Keith's Fisher 500C Project

[keithpgdrb]

Welcome to my official 500c project page.

My journey began when I found a local 500c for a decent price. I always wanted to try my hand at tubes, and this came at the right time. If you want to know the backstory, HERE is the thread where it all began.

So now my goal is to finish this project before the beginning of the school year. I have ordered the metalbone kit, and I think I have everything I need to get started. I will do my best to get some good pics in as the project moves forward. This will be my first attempt at recaping and really working on a piece of gear. I've done small fixes here and there, but never anything of this scope. I know I will look to all of you for guidance as I already have.

so, enough of the History here. On to my FIRST question.

I've read the metalbone instructions. But one thing has eluded me. While working on the unit, do I remove the tubes, or leave them in? I assumed I would remove them, but then I thought that they might have to be in for testing circuits.

now, I have cleaned the unit, but Here are the Before Pics!!!

also, a side question. how do I post pictures so they show up full screen instead of thumbnails? I hate thumbnails.

Her Beautiful Face

Under her Dress

Looking at her from above.

 

[Jailtime]

Nice 500C, that one should be a beauty when you're done. Good luck on the restoration!

Now to make the pictures big! Attach the pics, when they're done uploading, you'll see them listed by file name under the text box. Click on the file name, and the pic will come up in another tab/window. Copy that address, then paste it into the "Insert Image" button. You'll see the address bracketed with

 

[marloubow]

Hey Keith
I just started the metalbones restoration on my 800c. I have found alot of info here. I will have questions for the sages here as well seeing this is my first Fisher restor also. So I don't know how much help I can be.
good luck and ask away
martin
ps great looking unit!

 

[keithpgdrb]

Thanks guys! So. Tubes in or out while under the knife?

 

[tube-a-lou]

Thanks guys! So. Tubes in or out while under the knife?

Hi,
Tubes out, sometimes I leave in the pre-amp tubes when
I work on those areas to keep the wire and things in place.

Tube

 

[keithpgdrb]

Nice 500C, that one should be a beauty when you're done. Good luck on the restoration!

Now to make the pictures big! Attach the pics, when they're done uploading, you'll see them listed by file name under the text box. Click on the file name, and the pic will come up in another tab/window. Copy that address, then paste it into the "Insert Image" button. You'll see the address bracketed with , you can move that string around in your post.[/QUOTE]

Thank you for the info on how to manage the pictures here. so simple. Is there a way to get rid of the thumbnails at the bottom now?

 

[BinaryMike]

Removing the tubes will make it a bit easier to solder/desolder things at the tube sockets, because a tube in the socket sucks heat away from the terminal. This might not be a big deal if your soldering iron has plenty of juice.

 

[keithpgdrb]

Removing the tubes will make it a bit easier to solder/desolder things at the tube sockets, because a tube in the socket sucks heat away from the terminal. This might not be a big deal if your soldering iron has plenty of juice.

Good to know. haven't started soldering yet. Tubes come out this weekend. I'll get started on the amp on monday most likely.

 

[kpaxfaq]

Good luck! Take lots of pictures, do one section at a time then test. You'll be fine :thmbsp:

Kory

 

[Jailtime]

Is there a way to get rid of the thumbnails at the bottom now?

If you're uploading them to AK's server (please do so), I don't think you can remove the thumbnails at the bottom.

 

[bhamham]

Really nice 500C!

 

[keithpgdrb]

getting started with the Bias Power Supply. A few questions.

1. In the picture below, the yellow cap with "200" on it is the original, and marked C86 to be replaced. The instructions talk about orienting the inner shield to ground. The replacement cap I received is not the same as the one pictured in the "replaced" picture. the instructions say that when reading the cap value, the inner shield is to the right. I have pictured the cap I have. Is the right lead the one I should indeed connect to ground? I guess an easier way to ask would be, when I put in this cap, which way should the cap value face? towards the yellow can cap, or away?

Picture of the C86 cap, and R132 values.

2. Regarding the replacement of R132. I have been supplied with 3 different value resistors. how do I know which one to instal? Metalbone gives 3 options, but also states to use the one where you get the voltage at Pin 6 of the output tubes to -17 volts. Why would there be options here?

2a. There is no testing procedure outlined in the instructions that I see yet. is there a way to test to see which is the correct resistor before soldering it in place?

2b. If I indeed do need to solder one into place to test, what is the testing procedure?

Here is an overall shot of the bias supply area. my caps C102 and C103 do not look the same as in the instructions. mine are dark grey, originals in the instructions are light yellow, similar to the big can cap. should I be concerned at all here? I'm guessing not, but its always best to be safe. I only want to do this once.

 

[kpaxfaq]

1. That cap is non-polarized, I don't see why it would really matter.

2. Read the later posts of this thread starting at post #89 That should answer your questions about the bias resistor, how to measure the bias, and how to adjust it.
http://www.audiokarma.org/forums/showthread.php?t=373498&page=6

IMO you shouldn't be set on obtaining a specific negative voltage on the grids. It's more of a guideline. You want to pay attention to cathode current across the 10ohm resistors and tweak that resistor until you your 7591's in the 29-35ma range. Better yet ditch the resistor and put in a 10k pot. It can vary from tube to tube, there's nothing written in stone that -17, -19, or any other number means they are biased perfectly in their sweet spot.

Kory

 

[keithpgdrb]

1. That cap is non-polarized, I don't see why it would really matter.

2. Read the later posts of this thread starting at post #89 That should answer your questions about the bias resistor, how to measure the bias, and how to adjust it.
http://www.audiokarma.org/forums/showthread.php?t=373498&page=6

IMO you shouldn't be set on obtaining a specific negative voltage on the grids. It's more of a guideline. You want to pay attention to cathode current across the 10ohm resistors and tweak that resistor until you your 7591's in the 29-35ma range. Better yet ditch the resistor and put in a 10k pot. It can vary from tube to tube, there's nothing written in stone that -17, -19, or any other number means they are biased perfectly in their sweet spot.

Kory

Thanks for the guidance!

1. I know that the cap is not polarized, but is one side shielded, and the other not? thats what it sounds like in the instructions. From Metalbone instructions "the new capacitor (metallized polypropylene) was oriented with the inner shield to ground."

2. so it sounds like I'll have to instal one and test when I get this section complete. any suggestions on which one of the 3 to try first? 2.7, 3.3, 4.3?

 

[dsndblm]

Try the 3.3k first. Then you still have resisitors to tailor the voltage up or down if needed.

 

[marloubow]

Thanks Keith
I'll be watching yours as well
martin

 

[keithpgdrb]

Thanks Keith
I'll be watching yours as well
martin

Thanks Martin,

You've done a fantastic job on your thread, and I was using it today while working on phase 1 of the restore. so far, I've got the whole first section finished except for 1 resistor, and one cap. those should be really easy though, and I'll do those tonight.

one thing I was looking for that I could not find, was the method to test after I've finished section 1. you know, to make sure the right resistor was put in. I used the 3.3 as the first attempt.

I'm no electrician, I need to know where to put the multimeter probes. where are the test points.

anyone? I'd love to fire her up tonight and test.

 

[marloubow]

Great Keith, I'm glad it has helped somebody.

I'm pretty good about putting "tab A" into "slot X". But I need a lot more experience and understanding before I start to hand out advice.

My best advice at the moment is to read past post. Just today I found another that had info that I needed.

The longer I'm working on this project the more I understand it (tip of the iceberg). I go back and reread some threads that I read 2 weeks ago and I start to get it...it starts to fit together in my head.

I wish you good success. :thmbsp:
martin

 

[dsndblm]

Thanks Martin,

You've done a fantastic job on your thread, and I was using it today while working on phase 1 of the restore. so far, I've got the whole first section finished except for 1 resistor, and one cap. those should be really easy though, and I'll do those tonight.

one thing I was looking for that I could not find, was the method to test after I've finished section 1. you know, to make sure the right resistor was put in. I used the 3.3 as the first attempt.

I'm no electrician, I need to know where to put the multimeter probes. where are the test points.

anyone? I'd love to fire her up tonight and test.

What you are after with the 3.3K bias resistor is to get more negative bias voltage. This voltage is measured on pin 6 of the 7591s. Fisher ran the 7591s really hard to get every last watt out of the receivers at the expense of tube life. My schematic calls for -17 volts DC. I measure -19 volts on my 500C after the rebuild with a 3.7K resistor that I ended up with.

But, what you really want to do is install the 1/4 watt, 10 ohm cathode resistors I hope were included with your rebuild kit. These resistors are installed from pin 5 on the 7591 tubes to ground. Check current flow across the resistor. The 7591s seem really happy and sound great at 29ma-35ma of current flow. Do you have a good meter to check the cathode current?

I'm typing fast, Wife and Son are waiting to head out for the weekend. I hope this helps and will check in later.

 

[keithpgdrb]

What you are after with the 3.3K bias resistor is to get more negative bias voltage. This voltage is measured on pin 6 of the 7591s. Fisher ran the 7591s really hard to get every last watt out of the receivers at the expense of tube life. My schematic calls for -17 volts DC. I measure -19 volts on my 500C after the rebuild with a 3.7K resistor that I ended up with.

But, what you really want to do is install the 1/4 watt, 10 ohm cathode resistors I hope were included with your rebuild kit. These resistors are installed from pin 5 on the 7591 tubes to ground. Check current flow across the resistor. The 7591s seem really happy and sound great at 29ma-35ma of current flow. Do you have a good meter to check the cathode current?

I'm typing fast, Wife and Son are waiting to head out for the weekend. I hope this helps and will check in later.

-When measuring voltage from pin 6, do I measure from pin 6 to ground? I'm doing some more reading today to find out testing procedures for this unit. If I dont, I'll be asking this type of question for every measurement.

-Yes, the 10ohm resistors for the 5th pin are included in the kit. I see now that the first thing metalbone recommends is to add those resistors. I must have missed that, as I was just doing the restore in the order it is in the instructions. That is the next thing I will do.

-Yes, I have a nice multimeter that should be able to do the job measuring the cathode current. I just have to look up how to set it up.

thanks for your help!!:thmbsp: