| Author | Message | ||
     Brian M. Watson (mrroundel) Username: mrroundel Registered: 06-2007 |
Well, I ended up buying a bias tool from Asharp products over Ebay. The seller said that it was made in the U.S.A., and he had a nice website, so I pulled the trigger. It arrived today and I decided to go to work. I took my time and really thought about what I was doing. First I decided to check the bias just in case it wasn't going to need any adjustment. No such luck. Readings on the 6CA7's were from 18.6, 25.2, 19.4, and 27.6ma. Since I'd just purchased a single NIB 6CA7 over Ebay, I figured I'd throw it in and check if it was closely matched to the others. No such luck, that sucker was pulling 37ma. Is this because it really is NIB, or because it might be defective? Thoughts? I put the older tube back in its socket. Anyway, I got my made-from-chopstick adjustment tool and backed out the chassis. Of course it first had to hang up on the power cord. I would have seen it sooner but I didn't have good light in there. Anyway, no harm done. Pushed 'er back in, freed the power cord, and backed it out enough to access the bias trim pot. Unfortunately, there wasn't enough lead to have the cabinet speakers plugged in at the same time. So, I came on board here to see if it was o.k. to plug an external speaker cabinet into the chassis jack. Apparently, it was, so I plugged one of my PA speakers, marked 8 ohm, into the same jack that the combo speakers were plugged into and set the ohms switch to 8 ohm. Now, did I do something wrong here? I knew not to power up the amp without a speaker load, and it had a load. Again, this was into a 12" Yamaha driver that was marked 8 ohms. I read a post here saying that this was cool. Is it? The reason I ask is that when I powered the 212 HD 130 up in the "Hi" position, let it warm up, then flipped the standby switch, it started making a static sound that was disturbing. I quickly hit the standby switch and unplugged the amp. There was no burning smell or anything like that, it just got a bit noisy, despite having all volume pots on 0. Not loud, just a bit crackly. I figured that despite taking every precaution and doing what I thought was everything right, that I'd fried a transformer or something. I was bumming out that I might have messed something up in this fine 32 year old amp, and put her back together and prayed. Once back together, making sure that the speaker ohms setting was back at 4 ohms, etc., I fired her up in Low. It crackled a bit and then got quieter. The tubes were still glowing, so I plugged my guitar in, and she came to life. I'm not sure if it's any noisier than it was before, as it has made a little static noise since I got it a couple of weeks ago. Is there anything that I did above, that could have screwed something up? The only thing that I have some question about is the speaker hookup. Any input is appreciated. Since she needs a re-cap anyway, and I'm not going to risk her, or my, life doing it, I guess I'll take her in to a professional. I don't want to be afraid to bias my own amps, so I'm glad I bought the tool, but I'm concerned that on my first foray I might have botched something. Again, any input would be appreciated. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
OK- short course. ALL tubes are going to pull different amounts of current. The fact that the NIB Sylvania pulled so much more at a certain control grid voltage means nothing other that THAT PARTICULAR TUBE pulls more current and requires more negative voltage to control the flow of electrons. Chances are tha crackel is nothing more than dirty sockets brought on bu pulling tubesd in an out. Clean the sockets with a dab of something like CAG de-oxit. Let dry and retention. Problem will probably be solved. The first quad is actually fairly close but I personally would cool it off a tad to get the HIGHEST tube below 25mA. JMHO. Mike. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Geez, I tried to edit my own crappy typing- didn't work! | ||
| Brian M. Watson (mrroundel) Username: mrroundel Registered: 06-2007 |
Thanks Mike. I appreciate your response. I'll give it another go soon. I like your thought on biasing the tube quad that I have, as they are all Sylvania 6CA7's. While I don't get much chance to let the amp breathe, I would like to know that I've got those old workhorse tubes in there when I do. I have some De-oxit. Do you really think I need to re-tension the sockets? They seem to hold pretty well. Plus, I'm not sure how to do that. I'm also thinking that it might be best to avoid spraying the De-oxit gold on the sockets, as it would seem to lubricate/loosen them. Thoughts? Thanks. Brian W. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I use deoxit whenever I re-tube any amp. I spray a little on a q tip and wipe it on the pins of the tube and then work it in and out of the sockets. Then re-tension and tighten up the grippers and go. The grippers hold the tubes in, not the contacts. I've been using deoxit for as long as I can remember. Before that, I used tuner cleaner. | ||
| Brian M. Watson (mrroundel) Username: mrroundel Registered: 06-2007 |
Hi Mike, I did as you suggested, and cleaned the pins and socket with De-oxit. I then took the chassis out and laid it on the bench and proceeded to adjust the bias using my chopstick tool. The best I could get out of the quad set was 19.6, 25.0,14.9, and 19.1. Obviously that 14.9 is way out but I didn't want to bring the strongest reading tube up past 25. The amp sounds pretty good anyway. I'm not gigging with the amp. And, it is quite possible that I'm not as picky as most ;-) However, I'm guessing that with fresh cap's, and a matched set of JJ E34L's, that puppy would be pure sweetness. I've got it set up next to my '89 Marshall Bluesbreaker Reissue, and I will say that the MM holds its own against it. In fact, for clean headroom, it beats it handily. I guess that's its specialty. BTW: While I had the chassis out, I attempted to remove the filter cap cover (carefully), but it seemed to be held in place by something and I didn't want to force it off without knowing where the tension was coming from. I just wanted to get a look at how things are laid out in there, and the general appearance of the cap's. Any info on this would be appreciated as well. | ||
| Brian M. Watson (mrroundel) Username: mrroundel Registered: 06-2007 |
In case anyone is interested, I just stumbled upon a very informative article relating to biasing, and using mismatched tubes. http://www.lynx.bc.ca/~jc/bias.html |