Red Plating??

chinook9 OP


Yes you have 10v higher mains, you must be nearer the big pole/sidewalk mount transformers that have many taps and boost or lower the voltage on your mains line. The nearer you are to them the higher the mains voltage, the further away the lower the mains voltage gets.
You could complain to get it bought down, but they’ll just say the houses at the end will then be too low (100v). I got it bought down from 250v to 230v for Australia because I also service medical equipment and used that as a bargaining tool, the real point was, way back I was going though many incandescent light globes, they’d last sometimes not more than a month.

As for a different mains transformer, that’s for a specialist to do.

As I said before get it changed to a self adjustable bias much cheaper, that way you set yours bias 10% lower and will stop the plates glowing red, and you’ll get more power from the amp also compared to auto biasing.
Hope also your power supply capacitors have a bit of max volt head room for this extra mains voltage you have as they will be be up 10% also.

Cheers George

A bucking transformer is one day project. Built as I did, it covers from <115 to 126 volts
Changing bias will INCREASE B+ as current will drop.

A variac is a simple solution, but one I don’t recommend.

To drop the AC line voltage inexpensively you can use a device called a bucking transformer. This is simply a transformer that has the required voltage (10V) along with the current required. So if the amp is drawing 3 amps you could use a 5 amp 10volt transformer, like this one:
https://www.ebay.com/itm/Edwards-590-Transformer-10V-5A-Load-120V-Line/121034476341?hash=item1c2e378...

The input side of the transformer takes your AC line voltage. The output of the transformer is then put in series with the line voltage going to the amp. If its in phase, the voltage is boosted; you want it decreased so you would reverse the series connection. This will drop the AC line to the required voltage. Of course this should be done by a qualified technician. But the transformer is cheap, much more so than a variac which could do the same job.

What is peculiar here is that 115V has been gone a long time! 117V was the norm in the 1980s; autobias did not appear in tube amps until sometime in the 1990s.


Setting the amp up for a different bias scheme sounds expensive- we do a lot of work with audio equipment and work like that would take several hours- meaning that it would be $100s if done here in the US.

This all sounds scary to me the kt88 tube runs at a much higher voltage than the 6550 and the bias current is different also. They might sound good but i do not think that this setup will last long unless he ran the kt88 tubes at a lower voltage it would shorten the life of the tubes(6550) significantly. Also the fact that the amp is auto bias with a different tube in it eeven scares me more but like i said he could have designed it that way to work with both tubes ask him to be sure.

This all sounds scary to me the kt88 tube runs at a much higher voltage than the 6550 and the bias current is different also.

The two are not that different. In looking at the TungSol plate current, its rated at 140mA (max) where the KT88 is rated 100mA. They have similar maximum plate voltages too. This suggests that the dissipations are similar. I've not seen a circuit so far that supported 6550s that didn't also work with KT88s. My H/K Citation 2 has its power tube sockets marked 'KT88/6550'. Its generally accepted that if you design for one, the other can be substituted. The difference is that 'KT' stands for 'Kinkless Tetrode'; the kinkless bit being in the linearity curve. So 6550s should work fine but be higher distortion. Or KT88s should work fine but be lower distortion.