| Author | Message | ||
| Steve Daugherty (guitarer) Username: guitarer Registered: 01-2012 |
I own a 210-65 that I bought new, & used constantly until I got out of playing in '88. Recently, when JJ reissued the 6CA7, I swapped out the wimpy EL34s I'd been forced to use (after 6CA7s were discontinued in the late '70s). But when I went to set the bias, there was NO current reading, despite the normal 680+V plate voltage. There's no red-plating at all, & the amp sounds great, but something is amiss here. Bad bias pot, perhaps? Any ideas, anyone? Many thanks for any help. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Ok, first, HOW are you reading the bias CURRENT? Socket device or reading off of the cathode? Is this a SS driver version or the 12ax7 driver. If you're using a SS driver version, you have to measure VOLTAGE across the resistors, if you're using a 12ax7 version, you have to measure the VOLTAGE across the cathode resistor OR use a socket with the resistor built in OR install 1 ohm resistors BEFORE the built in cathode resistor and measure THEM for voltage. Tell us what you have first and then we will be GLAD to help. Always happy to help. Mike. | ||
| Steve Daugherty (guitarer) Username: guitarer Registered: 01-2012 |
Weber BiasRite, 12ax7 driver. A couple of weeks ago I put 6CA7s in an identical amp & it sounded like my amp (prior to EL34s), so I was inspired to do mine. After reading some general confusion on MM biasing, I set the other owner's at 10mA to be on the safe side. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
That IS kind of curious. You sure the Bias Rite is functioning correctly? I know they had some problems with them and that was why they discontinued them. You might try to put a 1 ohm 1 wat resistor BEFORE the factory resistor and read the voltage across THAT. It will give you a baseline to check your bias rite. Also, you can read the voltage across the factory 10 ohm resistor and do the "set it a 1/2 volt" thing but that's not very accurate. | ||
| Steve Daugherty (guitarer) Username: guitarer Registered: 01-2012 |
I've checked the Biasrite often & use it regularly on a variety of amps, it's reading fine - I set the bias on the other 210-65 with it. When I turn the bias pot on my amp, I can hear a very slight thump (for lack of a better word) at one spot, but the reading doesn't change - that's why I suspected it could be a burnt-out pot. Like I said, the amp has had heavy use; I played it 6-7 nights a week, 5-6 hrs. a night for 12 years. I'd be extremely lucky if a worn out pot is all it is! | ||
| Steve Daugherty (guitarer) Username: guitarer Registered: 01-2012 |
Oh, I meant to mention that the bias pot is VERY loose-feeling (compared to the tremolo pot, & it's very light blue (from the heat, that's probably normal). Frankly, I'd like to just put a fixed resistor to safely mate with 6CA7s & be rid of the bias pot. I'm not a hot-bias fan, especially in an amp that just doesn't do distortion. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
You COULD do that if you want. I have. I'd probably just put a new pot in and be done with it. | ||
| Steve Daugherty (guitarer) Username: guitarer Registered: 01-2012 |
Yeah, I'll just change the pot first & take it from there. Do you know it's value off the top of your head? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Should be 10k. What I like to use is the cermet multi turn pots. Gives you a better adjustment pattern. Like these: http://www.circuitspecialists.com/rectan gular-multiturn-cermet-sealed-trimpots.h tml The pins probably won't line up but just solder some little wires to one end to go through the board and epoxy the pot to the board. I've glued them to pots on Twins and works great. Mike. | ||
| Steve Daugherty (guitarer) Username: guitarer Registered: 01-2012 |
Thanks, Mike, I've done similar retrofits before, a multi-turn pot is great for fine-tuning. I'm pretty confident that it is the pot, as I fiddled with it a bit last night, & found that if I leaned on it with the screwdriver in different directions, it'd jump up to 1-3mA for a second. |
Tue, 03/25/2014 - 09:48
#1