Old Vintage Amp Servicing?

[big_teee]

Just some vintage questions on what gets done on old Amps.
I have some questions on working on old amps.
Do you always put 3 wire cords on amps with 2 wire cords and ground switches?
What is the normal on this.
Do you always replace all the Electrolytic Caps on old vintage amps.
What is the norm on what is changed.
Once you start replacing things on old Amps, where do you stop?
I have been asked to check and look over some old amps for a vintage shop.
I want to know primarily about what is the safety shock requirements.
These vintage shops are so concerned about keeping everything Original, but safety is most important.
I am a retired tech, just not on old amps.
Thanks,
Terry

[mikeboone]

Caps are a pretty standard thing to change. They may work OK but it just good policy for a variety of reasons.

Power cords are a dicy subject for me. The liability is tremendous if you do it and it's wrong and someone gets electrocuted. I don't repair old amp with bad power cords. I repair stuff in my home and do not have a separate policy to cover that liability.

[Resonator Guy]

It all depends on what the amp is to be used for. If it is going to sit on a shelf in a "collection" then I just dust off the tolex. If it is going to be played, I put on a three prong cord and replace anything that isn't functioning properly. My amps all get used and I want them to be safe and quiet. I don't have any 50s tweed Fenders, but a lot of Gibsons.

[soundguruman]

Just some vintage questions on what gets done on old Amps.
I have some questions on working on old amps.
Do you always put 3 wire cords on amps with 2 wire cords and ground switches?

YES, It's REQUIRED by our electrical Safety Codes, If you are going to "use" the amplifier.

What is the normal on this.
Do you always replace all the Electrolytic Caps on old vintage amps.
What is the norm on what is changed.
Once you start replacing things on old Amps, where do you stop?

We stop when the amp is safe and functional, we try not to over-do it.

I have been asked to check and look over some old amps for a vintage shop.
I want to know primarily about what is the safety shock requirements.
These vintage shops are so concerned about keeping everything Original, but safety is most important.
I am a retired tech, just not on old amps.

Thanks,
Terry

The main thing to look for is that the heater / filament circuit in old amplifiers is usually grounded to the chassis on one leg.
The circuit must be lifted from the chassis, so that the heater power is no longer referenced to ground.
(the filament can be balanced to ground with two 100 ohm resistors added, like a modern Fender Amp)
ALSO on antique amplifiers, sometimes one power leg is tied to the chassis, this has to be lifted also. You will find that in really old equipment.
It might be nice to "leakage test" the transformer insulation with a HIPOT tester...
Then the chassis can be grounded to the three prong plug.

THE DEATH CAP: ties the power line to the chassis, at the "ground switch."
The DEATH CAP should be REMOVED always. It can be replaced with a newer type UL tested / approved ground bypass capacitor. Although with a three prong plug and ground wire, it is really not needed any longer.

If the amp is "just" a museum piece, then the three prong modifications can be "skipped." However, I must WARN you that an amp built in
the old style can be quite dangerous, until it is updated as above.

Considering old electrolytics, I just fixed a 56 Champ amp, and the electrolytics still worked fine. BUT quite often we do replace them ALL.
This is because they become leaky, and draw too much current from the power transformer.

[Jazz P Bass]

I think, as far as the ungrounded 2 prong power cord goes, that the relevant authorities want this type of hookup off the planet. (as applied to vintage amplifiers)

[Tom Phillips]

J...I have been asked to check and look over some old amps for a vintage shop...

I think the shop needs to be more specific about what they mean by “check” and “look over.” Also what do they consider as vintage? Are you talking 50+ year old amps or just anything before 1990 with tubes in it.

Most shops just want to make sure that the amp is safe and that it works well enough to be demonstrated on the sales floor. An amp that doesn't work is a big unknown. A vintage amp with a few crackle noises is not a big concern to a knowledgeable buyer. The usual approach is to do no harm and leave the full overhaul and restoration to the buyer. The new owner can then decide what should be done to the amp after they take possession. As with many used items the cost of pre-sales work cannot usually be recovered by the seller. However, repair of basic problems to get the amp working and a checkout sheet noting that all the major parts such as transformers are original and no mods have been done can really help the sale. Cleaning and replacement of missing parts are important too. A vintage knob or fuse cap from you stash could make the amp much more attractive to a buyer.

[big_teee]

Thanks everyone.
You are confirming my own views of it all.
First up is a couple of Silvertone 1483s, I think in Head form.
I included the Schematic.
so I could just cut the Death cap loose at the AC, and insulate and leave for the original look.
Then install a 3 wire Cord.
Can someone explain the coil, tied to the Speaker wiring, the coil, maybe a choke right below the speaker wires?
For what purpose is the coil?
Thanks,
T
silvertone1483.pdf

[Tom Phillips]

The purpose of the coil was to roll off the low frequency content reaching the speaker. A very crude & unconventional design approach that puts extra load on the power amp at low frequencies. If it were my amp I would eliminate the choke across the speaker and upgrade to a better speaker. Then, if necessary, adjust the low frequency response of the amp via more conventional means.

[jrfrond]

Old electrolytic caps are just waiting to go bad, I don't care how they test. If the amp is 25+ years-old, they need to be changed as part of a normal restoration, period.

Plate resistors that are noisy need to be changed. Otherwise, leave them alone. Those may or may NOT go bad. This is highly-dependent on the moisture levels that the amp has seen over the years, as carbon-comp resistors are hygroscopic (Google that).

Proper restoration work demands that you use good quality as well as the correct type of components. Replace old carbon comps with new carbon comps, and use good electrolytic caps of the right package type (i.e. axial or radial). I use mostly F & T caps.

A two-wire AC cable, especially with a death cap, violates all current safety codes and MUST be upgraded, as it could be a matter of life and death. If customer hassles me on this, I refuse to work on the amp, regardless of what vintage-lover hyperbole they might spout in my direction. It's that simple. I don't want to be the last name connected with the inner workings of a potentially dangerous piece of gear.

[R.G.]

A two-wire AC cable, especially with a death cap, violates all current safety codes and MUST be upgraded, as it could be a matter of life and death. If customer hassles me on this, I refuse to work on the amp, regardless of what vintage-lover hyperbole they might spout in my direction. It's that simple. I don't want to be the last name connected with the inner workings of a potentially dangerous piece of gear.

Good point. Given our legal system, I can see a widow suing a service person for agreeing to work on an amp and NOT insisting on correcting it to safe 3-wire AC.

[big_teee]

Good point. Given our legal system, I can see a widow suing a service person for agreeing to work on an amp and NOT insisting on correcting it to safe 3-wire AC.

That is exactly why I asked the question.
I will have a talk with the Vintage shop owner.
We will at least fix the AC issues, or I will not work on them.
I don't want my widow to be suing either.
So what size 3 wire cable for a 2 Amp 50 watt amp?
Mojo Sells a 18/3, in 6 foot and 10 foot.
Probably my choice would be 16 gauge, but for 2 Amps the 18/3 should be large enough.
Also any 3 wire cord may require a larger hole and larger strain relief grommet?
T

[Tom Phillips]

My opinion:
18 AWG is large enough for just about any "normal" length (Up to ~12 feel long) guitar amp line cord.
16 AWG or larger is just a hassle fitting the strain relief and internal connections with very very little added bennifit.
Very high powered bass and sound reinforcement amps are another case but I don't think they are part of this discussion.
Tom

[soundmasterg]

One thing to keep in mind about 3 wire conversions of old amps is that people often do it wrong. The vintage amps usually had the fuse and switch on opposite sides of the AC line, and it should be done to where the neutral (white) wire goes direct to the power transformer, and the hot (black) wire goes through a fuse and switch before it gets to the other side of the power transformer. And also to be perfectly safe, the green wire should go on it's own bolt to the chassis instead of a transformer bolt, which means that you have to drill a hole in most amps to mount the bolt and solder terminal. Small price to pay for being safe though!

greg

[Gtr_tech]

carbon-comp resistors are hygroscopic (Google that).

So is most Ampex and 3M tape made from the mid 70s up thru the late 80s

[R.G.]

So what size 3 wire cable for a 2 Amp 50 watt amp?
Mojo Sells a 18/3, in 6 foot and 10 foot.
Probably my choice would be 16 gauge, but for 2 Amps the 18/3 should be large enough.
Also any 3 wire cord may require a larger hole and larger strain relief grommet?

See:
American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies
Conductor ampacity : PHYSICS OF CONDUCTORS AND INSULATORS

As a practical matter, 18AWG power cords are good for about 5-7A in normal use with normal thermoplastic insulation. The issue is not one of will it carry the current, but will it heat up noticeably with the current and degrade the insulation. 18/3 should be good for almost any non-mega amplifier with common kinds of wire/cable insulation. 16 is not needed unless you just like it.