New Member and Old Boogie Mark IV A

Hi!

My name is Es, I live in Germany, and I just got my first Boogie - a 1990 Mark IV A - this week!

The amp didn't work 100% when I went to check it out, so I managed to haggle the price down quite a lot :mrgreen:
After taking it apart and cleaning it, and actually understanding how it works (thanks to MESA for sending me a copy of the manual), it now produces some pretty awesome tones! The amp came with the MESA 420 "blu" 6L6s, 4x Sovtek LPS and a MESA 12AT7 for the reverb.



I do have some questions:

When playing, particularly or the tweed setting, the amp started losing gain and volume after 5 or minutes and after another minute or so the footswitch started playing up by not reacting :shock: . I read somewhere on the internet that "Russian" tubes were not up to the voltages in these Boogies, so went and replaced the Sovteks with an assorment of JJs I have - problem gone! Can anyone explain this issue?

We have 230V at the wall here in Germany, but the amp only has 220V and 240V taps on the PT. Which would be better to use? Or should I base the choice on the heater voltage (assuming that it should be 6.3V AC?)

What pre-amp tube are recommended for these - preferably current production, as I'm too stingy for NOS... I do have some JAN-Philips 5751s - would they be any good?

Cheers

Es

Hi again,

The problem is back.... it's also making all sorts of noises, including some clicking.
I swapped in a set of 6L6s from another amp, but that doesn't solve the issue.
Any idea where I should start looking?

Es

Hello.

It wasn’t evident in your posts: did you change preamp tubes also? Please test that first.

Hi!
I did change the pre-amp tubes, except for the 12at7 as I don't have a spare, but now the problem is back with the "good" tubes.
I am measuring the voltages at the moment, and I've also installed a 240V/220V selector switch. The heaters are at 6.44V with the 240V setting as opposed to 7.10V on the 220V setting with no tubes installed.

Es

The_Nuge said:

Hi!
I did change the pre-amp tubes, except for the 12at7 as I don't have a spare, but now the problem is back with the "good" tubes.
I am measuring the voltages at the moment, and I've also installed a 240V/220V selector switch. The heaters are at 6.44V with the 240V setting as opposed to 7.10V on the 220V setting with no tubes installed.

Es

Click to expand...

Well.. my experience is limited other than changing tubes... I had some intermittency in my Mark III but I don’t remember what was the culprit for that.

- Eevil

Hi again,

I suspect the problem may have been with footswitch! I took that apart and cleaned all the contacts, and since then the amp has been working fine!
The 220V / 240V mains switch I put in may also have helped, as the amp now hums less on the 240V seting. I measured all the voltages, and they all seem fine!
I've ordered new power supply caps, all the resistors around the tubes and new 6L6s (one of them rings ever so slightly) and will install all that once the bits get here - will report back!

Cheers

Es

I'm back!
I've replaced all the high-voltage filter caps, which has helped a lot in reducing the overall noise level.
However, the volume-drop problem reoccured last night during a jam with a mate. We pulled the chassis and discovered some funky soldering around the area with the low-voltage supply - which would explain the footswitch also olaying up! The 8200uF caps, the diodes and the R383 resitor had cold solder joints, so I've ordered new bits for that!

Will report back!

Es

I think I've solved the issues!

1. I replaced the 8200uF caps, diodes and 2 discoloured resistors in the low-voltage power supply, and now have solid 15V going to the footswitch. Haven't had any problems with flickering since then!
2. Volume drop in Tweed-Power mode: the amp is very picky with pre-amp tubes at the lower voltage levels! JJs seems to work fine, but (gold pin) EHs don't work at all! With the EHs you can turn the volume up to 5 an still be at bedroom level and the sound is all fizzly and crackly! I've now got Tung Sol in V1 and V5 and JJs in V2-V4 (V4 being an 12AT7) - sounds great!

Cheers
Es

Hi folks,

i'm also a new member, living in Germany, have a new "old MK4a" and - surprise, ist seems to be a similar or the same problem as the nuge had. After a while, 20 minutes- the amp drops down the level in all channels, footswitch looses control, the sound is thin and fuzzy.
Switching off, waiting for cooling down - new start up - the same issue apperars.

I got it yesterday with UPS, maybe on transport the amp got these solering problems - I have to take a look inside and i will check it out.

Do somebody have wiring diagrams of the MK4a?


Its not my first boogie - i play approx. 30 years the QUAD preamp for jamming or live on stage - so many hours and miles on the road. - never had any issue except one time tubes exchange. Long time ago i got my second Quad preamp, for backup....

I do, PM me if you’re interested. I’ve verified ~90% of the schematic against my Mark IVA so it is believed to be accurate. The switching Matrix is not in the set, but I do have one for the IVB as well