Quieting a Polytone MB II hissing/mild crackling noise.

[stanleyj]

Dropping a line on this forum for some input on the infamous Polytone hiss. I get a hiss/mild crackling noise from my Polytone MB II. It's a model from the 80's. I did take it in to and my tech and there were three microphonic signal caps that were giving the reverb lots of noise. Mostly, though, the
problem was with the grounding. Some of the shielded cabling was grounded
and also some of the grounding points were loose or had poor contact.

It did take care of the hum but it still has that darn hissing/crackling noise. It is present when nothing is plugged in and the volume does not affect the noise. It is present when the volume is all the way down. The noise does get louder when the reverb is turned up.

Could this be a result of maybe cold soldering joints? Will disconnecting the reverb tank quite this down?

[52 Bill]

Could this be a result of maybe cold soldering joints? Will disconnecting the reverb tank quite this down?

Welcome to the place. Cold solder joints can be the cause of many things. I don't know if this is the cause of your problem, but you should try to investigate and locate the problem to find out.

Typically hiss is more related to active devices like the ic opamps in your amp. If you say the noise increases when the reverb is turned up, that would be a good place to start looking for a problem. Does the noise also increase when the volume or tone controls are turned up?

Unplugging the reverb tank will likely not stop the noise and if left unterminated it may cause additional hum in the return circuit of the amp.

[stanleyj]

[QUOTE=52 Bill;358387] Does the noise also increase when the volume or tone controls are turned up?QUOTE]

No the noise does not increase when the volume or tone controls are turned up. The noise is present with everything turned down to zero. It only gets louder if the reverb is turned up past 2 and over. It's not overwhelming but it is noticeable especially when mic'd.
I've heard Polytones have been known to have some white noise. Just want to see if I can get some input to pass along to my tech for him to zero in on.

[stanleyj]

My tech also said because of the cramped design of those amps it is susceptible to noise problems.

[52 Bill]

Have him change the reverb return ic and see what happens.

The cramped construction can cause a few hum problems especially if the reverb tank is mounted with the output side closer to the power transformer than the input side of the tank.

[Mark Hammer]

I bought an old Heathkit TA-16 combo amp two years ago, and experienced a rather noticeable hiss. I read somewhere that the input transistors could degrade from a variety of factors over time, and simply replaced the 2N3391s that were in the front end with new 2N5089s. The hiss left with the old transistors.

My amp is perhaps a good 15 years older than yours, but consider the possibility that transistors (something one tends to think about long after pondering the caps) may play some role.

[stanleyj]

"Have him change the reverb return ic and see what happens."

So even though the hiss/mild crackling is present with the reverb is turned off to zero the return ic could be a factor?

[52 Bill]

"Have him change the reverb return ic and see what happens."

So even though the hiss/mild crackling is present with the reverb is turned off to zero the return ic could be a factor?

You bring up a good point. So the answer is it depends.

Exactly which model do you have? Polytone has used a number of different circuits through the years. The one thing that has been generally true of all of them is that they all use dual section opamps.

I looked up the schematics for a few different models and found that on the older versions, the chip that handles the reverb recovery is also used for the main summing of the reverb and straight signals. On the newer version the second half of the reverb chip is used for the distortion channel circuit.

It would help if we knew what model /version you have.

[J M Fahey]

Somebody post the schematic

My Ouija board is out on warranty repair

[stanleyj]

You bring up a good point. So the answer is it depends.

Exactly which model do you have? Polytone has used a number of different circuits through the years. The one thing that has been generally true of all of them is that they all use dual section opamps.

I looked up the schematics for a few different models and found that on the older versions, the chip that handles the reverb recovery is also used for the main summing of the reverb and straight signals. On the newer version the second half of the reverb chip is used for the distortion channel circuit.

It would help if we knew what model /version you have.

I will look tonight and see if I can find some #'s on the back/outside.
It is a Mini Brute II with the silver control plate. Bass/Treb/Volume/Reverb.
The volume has a red nob on top (which I assume is distortion) that does not do anything. Control panel does not say "distortion" anywhere.
I was told it was from the 80's.

[52 Bill]

I will look tonight and see if I can find some #'s on the back/outside.
It is a Mini Brute II with the silver control plate. Bass/Treb/Volume/Reverb.
The volume has a red nob on top (which I assume is distortion) that does not do anything. Control panel does not say "distortion" anywhere.
I was told it was from the 80's.

The red knob is the distortion volume control. It turns on with a rotary switch or I think some had pull switches. If it's not working it may be a problem with the circuit or the footswitch jack.

Is the covering black felt with a grey front panel? If it is as a test, plug your guitar (passive pickups) into the main out jack on the back of the amp. If you strum the guitar you should hear it through the speaker. None of the controls will change the sound of the guitar. Now turn the guitar volume control all the way down. Do you still hear the noise coming from the speaker?

[stanleyj]

The red knob is the distortion volume control. It turns on with a rotary switch or I think some had pull switches. If it's not working it may be a problem with the circuit or the footswitch jack.

Is the covering black felt with a grey front panel? If it is as a test, plug your guitar (passive pickups) into the main out jack on the back of the amp. If you strum the guitar you should hear it through the speaker. None of the controls will change the sound of the guitar. Now turn the guitar volume control all the way down. Do you still hear the noise coming from the speaker?

The front has been replaced with a plastic speaker grill like the Henricksen Jazzamps. It is not felt. Here's a link of an image I found on the web which looks like the one I have judging by the control panel and covering:
https://www.google.com/search?q=poly...ml%3B800%3B600

[52 Bill]

The link sent me to a page of photos of different MB 2 amps.

What is the amp covered with? Please describe.

Is there a "Main Out" jack on the back?

[stanleyj]

The link sent me to a page of photos of different MB 2 amps.

What is the amp covered with? Please describe.

Is there a "Main Out" jack on the back?

It is covered with the black tolex. Not the diamond pattern but something like this:
Marshall Style Black Elephant Tolex Vinyl Cabinet Covering Yard 54" Wide

The label on back (id plate) just says Polytone Mini Brute. The s/n is 1412.

Tried plugging guitar into main out test you mentioned and noise is still there.

My tech is going to change the op-amp out and take a look at the reverb circuitry.

[52 Bill]

Tried plugging guitar into main out test you mentioned and noise is still there.

My tech is going to change the op-amp out and take a look at the reverb circuitry.

If you plugged into the main out jack, turned down your volume control and the noise was still there, that points to a problem with the power amp.

The main out jack is directly connected to the input of the power amp. If you turn down the volume control the signal from the preamp is shorted to ground and shouldn't be heard from the speaker.