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

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

Post by yello »

I am having an issue with an old military projector amp. I've modded them before, with great results. I am experiencing a rattling speaker sound as I increase volume, and only the low notes. The internal Jensen 4" speaker is working REALLY hard and it is even rattling the combo chassis, everything. I've modded and played several of them and never had a problem like this, including with the small speaker.

Here are the basic mods I did to both amps:

1) Upped the negative feedback resistor (connected to OT secondary) from 75k to 220k. Goal was no negative feedback for more gain. It is R222 on the schematic.

2) Lowered the V1B cathode bias resistor from 5.6k to 3.3k for more gain. R208.

3) Replaced the two 3w 350ohm ohmite resistors with 5w 360ohm wirewound cemented resistors. R240 and R228.

4) Rebuilt new plug in can type Filter caps (they plug into tube type sockets). 2 original cans were each three 20MFD 400v. I made new ones with 22uf 450v caps. C214 A/B/C and C216 A/B/C.

On the previous exact amp I did for my friend, I did the same mods as above, but not step 4, didn't change the filter caps. It sounded really good and he loves it.

I have tried to check and tighten every screw and socket to make sure nothing is rattling. I have looked at every new (and old) solder joint. I have measured the new filter caps out of the amp and they are all in spec with proper continuity for the plugs. I have looked for misplaced wires, solder joints, etc.

Both amps are having an identical problem, after identical mods, so I'm thinking it is related to the parts I put in.

I even changed the speaker in one of the amps in case that was it, but exact same problem persists.

I'm wondering if I have bad caps, or maybe I have created blocking distortion in the preamp by lowering the cathode bias resistor too much, or ??? I'm in the early stages of learning about amps so my knowledge is limited, thus my post asking for insight, encouraged by an 18watt.com friend to do so.

Would love any advice you can offer, questions you can ask, etc. - attached is a picture of the schematic, and the amps, and the caps I built.

Image

Image

Image

Image

Found the schematic online, a little easier to see, page 19:

http://www.acofs.org.au/part_5_files/Be ... 614EVM.pdf

I can attempt to make a sound sample tonight and upload it tomorrow.
Last edited by yello on Mon 09/26/22 11:21 am, edited 4 times in total.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

yello wrote:
Wed 02/02/22 3:35 pm
I am having an issue with an old military projector amp, a Signal Corps model 614 or AQ2A. I've modded them before, with great results. I am experiencing a farty speaker sound as I increase volume, and only the low notes. The internal Jensen 4" speaker is working REALLY hard and it is even rattling the combo chassis, everything. I've modded and played several of them and never had a problem like this, including with the small speaker.

Here are the basic mods I did to both amps:

1) Upped the negative feedback resistor (connected to OT secondary) from 75k to 220k. Goal was no negative feedback for more gain. It is R222 on the schematic.

2) Lowered the V1B cathode bias resistor from 5.6k to 3.3k for more gain. R208.

3) Replaced the two 3w 350ohm ohmite resistors with 5w 360ohm wirewound cemented resistors. R240 and R228.

4) Rebuilt new plug in can type Filter caps (they plug into tube type sockets). 2 original cans were each three 20MFD 400v. I made new ones with 22uf 450v caps. C214 A/B/C and C216 A/B/C.

On the previous exact amp I did for my friend, I did the same mods as above, but not step 4, didn't change the filter caps. It sounded really good and he loves it.

I have tried to check and tighten every screw and socket to make sure nothing is rattling. I have looked at every new (and old) solder joint. I have measured the new filter caps out of the amp and they are all in spec with proper continuity for the plugs. I have looked for misplaced wires, solder joints, etc.

Both amps are having an identical problem, after identical mods, so I'm thinking it is related to the parts I put in.

I even changed the speaker in one of the amps in case that was it, but exact same problem persists.

I'm wondering if I have bad caps, or maybe I have created blocking distortion in the preamp by lowering the cathode bias resistor too much, or ??? I'm in the early stages of learning about amps so my knowledge is limited, thus my post asking for insight, encouraged by an 18watt.com friend to do so.

Would love any advice you can offer, questions you can ask, etc. - attached is a picture of the schematic, and the amps, and the caps I built.

Image

Image

Image

Image

Found the schematic online, a little easier to see, page 19:

http://www.acofs.org.au/part_5_files/Be ... 614EVM.pdf

I can attempt to make a sound sample tonight and upload it tomorrow.
Have you tried to plug it into another separate speaker cabinet and disconnect the internal speaker? Does it fart out then too?
I notice the coupling caps between the plates of the PI and the grids of the 6BQ5 power tubes are .1uf. This is one of the reasons why F&nder amps are farty in the low end. You might wanna try putting .022uf or even .01uf in that position... I'd start with .022uf.. if the farting out is gone you might wanna try .047uf and .068uf to see where the treshold is between fart and rock (IF that would solve the problem, that is...)

Changing the cathode bypass cap to 1000 or 2200uf tends to tighten the low end too, and it's recommended on 18 watt amps.

I'm also obliged to say we need a measurement of the voltages on:
- B+ nodes
- preamp tube plates and cathodes
- PI plates, grids and cathodes
- PT plates, control grids, screen grids and cathodes (we need to know the bias)
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

It is worth noting that I have had several of these amps, both in stock form, and with similar mods, and never had this problem with them. In the past I have left the original filter caps in place. Also odd that I am having the same problem with these two amplifiers at the same time, the common thing being they received the exact same mods. And I did play one of these amps before modding it, and it sounded fine, this problem was not there, so I either didn't notice it, or it is related to my recent modifications. All that to say there are some great suggestions on how to change the circuit to potentially alleviate the problem if it was circuit design based, but I don't think the problem is with the circuit as designed, as I've played 6 or 7 of them and the little speaker sounds fantastic and doesn't have this problem. I think I have unfortunately created the problem, I just don't know how. I suppose the easy step is to reverse the mods one by one, starting with plugging the original filter caps back in, to see if I can track it down.

The amp does have a seperate speaker out, and I am not experiencing the problem, at least that I discerned, with off-board speakers - though I was testing with a much larger speaker than the stock 4". I tested with an 8" and a 10".

The problem is not apparent on all settings of the 5-way tone selector switch, only on the bassier settings. It is not as prevalent on the brighter settings.

I thought of something last night as well, the original caps were held in place by a bracket and screw, so they wouldn't move or fall out. Maybe the new inserted caps I made have mechanical noise related to their sockets not being held down? I can see if the original bracket will fit over and hold the new caps.

Image

And finally, a video I made last night, first at medium volume, then higher volume. The louder it gets the farting and speaker rattling gets more prevalent. There is also a scratchy buzzy tail on the notes that fades as the signal fades.

https://www.youtube.com/watch?v=DTdXTBufufs
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

That's a mechanical noise..I don't hear anything bad in the signal. Something is rattling and buzzing. Kinda like a string on a bad fret..
Do you have something frequency adjustable to inject a constant signal in the input? That way you can set it until the amp starts buzzing. That gives you the opportunity to chopstick and track down what's causing this. There is part(s) resonating
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

It certainly seems mechanical, at least in how the speaker is overworking and causing the cabinet to rattle.

I don't have a signal generator or anything like that. I will go through them both again to make sure that every screws is tightened, etc. (all the tube and cap sockets are secured with nuts and bolts for example). And I will check the caps too.

I have already tried to isolate the rattle to the front speaker panel, so I removed it and tested, and the speaker was clearly working too hard both with the speaker panel isolated, and the speaker removed from the panel and isolated. In those situations the rest of the chassis was making a rattle.

I'll report back after further research into possible things that are moving and causing the mechanical noise.

Thanks for taking a look at the problem and the video!
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

UPDATE: I think I have isolated it to the speaker, but I'm not sure why the speaker is behaving this way.

When the speaker and baffle are attached to the amp, the whole things rattles/vibrates and speaker has a low farty sound.

When I remove the baffle and speaker, they vibrate as above but the combo case does not.

When I remove the speaker from the baffle, same sound coming from just the speaker. The cone is moving very hard on low notes, and with more volume, which is where the problem increases. It sounds like the voice coil is moving to the point of hitting the rear of its travel point, hitting the magnet I guess.

I just can't figure out WHY the speaker is behaving this way. It is happening in an identical way with both amps, and after trying 5 different identical speakers. And I have not had this problem before in playing and modding 6 or 7 of these same amps.

I'm wondering if there are parts missing in the circuit that attenuate the signal and/or low frequencies for the little onboard speaker?

*I did chopstick the amp, and did not find any tubes or parts that were microphonic.

*I also went through both amps and tightened every single screw/nut/bolt, including putting the old filter caps back in with their retainer that holds them tightly in place so they can't move. That did nothing different in regards to the sound or performance, its the speaker getting overworked.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

Bieworm wrote:
Thu 02/03/22 3:38 am
I notice the coupling caps between the plates of the PI and the grids of the 6BQ5 power tubes are .1uf. This is one of the reasons why F&nder amps are farty in the low end. You might wanna try putting .022uf or even .01uf in that position... I'd start with .022uf.. if the farting out is gone you might wanna try .047uf and .068uf to see where the treshold is between fart and rock (IF that would solve the problem, that is...)
I think the above advice is sound. It's hard to say the circuits are identical, I wouldn't assume they are unless you absolutely know. I would try what Bieworm suggests and see how it affects the flatulence.

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

Post by yello »

I can confirm that the circuits are identical between these two units giving me issues, and the last one I previously modified, and the unmodified units I have. I have reviewed the schematics and the amps themselves, identical, other than some have parts made by different manufacturers and transformers made by a different company.

I didn't have .01uf on hand, but I tried .022uf, and .047 uf and there was zero audible difference in the problem. The speaker is overworked to the same degree.

Here is a new video showing the speaker issue:

https://www.youtube.com/watch?v=gE8Z7TvwdLM

I have tested 5 identical speakers in place of the one's in the amps, and they all exhibit the same issue. I have never had this problem with this type of speaker or these amps.

I am quite stumped, and flummoxed, but persistent and open to ideas.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

No way near an expert on this, but did you ever hear about voice coil rub? This phenomenon occurs often when speakers have suffered from being in a moist environment. The baffle deforms and pulls, touching one side of the voice coil..
With a bigger speaker you can feel/hear it when you gently push on the baffle at one side. One side grinds, the opposite side doesn't...
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

My suggestion would be to start by taking voltages from both amps. If you have an amp with the same circuit not having the issue, then get those voltages too.

Voltages tell more than guessing ever can. You should calculate your output tube dissipation and see if the speakers can actually handle what's driving them.

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

Post by yello »

I don't think it is voice coil rub. The same phenomenon is happening with these two amps, testing them with 6 different speakers, all which do not show signs of voice coil rub outside the amp, or in other settings.

The speakers are small, but should be able to handle the wattage per their spec.

I spent the morning taking two more identical amps, and modifying/cleaning them to the point of plugging them in safely, 100% stock. I even used the same plug in filter caps from the problematic amps, and the same speakers I tested with the other amps. No problem, amps and speakers functioned fine.

That seems to marrow things down to three mods I did:

1) lowered value of the preamp cathode bias resistor from 5.6k to 3.3k
2) increased value of the negative feedback resistor from 75k to 220k
3) swapped what I think are the fixed bias resistors from 350 ohm 3w to 360ohm 5w

Is one more likely than the other to create too much gain/signal/bass for the speakers to handle? That is what seems to be happening.

Given I am a beginner, is there a good article on 18watt or elsewhere to walk me through safely taking voltage numbers? I want a good refresher before I do that.

I may just set these two amps aside for now, and mod the two amps from this morning for my friend instead. Though that leaves me wondering about the mods I typically have done successfully on these (cathode bias resistor and negative feedback resistor). My aim is to increase gain without making things unstable.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by zaphod_phil »

Bieworm wrote:
Thu 02/03/22 3:38 am
yello wrote:
Wed 02/02/22 3:35 pm
I am having an issue with an old military projector amp, a Signal Corps model 614 or AQ2A. I've modded them before, with great results. I am experiencing a farty speaker sound as I increase volume, and only the low notes. The internal Jensen 4" speaker is working REALLY hard and it is even rattling the combo chassis, everything. I've modded and played several of them and never had a problem like this, including with the small speaker.

Here are the basic mods I did to both amps:

1) Upped the negative feedback resistor (connected to OT secondary) from 75k to 220k. Goal was no negative feedback for more gain. It is R222 on the schematic.

2) Lowered the V1B cathode bias resistor from 5.6k to 3.3k for more gain. R208.

3) Replaced the two 3w 350ohm ohmite resistors with 5w 360ohm wirewound cemented resistors. R240 and R228.

4) Rebuilt new plug in can type Filter caps (they plug into tube type sockets). 2 original cans were each three 20MFD 400v. I made new ones with 22uf 450v caps. C214 A/B/C and C216 A/B/C.

On the previous exact amp I did for my friend, I did the same mods as above, but not step 4, didn't change the filter caps. It sounded really good and he loves it.

I have tried to check and tighten every screw and socket to make sure nothing is rattling. I have looked at every new (and old) solder joint. I have measured the new filter caps out of the amp and they are all in spec with proper continuity for the plugs. I have looked for misplaced wires, solder joints, etc.

Both amps are having an identical problem, after identical mods, so I'm thinking it is related to the parts I put in.

I even changed the speaker in one of the amps in case that was it, but exact same problem persists.

I'm wondering if I have bad caps, or maybe I have created blocking distortion in the preamp by lowering the cathode bias resistor too much, or ??? I'm in the early stages of learning about amps so my knowledge is limited, thus my post asking for insight, encouraged by an 18watt.com friend to do so.

Would love any advice you can offer, questions you can ask, etc. - attached is a picture of the schematic, and the amps, and the caps I built.


Found the schematic online, a little easier to see, page 19:

http://www.acofs.org.au/part_5_files/Be ... 614EVM.pdf

I can attempt to make a sound sample tonight and upload it tomorrow.
Have you tried to plug it into another separate speaker cabinet and disconnect the internal speaker? Does it fart out then too?
I notice the coupling caps between the plates of the PI and the grids of the 6BQ5 power tubes are .1uf. This is one of the reasons why F&nder amps are farty in the low end. You might wanna try putting .022uf or even .01uf in that position... I'd start with .022uf.. if the farting out is gone you might wanna try .047uf and .068uf to see where the threshold is between fart and rock (IF that would solve the problem, that is...)

Changing the cathode bypass cap to 1000 or 2200uf tends to tighten the low end too, and it's recommended on 18 watt amps.
I strongly agree with all of the above points
You definitely need to put in a bigger speaker, at least an 8" or preferably 10" or 12"
-Likewise, on reducing the values of the coupling caps
Changing the cathode bypass cap to 1000 or 2200uF is in fact recommended for any and all cathode-biased amps.

The resistors used with the 12AX7s in the preamp section are ridiculously large in value (560k anode/ 5.6k or 10k cathode) and will put those triodes close to starved mode 8O, which will sound ratty. I recommend sticking to a more standard 100k/1k configuration with 0.47uF bypass caps across the 1k cathode resistors. The bypass caps will both boost signal gain and cut out the low-end flub :D

You can then remove that negative feedback resistor completely and regain the amp's detail and sparkle at the high end :D

Try those mods and have fun!
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Thanks for taking a look and responding zaphod_phil!

The amp only has space for the tiny 4" speaker, but is also has a speaker out so any 16 ohm cab can be used, and without the issues.

I love your circuit suggestions (and Bieworm's) and will try them out and report back.

Quick note - the amp has a pair of 6AQ5 tubes, not 6BQ5's. Not sure if that makes a difference for the circuit suggestions.

So to confirm, looks like:

*560k becomes 100k for preamp anode (R206 and R209) (for both preamp tubes or just v1? also R215 and R217?)
*5.6k (R208) and 10k (R205) for the cathodes becomes 1k plus a .47uf bypass cap. (for both preamp tubes or just v1? Also R214 and R217?)
*.1 to .022 or .047 for couplings caps (C212 and C213).
*100uf (C224) to 1000uf or 2200uf. Current 100uf part is a Mallory type BS 621000, bolted to the side of the chassis. Is it electrolytic? I can get a pic if needed.
*remove negative feedback resistor R222 at 75k (and the related 51 uuf cap too, part C223?)

Also, what voltage for each of the above caps?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

yello wrote:
Thu 02/10/22 11:25 am
Thanks for taking a look and responding zaphod_phil!

The amp only has space for the tiny 4" speaker, but is also has a speaker out so any 16 ohm cab can be used, and without the issues.

I love your circuit suggestions (and Bieworm's) and will try them out and report back.

Quick note - the amp has a pair of 6AQ5 tubes, not 6BQ5's. Not sure if that makes a difference for the circuit suggestions.

So to confirm, looks like:

*560k becomes 100k for preamp anode (R206 and R209) (for both preamp tubes or just v1? also R215 and R217?)
*5.6k (R208) and 10k (R205) for the cathodes becomes 1k plus a .47uf bypass cap. (for both preamp tubes or just v1? Also R214 and R217?)
*.1 to .022 or .047 for couplings caps (C212 and C213).
*100uf (C224) to 1000uf or 2200uf. Current 100uf part is a Mallory type BS 621000, bolted to the side of the chassis. Is it electrolytic? I can get a pic if needed.
*remove negative feedback resistor R222 at 75k (and the related 51 uuf cap too, part C223?)

Also, what voltage for each of the above caps?
The 2200uf cap I always use 63V
Other cathode bypass caps 25V is plenty..
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by zaphod_phil »

yello wrote:
Thu 02/10/22 11:25 am

The amp only has space for the tiny 4" speaker, but is also has a speaker out so any 16 ohm cab can be used, and without the issues.
I love your circuit suggestions (and Bieworm's) and will try them out and report back.
yello wrote:
Thu 02/10/22 11:25 am
Quick note - the amp has a pair of 6AQ5 tubes, not 6BQ5's. Not sure if that makes a difference for the circuit suggestions.
Yes, I saw that. 6AQ5s are basically miniature 6V6s and should be capable of about 10W output in Class AB, push-pull operation - see http://tdsl.duncanamps.com/show.php?des=6AQ5 The circuit suggestions won't be any different, as they're focused around the preamp.
yello wrote:
Thu 02/10/22 11:25 am
So to confirm, looks like:
yello wrote:
Thu 02/10/22 11:25 am
*560k becomes 100k for preamp anode (R206 and R209) (for both preamp tubes or just v1? also R215 and R217?)
For both preamp tubes. R215 is also an anode load resistor and needs to be 100k. R217 (5.6k) is a cathode resistor in the cathodyne phase inverter and is probably best left as is.
yello wrote:
Thu 02/10/22 11:25 am
*5.6k (R208) and 10k (R205) for the cathodes becomes 1k plus a .47uf bypass cap. (for both preamp tubes or just v1? Also R214 and R217?)
You might like to try 1uF or 2uF for those bypass caps, if you find the 0.47uF is cutting too much bass. Use what sounds best to you.
For both preamp tubes. R214 needs to be 1k, with R215 again being 100k. That will give a much nicer overdrive sound.
yello wrote:
Thu 02/10/22 11:25 am
*.1 to .022 or .047 for couplings caps (C212 and C213).
yello wrote:
Thu 02/10/22 11:25 am
*100uf (C224) to 1000uf or 2200uf. Current 100uf part is a Mallory type BS 621000, bolted to the side of the chassis. Is it electrolytic? I can get a pic if needed.
Generally, all caps above 1uF are electrolytic.
yello wrote:
Thu 02/10/22 11:25 am
*remove negative feedback resistor R222 at 75k (and the related 51 uuf cap too, part C223?)
Yes, remove both. That cap will kill all your treble response

Also, what voltage for each of the above caps?
Coupling caps should be rated at about 400V

preamp cathode caps can be generously 5V rated

The 6AQ5 specs show a bias voltage of 15V for P-P Class AB operation. So around 30V should be good for the shared cathode cap C224. You can check that voltage in your amp with a meter, just to be sure.
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