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Improving the Fisher 400

eduarsan1 said:
Dear All

Due to this thread was started by Dave and talks about the EFB, unity gain buffer, the control section and output stage. I attached the drawings with this modifications ONLY.

PD: The 10 Ohm resistor to drop the voltage in the heaters powered from the DC Bias winding was added

Regards

Luis
Click to expand...

Luis, I noticed on your Mouser order list that you ordered some parts in greater quantities than what's required to do upgrade a 400 such as the transistors and mosfet. Do we have to sort though some to get ones that will work best with this build or are you doing more than one amp? Stocking up etc.

Thanks for taking the time to make the schematics and sharing your order history with us.

Dan
 
Dan

You´re right! I decide to have in Stock more pieces due to the EFB is planed to improve in another amp!

If you need any help don´t hesitate in ask!!!

I will start the update in my 400 this weekend, so keep connected to this thread!!!

Regards!!!


Luis
 
Luis, thank you for clearing that up. I'm sure that I'll take you up and ask for more help also once I get the parts and start the project.

Dan
 
I finally got what I hope is all the parts minus the pref boards to attempt this project. I have to hit radio shack.

Anyway, with IBAM how do I go about adjusting the bias with EFB also installed? I'm thinking I'd turn the IBAM pots all the way up, adjust the EFB so that the lowest tube is 21ma then fine tuning the IBAM's so the other tube or tubes are also 21ma?

Thanks
Dan
 
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Hi Dan -- That's not a bad way to go. When you get to building your IBAM, let me know what the component values are, and I'll see if any tweaking is needed to the EFB circuits.

Dave
 
Dave, thanks for the reply. I'm going to start off with the values in the schematic's Luis drew up. 10k 1/2 watt pots, 10k resistors and a 4.7uf caps.

Btw, how much fluctuation from 21ma within a set of 7868's would you consider to be matched/optimal with your EFB circuit?
 
I don't like to see a fluctuation that is greater than +/- 5%. Have you confirmed that your unit has a serial number of at least 48001?

Dave
 
Hi, I'm looking at the datasheet for the STF10NK50Z and I can't figure out 100% what leg goes to where on the EFB part of this project. Does pin 1 connect to the .47film cap, 2 to the 1.8k 4w resistor and 3 to the 22uf 350v capacitor?

Thanks
Dan
 
Hi Dan

Pin 1 (Gate) is comming from the 453V network going down thru the 100K resistor

Pin 2 (Drain) comming from 1.8 k 4W resistor

Pin 3 (Source) Is going to Bias Control and the grids on to the 7868 tubes.

Hope this helps

Regards

Luis
 
dannyr said:
Anyway, with IBAM how do I go about adjusting the bias with EFB also installed? I'm thinking I'd turn the IBAM pots all the way up, adjust the EFB so that the lowest tube is 21ma then fine tuning the IBAM's so the other tube or tubes are also 21ma?

Thanks
Dan
Click to expand...

I am also interested in your results. The IBAM by itself is slightly interactive. It usually requires some tweaking to get all the tubes to the desired ma. The addition of the EFB will add a new element.

By the way "turning the IBAM all the way up" should be all the way counterclockwise, I call this all the way down (lowest ma). This will be maximum negative voltage. The IBAM design is safe in that even if you turn it all the way up (lowest negative voltage, highest ma) it will not go to "0" volts. I added the safety after red plating a tube while turning the wrong pot.

Speaking of "0" volts. It is important to test your IBAM. Several people have red plated tubes from mis-wiring, bad solder joints, etc. Somewhere there is a thread with a proposed testing procedure I came up with. I don't have a running unit right now to try the procedure so perhaps you could be a beta tester? I try and find the thread and post a link to it.
 
drewbolce, thanks for the heads up. For the first test with tubes I'll put in the original 7868's that have no getter flash left. Just brown at the top. I wonder if I should use them with EFB until they totally go or not.

Anyway, let me know how I can help out and I'll be happy to do it when I get to the step.

Thanks
Dan
 
Danny -- nothing wrong with turning the unit on without any output tubes installed, and then adjusting the IBAM controls for max negative voltage at the control grids. That way, you know you've got them turned to the minimum current draw position before the tubes are in place.

Just realize that the power supply caps will not discharge themselves to a safe level nearly as fast from this procedure, but will still discharge themselves in about 5 minutes time to that level due to the design of the power supply. With the tubes installed and biased properly, the unit will normally discharge itself to safe levels in about 60 seconds.

Dave
 
Dave, I'll be sure to check the power supply cap voltage between fireups.

Would you retire a set of original output tubes that have only brown where the getter flash was or run em? I'm not sure if with EFB if they'd have a lot of life left because they are going to ran a lot less hard or if I should just start with a fresh set.

Dan
 
Lost gettering is a sure sign of very long use, but doesn't always mean a tube is completely dead either. However, more than likely, the tube is well worn, and well off of expected capability. Might be best to save these tubes as test tubes (literally). I'd start the modification with a fresh set of tubes, and enjoy the extended life they'll have.

Dave
 
Here is a link to my post on testing the IBAM. Dave, could you take a look at it? I am always reluctant to post an untried procedure. Rereading it I find it a little too much on the "safe" side.

http://audiokarma.org/forums/showpost.php?p=6504026&postcount=19

Lost gettering does not always determine output tube condition. The only sure way is to test them in a known good unit under full power and measure the distortion. I use a load box with a ST1000B distortion and power measurement.

White dust does indicate tube condition (tube is vented to atmosphere).
 
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I finally got off my but and started work on the amp. The power supply is all recapped. While I was soldering and figuring out how to fit the new capacitors a question came to mind. Can I use the Eico ST70 mod method of adjusting AC balance with injecting a .5Vrms 1kHz sinewave to the aux input and adjusting the AC pot for the least DC between the two 10ohm resistors between of a channels output tubes? If so what would be the recommended Vrms to have set the level of the speaker outputs/8ohm resistor? I have no scoop to see if it's clipping or not:(.

thanks,
Dan
 
Hi Dan -- It will work just fine. If you are using an 8 ohm load, then adjust the output for 12.0 vac, which will be cause the the amplifier you are adjusting to produce 18.0 watts RMS. As long as your line voltage is at least 117 vac, this level of power output will be safely below the maximum power output of the amplifier to prevent any clipping, yet still represent a strong signal swing to obtain a good balance with.

Keep us posted on your progress!

Dave
 
Dave, great. thanks for the help yet again.

I got the opamp buffer board done today. it took me a lot longer then it should have. I think that the EFB will be easier due to the parts being much larger. I hope that I didn't curse myself by typing that.

If anyone can point me on how to convert the tapehead input to another aux please let me know.

Dan
 
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