12xxx Larry Ground

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Strat78
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Re: 12xxx Larry Ground

Post by Strat78 » Fri Mar 23, 2012 8:31 am

ground #3.

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Re: 12xxx Larry Ground

Post by jimmyride » Fri Mar 23, 2012 8:48 am

Thanks a lot!

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Re: 12xxx Larry Ground

Post by Twistingcrow » Tue Jul 10, 2012 10:53 am

Hi there.
I'm building a 12XXX too... I started this morning. My heater CT is too short to go to point 1 of Larry's Star Ground (I'm using Marstran T2562 PT and Marstran C1998 OT). Do you recommend to solder a wire to it and shrink wrap to add the necessary length or do you recommend to solder it to another point?
Thanks!

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Re: 12xxx Larry Ground

Post by Strat78 » Tue Jul 10, 2012 11:10 am

Just extend the wire so it can reach #1. :toast:
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Re: 12xxx Larry Ground

Post by Twistingcrow » Tue Jul 10, 2012 12:01 pm

Sweet. Let's extend then... Thanks! :rock:

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Re: 12xxx Larry Ground

Post by Twistingcrow » Fri Jul 13, 2012 9:11 am

Image

I have a couple of questions on this:
The bias pot on the board is not shown. I have seen that it must be located between the two + sides of the 10uF caps. OK about that. But there is also a linking ground wire there. So does the pot replace that or must I still keep that wire? In the JTM45 build that same wire is kept s I guess I have to keep it here as well...

OK also about the 68K bias resistor that must be moved (and reduced to 33K I have read?)

Another one: is the PT bias wire soldered at the far left of the board (to the 27K resistor on the pot side of the board) or must it go to one of the bias pot legs? If so, which one? The one closest to the 27K or the other one?

Thanks!!!

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Re: 12xxx Larry Ground

Post by neikeel » Fri Jul 13, 2012 11:57 am

You keep the ground wire linking the two bias caps.

The bias pot is effectively replacing part of the 68k fixed bias resistor.

I would use a 47k resistor with a 25k pot (gives you minimum of 47k and maximum of 72k). If you cannot get in range you can always go up or down.

Two legs of the bias pot must be grounded (middle and one end) the other hooked up to the free end of the bias resistor. I am pretty sure there are pics in the wiki for the different shapes of bias pots.

No one of the bias feeds goes to the 27k bias feed resistor and the other to ground (it is 95vac so either wire will do).
Neil

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Re: 12xxx Larry Ground

Post by Twistingcrow » Fri Jul 13, 2012 12:30 pm

Thanks a lot!!!

Just one last thing (unless I need troubleshooting later that is...). I have a .0022uF (Sozo 222K) cap as per Robert's (Valvestorm) advice. But I don't know where it goes... And I haven't found any pic...

Thanks!

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Re: 12xxx Larry Ground

Post by vanhalen5150 » Fri Jul 13, 2012 1:02 pm

Twistingcrow wrote:Thanks a lot!!!

Just one last thing (unless I need troubleshooting later that is...). I have a .0022uF (Sozo 222K) cap as per Robert's (Valvestorm) advice. But I don't know where it goes... And I haven't found any pic...

Thanks!
.
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Re: 12xxx Larry Ground

Post by Twistingcrow » Fri Jul 13, 2012 1:41 pm

Awesome!!!! Thank you!!!! :rock:

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Re: 12xxx Larry Ground

Post by BoogieEngineer » Thu Oct 18, 2012 10:11 pm

Please someone tell me what brand the cap circled in blue is!
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Re: 12xxx Larry Ground

Post by vanhalen5150 » Fri Oct 19, 2012 6:44 am

I think its a Lemco like the others on the board.
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Re: 12xxx Larry Ground

Post by BoogieEngineer » Fri Oct 19, 2012 11:51 am

vanhalen5150 wrote:I think its a Lemco like the others on the board.
Where can I get those caps, I mean Lemco's with such high value? I searched over at familiar places like ValveStorm, TubeDepot and WattsTubeAudion, none has them

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Re: 12xxx Larry Ground

Post by neikeel » Fri Oct 19, 2012 12:16 pm

Why do you need one of those there?

A mustard is stock.

The RS silver micas are available in 22,000pF if you want a clear bright tone there
Neil

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Re: 12xxx Larry Ground

Post by BoogieEngineer » Fri Oct 19, 2012 12:36 pm

neikeel wrote:Why do you need one of those there?

A mustard is stock.

The RS silver micas are available in 22,000pF if you want a clear bright tone there
From the SL68 gut shot seems like John Suhr uses those for V1b and V2a (0.68uF). He uses Mallory's for couplers and others, so I think brand choice may matter. I mean different cap constructions matter in different positions

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