Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on June 21, 2014, 12:39:39 am
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I have an Ampeg VT-22 on the bench right now. I have replaced the electrolytic caps as the original cap cans
had black goop seeping from the tops. Turns out this is adhesive used to keep the cardboard cap covers attached.
I noticed that two of the cap cans are wired in series. That means one of the outside shells is HOT. You can see in my pic labelled "before", this is how the cap cans were wired before I replaced them. See the white jumper wire connecting + and - of the cap cans together?
The original mallory cap cans had cardboard covers. I suppose I can tape up the HOT cap can shell and re-install the cardboard cover but is it acceptable to leave the cap can HOT?
I wanted to keep to the originality of the Ampeg and not install individual caps. I went with CE Dist cap cans and they fit perfectly.
http://el34world.com/charts/Schematics/files/ampeg/AmpegVT22_V4_RevA.pdf (http://el34world.com/charts/Schematics/files/ampeg/AmpegVT22_V4_RevA.pdf)
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One of those shells is hot all the time. Both may be hot when the STBY switch is open! Better insulate both for safety. BTW, that goop on top of the original cans did not leak out of the can. That's the adhesive used to secure the original cardboard insulator shells.
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Yes, I found this out the hard way on a vintage PA amp. Fortunately it was only 300VDC. It was quite stimulating, but I wouldn't recommend it. (Foolishly, I didn't expect a shock from the outside of the amp, so my other hand could have been anywhere - I could just as well be dead.) Someone on this Forum suggested coating the cap can in Plasti-Dip, which worked quite well. For a cap can already installed, I masked the area around the can and "painted" it with liquid insulator. The Plasti-Dip coating is much prettier.
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I don't know how readily available large heat shrink is, other than mail order. We have some huge stuff at the plant.
That would be a safe fix, and you could cap the cans before slipping the heat shrink over it.
Jack
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The original Mallory cap cans had cardboard covers. I suppose I can tape up the HOT cap can shell and re-install the cardboard cover but is it acceptable to leave the cap can HOT?
Didn't you mean to ask "Is it acceptable to leave the hot cap can uninsulated?" The cap can is going to be hot per design. I wouldn't leave the hot can uninsulated. The Sovtek Mig60 has a similar setup. Stacking power supply with an B+ of 640VDC (hot upper can +) and a screen supply of 350VDC (lower normal can +). In the stock amp they use these really sturdy black plastic tubes (see the cap to far right in photo). When I recapped I couldn't find the same values in the same physical size so I installed axial caps in the chassis. It worked great but this was my amp - not for someone else. Kudos on keeping it original. Keep it really original - put that sleeve back on.
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Yes that's precisely what I was asking but in my head it sounded clear :l2:
I will definitely make sure all the cap cans are insulated.
I was just surprised they were hot. I think it'd be safer to use axial caps instead. That's what I'd do in a build.
Truth be told…I got zapped by them while powering up after the cap job. I never made it off standby. I powered down and decided to leave amps till today. It scared the sh*t outta me. Stupid thing was I knew it was going to be hot but I brushed up against the cap without insulation. Lesson learned.
I also learned about black goop. :icon_biggrin:
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I will definitely make sure all the cap cans are insulated.
I was just surprised they were hot. I think it'd be safer to use axial caps instead. That's what I'd do in a build.
Hewlett-Packard used exactly the same "hot can cap" setup in some of its test gear. The lower cap gets bolted right to the chassis, and the upper cap is attached via a phenolic insulator and has the cardboard sleeve.
I'm guessing axial electrolytics at that time (50's to early 60's) weren't as compact as now. I can tell you the test gear I saw already had large chassis, which were pretty full as-is. There may not be the room for 8 or 12 axial electrolytics under the hood.
I mention it so you know this isn't some odd occurrence with the Ampeg; it seems to have been standard practice.
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Hot cap cans are very common in just about every type of power supply that use's a - and + voltage .
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Well you know what they say when you assume!
I'll pay much more attention to cap cans from here on. I always assumed the shells would be grounded.
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What is the diameter of the cap can? Seems to me you might be able to find a coin shop in your neighborhood and buy a tube used to hold a stack of US silver dollars. They are cheap, about 50 cents each. There are two types: the transparent styrene type, and a nylon type which I would prefer for this use. The clear styrene type is kind of fragile. A US silver dollar has a diameter of 1.5" if that helps any. They might not be long enough, might have to glue two together, or, glue together one tube and the shorter cap piece that comes with it. Other cheapo suggestion: Length of PVC pipe, with a cap. Also cheap. And yes, ugly.
I myself am not that happy with heat shrink nor spray-anything when it comes to 300+ volts.
Of course there are several other sizes, for half dollars, Eagles, etc; Search for "specifications half dollar" (for example) to get the diameter of a half dollar in the event 1.5" is too big.
http://www.jpscorner.com/coin-tubes.html?gclid=CMaKh9OAjr8CFRAV7AodQBoABA (http://www.jpscorner.com/coin-tubes.html?gclid=CMaKh9OAjr8CFRAV7AodQBoABA)
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I always disliked how Ampeg used odd-parts in the VT-40 (VT-22 is similar).
Not just the hot-cans (which was modestly common in the day), but odd combinations of dual caps.
Fender got on a better foundation with the doghouses and strip-board.
I looked long and hard at doing an internal cap-board in "my" VT. Really a better plan.
However I was also de-rating from 590V to 400V, the original caps were working fine, so I let it be.
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What is the diameter of the cap can? Seems to me you might be able to find a coin shop in your neighborhood and buy a tube used to hold a stack of US silver dollars. They are cheap, about 50 cents each. There are two types: the transparent styrene type, and a nylon type which I would prefer for this use. The clear styrene type is kind of fragile. A US silver dollar has a diameter of 1.5" if that helps any. They might not be long enough, might have to glue two together, or, glue together one tube and the shorter cap piece that comes with it. Other cheapo suggestion: Length of PVC pipe, with a cap. Also cheap. And yes, ugly.
I myself am not that happy with heat shrink nor spray-anything when it comes to 300+ volts.
Of course there are several other sizes, for half dollars, Eagles, etc; Search for "specifications half dollar" (for example) to get the diameter of a half dollar in the event 1.5" is too big.
http://www.jpscorner.com/coin-tubes.html?gclid=CMaKh9OAjr8CFRAV7AodQBoABA (http://www.jpscorner.com/coin-tubes.html?gclid=CMaKh9OAjr8CFRAV7AodQBoABA)
I will re-use the original mallory cardboard insulators.
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Work has been completed on this amp. Time to test now.
I am seeing no bias current on my bias tool.
The amp is working and sounds fine.
Plate, screen voltages ~530 VDC
Grid is -61 VDC
I have checked the bias with 4 bias tools and every one of them is not measuring idle bias current.
Power tubes are JJ 7027A's.
When I monitor the bias current I see expected current values from 2mA to 140mA while playing guitar through the amp.
Why am I not seeing the idle bias current on my meter with this amp? What am I missing here?
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Thinking my meter is dead or something I measured by the transformer shunt method.
With the JJ 7027a's I get 12mA
Installed winged C 6L6GC's and I get 48mA.
Looks like i need to replace the quad.
The troubleshooting continues...
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With the JJ 7027a's I get 12mA
Installed winged C 6L6GC's and I get 48mA.
Looks like i need to replace the quad.
Was that with -61Vdc bias for both sets?
If so maybe that's too much - bias for the JJ's?
Brad :think1:
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I measure -61 VDC on each pin 5 of the power tube sockets. This is in line with the schematic.
I'm getting confused. :help:
I have the 7027A's installed right now. Using the transformer shunt method I am getting 10mA per set of tubes(per side of the transformer)
When I install my 6L6's and measure same method I get 50mA per set.
This leads me to think that the 7027's need replacing….
Maybe, maybe not. If I play guitar through the 7027's it sounds perfectly normal. In fact it sounds LOUDER than the 6L6's.
:w2:
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If I play guitar through the 7027's it sounds perfectly normal. In fact it sounds LOUDER than the 6L6's.
That's odd, 7027's at 10mA and 6L6GC's at 50mA's, and the 7027's sound louder?
I'd be inclined to lower the - bias with the 7027's to up the current and see how they sound then.
Brad :think1:
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If I play guitar through the 7027's it sounds perfectly normal. In fact it sounds LOUDER than the 6L6's.
That's odd, 7027's at 10mA and 6L6GC's at 50mA's, and the 7027's sound louder?
I'd be inclined to lower the - bias with the 7027's to up the current and see how they sound then.
Brad :think1:
They sound the same for volume. I went back and forth a couple times just to be sure.
Still confused why the difference in bias current between the tubes.
I think it's time for a new quad of 7027A's.
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Alright…I have things in working order now with the 7027A's.
I must have been having a huge brain fart.
I adjusted the value of the bias resistor from stock 75k down to 47k.
This has the the idle bias current now resting at 41mA per tube. Plate voltage is 520VDC. This gives me about 21W per tube. I'm happy with that.
We all have our good and bad days eh? :icon_biggrin: