmr is a push-pull type.  I'm not sure as I never worked with a HW.

I just re-read your question, I see the HW xfmr is a push-pull type.

Fred.

[KA2DZT]

Wayne posted this question in addition to the other thread about the same issues.

I'm bumping this up for some additional comments.

Fred