filter cap markings

[bagudan]

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

 

[Eli Duttman]

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?

 

[bagudan]

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

 

[DHT112A]

"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.

 

[bagudan]

Here is a more accurate link to the schematic of a teisco checkmate 20

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

I´m sorry, but checkmate 15 schem seems impossible to find...

Mine is a checkmate 15, but it seem they are quite similar. i´m not too good at reading schems 8Have succesfully built 4 tube amps...I´m 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!

I´ll try posting a picture of the original wiring of the caps...

 

[bagudan]

 

[bagudan]

 

[Eli Duttman]

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.

 

[bagudan]

Well. now I´ve 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 60´s 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?

 

[Eli Duttman]

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".

 

[bagudan]

Phew... I Will get started Then. Since i dont have an original schem, I willhave to just look at each single part... But these caps are old and different from what i have seen.
For instance: if a cap says: atlas Oil 0,5uf cp-c 400wv, am I looking for a cap with a 400 v tolerance Then?
Thanks for the reply btw

 

[bagudan]

Hmm maybe a 0,5 uf cap is not an Electro?

 

[Eli Duttman]

Ah ha! Old oil coupling caps. that are leaking. Replace them with 0.47 μF. Soviet surplus K40s, which are paper in oil (PIO), of at least 400 WVDC.

Polypropylene film and aluminum foil 716P Orange Drops will work, but the amp's voicing may be unfavorably altered.

 

[bagudan]

Tanks for the reply. Maybe a silly question, but how do i know wich ones are the coupling caps.... There are loads of old Oil caps.... Arent they supposed to be connnected to the grids of the power tubes... Or do I need to change All the nonelelctrolytics as Well?

 

[Eli Duttman]

All 'lytics MUST be replaced, as they have dried out over time.

If you have the instruments to test the "oilers" for leakage, some might still be good. Otherwise, just replace them. Fortunately, Soviet surplus PIO parts are reasonably priced.

 

[bagudan]

Have just ordered the fresh set of caps...looking forward
Wondering about one specific cap though. Looking at the schem, there is a 0,02 uf cap between the 12ax7 an the 12au7.
This cap has been replaced (Or at least it is different from the others) It is a Jensen cap, manufactured in denmark (Me and the amp are from denmark) No big deal changing this one, but it is huge, and it says on the cap that it has been tested for 5000 v? Maybe this is just what they had lying around, but do you see any reason to find a cap with this extreme voltage rating?

 

[bagudan]

It´s alive
First of all, thanks for all the good advice...

Changed every single cap in the amp....and now it´s breathing again.
The cathode voltage is 10v
cathode resistor is 128 ohms
plate voltage (pin 3-7) is 330v
Weber bias calculator gives me: 12 watts per tube / 37 milliamps per tube
Still a bit hot, bút no redplating on the old 7189a powertubes at all. It´s not too loud, and it´s a quite clean sounding amp. I´m not shure if this is beacuse of old powertubes or maybe the small 10 inch speaker isn´t too efficient.

There is an annoying whistling kinda noise.... gets worse when treble, volume or reverb pot is turned up?
Tremolo works and sounds sweet, The reverb is lousy or nonworking? It is there somewhere, far away in the mix.... very faint, but You can hear the springs rattling in there constantly...

I bought some russian powertubes to replace the original 7189a´s. They are called 6P14P-EV. I believe they are good substitutes for 7189a´s. But when I put them in, I get no sound at all, even though the heaters light up?

To sum it up: I´m a happy man, but not finished working on this thing.... Wondering what else is left to do?

Power resistors.... Do they wear out over time?
What about the Two 0,0022 caps in parallel with the diodes on the filter caps (in the picture earlier in the thread)..... could only find ceramics to match the 1000v rating....will they do?

 

[bagudan]

Found som pinout diagrams..
It seems that the 7189a pinout is a little unique

http://www.bustedgear.com/res_Tube_pinouts.pdf

Can anyone confirm that I need to jumper pins 1 and 2, and also jumper pins 6 and 9, to install a 6p14p-ev, in a 7189a-socket?

 

[Ronsonic]

There is an annoying whistling kinda noise.... gets worse when treble, volume or reverb pot is turned up?

Suspect a microphonic tube. Tap on things and see which are sensitive to vibration. May be related to ....

The reverb is lousy or nonworking? It is there somewhere, far away in the mix.... very faint, but You can hear the springs rattling in there constantly...

Check resistance of the transducers. Open it up and look in the pan, a broken spring can pick up noise and kill the pan.