Berning ZH270 help

Hi folks,
I've a problem with my ZH270 that maybe someone else has got and solved.
My ZH270 since few days after 10 about minutes from turn on starts to rustle and croaking on the left channel, you can see the red led of bias to flick.
If I turn it off and then on again in short time (less than a minute). I got the big bump but then I can play for hours but no more noise comes up.
This is systematic. Any idea of what could be? Maybe something I can fix myself whitout send it to service.
Thanks for any helpful replays.
Sounds like a terrible problem and I suggest you sell it me. Just kidding (sort of). Good luck getting it sorted out. Bob
DId you switch tubes from one channel to the other to see if the problem changes channels? If you did and it stays in the same channel, it's probably not tubes. In that case I'd get in touch with David Berning or Allan Bhagan.
If one of the tubes are not within it's spec, the amp will see it within 1/60 of a cycle and continue to try and correct it within every 1/60 of a cycle while limiting current to the tube, which turns the LED red, if you turn the amp off and then on again and the problem is gone for awhile, that could mean the tube warmed up and is now operating within it's specs.

So as Oneprof suggested, please swap the tubes from one channel to another and see what happens, I suggest only doing the outputs firsts and try it that way for a while and see what happens.
It would also help to know the age of the amp.

Please feel free to e-mail me.
Thanks for your kind and appreciated help. I've forgot to tell I tried to swap the tube, power first and pre after but the problem persist on just left channel. It remembers me once when I had one cap gone with another amplifier...
I'm in contact with a good service here in Italy, I'll take my ZH270 there.
Hey Allan, I'm Paolo from may not believe but I've not yet received your post. It should arrive anytime these days....Italian Post Office....
Paolo, the noise you describe really is conducive to that of a tube problem, you need to take your time and switch one set at a time, and make sure you are changing them from one channel to another, as well, did you do any mods to the amp?
The ZH-270 is not a user friendly mod device by any means.

I hear stories from Europe of people cutting off the wrappers on caps and other stuff.

When you get the other set of tubes, we can go through the switching, so we can make sure it's in opposite channels.
Thanks Allan,
Just received your tubes and kit today, I'll install and see if it solve also the Left channel problem.
Anyway, no mods made on my ZH270, only preampli stage tubes swap. Im my ampli there was 1x 12AV7 RCA Electron Tube, 1x 12AV7 Internation Service Master (2 different!) and as 12AT7 2x WC GE and 2x JAN Philips.

As mod I'm thinking on signal cabling upgrade with better cable (the one used is quite bad actually) and maybe a better potentiometer, I could heard astonishing improvement with an Imai (Audiotekne) precision carbon resistor one.

I had to service it one year ago when a transistor in PSU was dead, but in the case the ampli didn't wanted to turn on: always red leds.

OK, just replaced existing unknown output tubes with the Allan Sylvania and the problem seems over. I had only one hour listening, so i need more time for last word.
By the way, either the Sylvania and the ERS kit does inprove the ZH270 sound!

Paolo you might want to talk to Allan about the cryoed hybrid wire upgrade if you think the tubes and ERS improve the sound. The wire upgrade improves resolution and truly improves the sense of ease to this exceptional amp. The only problem is you are in Italy and you need to have very good soldering skills for this mod but it is sure worth the trouble.
Thanks for the hint Tubegroover, actually Iàve already contacted Allan for the that. Living in Italy doesn help this.
Anyway I ve found what I consider the best option for cables: Sublima an italian manufacturer, He's cables are mervelous to preserve harmonic richness and correct timbre reproduction. So sooner or later I'll have my ZH270 cabled by Alessandro Cereda of Sublima.
Another thing I'm thinking to is to get rid of the potentiometer an switches. A friend of mine has done it with the Berning Siegrfried I sold him and the results were quite big.

By the way, the noise on the left channel came out again, I definetely need to service it. Even if my suspect is onot on components but maybe a soldering point no more good or a signal path on the audio board with breaks.

Switch the input interconnects to the amp from left to right and right to left, see if the noise stays in the same channel.

The ZH-270 is different from the Siegfried, as is has different feedback settings, if you remove the pot, then you will have problems with sensitivity in low and maybe medium feedback, you will most likely hear pre-amp noise.

You might be able to do it if you are running a CD player direct but there are few pre-amps made, especially tubes ones that the ZH-270 will not pick up the noise without having to reduce the sensitivity of the input to the amp, the only other alternative is to find the exact resistance needed to which feedback you use and put a fixed resistor.
Thanks again Allan,
The noise on left channel appears even without anything connected to input, definetely not dependent on pre o input cables.
The way it disapear it make me thing about a cap that cannot full charge properly, with a quick turn off/on it probably doesn't discharge much and get the current peak again to force him charging full. Just an idea.

Yea I agree, it is not easy to mod ZH270 without loose its great flexibility. In real I seldom use other tha 12dB feedback, it is the setting better delivering bass performance and best highs extension (I've Tannoy monitor golds 15" drivers, and dumping is a matter of quality here even at 94dB efficency).

The 25K pot in the signal path does indeed effect the quality of sound. Maybe inprove it with a switch matrix of very good carbon resistors.