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Post subject: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 5:36 am
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How I "fine tune" a Deluxe Reverb 1966 AB 763 by reducing the B + voltage.

The sound of the amp was very "trebly" and a bit annoying.
The bias was 30 ma on each tube with 435 volts at plates. Far too high for RCA 6V6 GT NOS.
Colder bias will raise the voltage , not good for 6V6

Power transformer are made for wall voltage of 117 volts while in my country (Canada and same in USA I guest) is around 125 volts.

To make a long story short, after checking all the components of the amplifier I decided to lower the voltage B + with zener diodes.
With four 10 volts 5 watts zener I got the voltage of 415 volts plate voltage suggested in the Fender schematic.
The bias has been adjusted around 60% confirmed by listening test

The amplifier has completely changed his tone , warmer, rounder with more headroom while crystalin. In my opinion a real Fender.

The zeners mounted on terminal strip are bolt on a transformer existing bolt , easy to return to original wiring

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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 7:06 am
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Interesting. Are those zener diodes hooked to the secondary (AC) line of the PT? You have no overheating issues?


Thanks!


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 7:23 am
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BMW2002Ti wrote:
Interesting. Are those zener diodes hooked to the secondary (AC) line of the PT? You have no overheating issues?


Thanks!


Yes AC side . As picture show , zeners are wired on the B+ negative side wire , yellow red wire wich normaly goes to the chassis ground .

No heat with 5 watts 10 volts diodes. At 10 volts they are good to 500 MA

Some store on the web sell 30 to 50 volts zener wich are big with heatsink and expensive . Mine cost $0.75 each

I often use Zeners since long time on home made amps built from scratch. Power transformer are sometimes too powerfull for my desing.

Found the idea from The Ultimate Tone volume 2 , page 2-46


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 7:36 am
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Very neatly executed.

Rawk on!

8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 9:42 am
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Interesting concept... It looks like four diodes, all forward biased to ground.
What is the fifth diode hooked to?
It seems to me that in this AC circuit, the zeners would be forward biased for 1/2 the AC wave and reversed biased for the other half. This would regulate only half the power supply down to a lower voltage.
I think I would have hooked up the zeners between the output of the rectifier and the first filter caps. But then again, I probably wouldn't have tried reducing treble by lowering B+. That's another interesting concept.


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 9:56 am
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TimsAudio wrote:
But then again, I probably wouldn't have tried reducing treble by lowering B+. That's another interesting concept.


The objective was to decrease the plate voltage to V7 and V8 by 20VDC to save some wear and tear on the tubes. The sonic improvement is a byproduct of that modification.

Arjay

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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 12:02 pm
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TimsAudio wrote:
Interesting concept... It looks like four diodes, all forward biased to ground.
What is the fifth diode hooked to?

AhAh ! I'm waiting for this question , it is only if I need few ten volts less , I don't

It seems to me that in this AC circuit, the zeners would be forward biased for 1/2 the AC wave and reversed biased for the other half. This would regulate only half the power supply down to a lower voltage.
I think I would have hooked up the zeners between the output of the rectifier and the first filter caps. But then again, I probably wouldn't have tried reducing treble by lowering B+.
The amp is now working on voltage desing by Fender not by modern wall voltage wich are higher. I never had to do this on any Fender amp using 6L6 .

That's another interesting concept.


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 12:05 pm
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Retroverbial wrote:
TimsAudio wrote:
But then again, I probably wouldn't have tried reducing treble by lowering B+. That's another interesting concept.


The objective was to decrease the plate voltage to V7 and V8 by 20VDC to save some wear and tear on the tubes. The sonic improvement is a byproduct of that modification.
Voltage decrease also on all tubes very close to the AB763 shematic volts


Arjay


Last edited by stratele52 on Sun May 17, 2015 1:13 pm, edited 1 time in total.

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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 12:23 pm
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stratele52 wrote:
Voltage decrease also on all tubes very close to the AB165 shematic volts


The '66 Deluxe Reverb is an AB763 circuit.

Arjay

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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 1:12 pm
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Retroverbial wrote:
stratele52 wrote:
Voltage decrease also on all tubes very close to the AB165 shematic volts


The '66 Deluxe Reverb is an AB763 circuit.

Arjay



You are right that's the schematic I use .
Too many amps on my bench :lol:
I edit a new answer

Thank's Arjay


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 2:06 pm
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TimsAudio wrote:
Interesting concept... It looks like four diodes, all forward biased to ground.
What is the fifth diode hooked to?


It looks like one zener is not connected. Perhaps, added in case you wanted to drop the voltage a bit more?


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 2:10 pm
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BMW2002Ti wrote:
TimsAudio wrote:
Interesting concept... It looks like four diodes, all forward biased to ground.
What is the fifth diode hooked to?


It looks like all five diodes are forward biased to ground. The top one is in series with the other four, but flipped over on the terminal strip. With its cathode tied to ground.


:lol:
All diodes are in same position , kathode side to ground , the flip one is not electricaly flip . Look carefuly at the terminal lug


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 2:13 pm
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stratele52 wrote:
Retroverbial wrote:
stratele52 wrote:
Voltage decrease also on all tubes very close to the AB165 shematic volts


The '66 Deluxe Reverb is an AB763 circuit.

Arjay



You are right that's the schematic I use .
Too many amps on my bench :lol:
I edit a new answer

Thank's Arjay


I've had that same issue a time or two, mon amí.

8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 2:14 pm
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stratele52 wrote:
BMW2002Ti wrote:
TimsAudio wrote:
Interesting concept... It looks like four diodes, all forward biased to ground.
What is the fifth diode hooked to?


It looks like all five diodes are forward biased to ground. The top one is in series with the other four, but flipped over on the terminal strip. With its cathode tied to ground.


:lol:
All diodes are in same position , kathode side to ground , the flip one is not electricaly flip . Look carefuly at the terminal lug


A close up
Image


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Post subject: Re: 66 Deluxe Reverb ; How I fine tune
Posted: Sun May 17, 2015 2:53 pm
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Stratele, one diode is connected to the yellow-red wire doesn't seem to be connected to the string. Its anode seems to only be soldered to the terminal strip. But, nothing is connected to it?


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