1967 Bassman General Maintenance

[saltydogg]

Gents,

I recently picked up a 1967 SF. While I'm no guitar tech by any means, I'd like to try some "basic" maintenance on the old dog.

1st is changing the 2-prong plug. Where can I get a correct gauge wire with a grounded plug for this amp?

Is there some kind of amp manual for such maintenance items like cleaning pots and caps and alike? Say: "Amp Maintenance for Dummies"?

Thanks for any insight you can provide!

[triodeamps]

First and foremost change the old PS caps. That is the heart of your amp. Bad PS caps will cook your PT and all voltages throughout the preamp and power stage will be higher. Eventually a cap will explode like a shotgun blast (Hence the Doghouse over the caps) Or the PT will drop dead shorted out. If you have ever seen some old fender amplifiers you will see hardened droplets hanging from the PT. This will be an amp where the owner didnt check or change the PS caps out before the PT (Cooking) melted the varnish on the PT. Other amps have potted transformers and they make a real mess and ooze out the potted wax when they are taking too much heat from bad PS caps. Food for thought....

[saltydogg]

Where can I buy the caps and are there any suggested types to keep the amp as close as possible to the original tone?

Also, getting back one of my original questions- what kind of literature or manual can I buy (or read online) to educate myself on amp parts? How about the damn 2-prong plug?

Generally, I would send the amp out for the servicing, however when I was told it would be in the shop for 6 weeks, I thought I should learn more about these repairs. Which means it will be sitting somewhere else for 5 weeks and 6 days. I am mechanically inclined and am familiar with soldering- so it shouldn't be too much of a hack job- I hope.

Thanks.

[s2amps]

Just an FYI...

When you change that 2 prong plug (which is a very good idea), you also need to cut out the "death cap" from the ground switch. The ground switch is now obsolete, so nothing should be connected to it.

Gerald Weber of Kendrick fame has some good books on tube amp maintenance. Just be sure to stay away from Torres, and you'll be fine :)

[Blue Strat]

The "death cap" is only a death cap with a 2 conductor, ungrounded, cord. You can leave it in when you install a grounded power cord. In some instances, the ground switch "can" reduce noise.

Spraque Atom electrolytic caps (www.mojotone.com (http://www.mojotone.com) and others) are the old standard replacement caps but others are ok too.

[doctorx]

Ted Weber has all the stuff you need. (http://www.tedweber.com/)

Here is a page on how to install a grounded cord. (http://www.regiscoyne.com/tech/3prongconversion/)

I would still pull the death cap.

I have a 67 Bassman too, check it out (http://www.regiscoyne.com/Blackface_bassman/).

[AL30]

For what it's worth. Here's the general maintenance I do on old Fender's.

1) Change the power cord. Lose the death cap.
2) Cap Job. This includes ALL electrolytics - Power Supply caps under the dog house, main board caps, and don't forget the bias supply cap. There is no reason to swap out the coupling caps (unless they're bad). The blue molded caps are fine (and getting pricey).
3) New resistors on the power tubes. There are two resistors on the back of the power tubes. These get a lot of heat. It's worth my peace of mind to change them. 470 ohm 2 watt and 1.5K 1/2 watt. I try and raise them a bit to keep them away from the heat.
4) Convert the bias balance to a common bias. Some people leave the balance AND convert.
5) Clean and retension the tube sockets
6) Check for any drifted components. Check the resistors under the doghouse and the resistor on the bias board - I usually just swap these out but it may not be necessary (keeps me from worrying about it).

Hoffman has a lot of good info and is a good place to order from. Fast Shipping.

http://www.hoffmanamps.com/ main page
http://www.el34world.com/charts/bias_conversions.htm
http://www.el34world.com/charts/fenderservice1.htm
http://www.el34world.com/charts/fenderservice7.htm
http://www.el34world.com/charts/fenderservice2.htm

There are other places to order from as well:

http://www.tubesandmore.com/
http://www.tedweber.com/
http://www.mouser.com/

What caps to use? Everyone has an opinion. Sprague Atoms have been the standard for a while but they are getting pricey. They are good caps. But you can also use F&T's, Illinois, and even Xicons. It's up to you. BE CAREFUL and have fun with it. Those are great amps.

AL

[doctorx]

Good post by Al. ^^

[saltydogg]

Good post by Al. ^^

I agree. Thank you all for your expertise! :bow

[AL30]

I agree. Thank you all for your expertise! :bow

Well now let's not get carried away here. I'm certainly glad the post helped you - or will help you. But, expertise? If you want expertise I would listen to everyone esle who posted. I'm just a hack with a solder iron and nice comic book collection. It's a tough juggling act. :rolleyes:

AL

[saltydogg]

For what it's worth. Here's the general maintenance I do on old Fender's.

1) Change the power cord. Lose the death cap.
2) Cap Job. This includes ALL electrolytics - Power Supply caps under the dog house, main board caps, and don't forget the bias supply cap. There is no reason to swap out the coupling caps (unless they're bad). The blue molded caps are fine (and getting pricey).
3) New resistors on the power tubes. There are two resistors on the back of the power tubes. These get a lot of heat. It's worth my peace of mind to change them. 470 ohm 2 watt and 1.5K 1/2 watt. I try and raise them a bit to keep them away from the heat.
4) Convert the bias balance to a common bias. Some people leave the balance AND convert.
5) Clean and retension the tube sockets
6) Check for any drifted components. Check the resistors under the doghouse and the resistor on the bias board - I usually just swap these out but it may not be necessary (keeps me from worrying about it).

Hoffman has a lot of good info and is a good place to order from. Fast Shipping.

http://www.hoffmanamps.com/ main page
http://www.el34world.com/charts/bias_conversions.htm
http://www.el34world.com/charts/fenderservice1.htm
http://www.el34world.com/charts/fenderservice7.htm
http://www.el34world.com/charts/fenderservice2.htm

There are other places to order from as well:

http://www.tubesandmore.com/
http://www.tedweber.com/
http://www.mouser.com/

What caps to use? Everyone has an opinion. Sprague Atoms have been the standard for a while but they are getting pricey. They are good caps. But you can also use F&T's, Illinois, and even Xicons. It's up to you. BE CAREFUL and have fun with it. Those are great amps.

AL

Clearly, this is not the advice of a hack. Though I'm still not over the comic book comment.??..?

[AL30]

Clearly, this is not the advice of a hack. Though I'm still not over the comic book comment.??..?

Well everyone has their vices. Buy a Giant Size X-Men #1 or buy a couple of amps? Tough choices. :NUTS - Sorry - it's the Batman thread in the Pub. I gotta stop reading that.


AL

[kingsxman]

4) Convert the bias balance to a common bias. Some people leave the balance AND convert.


AL

Al, how do you do this? I'm actually placing a parts order for my 1967 Fender bandmaster. Going to be replacing some caps and a few resistors. I would love to be able to set the bias on this also...

[AL30]

Al, how do you do this? I'm actually placing a parts order for my 1967 Fender bandmaster. Going to be replacing some caps and a few resistors. I would love to be able to set the bias on this also...

Here is just the bias conversion. Make sure you double check everything with your circuit before you order.

http://www.el34world.com/charts/bias_conversions.htm

AL

[kingsxman]

Here is just the bias conversion. Make sure you double check everything with your circuit before you order.

http://www.el34world.com/charts/bias_conversions.htm

AL

Thanks. Mine is the AB763 blackface circuit. What is the advantage to doing this mod?

[AL30]

Thanks. Mine is the AB763 blackface circuit. What is the advantage to doing this mod?

I think you may already have a bias circuit in there. Someone with more experience (and a better memory) may want to chime in on this.

The advantage of changing from the later balance system back to the bias system is your ability to actually bias the amp.

It's explained much better here: http://www.lynx.bc.ca/~jc/fenderBiasChange.html


... around 1968, CBS engineers thought they'd introduce a differential bias arrangement on the output stage as means of cancelling overall power tube plate-spec mismatch by pairing the grid circuits with a tunable resistive divider circuit ... a nice idea in theory, in practice it doesn't work and most techs replace that circuit by an earlier true-and-tried Fender equivalent circuits ... more importantly the common-mode bias (absolute voltage levels) cannot be adjusted ... this means the amp cannot be properly biased as in the earlier Fender stock circuit (the circuit most tube amps use anyway) ... especially in the case where you mod the signal path of your Fender Silverface amp for heightened tone you'll want to alter this circuit so the power tubes can properly biased (this is crucial in my opinion) ...

[doctorx]

Thanks. Mine is the AB763 blackface circuit. What is the advantage to doing this mod?

You probably already have it. I have a 65 Bandmaster, if yours looks like this than you are ok. Here is the schem and layout too. (http://www.ampwares.com/amp.asp?id=18)

http://www.regiscoyne.com/Blackface_Bandmaster/03%20Bandmaster-modded.jpg

[kingsxman]

You probably already have it. I have a 65 Bandmaster, if yours looks like this than you are ok. Here is the schem and layout too. (http://www.ampwares.com/amp.asp?id=18)

http://www.regiscoyne.com/Blackface_Bandmaster/03%20Bandmaster-modded.jpg
Yep.
So whats the bias proceedure? I take it that trim pot is the bias adustment?

[doctorx]

Yes the trim pot sets the bias. Read here how. (http://www.regiscoyne.com/tech/bias/)

[Blue Strat]

Yes the trim pot sets the bias. Read here how. (http://www.regiscoyne.com/tech/bias/)

Or if you don't want to read all of that, install a 1 ohm resistor on each of your power tube sockets, or risk killing your meter, tubes, amp or yourself using the Shunt Method....get a bias probe.

[kingsxman]

Or if you don't want to read all of that, install a 1 ohm resistor on each of your power tube sockets, or risk killing your meter, tubes, amp or yourself using the Shunt Method....get a bias probe.

I've got that weber bias rite adapter that fits in between tube and socket. I've biased 2 amps before with that. It shoudl work.

thx