The AM Forum

My initial setup for the modulator here (K1JJ 813 rig) is to use a backwards audio output transformer between a 20w audio amp and the modulator grids.  (I will eventually and likely switch to a SS audio driver) I have a note that says to use an 8Ω 50w resistor on the input (8Ω side) but I failed to note where that advise came from, but thought for sure it was on the forum here somewhere.  After multiple searches I cannot locate that post again.

What is the purpose of the resistor?  Is it series or parallel with the 8Ω winding?  Is it relay-switched in/out with T/R?

Thanks,
Steve KK7UV

Hi Steve,

I have used a 4-10K load resistor on the OUTPUT of the backwards audio output transformer in the past for protection of the transformer secondary winding in case the modulator tube load was lost. (and for T/R switching transcients)   Spiked high voltage on the secondary winding can fatally arc it over. A resistor on the 8 ohm side (parallel with the 8 ohm winding and in all the time) would do little to protect the transformer itself.   The 4K position provides a better constant load for the whole system.

But the 8 ohm position could help to keep a more direct constant load on the audio ampifier if the extra power is available.  I would experiment with resistive loads and clip leads in both positions to see which (or both) gives you the cleanest audio in all dynamic situations.  A tone generator and scope on the RF output (audio) waveform is all you need. An SDR spec analyzer of the overall signal will help too.

Since every rig is different, experimentation is the bottom line to optimize things.

T

Thanks Tom!   

A properly terminated audio transformer exhibits less phase shift across the spectrum.

A partial load resistor can reign in some of the excursions found in the grids of class B modulators.

73DG

I have some more boards available for the WA1GFZ Mosfet audio driver if you want to go directly to the SS audio driver now.

John

Thanks John, but I already have all the parts to build the GFZ driver.   I am using the backward transformer idea for the initial setup and then explore the GFZ option later on.  I opted for FR4 turret-board construction but have not done much more than some initial layout drawings.

When I gathered all the parts and documentation on the GFZ driver a few years ago, I stuck a note in there to ask the forum:  if the 813 rig only needs 20w of audio input as per the original K1JJ schematic, why use the GFZ driver to deliver "hundreds of watts" ?    For your 813 rig, did you scale down your GFZ build to 20 watts drive?

I run it as is. I just don't have to drive it very hard. I set the idle at 20ma for each tube. I then drive it to 100% modulation watching the REA mod monitor. I leave the board powered and mute the input with a spdt relay to ground the input during RX. I have a relay on the center tap of the modulator filament trans as per the K1JJ schematic. That relay and the audio relay get actuated at the same time.

That board bring up was the easiest part of my build. It just worked.

I use the SS driver as it is cleaner than an audio transformer. The board can deliver a perfect triangle wave to the grids from 50hz on up.  But the reason I went that way is that it was simpler than the transformer with amp approach as I did not have a suitable high quality audio transformer and amp.

Boards are $35 each including shipping if you change your mind. There is a pic of the board on my QRZ page.

John

Yes, the GFZ board is a very worthwhile addition. Once installed, you can forget it and expect perfect audio from it.

I have three GFZ circuits in service right now.   One GFZ board powers a pair of p-p 813s in the "JJ Tesla 300" rig;  the second is an original first direct wire prototype from 2010 - it powers a pair of p-p 4-1000A modulators; The third is a GFZ board that powers a parallel pair of class A, series modulated, single ended 813 modulators which series feeds a single class C 813 RF final.

The 813 series modulated rig will pass a squarewave cleanly at 10 Hz since there is no audio transformer (no modulation transformer) in the system.  Unlimited high end approaching 50 KHz.  That's a series modulated rig for you.

The newest GFZ board version available from John has some improvements over the older boards. It will easily drive a pair of 833As; not an easy feat considering the grid audio voltage swing required. A work of art and a robust addition to any tube modulated rig.

BTW, there is very little heat, even from my p-p 4-1000A employed GFZ prototype.

T  

For my application I only need 20 watts (correct?) so instead of the +300/-300 supply for hundreds of watts, what kind of supply voltage is needed for just 20 watts?   Is all the extra power being dissipated as heat in the heat sink?

To be clear - there is the original unbalanced driver board. We now refer to it as UBAD. That is in my 813 rig and I have had a second run of boards made for it. It is the design that was sold a year or two ago and is the design that Tom has in 3 of his rigs. We are working on a newer, larger board that is a balanced audio driver board for larger applications. We are calling it the BAD board. Boards are not available for it yet. A few have been distributed for beta testing.

On the 20W issue. I built my supply to provide +/-285V. I run the grid at 20ma of idle current. I don't need to drive it very hard to achieve 100%+ modulation. Frank said that it should idle at around 100ma on the supply. The transformer is an Antek 1T230. So that is around 30W per rail.

We now have a power supply board with FET regulated rails. I send one out free with new boards and will give them to anyone that bought the original board, no charge.

Read this article for Frank's discussion on power supplies.

Thanks again John.   I noticed on the photo of your board in that attached document that the MOSFETS are arranged such that they would appear to stand vertically off the board.  Do you use heatsink(s) at your power level?  One on each device or like the other photo where it looks like a single large heatsink was used - presumably with a larger power level? 

The board is mounted on 1/2 in standoffs on a full-size heat sink (see pic). You take the unpopulated board and mark 6 holes for the mounting screws. You also mark a mount hole in the center of each of the large holes in the board for each mosfet. Drill and tap the HS. I used 6-32 for mounting and 4-40 for fets.

You then populate the board with all components except the mosfets, adding solder lugs for grounding on 5 of the 6 mounting holes. (I can expand on the solder lug errata later).

Carefully bend the leads of the mosfets 90 degrees up opposite the tab. Check the alignment with one of the holes on the board.

Then position the mosfets loosely on the heatsink, using silpads for insulation and carefully feed the leads up thru the board while mounting the board to the heatsink with standoffs. When all screws are tight, solder the mosfets.

Second pic is of a mosfet between the board and the HS.

Thanks for taking the time to explain that John.  It all makes sense.  I'll post a pic of my turret-board variation when complete.