Ampeg M-12 project

[sleepingAwake]

I'm looking for an opinion since I'm still new to the game here.

I just picked this M-12 up and obviously its not original.
The cabinet has been recovered once and the speaker grill material looks newer.
The Jensen C12R is also obviously not original.

The previous owner stated it was working fine and then one day let the smoke out.
The 1K 10W resistor is cracked and was probably where the "smoke" came from. However a lot of the paper capacitors dont look so hot and a couple look like they have split.

The previous owner also stated that Jess Oliver was the last one to work on this amp but I dont see it. There are several bad solder joints and some just plain scary things going on.
You cant see it in the photos but the hot wire attached to the fuse is basically just jammed in there with one strand of wire actually being soldered.

Basically I like to go with original components and a factory look when possible but I think this one is a lost cause.
I'm leaning towards a full rebuild. All new carbon comp resistors and all new caps everywhere.
What do you guys think? Any point in trying to keep some of those paper caps? Among-st other things.

[olddawg]

I have a 1960 Ampeg Rocket in worse shape than that one. I've been trying to talk myself into rebuilding it as a 18 watt Marshall clone with 6v6 outputs. Won't be worth much either way.

[sleepingAwake]

I pulled the trigger on the parts for a complete rebuild.
I'm gonna keep this one I think.

Otherwise if I sell it whoever buys it will get a reliable amp that should last another 30 years.

[sleepingAwake]

Well the magic smoke came from the 1k 5watt resistor. The can cap had a leg short to ground internally.

So after replacing those items and cleaning up a couple of bad solder joints it works!
But it has a bad amount of hum.

I added a center tap to the heater supply for poops and ha ha's no change with the hum.
I replaced a bunch of the astron caps as some seem to have melted partially.
Some of the carbon comp resistors drifted up and I replaced them as well.

I still need to replace the 2 bypass caps.

I think it's 120hz hum as it sounds the same as when touching the tip of the guitar cable.
When putting your hand near the first preamp tube and the input jack area the hum gets worse.
Swapping tubes has no effect.

[52 Bill]

Well the magic smoke came from the 1k 5watt resistor. The can cap had a leg short to ground internally.

So after replacing those items and cleaning up a couple of bad solder joints it works!
But it has a bad amount of hum.

And when you replaced the filter cap did you get a new can cap or did you go with individual caps? Did you ground the caps back at the original circuit points?

[sleepingAwake]

And when you replaced the filter cap did you get a new can cap or did you go with individual caps? Did you ground the caps back at the original circuit points?

New can cap. I didn't think there was really enough room on this one to use individual caps and stuff them in the chassis.
Yes the caps are grounded at the original circuit points.

Its actually hard to deviate from the factory wire routing as it has friggin glue all over it. Thanks ampeg!

[Jazz P Bass]

I would recommend actually measuring the Vac that is on the power caps.
I have seen brand new 'can caps' that have bad sections.
If you find a cap that has a lot of Vac on it, parallel a known good one over it.
If the Vac ripple drops, that cap is bad.

[g1]

I think it's 120hz hum as it sounds the same as when touching the tip of the guitar cable.
When putting your hand near the first preamp tube and the input jack area the hum gets worse.

Wouldn't that make it 60Hz hum?

[sleepingAwake]

Wouldn't that make it 60Hz hum?

I thought I read on here that was 120hz hum???

Jazz measuring for vac on the filter can was exactly what I was thinking of last night but unfortunately I don't have a scope.
I don't have a good filter cap laying around either.

[Tom Phillips]

...measuring for vac on the filter can was exactly what I was thinking of last night but unfortunately I don't have a scope...

You don't need a scope if you have a good meter. You just use the meter to measure both the AC component and DC component of the voltage at a given node. The percent ripple is the AC voltage reading divided by the DC voltage reading with the result converted to %.

I say "Good meter" because many inexpensive meters can't make the AC reading in the presence of high DC. It doesn't damage the meter (usually). It just gives a wacky high reading. This has been discussed in several other threads on the forum.

Tom

[sleepingAwake]

You don't need a scope of you have a good meter. You just use the meter to measure both the AC component and DC component of the voltage at a given node. The percent ripple is the AC voltage reading divided by the DC voltage reading with the result converted to %.

I say "Good meter" because many inexpensive meters can't make the AC reading in the presence of high DC. It doesn't damage the meter (usually). It just gives a wacky high reading. This has been discussed in several other threads on the forum.

Tom

Well I'm not sure my meter is that great.
It's a craftsman model 82140

Using the method you've suggested Tom I get a reading of around 2.2% ripple on all 4 taps of the can.

I actually found some old caps from a bassman I fixed up and they hold a charge.
Jumpering them across the nodes offered no difference in hum. I'm not sure if that part was a waste of time.

[sleepingAwake]

Also moving the heater wires away from the chassis and elevating them offered no difference. They are however only twisted for the first three inches coming off the lamp.

[Tom Phillips]

Well I'm not sure my meter is that great. It's a craftsman model 8214. Using the method you've suggested Tom I get a reading of around 2.2% ripple on all 4 taps of the can...

2 to 3% is the approximate ripple that I’d expect to measure on the first stage filter node so it appears that your meter is up to doing this task. When there is a meter problem the results are way way off. However, the ripple on the down line filter nodes should be much lower. I.e. just a fraction of 1%.
Does the position of the volume and tone controls affect the level of hum?

[sleepingAwake]

2 to 3% is the approximate ripple that I’d expect to measure on the first stage filter node so it appears that your meter is up to doing this task. When there is a meter problem the results are way way off. However, the ripple on the down line filter nodes should be much lower. I.e. just a fraction of 1%.
Does the position of the volume and tone controls affect the level of hum?

I actually measured 2.0-2.2% at each node. But who knows that could be my meter and its inability to be as accurate as I need.
The increases with intensity as I up the volume on channel 1 and 2. On channel one the hum has more treble to it and channel 2 has a deeper tone to the hum. Manipulating the tone controls seem to just roll off the highs of the hum.

I'm thinking something may be up with the jack wiring. Someone has been in there before. Don't let that steer anyone else away from suggesting something else. I'm probably the last opinion to trust.

[Tom Phillips]

The first thing I'd be investigating is why the ripple on nodes 2 & 3 is so high. It's almost as if the internode resistors are not there or the 2nd/ 3rd cap sections are open. The absolute ripple measurement accuracy (2% vs. 2.2%) is not important. The fact that it doesn't drop to somethng like 0.01% at the pre-amp node is the red flag.

If you pull the phase inverter tube I'd expect that the hum will be gone or very low.
Since the volume control affects the hum we know that at least part of it is getting in through the pre-amp.

Have you been able to determine if it's 60 Hz or 120 Hz hum. Is it clear low hum or some buzz too? I expect that it's 120 Hz but you should check that the 6.3 V heater center tap circuit is correctly working. Measure from each side of the heater circuit to ground. You should read equal voltages on each side of ~3.2V.