Polytone 104

[vintagekiki]

After 5 minutes (no signal) when the amplifier stabilizing, the values that you specified are typical (20-30 mA).
Check for DC offset at the ends of speaker. Typical value DC offset is 0 +/- 100mV (0.1V)
On the collectors is not allowed to exist any AC voltage (no signal).
Time of 5 min was needed to "fill" all the capacitors in the amplifier and to stabilize the operating point.

[lowell]

that's crazy. so the first 5 minutes that this amp is turned on and played the output transistors are not biased correctly?

[vintagekiki]

Each amplifier power on a few minutes earlier before playing to stabilize (tube amp have switch stand by).
For the transistors would be dangerous to over biased (Ic> 50mA) Low collector current or under biased is not dangerous for the transistors. When the transistors heat up (after 3-5 min), the collector current to achieve the value of 20-30 mA, transistors are biased correctly.
Are the output transistors without signal hot?
Is it clean or distorted sound when you play?

[lowell]

haven't played it yet as 1 of the power amps still cannot be biased: there is NO current no matter where I put the bias pot. I've checked all related resistors and everything seems fine. I'm questioning the LM391N-80. I cannot find any of these. Do you know if they are available anywhere or if there is a substitute?

[vintagekiki]

If R13/14/15 are correct and if the change R15 does not affect on collector current probably is incorrect LM391-N80. Replaced by LM391-N100.
Do you have the correct voltage to the IC
pin6 = -33 VDC; pin15 U = + 33 VDC; pin9 = 0 VDC +/- few mVDC

If you do not have a replacement for the LM391-N80, maybe it's a modern solution to make new power module, for example, with TDA7293. Larger work is to make a new solid state power module like Polytone power amp mod 378B

[lowell]

I swapped the LM391 from the working PA and it works. I cannot find any LM391 online. Any suggestions for finding one? I don't wanna redesign this amp. I anyone has any LM391 please let me know if I can buy one.

[vintagekiki]

I anyone has any LM391 please let me know if I can buy one.

hinkel-elektronik bg-electronics preisroboter

[52 Bill]

There on eBay from Hong Kong for about $4 plus shipping.

[lowell]

i always forget about ebay!!! thanks guys.

[lowell]

damn... I think I put the new LM391 in backwards. There's still no current. Would doing that fry the IC? Why did I only order 1??

[Chuchi]

HI, I am brand new on this forum, non native English speaker and may have chosen the wrong post. I've bough years ago a Polytone 104 ...and since, I try to obtain some more information about it. It is fully functional.
Thanks to those are used to deal with Polytne, they might be fair enough to give me some explanations.
Thanks a lot.
Chris