Line to grid driver transformers

[KE6DF]

My question is:

How well to modulators built with a high level line to grid driver transformer, like UTC CG-59AX or CG-512 work driving class B grids in large tubes like 805s and 810s?

Assuming you drive the modulator from a small solid state PA amplifier using the 70.7 volt output.

This approach just reflects the variable impedance characteristic of a class B grid back onto the PA amp.

Is one able to get reasonable audio this way?

Dave

[k4kyv]

This is probably OK with most solid state audio power amps, since the internal resistance of the directly-coupled power amp to voice coil amplifiers is inherently low.  At least as long as the line to grid transformer is of good quality and the coupling is tight between windings.  The UTC CG series transformers should be good enough for this purpose.

OTOH, using this approach with tube type amplifiers is another case altogether.  First of all, the output tubes must have low internal resistance. That means low plate resistance triodes like the 2A3, or in the case of power pentodes and beam power tetrodes like the 6L6, substantial negative feedback around the power output stage.  Even so, there is always some internal resistance thrown in by the line-to-grid transformer, and when using a plate-to-line output transformer back to back with a line-to-grid input transformer, you have double the losses and internal resistance of one transformer, so a direct push-pull plates to push-pull grids class B driver transformer is much preferred. Of course, you use what you have on hand, while maybe staying on the lookout for more ideal replacements whenever you cruise the flea markets and web sites.

[Steve - K4HX]

I've done it. It works.

A better set up when using a SS amp as the driver is to use an output transformer (one for a tube amp) turned around backwards (i.e. low-Z windings to the SS amp and the hi-Z push-pull side to the modulator grids).

My question is:

How well to modulators built with a high level line to grid driver transformer, like UTC CG-59AX or CG-512 work driving class B grids in large tubes like 805s and 810s?

Assuming you drive the modulator from a small solid state PA amplifier using the 70.7 volt output.

This approach just reflects the variable impedance characteristic of a class B grid back onto the PA amp.

Is one able to get reasonable audio this way?

Dave

[ke7trp]

I use a 10 watt TOA Studio amplifier Model, BG10.  Works like a charm.  Thats the model to get. They show up on ebay cheap.  They also have a bass/treble on the back which is nice to fine tune the audio the way you want it.

C

[k4kyv]

I've done it. It works.

A better set up when using a SS amp as the driver is to use an output transformer (one for a tube amp) turned around backwards (i.e. low-Z windings to the SS amp and the hi-Z push-pull side to the modulator grids).

When doing that, for best results, use a high quality, conservatively rated output transformer with plate winding designed for relatively low nominal plate-to-plate load impedance, somewhere around 2000 to 2500 ohms. That works out to about 500-600 ohms per 1/2 secondary (what used to be the primary), since you are driving only one tube at a time in class B from midtap to one end of the winding, and the equivalent load for one half the winding is a quarter of the load for the entire winding (Z is proportional to turns squared).  500/600 ohms is about the minimum grid impedance per tube as seen by the audio driver in class B service.  A transformer designed for a high plate-to-plate load, e.g. 12,000 ohms, not only gives too much step-up, thus increasing the effective internal resistance of the driver; it also has more leakage reactance and DC resistance in series with the driver working into the low impedance of the grids on audio peaks, possibly increasing distortion under the variable  load of the class-B grid.

One of the older Gates BC-1 series transmitters used a pair of separate push-pull audio amplifiers each running a pair of 6L6s-like tetrodes, for the class B driver stage.  They fed each 833A with the 500-ohm tap on the output transformer driving the grid of that tube, and fed the inputs to the two amplifiers 180 degrees out of phase to achieve push-pull audio.

[Opcom]

I drive 3-500Z's with a CG-512. It's not a great match due to their high grid current and low drive voltage but it works.

[KE6DF]

I've done it. It works.

A better set up when using a SS amp as the driver is to use an output transformer (one for a tube amp) turned around backwards (i.e. low-Z windings to the SS amp and the hi-Z push-pull side to the modulator grids).

When doing that, for best results, use a high quality, conservatively rated output transformer with plate winding designed for relatively low nominal plate-to-plate load impedance, somewhere around 2000 to 2500 ohms. That works out to about 500-600 ohms per 1/2 secondary (what used to be the primary), since you are driving only one tube at a time in class B from midtap to one end of the winding, and the equivalent load for one half the winding is a quarter of the load for the entire winding (Z is proportional to turns squared).  500/600 ohms is about the minimum grid impedance per tube as seen by the audio driver in class B service. 

The line to grid driver transformers are pretty much equivalent to what you say.

The LS-5 (the LS series version of the CG=59ax) has a 1:2 step ratio from the 500 ohm line to 2000 ohm full secondary winding.

Dave

[N0WEK]

I've done it. It works.

A better set up when using a SS amp as the driver is to use an output transformer (one for a tube amp) turned around backwards (i.e. low-Z windings to the SS amp and the hi-Z push-pull side to the modulator grids).

.

One of the older Gates BC-1 series transmitters used a pair of separate push-pull audio amplifiers each running a pair of 6L6s-like tetrodes, for the class B driver stage.  They fed each 833A with the 500-ohm tap on the output transformer driving the grid of that tube, and fed the inputs to the two amplifiers 180 degrees out of phase to achieve push-pull audio.

That'd be the BC-1J that I've got. It seems to be a weird way to do it but I guess it worked. I'm going to try to leave that as stock as possible. I haven't got the manual with me but if I remember correctly the tubes are 5881s.

[Opcom]

About this and other transformers, the sheets give a ratio but do not say if it is from the line side to half the secondary or the entire secondary. I see the full secondary mentioned for the LS-5. This is the convention for all of them?

[KE6DF]

About this and other transformers, the sheets give a ratio but do not say if it is from the line side to half the secondary or the entire secondary. I see the full secondary mentioned for the LS-5. This is the convention for all of them?

I measured the ratio of a PA-59ax I have (same as CG-59ax).

It's 1 to 2 full primary to full secondary. So 500 line to 2000 ohms sounds right for it also.

I too find the specs in transformer catalogs confusing.

Not only to they sometimes list primary to 1/2 secondary, but also sometimes the ratios are reversed.

For example, sometimes putting 3:1 and other times 1:3 in the catalog entry for a 3x step up interstage tranny.

I don't have a CG-512 so can't measure that one.

Dave

[KA2DZT]

About this and other transformers, the sheets give a ratio but do not say if it is from the line side to half the secondary or the entire secondary. I see the full secondary mentioned for the LS-5. This is the convention for all of them?

I measured the ratio of a PA-59ax I have (same as CG-59ax).

It's 1 to 2 full primary to full secondary. So 500 line to 2000 ohms sounds right for it also.

I too find the specs in transformer catalogs confusing.

Not only to they sometimes like primary to 1/2 secondary, but also sometimes the ratios are reversed.

For example, sometimes putting 3:1 and other times 1:3 in the catalog entry for a 3x step up interstage tranny.

I don't have a CG-512 so can't measure that one.

Dave

Dave,

You're right, the way the ratios are shown can be confusing.  Just remember that most all inter-stage xfmrs are a step-up.  Most all driver xfmrs are usually a step-down ratio shown as primary to 1/2 secondary.  The 500 ohm line input driver could be a slight step-up.  Usually only driver xfmrs are shown as primary to 1/2 secondary.

The xfmr you say is 1 to 2 primary to full secondary is really a 1 to 1 ratio if stated primary to 1/2 secondary. So it's considered to be no step-up/down.  This looks about right for a 500ohm line driver.

Fred