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Polytone Mini Brute II fix up

Started by jpcar, October 25, 2016, 07:46:53 PM

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jpcar

I'm looking to fix up a Polytone Mini Brute II amplifier.

There are a few issues I'm dealing with..

Firstly, the preamp is quite noisy. This is hiss that is present regardless of the volume being up or down and regardless of anything plugged into the input. I'm wondering if this is an inherent problem with this amp or is this something I can do to improve it. I have another Polytone Mini Brute (later model) which is quiet as a mouse.
I tested the preamp from the noisy Brute into the power amp of the other Brute and it's noisy. Then I tested the preamp from the other Brute into the power amp of the noisy Brute and it's quiet. Hence my conclusion is that the preamp is to blame.

Second problem is the reverb is very weak. With the reverb control up full the reverb is just audible but not particularly strong. At half way you can barely hear the reverb. Again this might be an inherent problem with the amp. I recently replaced the old reverb tank (which had a snapped spring) with an Accutronics 8DB2C1C. This was a recommended replacement. Input impedance of 310 ohms and output impedance of 2575 ohms.

There's a trick I've seen with other amps to get more reverb, you can take the input for the reverb driver from before the Gain pot so that it gets maximum gain to the reverb tank all the time, making the reverb louder. In this case that would mean disconnecting R23 from the wiper of R72 (Gain pot) and reconnecting it to the side that's not grounded. (Schematic attached)

Has anyone tried this? Should I give it a go?

Another question is... can anyone tell me about OP1, OP2, OP3 etc.. in the schematic? What are they and what's their purpose. They are covered in black rubber on the circuit board. Are they little current limiters perhaps?

Thanks for any help!

phatt

Before you start any mods, swap the reverb tank connections and see if it improves the reverb effect, wrong connections is a common mistake when replacing the tank.
If no improvement,, then be aware the new tank may not be correct type.
If you still have the old tank measure the resistance of the transducers and see if they are indeed reading same/similar resistance.

The black blobs marked OP are *Vactrols* they are made of 2 parts, a photo resistor and a diode.
So OP1 photo resistor is in front of R36 and its corresponding Diode is down the bottom in the switching circuit after R53. With Diode off the OP goes very high resistance ,, Diode on OP goes low resistance, usually close to zero Ohms. This is how the channel switching is done.

Regarding the Hiss,, well hard to know but if you can dig up the schematic for the other unit then a very good chance of finding the offending section. A lot of amps do tend to hiss and rather pronounced at low bedroom volumes but never noticed at gig levels.
Phil.

jpcar

Solid advice. Thanks Phil!

I swapped the reverb tank connections and yes indeed they were the wrong way around!
I marked which way around they were when I removed the other tank but they might have been wrong before that.
Now the reverb sounds pretty good! No mod necessary!
Out of curiosity, I tested the resistance of the transducers of both new and old tanks, and they are close, not completely matching.
Old tank was around 32 ohms input and 248 ohms output.
New tank was around 32 ohms input and 204 ohms output.

As for the hiss, you might be right. It's not a offensively bad hiss. But the other Mini Brute I have much quieter so I wonder if there is something that can be done.
I have attached the schematic for the another unit. 

And thanks for the info on Vactrols. Got some learning to do on that topic.