Red Plating??

Given the ambient light level, it is a fair bit more red-plating than a little.

IF you are competent to operate inside a tube amplifier, check rated voltage, B+ and the cathode resistors. !!!__ ALWAYS KEEP ONE HAND IN YOUR POCKET __!!!

Many US HiFi products are still rated for 115v. That's OK in Los Angeles where 118v is a very good day. In Oregon 118v is a brownout where 122v+ is typical. A bucking transformer solved Oregon overvoltage  ieLogical BuckTrans

B+ could be too high due to a mains transformer mis-wiring [in the US, 115v instead of 120v power transformer input tap].

Cathode resistors could be the wrong value. [I received a pair of monoblocks and one amp had 20k & 30k plate resistors swapped]

ieales

I just checked my circuit voltage and found it to be 124 Volts.  The amplifier indicates its designed for 115V, 

Could I build a transformer to reduce the voltage??  I have built a number of electronic devices and probably have the required equipment.

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.