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Post subject: Fender SS100 Autobias Replacement circuit
Posted: Tue Oct 13, 2015 8:28 pm
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Aspiring Musician
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OK, I edited this post to update the pics. Scroll down through the replies to them.
Well, it finally happened to me. A SS100 came in with no sound and an unresponsive autobias module. All three temp lights come on, but no action. Bias is at -71 VDC. V1-V4 LEDs red
So I reset it and it went through the self test and then the lights would respond to the switches. Still -71 volts bias.
So I explored around and found the sum and difference input servo... seems OK
Then I found the PWM output stage... Only works after a reset... Clamped at less then 10% duty dycle....
So I built a manual bias circuit. It accomodates the change from club to arena with two bias pots
The new circuitry is mounted near the speaker jacks. I siliconed the back of the pots to the chassis at final assembly.
It requires 3 10K resistors, 2 10K linear pots, and a 1K resistor.
I'm gonna warn Ya'll not to zap yerself at this point. Discharge!
Start by removing the two black wires of the Club/Arena Switch where they connect to the circuit board. Splice in wires to extend out to the new circuit.
The raw C- is @ -80 volts. The new circuit gets this at a empty hole at WJ21. This is located near the end of the board by the speaker jacks.

Wire it up as shown
Set Arena bias first, then Club.

The output is buffered by the existing 68K resistors R101-R104. If a tube grid shorts, it won't affect the bias of the other tubes as much.

The circuit works well, There's a little bit of settling time after switching the A/C switch as the capacitors change their voltages.
but I'm looking for ideas on a bias setting for the club setting
The tubes are worn out Svetlanas that will be replaced.
Arena bias is set to 28Ma with a plate of 452VDC.
Club bias is currently at 10Ma with 229VDC no crossover distortion, but how hot can I run it at a low plate voltage?


Last edited by TimsAudio on Fri Sep 13, 2019 9:58 am, edited 1 time in total.

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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Wed Oct 14, 2015 2:41 am
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That seems kind of low for the idle bias. 12 watts and 2 watts in arena and club respectively. I would lower your bias voltages to -40 (or less), and - 15 (or less). Or, whatever it takes to get at least 15 watts and 3.75 watts respectively, maybe even higher wattage than that. The schematic shows 33 mA through each tube, but doesn't have a different reading for when switching modes. 33 mA in arena mode would give approx 15 watts idle per tube, so if you cut the voltage in half, you would need the same current in order to cut the wattage in half. Half the current and half the voltage for 1/4 the wattage.

Otherwise, that is what the arena/club switch does, cuts the Plate voltage to approx half.

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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Wed Oct 14, 2015 8:03 am
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OK, thanks,
I'll shoot for that when the customer brings in his new tubes.


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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Tue Aug 15, 2017 4:17 am
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Hi there,
the automatic bias system on a friend's Supersonic just stopped to cooperate the other day. I found your post on this topic and I'd like to use your schematic to replace the curcuit. I also wonder what you ended up doing to the customer's amp in the end, perhaps you remember?

It's great that you added the pictures to illustrate the build. The only problem is that the links to photobucket don't seem to work anymore. Could you please send me the pictures or reupload them? That would help me immensely, especially since the schematic comes out rather blurry on google.


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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Tue Aug 22, 2017 3:18 am
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Hi there,
the automatic bias system on a friend's Supersonic just stopped to cooperate the other day. I found your post on this topic and I'd like to use your schematic to replace the curcuit. I also wonder what you ended up doing to the customer's amp in the end, perhaps you remember?

It's great that you added the pictures to illustrate the build. The only problem is that the links to photobucket don't seem to work anymore. Could you please send me the pictures or reupload them? That would help me immensely, especially since the schematic comes out rather blurry on google.


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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Sun Sep 08, 2019 10:29 am
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According to the schematic it is 44v for arena and 15 for club.

I would be interested in seeing the schematic for this mod. Have one of these with a similar problem.

Mike


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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Tue Sep 10, 2019 4:02 pm
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I fired up my old computer and found these...
Image

Image


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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Tue Sep 10, 2019 8:52 pm
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Good stuff Tim.

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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Wed Sep 11, 2019 4:50 am
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Goof job Tims

Auto bias original circuit ;

Image[url=https://flic.kr/p/2hdSdd1]

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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Thu Sep 12, 2019 9:52 am
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I finally figured out why I couldn't post on Imgur previously.
I got a notification that I didn't have a verified email.


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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Thu Sep 12, 2019 9:44 pm
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Aha! You are a bot! I knew it! :lol:

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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Fri Sep 13, 2019 1:26 am
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shimmilou wrote:
Aha! You are a bot! I knew it! :lol:


A bot is a robot ? I never read that :lol:

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Last edited by stratele52 on Fri Sep 13, 2019 11:08 am, edited 1 time in total.

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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Fri Sep 13, 2019 9:55 am
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Yes, I was a bot. I was secretly copying everything you wrote to pass it on to Fender.
:wink:


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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Fri Sep 13, 2019 4:35 pm
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TimsAudio wrote:
Yes, I was a bot. I was secretly copying everything you wrote to pass it on to Fender.
:wink:

:lol: Good one, Tim. That's funny. But, now that we know the truth...

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Post subject: Re: Fender SS100 Autobias Replacement circuit
Posted: Fri Sep 13, 2019 5:35 pm
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Aspiring Musician
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Marky Forrest wrote:
TimsAudio wrote:
Yes, I was a bot. I was secretly copying everything you wrote to pass it on to Fender.
:wink:

:lol: Good one, Tim. That's funny. But, now that we know the truth...


Yeah, they already have Google and Majestic 12 for that. :lol:

At least Tim provides useful info.

<edit>

After thinking about this for a few minutes, I didn't mean that the other regular posters don't provide useful info, just the Google and Majestic bots. :oops:

Cheers,


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