New 4X1 X 4X1's Rig - Problem with audio spikes - Questions - Pictures

[K1JJ]

Very good, Jim and Bruce -

Yes, I will try a non-polarized electroylic combo as you described.

I will also try the short-to gnd test too, Bruce.  Though, the SS amp's input audio pot turned all the way down had no effect. I think it probably puts the input circuit to ground - not sure yet.

Well, so far the RF is not coming in thru the input audio, NFB, AC line - or coming in thru the grid leads output as based on my tests.   I will put a scope on the ac line, Bruce, to check the AC dynamic stability. And, also poke around with the scope probe at the various in and outs to see what they look like.

If it's not fixed  the coupling cap, then it almost seems to be generated by magnetic coupling thru the steel case from the steel cabinet housing the 4X1 below it.   We'll see.


After dinner I will build a pair of spark gaps FIRST, install them and continue testing with all iron protected.

Thanks again for all the suggestions. I wuda run out of then by now...

T

[WA1GFZ]

Tom,
Rather than guessing, shoot a tone through the system and monitor the output of the SS amp with a scope and unkey to see what happens. Many times a little RF in an amp will reduce the gain. Remove the RF and gain goes back to normal. If the input stage of the amp is a bipolar op amp or bipolar transistor this is very possible.

[K5UJ]

Tom Bruce, did not know that about the TMC braid, thanks.  I used to use braid outside--big mistake but that's a whole different deal.  Time now for a noob question--what is NFB?  Can't decode that one.  Tnx

Rob

[W2XR]

Tom Bruce, did not know that about the TMC braid, thanks.  I used to use braid outside--big mistake but that's a whole different deal.  Time now for a noob question--what is NFB?  Can't decode that one.  Tnx

Rob

Hi Rob,

No problem! NFB stands for negative feedback.

73,

Bruce

[K1JJ]

Yes, in this case audio negative feedback derived from a LONG string of 220K resistors across one modulator plate cap to ground - then tapped at the low voltage bottom into an IC which mixes it with the regular 1 volt audio. The plate cap is selected to provide negative feedback - the other cap wud give positive feedback and take off.

Just to show how easy it is to make a set of arc gaps for your iron, here's a set I'm adding to the mod xfmr sec and Heising reactor for protection. Once set to arc very occassionally when a big, rouge modulation peak is hit, they can remain set as-is for a long time.

Shown installed in a place you don't want your hand when keyed-up...

Now to fire this baby up and start looking around wid the scope.


T

[N2DTS]

Maybe it was Derb then?

I think your outdoor blower setup is cool (pun), I could never get used to a loud blower in the shack, it kind of ruins the nice audio to have something like that in the background.
I really like rigs with no fans or blowers, the push pull 812 rig was like that, and so is the 3X4D32 rig, the 813 rig has blowers but not loud ones.
The new computer has the loudest fan in the shack, I should do something about that...

Brett

Brett - You heard the testing in the afternoon and morning - good. I was testing only in the evening after dark on about 3856 into the dummy.

Yes, no doubt the RF is being generated in the SS driver amp on unkey only.  I'm just trying to come up with a way to fix it.

I'll try some more filtering ideas tmw and think more about a relay sequencing during unkey too.

T

[KC2IFR]

Tom,
As usual your threads are the best.......this one IMHO is great.
Just a couple of points........
When Jamie was finishing my 4x1 by 833's, for a while I drove the 833's with a backward connected transformer as u are (Hammond). I drove it with a QSC SS amp (600 watts in the bridge mode into 4 ohms). The power of the amp has nothing to do with it.....its the voltage available that counts. I had no problems with the QSC amp. This amp is very forgiving.......will work into reactive loads with no problem. Finally Jamie finished the cathode follower ckt. to drive the 833's.......a pair of 807's driven by a pair of 807's.
Using the backward connected tranny was fine untill the grids of the 833's drew grid current. At that point the Z of the grid ckt went WAY down causing some drive problems.
Anyway.........I think u should consider some sort of cathode follower to drive ur modumalotors.....
Tom......I am NOT trying to tell u how to design ur rig.....u are a LOT smarter the me!
Again.....JMHO.

Bill

[N2DTS]

The 8 ohm to grids setup works very well with all sorts of tubes.
I have done it with 811a's, 100TH's, 4cx250b's with great results.

Both the 811a's and 100th tubes are class b with grid current, and I think an 8 ohm source is quite stiff, so it works well.

Most people terminate the secondary of the driver trans with resistors that match the output impiedance so its loaded all the time.

I have never had any kind of spike on unkey, the audio stays on all the time (to the grids).

There really should be no change in the audio amp or the mod grids when the transmitter is unkeyed.
The mod tube grids should look like an open circuit, while transmitting, or not.
The audio amp should see the same conditions, key up or down.
The only time things would change (in class B) would be on mod peaks when the grids draw current.
With a pair of 4-1000 tubes, I am not sure you would need to drive them to class B to get 100% modulation!

Now if you were running AB1, key up or down, full modulation, the audio amp would always be looking into an open, no load, no change at all, ever.

Brett

[Gito]

Hi.
I always use AC filters/surge suppressor in my Transmitters ,I think /hope it will cleanup the AC line from RF and surges

Regards

Gito

[K1JJ]

I added the back-to-back pair of electrolytics (1600 uf @ 450V = 800 uf) to the output of the SS amp.  It has made a huge difference, almost livable, though still not there.  The spike on the modulator current meter is now only about 200ma. (It was over an amp before.)  There is still an ocassional spark on the gaps. (The new gaps are working FB)

It was DEFINATELY the SS amp not liking what it saw without the capacitor - AND now being influenced by the RF or collasping inductor fields..  Dig this: I can disconnect the SS amp's input completely, disconnect the amp's output 8 ohms completely, and see no signs of pulse from the amp on the scope when unkeying, as you'd expect. BUT by adding a small 12" wire hanging off one of the 8 ohm terminals, the SS amp spikes and I can see the overload diode flash.  It is obviously very sensitive to mag fields. Maybe I will move it to the other side of the room and deal with that.

All in all, the 4X1 rig seems very stable for both RF and audio when the SS amp behaves. But the SS amp is squirrelly and that's when the arcing starts.

So, Bruce and Rodger, your suggestions were pretty close to the problem, as well as a few others in general.

Thanks for the comments Bill. I may take you up on it and axe for a schematic of the tube cathode follower if this doesn't pan out. But when I hear the audio in the monitor of this thing with the full NFB working, I am really excited... :-)

Thanks again for all the help, OMs! - I'm gonna try some different transformer taps and if still NG, I'll move the amp. I won't stop til it is totally clean on unkey. It actually did unkey a few times without sparks or current meter spikes, so I know it's possible with this rig and general config - with some more work.

Seez ya tmw.

T

[KC2IFR]

Tom,
This is a pic of the chassis that has cathode follower ckt........

[KC2IFR]

This is a wider view of the sub chassis........

[K1JJ]

Tom,
This is a pic of the chassis that has cathode follower ckt........

Oh my... what a nice looking driver!  I wonder how you drive it?  I have about 1-2 volts available for whatever audio driver I use.

I'll bet the requirements for the 833A's are very similar to triode-connected 4X1's.

Please send me (or better yet, post) the schematic and what voltages are required for it.  I don't have enuff 807's but wonder if I cud sub them for triode connected 813's?  Maybe the 807's have more gain as-is  - have to look it up.


General comment:  I just measured the grid bias on the rig when in operation and was surprised. I have 90V of fixed regulated bias, but at 40 ma of grid current generates an additional -215 volts of grid leak for a little over -300 bias. The specs call for about -200v, so the final is in DEEP class C, but seems to like it.

I'm using a 5K grid leak resistor which is a little high.

I heard it said here before that deep class C makes better modulation. (as well as the higher efficiency.) I wonder if anyone has experimented with this?

T

[W2XR]

I added the back-to-back pair of electrolytics (1600 uf @ 450V = 800 uf) to the output of the SS amp.  It has made a huge difference, almost livable, though still not there.  The spike on the modulator current meter is now only about 200ma. (It was over an amp before.)  There is still an ocassional spark on the gaps. (The new gaps are working FB)

It was DEFINATELY the SS amp not liking what it saw without the capacitor - AND now being influenced by the RF or collasping inductor fields..  Dig this: I can disconnect the SS amp's input completely, disconnect the amp's output 8 ohms completely, and see no signs of pulse from the amp on the scope when unkeying, as you'd expect. BUT by adding a small 12" wire hanging off one of the 8 ohm terminals, the SS amp spikes and I can see the overload diode flash.  It is obviously very sensitive to mag fields. Maybe I will move it to the other side of the room and deal with that.

All in all, the 4X1 rig seems very stable for both RF and audio when the SS amp behaves. But the SS amp is squirrelly and that's when the arcing starts.

So, Bruce and Rodger, your suggestions were pretty close to the problem, as well as a few others in general.

Thanks for the comments Bill. I may take you up on it and axe for a schematic of the tube cathode follower if this doesn't pan out. But when I hear the audio in the monitor of this thing with the full NFB working, I am really excited... :-)

Thanks again for all the help, OMs! - I'm gonna try some different transformer taps and if still NG, I'll move the amp. I won't stop til it is totally clean on unkey. It actually did unkey a few times without sparks or current meter spikes, so I know it's possible with this rig and general config - with some more work.

Seez ya tmw.

T

Hi Tom,

I'm pleased the suggestion I made was workable and things now appear to be going in the right direction for you! Glad to be of service, OM! Did you also bypass the caps with the 4 ohm/20 watt resistor as I had suggested? That may well be the icing on the cake.

Now, one more point. Personally, I hate to couple really clean audio through an electrolytic capacitor, particularly such a large value as what you are now using. They generally sound horrible, which is one reason, aside from cost, that virtually all high quality solid state audio amplifiers did away with the large electrolytic DC blocking caps at the output and went with an all-DC coupled architecture. Speaker crossover network designers did the same thing; they dumped the electrolytics once low-cost, high quality film caps became available.

That said, you may find the sonic degradation imposed by the back-to-back electrolytics (to effectively create a non-polarized cap) is not that significant, and that you are still pleased with the end result.

In my rig, all audio stages are coupled or decoupled with either polypropylene film or oil caps. The power supply filter caps in the 1250 VDC plate supply for the 845s are oil caps, and the 400 VDC supply for the input and driver tubes to the 845s is filtered and decoupled with polypropylenes. Come to think about it, the electronically regulated shunt bias supply for the 833A modulators uses all polypropylenes for power supply filtering as well. These capacitors are also much more reliable, long-term, when compared to electrolytic capacitors.

Personally, I would consider the use of a solid state (or vacuum tube amplifier) that does not require the use of the output cap/resistive network, and will tolerate the inductive/reactive load of the driver xfmr while still remaining unconditionably stable under any load condition or phase angle, etc.

Just my thoughts!

When can we hear this rig???

73,

Bruce

[KC2IFR]

Tom,
The driver ckt. using the 807's was not designed by Jamie. It is a ckt used by some BC transmitter and I cant remember which one.
Perhaps someone on the board will know.
Trying to contact Jamie is almost imposable!
Bill 

[W2XR]

Tom,
The driver ckt. using the 807's was not designed by Jamie. It is a ckt used by some BC transmitter and I cant remember which one.
Perhaps someone on the board will know.
Trying to contact Jamie is almost imposable!
Bill 

Hi Bill,

It was used with some variations during it's production life in the Gates BC-1T thru BC-1H rigs, and probably the 500 watt versions of these transmitters as well.

I have the 807 audio driver board assy. from a Gates BC-1G; I may consider using it in my next 833A modulator, which will be mated with my TMC GPT-750.

73,

Bruce

[KC2IFR]

Thanks Bruce,
Perhaps u can supply the sceezematic to Tom.........
It is a very good ckt.

Bill

[K1JJ]

OK, Bill. I'll bet someone will come up with a broadcash 807 cathode driver schematic.

* whoops modify - I see Bruce did already. Can you post it, Bruce?


Bruce - I decided to move the SS amp to the other side of the room.  Do you think I should feed the 8 ohm cables over to the rig, or mount the driver transformer next to the SS amp and run the high impedance modulator grid leads over to the rig?

Seeing how sensitive the amp is to the 8 ohms being exposed, maybe I shud run the grid leads instead.  I will use shielded cable for whatever I do, of course.

T

[KC2IFR]

Tom,
I drive the 807 stuff with the output of my BL40 Modulimiter. It takes about 2 Volts PEP.
Bill

[W2XR]

OK, Bill. I'll bet someone will come up with a broadcash 807 cathode driver schematic.

* whoops modify - I see Bruce did already. Can you post it, Bruce?


Bruce - I decided to move the SS amp to the other side of the room.  Do you think I should feed the 8 ohm cables over to the rig, or mount the driver transformer next to the SS amp and run the high impedance modulator grid leads over to the rig?

Seeing how sensitive the amp is to the 8 ohms being exposed, maybe I shud run the grid leads instead.  I will use shielded cable for whatever I do, of course.

T

Hi Tom,

Please make the long cable runs only with the 8 ohm amplifier output! You want to keep the high impedance (secondary) side of the driver transformer cable run as short as possible.

Further to my previous thread on polypropylene caps for power supply filtering, etc.; I have attached a photo of the underside of my electronically shunt regulated bias supply I use with my class B 833A modulator. The polypropylene caps are the large black capacitors, and are frequently used for AC motor starting or motor running applications. These caps are rated at 40 uf each, and at 630 VDC. The three meters on the front panel are for monitoring the 845 audio driver cathode current, 845 plate voltage, and the regulated negative DC output to the 833A modulator grids.

73,

Bruce

[K1JJ]

Bill,

Two volts is very do-able.  I will have to get this SS amp working temp and see about that 807 driver.  What various voltages does it require?


Very professional looking unit, Bruce.  As I remember, your work is indistinguisable from factory work...(American, that is) 

T

[W2XR]

Thanks Bruce,
Perhaps u can supply the sceezematic to Tom.........
It is a very good ckt.

Bill

Hi Bill,

Yes it is. It saved Gates a lot of cost when they were able to delete that massive push-pull class A 845 audio driver circuit along with it's large 1200 VDC plate supply, and the problematic and very costly class B audio driver transformer.

Unfortunately, I only have a poor quality copy of the original schematic. Let me see if I can redraw it along with the special accompanying DC power supply, and post it here on the board.

Bill: did you get my private message to you of late yesterday? I never heard back from you!

73,

Bruce

[KD6VXI]

OK, Bill. I'll bet someone will come up with a broadcash 807 cathode driver schematic.

* whoops modify - I see Bruce did already. Can you post it, Bruce?


Bruce - I decided to move the SS amp to the other side of the room.  Do you think I should feed the 8 ohm cables over to the rig, or mount the driver transformer next to the SS amp and run the high impedance modulator grid leads over to the rig?

Seeing how sensitive the amp is to the 8 ohms being exposed, maybe I shud run the grid leads instead.  I will use shielded cable for whatever I do, of course.

T

Tom,

Just a thought... Have you thought of bypassing the amp on the output for RF?  .1, .01 and .001 uF across the terminals, or from each speaker terminal to chassis ground?

Had good luck with that combo when running AM and linears in the past, when TV's where more radio receivers.  Got rid of more than a bit.  You also might wanna try running the cables from the amp through some ferrite RIGHT at the speaker terminals.

Good RF bypassing can be a bear sometimes...  I know, been there.  Eventually you'll find it, tho.  I LOVE the design.  See Thru is SECKZI, just ask Victoria's Secret


--Shane

[KD6VXI]

The polypropylene caps are the large black capacitors, and are frequently used for AC motor starting or motor running applications. These caps are rated at 40 uf each, and at 630 VDC.

Another good place to look for high value AC caps is lighting supply stores.  Metal Halide, High Pressure Sodium and associated lights take one to three caps to run correctly.

I've found many different sizes in ballasts across the years.


--Shane

[N2DTS]

I dont think the cap should be needed.
All you need is to move the amp someplace away from the rig, and find some shielded cable heavy enough to run the 8 ohms in.
I bet that cleans things up.

More might not be better here, as far as audio power goes.
I use a symetrics dual 20 watt amp I got at a pawn shop, because it is only one rack space high, and it was dirt cheap.
It takes less than half power to drive anything I have.
Each output runs in twinax to 1/4 inch trs jacks on the modulator cabinets.

No matter what antenna experiments I do (antenns are over the house) I do not get into the audio.

If a pair of 4-1000 tubes take 40 watts drive power, I would use a 100 watt amp, not a 600 watt amp.

Brett