V1 Hissy

[G._Hoffman]

I'm rebuilding one of my amps for like, the forth or fifth time (nothing wrong, just been playing around with it), and V1 is really hissy.  essentially , it is an 18W with a plexi preamp, so V1 is the two input tubes.  When I turn the volume up on either side, it has all kinds of hiss, but turn down the volumes and it goes away, so the problem is clearly in V1.  I've gone through and made sure every solder joint is good (nice, clean, and shinny!), I've even replace the cathode and anode wires with a twisted pair on one half of the tube, and there is no change.  I've tried every single 12ax7 I have in that spot, and while some are better than others, they all hiss.  A lot.

What am I missing?


Gabriel

[Willabe]

What kind of R's are you using?

MF would be the least noisy. Even if you are using MF R's you might have a bad one.


              Brad        

[HotBluePlates]

And while I was surprised by it at the time, Platefire once fixed a noisy input stage by replacing the cathode resistor. Turns out in his case, noise in the cathode resistor was being amplified as though the tube were a grounded-grid stage.

[G._Hoffman]

What kind of R's are you using?

MF would be the least noisy. Even if you are using MF R's you might have a bad one.


              Brad        

2W metal film.  Well, one of the cathode resistors is a 3 watt, cause I'm trying to do this with parts on hand.  I only had to buy a couple of filter caps.  The only thing is, the two halves of the tube have separate anode and cathode resistors, and both have essentially the same hiss coming out (the bright channel hiss is a bit brighter, but then, you would expect it to be!).


Gabriel


Gabriel

[jojokeo]

I keep thinking grounding...? Where is the preamp ground going to? I'm assuming everything to be "normally" layed out - ie - grid stoppers @ V1 socket using shielded wiring, cathode R's & bypass caps grounded directly at or close to node's filter cap, using isolating cliff jacks, etc...? Any other fancy switching or relays going on anywhere?

[G._Hoffman]

I keep thinking grounding...? Where is the preamp ground going to? I'm assuming everything to be "normally" layed out - ie - grid stoppers @ V1 socket using shielded wiring, cathode R's & bypass caps grounded directly at or close to node's filter cap, using isolating cliff jacks, etc...? Any other fancy switching or relays going on anywhere?

Switchcraft jacks with stepped isolation washers, but other wise yeah.  The cap is right there, and everything is grounded to the cap.  The grounds for everything up to the PI are bussed from their local "star" grounds (the caps), with the power amp's screen cap running separately to the main cap - basically, what Merlin recommends in his books, and a ground scheme I've used successfully for the last 3-4 builds I've put together.  I was thinking of sending the V1 ground back to the central point separately, but have not tried it yet. 


Gabriel

[super&plexi]

not sure if this helps, or even if it pertains to, but in my super 60,.....recent troubles included input jacks (nothing new to these amps),..... But centering of Switchcraft  style jacks,.........and consequently finding that they can,........... and do become off center,............... and contact chassis, intermittently, and/or  sometimes so slightly as to make a measurable R was quite Eye opening for me. I think of this as more of a glitchy sound, than a hissy one, but who knows, it may help you.     .

[jojokeo]

Damn there only so many things to go wrong. Try replacing the load resistor since it is common to both triodes.