65 Vox Dies after it warms up?

[Vox Rocks]

Hello all, New guy here. I just picked up a very cool 1965 Vox Student 5 watt practice amp built by The Thomas Organ Company in insane condition. The amp sounds awesome when first fired up, (no hum, noise or static) then 4 or five minutes later it begins to lose volume and fades to no sound at all rapidly. Let it cool off for a half hour and same thing happens.

All I have done is check the tubes on my TV-7 tester and they all test good.
I tried different tubes one at a time to elimanate any tube problems. I had the chassie out a few days ago to have a look inside. I cleaned & lubed the one volume & power on Pot and tube sockets. The amp uses Oddball Tubes as listed on the tube chart (1-35W5 Rectifier, 1-12AU6 Preamp & 1-50C5 Power). The 50C5 power tube appears to run a little hot, judging by the brighter redish-yellow glow then typical tubes.Same thing with a different 50C5 too. It's not glowing red hot, just appears to be brighter?
I am not an electronics tech but everything looked OK to me, no lose connections or fried components. It all looks original and not tampered with. It also appears to have had very little use. Clean, clean, clean!

I love the tone of this simple design. It sounds like a cross between a blackface & Tweed era Champ.

Is there a way for a novice like me to find the problem without much electronic knowledge? I don't know how to test resistors or transformers and don't have the equipment either.

It still has the two prong power cord but does not hum when first turned on. Infact it sounds excellent untill this loss of power and volume drop.

The original Vox Bulldog 7" Speaker sounds great and is in like new condition.

Any help would be appreciated

[Joe6v6]

If it all looks original then the filter capacitors & any oter electrolitic caps are 20 - 25 years past the end of their service life. Electrolitic capacitors will only last for so long then they die, even if the amp was never used they dry out and are no longer good, heat inside the chassie speeds up this process.

I am sure a good service tec will want to replace all the electrolitic caps in the amp befor checking anything else & chances are thats whats wrong with it. This will in no way de-value the amp, It simply HAS to be done to older amps.
JOE

[Steve Conner]

That "drop in volume after a few minutes" tends to be caused by a leaky interstage coupling capacitor. Having a tech replace all the capacitors in the amp (not just the electrolytics) is the no-brainer solution. Though having said that, I have two old Selmer amps that are still gigging on all their original caps, but I also saw other amps where I had to change every single cap to get them working. (Not to mention a tube signal generator whose electrolytic caps exploded on me, splattering goop all over the place.)

So if it were me, I'd find the faulty cap, and replace it as well as any others in there that looked to be the same type as it.

[Enzo]

The little glowing things in each tube are called heaters, also called filaments. The only part of them that matters is the part you can't see down inside hte works of the tube. And further, what matters is the temperature they can heat the cathodes to. SOme will look brighter than others, but this means nothing.

To answer your question, NO there is no simple thing to do for this that doesn't require some electronic knowledge. For a technician, this is a simple amp, and will be simple enough to find what part(s) is bad. The shotgun approach - replace all the parts - will certainly find the bad one. I prefer to find the problem myself. If I find any leaky caps, I just check them all, and usually if one leaks, the rest of that type do too, as said above. On the other hand, there are plenty of 40 year old caps out there still working fine, I don't replace them just because. But all of this is acedemic if you can't solder, use a meter, identify parts on a schematic. That is what the guy like me in your town is for.

[Vox Rocks]

Thanks for the help guys. I will have a buddy of mine check it out.
He is an electronics tech who lives out of state. I'll ship him the chassie
Fed Ex and have it back in no time.

Here's a photo of the circuit.

Vox Rocks

[Tom Phillips]

Looking at the photo you posted, I see a connection in the power supply that looks like is was never soldered. It's in the lower left area of the picture. The terminal strip connection of the two yellow wires and the three 1/2 watt resistors. If that joint is trully unsoldered it could be causing your problem and a fresh solder joint may get your amp working. I have seen this situation many time in old equipment. Check it out and let us know if it's really unsoldered or just looks that way in the picture.
Tom

[Vox Rocks]

Thanks Tom, I didn't notice that connection. I will take a look at it and solder it if needed.
Glad I posted that photo !

Vox Rocks