| Author | Message | ||
     ab3640au |
I have some experience in building tube amps but have not worked on MM amps before. A friend had an old battered up HD130 combo (2x12's), which sounded like cr*p (distortion at low volumes). The tubes (EL34's and 12ax7 PI tube) looked like the original tubes (Sylvania). I've replaced all tubes and rebiased the amp .. I've also re-tolexed the cabinet as well (looks like a new amp!). It now sounds MUCH better, however I'm not 100% sure that it's as loud as it should be. For example, with Master Vol on 8 and Volume on 3 (in my living room), it's about as loud as you'd want it to be (without having the neighbors bash my door down) .. certainly not piercing, but about the limit for a living room .. nice & clean warm tone. Does this sound right for a 130watt amp? With the Volume on about 8 and Master on 2-3, it's also fairly loud but quite overdriven. Sorry if I seem to be asking silly questions, but I'm not familiar with these amps and want to be sure. Thanks Barry | ||
| Anonymous |
Check the plate voltage (hi). Should be > 700V. If <, it probably needs to be re-caped. | ||
| michael kaus |
For gawds sakes, I hope you didn't bias like a marshall amp with el's. Most people don't take into account that it has 700v on the plates and bias it to around 35-40mA of idle current. This is NOT GOOD. 23mA is plenty and should allow it to live. I'm also not a fan of that 1/2 volt at point Y crap, as it only give you a total of 50mA of current draw for BOTH tubes. you could have one hot tube and melt your monkey. Some of those also had different rates of preamp power. I have a 65 that sounds flat out on 4! Like anon said also, caps are getting old in that amp. | ||
| ab3640au |
Thanks for the feedback guys .. I had the opportunity to crank the amp today and it sounded real good! Very quiet (no hum, noise at all!) .. but more importantly .. real good tone! Sorry Michael .. I did bias the amp at 1/2 volt at point Y .. but she sounds ok .. REGARDS Barry M | ||
| michael kaus |
I don't want to sound like a crusty old fart(ok, maybe I am!), it's just that with the current tech involved in tube production, the tubes are so far apart that proper matching and bias control is very inportant in these amps. If you have one tube pulling 30mA and one pulling 20mA, you have a total of 50mA or your 1/2v on the resistor. This will be followed by a zip, pop,melt, off, sound! Tubes today suck and will not take 30mA's of cathode current at 700v.It's just really important to verify that your HOTTEST tube is no higher than 25mA(really,21-23 is plenty). That will give you the best chance of long life and transformer health. Mike. | ||
| vcamargo99 |
michael kaus, I also biased my HD130 with the Y-Point method. How else do you do it? I would like to try your method. I have NO complaint with my amp and I just installed new EH6CA7's and rebiased the amp. Keep the info coming!! | ||
| michael kaus |
Well, the easiest method is to get some kind of bias adapter. It goes in the socket of your power tube and the socket plugs into it. The two leads coming out go to your digital meter and read either mA's or mV's, depending on which one you have. Then you can get a true reading of cathode current(which also has a little plate current built in too-I use that for a safety margin). The other easy way is to install 1 ohm resistors on each socket from pin 8 to ground-or in this case, to the input of that 10 ohm resistor. This allows you to look at the cathode current/voltage PER TUBE. This is a little more tube safe than just getting a total current per pair. Mike. | ||
| vcamargo99 |
I've been looking into a WEBER BIAS RITE. That would let me check each tube. But after I determine that a reading is too far off, do I change a resitor on the tube socket? Which one? Am I getting in too deep? Thanks! | ||
| michael kaus |
If you have the 12ax7a PI tube, you have a bias pot on the main circuit board. Adjust the pot to the correct bias. The other pot there is trem adjust. |