Vox AC15 H1TV tube replacement

[gorch]

The amp has two channels. Ch1 the old EF86. Ch2 the classic Vox tone.

So, on ch1 the tone phases out and doesn’ sustain anymore while ch2 still seems to work. Since I had replaced the EL84 output tube a while ago, I’d like to know which one to replace for ch1 now. Well, as I’m not really good at tubes, their behaviour and lifetime in various amps, it would be great to listen to the experts advice over here.

Tubeamp doctors offer a full replacement set. However, I am not a friend of full replacements if only one tube had a problem.

[texwest]

Hey Gorch,  You may have figured this out by now.  But you probably don't have a problem with the el84's. They are both used for both channels.  So if one channel works it's more likely to be  a preamp tube.  Just get a known good 12ax7 and substitute it in each slot until the amp works and you've found the bad tube.

[gorch]

13 hours ago, texwest said:

Hey Gorch,  You may have figured this out by now.  But you probably don't have a problem with the el84's. They are both used for both channels.  So if one channel works it's more likely to be  a preamp tube.  Just get a known good 12ax7 and substitute it in each slot until the amp works and you've found the bad tube.

Thanks for reply. I haven’t done anything due to business and a long guitar clinic weekend. 

Great idea! I might just swap tubes before ordering a new one.

[HAMERMAN]

I took a look at the schematic and CH1 uses the tube in position V1 (an EF86).

[ArnieZ]

Don't put  a 12ax7 in the ef86 slot they are totally different

ArnieZ

[gorch]

9 hours ago, ArnieZ said:

Don't put  a 12ax7 in the ef86 slot they are totally different

ArnieZ

Thanks for the warning! I’d check the schematics before action anyways. Tubeamp Doctors offer a full replacement set for about EUR159. I might order and safe time as I don’t have any tube stock in the shelfs.

[Jakeboy]

Try a replacement ef86 first...

[texwest]

Oops I thought he said the ef86 channel was working fine. My Bad!!! Yes try to replace just the ef86 tube first.  If the other channel is fine, then your el84's are working ok.

[gorch]

Well, bought a full replacement set from Tubeamp Doctors. Checked the tubes twice and replaced them one after another. Amp seemed to work fine after replacement. Great sound on all channels. Next day, some quirky noise at startup and now dead silence on both channels. Needs more treatment though.

[texwest]

Gorch I recently retubed a new production AC30 and it didn't like Russian made tubes in certain 12ax7 slots.  It did exactly what yours did.  It worked the first time around and then died on the second  start up.  The filament or heater wasn't lighting up on a 12ax7 so it was obvious what the problem was.   It happened twice before I really researched it.

Apparently the Russian made tubes don't hold up if it's put in a tube socket that's wired as a cathode follower circuit and it kills the heaters on the tube. Andits well known that Russian 12ax7 tubes don't work in cathode follower positions.  And lots of Voxes use this kind of circuit on certain tubes.  But the AC30 I worked on even killed the heater in the phase inverter. Look at the tubes and see if the heaters are lighting up on them.  If the heaters aren't lighting up, they are dead and I would recommend using NOS or Chinese 12ax7 tubes in those slots. They don't have this issue.  Here's a good article talking about where the new production tubes are made.

https://www.tubesandmore.com/tech-corner/12ax7-comparison-current-made-tubes

[gorch]

Many thanks texwest. I‘ll return to the original 3-set of 12ax7 and see what it does. 

As the original problem was supposed to be the ef86. It should work out right away. We‘ll see.

[gorch]

Long story short. Eventually, I took the amp to a local tech since I missed time to move on analizing and fixing and had it sit there for months. Problem turned out that the balanced set of power tubes from TAD were mad and caused the fuse to melt. Will write to TAD and see what they are willing to service.

Right now, essentially, the EF86 seems the only one tube replaced. Both channels are working again. Nice!

[texwest]

The new production tubes are pretty unreliable these days.  I've had several failures in the past year after installing them in some guys amps.

One problem with EL84's is that they are run so hot  in these voxes they will redplate and blow the fuse.  When you buy new tubes you should always buy from people who know specifically what mA rating will work in that amp.  Even if you do that you can still have problems.  EL84's are not plug and play anymore.  They need to be tested for current draw (mA) and then you can ask them:  Will these tubes work in this particular amp?

The regular old chinese EL84's that vox uses are more reliable in their amps than other new production tubes including JJ's.  This is from my own observations in testing the bias and watching for redplating in my own and customers amps.

[gorch]

16 hours ago, texwest said:

When you buy new tubes you should always buy from people who know specifically what mA rating will work in that amp.  

Well, I thought the TubeAmpDoctors could be trusted. The set were dedicated for the amp model. I had bought a set of JJ power tubes for the amp at Tubetown in 2017. They are working great. I would need to improve on my technical skills though in order to have more control about these things though.

[tbonesullivan]

Aren't the AC15 and AC30 cathode-biased? Though, honestly I've never understood how they decide what the "correct" bias is.

[texwest]

Yes they are cathode biased but you still need to be careful to not run the tubes too hot.  These new vox amps are often running tubes at 120-125%  dissipation.  It's really part of the sound but it will take out tubes fast if they aren't hearty.  And the new production tubes aren't that hearty.

If a cathode biased amp is running a too hot bias you can change it by adjusting the cathode resistor.

[Daveadd]

I found in an AC 30 I had with the EF86 channel that the Russian heavy duty EL84 tube were the only ones that did last. I called them the Russian Bomber tubes.

I actually think the Russian military stuff is made for some rough duty.

[tbonesullivan]

You can still pick up some nice EL84 military production tubes. The only thing to look out for is that on some amplifiers made more recently, they have out of spec wiring on the sockets that can cause the tubes to fry.

[gorch]

The amp in question is supposed to be the original circuits of the 59 and high gain. It’s an anniversary model.

Anyhow, it’s working now. But thanks for the tips.