Dynaco SCA-35 hum/ground issue

Hi everyone!

I'm new to the tube-world (and to this forum). I recently aquired a Dynaco SCA-35 that has a hum/ground issue. It worked fine at the sellers place, although he only demostrated the unit playing Radio and CD - but assured the Phono-stage was just as flawless.

At home I hook up my turntable and immediately there is a hum/ground issue present in both speakers. It increases past 12 'o clock on the volume knob. And goes way by unplugging the turntable (both power and rca-cables). No hum/ground issue present when on Tape/Radio/Spare. I begin with cleaning the unit from dust and Deoxit all pots & tube sockets. That didn't help (but well overdue).

So it's narrowed down to the Phono-stage. So how should I proceed? It has the original tubes from when it was assembled. Should I begin with two new sets of 12AX7 tubes? Or could this be a "deeper" problem? Longterm I'm planning on upgrading this unit with available mods. But for now I just need this hum/ground issue to disappear. Any thoughts?

Edit: I forgot to clarify that the hum is present at Phono with nothing connected as well.

Vimmy wrote:Hi everyone!

I'm new to the tube-world (and to this forum). I recently aquired a Dynaco SCA-35 that has a hum/ground issue. It worked fine at the sellers place, although he only demostrated the unit playing Radio and CD - but assured the Phono-stage was just as flawless.

At home I hook up my turntable and immediately there is a hum/ground issue present in both speakers. It increases past 12 'o clock on the volume knob. And goes way by unplugging the turntable (both power and rca-cables). No hum/ground issue present when on Tape/Radio/Spare. I begin with cleaning the unit from dust and Deoxit all pots & tube sockets. That didn't help (but well overdue).

So it's narrowed down to the Phono-stage. So how should I proceed? It has the original tubes from when it was assembled. Should I begin with two new sets of 12AX7 tubes? Or could this be a "deeper" problem? Longterm I'm planning on upgrading this unit with available mods. But for now I just need this hum/ground issue to disappear. Any thoughts?

Edit: I forgot to clarify that the hum is present at Phono with nothing connected as well.

Best way is to start with a couple of grounding plugs.  Or if missing connect one
cable with rca connectors on each end between phono left and phono right.
If hum is still there you have isolated to the riaa stage.

If so, get replacemet ECC83 tubes. The best you can get is E83CC from eurotubes.com,
they are not that expensive , or ECC83S from the same place.
Reason for this is that the riaa tubes are heated with AC and leakage
between filament and catode is common on old tubes.

In the meantime you could exersize the "hum pots" at the backside of the amp.
They will side by side try to eleminate must of the 60hz hum that is caused
by the AC filements. Find the point where the hum is less.

/PS
the tubes mentioned may be obtained at tubular-well.se
i have an affiliation, i hope that is ok since both vinnie and i live an
a distant remote where tube distributors are scarce
/DS

Also, try reversing the AC plug for the turntable, if possible. Sometimes this removes hum. (Unless of course, you still had hum with TT AC unplugged.)