Amplifier antics

[evetsfrance]

I use a modified Yaqin 100B amp driving Quad ESLs. It sounds OK. I noticed annoying hum from the RH channel but no other symptoms. This usually means the bias on the KT88s needs adjusting. When both KT88s in a pair are drawing the same current supply ripple is cancelled out. I put the DVM on the test point (across the cathode bias resistor) and one valve read 0.5V as it should, the other zero.
Pulled the base off and everything checked OK for continuity then found the zero current moved with the KT88. Resoldered the pins to no avail, the cathode had gone open circuit so the amp was running with one channel single ended with little or no impairment of sound quality!
No spares so some ElectroHarmonix ordered from the States at a sensible price. A rummage in my valve box turned up one KT66 so I shoved that in where the dead KT88 had been and set the bias on both to 35mA. Sounds absolutely fine, indistinguishable from the other channel at reasonable listening levels!!

[DF96]

Gnats and camels!!

[evetsfrance]

Originally Posted by DF96

Gnats and camels!!

Well, yes, you would think it should sound awful in either configuration.
I have to admit that changing the Chinese 6SN7s for some Soviet ones (on sale by me here, plug plug) made an enormous difference but this does tend to cast a doubt over some of the fine points of 'tube rolling'!

[Miles Prower]

Originally Posted by evetsfrance

Well, yes, you would think it should sound awful in either configuration.
I have to admit that changing the Chinese 6SN7s for some Soviet ones (on sale by me here, plug plug) made an enormous difference but this does tend to cast a doubt over some of the fine points of 'tube rolling'!

It casts doubts on a lot of audiophool folk "wisdom", including the obsession with tube matching. Doesn't surprise me, after all, the experiment the OP did -- plug a KT66 in one hole and a KT88 in the other -- was described in the Radiotron Designers Handbook IV, where they put a 45 in on one side, and a 2A3 on the other. The resulting imbalance had much less effect on sonic performance than expected. It's also something guitar amp designers did a lot (6V6, 6L6 PP pairs).

Tube rolling does have a basis in fact: VTs are low gain devices, and so circuit performance depends much more on the characteristics of the active device. Not so much for transistors. Whether or not it makes a significant difference... that depends. I wouldn't waste my time tube rolling if the amp in question included NFB. The whole point of NFB is to make the performance less dependent on the open loop performance, and more on the feedback loop itself. With NFB applied, unless a VT is defective, I wouldn't expect enough of a difference to bother.

[evetsfrance]

Hi Miles,
I agree entirely. I do think the low signal end might show differences (the Soviet 6SN7s are different from and sweeter than the Chinese ones) but I believe a properly biased KT88 is just a KT88.
The thing that amazed me was to have listened to many hours of music with an open cathode (hum - blast! must sort out the hum!) without audible distortion.
Steve

[Miles Prower]

Originally Posted by evetsfrance

The thing that amazed me was to have listened to many hours of music with an open cathode (hum - blast! must sort out the hum!) without audible distortion.
Steve

That is a surprise. You would think the severe core magnetization would have caused saturation. I'd expect you'd notice that, as it sounds quite nasty.

[evetsfrance]

Originally Posted by Miles Prower

That is a surprise. You would think the severe core magnetization would have caused saturation. I'd expect you'd notice that, as it sounds quite nasty.

I tend to listen at low levels since I find CDs less intolerable when they're quiet (even better, silent) They do record silence superbly, probably their only + point over vinyl!.