Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: zakman1111 on February 18, 2014, 07:33:39 pm
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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!
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> the resistor on the grid (pin 5 330 ohm 1 Watt) of the 7591 power tube for the echo circuit
Only (likely) way for that to burn is if *that* 7591 has developed an internal short.
Pull that tube and explore all non-reverb sounds. Does the amp hold up?
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That is what I would suspect but with only the rec tube installed the fuse pops. The pin 5 330R is actually the Cathode bias resistor I said grid earlier. Thanks for taking a look. I have tested to the best of my knowledge the transformers and they all seem fine. My first thought was an internally shorted cap can but with it being so new I just don't believe it. I did try before I opened her up new power tubes including the 7591 and the rec.... more smoke. Did not pop the fuse till I took all tubes out but the rec....it would have eventually anyways. I have attached a schematic. Thanks for any help!
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Don't replace any part if not sure it is bad .
Remove all tubes .
Disconnect one by one power supply feed wire ; B, C, D, wire to output tubes , wire to Output Transformer .
I write ONE BY ONE .
If short still there , disconnect filter cap one by one , starting with the last one .
If short is gone when you disconnect a wire , follow this path .
Why don't you use the current limitter 100 watts light bulb to save fuses and work safely ?
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Here's a link to the issues that I had with mine:
http://www.el34world.com/Forum/index.php?topic=14736.0 (http://www.el34world.com/Forum/index.php?topic=14736.0)
Not sure if that will be of any help. But, note that mine was not wired as any schematic that I found. Including one that was sent to me from the new owners. I'm posting my last revision of the drawing for you to have as a reference, just in case yours is like mine.
Jack
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Section B seems to be the culprit. I disconnected the Echo transformer and no blown fuses. (have a line limit at home but I am not there, have a bunch of 3A fuses though) Does this mean the echo transformer is bad?
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Not necessarily. Measure resistance from V9 pin 3 to ground. Should be high. If not check C45.
C45 shorted or very leaky would also explain V9's cathode resistor (330Ω/1W) burning up.
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Section B seems to be the culprit. I disconnected the Echo transformer and no blown fuses. (have a line limit at home but I am not there, have a bunch of 3A fuses though) Does this mean the echo transformer is bad?
Disconnect it you'll see . :laugh:
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Not necessarily. Measure resistance from V9 pin 3 to ground. Should be high. If not check C45.
C45 shorted or very leaky would also explain V9's cathode resistor (330Ω/1W) burning up.
Yes I think we found it!!!! Replaced C45 with a .0047 I had on the shelf and no blow. I need to put everything else back the way it was before I really test it . I did some rigging with the caps and temp wired the reverb driver transformer in. I will post when I know for sure.So far Sluckey you are the man!
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Thanks to everyone for your help!!!
Sluckey nailed it. C45 was the culprit.
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Thanks to everyone for your help!!!
Sluckey nailed it. C45 was the culprit.
Stratele52 did not help you too ?
"Disconnect one by one power supply feed wire ; B, C, D, wire to output tubes , wire to Output Transformer .
I write ONE BY ONE ."
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My bad buddy!! Stratele helped narrow down the section! You guys are awesome :worthy1:
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My bad buddy!! Stratele helped narrow down the section! You guys are awesome :worthy1:
Thank's :occasion14: