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"Grid leakage" problem common to tube pre-amps?

Despite my EE degrees, I'm quite new to vacuum tube technology. I'm using a BAT VK-5 pre-amp (love it) and have developed a scary screaching, static when I adjust volume. It seems to happen most when the unit is cold. It's scary because I'm presently using a Krell KSA-200S and it will add 26.5 dB to pretty much anything you throw on its input. The noise is very loud, not low level static common to potentiometer-based controls (the BAT definitely doesn't use pots for volume control).

Victor & Steve at BAT are great and very responsive. Victor indicates that the problem is likely due to grid leakage on one of the tubes (6922s), causing the presence of DC.

Is this issue common or well understood by others w/ tube equipment? Can anyone provide some more details or background on this phenomenon?

Thanx,
John B
murphthelab
5 responses Add your response
sogood51
2,157 posts
 
Thanks Marakanetz, I had read something about Krell amps and tubes but could not remember what it was.
marakanetz's avatar
marakanetz
5,473 posts
 
Sogood51,
Krell specifically warns to use DC-free proccessors or sources. It's completely direct-coupled even between input and output stages and considered to be a DC-amplifier i.e. ranges from 0Hz to ...100kHz??.
At the same time Krell guarantees an undisputed circuit stability to the tolerance of TT cartridge DC output levels at maximum i.e. 3..5 mV that realy should not do any damage to the voice coil.
As to small internal change i.e. output or input coupling capacitor it will ruin everything Krell had ever aimed for basicaly no capacitors in the signal path.
sogood51
2,157 posts
 
You should talk to Krell about this I think. Is there not some small internal change that should be made on your Krell amp to use with tube pre?
sean
6,231 posts
 
While i know nothing about the line or product design, it seems as if DC leakage and the BAT name appear in quite a few threads on this subject. While i believe Victor to be extremely responsive to customer concerns, he might want to start checking into this. Sean
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marakanetz's avatar
marakanetz
5,473 posts
 
I'd guess that it's not normal with tubes.
Without looking-up the circuit diagram of preamp it's hard to realy make a conclusion why it takes place or poised to take place and how often.
6922 is a dual-triode very often can be used as a differencial input(if you're familiar with transistors that will be similar to the differential stage on two identical transistors) and in regular voltage-gain stage that can use such dual triode in number of different ways. The gain stage is where the circuit stability is most on demand that pretty much depend on power supply and regime the tube is being operated.
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