Filter Cap Replacement - 1984 Mark II C+

[Henz]

Thanks for the reply! And the link. I'll do the same, get both and probably use the higher voltage. I think higher is better. The 680 was replaced by MB at Mesa. He rebuilt the ldr supply which that goes to. Now, my amp uses 4 470ohm for the plates of all four power tubes. I have replaced them.(I may get new ones anyway. Wire wound like you suggested. I've always used 2 watt but would 3 be better?) I always measure them. They are within specs. My outer two tubes have 220k resistors going to the grids and the inner tubes have 2.2k resistors going the those grids. In circuit, the 2.2k's measure fine. The 220's measure 188, all of them, even the two at the bias input( E ) feeding the two bias rails on pcb. The issue that I have is the plate voltages were always around 470v in the past which is in specs rite? Recently the voltages have been 504v. I have no idea what caused that unless OT is going bad. The tubes do run a little hot. I just did a complete recap on my 2:90. Those voltages have increased too inside (prior to recap). Maybe it's my electric coming to house. My C+ doesn't have lots of noise but over the yrs I may not be noticing it, but it's there. Follow? Since I re capped 2:90 I thought I would do C+( they are all original). Did you notice a huge improvement in tone? Sorry for the long reply. I love talking shop. Hope to hear from you, thanks, Jim

 

[Henz]

Just went to Mouser. Those caps are a lot shorter and fatter than the ones that are in there now. Does that matter? Also, forgot to mention bias voltages above. Voltages on pin 5 on outer tubes are - 48. Inner tube especially is - 60. In the past they were always -39 and - 50. Don't know why the jump. While I'm at it, I'm gonna get new power tube sockets too. Thanks, Jim

 

[woodbutcher65]

I recommend you get your electrolytics from Mesa. I don't have a particularly high opinion of IC brand caps. They're decent but you can get better.

The major caps (the 220s and 30s) found in the power supply are now made by BMI, Barker Microfarads Inc, which means they're Sprague Atoms in everything but name alone. BMI is the former Sprague Atom factory with Sprague tooling run by former Sprague employees. They're top quality.

So you've got a 15 year old cap that's already failed. And some people think that my recommendation to replace caps any time after 10 years is too conservative. Well, a 10 year replacement cycle is certainly conservative but it's highly unlikely that you'll ever find a failed cap on that replacement time frame. If you do, it was defective or the amp took a power surge that took it out.

I change tires BEFORE they are running on the cords, too. Same reason. Safety margin.

Definitely change out the bias electrolytics as well. I also recommend replacing the .1 uF coupling caps between the phase inverter and power tubes as well, because they're the single part that I've seen fail most often after recapping an amp. Recapping restores power supply voltages to their designed values and that increase in voltage is enough to push some old, worn caps over the edge.

The symptom of this is that you get one or more red plating tubes right after a recap job. So change those coupling caps when you recap the amp. Use CDE (formerly Sprague) 600 volt 0.1 uF series 715P caps. Don't get the knockoffs offered by some vendors. Get genuine CDE branded parts. There are other orange caps that look like orange drops but if they don't say Sprague or CDE on them, they are not to be trusted.


As for alternative cap brands, if I can't get CDE then I want Vishay or Nichicon electrolytics if I can get them. And I'll spring for long life/high temperature types.

I just recapped a Kendrick amp and because I could get them, I put in 100v/100uF Vishay electrolytics for the bias caps, which are rated 10,000 hours at 125c. I think they should last.

 

[Henz]

Thank you for your reply. And input. I'll get those two .1 caps as well. On the schematic, they are the ones going to the plates of PI correct? Should i replace the electrolytics on my EQ board? They are original as well. I'm gonna put new plate resistors and new power tube sockets too. I just don't understand why my plate voltages increased . And my bias voltages as well. Is there a way that I can drain my old caps with a volt meter? I re capped my 2:90 and didn't drain anything. I didn't get bit but I was careful , or lucky?I sincerely thank you for your advice. Jim

 

[woodbutcher65]

If there's no voltage bleed resistor in the power supply, add a 330K 1 watt resistor from the highest voltage node to ground. That'll bleed it down quickly enough and not draw appreciable current. (About 1 milliamp, total max dissipation on the resistor will be barely half a watt at most. No problem.) This can be a permanent addition to the circuit but there should already be a bleed resistor.

Why voltages increase when you replace old caps: Because the old caps have degraded and their capacitance has dropped, their leakage current has increased, and their ESR has increased. All factors of ageing.

The bias caps on a Mark IIC+ should be 50uF 75V if original. They'll be the two smaller axial lead caps right next to the big 220 uF 450/500 volt caps on the supply board.

Since those take the heat (literally) if a tube should short and red plate, I do recommend putting in the best caps you can in this application and adding capacitance would do no harm.

Thus, I would not hesitate to put these beauties in: 4258PHBK-ND (Digikey part number) 100 volt 100 uF Vishay axial lead capacitor, rated for 10,000 hours at 105c. That'll do it!

 

[Henz]

Thank you for your reply. Instead of what the C+ calls for ( 50uf 75v) , 100uf 100v is better? Ya know from what i heard from you, you are very intelligent. I;m gonna order the bias caps that you suggested. I do get red plated tubes at times. I do not know if there is a bleed resister in power supply, so im just goin to remove and solder. I don't care at this point in my life. If I get electrocuted, at least I died trying! Blessings to you. you are a great help. Sincerely, Jim

 

[woodbutcher65]

Stiffening up the bias supply with 100/100s is something I'd do on my own amps. There's no down side to it. While you should not be too aggressive in increasing capacitor values in the power supply, going to a bigger bias supply cap will just help keep the bias even more stable. Since 75 volt ratings on caps are comparatively rare, you go up a voltage rating. No harm in uprating a cap, but don't downrate one.

You COULD stick with 47 uf if you want. No harm in that either. So there's a wide range of possible caps that fit in the box defined between 47/75 and 100/100 ratings.

Pick any caps you like within those limits. I recommend 105c temperature rating, OR HIGHER, and a long rated life. Your standard capacitors are going to be rated for 1000 hours use at the design rated temperature of 65c, 85c, or 105c. 105c is a premium grade. But going with longer rated lifespan is even better. I picked the Vishay 105c/10,000 hour caps because I'm pretty sure those will be lifetime parts. I'd expect them to still be good 20 or 30 years from now.

If a tube does red plate, the bias cap is going to be where the current draw comes from. So it's going to run warmer. If it's rated for that, then it means that the shorted tube isn't going to blow up the caps. If the cap blows, it'll usually short out. And THAT is bad for the power transformer.

It's not great for the power transformer to have to feed current to a red plating tube, either. But a shorted cap is much worse.

 

[Henz]

Thanks for the reply. You are very helpful. Drew, did you change the caps on your eq circuit board? And was it hard to remove from the side of the chassis? Thanks, Jim