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Posted: Thu Jul 29, 2010 7:53 pm
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Thanks Shimmilou,

Great explanations, and solid information, this kind of stuff makes the forum a better place.

I was astounded to see that in the poll you ran for your previous and very informative article someone actually voted "not helpful", when in fact the forum becomes much more valuable to most of us when you take the time to add those type of posts.

Thanks again,

Cheers,

Snowy


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Posted: Thu Jul 29, 2010 8:12 pm
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Hey Shimmy, Thanks for the bias post! This next point prolly has gone around the board a few times before, but I was wondering what your idea is on FIXED BIASING with no adjustment versus with a pot? Reason I ask is I've had several ppls amps die on them when the BIAS adjustment pot went bad. I've also owned several Princeton Reverb with set BIAS points and they've been very reliable amps.

Anyhow, I know this is the crazy Randall Smith from Mesa-Boogie, but he does have a few interesting points. Thanks, for any reply:

http://www.mesaboogie.com/US/Smith/biasadjust.html


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Posted: Thu Jul 29, 2010 8:21 pm
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Yeah, very good input from everyone!!! For the sake of dicussion; I have never seen a temp rating for tubes on the spec sheets. How can we measure this and determine what is acceptable ? Also, getting back to the cold side, what kind of operating conditions are we seeing when the tube/s are operating well below optimum bias ? Art

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Posted: Thu Jul 29, 2010 11:40 pm
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Hi BMW2002Ti,

Yes, I agree with Randall about the fixed bias design of his Mesa Boogie amps to a certain degree. If the bias is set properly for one type of tube, and you only use that exact same tube type for replacement, then the bias would never need to be adjusted. Mesa has a "Robo Tube" tester to make sure that all of the tubes that they use are very consistent in meeting their standards before they will label them. Of course that fixed bias means that you are limited to only using their brand of tubes. ;) Fender does something similar in some of their amps, in that they recommend only replacing the stock Groove Tubes with only Groove Tubes of the same label color so that the bias will be not have to be adjusted, theoretically. But often times the bias is set improperly on some Fender amps to begin with....so a trim-pot would come in handy.

I understand that the adjustable bias trim-pot that is used on many fender amps is definitely a little weak to say the least. A plastic adjuster that is located near the hot power supply components can fail easily. But the idea of adjustable bias isn't a bad idea just because poor quality components are used for the design. I have a high quality 25-turn trim pot ordered for my blues junior so that I can adjust the bias in the amp very precisely and not have to worry about the pot melting. I wouldn't use the plastic type pot for any adjustment in an amp.

Hi aclempoppi,

Measuring the temperature of the tubes can be done easily enough with a temperature probe attached to your voltmeter. You would just have to measure the temperature when the tube has been properly biased to get an idea of the normal operating temperature range. Some people have learned what the normal operating temperatures are from experience and can tell just by putting their hand close to the tube whether it's too hot or not. Proper biasing only means that the temperature of the tube is cooler on average, the tube will still increase in temperature and be hotter when playing, but be cooler when idling resulting in less heat overall. If the tube is biased too cool (temperature), that means that the circuit that feeds the bias voltage to the tube is operating at a higher voltage, using more current, producing more heat, and working harder.

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Posted: Fri Jul 30, 2010 3:42 am
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Thanks Shimmy! That 25-turn pot is a good idea. You have any brand that you prefer? Do you use UF400(X) diode in the bias power supply?


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Posted: Fri Jul 30, 2010 8:07 am
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You know it's funny that you mentioned people putting their hand close to the tubes to feel how hot they are...I have done that for years after playing for awhile and still often look at the tubes to make sure there's no cherry red plates...it's an old habit.
This doesn't mean that my hands and eyes are able to tell accurate measurements though.... :wink:


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Posted: Fri Jul 30, 2010 10:52 am
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BMW2002Ti wrote:
Thanks Shimmy! That 25-turn pot is a good idea. You have any brand that you prefer? Do you use UF400(X) diode in the bias power supply?


Make it easy on yourself, order a kit with instructions. I wanted to change some caps while I was installing the adjustable trimpot. I also found a standby switch that I could use to replace the stock power switch. Billm Audio offers just what I needed in a couple of kits. I ordered the "Basic Mods kit", which includes some new caps and a bias trim pot with instructions and a drill bit. I also ordered the "Standby Switch kit" which has the wires pre-made so that you can simply plug in the new switch. There are other kits available from other sites if you do a search, but I prefer the Billm mods. I spent $47 total for the two kits including shipping. There is a long wait for the parts from Bill, maybe 4 weeks, but I think that it is worth the wait. You can order your own parts, but you are only going to save a couple of dollars and you won't have any instructions.

http://billmaudio.com/wp/?page_id=65

If you want to order your own parts Mouser is a good place. For the trimpot, you will be replacing a fixed resistor (R31?) with an adjustable trim pot. Probably a 50K, 1/4 watt, trim pot would work just fine. Here's a link to one trimpot that would work well, although this one is a 1/2 watt. Obviously a 1/2 watt will be better, but 1/4 watt will work, as the resistor it replaces is 1/4-watt. You could also simply change the 22K resistor to a 27K and your bias would be very close to the proper setting.

http://www.mouser.com/ProductDetail/Murata/PV36W503C01B00/?qs=sGAEpiMZZMukHu%252bjC5l7YfLgFzO12zoetcpJm%252bP4RvU%3d

Not sure if the UF400(X) diodes are necessary, but it couldn't hurt. It won't affect the tone, but the better the quality is of the parts that you use, the better quality the amp will be. Usually you see the ultra-fast recovery diodes in high frequency switching power supplies. The BJr doesn't have a switching supply, so...
:)

.

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Posted: Fri Jul 30, 2010 7:13 pm
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Thanks Lou, I'd forgotten about those probes. Keep up the good work ! Art

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Posted: Fri Jul 30, 2010 8:30 pm
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Rebelsoul wrote:
Hey shimmilou,great explaination.
Have you had a chance to write some things on the subject of capacitors and other components,and their functions in the amp like we had talked about a few weeks ago?
no hurry....


Hi Rebelsoul,

I have the capacitor post up on my site now. If you want to know a little about capacitors, click the "www" button at the bottom of any of my posts on this forum to go to the site. On the site, click the "Tech Stuff" link at the top of the page. I have posted an edited version of the first post, "Consider this before changing amplifier tubes", and below that is the one about capacitors, just scroll down to the title, "Capacitors in an Instrument amplifier". I will also enter this post as a new topic in the "Modern Guitar Amplifiers (1985 - Current)" section of the Fender forum, so that others will know that it's there now too.

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