Ampeg M-12 project

[sleepingAwake]

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.

The internode resistors are definitely there. I had to replace one of them that blew when the old can cap shorted internally. The other 2 tested in spec and I re-used them.
It may just be my meter.

When I pull the phase inverter tube all sound goes away.

I dont believe the amp came with a center tap or at least I didnt see one so I added one on the pilot light. When I measure AC on each side I get 3.0vac and 2.7vac. No change was noted in the hum.


I'll say I may be wrong and it might be 60Hz hum. I made a video to demonstrate. You might have to turn the volume up to hear it.
Hopefully it offers clarification. When I touch the preamp tube near the base the hum increases. Same thing with placing my fingers near the input jacks.

[Tom Phillips]

Good job with that posting.
After hearing your demo I must say that the hum pickup sounds like it could be normal for that amp.
It's a low gain amp that was designed without shorting jacks at the input.
Does the hum diminish if you plug a guitar into the ACC 1 input AND turn the guitar's volume control all the way down?

[sleepingAwake]

Does the hum diminish if you plug a guitar into the ACC 1 input AND turn the guitar's volume control all the way down?

Oh no. It does. Don't even tell me thats normal.
I cant deal. I'll have to fix it.

[sleepingAwake]

The thing is even with my guitar plugged in and it's volume up the hum is very loud.
I've never heard an amp hum so much with a guitar plugged in.

I should note the video is quiet compared to real life.

[sleepingAwake]

So I made another video showcasing the hum the amp makes as I dont think the last one really captures it well enough.

[sleepingAwake]

Well I guess I'll consider it normal for now. Although I do plan on adding shorting jacks and some shielded wire in addition to twisting the heater wires.

What does everyone think of my voltage readings? Valve Data Recording Program

The amp sounds great but breaks up very early and it seems to me my voltages are low. I'd love to get just a little more clean headroom out of it.

[stokes]

You say you "added" a CT on your heaters.What did you use?What was there before you did that?Does it have a hum-balance pot?Your heaters dont seem to be balanced right now.You said you are getting different voltages on each side.They should match,like 3.2 on each side.

[guitician]

6v?

6v on v1's cathodes seems odd to me with a 270k plate resistor and 253v. You may want to check the ground on the 2.2k ohm resistor and 100uf capacitor.
Another thing to check is how the input jacks are wired. I had an M-12 that had them mis-wired from Ampeg.

[sleepingAwake]

You say you "added" a CT on your heaters.What did you use?What was there before you did that?Does it have a hum-balance pot?Your heaters dont seem to be balanced right now.You said you are getting different voltages on each side.They should match,like 3.2 on each side.

This amp did not come equipped with a hum balance pot.
However I think I found a center tap although I'm unsure.

The artificial center tap I added is on the back of the pilot light using two 100 ohm resistors grounded to the same spot.

[sleepingAwake]

6v on v1's cathodes seems odd to me with a 270k plate resistor and 253v. You may want to check the ground on the 2.2k ohm resistor and 100uf capacitor.
Another thing to check is how the input jacks are wired. I had an M-12 that had them mis-wired from Ampeg.

I'll recheck the grounds.
I've reflowed all the solder joints as whoever serviced the amp clearly had no clue.

The input jacks are wired closely to the joe piazza schematics.
I do plan on changing them some. My humbuckers are way to hot for the current configuration.

To me the power tube voltages all seemed low as well.

Is there a page that lists what the actual voltages should be?
I came up short other than specs for the tubes.
That doesn't really tell me what ampeg intended for cathode voltage and bias.

[guitician]

The v1 cathode resistor value would be worth checking also. The voltages for a cathode biased P-P 6V6 seem normal, around 300-330v. That hum seems normal to me too. Having the foil shielded back cover on it may help some there.

[sleepingAwake]

The v1 cathode resistor value would be worth checking also. The voltages for a cathode biased P-P 6V6 seem normal, around 300-330v. That hum seems normal to me too. Having the foil shielded back cover on it may help some there.

I'm going to rewire the inputs with some shielded wire and twist the heater wires when I get a chance.
I'll try covering the back panel in foil too. Seems like a decent idea.

I'll see if I check the preamp voltages/components again later tonight.

[sleepingAwake]

Shame on me for never replying.

I've yet to add a hum balance pot or work on the amp for that matter.
I decided to play it yesterday and realized how disappointed in its tone I was... and the noise it has.
I must be spoiled by my Bassman Ten.

So I'm thinking of doing something about the tone and taking another crack at quieting her down. It breaks up way too soon for me. I am using a guitar with humbuckers.
Also it produces a ton of heat. More so than I think is normal.

If memory serves both output tube cathodes are tied to the same resistor. I'm thinking I will split them apart, do something about the lack of high-end, lower the gain, address the heat issue, and add a hum balance pot.

[olddawg]

Shame on me for never replying.

I've yet to add a hum balance pot or work on the amp for that matter.
I decided to play it yesterday and realized how disappointed in its tone I was... and the noise it has.
I must be spoiled by my Bassman Ten.

So I'm thinking of doing something about the tone and taking another crack at quieting her down. It breaks up way too soon for me. I am using a guitar with humbuckers.
Also it produces a ton of heat. More so than I think is normal.

If memory serves both output tube cathodes are tied to the same resistor. I'm thinking I will split them apart, do something about the lack of high-end, lower the gain, address the heat issue, and add a hum balance pot.

My 1960 Ampeg Rocket has the same tube complement and is bone stock. I have never really cared for the amp. It breaks up early and the overdriven sound is not great. It was probably great for accordion. Try running it through a different speaker cab. One of these days I'm going to just gut it, put the innards on a shelf, and build a sleeper in the ugly cab, lol. I pulled mine out of a dumpster...

[sleepingAwake]

My 1960 Ampeg Rocket has the same tube complement and is bone stock. I have never really cared for the amp. It breaks up early and the overdriven sound is not great. It was probably great for accordion. Try running it through a different speaker cab. One of these days I'm going to just gut it, put the innards on a shelf, and build a sleeper in the ugly cab, lol. I pulled mine out of a dumpster...

I'd rather just sell it than tear it down and rebuild it to sound like a fender.
But I'll see what I can get out of it with some simple mods.