Early 60's Ampeg B12XY trouble

[Ek_Amps]

I am having some trouble debugging an old ampeg B12xy that used to be a fav. I have had the amp well over 10 years. I have pretty decent knowledge of tube amps but am no pro. I have built a bunch. Modded a few and debugged several but this one has me scratching my head.
In 2006 (before I had any skills at all) I had the amp looked at by a reputable tech because of hiss etc.. he did a cap job and the amp never sounded better. (did not change coupling caps) In 2009 I had to put the amp away for a while. It has been kept in climate controlled home environment. I got it out the other day and pop goes the fuse and the resistor on the cathode (pin 5 330 ohm 1 Watt) of the 7591 power tube for the echo circuit. The 1K 5watt dropping resistor in the power section was also hot but not smoking. Replaced the two resistors. Everything else visually looked good.
I pulled all the tubes and powered her on. Fuse held looked good. Put in the a known good 5AR4 and tried again. Good. Turn on Standby switch, tube arched and fuse popped again.
Tested OT by checking ohms on primaries and tested Good. Sockets look great see no build up.
I did notice that the capcan used when replaced was 40/40/20/20 525V and the other was 20/600v. From the schematic I have found this should have been 40/40/40/40 and 30 at 600v. I cannot source a 40404040 or a 30@ 600 but I did see 404040 and 25@800 and I thought the other 40 I can just put next to the 30@800. There is room.
Before I go and replace all the caps again. Does anyone have any ideas?
Maybe coupling cap?
Thanks for any help!

[gingertube]

Pin 5 is the 7591 cathode so the 330R is a cthode bias resistor so I would suspect a shorted tube.
Does the fuse still pop with the 7591 pulled?

Cheers,
Ian

[Ek_Amps]

The fuse pops when all but the rectifier tube is out. Even the 7591. Fine with no tubes or with the rec in standby but on with the rec and no other tubes it will pop.

[Ek_Amps]

Thanks for taking a look. Yea 5 is cathode whoops. I have attached a schematic

[gingertube]

The fuse pops when all but the rectifier tube is out. Even the 7591. Fine with no tubes or with the rec in standby but on with the rec and no other tubes it will pop.

That means either shorted high voltage capacitors or shorted HV wiring down stream of the standby switch.

Check that all caps were replaced during that earlier recap. Check all electrolytics for bulged ends or ends showing leaking "goo".


Cheers,
Ian

[Ek_Amps]

No goo. Only .022 coupling caps remain. But the caps were replaced in 06. Could the short be inside the cap can? Could a coupling cap cause this problem. Could the fact that some of the caps were replaced with 50% less caused them to go bad faster?
Thanks

[gingertube]

This seems to be the schematic even though it shows 7027 output tubes.
http://www.el34world.com/charts/Schemat ... _B12XY.pdf
Check that C48 was replaced. Then check C49, it is a multi section can type. Depending on how good the previous tech was - it was probably left in place and wires to it cut with individual new caps fitted for each section, It is possible that a new multisection can was found and fitted. It is also possible that someone got lazy and decided it was too hard and left the old C49 multisection cap in there.

It is unlikely to be a coupling cap. That might cause a red plate or similar but generally won't cause a fuse to pop.

Actually just spotted the 7591 rebverb drive.

Cheers,
Ian

[Ek_Amps]

The power tube in question is the one for the V9 for the echo circuit the 7591 not the 7027. The can is new or 06 new but replaced with a 40/40/20/20 instead of the 40/40/40/40 that should have been there. C48 was also replaced but again a lower value. Still @600V but a 20 instead of a 30. Looks like clean work. If I were to do it again I would fit a 404040 available and on 40 on its own then C48 a 25@800 is easy to find.

[Ek_Amps]

Would you recap it if not shorts seem to be present down stream of the standby switch?
I love this old goat! Finally get a chance to get it out and here we go. Gotta love it though.

[gingertube]

You can try lifting one end of the 1K which feeds C49 and see if that eliminates the fuse pop, if not, then the problem is C48.

The standby switch turns out to be a "red herring". It is in teh centre tap of teh high voltage winding - effectively on teh AC side and before the rectifier.

I recently did a full restoration of an Ampeg Echotwin 1B for a local guy.

Like you, he loved his amp and wanted it restored, properly, money secondary to doing the job RIGHT.

Cheers,
Ian

[Ek_Amps]

I will try that tomorrow. Thanks for your help thus far! Yes this is a great amp and SHOULD be restored properly.

[Ek_Amps]

1K resistor did not help but I did disconnect the Echo transformer and no fuses are blown. Does this mean that it is bad?

[Leo_Gnardo]

It's looking like the first-stage filter cap is kapoot. What was the source of the replacement power supply caps? Often you find two caps wired end to end without balancing resistors inside a cardboard sleeve to make up a 600 or 800V "dynamite stick".

If that proves to be the problem, I've replaced first stage caps in Ampeg and similar amps with Solen polypropylene film caps rated 630V. 22 uF = enough. Not terribly expensive when you balance the cost against headache time and they will far outlast electrolytic caps. I get my Solens from Antique.

Can caps by CE have unfortunately proven to be problematic. I replace can caps with 50+50 / 500V from Ruby (Magic Parts), and F&T and JJ seem to also work well. Any other cap sections left over, I put in individual caps. Sometimes this can be an advantage. In some circuits grounding the - end of the lowest-voltage end of the B+ string near the signal input part of the chassis helps cut hum.

Whatever you do, you don't want to wreck Ampeg transformers in those cans. Expensive to replace, or a hassle to repot. Let's hope we don't have to go there.

[Ek_Amps]

The first stage is a Sprague 20 @ 600V. Yea I was thinking about redoing it anyway while it is apart.
Is there a way to test this Echo OT (reverb transformer)?
Temporarily I put in a 30 @ 500V (had it on hand) to see if fuses still went and it made no diff yet I disconnect the OT for the reverb and no fuses blow.

[Ek_Amps]

Update!
Thanks to all for the help .005 C45 was the culprit. The little sucker across V9 was leaking! Replaced it with a .0047. Working as expected now. Still going to recap though.

Thanks again!