I have a Pignose G40V tube amp, and a couple of months ago, the power transformer failed. The primary coil was open. I had overstressed and overheated the amp, and it smelled real bad. It was my fault. I bought a new transformer from Pignose, and installed it. The amp worked fine for about a month, and then failed in the same way. The only difference is that this time the amp was not stressed, and the transformer doesn't smell. Pignose sent me another transformer for free, since it was still under warranty. The first one they sent me could have been bad, but i'm afraid that maybe there's something else wrong with the amp, and if I put it in new transformer it'll just fail again.
Does anybody have any ideas on what else could be wrong? Should I just put the new one in and hope? About a year ago one of the 6L6 output tubes was arcing inside, but stopped after a few hours and I never replaced it, so maybe that's a clue.
Thanks for the help!
Yeah, if you have a power tube or tubes drawing too much current, the tranny could fry. If the tube(s) were drawing hugely excessive current, the fuse would blow, but there's a gray zone where the fuse doesn't blow but the transformer's overloaded. I think it's a pretty marginal tranny. Also, the one I have was set up for Japanese 100V house current, I believe. The heater voltage was 7.5V on my 120-125V current. That in itself can cause 6L6s to overheat the cathodes enough to make the signal grids heat up and start emitting electrons, which makes the bias more positive, increasing the plate current to excessive levels. If there's a little bit of air in a power tube, the gas molecules ionize, the positive ions are attracted to the grid and make it go more positive, the plate current rises, the plate gets hotter and more gas molecules are cooked off of the surface of the plate, they get ionized...
Change the power tubes.
Might also want to check the DC voltage on the 6L6 grids with the amp running. This amp has a truly fixed bias (fixed bias, with no adjustment.) If something's happened to the bias circuit and it's not putting out the specified negative voltage, it could do this even with good power tubes.
Steve Ahola has a schematic for this amp over on the http://blueguitar.org/ site somewhere. When you find it, note that it's basically identical to the Marshall JCM 800 2204 "Super Lead Fifty." But built with the cheapest possible components.
:D
"A cheerful heart is good medicine."
I'd definitely recommend taking the amp to a good tech and have him modify the negative-bias circuit to include a bias-adjustment trimmer potentiometer if no such adjustment facility exists, so that proper power tube bias can be set to prevent future problems. This shouldn't be excessively expensive to have done, and will extend the life of your power tubes and the amp as a whole.
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P.S. You might also ask him about replacing the stock power transformer with a higher-quality unit. I Googled "Pignose G40V power transformer" and came up with a bunch of hits about the stock transformer having problems.
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Good Luck!
Strat
Hmmm makes sense. I'm not really sure that I want to put out the money to put in a better transformer, since I already have money into another stock transformer. It's not my only amp either, but I really do like the tone from it.
As far as the heater circuts on the tubes drawing too much current, wouldn't that cause the secondary coil for the heaters to fry, as opposed to the primary?
At any rate i'm going to buy a new set of 6L6's. I follow how gas in the tubes could cause them to screw up, but Im not really sure that I follow everything about biasing. I can remember this amp with these power tubes making funny noises at times. You would have to whack the amp to get them to stop. I guess it would be beneficial to go over all of the connections on the chasis too...
The problem with running the heaters too hot isn't that too much current flows through the heaters. It's that the control grids, which are close to the cathodes, get too hot. They emit electrons, and that makes them less positive. That increases the flow of plate current. The power tubes are underbiased, and underbiased power tubes can easily draw enough current to fry the primary. Ideally the power fuse would blow first, but that's not always the case.
The situation I described with gas ionization causing runaway plate current is a tube defect. It's a common mode of death for power tubes. It often causes amp damage in the process. But running the cathodes too hot is an amp defect, which my G40V came with. I installed two 5W 1.8 ohm resistors in the heater leads to drop the voltage to 6.0V. 1.5 ohm resistors would've put it right on the nominal 6.3V. Slightly lower heater voltage is fine and extends tube life. Running them hotter shortens tube life, and the old tube manuals carried warnings to never run a 6L6 above 7.0V on the heater for the reason I mentioned.
"A cheerful heart is good medicine."
Man Ric, If I understood music theory as well as you do electronics, I'd be playing the jazz thing all day long. :shock:
If Pignose is shipping Japanese 100v transformer-equipped G40Vs to the US, then Pignose should fix the amp with swapping for the US transformer or supply a new amp...that's their shipping screwup. Not that I really expect that unless they get a hefty lawsuit.
Strat
Yeah, I figured it was easier just to put resistors in and drop the voltage. Actually, what I suspected was that it wasn't a screwup, that they'd decided to cheap out by using one transformer that would work on Japanese power and figured it'd be OK on US voltage, which it's not.
It also had a speaker that couldn't handle the power and was damaged the first time I cranked it all the way. (I replaced it with a 70W 10" Jensen MOD, which sounds terrific, is louder, and can handle the power.)
Another design flaw it has is unfortunately common: It uses Pin 1 of the 6L6 sockets as a tie point between B+ voltage and the screen grid resistors. That was specifically warned against in the Radiotron Designers Handbook, 4th Edition, which any amp designer ought to be familiar with, in 1956. The problem is, Pin 1 isn't connected to anything in the glass 6L6 types like the 6L6GC, but in the original 6L6 tube, which has a metal shell, the shell is connected to Pin 1 so it can be grounded and serve as a shield. So if it's used as a screen tie point, and someone not unreasonably decides to see how the old metal 6L6s he stumbled across sound in his amp, the exposed power tubes in the back of his amp now have +420V on their shells. A serious shock hazard. (Besides, if Pin 1 were grounded, the only other mod necessary to convert it to use EL34s if desired would be adjustable bias as Stratman_el84 recommended.)
"A cheerful heart is good medicine."
OK, i'm definately going to buy new output tubes.
Should I install the resistors in series coming off of one of the leads from the power transformer?
Luckily, I don't have that speaker problem. I'm guessing my amp is older than yours, because they used to put in a much heftier speaker.
No, you have to install the resistors (if yours needs it, measure the heater voltage first) with half of the needed resistance in each leg of the heater circuit coming off the transformer, to keep the AC heater voltage balanced around ground potential. Otherwise you'll increase hum (and the Pignose is a pretty bad hummer already.) Acceptable heater voltage would be in the 5.7-6.5V range. (The book says 7.0V is the never-exceed value.) Measure it with tubes in place and warmed up. My Pignose was bought in 2001, I think. The speaker was allegedly rated for 70W "peak sound power," which usually means they just doubled the real rating of the speaker. However, the amp is really a typical 60W dual 6L6 design that Pignose called a "conservatively rated 40W." (Marshall called their version of the circuit 55W.)
"A cheerful heart is good medicine."
Another solution would be to use a variac set to the appropriate voltage. You can find some here: http://www.action-electronics.com/variac.htm
Cheers! Happy Thanksgiving!
Strat
Yeah, but buying an expensive Variac for a cheap amp is something I hate to do. It's easy to splice in resistors.
The Variac is a useful piece of equipment to have around the shop, though.
"A cheerful heart is good medicine."
Ok, i'm in the process of tracking down new power tubes, and when I get them i'll put them, along with the new transformer, in the amp. Then i'll measure the voltage to the power tube heaters, and ask you what value of resistors to put in. Unfortunately this will have to wait until the end of the semester at school, the 3rd week of December.
That'll be here before you know it.
I just got my Piggie back. I'd lent it out to my kids. I like it a lot, despite its quirks and deficiencies.
"A cheerful heart is good medicine."
Yeah, but buying an expensive Variac for a cheap amp is something I hate to do. It's easy to splice in resistors.
The Variac is a useful piece of equipment to have around the shop, though.
Agreed, but the variac I was thinking of at the link I posted was only $45 + shipping, so I thought it worth mentioning as, like you said, it's handy to have around anyway. I bought one of the 500VA slightly more expensive models ($89) for my bench and it's surprisingly high-quality. You can even order replacement wiper brushes for them there also. Very inexpensive compared to the standard Staco, etc models.
Cheers!
Strat
