Filter cap question

[macphly]

1982 4010 (small chassis combo version of 2204). I removed it from it's 112 combo box and reinstalled it in a small, Lead 100 Mosfet box. The retrofit worked out great...except that the faceplate is upside down (Marshall JCM 800 combos hung from the top...heads bolt to bottom).

ANYWAY...I'm pretty sure it's time for a cap job...has ITT 50-50-500s.

What's best? I've seen ARS out there (only on ebay...know where to get them new?)...bought JJs for my JTM45RI (they seemed fine)...heard of another (F&T?)...

Also, what's the shelf-life of filter caps? If they've been sitting around for 3 years, are they likely to be going bad?

[VacuumVoodoo]

JJ and F&T are good quality caps and you should have no problems with these. 3 years on the shelf should not have deteriorated modern electrolytics.
If the caps are in an old amp that has not been used for the past 3 years then reforming them before first use might be a good idea. Reforming: charging the cap slowly up to rated voltage over 24-48hrs, see http://www.vcomp.co.uk/tech_tips/reform_caps/reform_caps.htm

[macphly]

Thanks a million. The 3-year-old caps I'm speaking of are some unused ARS on ebay. I hate to admit it, but the only reason I consider them is that they look like the existing ITT (which look like the old LCRs). Any experience with these? Would you suggest them or should I be more comfortable with new JJs? Ultimately, on the spectrum of things I'm concerned about re: filter caps, appearance is a distant third behind good sound and endurance!

[VacuumVoodoo]

Well, lets see it this way: capacitor technology has improved at least an order of magnitude since the original ITT caps in your amp were state of art. So if the factory-fresh look of the amp's guts is of low priority go for modern stuff.

One thing that is not mentioned often: large can electrolytic caps are sensitive to mechanical shock, dropping one on hard floor may damage it.
( This info is contained in cap manufacturers apps notes).

When you buy on ebay you don't really know the caps history even if it's unused.

[macphly]

All great information...thank you very much. I think you've verified my gut feeling and have helped me to be comfortable with the JJs...they will fit in the existing clamps, yes? I'm thinking of doing the change myself (and understand the dangers and need to drain the existing caps)...any reason why I shouldn't...I don't have a variac...would I need to "reform" the new JJs when installing them and power up for the first time?

[VacuumVoodoo]

You don't need to reform new production electrolytics.
Check the dimensions in datasheets on JJ website: http://www.jj-electronic.com/pdf/TC_TE.pdf

[macphly]

VaccuumVoodoo,
Forgive if this makes me a schmuck--especially after you'd convinced me to buy some European caps, but I bought the ARS caps and did the swap. After talking to a couple of folks (One guy--a trusted "expert" like yourself--recalled a recent 2204 changed to new small caps and said he felt the amp got tighter and stiffer feeling. The other had recently installed ARS caps and said his amp improved). I decided it was a crap shoot, and that I'd go with the ARS since they were similarly "old tech," figuring I could always swap in the JJs if that didn't work out.

I tested the old ones before I removed them and found no voltage (amp had been sitting for almost a week). To be certain, I did the good ole screwdriver test and got no sparks...feeling safe I pulled the PCB to gain access to one of the filter caps, then did all the swaps...it all went very easily and the clamps fit perfectly...looks great and works great.

In the end, I don't think I notice any difference...but I feel better because I have new filter caps instead of 24-year-old ones. I'd guess I did well. Is there anything else I need to consider from a maintenance standpoint? I've recently re-tubed and biased (but didn't re-check it after the new caps...any reason I should?).

[macphly]

Below is a sort of geeky self-pride party over some modifications I've done to this "fixer-upper" JCM800 I bought just for this purpose. However the tranny gets pretty hot...I've never paid attention before but since changing to 6550s I've noticed that the tranny gets hot enough that I wouldn't be comfortable leaving my hand on it, but not enough to create an instant blister, if that helps describe it. Is this normal? If so, question #2 is that it is now mounted in this small head box, so I'm concerned that it may not ventilate well enough. It has a perforated metal plate on the back...should I install a top vent or a fan in the side or something?

Not only did I decide to re-check the bias, but while I was in there, I decided to try 6550s. I removed a resistor that was piggybacked on the factory one (this was the mod an earlier owner did to change the bias range for EL34s). The main reason for this change was that I've been finding the RFT 34s to get mushy later in the gig. Also I've got two pairs of original Tung Sol 6550s I've been using in my Flexi...Long story short, they sound GREAT in this amp. I also tried a pair of new reissue TS 6550s and while different from the originals (not quite as deep in the bottom), they would have made me really happy if I hadn't had the real deal TS to compare.

Feeling ****y, I followed a web tip, and while I was checking to see that the NFB was on the 4 ohm tab of the impedance selector, I added a cap to increase the bottom end.

I also remembered that I've had some Mullard ECC83s that didn't knock me out for my THD amps nor two other Marshalls, but they were absolutely the icing on the cake for this amp! Perfect mids!

[VacuumVoodoo]

Well..

Heater current draw EL34 - 1.5A, 6550 - 1.6A so far so good.
Plate current draw:

Idle: EL34 ca 30mA, 6550 ca 85mA (each)
Idle power consumption: 2xEL34 ca 45W; 2x6550 ca 90W

Full power (ca 40W into speaker) EL34 ca 120mA, 6550 ca 150mA (each)
Power consumption :
2xEL34 ca 115W; 2x6550 ca 140W

This shows you that the amp with 6550s will draw almost as much power idling as same amp with EL34s delivering about 1/2 of it's rated output power to the speaker.

When idling the amp draws twice the amount of power with 6550s, the transformer will be significantly hotter.
If you can hold your hand palm down on the transformer for 5 sec without twitching and blistering it should be fine. Extra vents and/or a fan is a good idea. Another solution (which I prefer) is to install a separate small transformer for heater supply.

[macphly]

Thank you...that makes me feel better about it. Yes, I can leave my hand on the tranny for 5 seconds no problem. Can you suggest a small transformer to use?

[VacuumVoodoo]

Hammond type 167N6 will give you center tapped 6.3V at 4 Amps, suficient to run heaters on 2xEL34 or 6550s.