Amp Hiss Just Got Worse

[Jeff_H]

So this morning I replaced the V30's in my 2x12 with the original G12T-75's,and went back to 8 ohms (I had been running V30's @ 4 ohms). They sound a little more open, just like I was wanting. Tighter on the lows and crunchier on the highs.

I also took the advice I received in my other thread and pulled all of the pre-amp tubes, and replaced them with JJ's that I had laying around. Fairly new JJ's that I just kept for emergency replacements. Now the hiss is worse. The clean channel is almost tolerable, but anything with gain is just too much. It doesn't matter if I use the gain channel on the amp, or add gain from my GT-6 or SD-1....the hiss is out of control. I've tried swaping different tubes into each position....including some from my Blues Jr that I know to be new...nothing helps.

I took out all of the tubes again, and re-seated them. Still the same. Is there anything that I'm overlooking? My gut is that I'll need to ship it back to Jeff Seal to have it looked at, but if I've forgotten something simple, let me know.

 

[TheArchitect]

Re: Amp Hiss Just Got Worse

Have you tried putting the original set of tubes back? The JJ's may be a higher gain tube than what you had which is bringing out more hiss. I don't know what Jeff's mods are but if he added gain stages to the circuit he may be using a lower gain tube like 12at7's instead of 12ax7's. That brings up another point. Are they infact the same tube as the ones you took out?

 

[STRATDELUXER97]

Re: Amp Hiss Just Got Worse

Have you tried putting the original set of tubes back? The JJ's may be a higher gain tube than what you had which is bringing out more hiss. I don't know what Jeff's mods are but if he added gain stages to the circuit he may be using a lower gain tube like 12at7's instead of 12ax7's. That brings up another point. Are they infact the same tube as the ones you took out?

Good post buddy....


Jeff..The JJs are pretty gainey tubes...Have you tryed a set of Sovtek 12AX7As in there instead? Can you open up your chassis and check "lead dress" and see also about some solder or connection problems...Are the wires from your preamp tube grids "shielded" on their way to your input jacks?

Did you have all of this when you first got your amp back after the mods and using the same tubes that were in the amp at that time? I don't think you would like the 12AT7s in your Marshall....Sounds like there's a problem that needs attention strickly in the preamp though...Check the positioning of all wires(Especially the green)around your preamp tube sockets....

 

[PUCKBOY99]

Re: Amp Hiss Just Got Worse

This may be something stupid, but have you tried cleaning the pots & inputs?

Or tried the tubes from the Marshal in the Blues Jr.???

 

[Jeff_H]

Re: Amp Hiss Just Got Worse

All good suggestions fella's. When it comes to the inside of the amp chassis.....I am a complete retard. I need to learn more about it, but I would only hurt myself or the amp if I tried to mess with it. It was setup with all 12AX7's, and that's what I've always run in all positions...never anything else.

The amp was very quiet when I first received it....it had a very slight hiss in it right before my mom got really sick. I probably didn't turn it on for 3-4 months straight. The hiss sounded much louder when I went back to it, but perhaps it really wasn't...our ears have terrible memory.

I went back and checked all of the connections made to my speakers, to make sure they were all solid. Then I started swapping tubes again. I managed to find a combination of tubes that brought the hiss back to useable levels again. I did plug a bunch of these tubes into my Blues JR and it's as quite as a mouse fart with all of them. I had been running some of the JJ's in that prior to putting them back in my Marshall. I find it odd that out of the 16 tubes I have...all would produce hiss of some kind...but some do definately more than others. I'm also picking up radio interference, which I didn't before...could actually hear the music with some of the tubes in place.

My gut now is that there is something wrong in the pre-amp as some have suggested...perhaps a loose solder joint, or something needs cleaning, or maybe a cap is going bad. I'll talk to Jeff and see if he has any issues with me taking it to a local tech (to keep is mod work private), or if he has annother suggestion.

Oddly enough...the JJ's were much smoother and darker...not higher gain at all. My RFT 12AX7 has the most gain of any of the tubes I have....and oddly enough it's the quitetest in V1.

 

[PUCKBOY99]

Re: Amp Hiss Just Got Worse

That sucks Bro......but Scott Splawn is right down the road from you, right?

I'm sure Jeff wouldn't mind Scott checking out his handy work!!!

 

[Jeff_H]

Re: Amp Hiss Just Got Worse

Well, I was thinking that Terry. Scott is who I would take it to for sure. I just want to make sure the Mr. Seal would be ok with that first...out of respect.

 

[PUCKBOY99]

Re: Amp Hiss Just Got Worse

ABSOLUTELY!!!

He's been nothing short of a complete gentleman on the board when he visits...he deserves THAT much respect. And I'm sure Scott would say the same thing!!! :beerchug: