Military projector amp - speaker distorting/overworked

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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

JMPGuitars wrote:
Sun 04/24/22 2:16 pm
R240 being soldered to V203 or C216 C is electronically the same in your diagram. The only question is what is the dashed Y line you drew?

Some amps are made with the screens slightly higher than the plates. This is not ideal for guitar amps.
I tried to explain that above. In three of my amps C216C is connected to V203, represented by the line "X". Alternatively, in one of the amps C216C is connected to v204 represented by the dotted line "Y", which would be in place of "X" in that amp.

Thanks for the note on "electronically the same", that goes to the root of my question and what I am trying to learn here and in general. I will re-wire according to your diagram.

That makes more sense to learn some amps, like these film projector amps, would be designed like this with screens higher than plates, and to learn how to fix it for guitar.

My 5w 30v zener diodes should arrive mid-week. Where in the diagram do I insert it?

Trying to follow your diagram about dropping B+ on an 18w, it looks like the end with voltage stripes connected directly to C216A, the other end going to R228?

Then do I leave the current OT wiring in place? The OT currently goes to the first 6x4, then one wire goes to C216A, and another wire connects to the other 6x4? I imagine I just shift the connection at C216A over to be at the other end of the diode.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

yello wrote:
Mon 04/25/22 9:51 am
Then do I leave the current OT wiring in place? The OT currently goes to the first 6x4, then one wire goes to C216A, and another wire connects to the other 6x4? I imagine I just shift the connection at C216A over to be at the other end of the diode.
Ignoring your schematic, and speaking broadly, the Zener typically goes after the first filter cap. Which in our 18W amps is where the OT secondary's CT normally went. But instead we're putting a Zener diode between filter cap one (usually marked A), and the other end of the Zener goes to the OT secondary CT. As it says in the thread, the Zener stripe always points to the source voltage, A, and the non-stripe side goes to the OT secondary CT. I do this by adding a terminal strip, exactly like my diagram. I'm not moving the red CT wire to node B, it floats between A and B (the two halves of my filter cap can).
findingzener.jpg
Thanks,
Josh
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

JMPGuitars wrote:
Mon 04/25/22 1:29 pm
Ignoring your schematic, and speaking broadly, the Zener typically goes after the first filter cap. Which in our 18W amps is where the OT secondary's CT normally went. But instead we're putting a Zener diode between filter cap one (usually marked A), and the other end of the Zener goes to the OT secondary CT. As it says in the thread, the Zener stripe always points to the source voltage, A, and the non-stripe side goes to the OT secondary CT. I do this by adding a terminal strip, exactly like my diagram. I'm not moving the red CT wire to node B, it floats between A and B (the two halves of my filter cap can).

findingzener.jpg
Zener arrived in the mail - The following is how I plan to implement the 1K screens resistor, and the zener, if I am understanding things correctly. I added my OT, and rectifier tubes to the diagram, and put them after the zener.

Here is the original erroneous version I posted:

Image

Here is the corrected version after the insight given below:

Image
Last edited by yello on Fri 04/29/22 12:38 pm, edited 1 time in total.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Incorrect. The wire from the rectifier should land on the 1st filter cap A. From there the zener takes off to the dropping resistor, where the OT primary lands. Watch out with the striped side being correct. It should point at the filter cap point A.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Fri 04/29/22 10:34 am
Incorrect. The wire from the rectifier should land on the 1st filter cap A. From there the zener takes off to the dropping resistor, where the OT primary lands. Watch out with the striped side being correct. It should point at the filter cap point A.
Thanks for the correction, that's why I posted my plan!

I see my error now. Josh mentions in his B+ lowering instructions to place the OT after the diodes and I assumed when looking at the diagram that meant put everything after, but I see now in the full schematic that the rectifier stays right at Filter A, my mistake.

Here is the corrected version:

Image
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

I installed the screen resistor and B+ dropping zener diode this weekend, and tested tonight. No shorts, and amp operates, and I'll get some voltages I can supply tomorrow when I fill in my chart and upload it.

In the meantime I made a quick video of the strange noise I mentioned a week or two ago - namely a squealing kind of sound related to the jack. It was present with the original 1/4" jack, so I installed a shorting jack with 1m. Now the sound isn't present unless I lift the shorted tip.

In the following video you can hear the sound, mild at first when the amp volume is off and I lift the jack tip, then gets louder when I turn the amp volume to 1 and again lift the tip. Anyone recognize what it could be, or if my shorting jack is now hiding some issue? The previous suggestion was to check the OT primary leads, but I didn't explore further than confirming the right lead went to the right tube per schematic, so picking this up again as a (hopefully) final issue to sort in this long amp troubleshooting adventure.

https://youtube.com/shorts/kfpZFqRooLQ?feature=share
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Hey.. I assume you omitted the video section , like you were treating a while ago?
I suspect that area. It looks like there is no grid stopper for V1a, nor a grid reference resistor. That's a good coctail for noise... there might even be issues when the grid ref resistor isn't a MF type, let alone NO resistor at all!
The input is the most sensitive part of any amp. Remove what shouldn't be there. There should be a shielded wire directly from the input to the grid of V1a, with a grid stopper right at pin 2. The shielded wire should also have only 1 connection for the shielding, preferably at the input ground. The other end of the shielding should be clipped off and sealed off with heat shrink.

I would recommend you put a 1M MF resistor between the tip and ground of the input jack AND a 10k MF resistor at the grid pin of V1a. Also remove any connections to V1a other than the plate wiring, the input grid and the cathode resistor.
input noise Dayn.png
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Tue 05/03/22 12:27 am
Hey.. I assume you omitted the video section , like you were treating a while ago?
I suspect that area. It looks like there is no grid stopper for V1a, nor a grid reference resistor. That's a good coctail for noise... there might even be issues when the grid ref resistor isn't a MF type, let alone NO resistor at all!
The input is the most sensitive part of any amp. Remove what shouldn't be there. There should be a shielded wire directly from the input to the grid of V1a, with a grid stopper right at pin 2. The shielded wire should also have only 1 connection for the shielding, preferably at the input ground. The other end of the shielding should be clipped off and sealed off with heat shrink.

I would recommend you put a 1M MF resistor between the tip and ground of the input jack AND a 10k MF resistor at the grid pin of V1a. Also remove any connections to V1a other than the plate wiring, the input grid and the cathode resistor.input noise Dayn.png
Yes, I removed the complete film section. That film section had a shielded wire and resistor going to V1a, pin 2, along with the regular 1/4" audio in jack that was originally used for microphone. I assume the removal of the film components is what caused the problem, remedied as you have outlined, thank you for the diagram, that helps.

I already upgraded the jack to a shorting jack with 1M MF resistor, but I did not upgrade the shielded wire, or put a 10k resistor, or separate the ground (I assume the blue and yellow lines leading to the grey circle in the upper right of your drawing is a separate ground point?) . I will do that. I previously never did with these amps as I left all the film wiring and components in place and never had this noise issue.
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Re: Military projector amp - speaker distorting/overworked

Post by yello »

Here the the new voltages, taken after the screen resistor and zener diode were installed. This time I also captured the voltages at nodes A (before zener), A (After zener), B, and C:

Image

And here are the previous voltages for comparison:

Image
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Re: Military projector amp - speaker distorting/overworked

Post by Bieworm »

yello wrote:
Tue 05/03/22 10:35 am
Here the the new voltages, taken after the screen resistor and zener diode were installed. This time I also captured the voltages at nodes A (before zener), A (After zener), B, and C:

Image

And here are the previous voltages for comparison:

Image
Did you put in a 1k screen resistor?
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Re: Military projector amp - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Tue 05/03/22 11:42 am
Did you put in a 1k screen resistor?
Yes, installed per the diagram a couple posts up. 1k 3w resistor from C216C to screen of tube V204, with a wire then connecting to the other screen at v203.

The voltage numbers are have been lowered in several instances by the zener diode, but I don't know how to interpret the new numbers much. The power tube screens are still slightly higher than the plates.
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Re: Military projector amp - speaker distorting/overworked

Post by Bieworm »

yello wrote:
Tue 05/03/22 1:15 pm
Bieworm wrote:
Tue 05/03/22 11:42 am
Did you put in a 1k screen resistor?
Yes, installed per the diagram a couple posts up. 1k 3w resistor from C216C to screen of tube V204, with a wire then connecting to the other screen at v203.

The voltage numbers are have been lowered in several instances by the zener diode, but I don't know how to interpret the new numbers much. The power tube screens are still slightly higher than the plates.
I wouldn't worry too much about the screens being slightly higher than the plates. It's not ideal, but you get maximum headroom this way. It's better for tube life that the screens sit lower, but you've got a lot of those tubes in stock anyway. And the problem is only present at idle...
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Re: Military projector amp - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Tue 05/03/22 2:23 pm
I wouldn't worry too much about the screens being slightly higher than the plates. It's not ideal, but you get maximum headroom this way. It's better for tube life that the screens sit lower, but you've got a lot of those tubes in stock anyway. And the problem is only present at idle...
The other concern was being biased too hot (therefore potentially needing to change the cathode resistor) - how do I calculate that with the new voltages? I see an online calculator but don't think I'm doing it right.
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Re: Military projector amp - speaker distorting/overworked

Post by Bieworm »

Your tubes are biased at 90% dissipation, which is fine enough in a cathode biased amp.
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Re: Military projector amp - speaker distorting/overworked

Post by JMPGuitars »

Bieworm wrote:
Tue 05/03/22 10:30 pm
Your tubes are biased at 90% dissipation, which is fine enough in a cathode biased amp.
Tell him how you figured that out so he can do it. 😉
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