Help with Hum - PR Build

[indiehouse]

Hey there, I have a Hoffman PR and it's got a bit of a hum to it with volume at 0. Not a glaring, buzzing hum, but definitely more than my other amps. I am handy with electronic repairs if I could just get some tips about where to look. Grounding issue of some sort?

[glass54]

Hi indiehouse,
Do you have a good resolution photo (gutshot) or two. 
What schematic were you working to?
There are some very talented guys on the Forum who could probably help you
Regards
Mirek

[sluckey]

Did you build this amp? Has it always hummed? Have you ever built another amp?

[Latole]

Did you build this amp? Has it always hummed? Have you ever built another amp?

1- A must to know first !
2- If the amp never hum before and play well. IMO it is a filter caps issue

[Latole]

Hoffman Princeton Reverb

https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf

[indiehouse]

Thanks for the replies. I did not build this amp. It's hummed since I got it. I'm comfortable working on it myself, I just need some guidance about where to look. Here are some photo's and schematic info. Transformers are Allen TP25 Power transformer, TO35MT Output transformer.

[indiehouse]

The hum increases when turning the Reverb level up.

[acheld]

Just a guess, but I've built a couple of these and always had to pay attention to lead dress involving the reverb send and receive wires, including the foot switch, and the ground connection.    I'm never quite sure "what" has made the difference . . . but I've been able to quiet them down.

Also, it's hard to see what is actually happening around V1.  Why is there an extra resistor present? (Edit: I missed the second input jack! That's why there are two.  Hopefully the jacks are wired correctly -- see lower left of https://el34world.com/Hoffman/files/CommonHookups.pdf.)

One minor tip.  It's always easier, at least for me, to shield my cables from the preamp ground bus, ie on the jack or pot side of the shielded cable rather than on the tube side.  It is possible that, if the grounding is not right (eg, the signal cable is not shielded well), that can play a role in the hum.  I can't really see where the builder has grounded the cables. 

These amps should be quiet when properly set up.   And they sound lovely.   

[Latole]

Lead dress ; look at the layout ; no lead routing in parallel.
Most of lead should cross other lead in angle

[mresistor]

Also, it's hard to see what is actually happening around V1.  Why is there an extra resistor present?

It looks to me like he mounted the 68K grid leak resistors on the V1 socket.

[indiehouse]

Also, it's hard to see what is actually happening around V1.  Why is there an extra resistor present?

It looks to me like he mounted the 68K grid leak resistors on the V1 socket.

I'm looking at the schematic I posted. I don't see a 68k resistor. Are these on the dual input jacks?

[mresistor]

Well the schematic you posted has a single 33K grid leak because it is a single input, but a Fender PR chassis or clone chassis has dual hi/lo input holes like yours does and if two input jacks are used then they use two 68K grid leak resistors. It appears to me that whoever built this PR mounted the dual input 68K grid leaks resistors at the V1 socket. They were probably duplicating Dougs design where the 33K resistor is mounted near or on the socket.
Many people say it's better to mount the grid leaks near the socket, but Fender used them on the input jacks and we all know his amps were pretty well made and quiet.


Is your hum 60Hz or 120Hz?  60 hz would be power input or the heaters or a bad tube.  The wiring isn't real pretty and neat and I wonder why the builder didn't use flyover filament wiring like Doug did in his and Fender did in his amps.  120hz would be rectified noise and would tend to be on the PS end.  I also don't know why the builder would use a full size bias potentiometer and mount it in a peculiar place as he has mounted it in the OT wiring hole. Weird. Just delete it and use a trim pot like Dougs circuit has. 


Have you looked at Dougs PR wiring? 

[sluckey]

There's a wiring error on the board near the 3.3M resistor for the reverb circuit.

[mresistor]

I don't recall ever seeing that short green/yellow stripe wire jumped to the ground bus before.  Ah  that's the shield for the Rg coax.

[Latole]

There's a wiring error on the board near the 3.3M resistor for the reverb circuit.

Here ?

[sluckey]

There's a wiring error on the board near the 3.3M resistor for the reverb circuit.

Disregard. Even though you did not follow Hoffman's layout, the circuit is still electrically correct. No error.

[indiehouse]

There's a wiring error on the board near the 3.3M resistor for the reverb circuit.

Disregard. Even though you did not follow Hoffman's layout, the circuit is still electrically correct. No error.

Dag. Was hoping that could be a promising lead. Am I back to square one?

[punkykatt]

Every Princeton Reverb I have built using a 40/20/20/20 cap can, I never needed an additional filter cap. Maybe there is something funky going on with that 22/450v radial mounted on the board?

[dogburn]

Every Princeton Reverb I have built using a 40/20/20/20 cap can, I never needed an additional filter cap. Maybe there is something funky going on with that 22/450v radial mounted on the board?

I'd guess the extra cap was put there so that filter cap could be grounded to the preamp ground. But then one of the caps should have been left unconnected, and it also looks like there's an extra voltage dropping resistor.

[Bieworm]

I’ve found out that there is a band aid solution for reverb hum. While I understand some might not like this I’m throwing it out here anyway.
Lift both cathode bypass caps of V3 and experiment with lower values of those, just on pin 3. I often settle for 4.7 or 10uf there and leave the one on pin 8 out of the circuit. Do not mess with those cathode resistors.
IME there is still plenty of surfy reverb available. And the sweep of the reverb pot will be better with that linear 100k pot.
I also agree with the others… that lead dress is just yelling for noise issues

[indiehouse]

Oh man. I am bummed. Would it be worth it to strip it all down and rewire everything? I've never built an amp before, but it's something I've wanted to do. But I also have limited time. I'd rather just sell it to someone who could address the issue.

[punkykatt]

You have the layout in post#5, make it look just like it. try to take a little time each day or when you have to work on it a little at a time. Its not that hard, do a small section at a time and highlight the layout with a marker as you go so you know where you left off.. Make sure the filter caps are discharged before you start. You can do it!

[indiehouse]

Thanks for the encouragement! If I go down this route, are there any components (caps, etc) that would benefit from an upgrade? Figure if my hands are already in there, it'd be a good opportunity.

[Latole]

Follow the layout carefully, take your time.
Don't work for long time and take a break
And you will find the issue.   

[acheld]

are there any components (caps, etc) that would benefit from an upgrade?

I think that a stock Hoffman PR build would be the upgrade.   

There are some components that are needlessly uprated visible -- for example I think I see an N5400 series diode in the bias circuit -- a small 1N4007 is all that is needed (and actually it's technically over rated for this position).  Likewise the large bias pot --  a small trimmer is all that is needed and fits the turret board.  So your bias circuit can be easily upgraded by going back to stock!

Also in that same area, I see that the provision for standard 1 ohm resistors (from cathode to ground) to measure plate dissipation has been altered in a weird (to me) way, by routing both cathode to ground circuits through one 0.5 ohm resistor.   Easier, and simpler, to use the board layout as intended and route each cathode to ground through its own 1 ohm resistor.   

All in all, I agree with Punkykatt and Latole -- take this thing right back to the Hoffman layout as drawn and you will be happy.   

It will be messy!   I don't know your skill set, or the equipment you have available, but if it were my project, I'd probably strip every component (re-using what you can, especially the high value items) and build it to look like Hoffman's layout. 

[indiehouse]

I was thinking more in terms of using caps from Jupiter. Do you think it's worth the upgrade there? If so, which ones are in the audio path that would see the most value in upgrading?

[punkykatt]

I would get the amp working properly first. Its easy to change out the coupling caps with this style turret board. My$.02.

[Latole]

I was thinking more in terms of using caps from Jupiter. Do you think it's worth the upgrade there? If so, which ones are in the audio path that would see the most value in upgrading?

You have aways a issue ; hum and you want to upgrade ?
Answer to your question ; Not IMO

[sluckey]

I was thinking more in terms of using caps from Jupiter. Do you think it's worth the upgrade there? If so, which ones are in the audio path that would see the most value in upgrading?

Fix it before you mod it!

[Latole]

 
 The arrangement of the wires ( leas dress) in the reverb circuit is particularly important.
I don't like what I see on the photo

[Latole]

Check all solder ; move each wire in the turret to see.
Not sure ; re- heat an add solder.

You'd be surprised how easy it is to mess up a solder even when you're used to it. It happens to me who has made thousands, yes thousands.

[indiehouse]

Thanks for all the replies. And to be clear, I wasn't asking about modding the amp, I was asking about using Jupiter caps in place of others when rebuilding the amp. Though I get it. I should follow the BOM first, then swap in the Jupiter caps.

[tubeswell]

Try swapping out some/each of the preamp tubes first -in case you have a hummy tube. Try the reverb recovery tube first.