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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Thu Aug 29, 2013 6:16 am
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Joined: Mon Jun 17, 2013 12:59 pm
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Hi.

So I finally got around to doing this yesterday. Here are the pics (notes follow):

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- Get the amp to warm up for a while. Don't put it into standby. Turn it straight off. This will drain the voltage from the power section. After 30 seconds or so, take the black output from the multimeter to ground (chassis) and clip it on. With one hand touch the red output to the end of a power section capacitor (or all of them one by one) and read the voltage making sure it is very low. If so, great, you can touch anything you want (CHECK THIS and read up on it, I am not liable for your self-harm!)

- Take out the chassis. I took out the PCB. So either take off the chicken knobs, nuts and washers first, or when the chassis is out if you too want to remove the PCB. It's just the four screws to remove the chassis if you want to go that route only

- Careful of the ribbons when taking out the PCB

- Drill a whole in the chassis for a pot (if you want to do it like I did). A metal drill bit on hammer mode was even proving tough so I used a brick bit. I screwed in the pot first to make it easier to solder

- Wire from one R87 hole to the middle and right lug of the pot (if looking from the back of pot towards lugs)

- Wire i.e. solder the left lug to the other R87 hole via a resistor in series i.e. lug to wire to resistor to PCB

- I used a 25K pot and 10K resistor and my bias adjust is 30mA to 60mA - perfect!

What would I do differently?

- Go straight to the brick drill bit the chassis is tough and messing around with a small metal bit and building up the size is a pain and time consuming!

- [for my job] use a thinner wire to the left lug. It's a great solder connection to the resistor but it's thick gage and I don't like the look of it even though I cut some away with scissors too! It was all I had left for the job though

It looks like the chicken head knob will be moved around easily, but with the back on it's out of the way - even when grabbing leads etc if you store them there. I have marked where 43mA is with a sharpie just to check pre-turning on.

(I used the Weber bias calculator online, plate voltage 454V (measured with the amp-head dual bias tester)).

EDIT/ Since doing this job I have noticed the effect of biasing on the sound of the amp. And it's great to be able to react by just turning the pot. Actually, I have made it run slightly colder! /

THANKS FOR THE INFO I HAD FROM USERS HERE - out of appreciation I am posting this to help someone else if i can..


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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Thu Aug 29, 2013 9:52 am
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Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
Thanks for sharing your info sidepartings26, and great job, especially for a first time. It looks pretty good and sounds like you got a great range of current adjustment. 8)

Not to criticize, but offer a couple of suggestions. Get rid of the thinner black wire connected to the red wire at the board and to the pot (so that you only have the red wire to the board and the black wire to the resistor on the board), and instead, use a jumper wire at the pot itself, from the red wire terminal to the outside terminal where the thinner black wire is (was) connected. Also, remove the chicken head knob from the bias pot, as it would be very easy to bump the pot and throw your bias out without knowing until it's too late. I agree about the look of the big wire, although no one will see it when the back panel is on, and it is a solid connection. You could use the thinner black wire, after removing it, to replace the thicker black wire.

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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