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Post subject: Do 1984 concert amps need to have bias set after new tubes
Posted: Wed Mar 16, 2011 12:22 pm
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My 1984 Concert amp sounds great, but other players are telling me I need to adust my bias after replacing tubes. I replaced them about 1 year ago, and play on average 1 hour per day. Normally, I never get above volume level 4. So, not really pushing the amp hard. I purchased it new in 1984, and have changed the tubes a few times since then. Each time I notice a BIG difference. Mostly brighter. Like I said, I'm happy with the sound. However, if it can get better, I would be interested. Does anyone have a procedure to adjust bias. I tried the web, and got a lot of conflicting details. If I take it to a tech, I'm afraid they might screw up the sound and I would not know what they did to reverse the process.

Note: I use matched grove tubes. I thought this was all that was required for a tube change.


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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Wed Mar 16, 2011 12:59 pm
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Yeah, best to reset idle bias everytime you place different power tubes into the amp. Notice it's idle bias. That means the setting is done with the amp at idle (no input, no volume, speakers connected). So, even if you don't really crank the be-jeebers outta the amp, the idle bias settings still has an effect on the tube's tone and life expectancy.

If you feel uneasy about twisting pots & high voltage... may want to take the amp to a good tech.

Just 2¢ worth. May the amp have a long life & prosper. :mrgreen:


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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Wed Mar 16, 2011 1:21 pm
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BMW2002Ti wrote:
If you feel uneasy about twisting pots & high voltage... may want to take the amp to a good tech.


It's critical for tube life that the idle bias be checked (and adjusted if necessary) each time the power tubes are changed. Which is likely why you've had to change out tubes so often since the amp was new. Quality glass, properly biased, should last longer than merely "a few years".

HTH

Arjay

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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Thu Mar 17, 2011 11:10 am
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I'm not opposed to twisting a few pots, and High Voltage does not scare me. However, I have no idea what pots to twist. Guess, I really instructions. Does fender have a published procedure. I did a search and didn't see anything. Is there a book I can buy, or service manual. If someone can point me in the right direction, I would really appreciate it. Thanks.


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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Thu Mar 17, 2011 12:24 pm
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I've never had a Rivera-era Concert Amp cross my bench but here's the schematic......

http://web.archive.org/web/200610200957 ... _ii_83.pdf

From the looks of it, the Concert doesn't have a true adjustable bias -- it's an "output tubes matching" system, more akin to that found in the original AB165 Bassman.

Best of luck, HTH

Arjay

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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Thu Mar 17, 2011 12:29 pm
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apgarcw wrote:
I'm not opposed to twisting a few pots, and High Voltage does not scare me. However, I have no idea what pots to twist. Guess, I really instructions. Does fender have a published procedure. I did a search and didn't see anything. Is there a book I can buy, or service manual. If someone can point me in the right direction, I would really appreciate it. Thanks.

I know what you're saying,and you probably have a respect for it,but.... high voltage should scare you,ever see what it can do to a person?
I've worked with thousands of volts a/c,and the d/c inside an amp is not as high but deadly nonetheless,you have to be very careful.
The smell of burning flesh is sickening.


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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Thu Mar 17, 2011 1:23 pm
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Rebelsoul wrote:
The smell of burning flesh is sickening.


Doubly so when napalm is used as an accelerant.

:shock:

Arjay

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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Thu Mar 17, 2011 2:35 pm
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Retroverbial wrote:
Rebelsoul wrote:
The smell of burning flesh is sickening.


Doubly so when napalm is used as an accelerant.

:shock:

Arjay


I thought there was nothing like the smell of Napalm in the morning :?: :lol:

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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Thu Mar 17, 2011 3:42 pm
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There isn't.

Trust me.

Arjay

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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Sat Mar 19, 2011 5:06 am
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Retroverbial: Thanks for the schematic. If I understand the schematic correctly, I remove the 6L6s and put a probe between pin 5 and ground for each tube. Then set the meter for miliamps and turn the output tube matching pot until both 6L6s have the same miliamps. Is that the general idea.

Im not the best with schematics, but it looks like I can also set the max current at or near the output tube matching pot. If yes, what current should I be shooting for.

Thanks for your help.


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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Sun Mar 20, 2011 5:21 am
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What I do is measure the current flow, at idle, of the power tube. So you need the tubes to be installed into the sockets. The easiest method, for push-pull, fixed biased, & Class AB1 amps (like most Fenders), is to measure the voltage drop across a fixed known resistor tied from ground (chassis) to the cathode of each 6L6GC. Either across a precise 1-ohm, 1/2 watt or 10-ohm, 1/2 watt resistor, which replaces the grounding strap.

By Ohm's Law, for each mV of VDC drop across a 1-ohm resistor = 1 mA of current flow. Or say 40mV measured = 40mA of current flow. Which is about the basic ballpark for most Fender amps using 6L6GC power tubes, with 450-475VDC applied to the anodes.

Another quick & simple way is to use a tool such as the Bias King. You plug your power tube into the tool's socket & this is then plugged into the amp. Whne you turn on the amp, the meter reads the mV across a resistor which is built into the kit/tool.

I hope this helps, somewhat. Kinda late in a weekend graveyard shift, so brain is a bit fuzzy.

http://biasking.com/index.html


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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Sun Mar 20, 2011 10:03 am
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apgarcw wrote:
...If I understand the schematic correctly, I remove the 6L6s and put a probe between pin 5 and ground for each tube. Then set the meter for miliamps and turn the output tube matching pot until both 6L6s have the same miliamps. Is that the general idea.

Im not the best with schematics, but it looks like I can also set the max current at or near the output tube matching pot. If yes, what current should I be shooting for.

Thanks for your help.


Output tubes must be installed, and you will have to measure the idle current (I) through the Plate of each tube to know if they are matched or not. The reading at pin 5 will be a voltage reading, NOT mA, but measuring that voltage won't do any good as you still have to measure the current going through the Plate of each output tube to check the matching.

The Concert amp has no Cathode resistor to measure current, so using a bias probe might be the safest/easiest way, but I have another method which is accurate and only requires a good multimeter. With the amp off and the caps discharged, measure the resistance (R) of each half of the output transformer (OT) primary and record the ohms readings. Red wire to blue wire is one half used for V8, red wire to brown wire is the other half used for V9.

Then turn on the amp and let it warm up for ten minutes or so. Then measure the DC volts (V) across each half of the OT primary, in the same places that you measured the resistance before, looking for approx 3 volts or so (more or less). Record the volts reading for each half of the OT primary, red wire to blue wire is the half used for V8, and red wire to brown wire is the half for V9.

Using the ohms and volts readings, you can calculate the idle current and wattage for each tube using ohms law. Volts divided by ohms equals amps (V/R = I). The resistance of the OT primary won't change, but the voltage across it will change while adjusting the matching pot. So adjust the pot, remeasure the voltages, and recalculate the current on each half after adjustment, to achieve a balance/match between the two tubes.

If your OT primary has the same resistance (or very close) for both halves, then you only need to adjust the pot to get the same voltage across each half. But if the OT primary resistances are not the same for each half, you want to go by the current calculations for the matching, not the voltage readings. For example, if you measured 80 ohms resistance (R), and 3 volts (V) from red wire to blue wire, that would be V/R = I, or 3/80 = .0375 amps (I). That would be about 37 mA of idle current for V8. The procedure is the same for V9.

Then, measure the Plate DC voltage at pin 3 of each tube and record the volts (approx 470 volts on each tube). Multiply the Plate voltage by the calculated current to get the idle wattage of each tube. For example, if you have 37 mA and 470 volts, that would be 470 x .037 = 17.6 watts, which would be roughly 60% of a 30 watt tube. :idea:

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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Sun Mar 20, 2011 5:29 pm
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Post subject: Re: Do 1984 concert amps need to have bias set after new tub
Posted: Mon Mar 21, 2011 8:01 am
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shimmilou: Thanks for the procedure. I hope the resistance on both sides of my OT is about the same. I won't have a chance to do the job until next week, but I will post an answer after it is done. Thanks again.


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