HT fuse blows in SVT 3 Pro

[shughey]

Visual inspection is not going to be helpful in this case.

Would your next step be to look at those electrolytics out-of-circuit? Just trying to get an idea of how to go about this methodically and appreciate and am interested in your advice. So far I have tube plate VDC readings that fall in spec per the table in the schematic.

 

[FunkHead]

I had a svt pro 4 for a while back in 2000s. It had similar problems. Had it repaired by qualified Ampeg service several times, never was really fixed. Had another electronic amp guy look at it and he declined getting in there because of the supposed safety risks. I sold it for parts and never looked back. It may not be worth fixing at all, those Ampegs are prone with failures.

You simply got screwed over by 2 under qualified amp techs.

 

[agedhorse]

Provided the plate supply does not rise above the voltage of the electrolytic, I would see if the HV fault goes away by removing the preamp tubes… BUT there are a few models that depend on the preamp tube heaters as the dropping elements for the fan circuit so if this is the case, substitute a known good set of tubes for the existing ones.

Does the failure occur at turn-on or while the amp is running?

 

[shughey]

Provided the plate supply does not rise above the voltage of the electrolytic, I would see if the HV fault goes away by removing the preamp tubes… BUT there are a few models that depend on the preamp tube heaters as the dropping elements for the fan circuit so if this is the case, substitute a known good set of tubes for the existing ones.

Does the failure occur at turn-on or while the amp is running?

I will try that next. I have a bunch of 12AX7s to try.

I've only been able to replicate the issue during power on. That's not to say it won't happen while running because I haven't logged enough hours into it in its current state to see if it'll blow while running.

 

[landrybass]

You simply got screwed over by 2 under qualified amp techs.

Yeah maybe so. It worked out though because that 75 pound sled was no fun. I was happy to get rid of it.

 

[FunkHead]

Yeah maybe so. It worked out though because that 75 pound sled was no fun. I was happy to get rid of it.

I was thinking about what I posted and I am not trying to insult the Authorized Ampeg service center. I apologize to anyone that may have taken offense to that. But I know from experience that not all Techs are created equal. It's just like Auto dealer Technicians(Which I was for 10 years~). Some Techs are great at diagnosing problems and others are parts changers. I was admittedly somewhere in the middle.

As far as the second Tech that wouldn't even touch your amp, He should have been honest and said it was over his head(No pun intended). I have no doubt that the "Right" Tech could have properly fixed your SVT4 Pro.

About weight: I am actually trying to buy a Peavey Fire-bass 700 that is probably even heavier than your Pro 4 was. Sure it's a bear to move but I would probably leave it in one rehearsal space or another. It's kind of sad to think about the older heavier Amps that have basically been discarded...

 

[Old Garage-Bander]

I was thinking about what I posted and I am not trying to insult the Authorized Ampeg service center. I apologize to anyone that may have taken offense to that. But I know from experience that not all Techs are created equal. It's just like Auto dealer Technicians(Which I was for 10 years~). Some Techs are great at diagnosing problems and others are parts changers. I was admittedly somewhere in the middle.

As far as the second Tech that wouldn't even touch your amp, He should have been honest and said it was over his head(No pun intended). I have no doubt that the "Right" Tech could have properly fixed your SVT4 Pro.

About weight: I am actually trying to buy a Peavey Fire-bass 700 that is probably even heavier than your Pro 4 was. Sure it's a bear to move but I would probably leave it in one rehearsal space or another. It's kind of sad to think about the older heavier Amps that have basically been discarded...

Maybe that's part of the problem. An "Authorized" Service Center that can't repair equipment they claimed to be authorized to repair, should be called out and subject to insults. Even the best techs get it wrong now and then, but will make good on it. But to have a place like this not be able to repair an item is cause to call their authorization into question. But then I don't know what "authorized" really means in this instance. Is it just that they can buy parts from the manufacturer at a discounted rate?
If it's my money, why should I just let anyone roll over me like that without raising questions? I would have at least brought up the issue with the manufacturer. Maybe they need to rethink who they allow to call themselves authorized.

 

[Wasnex]

Are you sure this is the pro version?

Apparently not

Try this one (and hopefully I caught everything). Orange is the fuse. Pink is HT, Red is B+, Purple is modified HT, and Green is the control signal for the modified HT.

 

[shughey]

Provided the plate supply does not rise above the voltage of the electrolytic, I would see if the HV fault goes away by removing the preamp tubes… BUT there are a few models that depend on the preamp tube heaters as the dropping elements for the fan circuit so if this is the case, substitute a known good set of tubes for the existing ones.

Does the failure occur at turn-on or while the amp is running?

I swapped in another known good tube into the 12AX7 I had in the power amp. So far the fuse hasn't blown, but I need to give it time to be sure. What I'm curious about is the tube on the right (the tube that may have blown the fuse) in the photo above has a black ring under the getter. Most of the getter has the typical silver appearance, but underneath that is a black ring that I haven't seen on my other 12AX7s. I checked continuity on the pins and couldn't find anything out of the ordinary there. Is that black ring indicative of something (I know sometimes the getter flash can be black in appearance, but it's clearly silver with black below it.)

 

[mike buzzetti]

View attachment 4793334

I swapped in another known good tube into the 12AX7 I had in the power amp. So far the fuse hasn't blown, but I need to give it time to be sure. What I'm curious about is the tube on the right (the tube that may have blown the fuse) in the photo above has a black ring under the getter. Most of the getter has the typical silver appearance, but underneath that is a black ring that I haven't seen on my other 12AX7s. I checked continuity on the pins and couldn't find anything out of the ordinary there. Is that black ring indicative of something (I know sometimes the getter flash can be black in appearance, but it's clearly silver with black below it.)

that tube on the right looks burnt to me , they look like they are both JJ tubes ? , if so I think you MAY have found your issue , or maybe not ?? time will tell .

 

[shughey]

that tube on the right looks burnt to me , they look like they are both JJ tubes ? , if so I think you MAY have found your issue , or maybe not ?? time will tell .

yes, both JJ ECC83s. I have about five others (new) that look like the one on the left.

 

[mike buzzetti]

yes, both JJ ECC83s. I have about five others (new) that look like the one on the left.

every time I have seen tubes like that they either sounded like crap or were a problem in some way. Only going from personal experience .

 

[agedhorse]

It’s possible that there was some internal contamination that consumed the getter material.

This MAY be the cause, sometimes contamination can cause the tube to runaway into saturation (like a short). This is especially possible on cathode followers, which means that on occasion the cathode resistor can also be damaged.

 

[crazyBassClown]

But to have a place like this not be able to repair an item is cause to call their authorization into question. But then I don't know what "authorized" really means in this instance. Is it just that they can buy parts from the manufacturer at a discounted rate?

It means that they can do warranty work. Which may consist of nothing more than swapping out entire suspect board assemblies with replacements from the manufacturer. If these become no longer available after expiration of warranty then the authorised service centre may no longer be able to fix them.
Why would a manufacturer do that? Because skilled service people are expensive and difficult to find, and it's just easier/cheaper to provide new boards than find people all over the world skilled enough to troubleshoot and component fix for their warranty claims.