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Old 7th April 2012, 03:38 PM   #1
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Default filter cap markings

Im performing a cap job on an oldteisco amp. The old chassis mounted caps have lugs marked a and b. The new jj caps have + and - markings.
Im guessing a is the positive side? It makes sense according to the wiring. Between thectwo original caps, there is a jumper wire from an a lug to a b lug? Is this a normal setup?
A lot of questions, i know
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Old 7th April 2012, 04:20 PM   #2
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It seems the OEM part is multi-section. A jumper between 2 sections, places them in parallel. The aluminum case is a common negative terminal, which is grounded.

Post the schematic, please. What value JJ parts did you buy?
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Old 7th April 2012, 05:54 PM   #3
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Hi. Thanks for the reply

I bought two of these:
JJ 100uF, 385V radial :: JJ Capacitors :: Electrolytics :: Capacitors :: Passive Components :: Electronic Parts :: Banzai Music

My teisco is the checkmate 15, but I can only find a schem for the checkmate 20... I think it is quite similar:
Omdirigering
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Old 7th April 2012, 11:17 PM   #4
DHT112A is offline DHT112A  United States
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"a lug to a b" or a to b would NOT parallel them. That would series them. Looking at some
Teisco amp schematics it appears they did series the filter caps.

Last edited by DHT112A; 7th April 2012 at 11:22 PM.
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Old 8th April 2012, 11:47 AM   #5
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Here is a more accurate link to the schematic of a teisco checkmate 20

http://www.preoccupationrecords.com/...ckmt20_sch.gif

Im sorry, but checkmate 15 schem seems impossible to find...

Mine is a checkmate 15, but it seem they are quite similar. im not too good at reading schems 8Have succesfully built 4 tube amps...Im a silly paint by numbers kinda guy)

BUT, looking at the schematic above: I think it looks as they are two caps wired in series, And the a-terminal MUST be the positive one, as the b-terminal of the last cap is connected to ground!

Ill try posting a picture of the original wiring of the caps...
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Old 8th April 2012, 11:52 AM   #6
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Click the image to open in full size.
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Old 8th April 2012, 11:53 AM   #7
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Old 8th April 2012, 01:13 PM   #8
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Yes, that looks like series wiring. I was thinking the jumper was across sections of a twist-lock multi-section part, not 2 separate parts. The caps. in the amp are clamp mounted and (apparently) single section.
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Old 8th April 2012, 08:12 PM   #9
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Well. now Ive installed the new caps, and I have sound coming through. The output is very low (It was also low before the cap job, actually nothing has changed)

I have serious redplating on the output tubes. The cathode resistor measures 128 ohms, the plate voltage (pin 3-7) reads 250 volts, and the voltage measured across the cathode resistor is 28 volts.
Using the weber bias calculator gets me in the area of 26 watts dissapation per tube.... I bit hot maybe

This is an old amp, from the late 60s I think, so a lot of things could go wrong. Where do i go next to try to solve my problem....all filter caps in the power supply are replaced with fresh ones... and the cathode resistor is supposed to be 130 ohms, so i guess a 128 ohm reading is ok?
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Old 8th April 2012, 08:29 PM   #10
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Replace ALL electrolytic caps., as they literally dry out over time.

The interstage coupling caps. can become "leaky", which throws operating points totally out of whack. Replace those parts with 716P series "Orange Drops".
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