Help please

[NEWGUY]

I have a KAT 6550 power amp,i have a problem with it?The front pair of 6550 now run alot cooler than the back pair,you can see the back pair glowing blue like the fronts used to,!! you can now touch the front tubes when powered up??.I have just installed these 6550's and the problem stays with the front pair when i swap them.The rectifiying tubes are about two years old , both of these glow the same.I took this amp on pre built but i had to almost rebuild most of it,ive done the pi filter mod black gates etc.I have been inside with a iron to re do any possible dry joints,and i cant see any leaky caps or shorts or burnt resistors,i suppose the next step is to start measuring some points.Any help guys ... could i need new rectifiers?????? thanx ....James

[andrew ivimey]

You are right, you have to do some measuring.

But how do you know the front 6550s are hotter than the rear 6550s?

As they are PP pairs the left front and back should be running at the same voltages, as should right front and back.

Someone here will now what the voltages should be.

[NEWGUY]

It also sounds like its clipping on loud music,it manages to image well with both left and right outputs ok ....James

[NEWGUY]

well the front valves are alot cooler to the touch and also they dont glow blue like the rear valves do...james

[andrew ivimey]

The 'cooler' bit is interesting. 6550 valves just don't work 'cool'.

Also if you have the blue incandescence you are lucky.

Did you say you have blue glow on the rectifiers too? Do the anodes glow dull (or bright!) red?

But only the rear 6550s?

New rectifiers won't do anything.

We need voltage checks.

[Les Reed]

A power valve like 6550 running so cool you can touch, can only mean one thing - it isn't conducting much current. This could be due to faults in the circuit eg. open circuit cathode resistor, or screen resistor; poor grounding on the affected side etc. We need voltage measurements.

[Mike G]

Hi,

You say you took the amp on ready built, do you have a copy of the original WAD build instructions, and the voltage checks? You will need these to refer to, if you are going to do some testing. I think I saved a copy on my PC before the demise of the old WAD site, should you need them.

Mike.

[NEWGUY]

The anodes all glow bight red both fronts and rears,no the rectifiers dont glow blue,i will measure soon and get back to you,thanx,,,too hot for messing with valves at the moment...28 deg at 9.30 silly....thanx for replies ....james

[NEWGUY]

do you thik its strange both front 6550's are affected the same....james

[Greg.]

Hi James,

You should not draw any conclusions from the incandecent blue glow of your valves. This is probably down to impurities still present within the vacuum and actually IMHO deminishes with time. Furthermore, on a paralleled output, the first valves in the circuit initially glow blue brightest, but over the passage of time this may reduce and the second pair blue glow more thereafter. The blue glow is a red Herring. The temperature change is more significant and therefore voltage testing is essential for you to move on and others here to advise. Do a full voltage test, publish here and you can be sure of some sound advise and direction.

Best wishes,

Greg