Need help with Eico HF-81 power supply filter caps

breadhead

Active Member
Hello all and please don't laugh to hard as I am real new to this game of tube audio but I love it! I have replaced a lot of capacitors on this amp with great success until I tried to replace the 3 section electrolytic can capacitor with individual Sprague Atoms. I now have a LOUD hum in one speaker. I have double and triple checked my connections and all seem OK. Could it be that I grounded the two 40uf caps to the ground lug on one of the 6BQ5 tubes? I grounded the other 20uf cap to one of the ground lugs on the old can capacitor. You can see all of the connections here http://campbellsbargainbarn.com/new_page_1.htm. Please have a look and let me know what I did wrong.

Thank you very much,
Jim Campbell
 
From your picture, it looks like the right side of the two taped together, the postive lead is soldered to one of the twist lugs of the old multi-section can. Thats a ground(neg). Can't tell about the other one.
 
Welcome to AK!! We were all newbies at one time!

Hope you will find some interesting reading and discussion here.

In regards to your problem, there are a few possibilities and some questions for you based on your description and your picture.

1) If you originally had the can cap, all the grounds were at the same place, namely the chassis surrounding the can cap. If you ground one 20uf cap at the original ground, that's probably ok. The dual 40uf's being grounded at the ground PIN connection of the 6BQ5 is probably the culprit. This is because the two grounds are not necessarily at the same common lowest ground potential, and this can set up a ground loop which can cause hum. Because I don't have a schematic, I can only generalize - I'm assuming you're working in the B+ power supply here. I would try putting the ground back directly on the chassis as close to the original can as possible and see whether that solves the problem.

2) A couple of things from the picture that I'd comment on. - With what looks like vinyl electrical tape holding the two caps together, I can't clearly see the polarity markings on the caps or the values. Electrical tape, especially in warm or high voltage situations can be unreliable to hold for long and leaves a sticky residue. I'd recommend desoldering, cleaning the exteriors and then repackage them using tie wraps to hold everything down. Looks cleaner that way and much more structurally sound.

Also, at least from the picture, is the right side blue cap (+) side down? If the lower right hand corner is the 6BQ5 you were talking about and you are connecting the cap to the GROUND lug of the tube, that may be reversed polarity. I'm not sure since it appears the upper part of the picture has the (-) side of that cap being grounded to the old can chassis ground. If this is correct and the (+) side of the right cap is supposed to be handling high voltage from the tube socket (B+), then you should better insulate the (+) lead of the cap, it looks perilously close to the chassis and one slip and its bye-bye cap and probably transformer.

So we'll need some clarification here as to what's going on and how you've wired these in. Also, if you are replacing the old can cap, you should desolder the connections to the old cap and recreate the circuit using your new caps. Not real clear whether you've done that.

Hope that's a first step in your trouble shooting.

Regards,

Bart
 
I've been told kind of a rule of thumb is to put the HV PS cap grounds to where the PS transformer CT is grounded.
 
Thanks for all your help so far. The two 40uf caps that are taped together, (and thanks for the advice about vinyl tape), do both have (-) side down. I soldered the negative lead from cap #1 to the negative lead from cap #2. I then soldered the negative lead from cap #2 to one of the ground leads on the 6BQ5 tube socket. In the upper left hand corner of the picture is the 20 uf cap which is the one that I grounded to the old can cap ground lug. I did disconnect the leads from the old can cap except for the resistors which I replaced with new. As I said I did recheck the circuit several times and it appears that it is correct.

Thanks,
Jim
 
I just looked at the picture again and I can see why you thought the right hand cap of the two that are taped together has a grounded (+) lead. It looks as though it is soldered to the ground lug of the old can cap but actually the positive lead of this cap, one terminal of each resistor and the red wire are all soldered together. This solder joint rests directly above one of the ground lugs for the old can cap but there is a good inch clearance between them.
 
Hum in one channel is almost never a power supply issue. I would start looking for something that may have moved or lost its proper connect near the area of the work you just performed.
Also I would of used a new modern clamp mount dual 50uF can from JJ Telsa and one under chassis to perform this replacement it would of fit much better.
Get that electrical tape out of there and use wire ties. Electrical tapes adhesive will melt with the heat present under the chassis and unravel. Never use electrical tape in a tube amp.

The ground setup you used while not Ideal will work just make double sure that it measured tube be absolute ground. Tighten the screws up that hold that tube socket in.

Craig
 
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