'62 JMI era Vox AC30 rebuild help!

[TravisThatcher]

Hoping to harness the collective knowledge here to help solve a problem that I've been banging my head against the wall about for some time. Helping a friend rebuild his '62 JMI Vox AC30 and we've run into some puzzling issues. At one point not too long ago the amp worked but was getting increasingly noisy. Eventually it just put out noise and my friend started to look into a rebuild. We recapped everything in the amp and replaced pretty much all of the resistors. We've been double checking the wiring a ton as well. Currently the amp starts up and passes the vibrato channel but really quietly. The normal channel passes no audio and the bright channel which has the factory fitted top boost will only pass a tiny bit of signal if the top boost controls are cranked. Yesterday I started checking with a scope and a signal generator and I found that there's no signal coming into either volume pot for the normal and bright channels. We replaced V1 with still no real change. Upon measuring some voltages I found that pins 1 and 6 of V1 - the 12ax7 - are getting the full 320v from the power supply, not 170v as noted in the schematic I've attached. I'm wondering if this is the reason V1 isn't passing any signal? I checked the resistors coming off pin 1 and 6 and they are both measuring correctly at 220k and 270k respectively.

If anyone has any experience rebuilding these suckers I'd love some help here. We've exhausted pretty much everything I can think of and while I have a general understanding of the electronics I can't see why that voltage is so high.

[52 Bill]

Welcome to the place.

Upon measuring some voltages I found that pins 1 and 6 of V1 - the 12ax7 - are getting the full 320v from the power supply, not 170v as noted in the schematic I've attached. I'm wondering if this is the reason V1 isn't passing any signal? I checked the resistors coming off pin 1 and 6 and they are both measuring correctly at 220k and 270k respectively.

The next thing to check here is the grounding of the cathode resistors going to pins 3 and 8 of V1. If the grounds are open, then the tube will not turn on.

This of course assumes that you have already checked the filament voltages going to the tube, because again if the heaters don't heat, the tube will not turn on.

[Jazz P Bass]

To add, make sure the 25uf bypass capacitor, across the 1.5K cathode resistor, has the polarity orienred correctly.

Plus to cathode, minus to ground.

[pdf64]

The solid core wires used tend to fracture / be prone to dry joints; especially the case for the heater wiring, as it's a heavier gauge.

[drewl]

All of the above and oxidized/corroded tube sockets not making a good connection

Look for proper cathode voltages AND filament voltage.

I see this alot with ancient Silvertones.

[Chuck H]

All of the above +

And you can take this any way you like... You messed up when you started replacing handfuls of parts for no complete and diagnosed reason. This causes a huge increase in the likelihood of miswires and cold solder joints. Especially in a spaghetti factory like an AC30TB and especially with older amps where the lead wire may develop an oxide coating. When initial problems change or get worse after such action it usually means you have compounded the problem with additional problems. Many novice techs have done this sort of thing and utterly shelved projects because troubleshooting eventually became impossible. Some started with working amps that just needed the power supply caps replaced. Some have even destroyed the original value of their amps because all the additional problems, testing and bench time eventually leads to a mistake like failing to plug in a proper load or a shorted power supply and an original transformer gets toasted.

You can get crabby with me if it makes you feel better. I'm just encouraging you to stop fixing anything you don't know is wrong. I know you came here to make things better. And I'm going to start by asking you not to make anything worse.

With the amp in it's current state it will be tough, but not impossible to troubleshoot. Please check suggestions and perform any tests requested.

I'm not trying to be the turd in the punchbowl, but I've seen many cases like this here and they don't all get resolved. I (we) want your amp to get fixed.

[TravisThatcher]

We actually replaced the tube sockets so they should be fine. I called my friend and asked him to verify that the cathode resistors and the bypass cap are attached properly to ground on the other side and that the cap is oriented the right way - though from memory I think they are. The tube does light up a bit as do the other pre-amp tubes so the heater should be okay but I'll get the voltage measured. Thanks for the help so far! Any thoughts on the voltage being so high on pins 1 and 6? Its the same if the tube is removed as well.

[TravisThatcher]

No offense taken - I have a lot of experience with synths but I'm still learning a lot about tube amp designs. We replaced everything against my better judgement/gut feeling but my friend was wanting to go for it. I definitely see how it could complicate things. We have gone over the wiring and layout many times looking for errors in how things are put back together. We've been using the layout here: http://turretboard.knucklehead.dk/wp...normalrev7.pdf as a reference. I can see that might be a problem if it isn't completely correct. I've been cross checking connections with the schematic but I wonder if there's any glaring errors here.

[Chuck H]

No offense taken - I have a lot of experience with synths but I'm still learning a lot about tube amp designs. We replaced everything against my better judgement/gut feeling but my friend was wanting to go for it. I definitely see how it could complicate things. We have gone over the wiring and layout many times looking for errors in how things are put back together. We've been using the layout here: http://turretboard.knucklehead.dk/wp...normalrev7.pdf as a reference. I can see that might be a problem if it isn't completely correct. I've been cross checking connections with the schematic but I wonder if there's any glaring errors here.

I'm relieved to find out your experience level. That improves the odds enormously and I'll take that the circuit has been checked and is probably correct. There could still be a miswire, but probably not an obvious one.

Since you aren't getting signal into the volume pots then, the next thing I'd check would be plate and cathode voltage for the previous stage. See if the tube is conducting. This assumes you DID see signal at that amplifier input grid.

[52 Bill]

Any thoughts on the voltage being so high on pins 1 and 6? Its the same if the tube is removed as well.

I guess I should have explained it better, but if the tube is not turning on, there will be no current flowing through the tube and therefore no voltage drop across the plate resistor. Just like when the tube is removed from the socket.

The voltage will only come down once the tube starts to conduct. It needs filaments to heat up and it needs a complete circuit through the cathodes to ground before it will conduct.

[drewl]

What he said.

In the future try replacing caps or sockets one at a time and verify the unit still works after each part replaced.

May take a little longer but will cut down on troubleshooting if there is a problem.

[jaysg]

At one point not too long ago the amp worked but was getting increasingly noisy. Eventually it just put out noise and my friend started to look into a rebuild. We recapped everything in the amp and replaced pretty much all of the resistors.

Assuming mostly carbon comp resistors, what did you replace them with? btw, I really hate the way these were built and I agree that you're going to have to systematically ring out all the wiring looking for cold solder joints and open wires -- good solder on both ends, but a break inside the sheath.

[TravisThatcher]

Thanks Bill - that makes sense. Definitely seems that V1 isn't turning on. Just asked my friend to actually measure the heater voltages - I know the lines are more or less working because the top boost tube lights up but perhaps the voltage is wrong or V1 isn't seeing it for some reason. The cathode resistor and bypass cap is connected to ground properly. I'm also having him check the inputs to the pre-amp. I had checked for signal from the jack but not necessarily checked to see if the shield was interfering or shorting anything out. Will report the results...

[TravisThatcher]

Replaced with metal films - I might get yelled at for this, I know. Most of the original carbon comps that were there were out of spec.

[g1]

The cathode resistor and bypass cap is connected to ground properly.

To rule out any other issues in that cathode circuit, what is the resistance measurement from the tube socket cathode pin to ground?