I'm back to work on my DX-100b gain, here's the update:

[W7XXX]

Why 797 v on AM and 750 v CW? Are you loading the final the same? This voltage may be higher if loaded lighter for AM. It will drop with modulation peaks as will the screen voltage.

How much screen current are the 6146's drawing loaded up on AM?

What is screen voltage on 6146's loaded up on AM?

These are questions that need to be answered before you proceed further.

Forget about the 390 v on the 1625 screens and think what else is wired to this point ... the clamp tube and the 6146 screens. Also remember that under full load and modulation peaks this 390 v is going to drop, so if you leave the 1625's wired in class AB2, then this voltage will probably drop to 350 under mod peaks. If this still concerns you, then a dropping resistor can be wired in the screen circuit.

I still say leave the 1625's pulled and get this xmtr to load up right in AM position. Later modify the 1625's to triodes and forget about the extra 40 volts as it is no problem if the SCREEN voltage and or CURRENT draw of the 6146's are not excessive. Modifying to class B triode connection gets rid of the bias voltage that also effects grid current on modulation peaks. Modifying to triode connection SIMPLIFIES the modulator circuit and should put your mind at ease.

Now why does an xtal work on CW but not AM? Are you using the same xtal and SWITCH SETTING? If not then this is your problem, dirty switch or bad xtal. If so this makes no sense ... nothing in switching from cw to am effects this circuit. The 12BY7 as an xtal osc works the same whether on am or cw setting.

[kf6pqt]

Well, the final never got mucked with by Mr. Golden screwdriver, and the 6146's in there look OLD. That, and I never had a problem with the final amp.

Its back together, its working FB when using the VFO, I've done some yelling into the mike, and its taking it just fine. Sounds like plain 'ol communications quality audio (D-104) listening with a set of decent headphones on the portable ricebox, but hey, thats a pretty subjective test listening to myself... and at this point its a very-much stock DX-100b.

I do think I see some blue glow on one of the 1625's on heavy audio, I need to do a quick repair on my ancient Heath TC-1, make sure I've got a good pair of 1625's in there. Then I'll throw it back in its case, drag it back into the shack next to the NC-300 and the scope, and see if I can scare up a QSO and see what they think.

Hopefully whoever that is isn't too much of an audio snob.

[W3SLK]

Jason said:

I do think I see some blue glow on one of the 1625's on heavy audio, I need to do a quick repair on my ancient Heath TC-1, make sure I've got a good pair of 1625's in there.

I have a saying that goes like this: "Don't sweat the petty things and don't pet the sweaty things!"  Jason, I have yet to see a pair of 1625 NOT glow blue. There was a thread a year or two ago about this very subject. I seem to remember a brand new pair of 1625's were ok for about a week or two before they started glowing blue with voice peaks. If it sounds good, it is good. If you want to tinker with the audio, put bigger caps at the coupling points. Beware of that tiny driving transformer. Its one of the weak points in the audio chain.

[Jim KF2SY]

Jason:
Congrats!  You got the beast working.  Use it, enjoy it, get plenty of audio reports. All the while you will learn its care & feeding, idiosycrasies, shortcomings, etc.  It is what it is.
A durable 100 watt Plate modulated 160-10 meter transmitter with punchy audio.   You tamed the beast,you are its master  
Later, you can do some audio mods.  I would suggest go slowly.  Don't do all the mods A - Z.
If their is a problem it will be more difficult to find where and at which step/mod.  Easier
to get back home from a troubleshooting standpoint in small mod. stages

 
P.S.
I won't mention what the breathtaking sound of chimes thru a stock DX100 modulator sounds like.
 

[kf6pqt]

Allright, I put it all back together, and got it on the air on 3870 last night, QSO with some guys in AZ, and some in farther extremities of the Los Angeles area.

You know how when you're listening to your own transmitter in the same room as you're receiver, it sounds like you've got a bad 60hz hum?  Well, I really DID have one.

"Yeah, it sounds kinds bad, but we can understand you, so keep talkin!"

I tried unplugging the mic while transmitting, that apparently didn't help, so it isn't my questionable looking mic cable. (which I'll likely swap out today.)

By the next time it was my turn, they said that the 60hz hum was GONE!

So something must have just needed a burning in.  I'll try it on 7290 today. I'll also work on my scope and have that ready too, its got a noisy-as-hell fan bearing that wouldn't sound too good on the air. I don't want to be asked why I'm running a meat grinder in the shack.

As far as the mods, maybe, if something zorches and I have an absolute need to open it up. Theres a lot of audio snobs out here on the west coast. Personally, I think if you have to have a 6-foot rack of gear between your mic and your transmitter, you're missing the point. They can learn how to deal with a stock DX-100!

Thanks for all the help! 73!
Jason kf6pqt

[W7XXX]

Theres a lot of audio snobs out here on the west coast. Personally, I think if you have to have a 6-foot rack of gear between your mic and your transmitter, you're missing the point. They can learn how to deal with a stock DX-100!

Thanks for all the help! 73!
Jason kf6pqt

True some of the audio over achievers miss the point of vintage ham radio communications, but they are still AM'ers and high power, big audio with low frequency response, big groups, etc. are the modern trend. It is sad some refuse to acknowledge stations using stock lower powered vintage rigs, but that is their loss in my opinion. I have no problem talking to anyone if their audio is understandable.

I am on morning at 3875 0500-0630 PST and feel free to give me a call or join in on the qso anytime.

[W3SLK]

Jason said:

Allright, I put it all back together, and got it on the air on 3870 last night, QSO with some guys in AZ, and some in farther extremities of the Los Angeles area.

You know how when you're listening to your own transmitter in the same room as you're receiver, it sounds like you've got a bad 60hz hum?  Well, I really DID have one.

"Yeah, it sounds kinds bad, but we can understand you, so keep talkin!"

I tried unplugging the mic while transmitting, that apparently didn't help, so it isn't my questionable looking mic cable. (which I'll likely swap out today.)

By the next time it was my turn, they said that the 60hz hum was GONE!

Most excellent! Now you have yourself a little bit of confidence and soon you will be a DX-100 guru  I would recommend getting the scope up and operating. That way you will be able to tell if you have any audio or RF issues at a glance. That 60Hz hum could very well had been a bad ground connection on the mic connector. Leave it until it manifests itself to the point of unbearability. Then you will learn another lesson about your DX-100. Congrats man!