• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube recommendations

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Sfpeter68

Member
Joined 2018
I have one of those "Nobsound" single ended amps that uses the following tubes:

X2 PSVane KT88's
X2 6N8P Preamp tubes
5Z3PA Rectifier

In general I've been happy with it but have wondered if it could be improved. Are there any drop-in replacement tubs that would give better sound quality? I'm not looking for power (it's a low power amp anyways) as much as clarity and "feel", and I'm using two Sony full height speakers.
 
kodabmx

kodabmx

Member
Joined 2011
If you're bored and have lots of money to waste, the 6N8P can be replaced with 6SN7 or Soviet 6N8S, the 5Z3PA can be replaced with a 5U4GB, and the PSVane KT88 can be replaced with NOS Genalex.

IMHO you won't notice a lick of difference except your wallet will be lighter.

Cheers.
Koda
 
kodabmx

kodabmx

Member
Joined 2011
Those 6N8S "crappy" soviet tubes are just as good as NOS RCA black plate 6SN7GTB in my testing... I don't waste money on audiophoolery... I run them at 450V+ volts (they are rated for 250V) and they never break a sweat. In 7 years I've never even had a failure which is more than I can say for NOS American tubes or especially the Chinese "arc bottles" than are "6550". Many screen grids have opened up on those for me.
 
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kodabmx

kodabmx

Member
Joined 2011
No, it's indicative of the Soviets tubes being understated on the datasheet. They basically copied the 6SN7 datasheet, but built the tubes like a 6SN7GTA/B... It started as an experiment to see how they compared to the RCA, and they are just as good if not better IMHO.
 
kodabmx

kodabmx

Member
Joined 2011
The power spec should be respected, but the voltage spec is very conservative in my testing. For all I know even the US 6SN7 (not GTA/B) would also work at 450V.

The Soviet 6P6S (6V6 equiv.) is also only rated to 250V, but it's been used at 450V in guitar amps without issue either.
 
2

20to20

Member
Joined 2010
kodabmx said:
For all I know even the US 6SN7 (not GTA/B) would also work at 450V.
Click to expand...

I had to look it up. The max plate voltage for 6SN7 is 450v. not 250v. So if you are running the Russian tube by 6SN7 specs as opposed to the Russian sheet for its tube, you said rated at 250v., then it just an experiment disregarding the maker's recommendation.
 
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2

20to20

Member
Joined 2010
kaputt said:
Anode voltage limits don't matter that much as long as you respect maximum dissipation.
Click to expand...

It does matter. If you push the plate voltage way above the max then you have to bias the tube so far toward cutoff it operates down near the bottom of the curves. Look at 6SN7 curves and then imagine the max plate is 250. not 450v. And then imagine where the bias is if 450v is applied. That is just operating point concerns. Operating past the max design voltage is asking for tube failure, out of the blue.
 
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huggygood

huggygood

Member
Joined 2011
the French tube guru "Francis Ibre" explains this well about Russian hits. Russian manufacturers kept a large margin in the data of their tubes for can announce very long life. And I agree with kobabmx, the 6n8s works very well beyond the manufacturer data. moreover, they are the best 6sn / 6sl7 that I have been listening to until now without having to sell one of my kidneys to buy them.
 
pblix

pblix

Member
Joined 2011
20to20 said:
Look at 6SN7 curves and then imagine the max plate is 250. not 450v. And then imagine where the bias is if 450v is applied. That is just operating point concerns. Operating past the max design voltage is asking for tube failure, out of the blue.
Click to expand...

I just pulled up the curves, but rather than imagine, I drew up some load lines. At about 20kOhm @ 450V B+ you're right on the edge of the dissipation limit. At, say 27kOhm you're plenty safe with about -8V bias. The load line never intersects the 2.5W dissipation zone. In fact, looks almost ideal. As long as you intend to operate in the linear region I don't see the issue.

Sketch it up yourself and you'll see.
 
2

20to20

Member
Joined 2010
For the tube in question, you can't actually use the curves of 6SN7. And we don't have the curves of the Russian tube that supposedly has a max plate voltage of 250v. The excersize is to look at the 6SN7 curves and then shift everything to the left as it would be if the max plate voltage was 250v. and a typical plate voltage in operation was 200v. or less. Then imagine where the current and bias points would be to keep it from redlining if you applied 450v on the plate. Any usable loadline would be useless for a practical input signal.
 
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20to20

Member
Joined 2010
pblix said:
.. As long as you intend to operate in the linear region I don't see the issue.

Sketch it up yourself and you'll see.
Click to expand...

So if we assume the tube in question has a 250v design maximum plate voltage, and is being run at 450v. then you could use the 6SN7 curves by shifting right. You would approximate the overvoltages by using the 250/450 ratio of 55%. So lay up your loadline as if the plate voltage on the 6SN7 was 700v.
 
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