ANOTHER 4D32 question.

[W4RFM]

So with the seasons change, I am gathering my thoughts and parts for a build.  This will be an award seeking 2x 4D32 Class C stage modulated by a pair of 807's.  Before you want to change my plan, I have the Berkshire mod iron for the two 807's and the power supply for it, and the tubes.
My question is, using an 1800 volt CT 400 ma power transformer, I can run 900-ish volts at ample current to the two 4D32's, with an input power of 350-360 watts.  If you have had experience with these tubes, will they tolerate the overabundance of voltage for a lengthy period of time?

[WBear2GCR]

Probably, but you are plate modulating... Think that a pair of 807s is not going to modulate to the tune of
a 300 watt carrier? More like a pair of 811a and maybe not then...  The 807s at ~750volts in AB2 will do
about 100watts, not continuous though. iirc.

Think I'd consider the 4D32s to modulate a pair of 813s...

What is the spec on the mod iron?

[W4RFM]

The mod iron is rated for full tilt boogies of 120 watts as per the RCA transmitter tube manual design.  A triod connection with 750V @ 240ma, also on the data sheet with those same numbers.  I am only looking for 220 to 250 carrier. Thanks for your reply. I feel likt the 4D32 thing may have been discussed to death...sorry bout that.

[K1JJ]

Hi Bob,

While the 4D32 plate modulated shows about 650 VDC in the data sheets, I have run them at the equivalent of 800-900V  when using my old tube PDM rig or the current plate modulated 4D32 rig. So yes, no problems with the 4D32s at somewhat higher voltages.  Beautiful tubes.

I am also building a series modulated 4D32 rig now that will be the equivalent of 1500-2KV peak voltage.  (~650V DC dead carrier)

Agreed that the 807 modulators will be a little light for full, clean modulation at that voltage  and carrier power level. (250 watts out)    As said, 811As may be a better choice, but I understand your desire to use the 807s - already having the parts.   As a thought, can you do push-pull parallel 807s plate modulating a pair of 4D32s?

I would put some air on a pair of 807 modulators when running two 4D32s (muffin fan blowing straight down from above with a Variac)  and keep the final heavily loaded (less tank C2) and at a lower  carrier level to find the most robust settings with the cleanest tones.  You may even find a single 4D32 at ~150 watts out  (unplug one tube) is the best combo for your mod xfmr ratio, etc.  Experimenting with sine and triangle tones will tell you everything you need to know.

I had four 4D32s in parallel at one time and now run a single one in both rigs. Nice 150 watt class rig.  The thing you need to find is the compromise between audio cleanliness and power output when using the 807s.

Keep posting your progress and results, cuz it will help others in the future who will be doing the same thing from the archives.

Tom, K1JJ

[W4RFM]

Thank you Tom. I took away several good thoughts from your post.  I have 811a's as well, I just got the "hanker" for some 807 action.  I was looking thru an old ARRL handbook and saw the 4x 1625 modulator in the 1964 issue. Could be done of course with 807's.  They advertise 140 watts at 750 volts.  Hummmmm. 4 tubes behind a piece of glass would sure look cool. 

[K1JJ]

Bob,

I wonder how you plan to hook up the 807 screens?  Will this a combo triode-connected grid/screen driven modulator or standard reg/fixed + voltage on the screen and grid driven?

Since you have 811As in stock, 811As as triodes may be a better choice and bullet-proof for abuse and easy to get sounding FB... vs: p-p parallel 807s. Though with work the grid driven 807s can sound as good with less output.

T

[WD5JKO]

Bob,

  From what I read here, the key is that Berkshire Mod iron. Other things like tubes, and voltages are easier to change.

Would be nice to know what the turns ratio is of that Mod iron, primary to secondary. Knowing that, we can determine which tubes to put on each side of the Berkshire.

So, look at the carrier DC input/RF output you desire. Then what is the maximum modulation percentage in the upward direction do you want? 80% 100% 120% 140%?

Remember distortion rises rapidly as the clip point is approached. You want some headroom.

Perhaps something like two 4D32's in the final, and four 807's in the modulator would be the right balance? This would be like two Viking I's combined.

Do you want to run with two sources of Mod B+ and RF B+), or combined B+ (RF + MOD)?

If the Berkshire has a gapped core, then it is designed to handle the RF Amp plate current through the secondary winding. If you pass that current through a big choke, and capacitor couple the audio, then the wattage rating of the Berkshire will go up at least 2X.

Look at this as a journey, where only you have the final say so.

Here is a good reference from some years back:
http://amfone.net/Amforum/index.php?topic=27244.msg207633;topicseen#msg207633


Jim
Wd5JKO

[W4RFM]

I love this advice guys, really.  I already have a 2 x 811A modulator built, so I may go with it first and "think" about building the 4x 807 for later. I plan to use separate power supplies for everything, that way if one craps out, I just have to replace it, also makes construction lighter, and modular (sort of). I have multiple choices on mod iron. (Too much ham festing). I just saw the Berkshire on e bay, and it was hefty so I snagged it.  I will have to look up the details again, that was a pre-Covid purchase.