Red Plating??

I have a CLASS A KT88 amp that has an autobias circuit.  The company rep recommended SED 6550C Winged C tubes.  I purchased a quad of them.  They bias OK and they sound wonderful but I just noticed that they red plate a bit. If I can figure out how to do it I will attach a picture.

The red plating is just in the inner corner of one of the plates on all 4 tubes.  It looks as though I am looking at the filament through a translucent plate but I know that its red plating.

Can I operate like this safely?  The bias does hold steady and does not fluctuate. The red plating does not change.  As a Class A amp this status should hold steady.  Of course, it does run hot. If all that it hurts is tube life, thats OK.  These are my best sounding tubes without a doubt.

You should ask the manufacturer of the amp. Send the pic of the tubes.

I just noticed that they red plate a bit.
Drop the bias 10%

Cheers George
+1 George. But how do you  manually bias?

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.

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

Thank you all for your input. I have sent a message to the company rep.  He was the one who recommended these tubes. 

atmasphere, you mention "a bit of red plating."  Would you consider the red plating shown in my photo a "bit"?  The amp "bias- checking" lights/circuit indicate that it is correctly biased. All four tubes have a similar amount of red plating.
@atmasphere some really good info in your post.

Thank you.
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.
chinook9 OP

Red plating is not good even slight, because both are doing the same it means the tubes are being pushed to their max, either because the tubes are overrated or the mains voltage in your area is higher, than where the amp was built, in Au we can have up to 30vac mains differences in different areas.
Best is do what I said and get it converted by a competent tech to a bias system you can adjust yourself (to 10% less), which we in Au call fixed biasing as you fix the bias point manually, and you'll get more power.

Cheers George
@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. 
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]

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:

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.
Great information being posted in this thread.  For the time being I am waiting to hear from the company rep but I'm not holding my breath. I'll use the KT88s until I get this resolved. 

Not sure if the bucking transformer will solve the problem but its worth probably worth the investment of $90 if it might work. 

I'd like to hear opinions on potential of the bucking transformer to solve the problem.  They are available in 3% or 7% reduction in voltage and it looks as though I should go for the 7% reduction if I get one.
chinook9, log your voltage over a few days. It is not uncommon to have a higher level for short periods. Base your required drop percentage on average. Note that voltage levels can vary drastically by season dependent on the local requirements. In L.A. summer is the high load, but the Oregon coast winter load is higher.

A device rated for 115v is typically ±5% which is the line voltage standard. However, local utilities may struggle to maintain. This is a good read  Voltage Tolerance Boundary (

The eBay link above has no current spec. The box is plastic. Pass...

Build a variable bucker and be done with it!
That is 5VA, not 5A. This is a low voltage transformer for things like doorbells.

Ebay has premade bucking transformers:

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.
It looks like the 6550 is back in production. This place has them listed as new!
Yes, there are now several varieties. Electric guitar players like them for the distortion they make.
You could use a "Constant Voltage Transformer" (Good small one about $200) which would even out Voltage Spikes & Noise, assuming it supplied sufficient current for your AMP..

I utilize a single CVT for all but my Main Power Amps (which are plugged directly into a dedicated 20 amp line), the CVT provides the best results for Sound Quality from smaller current draw complnents
A CV or Isolation Transform is a 1:1 device. It does not reduce the input voltage unless running above rated current. For proper operation, the load must match the device capacity.

It provides isolation and reduces some noise components, some by a lot.

I think you mean running above its rated voltage.  A CV transformer will work over a fairly large load range. It saturates based on voltage.  It is typically 1:1, and typically the output is a bit higher than the input, which will be maintained with voltage sags (and peaks).  Sag performance is better at lighter loading.