Princeton Reverb Hiss and Hum

[dajazzman]

I just finished building a Princeton Reverb and I'm wondering just how much hiss and hum normally comes out of the amp with the volume, treble, and bass turned all the way up.

The hiss and hum are both entering in the first preamp, with the hiss starting at the grid and the hum is on the plate voltage. I can ground the grid of the first preamp and the hiss goes completely away and I can ground the grid of the second preamp and the hum goes completely away.

 I just wondering if some hiss and hum is normal, because when I turned the volume control down to about 7, treble around 5, and bass around 6 everything is quite. I've done extensive searching and I can't find what the norm is on these amps.

Thanks,
Jim

[p2pAmps]

I have built many Hoffman turret style PR and they are all dead quiet...

Can you post pictures please...

[Fresh_Start]

Did you use shielded cable from input jacks to V1?

Have you tried another tube in V1 spot?

Apologies if those are too obvious. Hum could be lots of things. Heaters too close to signal wires, etc. Or a weak solder joint on ground.

Also, how does it sound with knobs at 5?  Most amps make a little bit of noise at 11 ;)

Chip

[dajazzman]

I'll post the pictures tonight. Was yours quiet even with the controls full up?

[p2pAmps]

I'll post the pictures tonight. Was yours quiet even with the controls full up?

quiet wide open

[p2pAmps]

Did you use shielded cable from input jacks to V1?

Have you tried another tube in V1 spot?

Apologies if those are too obvious. Hum could be lots of things. Heaters too close to signal wires, etc. Or a weak solder joint on ground.

Also, how does it sound with knobs at 5?  Most amps make a little bit of noise at 11 ;)

Chip

Chip is correct, check all these things...  Take a wooden chopstick and move around inside your amp with the noise and see if certain wires are causing this too.  Be careful

[dajazzman]

I used Mogami cable for the Input with the ground connected at the jacks only. I used Dale resistors for the whole amp. I've tried some different tubes, right now I have a Mullard 7025 in V1.

The amp is quiet when I turn down the controls. I just wondering what is the normal hiss and hum with the controls full up. Maybe that's normal. I had an amp builder redo a Deluxe Reverb with a Hoffman board and I've been comparing it to that. The DR seems to have the same and maybe a little bit more hiss with the controls all the way up and a little different hum but still present.

[p2pAmps]

I used Mogami cable for the Input with the ground connected at the jacks only. I used Dale resistors for the whole amp. I've tried some different tubes, right now I have a Mullard 7025 in V1.

The amp is quiet when I turn down the controls. I just wondering what is the normal hiss and hum with the controls full up. Maybe that's normal. I had an amp builder redo a Deluxe Reverb with a Hoffman board and I've been comparing it to that. The DR seems to have the same and maybe a little bit more hiss with the controls all the way up and a little different hum but still present.

Well I think the truth is you will always get some noise wide open but it shouldn't be glaring at all.  Those preamp tubes will cause you grief sometimes even if they are NOS, which are sometimes more problems than not. 

Still want to see pictures cause I like looking at them :)

[dajazzman]

I thinking the hiss is getting into the grid of the first preamp, because I can ground the grid and it's dead quiet. I also think the hum is getting the plate of the first, because I can ground the second tubes grid and it's dead quiet.

I can see some ripple on the B+ for the preamp and it looks like it just gets amplified all the way down. It's not much maybe less than 5mv on the scope.

[dajazzman]

Yes I'll get the pictures up as soon as I get home.

[sluckey]

I can ground the grid of the first preamp and the hiss goes completely away...

Is there any hiss with nothing plugged into the inputs? If so, this would indicate that the input jack switch terminal is not wired correctly. Take a close look.

[dajazzman]

Yes hiss is present with no Input, but guitar works through Input. I will double check them. I used an ohmmeter when checking them before. I measured from ground to the V1 input. I saw the changes of the ohms as I inserted a plug into each jack, but maybe what I saw wasn't right. I don't have the ohms off the top of my head but in looking at the schematic what I would expect measured the same, but maybe I was wrong in what I thought I would measure.

[sluckey]

Use a guitar cable (or new unwired plug) but don't plug into the guitar for these checks. Expected theoretical grid to ground resistance readings...

No plug = 34KΩ
Plug in HI (#1) jack = 1.034MΩ
Plug in LO (#2) jack = 68KΩ

If you get these three readings your jacks are wired correctly and the switches are working properly.

[dajazzman]

Now that you posted them, I believe that's what I read. I'll double check them tonight. With the volume control all the way up, the hiss starts to not be noticeable at around 6 on the knob. I wish the amp was as quiet as when I ground the first grid.

[Ed_Chambley]

Check your tube if you have not.  I did not read where you tried a different tube in V1, if you have sorry to mention such an elementary solution.  One thing I ran into with a couple of princeton's last year were the artificial center tap resistors.  Both has 100 ohm carbon film resistors and in both cases one had drifted low.  One was actually shorted and the amp was doing all sorts of crazy things.


On a Deluxe Reverb a couple of weeks ago I had a hiss problem which was being caused by the switchcraft jack not making a solid ground connection.  I pushed on it and the amp went quite.  Just a couple things to talk a quick check.


Lastly, my personal Tweed Deluxe hum finally got where I no longer could stand it.  Here the problem was the wiper tab of the volume pot had gotten bent, how I do not know.  It was not touching the back of the pot but as soon as I hit it with a stick it sounded like I shut off the amp.

[dajazzman]

Good stuff to try. I didn't use the 100 ohm resistor setup, I just grounded the center tap of the filament windings. I measured 3.2vac from each filament leg to ground at the lamp. 

[dajazzman]

Some Pictures

[dajazzman]

More Pictures

[dajazzman]

Took me 45 min to figure out how to post pictures at the right size. Ha!

[sluckey]

Very neat! I don't see any hiss. 

[Willabe]

Yes that's beautiful work. 

There is 1 thing I don't trust, but I don't think it would cause hiss.

You have preamp signal wires running parallel to/with the B+ string.

[HotBluePlates]

Now that you posted them, I believe that's what I read. I'll double check them tonight. ... I wish the amp was as quiet as when I ground the first grid.

Did you do the measurements Sluckey suggested?

I think you already know the noise is associated with either components/wiring connected to that first grid, or it's the tube itself.

[p2pAmps]

Yes that's beautiful work. 

There is 1 thing I don't trust, but I don't think it would cause hiss.

You have preamp signal wires running parallel to/with the B+ string.

First off, great work...

Willabe makes a good point about the signal wires running parallel to/with the B+.  I have been burned myself by this several times but mine was more humm than hiss. 

How about a recording showing everyone the actual sound?  It would be good to hear what you hear.

[sluckey]

I see you used shielded cable on the input jacks and volume control, but I don't see the shield connected to ground. Is it?

[sluckey]

Willabe makes a good point about the signal wires running parallel to/with the B+.  I have been burned myself by this several times but mine was more humm than hiss. 

I want to use this thread and dajazzman's pic to make a point about signal wires and B+ wires. This amp has 4 B+ nodes. Node A is very dirty and Node B is dirty. You should avoid running low level preamp signal wires or pots or components close to these two nodes. Node C is clean but I still would not want preamp signals running close to it. Node D is very clean. There is no harm in running any guitar signal close to Node D. Node D is just as clean as the preamp ground.

So maybe keep this in mind when running wires. It's bad to mix signals with dirty B+, just like it's bad to mix preamp grounds with dirty power amp grounds. Mike, I think your burn was because your very dirty Node A was very close to your tone pot.

[p2pAmps]

Willabe makes a good point about the signal wires running parallel to/with the B+.  I have been burned myself by this several times but mine was more humm than hiss. 

I want to use this thread and dajazzman's pic to make a point about signal wires and B+ wires. This amp has 4 B+ nodes. Node A is very dirty and Node B is dirty. You should avoid running low level preamp signal wires or pots or components close to these two nodes. Node C is clean but I still would not want preamp signals running close to it. Node D is very clean. There is no harm in running any guitar signal close to Node D. Node D is just as clean as the preamp ground.

So maybe keep this in mind when running wires. It's bad to mix signals with dirty B+, just like it's bad to mix preamp grounds with dirty power amp grounds. Mike, I think your burn was because your very dirty Node A was very close to your tone pot.

Glad you pointed that our Steve as that is great info.  And you are correct about that Node A and tone pot...  It was a good lesson learned for me...

[dajazzman]

Yes the shielded cable from the Input jacks is grounded, it's coming from underneath the board back to the Input jack to ground. I lifted the wires from the pots away from the B+ buss, but didn't really hear any difference in the hiss or hum.

I think what I will do is removed the shielded cable from the Input jacks on V1 pin 2 and just put a 34k resistor to ground and see how much hiss is there compared with having the Input cable. I can see the hiss on the grid with a scope and I also see the hum which is exactly the same as the ripple on the "A" B+ on the plate of V1 just smaller. I thought about adding a another cap in parallel with the "D" B+ cap to see if that helps.
 
Most of the wires near the "A" B+ are from the Tremolo circuit. The wires from the Volume, Treble and Bass are not near the "A" B+.

[HotBluePlates]

I think what I will do is removed the shielded cable from the Input jacks on V1 pin 2 and just put a 34k resistor to ground and see how much hiss is there compared with having the Input cable. ...

Sounds like a smart troubleshooting move.

[Willabe]

Yes the shielded cable from the Input jacks is grounded,

You only grounded 1 end of the cables shield, right?

[dajazzman]

Yes, just at the input Jack end

[h4ck.b0x7]

What is the reasoning behind choosing a 34k resistor? Is that because of input 1 normally having a parallel 68k connection, hence 34k?

[dajazzman]

If you measure from pin 2 to ground with nothing in the input Jack's that's the ohms you'll measure with the two 68k in parallel.  When I insert a blank plug into Jack 1 it goes to 1.34M.

[dajazzman]

Whoops that is 1.034M

[vibrolax]

I just finished building a Princeton Reverb and I'm wondering just how much hiss and hum normally comes out of the amp with the volume, treble, and bass turned all the way up.

According to Duncan's tone stack calculator, with the treble on 10, there's about an 9 dB boost (-4 dB) compared with treble on 5 (-13 dB).  I have a genuine 1967 Princeton Reverb (replaced all electrolytics a while ago).  It's not the quietest amp I own, but it's completely acceptable to me with the volume and tone settings I actually use.  The amps I've built with Hoffman-style grounding are quieter.

[h4ck.b0x7]

Can you post a video of a Hoffman amp you have maxed out, treble and volume with nothing plugged in. And your princeton maxed out the same?

[dajazzman]

I tried removing the input Jack cable and sub a 34k resistor and the hiss was still there. So I'm thinking it's either outside interference or I need to get my tube caddy out and try more preamp tubes.  I will try posting my Princeton and the Deluxe Reverb that was built by somebody else.

[h4ck.b0x7]

Sounds good.

[HotBluePlates]

... the hiss was still there. So I'm thinking it's either outside interference or I need to get my tube caddy out and try more preamp tubes.  ...

Or the issue is the plate and/or cathode resistor of that 1st tube stage. But yes, the tube needs to be ruled out.

[h4ck.b0x7]

Would outside interference cause hiss? I know it would cause hum. With mine I've tried all kinds of tubes. The less gain a tube has the less hiss there is.

[HotBluePlates]

Cool Amp Tools (Library of Information)

Scroll to the bottom, there's a "stethoscope listening amp" for you to probe your amp and actually pin down where noise is originating.

Would outside interference cause hiss? ...

If I were minimizing hiss in an amp, I wouldn't be chasing external interference.

[h4ck.b0x7]

If I were minimizing hiss in an amp, I wouldn't be chasing external interference.

I didn't think so.

[dajazzman]

If I were minimizing hiss in an amp, I wouldn't be chasing external interference.

I didn't think so.

Outside Interference would be highly unlikely, but you would be surprise the Interference your local electric utility system generates and I'm not talking about 60Hz hum. They have technicians that all they do is track down Interference from bad Insulators, pole grounds, loose mounting hardware and the like. Some of the stories about the problems it's creates is unbelievable.
 
I think my next step is to try a lot more tubes and see if it makes any difference.

When I first powered it up I had some cheap tubes in there just in case they blew up. I didn't want to destroy my good ones. The amp actually had more hum at that time until I replaced the first tube with a better one. I don't have  a lot of faith that I'll get rid of all the hiss with a tube change. I have now heard three amps built by three different people, one that was built by a guy who that's all he does and they all have hiss with the volume and treble control full up with the standard Fender design.

I'll keep trying, but at some point I'm just going to Install it in a cabinet and call it good. When I set the controls like I like which is the volume at around 6 or 7, treble at 3 or 4, and bass at 6 or 7, I don't hear any hiss or hum.
 

[h4ck.b0x7]

I did some rerouting of wires today and all is good. There is a bit hum when the reverb is maxed out but that's so far into Dick Dale territory it serves me no purpose. As for hiss, there is some hiss but I compared it to pretty much every amp in Guitar Center today and it is nothing out of the ordinary, this included some top notch vintage Fenders. A/B'd with a stock DRRI and there is NO comparison.

[Willabe]

A/B'd with a stock DRRI and there is NO comparison.

Meaning what? 

[h4ck.b0x7]

This thing knocks the socks off a stock DRRI in pretty much every category when it comes to tone. As for difference in background noise, this is much quieter than the DRRI was. Especially the hum.