Eico HF81: which of the can caps is the 20uf?

Raul

Active Member
I put here a question that remained suspended in my last thread. I am going to replace the 3 can caps as mine were replaced with a rather random can cap by the previous owner, so anyone that has an HF81 with original can could 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.
8DXRCjY.jpg

I am asking because there is a bit of confusion between sams and manual. Sams says C3a (half-round) is the 20uf cap:
9kgAUeW.jpg

while the Eico manual in parts list identify the can as C37 40/40/20 (that should logically be labelled A,B,C in this order but it doesn't specify it) 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. From the part list the resistors here are R60: 68K, R61: 22K, R62: 4.7K, R63: 350 ohm, C38 and C39 are two separate single 30uf caps.
All this is very confusing to me, any definitive clarification would be very welcome. Thank you
LL7xrkU.jpg
 
What does the 20uF cap feed in your amp? I would use that as a guide, providing the amp works...
 
The amp works, but as I wrote the can is not the original one, the previous owner put this one, for this I need to know correct capacitances in the circuit:
FOlKK94.jpg
 
The 22uF cap on the first schem goes to V2, V7... Is that the same as your amp?

The problem is that I don't know how tubes are numbered in that sams schematcs, they are different than in the eico manual, however here is my amp under-chassis:
2A9AZUQ.jpg
 
All tube numbers refer to this diagram.

HF81diagram.jpg



HALF CIRCLE
Original: 20uF
Yours: 50uF
(single orange wire off cap, 280V supply, to phase inverter V5, V6)

TRIANGLE
Original: 40uF
Yours: 10uF
(two orange wires off cap, 115V supply to phono preamp, V1, V2)

SQUARE
Original: 40uF
Yours: 100uF
(single red wire off cap, 175V supply to tone amp, V3, V4)


The two orginal 30uf sections of the can straight off the rectifier have been replaced by the blue caps below the chassis.
 
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Hi Raul - I thought that was answered in the previous thread, but maybe not so clearly.

The shapes which designate each section are not really important. And, it was suggested that Eico probably used a can with those values simply because they got a good deal on them in a bulk purchase. There is no real reason why one of the three should be 20uf.

My earlier suggestion was to just replace all three sections with individual 47uf caps. The main consideration is that the voltages of the replacements should be higher than the voltage which they supply to the various parts of the circuit. The highest voltage supplied by those is 280v, so a 350v (minimum) cap should be used. You could use 350v caps for all three or you could use different voltage ratings such as 350v, 250v (for 175v source) and 150v (for 115v source).

If you insist on including a 20uf section (modern equivalent 22uf), I would use it for the 115v source. Early stages of an amp that supply preamp tubes don't usually draw as much current during operation and can get by with lower capacitance than output tubes.
 
Shape Original Yours . Goes to
half circle . 20uF 50uF . single orange wire V8 . 280V supply to phase inverter
triangle . 40uF . 10uF two orange wires . 115V supply to phono preamp
square . 40uF

So are these from an original HF-81 can? Mine is not an original can. In this case at least the orientation of my "shapes" is correct. I have correct voltages notwithstanding the different capacitances in the can that was used: half-circle 284 (280), square 177 (175), triangle 123 (115) . So if this is correct I will put the 20uf in the half-circle and the 40 in the other two places.
 
I think I confused you! The amp works now,,, so I would just follow the wire from the 22uF cap in the amp to the tube it powers... then compare that to one of the schematics, to check the schem for accuracy... I think the other posters may explain more clearly...
 
If you insist on including a 20uf section (modern equivalent 22uf), I would use it for the 115v source.

This is what the Eico schematic shows. The 115v source powers the tubes they designate as V1 and V2. The position of those tubes is shown on the diagram posted above. I would trust the Eico schematic over the Sams but, as I stated earlier, if you just use 47uf in all three positions the issue becomes moot. Again, the main thing is to make sure the voltage ratings are correct.
 
So are these from an original HF-81 can? Mine is not an original can. In this case at least the orientation of my "shapes" is correct. I have correct voltages notwithstanding the different capacitances in the can that was used: half-circle 284 (280), square 177 (175), triangle 123 (115) . So if this is correct I will put the 20uf in the half-circle and the 40 in the other two places.

Sorry, my post wasn't complete. I organized it a bit better. Hope it makes sense.

The values in your replacement can do seem a bit "random". As FlaCharlie says, putting 47uF in all three positions would be fine. I would probably go for 400V rated caps.
 
OK, will put 47uf 450V in all three positions so everything is solved- Thank you
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update: caps just replaced, amp works fine but voltages at PS caps are a bit higher than before:
half circle now 292 it was 284 (schematics 280), square 186 it was 177 (175), triangle 132 it was 123 (115). Voltage from my stepdown transformer is 115V. Should I worry about these values or may I consider them still good as two are less than 10% off and one is 15% off from schematics?
 
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Should I worry about these values or may I consider them still good as two are less than 10% off and one is 15% off from schematics?

Since +/- 15% is considered "in spec", no problem.

If you really want to drop the voltage a bit on that last section, you could just put in a higher value resistor in place of R60 (Eico Schematic).

Using Ohms Law . . . you are now dropping 54v across R60, which is 68k. That's 186v - 132v = 54v. Current for that 115v section is I = V / R, so current is 54v / 68,000 = .0007941 amps. V (voltage drop) = I x R. Plugging in standard values for R shows that .0007941 x 82,000 = 65.12v.

So if you substitute an 82k for the 68k you'll drop 65v which will get you down to about 121v.
 
Thanks Charlie, I think I can live with those voltages, but why before with the same resistors voltages were all lower? For the different capacitances of the previous caps?
 
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