AC30 (TBX) stopped working.

[Chuck H]

Hi there Pete.

Thanks this clarifies & gives me a bit of confidence to keep using it like you suggest, IE keep Standby in the 'on' position & forget it. Even if it 'kinda feels like' this way is putting undue stress on. Ok so in fact the very opposite using it like so.

Remind me, do you have one of these AC30/6 TB's? Or do you know them from fixing some maybe- you've obviously got a very comprehensive knowledge of the circuit, flaws & all.

Thanks, SC

I know I'm responding to your address to Pete, but...

With the info he provided it should be pretty easy to look at your rectifier tube socket and see if it's one of the amps with the incorrect wiring. If your red lead on the rectifier socket is on pin 2 you should swap it to pin 8. Easy enough I think and of great benefit to the amp. You'll still want to just leave the standby in play position but fixing the factory miswire is one less thing to worry about going forward. Since the amp is open anyway.

[loudthud]

Since the output stage is cathode biased with a single resistor for all the tubes, (and the bias is pretty hot) won't the tubes red-plate with just one or two plugged in ?

Edit: OK, it's a pair of resistors in parallel...

[g1]

I will draw the analogy of putting a 120R cathode resistor in a Champ.

[Helmholtz]

It's 50R with 4 tubes. For the same bias one tube would want 200R.
So yes, I think a single tube will redplate.

[nosaj]

Hi Helmholtz,

grateful for your input, as always. Cold.. do you think there's any relevance to the amp being very cold (I turned on at 1am, a cold night, & always a very cold room where the amp is) ? Or are you just using cold more generally, as in simply a turned-off amp.

I was in fact going to do a thread on keeping the amp in a cold room/ any issue? etc.

Thanks, SC

Based on your question. i will tell you my experiences with el84 amps. Cold rooms are ok. Getting the amp fired up and playing hot for a while what you don't want to do it move it into another enviroment with a quick temp change for example playing a show then moving the amp to the van without a cooling off period can cause condensation to form and break the el84s.
nosaj

[Greg Robinson]

Just to note standby in these amps is notoriously awful.
The design is hot switching, but even worse is a manufacturing error in the initial few years production that has the rectified DC taken from the pin 2 heater, rather than pin 8 cathode (and heater).
So the hot switched surge current passes through the rectifier heater, resulting in it having a short life if standby is used.
if an amp has a GZ34 rectifier, the use of standby has absolutely no technical merit, and possibly significant disadvantages, especially so if it’s a hot switching type.


Its completely normal.
The GZ34 warms up and gradually ramps up the HT voltage. When it gets over 90V the neon standby bulb strikes, illuminates.
Ignoring the standby switch minimises stress at power up.

This is not the circuit with the flawed standby switch wiring. That is the AC30CC2. The AC30 Top Boost Sea Cheif has is correctly designed with no hot-switching, however as noted the standby serves no real purpose.

Use 6.3A fuses for F3 and F4.

Buy a few in case you have a power tube problem. And I'll reitterate, install the fuses and plug the power tubes in one at a time. Waiting for each to warm up for twenty seconds before plugging in the next. If one has a short it will pop the fuse soon after being plugged in.

Definitely don't want to put in tubes one at a time, the single cathode resistor of 50Ω will result in anything but the full compliment of EL84's to redplate and be damaged. The bias is already quite hot even with all four, dissipating about 120% of EL84 rating.
FWIW I regularly change the 50Ω (2x100Ω in parallel this amp) to 68Ω for increased tube life with no ill-effect on tone.

[Chuck H]

Well good call on the bias condition regarding one tube at a time installation. I've never dealt with this kind of bias condition that's over 100% dissipation and since the the circumstances are VERY temporary I had hoped it would work as it has for AB1 biased amps. But we don't want melted tubes so that's why I asked. And I'm glad for the intervention.

So how to determine why that fuse blew? Just put in a fuse and see if it pops? Then what? I'm not sure SC is equipped or experienced enough to use a current limiter and take radings on start up but maybe he'll chime in to say otherwise.

What about lifting one of the 100R resistors before installing tubes one at a time? Yes it would be hot, but not damage in twenty seconds hot. And if there's a shorted tube we have a chance to discover that.

What I'm seeing is problems with my suggestion and no other suggestion on how to proceed with diagnosis. Not that the fault seen in my suggestion is wrong. But we still need a way to go forward.

[Greg Robinson]

Nominal heater current with 6xECC83@300mA (1.8A) +4xEL84@760mA (3.04A) for a total of 4.84A - fuses of T6.3A are too far on the conservative side IMO. ECC83 typically have a cold heater resistance of ~4Ω and EL84's usually around ~2Ω, that's a parallel cold resistance of ~300mΩ or almost 21A peak current (ignoring transformer impedance which will limit the inrush somewhat), or over 3x the fuse rating. The heaters will take roughly 30 seconds to stabilise at their nominal current, that's quite enough to stress even a slow-blow fuse. Every time a fuse power-cycled, that thin piece of fuse wire is subject to magnetostriction - if you've ever watched a glass fuse while you apply power you will have seen how the fuse wire "jumps" inside the cartridge. Each power cycle stresses the fuse mechanically, and it WILL eventually fail, even without being overloaded. Combine that with a fuse rating that is marginal to begin with, and nuisance fuse blowing becomes a reality, I see it often on this model. If it were my design I'd want more like T8A for heater fusing.

I'd recommend Sea Cheif just replace the fuse with the correct type, make sure all EL84's are installed, and turn on. If there were a heater to cathode short then the heater winding center tap would have burned out, not blown a fuse. I'd give very good odds that the amp is absolutely fine, and it's just a case of nuisance fuse blowing.

[Sea Chief]

Fantastic help here chaps. I'll wait to see if Pete concurs about the problem one being the chinese new CC2.

Mine is the " AC30/6 TBX Top Boost ". Marshall factory built 1995 (the additional X simply denoting it has the better Alnico Blue spkrs in).

No problem to switch a wire though even so, amp is out, wires easily accesible. Perhaps not so easy to remove pcb to do the 68r resistor job.. but I'll evaluate.

I do see one el84 is more 'worn' looking than the others. Seems to have been hotter perhaps, the red JJ markings gone kinda grey in places, unlike all the other 10 JJ's. This tube is the 2nd el84 onwards from the rectifier. No.2 of 4 as it were.

Thanks SC

[Chuck H]

I do see one el84 is more 'worn' looking than the others. Seems to have been hotter perhaps, the red JJ markings gone kinda grey in places, unlike all the other 10 JJ's. This tube is the 2nd el84 onwards from the rectifier. No.2 of 4 as it were.

Could be due to THAT tube being too hot at the existing bias. But it could also be that the tube right next to it (tube 1 as it were) isn't conducting so that tube 2 is carying all the current for that half of the OT primary. I think this is more likely.

[Helmholtz]

Every time a fuse power-cycled, that thin piece of fuse wire is subject to magnetostriction - if you've ever watched a glass fuse while you apply power you will have seen how the fuse wire "jumps" inside the cartridge.

Are you sure there's magnetostriction involved?
I'd think it's simply thermal expansion.

If there were a heater to cathode short then the heater winding center tap would have burned out, not blown a fuse.

With a heater to cathode short the current would be low due to the cathode resistor.
If anything shorts the heater voltage to ground, a fuse will blow and protect the PT.

[Greg Robinson]

Are you sure there's magnetostriction involved?
I'd think it's simply thermal expansion.

You know I never really stopped to think about that, someone once told me magnetostriction was involved - and I'm sure it would play some small part, but themal expansion would surely be the dominant effect.

With a heater to cathode short the current would be low due to the cathode resistor.
If anything shorts the heater voltage to ground, a fuse will blow and protect the PT.

Quite right, had my fixed bias/grounded cathode thinking cap on when I wrote that. Wouldn't expect many opportunities for heater to short to ground though.

[g1]

What I'm seeing is problems with my suggestion and no other suggestion on how to proceed with diagnosis. Not that the fault seen in my suggestion is wrong. But we still need a way to go forward.

In this case it was the heater fuse, but the question has come up before with regard to HT fuse blowing. Perhaps it warrants a dedicated thread.
I would say you need to disconnect the cathode resistor and tack in a substitute or series resistor to check single tubes in a cathode biased multi-tube power amp like this.

[Sea Chief]

Nominal heater current with 6xECC83@300mA (1.8A) +4xEL84@760mA (3.04A) for a total of 4.84A - fuses of T6.3A are too far on the conservative side IMO. ECC83 typically have a cold heater resistance of ~4Ω and EL84's usually around ~2Ω, that's a parallel cold resistance of ~300mΩ or almost 21A peak current (ignoring transformer impedance which will limit the inrush somewhat), or over 3x the fuse rating. The heaters will take roughly 30 seconds to stabilise at their nominal current, that's quite enough to stress even a slow-blow fuse. Every time a fuse power-cycled, that thin piece of fuse wire is subject to magnetostriction - if you've ever watched a glass fuse while you apply power you will have seen how the fuse wire "jumps" inside the cartridge. Each power cycle stresses the fuse mechanically, and it WILL eventually fail, even without being overloaded. Combine that with a fuse rating that is marginal to begin with, and nuisance fuse blowing becomes a reality, I see it often on this model. If it were my design I'd want more like T8A for heater fusing.

I'd recommend Sea Cheif just replace the fuse with the correct type, make sure all EL84's are installed, and turn on. If there were a heater to cathode short then the heater winding center tap would have burned out, not blown a fuse. I'd give very good odds that the amp is absolutely fine, and it's just a case of nuisance fuse blowing.

Great, then unless otherwise suggested, I'll try this. "Nuisance fuse blowing" would be a real pain, for me, should this be the case as I couldn't find the cause & be permanently frustrated by it. But maybe liveable with.

This gives me -hope- you see. I'm down on my luck with tube amps: all 3 of mine are currently faulty: this AC30, my Twin Reverb languishing on the bench for a year (conductive board? needs a huge board rebuild to establish if this IS the case of a dreaded persistent HUM).. & my simple 5w Randall combo's down too.

Many thanks indeed Greg. SC

[Helmholtz]

I'm with Greg Robinson that the amp is probably fine and the heater fuse died from fatigue, maybe in conjunction with high mains voltage.

To test the tube heaters only, individual tubes can safely be tested in standby mode (or with rectifier tube pulled).