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Old March 14th, 2010, 09:04 AM   #1 (permalink)
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Re-biasing and amp without a bias probe?

I had my Deluxe Reverb converted back to 6V6s from 5881s and returned it to it's original bias circuit. The shop it was done in didn't have a functioning bias probe. So I took the amp home and biased the amp by turning the amp on and monitoring the tubes as I adjusted the pot and using a guitar to monitor the sound. I'm guessing this isn't the recommended method, but the amp came out sound better then ever.

I was told I can go back whenever I want to get it biased with a probe when the shop gets it probe working again, but should I bother?

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Old March 14th, 2010, 01:07 PM   #2 (permalink)
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If you need to go to a shop to get the bias adjusted, then you can't adjust the bias properly without some equipment - a bias probe is one option.

You could easily have the amp modified to have the cathodes on the power tubes ground through a 1 ohm 1% resistor, and then after measuring the plate voltage on the power tubes, you can set the bias current to the proper range by setting a meter to millivolts and reading from the "top" of that resistor to ground on each tube (this is in essence what most bias tools do). You can also have that 1 0hm resistor connect to a jack on the back of the amp for each tube, then you can read the voltages through the jacks with a meter and you don't have to open up the amp to set bias. Many amps come with bias jacks like this.

If you don't know what I'm talking about, do NOT attempt to do this yourself- but any good tech could do it in a few minutes.

As to your tech, change, and don't go back. Any good tech should not need a "bias probe" to check bias, and should NOT have given you back an amp with the bias not set.

If you're going to set the bias by sound (not recommended), make sure to do it in a dark room and play the amp loud, make sure the power tube plates are not glowing red. That will destroy the tubes and some expensive parts of your amp very quickly. If they are, adjust the bias to where the power tubes are not redplating, then it's safe at least.
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Old March 14th, 2010, 07:08 PM   #3 (permalink)
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Quote:
Originally Posted by Wayne Alexander View Post
If you're going to set the bias by sound (not recommended), make sure to do it in a dark room and play the amp loud, make sure the power tube plates are not glowing red. That will destroy the tubes and some expensive parts of your amp very quickly. If they are, adjust the bias to where the power tubes are not redplating, then it's safe at least.
Essentially that is what I did. Basically ran the amp at 10 with in a room with no lights on and the blinds drawn so I could monitor the colour changes in the tubes. I then turned the pot that adjusts the bias until I saw the faintest red on the plates then I drew it back right away until I thought I liked the sound.

My one worry was while the amp sounds great and is behaving well, it might be doing unseen damage. Or that I was going to bias the tubes too hot so it reduced their lifespan significantly. I won't use it until I get to a tech if either of these were the case.
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Old March 15th, 2010, 01:33 AM   #4 (permalink)
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If it's not redplating and you like the sound, that should be safe, so it should be ok to play the amp. I'd still recommend an actual bias measurement, and setting the tubes to maybe 70% dissipation, but what current level that is depends on the plate voltage.
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Old March 15th, 2010, 03:06 AM   #5 (permalink)
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You can get the current by reading resistance of red to blue from OT and write it down. Then measure the live voltage across red to blue with amp on. Divide the voltage by the resistance and you have the current. For instance, 3v/130 ohms is .023 (23 mA). Do the same for the red to blue to see which is higher, then adjust accordingly. You can multiply the current you're shooting for x resistance and adjust for that voltage. Be sure to re-check the plate voltage after adjusting.
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Old March 15th, 2010, 09:48 AM   #6 (permalink)
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Cool, thanks for the advice. I don't have any equipement (hence using eyes and ears to bias for the moment) like meters at hand. I'll just take the amp to someone that does and get them to do that this weekend.
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