k12 question

[Webelo]

I ordered some Wima caps to replace some of the caps on my s5 k12 amp as recommended here.

The stock value for caps c1, c2, c3, c4, c6, c7 is .22 uf/250v

The caps I bought are .15uf/400v.

are these a suitable replacement? Or should I be replacing them with Wima .22uf's?

thanks.
john.

[Shannon Parks]

John,

Can you point me to a schematic? I assume it is somewhere in the forums, but if you have a link already handy...

[mesherm]

http://webpages.charter.net/mesherm/k12m_schematic.jpg

[Shannon Parks]

I would omit C1 and C4 and use wire jumpers. The front end will not be AC-coupled, but I think even with a .22uF cap you would suffer too much roll-off at the low end, if you had the audio pot turned low. The other .15uF caps are fine subs.

[Webelo]

I have the newer k12 'g' model. I guess it's a different tube than the 'm' model and a few circuit changes to accomodate for that, but I can't seem to find the 'g' schematic online. Does this change your answer?

What does substituting lower value caps mean in terms of the circuit? In terms of sound? Am I still able to use the wima caps if I don't jump c1 and c4?

john.

[Shannon Parks]

What does substituting lower value caps mean in terms of the circuit? In terms of sound?

Extremely small difference. The amp could possibly work better. It will shift a LF pole around below 20Hz.

Am I still able to use the wima caps if I don't jump c1 and c4?

I would only use C1 & C4 if you intend to operate this amp with a preamp and these volume pots will be wide open.

[EWBrown]

I used the 0.15 uF caps and they sound fine. Even 0.1 uF would be OK, the difference in LF response is still far below normal ranges of human hearing and most OPTs Yellow_Light_Colorz_PDT_11

Those original brown mylar 0.22s are the "weak link" in the original design.

C1 and C4 (the input caps) are there just in case that the audio source has DC riding on the output -a not-too-likely scenario.

/ed B in NH

[Webelo]

cool... thanks for the heads up guys.

The WIMAs don't fit the pcb that great anymore. And with the short leads, I've had to shape them a bit.

I'm in the process of making mods to the board and putting the whole thing in an enclosure. Expect more questions :-) and photos soon.

ps... you guys are pretty wicked helpful without the "smug" that comes with another popular tube forum.

[EWBrown]

THe ones I used fit the board perfectly, perhaps these are a different cap (MK4 or MK-10?) or the board layout may have changed.

Either way, the WIMAs give a lot of "bang for the buck" improvement.

/ed B in NH

[Webelo]

I'm looking forward to powering her up again.

Mr. Brown, Where abouts are you in NH?

I'm just south of Worcester, MA.

[EWBrown]

I'm in Greenville, about 15 miles north of Fitchburgh. In the frozen wastelands of NH Yellow_Light_Colorz_PDT_02

/ed B in NH

[Webelo]

C1 and C4 (the input caps) are there just in case that the audio source has DC riding on the output -a not-too-likely scenario.

When would DC be riding on the output?

[Webelo]

Here is the layout I'm considering. Any major no-no's? The power transformer and PCB (obviously) will be mounted under the top plate.

Would someone mind running through wiring up the new OPTs... So many colors/wires!

[Gingertube]

Weblo,

Your picture shows a Hammond 1608 Output Transformer.
Check this out:
http://www.hammondmfg.com/1608.htm

Primary Side Wiring:
P1 - Blue Lead
CT - Red Lead
P2 - Brown Lead

The Blue/Yellow and the Brown/Yellow leads on the tranny are Ultralinear Taps and are not used (put a bit of heat shrink over the lead ends to stop them shorting to anything and coil them up and stow them out of the way)

Secondary Side Wiring:
For 8 Ohm Output

S1 - Yellow Wire
S2 - Black Wire AND Black/Yellow Wire

Connect Green Wire and Green/Yellow Wires together and stow these (These 2 wires are the 4 Ohm Output if you decide that you want 4 Ohms rather than 8 ohms). If you do want to use the 4 Ohm Output you still need to take the feedback connection from the 8 Ohm tap.

That should be fine BUT if you find that the amp oscillates when you've wired all this up then:
Disconnect the feedback from the 8 Ohm tap - if that stops the oscillation it means that the phase of the feedback is backward. To fix that swap the Blue and Brown leads on the Primary side and then reconnect the feedback.

Hope this helps,
Cheers,
Ian

[Webelo]

Just want to make sure I have this right. I need to connect the green and green/yellow wires together...as in electrical connection?

S2 is to ground... so I should ground both 0ohm taps?

I think I just figured it out how secondary connections work... there's a nice diagram on the hammond website :-)