Alternate Individual Bias Adjustment Modification

I will hopefully get to installing the IBBA this weekend. From your schematic for the EFB circuit you reference biasing output tubes to 21ma. I first rebuilt my power supply and the majority of the 400 with Jim Mcshane's kit & then followed with your EFB, Power Supply, Control Section & SS Buffer. I currently have the original fisher output tubes installed what would you recommend for bias current? im also considering a new set of TungSol 7591's what would they get biased at?

 

OK. I had no idea you were talking about a 400 with EFB installed. I'm sorry, I must have missed that memo somewhere! If you have EFB installed in a late version 400, then 21-22 ma per tube is ideal. It doesn't matter what brand of tube you are using -- the bias specification stays the same regardless of the manufacturer of the tubes.

You really went full out on your 400 -- congratulations!

Dave

 

Thanks Dave your help and your designs have made this 400 amazing compared to the stock design. Im also going to install your RIAA mod when I can locate 350pf silver mica caps. every big distributor only offer 360pf. Not sure what will be left to do other than get some better matching tubes for the preamp & power amp sections.

 

Dave Help! Im having trouble with the IBBA board. I have it installed and I can't get the tubes to go lower the 48ma? I don't know why the bias has risen so high Ive attempted a couple of times to adjust but no luck bias only goes higher?

 

What im gathering is the 15k resistors need to be removed from the board or values changed. Is that correct since the EFB also allows for an overall adjustment before the IBBA board. Checking voltages at the 1k resistors going to pin 2 the best I can adjust to is around -10volts to -8volts

 

Your problem is due to the fact that the IBBA modification was never designed to operate with the EFB(tm) circuit for late version 400 receivers. Rather, it was designed to be a drop in fit for stock 500C/800C receivers.

To adapt the EFB circuit for use with the IBBA, on the EFB board:

1. The 22K resistor becomes 18K (.25 watt is fine)

2. The 39K resistor becomes 75K (.25 watt is fine as well)

Use the bias control on the EFB board to tweak the IBBA bias control into best range. Worst case, you may need to adjust the 75K resistor slightly to suit. Lowering its value reduces bias voltage output from the EFB board, while raising its value increases it. Make sure that the small electrolytic on the output of the EFB board is rated for 35 vdc or more.

I hope this helps!

Dave

 

I used an 1100 in parallel with a 250 to get the required 1350 pf. I couldn't find 350pf either, or an actual 1350pf cap. Its hard to read, but I posted a list of part numbers that worked out for me when I did the RIAA fix on my TA-600

The Fisher Phono Preamp, and RIAA

If you can't read it (I can't), I'll look at the full size pic when I get home to pull the numbers. They're all Mouser items.

 

thanks gadget I already ordered parts. I think some of the ones you listed are no longer available. I found 1% silver mica, 1300pf, 300pf, and 50 pf on mouser.

 

and thanks Dave I ordered parts I will try out with the changes Friday

 

to follow up I used the 18k in place of the 22k and two 75ks in series to replace the 39k and get in the proper range. I also figured out from doing some research on the site that I had to move the 220 ohm resistor from pin 4 of an output to ground to remove a hum and properly bias the right side. One more thing ive noticed that I would like to sort out is that on the left channel there is a lower hum than the previous one stated in the right channel. if I touch the phase inverter tube the hum gets louder? any ideas what I should adjust to get this to perform correctly?

 

First, are you saying that you originally had two 75K resistors in PARALLEL to achieve 37.5K as a close replacement for the original 39K value? If so, then simply removing one of the 75K resistors leaves the other 75K still in place, representing its actual value then, and the value needed for the adjustment to the EFB board. Is this correct? Two 75K resistors in series would add to produce a total resistance of 150K.

Dave

 

two 75k resistors in series to create 150K worked to get in the correct range making 21ma. A single 75k had me in the area of 35ma. any ideas on the left channel hum?

 

If the tube itself is sensitive when you bring your hand close to it, have you tried a different tube?

 

yes it still does it, I switched the phase inverter tubes. it seems to be a little quieter with the other tube yet it only happens in that channel

 

Some pics of the wiring for the phase inverter tube in question would help, as otherwise I'm grabbing at the wind. What is the negative voltage coming from the EFB board to the IBBA board?

Dave

 

Dave I figured it out I tightened up pin 2 with a dental pick and the hum has gone.

 

Glad you found it!

Dave

 

I've come to the point in my learning curve where I need to master building these adjustable bias boards. I have about six units that would benefit from these and make them prolong tube life. I have all the parts ordered and plan on digging in as soon as they arrive, I'm sure I will need a bit of guidance along the way.

 

I would love it if someone would produce these adjustable bias boards (all set to go) that would fit on my fisher tube stuff with minimal wiring. If anyone knows of someone making these, I sure would like to purchase a few, if they came with some install instructions.

 

More fun learning to do it ourselves,
At least I will try and keep that attitude and see how I feel after putting 6 together.