Tube Amps / Music Electronics
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|12/27/2003 12:06 AM|
||I killed my Traynor - debug help?|
Hello, I'm a first time poster.
Yesterday, I performed a few jobs on my amp, Traynor YGM-3 Guitar Mate Reverb. The first thing was a Eminence speaker, and that was installed correctly. Then the 2 prong power cable was replaced with a 3 prong, and the polarity switch was removed. After this, I tested everything, and it worked fine.
Then I moved onto a cap job. There were 3 25uf/25V, 1 4.7uf/63V, 1 8uf/250V, 1 68uf/250v, and the 2 hanging 40uf/40uf/450V. I installed a cap canister type board because I bought 4 40uf/500V on advice from amp tech on the Fender Forum. After everything was installed, I tested the amp, and no sound. So I took out all the new caps, and put the old ones back in. Still no sound.
There is a VERY faint noise that comes out of the speaker. Messing around with any of the controls does not affect the sound. So I have come to the conclusion that I screwed something up doing the cap job.
Here are some facts that I know-
-Pilot lamp lights
-All tubes glow, maybe brighter than normal
-I tried two sets of preamp tubes, including new JJ's.
-Tremolo is bias modulation type, but turning the controls does not change any of the tube glow in the power tubes. I'm not sure if they normally would either.
This is my first amp project. I am pretty skilled in pedal building, in fact I heard about this forum from Arons stompbox forum. I haven't done any live amp measurements, mainly because I wouldn't know what to measure.
I'm hoping that this forum can help me debug my amp.
Thank you very much. Any help is appreciated.
PS. Since the Traynor YGM3 isn't really a popular amp, I think I should give a description of it. It is a 2 x el84 power section, and has 4 12ax7 preamp tubes. It has reverb, and tremolo. The controls are 'volume, treble, bass, reverb, speed, intensity.' My model is a 3 input without the bright switch.
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|12/27/2003 5:45 AM|
Hi, check out R.G. Keen's tube amp debugging page, a very nice resource:
|12/27/2003 4:11 PM|
Never fear, all is not lost! You have a great amp there. It just needs a little help.
I bet you've just missed a connection with the B+. You see, the 25/25v caps are preamp cathode bypasses. They make sure there's tons of gain. If you screwed them up or put them in backwards things would still work but the gain would be low.
Likewise with the bias cap. It's a common mistake to wire this one backwards, especially when working too late at night. This is about the only time the + terminal is grounded, since it supplies a negative bias voltage. If this was the problem you'd hear some sound and see the output tubes glow red on the plates. The bias cap usually explodes and sprays gunk around inside. I'm betting you would have noticed!
So it's probably the 40/500v's that are the problem. No B+, no sound at all. I suggest you go to the Schematic Heaven website and d/l the schematic. You'll find the Traynor stuff under the "Bargain Bin" amps. Measure the voltage at pts A and B. If you're not experienced with high plate voltages then I echo the other reply's suggestion of reading R G Keen's debugging FAQ. I usually gator the meter leads while the amp is off and then turn on the amp - but then, I'm a bit of a wuss sometimes! Get enough shocks these days reading the newspaper.
Keep at it - you'll find it!
|12/27/2003 6:48 PM|
Thanks to both of you for replying. I have gone to Geofex, and got some ideas, but nothing conclusive. It pointed to maybe a output transformer problem, but I do not think that would be an issue because it was working right up until I replaced the caps, and I always made sure to have a speaker load attached. Still a possibility though.
I started measuring some voltages, and I have a few findings:
-Power input is 124VAC, everything lights up.
-After the diodes, all I could measure were DC voltages... normal for rectification stage. Everything after this is in DC.
-All 4 of the 40uf filter caps were within normal voltages (+ or - 20% of schematic)
-The first preamp stage was within range
-The tremolo intensity pot was modulating from 225-235V
-The only "off" voltages were right where the 8uf and 64uf junction and surrounding areas are. The schematic calls for -28 following a diode, where I measured +8. That -28 is strange to me - negative voltages? I will definately need to check this area out.
I had to stop measuring because my power tubes were glowing bright red. It didn't even look like it was from the inside of the tube - it was really more of a blob of red on the sides. This is wierd because I have been using these same power tubes for the past 6 months. I'm starting to think that maybe they are receiving too much voltage from the 8uf/64uf because pins 2 of the power tubes come in contact with these caps, only being seperated by 3 resistors. This could be the problem. With that being said, shouldn't there still be sound coming through? I might be looking at multiple problems.
I can send anyone a schematic of my model. It's a little different from the Schematic Heaven ones - mine has the 3 inputs with no bright switch - but the rest of the circuitry is pretty close.
Thank you for reading.
|12/27/2003 9:22 PM|
You've got a problem in your bias circuit! The voltage has to be negative, approximately the -28V you indicate. As the bias voltage moves more positive, the tubes draw more current and hence the red plating. That cherry red glow will destroy the EL84's.
Take all the tubes out and power up the chassis. Check the bias circuit until you can locate the reason it's not producing a negative voltage.
Since you only performed a cap job, you'd have to expect the cap in the bias circuit to be reversed.
In this part of the circuit, the +ve of the electrolytic is tied to ground since the V is negative.
|12/28/2003 12:22 AM|
one thing I do for straight replacment jobs (filter cap, power transformer, output trans., etc.) is to pencil and paper out a picture with everything clearly marked so I know exactly how everything should be connected. Or if you have a digital camera, that should be useful for that.
re: the electrolytic caps, it's my understanding that they can take a certain amount of reverse voltage (20%?) and beyond that I guess they are permanently damaged(?), so you may have to use new ones. Don't take the bias cct. lightly, it's extremely important.
|12/28/2003 5:02 AM|
I took the tubes out, and those bias caps were measuring right where they should be, -30 and -18. The caps are in with plus side to ground. So I put the tubes back in, and when I flipped standby to operate, there was loud static and blue lights coming from the power tubes. I'm not sure if it was arcing, or from inside the tubes, but I shut that thing off right away. It only did this when I had my negative probe clamped to the chassis.
So I took out all the tubes, and began measuring. One odd thing was that 3 of the 4 40uf filter caps were measuring 455+, and the fourth was at 442. These should be quite a bit lower, one should be at about 220. Could I be looking at cap failure in addition to the problems I already have on hand? (If I didn't mention before, I took out all the new caps and put the originals back in.)
I was thinking tomorrow maybe I will take detailed pics and label each voltage I measure. If anyone wants to take a look, let me know and I'll send it as soon as I finish.
Thanks in advance.
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