Red Plating??

Yes, there are now several varieties. Electric guitar players like them for the distortion they make.

That is 5VA, not 5A. This is a low voltage transformer for things like doorbells.

https://www.ebay.com/itm/Edwards-590-Transformer-10V-5A-Load-120V-Line/121034476341?hash=item1c2e378...


Ebay has premade bucking transformers: https://www.ebay.ca/itm/Bucking-Transformer-Voltage-Reducer-For-Fender-Marshall-Gibson-Vox-Ampeg-Etc...

Good catch! - thanks.


I was not aware of the product for guitar amps but its nice to know about! That would be perfect for this sort of thing, if line voltage is all there is to it.

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.

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.

@georgehifi  When you go from an autobias to adjustable (by definition both are considered 'fixed bias' as the bias is 'fixed' by the voltage on the grid), regardless of how the amplifier is biased, the output power is unchanged (the distortion created is another matter entirely). The bias by itself does not affect the power- it simply sets how much quiescent current is flowing through the tube.

To increase amplifier power the B+ would also have to be increased. Then of course the amplifier could not be biased for class A operation, because the dissipation of the power tubes would be exceeded. With any power tube when you look at the specs, there is a maximum plate current and maximum plate voltage that are shown. These two maximums should not occur at the same time.


Performing modifications on a new amplifier is likely to void its warranty. Its one thing to ask for support of a warranted product, its entirely another to ask for it when the product has been modified by unauthorized personnel. You might want to take this under advisement; I hope you now see that you've offered bad advice. In this case the best course is simply contact the manufacturer to see if the amp needs service or not. 

Would you consider the red plating shown in my photo a "bit"?

@chinook9  Technically this would qualify as 'a bit' :)  It would certainly have me nervous! I would contact the manufacturer of the amplifier.

Interesting, the manufacture designs an auto bias amp around the kt88 tube and then recommends the user to use 6550 tubes.

Functionally they are nearly the same. No-one makes 6550s anymore; KT88s work in all the same places. But a 6550 will be a vintage tube and most vintage tubes are better than new production.

Sorry didn’t see you have cathode biasing, (auto bias).
You could get them converted to fixed biasing, by someone close to you that’s good with tubes, could cost around 2-3hrs labour, up side is you’ll get a little more wattage also.

Its important to understand the difference between autobias and cathode bias. Autobias is a form of ’fixed bias’ in that usually there is a servo circuit of some sort that monitors the bias of the power tubes and applies correction. ’Cathode bias’ is often confused as being the same, because it is also ’automatic’. But to do cathode bias you need a fairly large resistance in the cathode circuit; as the tube conducts the voltage dropped across the resistor is the bias voltage (since the grid resistor is tied to ground).


If you were to convert to some form of fixed bias (such as autobias...) from cathode bias, the cathode resistor value would be vastly reduced and the resistors in the grid circuit tied to a negative voltage source instead (which might be controlled by the autobias servo). Because the voltage drop across the cathode resistor is gone, you would have more plate voltage on the power tube and so ’more power’. But if the amp is supposed to be class A, at this point the dissipation in the power tubes would be exceeded, leading to short term failure!

For this reason, I would ignore George’s comments entirely!


Since the tubes are clearly running a bit hot, I think it would be a good idea to contact the manufacturer as others here have advised and see if this is OK. Its not surprising that with some modern tubes they might be a bit fudged on some of the numbers; if the plate dissipation of the tubes is lower than spec, you would see a bit of red-plating even if the tube is biased correctly for class A.