Fisher 500-B - Help w/ Can Cap Replacement in Chicagoland Area

All the new parts are in and everything appears to work. My solder work on the can cap to chassis isn’t pretty; my 40w Weller Pencil and 140w Weller gun weren’t really up to the task. That said, re-stuffing the can caps was easier than I thought it’d be. I’ll post output tube pin readings and an updated Mouser order list later (some of the caps in the initial order didn’t fit) but thought I’d post a few pictures of the progress in the meantime.

Great work, looks awesome!
 
EA668D3B-30B5-4C67-BC1F-3093AF6A95EC.jpeg Thanks! Here's the updated Mouser parts list I used for three of the multi-section caps (caps for the other two cans came in my Metalbone kit). The majority of the caps in this list are the thin pencil style. I had to go up quite a bit in voltage for C77 to get this style; Nichicon makes a 56uF/250v model but it's non-stocked at Mouser with a long factory lead time, which is why I just opted for the 56uF/400v.

Here's my current pin readings on my current old/unmatched tubes:
V13 V14 V15 V16
P3 415 414 412 414
P4 384 384 384 384
P5 0.238 0.249 0.280 0.308
P6 -14.40 -14.41 -14.41 -14.4
 
Last edited:
So I reached out to Jim McShane for new tubes and he was saying I really need to install a bias adjustment mod first, so my plan to hold off on additional modifications just went out the window.

I've read a number of threads on IBAM and IBBA; I'm going to install an IBAM if for no other reason than it seems a little more in line with my skill level to build/install. I picked up 4 x 12k resistors 1/2w, 4 X 33uF/50v e-caps, and 4 X 20k 1/2w trimmers, and I'm currently in process of building the board.

I understand breaking the connections between R102 - R105 and how the board ties into the grid resistors. However, I'm still a little confused on where to tie in the IBAM's power supply. I believe I remove C85, R129, R135, and the white wire that led from that junction to the grid resistors,but then where do I attach the board power supply? Also, can the ground wire from the IBAM board tie in to any ground connection?

InkedFisher 500B IBAM_LI.jpg
 
Follow the lead from C85, R129, R135 back to the DUAL BIAS CAP underneath. Tie it in to the NEGATIVE Voltage (Positive is Grounded)side. That's the Power Source for the IBAM. Ground from the IBAM can go to any CLOSE and available grounding point, the closer the better. Make sure it reads <1ohm. Then tie each of the individual leads to each point where you removed the White lead from the Grid resistors. SEPARATE the GRID Resistors 1st.
 
I apologize but I’m still confused. C85, R135, and R129 all meet at the last tab on the rail on the picture above, the same tab that begins the original white wire going to the grid resistors. On the opposite side of R129 from that rail tab is one of the negative twist tabs from C79, the twist tab that is also connected to a purple wire in the picture above. Is this the point where I should connect the IBAM poster source (the C79 twist tab)? If so, I also completely remove C85, R135, and R129, correct?
 
Get rid of C85, R135 and R129 - the IBAM board takes care of their functions. The negative bias supply comes from the negative side of C79A & B (C60A & B in the schematic below). I think it's the white wire but make sure it comes off the negative side of those caps, which then feeds to one side on all four of your 10k trimmers - see below.




index.php
 
Excellent! Thanks - another question or two. First, I built my IBAM with 20K trimmers - just want to double check that using those is OK. Second, can anyone point me to instructions on using the IBAM once installed or is it as simple as starting on one side and just dialing in the readings? Should the trimmers be at any specific point at start out i.e. further to one particular side?
 
Set the trimmers so they are showing 0 ohms or as close to ZERO OHMS from the input to each output. This will allow the tubes to have MAXIMUM Negative voltage (A VERY GOOD IDEA) when starting up 1st time. Adjust in increments (a few ma at a time) until reaching desired output dissipation. As the controls are interactive to a point, expect to see one adjustment affect all 4 tubes. As you dial it in, this will lessen.
 
"from Thorne in E-mail"
Larry - Isn't the process for using IBAM to set the bias for all tubes at the same level? e.g. when I set my 4 tubes, I ensure all 4 are .33mV.

Yes. But on initial startup you want MAX VOLTAGE (-24v or whatever your bias circuit is producing) so you aren't overloading the tubes with current. Then determine what the initial bias ma is per tube and equalize them at a low setting. then bring them up 2-3ma at a time until you get to your target reading. This keeps them pretty much balanced and lessens the chances of red-plating any one tube. Plus you're balancing (as much as an IBAM will Balance) at a lower setting and bringing them up in steps (as the readings interact with one another, getting them balanced as much as possible at a lower setting will help with lessening the interaction at the final settings.)
 
If you have a variac set the mains voltage to around 90vac and start adjusting your trimmers. Then slowly increase your voltage to 117vac and set the trimmers for 34-35ma.
 
Ok, finished and installed the IBAM (attached a picture).

If I balance the tubes to 34-35ma I’m averaging about -11v on pin 6 for each tube, which I’m guessing is the result of my very used/old tubes. Until I get a new quad, should I leave the current tubes at 34-35ma or is there some compromise to shoot for to bring up the pin 6 voltage?
 

Attachments

  • F7791D48-B196-4B93-AE41-D1BEA6907E87.jpeg
    F7791D48-B196-4B93-AE41-D1BEA6907E87.jpeg
    137.4 KB · Views: 14
32ma should keep you in the 70% to 80% range. Any lower and you're increasing distortion and the tubes are out of their best operating area. @ 415 on the plates, 32ma gives you about 13.28W per. that's before screen dissipation. Subtract 3ma for the screens and you're down to 29ma total for about 12.03W. This would be the lowest I would take the tubes.

Right now your actual wattage is, subtracting for the screens is around 32 to 31ma. @ -11V or there abouts. Best way to measure the tube output is by the load (ma) not the voltage. You want the load equalized from tube to tube, with attendent differences in pin 6 volttages. You really don't want the tubes "balanced" by the PIN 6 voltage as one may be at 34ma, one at 31ma and the other two somewhere in the area. Balance the loads and let the voltage fall where it does. But I agree with you that -11 is getting close to the edge of the cliff. I generally retire 7591's to testing units, when the Cathode voltage hits -13V with them running @ 80% of max dissipation.

One thing I STRONGLY RECOMMEND is installing 100ohm 1/4w Screen resistors to prevent any arcing events. See drawing on page 2 of the STICKY called "COMMON PARTS FOR FISHERS" ( http://audiokarma.org/forums/index....needed-for-fishers.327561/page-2#post-6480170 ). Basically you remove the inter-tube jumper from between the tubes at pins 4 & 8. (Leave connected on the tube sockets themselves). Then install a 100ohm 1/4w resistor from pin 1 on each tube to pin 4. Take the Screen supply on the 1st tube at left, and move it to pin 1. Add wire in a Daisy chain to the other 3 tubes pin 1. Solder, and you're done.


index.php
 
Last edited:
Screen stabilizers installed and I got my new tubes from Jim. However, I think I’ve got an issue - I’ve been told a new set of Tung Sols would need about -22v at pin 6 once they were dialed in - I’m getting nowhere near that.

Maybe I’m not unstanding and/or reading the pin 5 ma measurements correctly. Started hunting around for other info on pin 5 measurements and saw Dave G mention pin 5 should be in the .3-.35vdc range. At the low end of that spectrum (.3vdc) I’m averaging -16.0 at pin 6. On average, pin readings are as follows:

Pin 3: 408
Pin 4: 380
Pin 5: .301vdc
Pin 6: -16.0

Maybe I’ve got a problem with my IBAM? The line in is at -23.7. If I fully open the trim pots the max reading I’m getting at pin 6 is -20.3.

I used 20k pots in the IBAM rather than 10k pots that are normally mentioned. Did I need to make a corresponding change to the resistors on the board? I used 12k resistors per original instruction/schematics I saw on AK.

I apologize that I’m a all over the place in this post - I’m just really not sure what to do/where to look for an issue at this point.
 
K... What matters is the plate voltage at pin 3 and the cathode voltage at pin 5. What you do is use formula P=I*V (known as Ohm's Law), power in watts = pin 3 current times pin 5 voltage. Assuming you have the 10 ohm cathode resistor at pin 5 and you measured the voltage on the grounded side to get .301 VDC, divide pin 5 volts by the 10 ohms and you get .0301 which is your amperage or current. Substitute the amps into the IV part of Ohm's Law and you get .0301 * 408 = 12.28 which is your wattage, P. So currently your 7591 tubes are pushing 12.28 watts. Most people would say the "sweet spot" is around 70% of max wattage (19 watts) for a 7591. 70% of 19 = 13.3 watts so you're close. You're running a little cool, about 65% of max wattage.

What you want to do is figure out what value to get at pin 5 so you can have 13.3 watts. Using P=IV again and the same math as before, we have 13.3 = I * 408. Solve for X in 13.3 = (X vdc/10 ohms) * 408.

13.3 = (x/10) * 408
13.3 / 408 = x/10
.0326 = x/10
.0326 * 10 = x
.326 = x

So to get 13.3 watts, you want to adjust the bias so you measure .326 VDC at pin 5.

Thorne
 
Last edited:
Back
Top Bottom