Boogie Mk I - need help with filter caps

shakti · Post by **shakti** » Mon Nov 07, 2005 5:30 pm

I bought a '76/77 Mk I this summer - been wanting one for years and years, since I was never 100% happy with my reissue. The original definitely has something the reissue doesn't - it's much more clear and articulate, not just the congested and gainy sound that the reissue has. With the original, you can much more clearly hear that it's derived from a blackface Fender, which means a much better clean sound and a reverb that stomps all over the reissue. BTW, mine is an export model 60W with EQ and reverb.

However, I don't think my original is sounding the best it could. Curiously, the highs seem more shrill than on the reissue. It has quite significantly less gain than the reissue as expected, but to the point where it sounds a little bit weak. It's also a tiny bit noisy, and seems to lack a little punch and power. Sounds like a classic case of an amp in need of new filter caps. Also, I can't get it in the right bias range even with NOS JAN Philips 7581s specially selected for "Boogie" bias. Someone had added a bias pot in there - once removed, the amp came closer to bias range, but is still way off, with an ice cold -22mA or so @ 450V.

So I want to take this amp back to its best possible sound. Unfortunately, I am far away from the amp right now, but would love to order the right parts for when I'm temporarily back home for 10 days in the end of the month. Looking at various schematics on the net, I am a little confused as to what I need... so basically, I would love any help to order the right parts. If you were going to do a full-over on an amp like this, what would you do? I am thinking I need to change the filter caps and possible the bias cap(s)? I am also planning to change the rectifier diodes to FREDs, and possibly add a bias pot. Any hints or ideas on how to go about this would be much appreciated.

Looking at the schematics from www.tubefreak.com it looks like I need these caps:

3 x 30 uF @ 500V (these are the filter caps, right?)
2 x 60 uF @ 350V (also filter caps??)
1 x 60 uF @ 500V (what is this cap?)
1 x 30 uF @ 70V (bias cap? a small cap, I guess by looking at photos of my amp)

However, I seem to have trouble finding caps of the right ratings and with the right size. The 30/500 ones are OK - Sprague Atom should do the job. Can't find any 60uF oones, though...would 80uF be terribly bad, or should I go in the other direction with 50 uF instead? Lastly, where to find a small cap rated 30 uF/70V for the bias cap?

Once again, all hints and help is much appreciated! I need to get this amp back in shape, and the local shop never has much in stock as far as parts (in fact, I'd rather even do this myself if I can...).

Billy Batz · Post by **Billy Batz** » Mon Nov 07, 2005 5:51 pm

Thed 60/350v is the mains filter, the 3 30/500v are the others. Looking at the schems on that site I dont see the other 500v your talking about but it could only be a filter cap. If sure it cant be a bad ida to replace the bias filter cap as well. Some epople think you should replace all the electrolytics (even the preamp cathode bypass caps) but thats up to you. They all dry out though.

Flames1950 · Post by **Flames1950** » Mon Nov 07, 2005 6:44 pm

Seems like everyone ends up with a 100uF/100V cap in the bias section on Fenders. It seems likes the only option out there in a high enough voltage.

Guest · Post by **Guest** » Mon Nov 07, 2005 6:47 pm

Thanks!

Check the schematic called Mk IA, there's a single 60/500 going to ground in the power amp. I have a photo of my amp on my iBook, I think I can guess which one it is (I can't make out the value or where the leads go, though).

But what about the values? The 30/500 are easy to find, but I can't find any 60/350 or 60/500, nor a 30/70 (I am guessing one with a higher voltage rating should be fine?).

I am a little hesitant to change signal caps - I mean, would you throw out your mustards?

Also, they're PCB mounted, which makes it so much more of a hassle. The filter caps are on separate boards, so they're much more easily replaced.

Billy Batz · Post by **Billy Batz** » Mon Nov 07, 2005 6:50 pm

Just to clarify something. The main filter is 2 60/350v in series so its 30u total. According to the schem and it sound that way in your amp.

Billy Batz · Post by **Billy Batz** » Mon Nov 07, 2005 6:56 pm

Anonymous wrote:Thanks!

Check the schematic called Mk IA, there's a single 60/500 going to ground in the power amp. I have a photo of my amp on my iBook, I think I can guess which one it is (I can't make out the value or where the leads go, though).

But what about the values? The 30/500 are easy to find, but I can't find any 60/350 or 60/500, nor a 30/70 (I am guessing one with a higher voltage rating should be fine?).

I am a little hesitant to change signal caps - I mean, would you throw out your mustards? Also, they're PCB mounted, which makes it so much more of a hassle. The filter caps are on separate boards, so they're much more easily replaced.

Actually its a bit wierd in that schematic. Theres the main input filter section, then the standby, then that 60/500v is directly coupled to the main filter before the choke. The total youd have would be 20u on that mains section. Im not sure what thats for to be honest. Theyre all filter caps though