Tube replacement troubleshooting question

Hi all, been a while since I've posted here, but always looking at the many builds and threads.

I have a small 15watt tube amp which has been great until it stopped working. No sound

the tubes are 2 - 12ax7 and 2 - EL84's - amp =(Stauss SV TNTB chinese make)

The amp powers up and the tubes all glow ( I have noticed that one of the 12ax7's glows on one side only. I belive there are 2 filiments in each tube and this is possibly the problem)

I have checked the fuses which appear good.

I have some mesa boogie tubes on the way both the 12ax7 and el84.

Now I have learned a little about the need to 'bias; the tubes correctly.

It's probable that this amp is cathode biased meaning i can change tubes easily.. I think

My main question is can any of the amp gurus establish if my amp is actually cathode biased from the photo below?

Attachment 33401

If both of the cathode pins of the two EL84 power tubes are tied to ground via a 5W resistor (of at least a couple of hundred ohms in value), then it's highly likely that the amp uses cathode-biased power tubes, it's even more likely if there's a single HT winding on the power transformer with no 50V -V Grid Bias tap for the power tubes.

See if you can find a schematic for the amp on the internet.

Thanks for the rply Doc !

I must admit to having no real knowlege of what Im looking for.
I was at least wise enough to drain all the power and took great care when taking some photos of the internals
I may need to find a local tech even if it turns out to be an easy job.

Im unsure if i can just drop news tubes in or not.. me thinks probally not

Regards Gavin

On amps of that size it is typical that you can just pop the tubes in and be done with it..If the designers were concerned they will usually install a pot so the bias can be adjusted, or have relatively easy access to the cathode bias resistor or grid bias resistors. With a pot you just need a multimeter and a adjustment tool, but changing cathode or grid bias resistors should be done by a trained tech. Swapping the 12AX7's will always be fine, and swapping out the EL84's should not pose an issue however if you are concerned then do it anyway and at some stage down the track take it to a tech and have him check it over.

What concerns me is that it is not producing sound, and you describe one of the filaments in one 12AX7 not glowing. Before doing too much else I'd be replacing that 12AX7 and see what happens. In near all current day uses of 12AX7's they employ cathode bias and can be swapped out as required with nil re-biasing needed.

Thankyou Marcel and Doc for the assitance.
I think i will start by replacing both 12ax7's in hope that the sound returns.

Really should only need to replace the one with the blown filament... The rest should be okay, or kept as spares.

would it be worth swapping the current 12ax7's to check that the good one (2 filiment glow) works in the other socket?

maybe that would test to see if the tube is the issue?

thanks again
Guv

You could try, it can't hurt, but often on those small amps you need all stages to work to get any output so don't be surprised if there is nil difference.

Yes, the failed filament will travel with the tube regardless of the socket it is in.....

Looking at the configuration of the amp, I'd guess that it's a direct copy of the original Vox Night Train NT15 head (though with their own circuit board layout). Do a web search and you will be able to download a copy of the circuit diagram for it e.g. https://elektrotanya.com/vox_night_t...wnload.html#dl

No adjustable bias as far as I can see on the power valves.

Hi Simon,
I hope your doing well mate, and I think you are on the money again.

I have read on the very limited web searches, that this amp is said to be a night train nt15 clone..

Ive been mad googling el84's, 12ax7's, biasing , schematics, cathodes, (and learning at least 25% of it all) ....loving it, therapy for me

regards
Guv

One other thing to do is to check that there aren't any cracked solder-joints on pins 4,5, and 9 of the two 9-pin sockets for the 12AX7 tubes, pins 4,5, and 9 are used for the heater connections, with pin 9 being the common connection for each triode's heater, the 12AX7 is what's called a high-gain twin-triode tube (theoretically capable of a gain of 100) and is a very common tube in guitar amps, each of the 12AX7 tubes in your amp can have it's heaters wired to operate on 12.6V AC (only pins 4 and 5 used with no connection to pin 9, both triode heaters in series), or 6.3V AC (pins 4 and 5 tied together, with pin 9 being the common, both triode heaters in parallel).

Also check pins 1,2,3 and 6,7,8, on both sockets for cracked solder-joints as well, you can do this by examining the solder-joints under a strong magnifying-glass, if you see any joints that look cracked or suspect, just heat the joint up with a soldering iron and add a little extra fresh solder to it, that should fix any cracked solder-joints, but make sure the amp is fully powered-off and disconnected from the mains, and that no dangerous voltages are present first before you do anything else.


Sometimes those amps with board-mounted 9-pin sockets can suffer from cracked solder-joints on the socket pins, usually due to heat and vibration when the amps are operating, or when transported on the road in a car, if it turns out to be a faulty 12AX7 tube with a burnt-out heater, then it's just a simple matter of replacing the faulty 12AX7 with a good one.

From the photo the 12AX7 solder connections on V3 and V4 look to be fine...

The cathode bias resistor looks to be R4 a 120 ohm (Brown Red Brown).... a little hot but the HT voltage will also be a factor.... Multimeter voltages from everywhere will give proof.

My other thought is that maybe there are some socket pins that need re-tensioning in the socket for the 12AX7 tube that's faulty.

Happy Days, my amp lives !
Now appears the faulty 12ax7 was the culprit.

I found a Night Train schematic which displayed a 5w 12ohm between the EL84's consistant with Marcels, Simons & Docs input confirming amp is cathode biased.

I swapped all tubes with near new mesa boogies (which could be any manufacturers from researching) and Boom. Sound again!

Thanks all for the assistance.

regards
Guvna