• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Help with Fisher 660A voltages

Derek -- What is the stabilized voltage across the 400 ohm resistor? Also, we need to know what the actual resistance value is of your 400 ohm resistor/R129 combo, so that with the voltage measurement you make, we can determine actual output stage current flow.

Dave
 
dcgillespie said:
Derek -- What is the stabilized voltage across the 400 ohm resistor? Also, we need to know what the actual resistance value is of your 400 ohm resistor/R129 combo, so that with the voltage measurement you make, we can determine actual output stage current flow.

Dave
Click to expand...

The voltages are as follows (measured while 'Ziggy Stardust' was playing):

Bias voltage (at the live end of the 400 ohm resistor) = 44V
OPT voltage: 432V
7591 voltage: 412V
12AX7 voltage: 310V

Not sure how to calculate actual resistance between the 400 ohm and the 15k (R129) resistor.

-D
 
Well, that answered that. I never really could see what R129 was. On this end, it looked by 1.3K, but now that you are indicating that it is 15K, that changes things a little.

With a 400 ohm resistor and a 15K resistor connected in parallel, you get (400 X 15000)/(400+15000) = 390 ohms. 44volts/390 ohms = 113 ma/ 4 tubes = 28 ma per tube. This is a little cool to operate these tubes at.

Using a 350 ohm resistor, that in parallel with a 15K resistor = 342 ohm. With a cathode voltage of 44 volts, this equates to 32 ma per tube, which is should deliver very good performance.

Using a 300 ohm resistor, that would equate to 294 ohms, or 37.5 ma per tube, which would be about the max you'd want the tubes to draw under quiescent conditions.

325 ohms will bias the tubes at 34.5 ma each.

Therefore, anywhere between 325 - 350 ohms should be about ideal. At the current level that this resistor range will cause the output stage to draw, output tube dissipation is about 13 watts per tube, which is just fine for these tubes.

I'm sorry for moving you around on these values, but the slight shift downward is because without anything else to go on, I could only go on what R129 appeared to be, which was 1.3K. Now that you have confirmed the actual value as much higher than that, that has shifted the required value of the "heater resistor" lower to compensate.

If you have a 350 ohm resistor, that should suffice just fine. The resistor will dissipate 5.5 watts, so again, a 20 watt resistor would be optimum.

Dave
 
dcgillespie said:
Well, that answered that. I never really could see what R129 was. On this end, it looked by 1.3K, but now that you are indicating that it is 15K, that changes things a little.

With a 400 ohm resistor and a 15K resistor connected in parallel, you get (400 X 15000)/(400+15000) = 390 ohms. 44volts/390 ohms = 113 ma/ 4 tubes = 28 ma per tube. This is a little cool to operate these tubes at.

Using a 350 ohm resistor, that in parallel with a 15K resistor = 342 ohm. With a cathode voltage of 44 volts, this equates to 32 ma per tube, which is should deliver very good performance.

Using a 300 ohm resistor, that would equate to 294 ohms, or 37.5 ma per tube, which would be about the max you'd want the tubes to draw under quiescent conditions.

325 ohms will bias the tubes at 34.5 ma each.

Therefore, anywhere between 325 - 350 ohms should be about ideal. At the current level that this resistor range will cause the output stage to draw, output tube dissipation is about 13 watts per tube, which is just fine for these tubes.

I'm sorry for moving you around on these values, but the slight shift downward is because without anything else to go on, I could only go on what R129 appeared to be, which was 1.3K. Now that you have confirmed the actual value as much higher than that, that has shifted the required value of the "heater resistor" lower to compensate.

If you have a 350 ohm resistor, that should suffice just fine. The resistor will dissipate 5.5 watts, so again, a 20 watt resistor would be optimum.

Dave
Click to expand...

Crap - I misread R129 as R123. R129 is, in fact, 1.3k, so your assumptions were correct in the first place. That, and I'm a nearsighted dumbass.

-D
 
derekva said:
Crap - I misread R129 as R123. R129 is, in fact, 1.3k, so your assumptions were correct in the first place. That, and I'm a nearsighted dumbass.

-D
Click to expand...

So, using your math above:

400*1300/(400+1300) = 305.9 ohms

44 volts / 305.9 ohms = 143.8mV

143.8mV / 4 tubes = 35.95mA

So I should be slightly less than maximum current with a 44V bias - if I adjust the bias voltage slightly lower (say 40V) using the bias pot, that'll put me down around 32.7mA.

Hopefully, I'm not missing anything,

-D
 
Derek -- You've got it perfect! With R129 in fact being 1.3K (so glad it was!), then it sounds like 400 ohms is just about perfect -- allowing you to adjust the bias now at will to where you want it.

By the way, there is nothing stopping you from still adding the 10 ohm sampling resistors at each cathode connection to measure the current of each tube for matching purposes if you want to. If you do, just remember that you would need to measure the voltage for each tube directly across each resistor, rather than from the cathode to ground, because in this case, these resistors are not connected directly to ground.

It sounds like you've about got this one all wrapped up. Congrats!

Dave
 
dcgillespie said:
Derek -- You've got it perfect! With R129 in fact being 1.3K (so glad it was!), then it sounds like 400 ohms is just about perfect -- allowing you to adjust the bias now at will to where you want it.

By the way, there is nothing stopping you from still adding the 10 ohm sampling resistors at each cathode connection to measure the current of each tube for matching purposes if you want to. If you do, just remember that you would need to measure the voltage for each tube directly across each resistor, rather than from the cathode to ground, because in this case, these resistors are not connected directly to ground.

It sounds like you've about got this one all wrapped up. Congrats!

Dave
Click to expand...

So the 10 ohm resistor would be in series with the bias string (e.g. insert resistor between cathode and pin 5 for each tube), yes?

And the amp sounds fantastic - I need to figure out a bottom plate for safety and put a nice piece of exotic wood on the 'front' to make it a little less industrial looking for the living room...perhaps even with a indicator bulb on the front so my wife & kids know when it is on (and hot). I think this amp has permanently displaced my 800c as it sounds that great.

Thanks again for all your help, Dave (and Larry!). You guys are my Fisher gurus!

-D
 
I think you've got it. As originally designed, the end of the heater bias string (now the 400 ohm resistor), R129, and the + of the cathode bypass cap all connected to the cathode terminal (pin #5) of all four output tubes which had their pin #5s all strapped together.

To install the sampling resistors, the pin #5 cathode terminals would all have to be disconnected from each other and the top of the bias string, R129, and bypass cap. The bias string, R129, and the cathode bypass cap would then need to be reconnected together at a new tie point, and then the sampling resistors each installed between the new tie point, and pin #5 of the particular tube it serves.

If you install bias test points, the negative test point would be connected to the new tie point, and of course the positive test points would connect to pin #5 of each output tube as usual.

If you install such test points, be certain that they are all carefully isolated from the chassis, since all of these points represent a voltage of about 44 volts above chassis ground.

Dave
 
dcgillespie said:
I think you've got it. As originally designed, the end of the heater bias string (now the 400 ohm resistor), R129, and the + of the cathode bypass cap all connected to the cathode terminal (pin #5) of all four output tubes which had their pin #5s all strapped together.

To install the sampling resistors, the pin #5 cathode terminals would all have to be disconnected from each other and the top of the bias string, R129, and bypass cap. The bias string, R129, and the cathode bypass cap would then need to be reconnected together at a new tie point, and then the sampling resistors each installed between the new tie point, and pin #5 of the particular tube it serves.

If you install bias test points, the negative test point would be connected to the new tie point, and of course the positive test points would connect to pin #5 of each output tube as usual.

If you install such test points, be certain that they are all carefully isolated from the chassis, since all of these points represent a voltage of about 44 volts above chassis ground.

Dave
Click to expand...

Thanks again for the advice, Dave.

To run with these modifications, hypothetically, if I wanted to add the capability to individually bias each tube, then I'd simply add a small trimpot (since the main bias pot is 5k) for each tube between the pin #5 and the resistor (or the resistor and the tie point), correct?

Still need to post photos. I'm debating whether I want to wait until all the aesthetics are done as well, or post what I've got and update later.

-D
 
Derek -- On the fly now, but wanted to give quick answer.

First, most importantly, in my last post regarding the installation of cathode sampling resistors, I failed to mention that there is a 4.7K resistor also currently attached to the output tube cathodes that feeds the bias control. Like the other components I mentioned, it too would need to be disconnected from the cathodes and tied to the new tie point as well, so that the connections at the new tie point would consist of:

1. R129.
2. The cathode bypass cap.
3. The new 400 ohm bias string resistor.
4. The 4.7K feeding the bias control.
5. One end of all four new sampling resistors.

The other end of the sampling resistors would of course each go to their respective output tube terminal #5.

As for independent adjustment of each tube, the best way to accomplish that would be to replace the existing 5K bias control with four 20K pots all wired in parallel to replace the existing pot. For the purposes of the circuit, this still represents a value of 5K, so no other values would need to be changed. Then, the four 330K grid return resistors would all need to be separated from each other, with one of each of these resistors then returned to a wiper of its own new bias control. In this way, each tube could then be measured, and adjusted separately.

Dave
 
Fisher 660a Schematic

Hey there, sorry for butting in on this thread but the most comprehensive information I can find on rebuilding a Fisher 660a, which I recently acquired along with the pre-amp from an Ambassador counsel, is on this thread. It sounds like you have the full wiring schematic for this unit and if so, would you be so generous to pass it along to me? This will be my first tube restoration project and beginning with the proper data about this unit will be a good start. Thanks for the help!!

Joshua
 
You must log in or register to reply here.
Back
Top Bottom