ST70 HUM on New Build Single Channel

Let me preface this buy saying I have read about 200 posts on hum.

I have a new build ST70. Everything is new. Transformers, tube sockets, tubes, bias circuit, can caps, resistors, etc.. all new components. Not a single old part on it. Using a Dynakit PC-3B board.

Fired the unit up and checked voltages and everything was to spec per the dynaco documentation. Used a variac to set voltage to 117 VAC and biased each side to 1.25 VDC.

Left side is dead quite. Not a single noise. Right side has a significant amount of hum. I went back over the wiring and all looks good. I rotated tubes (Tung Sol EL34, Tung Sol 5AR4 and NOS RCA 6U8G) and hum stays on the right side.

I shorted the RCA's and the hum gets louder. Also, I noticed that the impacted side has significantly less output by about a factor of half when hooked to a source.

Tonight I am going to hook up my signal generator and dummy loads and use a scope to try and pinpoint if there is a section in the amp that is producing the hum but I wanted to see if anyone has any other troubleshooting tips I should try. Especially with the dimished output which I think is a definite sign something is not right.

moodyj2000 wrote:Let me preface this buy saying I have read about 200 posts on hum.  

I have a new build ST70.  Everything is new.  Transformers, tube sockets, tubes, bias circuit, can caps, resistors, etc.. all new components.  Not a single old part on it.  Using a Dynakit PC-3B board.

Fired the unit up and checked voltages and everything was to spec per the dynaco documentation.  Used a variac to set voltage to 117 VAC and biased each side to 1.25 VDC.  

Left side is dead quite.  Not a single noise.  Right side has a significant amount of hum.  I went back over the wiring and all looks good.  I rotated tubes (Tung Sol EL34, Tung Sol 5AR4 and NOS RCA 6U8G) and hum stays on the right side.

I shorted the RCA's and the hum gets louder.  Also, I noticed that the impacted side has significantly less output by about a factor of half when hooked to a source.

Tonight I am going to hook up my signal generator and dummy loads and use a scope to try and pinpoint if there is a section in the amp that is producing the hum but I wanted to see if anyone has any other troubleshooting tips I should try.  Especially with the dimished output which I think is a definite sign something is not right.

Sounds to me as a good plan.

You could also switch the pre ( both the outputs AND the NFB ) to identify if the hum comes
in the PC3 board or in the power section.

That would be switching the wires on the PCB at points 12 & 13 for the NFB and 7 & 17 for the pre-inputs correct? What about the two wires from the RCA ground to the PCB (non-numbered)?

moodyj2000 wrote:That would be switching the wires on the PCB at points 12 & 13 for the NFB and 7 & 17 for the pre-inputs correct?  What about the two wires from the RCA ground to the PCB (non-numbered)?

Only switch the 2 outgoing to the EL34 grids and the NFB connection Keep the input as is .

I would use the clue you gave “hum gets louder with grounded RCA input” as somewhat significant. This might suggest that there is a suspicious grounding point in the amp. And I bet it is in the input section of the noisy channel. A loose ground will cause the input to “hear” a good ground at the RCA connector because they won’t have the same reference. If your amp uses screw eyelets to the chassis, you might try retightening those and looking for cold solder on them. They tend to suck the heat out of an iron.