AC30 (TBX) stopped working.

[Sea Chief]

Hi chaps, happy new year (sort of, bit late!)

help needed please. UK-built '95 AC30 tbx not working. The consensus with these it is said, is keep the Standby on, & just use the power switch. Today I turned the power switch on, & waited for the Standby lamp to light.. all as usual then.. but this time, I only see the rectifier tube lit. All other tubes not lit.

I've never experienced this with any amp before. Weird. Nothing at all from speakers (obviously). Just a dull hum presumably from the 6,3v going to the 5AR4.

Any ideas? Thanks SC

[Pixel]

Isn't the heater on the rectifier not on the 6.3V?

[nosaj]

Isn't the heater on the rectifier not on the 6.3V?

Probably not it's a 5v tube and rectifier tubes typically have their own secondary.
nosaj

Now I'm confused about Pixels statement and I think it might be the double negative that got me.

[nosaj]

Seachief , Just to be clear because I am not. Was this amp working in your possession before or is this a new amp to you just not working?
nosaj

This link show location of fuses to see if any are blown.
https://www.voxshowroom.com/uk/amp/tbxservice.html

[nosaj]

Schematic https://www.drtube.com/schematics/vo...60-02-iss5.pdf

[Chuck H]

I can see by the schematic posted by Jason that there are fuses on the PT filament secondary for the 6.3VAC supply. This is a different secondary from the filament supply for the rectifier tube (as noted above). Typically when a fuse blows you want to know why before affecting other repairs. But in this case... Considering the lack of reports about the filaments on tubes being affected by other failures and the fact that almost no other amps use a fused 6.3VAC supply I'm going to say this is just over engineering and you should check and repair at F3 and/or F4 as needed. If neither of these is the culprit then you may have a bad 6.3V seconday. Which is very rare for other guitar amps but being unfamiliar with this model I won't discount the possibility.

To see if either of these fuses is open you'll need to locate them on the board and do a continuity test across each fuse. I've never seen the innards on one of these amps so this is all I can report... Test for continuity across F3 and F4 on your board.

[loudthud]

Because of the low resistance of the 6.3V winding and the parallel load of all the tubes, measuring the resistance across a blown fuse won't give a meaningful result unless the fuse is removed from the circuit. If you are OK with working on a live circuit, measure the AC Voltage across the fuse.

[Chuck H]

Because of the low resistance of the 6.3V winding and the parallel load of all the tubes, measuring the resistance across a blown fuse won't give a meaningful result unless the fuse is removed from the circuit. If you are OK with working on a live circuit, measure the AC Voltage across the fuse.

Good catch. Thank you. I suppose one could pull the tubes or the fuses and test for continuity if they didn't want to work in a live amp.

EDIT: Not sure how I missed that Jason posted board images. I did go right for the schematic. My usual inattentiveness. If they're clear T6's (as shown) it might possible to visually inspect them. I think it's the two at the lower right on the board as shown.

[Sea Chief]

Seachief , Just to be clear because I am not. Was this amp working in your possession before or is this a new amp to you just not working?
nosaj

This link show location of fuses to see if any are blown.
https://www.voxshowroom.com/uk/amp/tbxservice.html

Hi nosaj.

Yes the amp's been with me & working fine for 6 months.

One other odd thing: when I found it not working, as said, I turned OFF (leaving Standby in it's usual on position).. but the Standby lamp was still illuminated.

Very strange. Usually it goes off simultaneously with the OFF lamp. Panicked a bit & pulled the mains plug (thinking some mains must be still going to it). The Standby lamp was -still- lit for a further 30 seconds, then went out.

Thanks, SC

[Sea Chief]

I can see by the schematic posted by Jason that there are fuses on the PT filament secondary for the 6.3VAC supply. This is a different secondary from the filament supply for the rectifier tube (as noted above). Typically when a fuse blows you want to know why before affecting other repairs. But in this case... Considering the lack of reports about the filaments on tubes being affected by other failures and the fact that almost no other amps use a fused 6.3VAC supply I'm going to say this is just over engineering and you should check and repair at F3 and/or F4 as needed. If neither of these is the culprit then you may have a bad 6.3V seconday. Which is very rare for other guitar amps but being unfamiliar with this model I won't discount the possibility.

To see if either of these fuses is open you'll need to locate them on the board and do a continuity test across each fuse. I've never seen the innards on one of these amps so this is all I can report... Test for continuity across F3 and F4 on your board.

Hiya Chuck. (I can actually see you now!)

Great help & nosaj too.

Ok I find F3 is blown. It measured continuity when sitting in the slots on the board, but I saw a tiny singe mark & severed wire in the glass. Removed it & no continuity/ defo blown. It says T6.3L 250v

So why might this have failed is the question.. I guess.

TBH when it was working I don't feel too happy about leaving the Standby always 'on', & just using the ON/ OFF. I know this is recommended, but watching the yellow Standby lamp light on it's own, after 30 seconds or so, seems a bit odd to me. Could this method be causing too much 'strain' perhaps ?

Thanks chaps, Capt

[Chuck H]

Hi nosaj.

Yes the amp's been with me & working fine for 6 months.

One other odd thing: when I found it not working, as said, I turned OFF (leaving Standby in it's usual on position).. but the Standby lamp was still illuminated.

Very strange. Usually it goes off simultaneously with the OFF lamp. Panicked a bit & pulled the mains plug (thinking some mains must be still going to it). The Standby lamp was -still- lit for a further 30 seconds, then went out.

Thanks, SC

Just thinking out loud. Corrections imminent ...

With no tubes conducting for a time as they cool after shut off it will take a while longer for C45/46 to discharge. This may be keeping the standby LED illuminated.

[Chuck H]

Hiya Chuck. (I can actually see you now!)

Great help & nosaj too.

Ok I find F3 is blown. It measured continuity when sitting in the slots on the board, but I saw a tiny singe mark & severed wire in the glass. Removed it & no continuity/ defo blown. It says T6.3L 250v

So why might this have failed is the question.. I guess.

TBH when it was working I don't feel too happy about leaving the Standby always 'on', & just using the ON/ OFF. I know this is recommended, but watching the yellow Standby lamp light on it's own, after 30 seconds or so, seems a bit odd to me. Could this method be causing too much 'strain' perhaps ?

Thanks chaps, Capt

I don't know that anyone reports failures with these amps relative to any use of the standby switch. I wouldn't worry about it. And as I mentioned earlier, ordinarily you WOULD want to know why a fuse has blown and try to affect repairs before replacing that fuse and powering up. But in this case I suspect it's just a tender filament circuit and that the fuses just blow sometimes. I know I'm going out on a limb saying this. But there it is. Excluding that possibility an internal short to the filament on any tube would open that fuse. This is very rare with preamp tubes. It does happen with power tubes but I've sever seen it with EL84's. Just to be safe you could pull your power tubes before replacing the fuse. Then fire up the amp and plug the EL84's in one at a time. If one has a short to the filament it will pop a fuse as soon as it's plugged in.

[Sea Chief]

Because of the low resistance of the 6.3V winding and the parallel load of all the tubes, measuring the resistance across a blown fuse won't give a meaningful result unless the fuse is removed from the circuit. If you are OK with working on a live circuit, measure the AC Voltage across the fuse.

Hi Mr. Thud,

great- this info helps me understand things a bit more. Appreciated.

SC

[Sea Chief]

I don't know that anyone reports failures with these amps relative to any use of the standby switch. I wouldn't worry about it. And as I mentioned earlier, ordinarily you WOULD want to know why a fuse has blown and try to affect repairs before replacing that fuse and powering up. But in this case I suspect it's just a tender filament circuit and that the fuses just blow sometimes. I know I'm going out on a limb saying this. But there it is. Excluding that possibility an internal short to the filament on any tube would open that fuse. This is very rare with preamp tubes. It does happen with power tubes but I've sever seen it with EL84's. Just to be safe you could pull your power tubes before replacing the fuse. Then fire up the amp and plug the EL84's in one at a time. If one has a short to the filament it will pop a fuse as soon as it's plugged in.

On the 'Vox showroom' page nosaj kindly put up, where it shows the fuses, it says "if it's blowing fuse it's almost certainly the rectifier tube- replace".

Is there any way of determining if this tube is faulty? I've got amp out, & given tube pins a good contact-clean: I do suffer from a -little- house damp near amp, & general atmospheric damp out here (wet Wales valleys) the main bugbear making the input jacks need a regular squirt/ clean. I read that Florida suffers similarly.

I only have T1A fuses: could these be used? (T6.3A needed for F3, F4). Fwiw the single 2A fuse at the chassis back is ok.

Thanks, Capt

[Chuck H]

On the 'Vox showroom' page nosaj kindly put up, where it shows the fuses, it says "if it's blowing fuse it's almost certainly the rectifier tube- replace".

Is there any way of determining if this tube is faulty?

On the Vox Showroom page they are not talking about the circuit in question. The rectifier tube is on a different circuit from the other tubes. This was already mentioned a couple of times earlier. Please don't trouble about the rectifier tube right now.

I only have T1A fuses: could these be used? (T6.3A needed for F3, F4). Fwiw the single 2A fuse at the chassis back is ok.

Don't install the 1A fuses. You'll need to get the correct value fuses before continuing. The schematic indicates 6.3A for these fuses but the Vox Showroom specifically states that F3 and F4 are 3A. 3A seems more appropriate to me. These must be time delay fuses.