| Author | Message | ||
| Pat Ulanowicz (pat) Username: pat Registered: 10-2010 |
I'm working on a RP-100 with a GP-2 board. One of the original problems was one of the JE1692 transistors was physically broken. Both the base and emitter leads were broke. Somebody tried to solder-bridge them together which failed and then I tried and failed. Following this forum I tried to get a 2N6488 replacement, but, they are discontinued also because of lead content. So I got a 2N6488GOS-ND lead free replacement. I can not find the specs on a JE1692 for comparision but the rating looked close so I dropped two of these in. Now I have three issues: (1) The bias is now very touchy. It pretty much slams from 10mV to 50mV by breathing on the bias pot but it seems stable once it is set. Could this be from the new transistors having a higher beta? (2) When playing at high volume (yes, I test amps at high volume) I smell the distinct smell of hot components. Is this the new transistors? (3) At clean settings I can hear a distorted sound under the note, faintly in the back ground. Any takers on this? | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Hi Pat. The GP/GD-2 board takes the 2N6292 as a replacement for the JE1692, and this one is available from Mouser (and no doubt elsewhere). When I play either of my MM amps, at some point it will give you a nose-reminder of every stinky bar and rehearsal space I've been through. Seriously though, it's very subjective - because the amps run very cool at idle, they will not smell of anything until you start opening them up a bit. If the smell is that of burnt silicon, it's time to shut the amp down and inspect further. As for the faint buzzing: It can happen in MM amps when either the driver transistors or the output tubes (or both) are not matched closely enough. One tube goes into cutoff producing distortion. As you have discovered, it is sometimes touchy to adjust the bias point: Too hot and the amp draws waaaay too much idle current, too cold and you get cross-over distortion. When it really comes down to it, a scope and tone generator may be needed to get it right. Hope this helps -Lars Verholt www.captain-foldback.com | ||
| Pat Ulanowicz (pat) Username: pat Registered: 10-2010 |
Thanks for the reply Lars. It smells like burning semiconductor alright. I got the 2N6488's because in another thread somebody said the 2N6488's perform better than the 2N6292's and, at the end, MM's where shipping with 2N6488's. I'll try some in there anyway. Maybe that will fix #1 problem. - Pat. | ||
| Pat Ulanowicz (pat) Username: pat Registered: 10-2010 |
Well Lars, after comparing specs between the two, the 2N6488 max power is 1.8W while the 2N6292 max power is 40W - quite the difference. Doing some calulations in the RP100, the transistor is trying to disapate around 3.3W so this is probably the source of the smell. I guess we should wipe out the recommendation of the 2N6488GOS as a replacement transistor in these amps. | ||
| Pat Ulanowicz (pat) Username: pat Registered: 10-2010 |
After further review, the 1.8W rating on the 2N6488 is the un-heatsinked rating while it is actually rated for 75W with infinate heatsink. The 2N6488 should out-perform the 2N6292. Now I'm back to where I started. I'm now wondering if I'm smelling the rectifier diodes heating up? I'm still going ahead with installing 2 2N6292. | ||
| Pat Ulanowicz (pat) Username: pat Registered: 10-2010 |
I installed the two 2N6292 transistors and I don't get this burning semiconductor smell at high volume any more. The thing I do notice is that I lost some headroom volume wise. Maybe the 2N6488 drove the hell out of it, beyond design. I don't know. | ||
| Pat Ulanowicz (pat) Username: pat Registered: 10-2010 |
I convinced the owner to buy some output tubes and this cured the "hash" sound problem. Thanks fo the input. - Pat. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Kind of late to the party here but kind of curious about the tube choice. I hate to be the ahole but I could almost guess that they were Sovtek's. They always sound "brittle" to me in almost anything. ESPECIALLY the old wxt plus. If you were overdriving the drivers, they CAN get rather nasty smelling. Kind of like that smell RIGHT before the amp goes chernobyl. Friend of mine that I worked with last week had an rp that honestly, was the BEST sounding MM that I ever heard. Little 1-12 and played a strat through it. Doesn't hurt that he's a GREAT player though but DAMN, it sounds good. Think it was an rp. Maybe rd. Same amp, just dist instead of phaser. Like I said, REALLY sounded good though. | ||
| Pat Ulanowicz (pat) Username: pat Registered: 10-2010 |
It had some Sylvania 6CA7's in it originally. They were pretty old. The hash or faint distortion sound was from mismatched and/or old tubes. (thanks Lars). A friend of mine had some Tesla (pre JJ) EL34's that were packaged for Marshall, probably in the 80's, that I threw in. I've shipped the amp back to a happy owner. The remaining issue I have is the bias adjustment pot. Trying to bias this thing is a pain. It slams from too hot to too cold just by breathing on the pot. I just can not get any kind of fine adjustment out of it and this is not the only MM with the 100 ohm pot I've had this trouble with. Has anybody tried a 10 turn or maybe installing some voltage dividing resistors for this problem? Is it just me? Unlucky maybe? Is the pot messed up? - Pat. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Could be a cratchy pot. If you want, you could juggle the bias supply to give a smaller range. Tha would give you a little better control or try a munti turn pot. Either will make it easier. Or measure the pot and sub fixed resistors in it's place. A lot of thing COULD be done. Don't remember running into any of the earlier ones that were touchy but haven't worked on many rp's. |
Tue, 03/25/2014 - 09:34
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