Ground questions st70/SP8

I have been working through some noise issues with my setup. Some of the noise I was experiencing was related to bad power tubes, and some additional noise was microphonic preamp tubes. I also discovered I was getting a hum from setting my st70 on top of the preamp. Now that I have separated the pieces and fixed the above problems, I am still getting a slight hum.

With the ST70 not hooked up to anything, you can only tell it is on by a very slight hum only audible in front of the speakers. I am hoping I can get it this quiet with the sp8 hooked up as well.
With the SP8 hooked up, everything on but with the volume off, there is a decent amount of hum noticeable from my listening chair on the phono side, and on the other selections it is less. I expect the phono to be the noisiest position, but I hope I can cut this down some more.

Things I think may be the problem:

I have the grounds hooked up with a buss bar between each RCA, and both wire going to the output have the grounds attached at both the buss bar and on the PCB. Each input wire has a wire attached to the buss bar and wrapped around the input wire but cut off at the selector switch. the output from the balance control is connected to the balance control ground on one side, and the PCB on both sides. There are also ground wires going from the volume control ground to the PCB ground, and a wire off of this ground to the chassis. The chassis is not connected to the ground at the power jack, so the 3rd leg of the power cable is floating. The phono grounds are both connected at the PCB, and the phono ground is connected to the chassis.

I cut the ground from the power cable because this reduced hum and a buzz from the connection to my TV.

The hum level drops slightly hen I touch the SP8 Chassis.

My shielded phono input wires are about 8" long, as I built this in a larger case.


When I touch my ST70, the hum is also slightly reduced.

My st 70 has the original 2 prong power cable.

This hum I am hearing from the SP8 is not affected by the volume setting, which means to me it is occurring after the phono stage and probably related to the output wiring, the wiring of the volume and balance control, and the power cable grounding.

Is the circuit ground supposed to have continuity with the chassis?

I am also thinking maybe the st70 needs a more modern power cable and needs to be grounded. Maybe I am also getting some noise from the original st70 input jacks..

Any ideas?

I can only relate some of the issues I had to work through with my ST70 and Mark IIIs. You did not mention whether you had recapped it especially the multi section cap which can be troublesome. The input and output jacks should be isolated from the chassis (I learned this the hard way) with the ground side of the input jacks wired directly into eyelets on the PC board. You can't rely on the chassis to act as a ground...again, inputs and outputs should be isolated. I kept my two-prong cords. I had bad hum on the ST70 and kept blowing fuses and trashing tubes on the Mark III until I resolved the chassis ground issues. Good luck.

That sentence I wrote is a little confusing! I should have said the RCA phono grounds go to the board, and the single ground attachment for the phono goes to the chassis.

The st70 is wired with the jacks grounded to the vta board. I have the original input and output connections on the st70. The only other remaining original parts on the st70 are the transformers, choke, chassis and tube sockets (they were in good shape so I just cleaned and retensioned)

Are the signal grounds (from the rca jacks, etc.)on the SP8 supposed to tie to chassis ground?

I figured out what the problem is/was...

I am an idiot!

Kidding, sort of.

It was the 12au7 tubes in preamp.

I come from the guitar amp world, and it seems that over there humming tubes are maybe less noticeable/more acceptable.

Every issue I have had with my VTA stuff has ended up being the tubes. I haven't/don't have the money for good old tubes, and was assuming that I was doing better using the tubes I have than buying JJs or EH.

I bought a lot of 20 RCA long plate 12AU7 tubes on ebay, that were sold as untested for $30, which I thought was a good buy if even one or two worked out okay. It turns out that almost all of them were very noisy, and I assumed it must be the wiring I did, grounding, something else, rather than going to the source.


For anyone else going through hum problems, I determined it was the tubes by grounding the input jacks of the ST70, and listening. It was silent. I then plugged in the SP8 but didnt turn it on. No change. I turned the SP8 on, but left the volume off. This is when the hum began, and at this point I realized that the 12au7 tubes were after the volume control in the circuit. I switched to a different pair and the hum increased, meaning the hum is related to the tubes. After about 10 tube swaps, I found a pair that are acceptable. I think I may ask for a pair of new tubes for christmas though....

As always, thanks for your help.

Lesson learned, Bob!

I think I also let the brand name cloud my judgement. My st70 had the original XF3 Mullard tubes, and with how much praise people blamed on those, it was hard to think that they could have been the cause of my other problems, but when I bought new EH 6CA7 tubes, it sounded significantly better.

I found some much more acceptable 12au7s, and this is really sounding nice now.

Bob's description of tube sellers on ebay is 100% accurate, especially when tubes are sold in quantities as untested.
Those are the bad tubes they are trying to get rid of !!