PAS Hiss - Gets Softer as Unit Warms Up

Hello.  I have a PAS that I restored using the Z-Mod PC5, PC6, and PSU kits sold by Tubes Nirvana.  I run it through an ST-70 that I restored using the VTA 70 kit sold by Tubes 4 Hifi.  Both have new JJ tubes (less than 24 hours on them).  The ST-70 runs quiet, i.e., nothing through the speakers when the PAS is not connected.  When the PAS is connected, I get a noticeable hiss through the right channel when the units are first turned on.  This happens regardless of whether any components are connected to the PAS, i.e., the hiss is the same whether or not a component is connected, and regardless of what position the selector knob is in.  After one minute, the volume of the hiss has faded by about 50%.  After three minutes, the hiss is basically gone with the volume knob set at normal listening level, i.e., at about the 3-4 level.  If you put your ear up to the speaker you can still hear it.  You can also still hear it if you turn the volume up high, i.e., at an 8+ level.  Because the hiss basically goes away at normal listening volume and distance, I have not been overly concerned about it.  However, I thought I would see what others have to say.

If the pas has DC on output ( PAS 2and PAS3 has, PAS3X has not) this might upset the input
of VTA70 ( which by the way lacks an isolating cap on input).
An isolating cap might fix this , easiest place is on the output RCA connector in the PAS3, 0.1uF would do just fine.

Thank you for this response. So, add a 0.1uF cap where the wire connects to the output RCA? What kind of cap? Aluminum?

flip69 wrote:Thank you for this response.  So, add a 0.1uF cap where the wire connects to the output RCA?  What kind of cap?  Aluminum?

No, no electrolytic. Some kind of film/plastic would do. ( 0.1 uF 200V - 0.47 200V )

OK. I added 0.1 uF 250V film caps to each of the signal wires where they meet the RCA output connectors. I did not add a cap to the ground connection on the output connector. Was I supposed to? The caps seems to have helped the hiss. It is still evident when I turn everything on. However, it is now closer to gone after things warm up for several minutes. That said, you can still hear it on the one side when you put your ear up to the speaker, or turn the volume way up.

A couple of notes. The hiss stays in the right speaker even when I use the balance knob to put all the sound into the left speaker, i.e., it is not controlled or affected by the balance knob. Also, there appears to be some kind of grounding issue. It appears to be separate from the hiss. When I turn the volume up high, you can hear a hum in both speakers. The hum goes away when I touch my finger to the chassis. Touching the chassis does not affect the hiss. I have replaced the original I/O RCA connectors with the upgrade I/O kit sold by Tubes Nirvana. The installation instructions show only one ground wire running from one of the ground holes on the PC5 preamp board to the top RCA output connector. I tried running a line from the second ground hole on the board to the ground terminal on the other output connector. It did not affect the hiss or the ground hum. I note that I installed a new two-prong plug and power cable when I did the restoration.

So, the two follow-up questions are: 1) What else, if anything, should I do to troubleshoot the hiss; and 2) What about the ground hum? Again, both the hiss and the hum are soft enough that you really only hear them when nothing is playing and the volume is turned way up. I am sure I could live with them as is. However, since one channel has no hiss, it seems like I should be able to get rid of the hiss in the other channel as well. As far as the grounding hum, it seems like a general chassis grounding issue. That also seems like something I should be able to fix.

Hum, in my experience, often comes from not connecting together the units chassies. This does not
preclude that the signal ground also should be joined but separated. The separation might be
a 10ohm resistor in each chassies.
The chassies in turn should be connected to "real earth" in order not to swing with the mains cycles.
As for the hiss i have no explanation. Seems to be a component that is adding some randomly item.
Do you have a scope to "look at" the hiss ?

flip69 wrote:Both have new JJ tubes (less than 24 hours on them).

Regardless...did you try swapping tubes between channels to see if the hiss "follows" a tube?  
Three minutes sounds tube warmup-related.

This assumes that those boards use both halves of each tube for one channel, and both halves
of the other tube for the other channel. I'm not sure how those boards do it; the stock
Dynaco circuit does it that way.

Thanks again for the comments. With regard to the hiss, I do have a scope. However, I am inexperienced at this and would have to be taken by the hand and walked through the process of looking at the hiss. I will try switching the tubes to see if that has any effect. At this point, the hiss is minor enough that it does not worry me much.

Regarding the hum. I do not understand what peterh means when he says the signal ground "should be joined but separated," or where to add the 10ohm resistor in each chassis. Please clarify. How do I connect the two units together for grounding purposes? How do I connect the chassis to "real earth"? Again, my inexperience is showing. As with the hiss, the hum is not a big enough deal that I am too worried about it. However, for purposes of my own education, I would like to understand these issues.