New Mark IV version A owner questions

Hey guys,

I just got my Mark IV the other day and after going through it, I have a couple of questions for the gurus.

1. The reverb is not working. I pulled the preamp tubes to make notes of what were installed, and I noticed that one of the previous owner had put an 12AX7 in the reverb slot (#4) and a 12AT7 in the input stage slot (#1). Swapping these didn't fix the reverb. The amp still works, so I'm assuming that the 12AT7 is fine. I've looked into all the simple causes I could think of. The cables are plugged into the chassis and the tank, the reverb knob is turned up to 10, etc.. Any idea of things I could try?

2. There is a mystery input jack on the bottom of the chassis. It's located just below the eq rocker switch. I'm not talking about the footswitch remote jack, this is a 1/4" jack. No label that I can see. Any other Mark IV ver. a owners know what this is for? Is this a standard jack, or some mod? My first thought was that it's a pedal input for the reverb, but I haven't explored that theory yet.

3. There is a quiet fuzz sound that accompanies the low notes of the guitar. This is mainly noticeable with a clean setting. It's tolerable and you'd never hear it at stage volumes, but it is bugging me. I've looked for anything in the back of the amp that could be vibrating, but no luck. Any ideas what could be causing this?

Sorry for the novel, and thanks in advance for any help.

Congrats on the MkIVa.

1) Reverb
Try reversing the reverb cables.
On my amp, the thicker cable (grey on mine) is plugged in the RCA closer to the amp front and the thinner (white) is plugged into the RCA closer to the back. Also check the cables for continuity.

BTW the tubes seem reversed (That's not the cause of the lack of reverb, but the amp will not sound as designed).
When facing the open back of the amp, V4 is the leftmost preamp tube and V1 is the rightmost.
V4 is spec'd as a 12AT7 and V1 as an SP12AX7 with a tube shield. (a regular 12AX7 and/or no shield may result in more noise). The rest of the preamp tubes are 12AX7

2) mystery jack
Mine has the same jack labeled as "Remote" and a small triangle. With the std pedal unplugged, an open 1/4" plug inserted in that jack will enable the GEQ, shorting it will turn it off.

3) fuzz noise when low notes played
Multiple possibilities. Noise when you tap the chassis suggests that tube sockets need cleaning.
Tap each tube with a pencil, dowel or something similar. Try replacing any noisy ones.
Is it farty bass? Bass should go down as gain and volume goes up (you can add bass with GEQ if you want).

Thanks for the response SquierJVs!

1. Good idea about testing the continuity on the reverb cables. I believe they're plugged in correctly, but I'll have to double check.

As for the tubes, I swapped the v1 and v4 when I saw the previous owner had them reversed. I didn't notice any difference in tone, at least not at bedroom volume. But it's back to spec now.

2. Interesting... So is there a difference between the external jack for the eq in the right corner of the back panel and the one on the bottom of the chassis?

3. I've tapped the tubes, but I didn't find any that were noisy. I don't think it's farty bass. I've got the bass down around 3 or so. Also, it's there with the amp volume turned way down as well. The best way to describe it would be that it sounds like I have a really nice sounding clean guitar with another fuzz guitar doubling very very quietly. You really don't notice it until you softly play a note and let it ring. Then it's there in the background. I'm going to see if I can find a cabinet I can plug it into so I can rule out the speaker at least.

2) mystery jack
I found the answer in the Mark IVa manual. I should have looked there before experimenting.

The "Satellite" powered amp & speaker systems have a similar GEQ jack. By connecting the two via a cable, the GEQ on the Satellite is enabled whenever the GEQ on the Mark IVa is enabled.

3) fuzz noise
Troubleshooting with another amp and another cab would be a good idea. Driving a cab with another amp will rule anything physical in the Mark IV combo. And using the Mark IV with another cab will tell you whether the problem is caused by the vibrating Mark IV combo interacting with the chassis and its electronics.

PS
A previous owner of my amp used a 12AX7 instead of a 12AT7 in the reverb position. When a 12AT7 was used, there was better control of reverb, but the difference in sound was more subtle ... better clarity of reverb. I haven't heard a 12AT7 in V1. Given that its gain is less than that of a 12AX7, you should hear a difference, but that depends on the other stages and their settings.