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 »

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

Post by JMPGuitars »

yello wrote:
Thu 04/21/22 2:48 pm
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.
You could use Ohm's Law to calculate the resistance you want. I would still use a similar method that you see in the 18W drawings with 1K resistors to the screens after the node A to B dropping/decoupling resistor.

Thanks,
Josh
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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 3:01 pm
yello wrote:
Thu 04/21/22 2:48 pm
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.
You could use Ohm's Law to calculate the resistance you want. I would still use a similar method that you see in the 18W drawings with 1K resistors to the screens after the node A to B dropping/decoupling resistor.

Thanks,
Josh
I've never used Ohm's Law so would need to figure out how to do those maths...and I don't know what resistance I would want as an end.

I have some 3w 470ohm and 5w 1k ohm resistors on hand that I had purchased for screen stopper use but hadn't implemented yet. I already put 1.5k 1/2w control grid resistors for each 6aq5.

I'm still wondering about the overall bias being too hot. Is that related to removing the unused 6aq5 so now more current is going to the other tubes?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

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.
Thinking back, I have a nagging suspicion there is a wiring issue. The only solder joint that looked non original in the wiring of the amp when I first opened it, was at the first power tube 6aq5 pin 6, the screen. The wiring deviated slightly from the other amp I had in how it connected between the turret board component and tube socket and Cap, but I thought logically it was the same thing. I'll take a second look today and rewire it to match my other one, then test voltages.

The screen stopper might still be called for, but at least looking at and correcting the potential issue first would help!
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

I would try this:
Change the 350 ohm resistor between C216 B and C to 1k5 5W
Add a 1k 3W resistor between C216 C and each screen
Change the cathode resistor to 330 ohm 5W

Deleting the futile current draw of the unused tube doesn't affect anything IMHO, except a little more gain in the preamp.. maybe
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Thu 04/21/22 4:14 pm
I would try this:
Change the 350 ohm resistor between C216 B and C to 1k5 5W
Add a 1k 3W resistor between C216 C and each screen
Change the cathode resistor to 330 ohm 5W

Deleting the futile current draw of the unused tube doesn't affect anything IMHO, except a little more gain in the preamp.. maybe
I checked this current amp against another in my shop, same exact circuit. Same mods (short of preamp bypass caps). The wiring is correct, there was not an error as I wondered, there are two variations of how wiring gets from C216c to the power tubes: Cap wired to power tube 2 screen, then wire to power tube 1 screen OR cap wired to power tube 1 screen, then wire to power tube 2 screen. Either case has the wire from R240 (350r) going to power tube 1 screen.

Also, I swapped the caps, tubes, and fuse from current amp to the other one, tested voltages, and I get the exact same scenario - ~330 on screens and ~320 on plates. That must just be how these amps are, though maybe not ideal as Josh pointed out. They certainly lasted through decades of use by the military, and I have several friends playing them without issue both in stock and modded formats.

Is it R240 that is between C216B and C216C? That what it looks like. I can change that to 1k5 5w, will just need to order that part.

When you say to add a 1k 3w resistor between C216C and EACH screen, do you mean 1 resistor and connect pins, or separate resistors for each screen and therefore separate wires from the cap to each screen? I'm trying to visualize it, as my 18w diagram for my ef86 build just uses 1 resistor from cap, then wires screens together. I don't fully understand the concept thus my confusion - I'm trying to increase resistance between C216C and the screens right?

Then it looks like if the above is correct for R240 swapped to 1k5, then that leaves R228 changed to 330r right? Or do you mean change R241, the 220r 8w resistor to a 330r 5w?

If you mean R241, I'll have to think through how I implement that and where I put it. R241 is a long ohmite that doubles as a standoff to hold the turret board in place. I could leave it in place for its mechanical function and add the cathode resistor elsewhere, like down with the 2200 uf cap in the new terminal I put in. Alternatively could I put another resistor in series with it to get it up to 330r or close? I have about 50 10w 50r laying around from a Hammond organ. Probably easier to do option A.
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