Can a choke get too hot and cause a filter cap to fail?

[jukelemon]

Hi all.

About a month ago, my 1954 Gibson GA-77 failed in the middle of a gig. It would blow a fuse then after going from standby to ON.

The amp still has the original cap can for the 2x20ufd between the choke and still has the original rectifier so I suspected a failed can or rectifier given what it was doing and the age of that section and that I removed the power tubes and still got a blown fuse.

Put the cap can though my cap tester and sure enough, one side showed a failure/short.

So, bought a new cap can and wired it up (same exact way as far as the choke leads). Worked like a charm through 3 shows (3-4 hours a piece).

Playing on Beale St this past weekend and the amp does the same thing as before. Exactly. Amp died/blows fuse.  New fuse. Standby to On blows the new fuse.

Luckily I had a back up.  Got home and the new cap can failed on the SAME side i.e. the same choke lead as the other original can that failed/the one hooked to B+

I tested the entire cap can and only that side had failed (B+ side).

The cap can I bought (due to size and not wanting to remove the original can mount) came with 3 x 22 ufd.  So, I hooked up the non used one and it was back to normal again.

Prior to doing the above, I removed the B+ connection from the rectifier just to see if anything funky was going on with the PT.  Nothing unusual.

Any ideas? Logically, I know it is possible for a new can to die...but it seems unlikely.

Thanks

[eleventeen]

Sounds to me like you may have a choke on the verge of failing, which is what I thought your first thought was. Usually, once a choke beings on that path it just gets worse until it becomes a stinky, burnt shorted tranny.


It's not clear from your description: did you replace the filters caps TWICE? Did you replace your replacements?

[HotBluePlates]

The amp still has the original cap can for the 2x20ufd between the choke and still has the original rectifier so I suspected a failed can or rectifier ...

... the new cap can failed on the SAME side i.e. the same choke lead as the other original can that failed/the one hooked to B+

I tested the entire cap can and only that side had failed (B+ side). ...

"B+ side" is unclear (to me at least).

I assume the 2 cap sections are on either side of the choke, one to the rectifier and the other to the output transformer center tap. Is this correct, and do you mean the section connected to the rectifier failed?

If so, sounds like a rectifier tube which is intermittently shorting and passing a.c. to the first filter cap. The choke should protect the second filter cap from damage, at least somewhat. Sounds like you need a new rectifier and/or to add solid-state diodes between the PT winding and the rectifier tube's plates to pre-rectify the incoming voltage.

[jukelemon]

The amp still has the original cap can for the 2x20ufd between the choke and still has the original rectifier so I suspected a failed can or rectifier ...

... the new cap can failed on the SAME side i.e. the same choke lead as the other original can that failed/the one hooked to B+

I tested the entire cap can and only that side had failed (B+ side). ...

"B+ side" is unclear (to me at least).

I assume the 2 cap sections are on either side of the choke, one to the rectifier and the other to the output transformer center tap. Is this correct, and do you mean the section connected to the rectifier failed?

If so, sounds like a rectifier tube which is intermittently shorting and passing a.c. to the first filter cap. The choke should protect the second filter cap from damage, at least somewhat. Sounds like you need a new rectifier and/or to add solid-state diodes between the PT winding and the rectifier tube's plates to pre-rectify the incoming voltage.

Sorry ..

By B+ I meant the rectifier side of the choke/1st power supply cap. That is the side that has failed 2 times now-once on the original can and now on the new can.

The rectifier is a very old 5v4 so it could very well be that. When I had it tested, it was "fine" but of course it could be failing after long use.

[jukelemon]

Sounds to me like you may have a choke on the verge of failing, which is what I thought your first thought was. Usually, once a choke beings on that path it just gets worse until it becomes a stinky, burnt shorted tranny.


It's not clear from your description: did you replace the filters caps TWICE? Did you replace your replacements?

The original can was a 2x20uf. One side failed. It was original. I replaced it with a same size (diameter) CE can. The only CE can that had anything close (diameter and farrads) was a 3x20uf.

Thsi last time, the rectifier side of the can failed again. So, I used the unused (3rd cap in the can) as a replacement.

Hope that is more clear.

Thanks all for the insights and help.

[DummyLoad]

did you replace the original rectifier? or did you assume the cap was bad and rectifier survived and re-used it?


--pete

[Willabe]

When you get this fixed I'd like to ask you some questions about this amp as I have built one. 

[eleventeen]

Some failure modes of indirectly heated GZ34s and 5V4s can do very weird things. If one old 5V4 saw both of these ecaps fail...I would have to suspect the tube. at some point. HBP's idea of the SS diodes in front of the tube is excellent.

[jukelemon]

did you replace the original rectifier? or did you assume the cap was bad and rectifier survived and re-used it?


--pete

When the failure first happened, I removed the 6l6s and 5v4 and sarted troubleshooting the power supply.

I started with a new fuse and then the old rectifier. That = immediate popped fuse from Standby to On.

I then sub'd in a 5r4 I had lying around to see if I would get a similar result. Same thing.  So...at that time I assumed something down the line AND since the cap can was original, I assumed a failed cap.  Sure enough, I found the shorted cap on one side of the can.

So, at that time I didnt think the 5v4 was the issue.

Replaced can, put in old 5v4, and the amp was stable for ~15 hours or so.

My gigs in Memphis had the amp on for ~5 hours at a time and at constant playing. We do a gig/jam thing so someone is/was always on the amp.

[jukelemon]

When you get this fixed I'd like to ask you some questions about this amp as I have built one. 

Sure thing.

This is likely my favorite large combo Gibson of all time. I have owned/played most of the 50's units as of this point. I have sold off all of them (including the fabled GA-40) minus my GA-6, GA-77 and GA-86 because I have found those 3 to be the best of all of them.

The GA-77 has a monitor out (not sure if you built one in on your clone) and I use that to drive a second amp (usually the GA-6) for some additonal bite.

[jukelemon]

Some failure modes of indirectly heated GZ34s and 5V4s can do very weird things. If one old 5V4 saw both of these ecaps fail...I would have to suspect the tube. at some point. HBP's idea of the SS diodes in front of the tube is excellent.

Yeah, it is starting to look like that.

I have an extra 5v4ga so I can use that from this point on and see what happens.

[Willabe]

Which GA-77?

Gibson made 4 maybe 5 different GA-77's and 2 or 3 GA-70's (Country & Western) amps, that were the same amp. 

Some had octal preamp tubes, some had 9 pin pre tubes, some had a presence control and some had a 2nd single knob tone control on 1 channel like on a Fender tweed Deluxe.

Problem is that Gibson a some point had a flood in their basement where they kept their schematics and lost many of them. So there are Gibby's out there that there's no schemo for.   

And many Gibby schemos have errors on them.       

I built this 1; http://el34world.com/charts/Schematics/files/gibson/Gibson_ga77_vanguard.pdf

[Willabe]

Also, have you ever owned or played through a GA-55V and a GA-83S? (Vox copied the 83S vibrato for their AC15 and AC30. Vox's top boost tone circuit was lifted from the GA-77.)

If so did you try their vibrato? They both have true pitch shift vibrato. 

[jukelemon]

Which GA-77?

Gibson made 4 maybe 5 different GA-77's and 2 or 3 GA-70's (Country & Western) amps, that were the same amp. 

Some had octal preamp tubes, some had 9 pin pre tubes, some had a presence control and some had a 2nd single knob tone control on 1 channel like on a Fender tweed Deluxe.

Problem is that Gibson a some point had a flood in their basement where they kept their schematics and lost many of them. So there are Gibby's out there that there's no schemo for.   

And many Gibby schemos have errors on them.       

I built this 1; http://el34world.com/charts/Schematics/files/gibson/Gibson_ga77_vanguard.pdf

Hi there. Mine is the 1954 i.e. the same circuit that Vox supposedly took the Top Boost from.  It has an additional octal in the circuit and is known as the "7" in my Gibson circles.  2 cannels - one with a Tone knob and with a Bass/Treble stack.  I have a copy of not only the original circuit but also the Owners Manual that would have come with it.  It is spot on to what is in the amp.

Pretty interesting...the Monitor out was designed for crystal headphones and recording applications.  Pretty cool thinking for 1954. The preamp passes signal into the Monitor out and in StandBy. My GA-85 does the same.

As far as the GA-55V, it is an exceptional amp. I just dont like 2x12 configurations in general. And the GA-83 sounds exceptional as well...just weak and underpowered in my mind unless you are dedicating it to a recording studio.

I have found that the 50's Gibsons all sound good...just that some can actually be gigged withough a Mic and some cannot.

I would not pay the prices on the GA-83s given their lack of volume.

Honestly, I dont think I have heard a better amp than the GA-86. I may have the only one in existence. I dont know for sure. But I have never seen another one and the pic of the one on line is mine lol. The internal wiring is a joke to work on. Double sided PTP boards that can only be removed by removing a bunch of other stuff. It is another 5v4/6l6 circuit with 2 channels and 9 pin pre's.  I am not sure why it sounds so good but it sounds to me better than any amp I have ever played. It is a perfect combination of Fender clean and Gibson dark/woody tones. Not sure else how to describe it.  It lies somewhere on the spectrum between a BF Vibrolux and a Low Power Tweed Twin. I know...try to wrap your head around that combo lol.  The issue is that the GA-86 is in an odd EH-150/180 kind of set up so it is not light and larger than most amps. But you can open/close the cabinet to get all sorts of cool tones. I am really glad I have it and if you want to build something, THAT would be a very cool project.

Thanks all for the suggestions. I have another 5v4 in the amp.  Hopefully things will stay stable!

[HotBluePlates]

When the failure first happened, I removed the 6l6s and 5v4 and sarted troubleshooting the power supply. I started with a new fuse and then the old rectifier. That = immediate popped fuse from Standby to On.

I then sub'd in a 5r4 I had lying around to see if I would get a similar result. Same thing.  ... since the cap can was original, I assumed a failed cap.  Sure enough, I found the shorted cap on one side of the can.
...
Replaced can, put in old 5v4, and the amp was stable for ~15 hours or so. ...

Here's what I think happened in the above sequence of events:

1. Intermittent shorted-5V4 -> passing a.c. to 1st filter cap -> 1st filter cap damaged/fails as short-circuit -> fuse blows.
2. Replaced 5V4 with 5R4 -> shorted cap still in-circuit -> fuse blows at power up.
3. Replaced filter caps -> re-used 5V4 -> time elapses until next intermittent short
4. Shorted 5V4 -> a.c. to 1st filter cap -> new cap pops again.

The moral of the story is swapping parts until the amp works may not be a lasting repair. You have to think about what happened and why. The most obviously damaged part (shorted cap) may be only a symptom of the real problem (shorted rectifier).

[uki]

Some pictures of the amp would be really cool !!   

[jukelemon]

When the failure first happened, I removed the 6l6s and 5v4 and sarted troubleshooting the power supply. I started with a new fuse and then the old rectifier. That = immediate popped fuse from Standby to On.

I then sub'd in a 5r4 I had lying around to see if I would get a similar result. Same thing.  ... since the cap can was original, I assumed a failed cap.  Sure enough, I found the shorted cap on one side of the can.
...
Replaced can, put in old 5v4, and the amp was stable for ~15 hours or so. ...

Here's what I think happened in the above sequence of events:

1. Intermittent shorted-5V4 -> passing a.c. to 1st filter cap -> 1st filter cap damaged/fails as short-circuit -> fuse blows.
2. Replaced 5V4 with 5R4 -> shorted cap still in-circuit -> fuse blows at power up.
3. Replaced filter caps -> re-used 5V4 -> time elapses until next intermittent short
4. Shorted 5V4 -> a.c. to 1st filter cap -> new cap pops again.

The moral of the story is swapping parts until the amp works may not be a lasting repair. You have to think about what happened and why. The most obviously damaged part (shorted cap) may be only a symptom of the real problem (shorted rectifier).

Yes, I follow and it makes sense.

In the past, when a rectifier has shorted it was not intermittent.  It just simply failed. That is why I took the steps I did. I am hoping this spare lasts/resolves the issue.

Thanks for the help

[Willabe]

It has an additional octal in the circuit and is known as the "7" in my Gibson circles.  2 cannels - one with a Tone knob and with a Bass/Treble stack.  I have a copy of not only the original circuit but also the Owners Manual that would have come with it.  It is spot on to what is in the amp.

Would you be kind enough to post any and all Gibby schemos you have that are not all ready in Doug's tube amp library?

What Gibson circles, where? Do they have more Gibby schemos that this forum doesn't have?

Mine is the 1954 i.e. the same circuit that Vox supposedly took the Top Boost from.

OK, interesting, yes Vox lifted the Gibby TS from the GA-77 for their top boost circuit. BUT, some say the Gibson schemo drawing had a flaw in the TS and that Vox copied it flaw and all, others say there was not a flaw in the Gibby schemo and that Gibson wired/shipped them as in the Gibby schemo.

It is spot on to what is in the amp.

So your GA-77's TS   ~ IS  ~  wired exactly as in the Gibby schemo I posted a link too, just like in the Vox AC30 schemo?
 

Pretty interesting...the Monitor out was designed for crystal headphones and recording applications.  Pretty cool thinking for 1954. The preamp passes signal into the Monitor out and in StandBy. My GA-85 does the same.

I saw that on the GA-85 schemo, I have not seen/heard about that on a GA-77/GA-70 before.

As far as the GA-55V, it is an exceptional amp.

Did you play through the vibrato on that amp? (Did you ever play through the vibrato on the GA-83S?)

Do you know if the vibrato circuit in the GA-55V, as drawn in the Gibson schemo is correct? Link below;

http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-55V_true_pitch_shift_vibrato.gif
 

[jukelemon]

Hi.

The schematic link you posted is for a later 50's early 60's Vanguard (Tweed) circuit. I looked at Doug's library and what he shows for the 53/54 Gibson GA-77 is actually for a 54 (late) not for a 53 (which didnt exist at least according to Gibson and certainly not an early 54).

My schematic is for 1954 (1st year/model) as I understand it and it only lasted several months and then went to a 3 x 12aXX pre amp. My model has 2 x 12ax7s and 2 octals.

I have heard the same thing about the Vox's but honestly I dont play Vox's and never really compared the circuits.  What I do know is that what I have for the schematic for this 1954 is exactly what is in the amp minus the upgraded power supply caps that someone changed out many moons ago.

I have attached the GA-77 info for the benefit of the group. 

As far as the Vibrato questions, I dont really recall anything other than thinking they sounded good. Sorry, it was a while ago.

There is a FB Gibson amp forum with a lot of knowledge base. That is what I was referring to in my post.

There are a few guys that have recreated the tolexes, logos, and have essentially focused on Gibsons as opposed to any other amps.  Small group but know Gibsons very well.

It looks like Doug has a lot of the Gibson schematics.  The one I posted is what most dont have or if they do they dont tell since it is supposedly the fabled GA-77 with 7 tubes.  I dont know...that seems a little silly so I attached it. Many of the guys on the Gibson site have drawn out the schematics after a restoration occurred. There is one guy in particular that loves 55's and has restored a few. He would know the answer to your question immediately.

[Willabe]

I have attached the GA-77 info for the benefit of the group.

My schematic is for 1954 (1st year/model) as I understand it and it only lasted several months and then went to a 3 x 12aXX pre amp. My model has 2 x 12ax7s and 2 octals. The one I posted is what most dont have or if they do they dont tell since it is supposedly the fabled GA-77 with 7 tubes.  I dont know...that seems a little silly so I attached it.

THANK YOU!         Yeah, I think it's silly too.

The schematic link you posted is for a later 50's early 60's Vanguard (Tweed) circuit. I looked at Doug's library and what he shows for the 53/54 Gibson GA-77 is actually for a 54 (late) not for a 53 (which didn't exist at least according to Gibson and certainly not an early 54).

I posted the link to the Vanguard version because it's easier to read. The other GA-77 in Doug's library is the same circuit, just harder to read and has a few drawing errors. On/Off/Standby and presence circuit are wrong, I forget any others.   

I have heard the same thing about the Vox's but honestly I dont play Vox's and never really compared the circuits.  What I do know is that what I have for the schematic for this 1954 is exactly what is in the amp minus the upgraded power supply caps that someone changed out many moons ago.

Your GA-77 has completely different tone control circuits that in the later GA-77's. 

As far as the Vibrato questions, I dont really recall anything other than thinking they sounded good. Sorry, it was a while ago.

Ok, thanks, I am very interested in how the Gibby vibratos sound.

There is a FB Gibson amp forum with a lot of knowledge base. That is what I was referring to in my post.

Many of the guys on the Gibson site have drawn out the schematics after a restoration occurred.

Thanks, I'll do a search for them. 

There are a few guys that have recreated the tolexes, logos, and have essentially focused on Gibsons as opposed to any other amps.  Small group but know Gibsons very well.

There is one guy in particular that loves 55's and has restored a few. He would know the answer to your question immediately.

Is it the guy who owned/built the Buffalo amps? I forget his name, nice guy. I found him on face book after he had closed down and was talking with him some. He told me about the GA-55V and was looking for the schemo he had for it, but had to go. When I tried to contact him again after the Christmas holidays (2014) I guess he had un-friended me?     

Someone told me there's a copy of the GA-55V in the Jack Darr 'Electric Guitar Amplifier Handbook'. I have a copy and sure enough it's in there.

[jukelemon]

Not sure if it is the same guy who did Buffalo Amps. I think it is though.

As far as the Gibson trems, I don't like them as much as say a Fender Brown Pro or even a BF roach type.  They never seem to have the right amount of balls behind the effect and it is always suffers at performance levels. Not sure why. Never took the time to think about it/look at it.

For my Gibsons I run through a 1961 Fender tank and a Boss TR-3. Does the job for shows.

Glad you found the 55v schem

[Willabe]

As far as the Gibson trems, I don't like them as much as say a Fender Brown Pro or even a BF roach type. They never seem to have the right amount of balls behind the effect and it is always suffers at performance levels.

I've not heard the Gibby vibratos in person or on youtube. But I did hear a clip on youtube of a guy playin through an AC15, which was lifted from the GA-83S, and it sounded GREAT to me. Sluckey did an AC15 build last year with the Vox/Gibby vibrato and his sound clip sounded GREAT to me also.   

For my Gibsons I run through a 1961 Fender tank and a Boss TR-3. Does the job for shows.

Sounds like a killer rig! 

Glad you found the 55v schem.

Me too! It's now in Doug's schemo library. One of these days I'm going to put the vibrato circuit on my bread
board and see how she sounds.

You wouldn't know the OT primary Z from your GA-77 would you?   

[jukelemon]

I dont know the OT primary specs.  Sorry.

[Willabe]

Ok, took a shot, thanks. 

[uki]

Some pictures of the amp would be really cool !!   

QFE !

Could we have some nice pics of that marvelous amp, pretty please !! 

I picture say more than a thousand words !! 

[DummyLoad]

as a rule but more for peace of mind really, if the main filter fails then i replace the rectifiers - even if they test good. 


--pete

[DummyLoad]

Some pictures of the amp would be really cool !!   

QFE !

Could we have some nice pics of that marvelous amp, pretty please !! 

I picture say more than a thousand words !! 

+ 1


share the gear pr0n!~


--pete

[jukelemon]

as a rule but more for peace of mind really, if the main filter fails then i replace the rectifiers - even if they test good. 


--pete

I follow.  However, I was less concerned with the rectifier given the age of the original cap can.  You know?

I will post up some pics when I can figure out how to resize them.

[uki]

I will post up some pics when I can figure out how to resize them.

Gimp is an awesome program to manipulate images !! Free and open source. www.gimp.org

[Paul1453]

I will post up some pics when I can figure out how to resize them.

Gimp is an awesome program to manipulate images !! Free and open source. www.gimp.org

+1 for Gimp

That is what I use, but it has a lot of features and took me a little while to figure out how to simply reduce the size of the image and save it as a .jpg