Polytone 103 Reverb Problem

[mokomon]

Thanks to all for help getting this amp working , from previous posts. After using it for a while I notice that the reverb goes into oscillation or feedback after anymore than 4 on the control. I checked all the resistors in the reverb circuit and diode tested the two transistors (nse170 & nse180),changed the electrolytic cap.I have not checked the coupling caps. Any thoughts as to what might be causing the feedback ? Could it be the transistors are just not up to par , or the IC is not really good ?The reverb works very well even at high settings for a brief moment until the oscillation starts. The control has to be brought right back to 0 to stop it. Any guidance would be much appreciated.

[soundguruman]

Bad reverb tank, broken spring or bad coupling coil(s) in the tank.
Reverb hooked up backwards, or bad shielded cable to/from the tank.
mmmmSometimes the tank needs to be more isolated, in a cloth bag, or wrapped in some type of foam, padding, etc...
But the reverb on polytones is awful, and ringing in the reverb was a normal factory mistake.
Polytones have a number of bugs in the design.
Like the bug that causes the output transistors, and drivers to blow, on a pretty routine basis.
(it's really that the bias is too hot, and not enough heat sinking...)
This is why almost all polytones wound up in the landfill.

[mokomon]

What was the actual cause of the output and driver transistors blowing ? I will check the reverb tank and cables.

[Chuck H]

If the pan itself is feeding back, as in the springs are talking to the speakers, then isolation is the issue. If it's an old pan there are bushings in the transducers for the springs that dry out and stop damping as they should. If it's a new pan the problem could be the same thing. As in my observation new pans seem more prone to acoustic feedback than the older pans were when new. One tip I got here (but haven't tried yet) was to go with a three spring short pan. Supposedly gives a good, full reverb but much less prone to acoustic feedback.

[soundguruman]

What was the actual cause of the output and driver transistors blowing ? I will check the reverb tank and cables.

The issue is not enough thermal dissipation.
The heat sinks are inadequate. The drivers and outputs run too hot, and eventually break down.
This is the downfall of polytone.

But I'm not saying that it's impossible to add proper heat sinks...That is entirely do-able.

Another issue was the bias settings.
They biased it really hot, to get very low distortion. That's impressive, but also self destructive.
The bias needs to be set cooler, so the transistors survive.
Low distortion, OK....but what about reliability? Wanna put a fan in it???

Last- the REVERB
It's normal for the reverb to ring in a polytone. They ALL do it.
A basic design mistake.
The "reverb" control adjusts the reverb SEND. The reverb return is ALways ON. Therefore, you have no control over the ringing, unless you shut the reverb OFF, with a foot switch.
The reverb control "SHOULD" adjust the RETURN, not the SEND.
Same problem with Kustom, and other amps...designed by people who lack the basic understanding of GUITAR AMPS.

I'm not saying that the problem cannot be fixed. But you are going to have to do a bit of re-designing and modifying.
Typically, the reverb tank is mounted in a cloth bag, and acoustically isolated, to prevent excessive ringing...but polytone did not do that.

So, you are looking for something "wrong" -- but there is nothing wrong.
You are just looking at the design errors...Which "can" be corrected.

[mokomon]

Thanks for the replies . I solved the reverb problem. The reverb tank originally had foam tape on the reverb tank so it wouldn't rattle on the cabinet. Also the screws that held the tank in only had a soft rubber washer to cushion them against the tank. The foam deteriorated and the tank was sitting on the shanks of the wood screws holding the tank to the cabinet . So major vibration and feedback. So I placed the reverb tank into a vinyl bag and screwed it to the cabinet just through the vinyl bag. Problem solved , I can crank the reverb to max with no feedback at all. Thanks again for all your replies to help me fix this problem. Hopefully this will steer someone else in the right direction if they have a problem with any amp reverb.

[soundguruman]

The bag and foam is pretty much used on all reverb tanks, in all guitar amps...

[mokomon]

How would you go about setting the bias on this amp ? Changing R10 (2.2k) ?

[Jazz P Bass]

How would you go about setting the bias on this amp ? Changing R10 (2.2k) ?

After looking at that bias setup, my first thoughts are 'don't mess with it'.

[52 Bill]

How would you go about setting the bias on this amp ? Changing R10 (2.2k) ?

They are using R24 (47R) to set the bias. The smaller the value the colder the bias, the larger the value the hotter the bias.

[olddawg]

Many times replacing the rubber mounting grommets and using an appropriately cut piece of 1" foam under the tank with slightly longer mounting screws will resolve a lot of reverb tank microphonic evils.

[Sredna]

A basic design mistake.
The "reverb" control adjusts the reverb SEND. The reverb return is ALways ON. Therefore, you have no control over the ringing, unless you shut the reverb OFF, with a foot switch.
The reverb control "SHOULD" adjust the RETURN, not the SEND.
Same problem with Kustom, and other amps...designed by people who lack the basic understanding of GUITAR AMPS.

You are wrong about that. The reverb control (knob) adjusts how much of the signal from the recovery is going to the last summing amp before the power amp.

[Chuck H]

That's what he said. He used common effects loop type circuit references in which the "send" would be the signal being sent to the pan and the "return" would be the pan signal going to the recovery circuit. Rather than say "You are wrong about that", which is instantly confrontational, it might have been better to say "Do you mean to say?.." or maybe "I don't see it that way", which makes it your own problem rather than indicating another persons failure, and therefor less confrontational.

In this particular case I think you have misunderstood the effects loop terminology. Which would indicate a misunderstanding of guitar amps. Which is what SGM has warned about.

[Sredna]

First of all: No offence meant! And you're right, I could/should have expressed myself in a less confrontional way. Sorry about that!

Back to the topic:

Studying the schematic in the first post clearly shows that the signal is tapped from the volume wiper and goes to the reverb driver circuit and from there to the reverb tank input. Then from the reverb tank output to the recovery circuit ("reverb pickup" in the schematic) and first after that comes the REVERB control. That means that the REVERB knob controls the RETURN of the signal, not the SEND. Similar design in 4 other (different) Polytone schematics I've checked.

If there is a different way to read/understand the schematic, please entlighten me! Peace!

[52 Bill]

Yes you are correct, all of the Polytone amps that I have ever seen control the reverb return signal and not the send signal.