Princeton Reverb Clone Build Hum Problem

[justsoldern]

Just built a PR clone using a replica chassis, eyelet boards, brass grounding plate etc. It's the B1270 circuit. I've repaired/restored a number of original Princeton and other BF amps and wanted to do this one from scratch, so I know exactly what the originals are like inside. I strived to have the layout and wiring as close to the originals as humanly possible (for better or worse!) right down to the grounding scheme and cloth wire and how every wire and tube hookup is routed. The amp sounds great but I have a faint 60 Hz hum at Idle that has turned into a challenge! Not a problem live - but it bugs me at home. I'm getting about 10-12 mv of 60Hz from the speaker at idle with nothing plugged in. I'm sure it's 60 and not 120 Hz. I know it's coming from V1B.

No change when Volume is down or V1 pin 7 grounded
No change when reverb is turned down (with or without tank disconnected)
No change when grounding V2 grid pin 2 on the reverb driver
Dead Quiet when grounding V1B output after the 0.022 Cap
Dead Quiet when grounding V3A grid pin 7

So first I suspected heaters/, so I installed a voltage divider and raised the heater CT to +30V - No change (That has worked well for me in the past with noisy 5E3 circuits)
I also disconnected the heater to V1 and connected to a DC bench supply - no help there either. So I've ruled out heaters a couple of ways
I've swapped in numerous known quiet tubes - No Joy
I've pushed wires between the board and sockets around every which way
I've replaced the cathode 1.5K resistor and 25 microfarad cap on V1 -No Joy
I've replaced the 100K plate resistor and the 0.022 Cap along with the 500 pf cap to the reverb driver - No Joy
I rotated/moved the reverb transformer to see if it was inducing a hum - no help

I'm assuming a ground loop now but my grounding scheme is exactly like an original. Which maybe is not the best but should work.
I've tried jumpering the V1 cathode grounding point on the eyelet board to other spots on the chassis but it hasn't helped. Any ideas out there?

[g1]

I'll start with a very obvious question but one that needs to be ruled out before proceeding.
Have you had other stock amps of this model in the exact same environment and found them to be quieter?

[Chuck H]

And I'll add, have you tried a couple of different tubes in the V1 position? I once wasted half a day chasing hum in a new build because one of the brand new tubes hummed like it didn't know the words. So did the brand new replacement. So after wasting all this time I'm never getting back I threw my hands in the air and though "Ok, fine, there's nothing wrong and yet..." So I tried a KNOWN GOOD tube in the hummy circuit and viola. I promptly went and stuck my head in the oven. Unfortunately I have an electric oven so that was also cumbersome.

[justsoldern]

Hi guys, Thanks for the thoughts. Yes I've worked on many amps on the same workbench. I've done a bunch of repairs to friends amps over the years, mostly black/brownface, and tweed vintage fenders. I just finished some work on a 59 bassman reissue. (I must say I don't enjoy working on the printed circuit board amps much!). I'm not a super guru - but I'm fairly competent with tracking down the usual issues with leaky caps and bad tubes, solder joints etc. Never had this problem. This amp is humming no matter where I have it in the house. I also own a nice "65 Pro Reverb which is quiet as well. It's not a loud hum at all. only about 12 mv at the output. I shut down my central AC and basement dehumidifier while working on it just so I can hear it clearly

Yes I've tried many tubes. I was hoping that would be it after reading many posts here and elsewhere. A couple of JJ's, a JJ Mid Gain, Tung Sol, and then I bought a special select low noise Tube Amp Doctor tube, all of which are dead quiet in my Pro Reverb. I haven't tried swapping out the rectifier yet, but my understanding is that if that were the problem I would be seeing a 120 hz issue. I have a few rectifiers lying around I guess I could try it. I feel I've ruled out V2, and everything after the 3.3M resistor going to V3. The ampis dead quiet if I ground after the 0.022 cap leaving V1B

[Chuck H]

If it's positively 60Hz then it's either induction from the PT or the filament circuit. All the cathodes are fully bypassed AND all the filaments are in parallel, though only one triode is offending, so it seems unlikely to be the filaments, but I might try separating the cathode grounds for the reverb driver and V1B. Ground the V1B cathode with the input on it's own, unshared lead. It's a long shot, but easy enough to try.

[loudthud]

Schematic I found: http://el34world.com/charts/Schemati...verb_b1270.gif

Is this the schematic you used?

Did you use a quad can cap, vintage style soldered to the chassis?

Using a 5U4 rectifier tube with DC output on pin 8?

Did you eliminate the Death cap and use a 3 wire mains cable?

The schematic shows no standby switch. If you pull the rectifier tube while the amp is on, will the hum stop immediately or fade out as B+ drops?

[justsoldern]

yes to all of the questions. If you look at the layout for that schematic, it is identical to my layout. (but no death cap and I have a 3 wiregrounded cable) No standby switch. I tried hard to do an exact clone in every other way possible. The only deviation from an original amp is that I've added a pot near the bias circuit to adjust bias, and now I've also added a little terminal strip with some resistors and cap to get a raised DC voltage for the heater center tap.(From Tubeswell) My transformer has a heater center tap which I used at first, then traded for the two 100 ohm set up, (No Joy)then got rid of them and added the voltage divider off the first B+ (about 30 volts). None of it has helped so in the end if I find the problem elsewhere, I may take it all out and just ground the center tap again. I've Run V1 from a DC bench supply with no luck, so I feel like I've beat the heater problem to death.

Very cool idea on the rectifier BTW!. When I pull it the hum stops pretty quickly! The voltage divider network for the heater CT may be acting as a bleeder however. perhaps I can take it out and repeat the experiment. I did just replace my JJ 5U4GB with a different Sovtek 5U4G and even a Ratheon 5Y3GT. Still have the hum with all of them. Wouldn't hum from the B+ be 120Hz? Thanks for the support!

[Chuck H]

Clever to consider the fade might be bleed from the elevation circuit. Do try your experiment as outlined. Mr. Thud was probably considering the 5V supply being the cause rather than the HV rail. Have you measured AC on your HV rail?

[loudthud]

Wouldn't hum from the B+ be 120Hz?

That's the case about 99% of the time, but I feel this is an unusual case. An imbalanced PT or rectifier could case 60Hz noise on the B+. 60 Hz noise usually sounds like a low organ note, a pure tone with no higher harmonics. 120Hz noise is usually more of a buzz with lots of higher harmonics. You seem to eliminated most of the common causes.

30V might be a little low for heater elevation. I like 50V to 60V. A little experiment I like to try is this: Ground the heater CT voltage divider (the point where the CT is connected) with a clip lead. Power the amp up and listen to the hum. Now listen as you disconnect the ground and let the heater float up to the standoff Voltage. If it's working you'll hear the 60Hz portion of the hum diminish, but the 120Hz portion will be mostly unchanged. If the 120Hz is too loud, it's hard to hear any difference.

Edit: Sometimes those HiFi types go to extreme measures to remove the last bit of hum. Things like reversing the red or yellow wires to the rectifier tube can make a difference.

[justsoldern]

Thanks guys, I'm not home for a day or two buy You've given me some things to try. I'll ground that center tap voltage divider resistor and listen for change, if it has an effect I'll bump up to 50-60 volts elevated. I'll take out the elevated divider -go back to a grounded center tap and repeat pulling the rectifier. I still need to try Chuck H's idea of lifting the V1B cathode bypass cap and resistor from it's common ground with V2 and ground it on it's own with the V1B grid ground (which is now on the volume pot case). Then I can reverse the reds and yellows. I figure I should first just do the reds, check it then do the yellows - I think. Looks like I have some soldering this weekend! I sure I checked the HV rail for signs of AC and didn't see any, but I'll check again. Wouldn't a lot of this power supply hum or even fillament issue show up even more on V1A?

I don't own a scope but I knew I needed to find out if it was 60 or 120 hz. So I used my little Zoom H1 recorder to record a few minutes of the hum, then imported the file into Audacity, which is an audio editing program where you can blow up the gain and zoom in the time scale so that you are looking at one second of recorded time. Then I just counted the cycles. Came out right at 60 (Crude I know - don't laugh!). Some day maybe I'll get a scope!

[eschertron]

I don't own a scope but I knew I needed to find out if it was 60 or 120 hz. So I used my little Zoom H1 recorder to record a few minutes of the hum, then imported the file into Audacity, which is an audio editing program where you can blow up the gain and zoom in the time scale so that you are looking at one second of recorded time. Then I just counted the cycles. Came out right at 60 (Crude I know - don't laugh!). Some day maybe I'll get a scope!

Not crude at all! Now, was the wave pretty well sinusoidal, or sawtoothed? That would indicate if there were harmonics riding on the 60Hz wave. As discussed, the noise may be from multiple sources, mixed together for the waveform you captured.

[justsoldern]

Looking again at it - it's not a perfect sine wave, so there could be something else riding along. The largest peaks are definitely 60 Hz. I'll get a screen shot from Audacity, convert it to a jpeg and post it - (after I learn how to post photos in this forum!) I was also probably taxing the signal to noise of my recorder a bit! If I record a new file with the amp off first, then on, maybe I could use Audacity to do a background removal. That might clean it up a bit! I'll also post a pic of the amp guts. I'll try photobucket and get a couple of links going. Might take a couple of days. Every experiment is leading to another! But I understand that this 60 vs 120 question is very important in tracking this down. I thought I had it answered - but it's worth a second look

[justsoldern]

Ok guys, sorry I've been away for a few weeks but I'm back on the case with new info. I've tried a lot of stuff and nothing has worked. I raised the dc heater offset to 50 volts, I've reworked the grounding around V1 separating the cathode and 1.5K resistor from it's spot and grounding it to the input jack plus lifting the volume ground from the pot and grounding to the input jack. I tried a lantern battery on V1. I put in a humdinger with 50 volt Ct. I borrowed a scope from a guy at work for an evening. Definitely 60 Hz at the speaker. 120 hz on the B+. Measuring A/C with my Fluke on the B+ I get 4.6 v at the first cap, 0.3 at the second, 18 at the 3rd and 5 mv at D (after the 2nd 18K resistor - no stokes mod) I had to use a 0.22 cap and 1 meg resistor to get that last one.
Then I pulled V1 and got a surprise. It still hummed. I've been focused on V1B (1270 Princeton schematic) So let's reset a minute. I think I may be confused

With V1 pulled - Buzz (low level as described before about 10 mv at speaker)
With V2 pulled - Buzz
Ground after 0.022 Cap outlet side of V1B - Quiet
Ground reverb wiper - Buzz
Ground V3A pin 7 grid - quiet
Ground V2 Grids 2/7 - Buzz
Pull V3 - Quiet

I'm really getting frustrated!

[Chuck H]

When you grounded the signal after the V1B coupling cap, was that before or after the 3.3M resistor?

[justsoldern]

Grounding before the 3.3M will silence the amp. Also after the 3.3M (which is also like grounding the V3A grid.) Grounding the reverb wiper will not silence the hum.

I've done some other suggestions also. Loudthud asked me to ground/unground the elevated voltage divider and listen for a change between elevated and grounded. There was no change. I also suspected the incoming AC wiring layout, so I disconnected the PT primary from the fuse and on/off switch and routed it off the front of the chassis to another power cord eliminating all the wiring around the on/off switch and the old ground switch and A/C receptacle area. No help there either.

I'm thinking next to try and disconnect the 100K plate resistor going to V1B. First at the socket and then at the board leaving the run to the socket connected. It seems it has to be coupling in on that wire somewhere???. There is the underboard trace that goes from the 0.022/500 pf junction to the 3.3M and another wire that goes from after the 3.3M to the grid of V3A. It has to be coming in on one of those high impedence lines?? Right?