Vintage amp maintenance and value.

[HSB0531]

Does replacing old dried out electrolytic capacitors change the value of a vintage amp?

Specifically a 1967 Ampeg BT-15.

[ArnieZ]

If the amp is going to be used replace them and keep the old ones for the buyer

ArnieZ

[gorch]

Any need to do this? Never touch a running system!

[HSB0531]

26 minutes ago, ArnieZ said:

If the amp is going to be used replace them and keep the old ones for the buyer

ArnieZ

That was exactly what I was thinking. That way they they could replace the originals.

[HSB0531]

24 minutes ago, gorch said:

Any need to do this? Never touch a running system!

Yes, the amp has a 60 cycle hum (p/s filter caps) and the amp fizzles and pops randomly (tone circuit caps - electrolytic and ceramic).

I learned Not to touch a running or non running system when I was 12.

I had my Ampeg Jet amp chassis open and I had my thumb on the chassis and I accidentally touched the business end of a 600 volt filter cap.

It was unplugged, but caps hold a charge for quite a while.

Thank god for quick reflexes 

[chromium]

You hit the nail on the head with the term "maintenance".  As for vintage value, IMO, it would kinda be like running a '70 Chevelle on its original, worn out brake pads - just because they are original.

After about a decade, the dialectic material between the plates inside of electrolytic caps dries out, and they can fall way out of spec. Being that they serve important jobs, such as filtering unwanted ripple out of the power supply, replacing them can reduce noise, tighten up bass response, and prevent failure of more expensive parts downstream.

Most of the stuff I work on is low voltage DC (old effect units, tape echoes, keyboards, etc), and I always start by replacing the power filter and decoupling caps for this very reason.  Like Arnie stated, I just bag up the original caps in case some buyer down the road is concerned about preserving "vintage integrity".  I would rather have the stuff working tip-top, and I can't see a reason anyone would ever want to put those old caps back in.

[Jakeboy]

Chromium nails it perfectly.

[HSB0531]

3 hours ago, chromium said:

You hit the nail on the head with the term "maintenance".  As for vintage value, IMO, it would kinda be like running a '70 Chevelle on its original, worn out brake pads - just because they are original.

After about a decade, the dialectic material between the plates inside of electrolytic caps dries out, and they can fall way out of spec. Being that they serve important jobs, such as filtering unwanted ripple out of the power supply, replacing them can reduce noise, tighten up bass response, and prevent failure of more expensive parts downstream.

Most of the stuff I work on is low voltage DC (old effect units, tape echoes, keyboards, etc), and I always start by replacing the power filter and decoupling caps for this very reason.  Like Arnie stated, I just bag up the original caps in case some buyer down the road is concerned about preserving "vintage integrity".  I would rather have the stuff working tip-top, and I can't see a reason anyone would ever want to put those old caps back in.

You, ArnieZ, Velrush etc have the right idea.

I'm going to replace all the bad caps, bag the originals and clean all the pots and switches.

 

[Hfan]

Great advice, and an easy way to discharge the amp is to power it up with a load (guitar) attached, then pull the plug from the wall outlet.  Then to confirm it is discharged you can check voltages from chassis ground to the electrolytic / filter caps, and the tube connections if you want. Happy soldering.

[chromium]

If it helps, I came a across a nice video recently when I was digging through Bill M's extremely helpful site on Blues Jr. mods.  This shows first hand how tube amps discharge, and a provides a few helpful tips.  It's never a bad idea to verify with a DVM before digging in, but there's really nothing to fear if you take these precautions.

Setting bias while the amp is lit up still puts me on edge, though...  

 

[HSB0531]

58 minutes ago, chromium said:

If it helps, I came a across a nice video recently when I was digging through Bill M's extremely helpful site on Blues Jr. mods.  This shows first hand how tube amps discharge, and a provides a few helpful tips.  It's never a bad idea to verify with a DVM before digging in, but there's really nothing to fear if you take these precautions.

Setting bias while the amp is lit up still puts me on edge, though...  

 

Thank you.

Although the BT15 is solid state, there's a schematic I dug up that shows how to set the bias.

Theres a varistor on the PCB that needs to be set to a specific voltage.

I have to check, but I believe they said with a specific signal level at the input.

I own a DVM, an audio analyzer (Impulse response, time delay, frequency distortion etc) and an NTi MR-Pro  http://shop.nti-audio.com/pd_minirator_mr-procfm.cfm So I have the test gear covered.

I Do hate being around high voltage and current though.

Just have to be careful. Bzzzzttt!!!!!

BTW: if you don't see me posting after next week, I done went and fried myself ZZZZTT

 

[chromium]

1 minute ago, HSB0531 said:

Although the BT15 is solid state

Ah ok - I wasn't familiar with that model... just assumed it was a tube amp.

Less to worry about working on solid state, and you don't have to deal with setting bias for power tubes, etc.  I usually just make sure the big power filter caps are completely discharged using a test lead w/resistor wired into it - same as he demonstrates in the video.

There is a strong possibility that recapping that power supply will help address the 60-cycle hum you are hearing.  That's where I would start, at least, and then check the outgoing DC voltage(s) to ensure that they are within spec (depending on the power supply design, there may be trimmers to adjust that).

[HSB0531]

8 hours ago, chromium said:

Ah ok - I wasn't familiar with that model... just assumed it was a tube amp.

Less to worry about working on solid state, and you don't have to deal with setting bias for power tubes, etc.  I usually just make sure the big power filter caps are completely discharged using a test lead w/resistor wired into it - same as he demonstrates in the video.

There is a strong possibility that recapping that power supply will help address the 60-cycle hum you are hearing.  That's where I would start, at least, and then check the outgoing DC voltage(s) to ensure that they are within spec (depending on the power supply design, there may be trimmers to adjust that).

You got it exactly right on all points.

I was very surprised when I saw no tubes.

Ampeg was known for tube amps for as long as I can remember in the 50's through the 70's.

I can check voltage on all caps, at the leads with the DVM before Unsoldering.

yes, 60 cycle hum on a SS amp is directly related to bad caps, but I'm also going to test the rectifier diodes.

Yes there's actually a trimmer resistor to set bias.

You see bias adjustments on tube amps all the time (some auto-bias).

This will be a fun project.

Pics to follow.