Help with Eico HF-81 power supply capacitors

[Raul]

About 10 years ago I bought an Eico HF-81 in good conditions, the seller said it was already electronically restored, all coupling caps are orange drops 716P, I only had to replace a few small electrolytics, and take care of the cosmetics. I had it repainted and cleaned the faceplate, here it is as it became after restoration:

The amp sounds fine, voltages are all within specs except for some on 12AX7s that are a bit higher 94 (80) 80 (60) 105 (94) 110 (95), as all resistors are still originals I will have to check them all one of these days. Frankly I did not use this amp much during these years and I didn't care about PS caps until today when I opened it ready to replace the can caps. I noticed that the PS caps had been already replaced.

The two single caps that according to schematics were 30uf 400V were replaced with two 47uf 450V (are these capacitance correct or too big?) while the can was replaced with this Mallory PFP:

It seems that he changed something inside here too, "semicircle" is 50uf 350V (on schematics it was 40 and I measured it at about 58uf), "square" is 100uf 350V (measured 108, schematics 40uf), "triangle" is 54 (measured, as on can it is indicated at 10uf 300V while on schematics it is 20uf 400V, so if it's 300V or even 350V it seems a too low voltage anyway).
Then there is the 4th cap in the can, unused in the amp, that measures 30uf (probably it's the one hand-written in red 30uf 350V on the can).
My question now is: what should I do with these caps? Which ones are OK and which ones I should eventually replace (also for safety reasons) and with what?
Thank you very much

 

[sKiZo]

The 350v rating on your cap should work, but is bound to stress something over time. I'd put that on the to do list.

Hard part is finding an appropriate can cap for the Eico's that will fit the chassis. Not a lot of height to work with. I used regular axial caps on my HF12a and left the original in place just to fill the hole in the chassis. I've also heard of folk using a JJ 50uf/50uf/500v two stage can and adding a 100uf/500v axial below the chassis for the big stage.

 

[primosounds]

Although it is best if you follow the recommended voltage on the schematic. There is a generous surge allowance on top of the working voltage. So if the working voltage is not more than the rated voltage on the cap you maybe ok. Just measure the voltage after the amp has warmed and if falls below the rating on the cap then you are ok. One thing you can check for is the heat build up in the cap soon after you first turn it on. If the can heats up quickly say within 5 minutes then it is definitely going bad and needs replacement. Otherwise if there is no hum or odd noises i would just leave it. Cheers
BTW, those Mallory blue label caps are highly coveted and are believed to be the better sounding of the can caps.

 

[FlaCharlie]

The two single caps that according to schematics were 30uf 400V were replaced with two 47uf 450V (are these capacitance correct or too big?)

I'm out of town again but I found a schematic online. This amp uses dual EZ81 rectifiers. I assume that doesn't affect the maximum first cap rating, which is 50uf. If so, those replacements should be fine and the higher voltage rating is preferred because the voltage supplied by those caps is 340v and 330v and will likely be higher due to modern wall voltages.

the can was replaced with this Mallory PFP: It seems that he changed something inside here too, "semicircle" is 50uf 350V (on schematics it was 40 and I measured it at about 58uf), "square" is 100uf 350V (measured 108, schematics 40uf), "triangle" is 54 (measured, as on can it is indicated at 10uf 300V while on schematics it is 20uf 400V, so if it's 300V or even 350V it seems a too low voltage anyway).
Then there is the 4th cap in the can, unused in the amp, that measures 30uf (probably it's the one hand-written in red 30uf 350V on the can).
My question now is: what should I do with these caps? Which ones are OK and which ones I should eventually replace (also for safety reasons) and with what?

The schematic shows C3a (semicircle) as 20uf and it supplies 280v, so you could leave the replacement (58uf 350v) in, although that's a pretty big increase in capacitance. C3b (square) is shown as 40uf and it supplies 175v, so your replacement (108uf 350v) should also be OK. On the schematic I found, C3c (triangle) is shown as 40uf and supplies 115v. If your replacement cap is supposed to be 10uf and it actually measures 54 I would replace it. I'd probably use a 47uf rated at 150v to 200v.

 

[Raul]

As I have at hand two 47uf 450V and one 22uf 450V I am tempted to replace them all, as that can has at least 10 years so it won't hurt to replace those caps anyway.
What about the two singles caps (C1 and C2) that now are 47uf and were 30uf? Aren't these the first after the rectifiers? May I leave these alone?

 

[Raul]

Although it is best if you follow the recommended voltage on the schematic. There is a generous surge allowance on top of the working voltage. So if the working voltage is not more than the rated voltage on the cap you maybe ok. Just measure the voltage after the amp has warmed and if falls below the rating on the cap then you are ok. One thing you can check for is the heat build up in the cap soon after you first turn it on. If the can heats up quickly say within 5 minutes then it is definitely going bad and needs replacement. Otherwise if there is no hum or odd noises i would just leave it. Cheers
BTW, those Mallory blue label caps are highly coveted and are believed to be the better sounding of the can caps.

voltages at the can caps are square 181 (schematics 175), half-circle 296 (280), triangle 125 (115)

 

[primosounds]

voltages at the can caps are square 181 (schematics 175), half-circle 296 (280), triangle 125 (115)

Well those voltages are well within the 5% tolerance level so you are in fine shape there. What are the voltage rating on the can cap for those 3 posts? Are those rated at 350vdc ? If so, you would have nothing to worry about. Many times especially for kit amps, the manufacturer would use what caps they have on hand that 'fit the bill'. In other words as long as they satisfy the engineering requirements never mind that it is actually over specc'ed, they would install that part. In this case a 350v part for the original design could have been installed. But, if they had a whole truck load of 400v caps in the warehouse they would use that part instead of ordering new ones.

 

[Raul]

Well, at the end I decided to replace the first two caps (C1 and C2 in sams schematics, as their reading with my ESR meter weren't clear) and I put two 33uf insted of the 47uf that the previous owner put there, these are closer to the original 30uf in Sams, while I am leaving alone the can caps for now. I replaced also about a dozen of resistors that were sligthly off specs, voltages are all within specs except for pin 1 and 6 of V1-V2 on sams (V1-V6 on eico manual) that are a bit high at 94 and 80 instead of 80 and 60.
Another thing is some noise/hum that is present on one channel but it stops by moving a bit the tube on V8 (V5 on eico schematics) phase inverter, it doesn't cange by swapping tubes and when tapping with a wood stick I found that the most "sensitive" point seems to be connection between pin 7 of V8 and the switch on the chassis (red circle below):

I frankly don't know what that swich is, a stereo mono? Is it possible to remove/bypass this switch? Pros? Cons? It seems that someone does this mod when restoring the HF81. About doing it I understand that wire from pin 7 should go directly to the green circle but what about the other connection, the wire in the yellow circles?

 

[FlaCharlie]

Well, at the end I decided to replace the first two caps (C1 and C2 in sams schematics, as their reading with my ESR meter weren't clear) and I put two 33uf insted of the 47uf that the previous owner put there, these are closer to the original 30uf in Sams, while I am leaving alone the can caps for now. I replaced also about a dozen of resistors that were sligthly off specs, voltages are all within specs except for pin 1 and 6 of V1-V2 on sams (V1-V6 on eico manual) that are a bit high at 94 and 80 instead of 80 and 60.
Another thing is some noise/hum that is present on one channel but it stops by moving a bit the tube on V8 (V5 on eico schematics) phase inverter, it doesn't cange by swapping tubes and when tapping with a wood stick I found that the most "sensitive" point seems to be connection between pin 7 of V8 and the switch on the chassis (red circle below):

I frankly don't know what that swich is, a stereo mono? Is it possible to remove/bypass this switch? Pros? Cons? It seems that someone does this mod when restoring the HF81. About doing it I understand that wire from pin 7 should go directly to the green circle but what about the other connection, the wire in the yellow circles?

First, what do you mean when you say your ESR readings were "unclear"? I would probably have leaned toward keeping the 47uf caps in place since they will offer a bit more filtering and they are not too high for the rectifier. Going back to stock values is OK too but I would definitely use 450v parts for reasons I mentioned earlier. I have a cheap ESR meter so I don't really trust, or even care about, the actual reading. I just compare the reading of the old cap with the reading of a replacement cap. Did you try that?

As for the can cap, I'd be concerned about the section that was supposed to be 10uf and actually measures 54uf. Even though the schematic calls for 40uf, that section is waay out of spec so I'd replace it. Again, I'd use a 47uf cap. The voltage ratings I suggested earlier are based on the level of voltage each cap is supposed to supply to the circuit. Since that section supplies only 115v, a cap rated at 150v or 200v is all that's needed. As Primo mentioned, the values of the original can were probably the result of them getting a good deal on a bulk purchase of caps, not engineering decisions. And the previous owner replaced the original can with a totally random unit. I'd probably replace the three sections of C3 with individual 47uf caps rated at 350v, 250v, and 150v.

The higher voltages on those pins are likely due to a combination of factors including increased wall voltage, increased PS capacitance, resistor values and condition of the individual tubes. To chase it down, I would start by putting the amp on the variac and adjust it so that the voltage supply for that tube is exactly right and then measure again. If the voltages are OK now, you could adjust the value of the resistor going to that section of the PS. This will, however, affect PS voltages downstream. You could also adjust the value of the plate resistor for each tube.

The noise/hum is probably just a dirty socket/pin connection. Clean them and tighten up the socket contacts. Sometimes putting a very slight bend in a pin will also help it to make a better contact.

Here's a post that refers to that switch. https://www.audioasylum.com/forums/vintage/messages/22/228366.html I forget what I did when I rebuilt my HF-81 but I'd suggest just cleaning it and leaving it alone. The cause of your noise is more likely the socket / tube connection.

 

[Raul]

"unclear" means that my meter had difficulty in reading those two caps and after some time it said "damaged or missing part" so even though the caps were probably good (it was difficult that both had gone bad together so probably it was difficult for the meter to read them in that circuit) I replaced them and fearing a too high capacitance after the rectifiers I put two 33uf 400V that I had at hand. When measured off circuit both 47 caps were correctly read as 48 and 51uf, 0.9 ESR and low V.loss, so they are good and I could put them back in the amp but I think I will leave the two 33 in their place (I'm lazy sometimes)
About the can, as I don't want to mess things too much I think that I will follow your advice and replace all 3 caps. One thing is unclear from the schematics, which one is the 20uf cap, Sams says C3a (half-round) is the 20uf cap, while the Eico manual in parts list the can as C37 40/40/20 (that should logically be labelled A,B,C) and in its schematics it shows A as the first after the two single 30uf caps, in this case if A is the first one it should be 40uf and the last one (C) the 20uf. Very confusing. Putting as you advised a 47uf cap in every section would solve the doubt in any case.
P.S. I cleaned the switch and socket with deoxit and that noise seems gone

 

[FlaCharlie]

"unclear" means that my meter had difficulty in reading those two caps and after some time it said "damaged or missing part" so even though the caps were probably good (it was difficult that both had gone bad together so probably it was difficult for the meter to read them in that circuit) I replaced them and fearing a too high capacitance after the rectifiers I put two 33uf 400V that I had at hand. When measured off circuit both 47 caps were correctly read as 48 and 51uf, 0.9 ESR and low V.loss, so they are good and I could put them back in the amp but I think I will leave the two 33 in their place (I'm lazy sometimes)
About the can, as I don't want to mess things too much I think that I will follow your advice and replace all 3 caps. One thing is unclear from the schematics, which one is the 20uf cap, Sams says C3a (half-round) is the 20uf cap, while the Eico manual in parts list the can as C37 40/40/20 (that should logically be labelled A,B,C) and in its schematics it shows A as the first after the two single 30uf caps, in this case if A is the first one it should be 40uf and the last one (C) the 20uf. Very confusing. Putting as you advised a 47uf cap in every section would solve the doubt in any case.
P.S. I cleaned the switch and socket with deoxit and that noise seems gone

That message would make me nervous too. I'm sure the 33uf caps will work fine. Glad to hear you got it all sorted out.

 

[triode17]

except for some on 12AX7s that are a bit higher 94 (80)

Which AX7, V1 and V2 or
and V6? Also, do you have the owner's manual? If not, here it is. http://www.vintagevacuumaudio.com/schematics-manuals/eico/eico-hf-81-owner-manual-schematics.pdf

 

[Raul]

V1 and V2, that are 91/93 and 78/79 instead of 80 and 60

 

[Raul]

I am going to replace the 3 can caps as mine were replaced with a bit random values by the previous owner as I wrote above, so anyone that has an HF81 with original can can please tell me if the positions of the 3 shapes (half-circle, square, trianle) are at least correctly oriented in mine (pic below) and which values correspond to each shape? i.e. is the half-circle the 20uf cap and the two other are the 40uf? Is half-circle the one above that shares the 22k 1W resistor with square on the right like in my pic? I don't want to put the 20 in a wrong place.
This because, as I wrote before, there is a bit of confusion between sams and manual. Sams says C3a (half-round) is the 20uf cap, while the Eico manual in parts list the can as C37 40/40/20 (that should logically be labelled A,B,C) and in its schematics it shows A as the first after the two single 30uf caps, in this case if A is the first one it should be 40uf and the last one (C) the 20uf.
Any definitive clarification is welcome

 

[FlaCharlie]

If you're going to use individual caps mounted underneath, just use 47uf for each. You could use caps with voltage ratings of 350v, 250v, and 150v, depending on the section of the PS each cap is supplying, see the schematic. Or you could use all 350v (or higher) rated caps.

If you're going to use a replacement can, I'd suggest a 40-40-40 with similar voltage ratings. You could go with a bit more capacitance if you like because these are not connected directly to the rectifier. As mentioned earlier, Eico probably chose these values because they got a good deal on a bulk purchase of cans with that combination. If I was using one cap that had lower capacitance than the others, I would use it in position C3c.

 

[Raul]

If you're going to use individual caps mounted underneath, just use 47uf for each. You could use caps with voltage ratings of 350v, 250v, and 150v, depending on the section of the PS each cap is supplying, see the schematic. Or you could use all 350v (or higher) rated caps.

If you're going to use a replacement can, I'd suggest a 40-40-40 with similar voltage ratings. You could go with a bit more capacitance if you like because these are not connected directly to the rectifier. As mentioned earlier, Eico probably chose these values because they got a good deal on a bulk purchase of cans with that combination. If I was using one cap that had lower capacitance than the others, I would use it in position C3c.

will put separate caps, but which one in the original amp (not schematics) has the lower capacitance?