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 JMPGuitars »

Yay! Progress!

A muting jack would be a good idea. But you're right to go after your lead dress. Lead dress is typically the cause of noise issues. Chopstick time!
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Try swapping OT primary leads from position. That's a common oscillation source
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

JMPGuitars wrote:
Mon 04/18/22 3:46 pm
Yay! Progress!

A muting jack would be a good idea. But you're right to go after your lead dress. Lead dress is typically the cause of noise issues. Chopstick time!
Funny thing is, the lead dress in these amps is always a mess from the factory 80 years ago. And they are typically without noise issues. The current issues I have are definitely new/wierd in my experience with these amps, but I will for sure chopstick to see if I can improve things.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Mon 04/18/22 4:45 pm
Try swapping OT primary leads from position. That's a common oscillation source
I just checked, the OT primary leads are going to the right spots, both per schematic, and per another same amp and the amp I pulled this OT from. That said, maybe that is needed in this instance, its one of the easier things to do so I'll utilize that along with my chop sticking as needed.

Gonna go take a video now.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

The noise is somewhat better now that I arranged the wires and bolted the turret board back in its proper place.

Here is the video:

https://youtube.com/shorts/ycipZeNJfw0?feature=share


First is amp totally turned off, nothing plugged into input. You can hear the noise, then I turn the amp volume up a little, not even to 1. Noise is quite loud. Then I plug in guitar and that noise goes away. What is left I think is roughly typical noise for having an Esquire (single coil pickup) plugged in.

Is that particular noise symptomatic of something?

I've never heard that noise before, even without an instrument plugged in, in these types of amps with the same original jack. I imagine it would benefit as noted from a proper shorting jack with 1m to ground. Possibly from a shielded input wire as well?
Last edited by yello on Mon 04/18/22 5:47 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 JMPGuitars »

Install a muting jack.

Lead dress doesn't necessarily mean "neat and tidy." What it really means is that certain wires can't be near or parallel to other wires. Some wires, in proximity to others, can cause loud noise or oscillations, or cause hum.

It's hard to tell from the video, but the tubes look like they're biased too hot / "red plating." That could be the camera lying, but if they are, then fix the bias.

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 04/18/22 5:45 pm
Install a muting jack.

Lead dress doesn't necessarily mean "neat and tidy." What it really means is that certain wires can't be near or parallel to other wires. Some wires, in proximity to others, can cause loud noise or oscillations, or cause hum.

It's hard to tell from the video, but the tubes look like they're biased too hot / "red plating." That could be the camera lying, but if they are, then fix the bias.

Thanks,
Josh
Yes, totally understand the lead dress thing. Just getting wires in the right place to avoid noise, hum, etc.

I edited the above post with more info. Some of the noise is gone, but not the input jack thing. That is new, never had that in these amps. Very apparent and very loud, without a guitar plugged in. Not sure why it would do that other than this amp has different mods than the stock and slightly modded versions I've done in the past. What causes that type of noise, that is apparent even with volume off, and is LOUD when volume turned on?

I'll take a look at the tubes, but that looks typical to me, and was likely the video. The two tubes glowing brightly not he right on the two 6x4 rectifiers. They glow are glowing brightly on the inner portion but not on the plate. The power tubes also do not show any red-plating that I can see.

I imagine I'm finally to the place where I can take voltages inside the amp to see where things are with bias, etc.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Shorting jack with 1m resistor fixed the noise issue that happened with nothing plugged into the input.

Amp sounds fantastic, albeit a little noisy. I really dig the result of the mods suggested to me for more gain and tighter bass response, it’s a whole different amp in a great way. The mods added more gain, and also negative feedback was removed, so maybe those things are the cause of the noise/hum, but nothing super problematic anymore thankfully. I’ll chopstick to see if I can improve it.

Then I need to take voltages. Need to figure out what voltages to take and where.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

ok, now you sorted that out...

try building back the donor amp with the transformers from the one you pulled out your hair these past few weeks. Maybe there will be no problem whatsoever and the riddle continues :D :D :D
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

yello wrote:
Mon 04/18/22 5:28 pm
Bieworm wrote:
Mon 04/18/22 4:45 pm
Try swapping OT primary leads from position. That's a common oscillation source
I just checked, the OT primary leads are going to the right spots, both per schematic, and per another same amp and the amp I pulled this OT from. That said, maybe that is needed in this instance, its one of the easier things to do so I'll utilize that along with my chop sticking as needed.

Gonna go take a video now.
The identification of the wires is not arbitrary. It's impossible to determine the right connection just by the colors of the wires. There is a 50% chance you get it right the first time. Both sides of the center tap are never 100% the same. That's why swapping the wires sometimes resolves the oscillation. Often you don't have a problem when there's no NFB loop...but your amp has one, so it's a candidate for oscillation when wires are connected the wrong way.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Tue 04/19/22 5:58 am
The identification of the wires is not arbitrary. It's impossible to determine the right connection just by the colors of the wires. There is a 50% chance you get it right the first time. Both sides of the center tap are never 100% the same. That's why swapping the wires sometimes resolves the oscillation. Often you don't have a problem when there's no NFB loop...but your amp has one, so it's a candidate for oscillation when wires are connected the wrong way.
Interesting, I would have thought the wiring of the OT would follow the schematic. It shows primary terminal 1 goes to V203, and terminal 3 to V204. And that is how it is in reality on these amps, so I wired the same. I wired it the same as this PT was in the amp it came from. Both wires are same color, blue.

I suppose I could try switching the wires and unshorting the jack with no instrument plugged in to see if that fixes it, but I no longer hear the sound, as it was only happening with no instrument plugged in and a normal 1/4" jack, but I now have a shorting jack installed. I don't know if that sound is oscillation or not but it was loud. Is my shorting jack now hiding a problem, or is it fine given I don't hear it?

The negative feedback loop was removed from the amp as part of the suggested mods earlier in the thread which I imagine adds to some of the noise and gain the loop was there to tame.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

I'm doing some reading about taking voltages. As I understand it, I need to get DC voltages on power tube plates and screens, preamp cathodes and grids. What else - measure every non-heater pin on all tubes? Measurements elsewhere?

I made a reference chart to help me chart what I measure:

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

Post by JMPGuitars »

For what we typically look at, check out my voltage charts: viewtopic.php?t=24418

Your target voltages can be determined by either looking at the datasheets for the tubes, and/or testing other amps with the same circuit and determine an average. Then tweak to your auditory enjoyment.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Here are the voltages I measured last night. Amp was warmed up for several minutes, with all tubes in, speaker load, and with volume up about a 1/4. I didn't know if amp volume needed to be wide open. Also my DMM was set to the DC volts setting, not sure if I had it in the correct range.:

Image

And here are pages from a manual about these amps from 1954, although that particular version of the amp is biased in a different way than mine which is from the 1960's. 98% the same circuit. I would think the voltages could still be a guideline given I don't have the manual from my era, or voltages from another of these amps to compare to:

Image

Image

Image

Image

The above charted voltages are all based off of 117volts at the wall. My house is putting out about 122-124v.

Several of my voltages seem high, in the range for the max of the tubes or beyond. Would love to hear your thoughts on next steps if it needs different bias resistors, or if other sections look off and why that would be. This is the first time I've taken amp voltages so need to learn to interpret the results.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Preamp voltages look fine.
The power tubes are really odd. The screens should be 10V less than the plates.. yours are 10V higher than the plates. That's bad...
Are you sure you didn't switch them?
Also... what is the cathode resistor on the 6AQ5 tubes? That 20V cathode voltage is pretty high...
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