Mark IV owners

brianf · Feb 14, 2006

Is it normal to be getting more "hiss" from the amp when it is set in tweed power??? I'm certain my Mark IV head was not like this but my new combo sure is. Flick to full power and the noise is gone.

Thanks for any comments.

brianf

Boogiebabies · Feb 15, 2006

It's probably a preamp tube acting funny with the lowered voltage.
They can be really strange beasts that exibit hiss, pops, sqeals, ticks and motor boating. I love the tweed setting on the MK IV. I get all kinds of hiss and pops even with GT 12AX7M's. Sometimes it clears up when you switch to standby as well, but sometimes even a new tube can be bad. I have had a horrible time with the 12AX7M. Out of the 25 I bought, at least half of them went bad.

brianf · Feb 15, 2006

I'll start there then. I have a box full of various 12AX7's. Actually my RoadKing is dead quiet so I'll pull those ones out and see if that is the problem. Thanks.

brianf

Boogiebabies · Feb 15, 2006

My RK is very quiet too.

aenigma · Feb 17, 2006

I have a little off topic question.I hear older Mark IV's had a 12A7T instead of the 12AX7 in the phase inverter slot I believe.Someone said it sounded alot better in his amp than the 12AX7 but that it may only sound good in older amps like his that were designed to use that tube.

Any information on this?

Boogiebabies · Feb 17, 2006

It used an AX7 in the PI and an AT7 for the reverb.

You can use an AT7 on any PI. It cuts down on the gain to the power tubes.
I actually like a NOS Westinghouse or an RCA and have used this on the Diezel VH4. Try it, you may or may not like it.