Polytone Amps

Most of the Polytone amps are quite similar, even amongst different model names. There are only a few types of power amps used, and the most variety is probably in the preamps.

Most unusual about the design is that you usually have to remove the speaker first before you can do anything. That can be an upsetting realization when trying to disassemble one for the first time.

Replacing the fuse assembly might be the most common thing I've had to do on Polytone amps. I often end up replacing the whole assembly because the old fuse cap is gone, and I have no idea where to order a replacement cap from, and its simply quickest to just change out the whole assembly. The assembly must be short enough inside the cabinet because it sits very close to the power amp chassis. Further, if the power amp screws rattle loose (see below) I've had at least one amp brought to me where the fuse was randomly blowing due to the chassis touching the fuse holder. For this reason, I like to heatshrink the connections.

Even for changing the fuse assembly, you have to remove the speaker, and the power amp. You'll probably remove the preamp too, as it is just easier to work with the less stuff you have in there. Note that you probably won't remove the power amp completely, due to the wiring, but you can take out the 4 bottom screws and then swing it out over the side.

Most of the Polytones I see are very abused and/or neglected. Here in NYC, they are often strapped to luggage carts (which is a very dubious idea for any amp!), and then violently rattled as their owners drag them up and down streets, stairs, and whatever else. Screws fall out and get lost. When the power amp screws get lost, the metal chassis can bend, tear, and/or break.

There is usually a bias trim pot, and this should be checked like any other amp. I'll have to post my notes gathered from music-electronics-forum at some point, but for now just search through those posts for biasing advice.

Some amps have a "STV-3H-Y" biasing diode that has a screw hole to mount to a TO-3 transistor for thermal tracking. (The proper package name escapes me.) I received an amp that must have had a power amp meltdown before because every semiconductor had been replaced. Instead of replacing the STV-3H-Y, the previous tech had just heatshrinked 3 diodes together in series and left it just hanging next to the heatsink... perhaps naively thinking that dangling the diodes near the heatsink would provide thermal tracking, or perhaps just not caring about that. The symptom was humming, and sure enough the amp was running way hot.

To solve the mounting issue, I drilled holes right into the heatsink to accept some regular through hole diodes, and glued them into the heatsink for good thermal tracking. I found that the real problem was that only 2, not 3, diodes were needed to bias up the NTE replacement transistors. The original part, the "STV-3H-Y," is drawn as 3 diodes on the schematic, and the previous tech may have just gone with 3 diodes blindly, and not checked the bias afterwards.