New tubes Vox Night Train 50

[gingertube]

My IT Manager at the day job has a less than 12 months old Night Train 50.
He was talking about selling it as it was "farting out" on bass notes and when using the high gain channel driving hard (he's a bit of a shreader type player).

He asked me to take it home and have a look at it.
Confirmed his "fault description" so checked the output tube bias. One at 46mA and one at 21 mA idle currents.

Put the low idle current tube into the AVO MK3 tube tester and showed that the tube was duffed - I could not set a stable test current, the longer the tube was on the more its anode current faded away.

Looked in the EL34 bin, lots of JJ's but I knew he did'nt like those, so kept looking and found a pair of Winged C Svetlana that had seen only about 4 hours use in a HiFi Amp prototype.

I put those in and re-biased for 40mA each output tube. The board is VERY sensibly laid out. Only 2 pots on the board, and they are labelled VR3 and VR4 (VR1 and VR2 don't exist) these pots being the individual bias adjustments for teh 2 output tubes. The pot physical location and the bias test points are sensibly arranged such that their order matches the output tube order physically.
The test points are across 1 Ohm resistors in the EL34 cathodes so 40mV indicated 40mA idle.

Dropped it back to him Monday and he is thrilled with the result - says it sounds 400% better than when brand new. I take that with "a grain of salt" but it does indicate the the NT50 does respond well to having superior output tubes fitted.

I was a little worried about tackling this without a schematic but without reason. Once I noted that it was fixed biased with a separate bias adjustment for each EL34 it was straight foreward.

So if you have some nice EL34 I can recommend swapping out the Chinese jobbies that the amp has factory fitted. It is a simple job.

I originally fitted some vintage NOS Philips 6CA7 but they were a little shorter and the tube spring retainers were flapping loose - so he ended up with the Winged C Svets instead.

Cheers,
Ian

[Stevem]

If your a fan of tight Bass then saying you do not like JJ brand 34s
( especially the 30 watt rated Groovetubes version!) is like saying you would not like winning the Lottery if you ask me!
May be he did not like how hot or cold they may have biased in the amp he played them thru?

Groovetubes JJ EL34s are all I will use when I retube a duel or triple recto amp for a metal player unless they want to spring for KT88s!