Military projector amp - last edited 9/26/22 - layout help needed

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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:

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

Post by yello »

Bieworm wrote:
Wed 04/20/22 11:22 am
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...
I will re-measure when I get home from work to be sure I didn't switch the screen and plate voltages when I wrote them down. If I'm reading the schematic correctly, R228 and R240 are the resistors going to the screens. Schematic calls for 350 ohm 3w, and closest I found was 360 ohm 5w.

The 6aq5 cathode resistor is a shared 220 ohm 8w Ohmite resistor. The accompanying cap was changed from 100uf to 2200uf. Here is a photo of the resistor, it is the stock resistor and measures right at 220 ohm:

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

Post by Bieworm »

When your voltages are correct the amp is biased at approx 110%.. you should bring that to 85%, thus raise the value if the cathode resistor.
Your screen voltages should be lower than your plates!
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Wed 04/20/22 12:08 pm
When your voltages are correct the amp is biased at approx 110%.. you should bring that to 85%, thus raise the value if the cathode resistor.
Your screen voltages should be lower than your plates!
How do I accomplish both of those things? -> What size cathode resistor should be used to get the bias right? And how do I lower the screen voltage?

Those part of the amp are basically stock so I am surprised they are presenting as problematic. I've used lots of these amps in their stock format, and sent them along to friends, and there hasn't been any issues with them, though I've never taken voltage measurements on them.

I might have a matching amp to this amp in my shop, that I have already done a few of the same mods to. I think it is done if I remember correctly and I just need to pop in a fuse and tubes to test it. I could take voltages on that one and compare results to this one.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

The first thing to do is to lower those screens. On a normal 18W amp the plates are at node A. That's the 1st filter cap after the rectifier. The screens are at node B, the 2nd filter cap. There's a dropping resistor between those nodes in the range of 1.5k to 2.2k 3W to 5W. That puts your screens about 20V lower than the plates.
Try moving your screens to that node. There should be a 1K 3W resistor to each screen from that point.
I need a clearer image of this part of your schematic. The values are very blurry like this..
Screenshot_20220420-201753_Chrome.jpg
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Wed 04/20/22 1:18 pm
The first thing to do is to lower those screens. On a normal 18W amp the plates are at node A. That's the 1st filter cap after the rectifier. The screens are at node B, the 2nd filter cap. There's a dropping resistor between those nodes in the range of 1.5k to 2.2k 3W to 5W. That puts your screens about 20V lower than the plates.
Try moving your screens to that node. There should be a 1K 3W resistor to each screen from that point.
I need a clearer image of this part of your schematic. The values are very blurry like this..Screenshot_20220420-201753_Chrome.jpg
Here is a clearer version:

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

Post by Bieworm »

D...
You must have messed something up with the wiring OR the measurements aren't correct. There's 2x 20uf filter caps and 2x 350R dropping resistors between the plates and the screens.
Your voltages don't make any sense...
20220421_064259.jpg
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Wed 04/20/22 11:45 pm
D...
You must have messed something up with the wiring OR the measurements aren't correct. There's 2x 20uf filter caps and 2x 350R dropping resistors between the plates and the screens.
Your voltages don't make any sense...20220421_064259.jpg
Yep, could be either one!

I'll doublecheck wiring, but fairly confident I got everything right. I did not have to unsolder many wires, it was mostly component replacement on the turret board. I can also take a pic of my work and upload it.

For voltage measurements, is there as certain procedure? I want to make sure I am measuring correctly. I assume it is tested in playing conditions (speaker load, tubes in, amp warmed up), and simply DMM set to DC and one end clipped to ground (like at HV CT), other end measures tube pins - does amp volume need to be turned on or at a certain level?

The additional check I can do is to test my other similar amp's voltages, hopefully can do that tonight.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

The most accurate way to measure is after playing it for a while, so it's warm enough and caps have done some labour. The position of the pots is less important, since you're not measuring signal going through the amp. You measure reference voltages at idle.

I tell you, it's not making any sense .. the screen voltages being higher than the plates. There must be something wrong, like I said earlier.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

Bieworm wrote:
Thu 04/21/22 11:07 am
I tell you, it's not making any sense .. the screen voltages being higher than the plates. There must be something wrong, like I said earlier.
The simple answer is that the resistor values are too low to drop the screen voltages. Just because something is in a schematic, doesn't mean it's good.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

JMPGuitars wrote:
Thu 04/21/22 12:56 pm
Bieworm wrote:
Thu 04/21/22 11:07 am
I tell you, it's not making any sense .. the screen voltages being higher than the plates. There must be something wrong, like I said earlier.
The simple answer is that the resistor values are too low to drop the screen voltages. Just because something is in a schematic, doesn't mean it's good.
What value resistor would be better there to get the screen voltages right?

Would the removal of the circuit to the left of the R228/R240 affect both the plate/screen issue as well as overall tube bias? That was the 6aq5 oscillator section for film projector purposes that isn't needed for guitar. It is fully removed so the transformer and circuit have one less tube.
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