Hello folks,

I've a Zenith twin 6V6 power amplifier puller from an old Zenith 12H093 cabinet radio. Currently I've only the amp portion with me. I'm trying to convert it to an amp with a tube radio. I had previously also asked some questions about this amp when trying to restore it. As of now I've changed all the capacitors and the and resistors. Added a power cord and added an output transformer to couple it with a Zenith 8 ohm speaker. I'm testing it before making a case enclosure for it.

The issue now is that the music plays fine when fed from a smartphone. But there is a background hum along with it that can also be heard when the music is muted. If I touch my hand/ touch the ground of audio input, on the metallic can capacitor body (circled in the picture of the amplifier) the hum is completely gone and the amp is perfectly silent and even when playing music the hum is gone. I measured the voltage between the ground of audio input and the can capacitor body which also happens to be the negative terminal shows a voltage of -7.2V. My question is if it's okay to connect the ground (-) of the can capacitor to the body of the tube amp?

One more thing I noticed is that as I hold the can capacitor body tight with my hand the hum dies down and the audio becomes perfectly clear and louder. If I let my hand go the hum overwhelms and greatly reduced the music. Its like its getting drowned by the hum.

Thanks in advance.

Did you restuff the existing electrolytic, or mount the new capacitors on the bottom?
Is there still any connections to the old electrolytic can?

Sorry, I forgot to mention that critical information. The original capacitor is disconnected from the circuit. I left it there only to keep the original connection between the C2 (0.1uF) & R7 (200 ohm) on the capacitor negative lug and also to keep the original look . All the other lugs are disconnected. The replacement caps are mounted underneath.

Also I have added a small update:

One more thing I noticed is that as I hold the can capacitor body tight with my hand the hum dies down and the audio becomes perfectly clear and louder. If I let my hand go the hum overwhelms and greatly reduced the music. Its like its getting drowned by the hum.

You have to look at the full schematic. This radio had a convoluted design for grounding the power supply to minimize hum or other unwanted issues occurring when it was connected to the radio and phono chassis, and also to provide some negative bias in the radio chassis and to the audio circuits.

If you look at the partial schematic of only the power supply that you posted, there is NO ground connection from the negative side of the filter caps or power transformer CT or the common level bus in the chassis you have, going to the chassis. That connection was intentionally made at the other end of the interconnecting cable, through pin 4 of the connector and a wirewound resistor inside the radio chassis. It's impossible to read on the schematic, but could be R32 and 33Ω.

I believe you can get away with adding a wirewound resistor of the correct value from that point in the circuit, grounding it right at the audio input jack, making sure it's well soldered to the chassis. That should solve your issues. You cannot connect the negative side of the electrolytics to the chassis. That's not how this circuit was intended to be wired.

However, if you want to make a simple change, I believe that if you increase the cathode resistor of the 6V6's to around 270Ω and 5 watts wirewound, you can then connect the negative side of the electrolytics and that common bus to ground right at the audio input jack and not worry about the other resistor. Doing this change will increase the bias on the 6V6's enough to put them in the safe operating range.

Hello Dennis,

Thanks for the detailed response. Regarding the pin 4, you are right. If you see the complete schematic for the whole radio the pin 4 has R32 which is 33 ohms grounded. I'll attach the Page 1&2 of the full schematic below. BTW in the original schematic you will see the power amp section has a slight variation for speaker field coil. Mine is the permanent magnet version so the original post schematic is for that.

Now regarding changing the cathode resistor I will give it a try and see how that goes. But before that I would like to clarify something. When I used a multimeter to make measurements in DC mode (with multimeter negative lead on capacitor body) between the following points I got as below:

Audio input jack (+) & can capacitor body = 0V

Audio input jack (-) & can capacitor body = +9.2V

Now, when I hold my hands between the audio (-) & can capacitor body my body acts like a capacitor right? Because this thought gave me an idea. I tried to connect a film capacitor 600VDC 0.25MFD between the audio (-) & capacitor body and the hum died immediately. Do you think adding a film cap will solve the problem here.

Also I suspect the C4 450VDC 40MFD electrolytic cap to be leaky. Because when I went to buy replacement caps most shops around my place only sold up to 400VDC caps only. Only in one particular shop they had 450VDC. This particular shop had only two brand new 450VDC 47MFD caps, the last one was quite old like 2001 manufactured. Since it was old the shop guy gave it to me for free and charged only for two. I have a feeling that the C4 capacitor if leaky could induce AC 60Hz hum into the audio part right?

I've ordered a brand new cap just now.

The big question is: what do you intend to do with this amplifier? If you want to use it as a stand-alone amplifier, then connect the negative side of the filter capacitors to the chassis and the shield of the audio input.

On the other hand, if you intend to use it for it's original purpose, then follow the original schematic and use it as part of the radio. If you want to add an audio input, do it on the radio chassis.

Your body was mostly not acting as a capacitor but as a resistor.

Hello Jim,

I plan to use this as a stand alone amplifier. I don't have the remaining part of the radio. I'm feeding the audio directly to the amp from another ECC83 pre-amp.

Also as Dennis has mentioned I added the R32 33ohm resistor from the capacitor ground to audio shield. Now the hum is very very faint. The audio is fine now.

Also I would like to add a volume control to the amp. Can anyone please suggest a good volume control that can be added to this amp?

Your volume control needs to be part of the preamp, since this chassis does not actually have a low level gain stage. The 6J5 (or 6C4 in some late versions) only serves as a phase inverter.

Doesn't your preamp circuit include volume and tone controls?

My pre-amp is a single stage ECC83 with no volume control. The circuit for the pre-amp is attached below.

The tuner I've is a Pilot FA-540 tuner. The only volume control that I'v at present is on the "Level Set" knob at the back of the tuner.

Circuit diagram of the tuner is in link below for reference:

https://drive.google.com/drive/folders/1sA3hlHPTCeM0I6zk0MUVFfL8KI3Vtxd-?usp=sharing

That's the right place to put the volume control. I don't see a schematic attached, only the one for the tuner.

That's the right place to put the volume control. I don't see a schematic attached, only the one for the tuner.[/quote]

Sorry Jim, I missed that. Have attached it now.

can a grounded power cord be added to this?

For a volume control, remove the 1M resistor (R49?, the schematic is blurry) from the grid of the ECC83. In it's place, connect a 1M audio taper pot. Connect the grid of the tube to the arm of the pot instead of where it is connected now.

Well I don't remember exactly, but as per the schematics the Pin 7 is grounded. I think I should have added ground to that pin. I had followed the schematic exactly to make the power outlet connection.

Thanks for the information Jim. Yes, the resistor is R49. I'll add the 1M pot and give it a try. I'll post the result, once I get it done!