The Amp Garage

Best youtube-video I've seen in a long time!

Just tried lifting the resistor and cap leads and run a wire to the speaker ground as you suggested. That actually made the hum worse; back to more or less the same level as when the OD section of the preamp ground was connected to the speaker/OT ground. Not sure why that would be, maybe some other...

you should beleive only to yourself not to some random grounding methods from the internet.. i never use any of the proposed gnd methods because they are often way out from optimum or full of mistakes. ha, now question how to do that? you are on the good way i see already.. and one day when you wil...

There is one part of the above experiment that won’t be optimum, and that is the phase inverter ground, at the junction of the 390 ohm resistor and 2.2 uF cap. That belongs to the power amp, and should be grounded where the OT secondary common is grounded. The short under-board jumper tying that ey...

I think you can try this as an experiment. Disconnect the wire at the red X, and add a jumper on the main board as shown by the red arch. This seems to be the least invasive way to test the theory. Once you do that, you can try different locations to ground the OT common. The location shown in the ...

Ok, will separate and ground the OD trim pot where you suggested.

But what are the "shields"?
And should I leave the bias ground where it is, or move to filter ground?

Grounding speaker jacks at or near the jack is always the best option. If you are using jacks that do not self ground just place a terminal solder lug at the floor of the chassis near the speaker jacks. Cheers Guy Thanks. Actually, that's where the speaker jacks were grounded. Maybe the bias pot gr...

martin manning wrote: ↑Thu May 02, 2024 1:22 pm I see reductions in the fundamental (50 Hz), and a significant reduction in the third harmonic (150 Hz).

Yes indeed. It just sounded much worse than it looked

Thanks again, Martin! Yes the OT secondary common is grounded at the speaker jacks. So I'll try connecting the speaker jack/OT secondary common to the mains filter ground first, and if that doesn't work, I'll try different locations. It's has been quite an extensive but ultimately satisfying process...

Ok, so I started chasing that last remnants of mains hum. 100 Hz hum in the speaker when all volumes turned down. Also present with preamp tubes removed, I concluded that the problem is not in the preamp. I measured AC ripple with my scopemeter before and after the choke. Before choke Before choke.b...

I would appreciate if you could check if my voltage readings are as they should be (quality check :) BTW, with 35 mV on the cathodes your bias is pretty cool at 0.035A X 432V = 15.1W, or just over 50% of max anode dissipation (30W). You could bring that up to 60%, around 42 mV. Thanks! I didn't do ...

It's possible that the magnetic field from the choke was inducing current in the OT, as it receives a pretty noisy DC voltage with a sawtooth waveform on it, or maybe more likely that the original OT was not doing a very good job of cancelling the AC on the B+1 feeding into the primary center tap. ...

In the quest of getting rid of humming noise, I finally got the C-lator to work properly and started working on the amp. I got the Mercury Magnetics transformers and choke. The reason I wanted to change transformers was that the amp made an AC hum through the speakers even in standby mode and also w...

martin manning wrote: ↑Mon Apr 01, 2024 12:06 pm It won't hurt to oversize the PT a bit, and yes torroids are better behaved re core vibration.

Got those toroidals and installed them today. Works perfectly! No vibrations at all! Those 25 years old Events are rescued and sounds good!

Thank you once again, Martin, for your advice