amp pop question

[madbean]

Thanks guys this is all super helpful. I'm going to take another look b/c I took it apart at like 7:30 this morning. I probably missed some things! But, it did appear to me that they used a JJ filter cap like this one: https://www.tubesandmore.com/products/capacitor-jj-electronics-500v-5050-f-electrolytic in the first two spots and the F&T on the third. So, maybe I should just convert it back to 20/20/20.

Digging through the Psionic Audio video, too.

[Zigcat]

That roasted resistor looks like someone added it to drop the b+ off the rectifier. Can just remove it completely as probably not needed. Not the best way to accomplish that either.

[Zigcat]

Okay, last comment

Change the cap can to a 32uf/32uf. A 5y3 can handle 32uf in a capacitor input power supply. That 22uf to the preamp is fine as is. All of your bypass caps looks new and don't need anything. The 1k 1w dropper resistor on the left side of the board could be increased in wattage for piece of mind. Remove the 470 ohm 5w resistor and go straight to the cap from the rectifier. Make sure the 6v6 is biased close enough and not over dissipating. New tubes. Should be good to go.

[madbean]

Okay, last comment

Change the cap can to a 32uf/32uf. A 5y3 can handle 32uf in a capacitor input power supply. That 22uf to the preamp is fine as is. All of your bypass caps looks new and don't need anything. The 1k 1w dropper resistor on the left side of the board could be increased in wattage for piece of mind. Remove the 470 ohm 5w resistor and go straight to the cap from the rectifier. Make sure the 6v6 is biased close enough and not over dissipating. New tubes. Should be good to go.

TYVM. I'm going to follow all your suggestions. For the 1k and 10k drop resistors, would there be any to not swap those out for 2W metal oxide? I don't see any reason to keep them as carbon comp (I don't care about preserving the original specs since I'll never sell this amp anyway).

[Zigcat]

The 1k and 10k can be upgraded to 2 or 3w metal oxide. If they are in spec, they are probably fine. But you can shotgun anything if you're not worried about originality. You could easily get into a rabbit hole with changing stuff and it probably won't make a noticeable difference in the end.

[gordo]

I sort of understand the B+ drop now.  I got the chance to play an original blackface Princeton a while back running at wall voltage.  Then the same amp plugged into a AmpRX BrownBox dialed back to 110v.  The little amp just softened up and had such a sweet overdrive.

Alas, I can't have either the Princeton or the BrownBox.

[madbean]

I sort of understand the B+ drop now.  I got the chance to play an original blackface Princeton a while back running at wall voltage.  Then the same amp plugged into a AmpRX BrownBox dialed back to 110v.  The little amp just softened up and had such a sweet overdrive.

Alas, I can't have either the Princeton or the BrownBox.

I put some more thought into that mod today and here's what I came up with: that 470R 5W has definitely been cooked a bit (the insulation on the red wire below it has a small black mark on it) so I'm wondering if that's the culprit here. And, that resistor along with the 50uF decoupling cap does come a time constant to charge up the cap. So, maybe the difference in current draw on the power tube and charge time of the filter cap could cause the pop as the resistor starts to fail? I'm not really sure.

But, I did go ahead and include a 5W wirewound 470R in my order to replace the Ohmite one if I decide to leave it as is. I think it's just a matter of checking all the B+ voltages and comparing, and listening. This is actually getting fun. Even though I can completely understand a pre-amp schematic I lack the knowledge of proper practice/tricks at high voltage stuff. So, that's something to correct.

[fair.child]

I sort of understand the B+ drop now.  I got the chance to play an original blackface Princeton a while back running at wall voltage.  Then the same amp plugged into a AmpRX BrownBox dialed back to 110v.  The little amp just softened up and had such a sweet overdrive.

Alas, I can't have either the Princeton or the BrownBox.

I put some more thought into that mod today and here's what I came up with: that 470R 5W has definitely been cooked a bit (the insulation on the red wire below it has a small black mark on it) so I'm wondering if that's the culprit here. And, that resistor along with the 50uF decoupling cap does come a time constant to charge up the cap. So, maybe the difference in current draw on the power tube and charge time of the filter cap could cause the pop as the resistor starts to fail? I'm not really sure.

But, I did go ahead and include a 5W wirewound 470R in my order to replace the Ohmite one if I decide to leave it as is. I think it's just a matter of checking all the B+ voltages and comparing, and listening. This is actually getting fun. Even though I can completely understand a pre-amp schematic I lack the knowledge of proper practice/tricks at high voltage stuff. So, that's something to correct.

Maybe it's time to start new venture with MBP amp project?

[madbean]

I was able to get rid of the pop finally!

I took voltage readings this morning on all the relevant tube pins to compare to what's on the schem. B+ was ~390v and with that added 470R 5W resistor it dropped to about 366v. So, I decided to keep it and just replace it with the new one. Now I have about 361v which is very close to what it should be. I replaced the can cap with the (2) 32uF and also the 1k and 10k drop resistors. They were a bit melty on the bottom so I guess it was good to switch them out for the new 2W metal oxide.

I used a bias calculator for the plate dissipation on the 6V6. It came out to 107% and it should be under 80% from what I understand. I didn't have much in the way of 1W resistors, but I did have 150R. So I put that in series with the 470R cathode resistor already there and measured it @ 608R. After taking voltages and calculating again it the dissipation dropped to 97%. So, I'll get maybe a 750R, 820R and 1k on my next parts order and see if I can get it in range.

I'm pretty satisfied with the experience. I learned a bit and solved a problem. I think what's in store for me is just building a VibroChamp.

[gordo]

That's cool.  I'd really like to find out what a 110v circuit is expecting to see.  Are the 60's/70's schematics taking that into account because that's all they had or is there more than that.  I'd never put much thought into it and most amps I've built I'm just happy to see stuff in range but after listening to this little Princeton it kind of set me back on my heels.  I got a chance to hear the little beast live by one of my fave guitar players on the BrownBox and it sounded like magic.

[gordo]

When I sold my tweed Champ clone I got it to within what the 50's version(s) specs were.  I thought it sounded really good and although I haven't heard the guy that bought it play it he's a killer player and has a real tweed so he obviously found something in common with a tour-able version.  Would downing it on the BrownBox make a difference though?  In my mind it would push the voltages too low, which might sound cool.

[Zigcat]

Super awesome, Brian! Pedals ARE a gateway drug.

[danfrank]

Those blue drop caps are notorious for becoming leaky. The tone ones aren't that critical but the one in the attached pic that feeds the 6V6 is. Take voltage measurements on both sides of it with reference to ground. The 6V6 side should be at ground (0v).
Also, see what you can get a 5R4 rectifier tube for... They are beefy yet only take 2 amps filament current like the 5Y3. I wouldn't use the GZ34 because of the voltage increase.

[madbean]

Those blue drop caps are notorious for becoming leaky. The tone ones aren't that critical but the one in the attached pic that feeds the 6V6 is. Take voltage measurements on both sides of it with reference to ground. The 6V6 side should be at ground (0v).
Also, see what you can get a 5R4 rectifier tube for... They are beefy yet only take 2 amps filament current like the 5Y3. I wouldn't use the GZ34 because of the voltage increase.

Ok, will do thanks. I may have a 22nF orange drop around I could put there. Probably the only high voltage cap I have in that range.

[cooder]

Great that its going again!
The 5R4 rectifier while only having a 2amp heater draw, so in that sense comparable to 5Y3, it has as first filter cap allowance according to datasheet only 4uF max rating for first node filter cap, so that might be a problem if you substitute that in.
If you don't want to increase voltage I would stick with 5Y3.