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1

Moreplay / Re: Can balance knob be bypassed

« Last post by Paul Birkeland on Today at 05:32:47 AM »

The yellow caps present a very low impedance between audio ground and the chassis but only at very high frequencies.  This is a way to drain off high frequency gunk off the audio ground before it gets into the circuit.  These are not necessary, nor useful on the output jacks. 

We have about three decades of preamp kits that never used these, so don't fuss too hard over them.

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Moreplay / Re: Can balance knob be bypassed

« Last post by Finkle on Today at 05:17:36 AM »

This has been greatly helpful and informative. Thank you guys! Feel like I've gained more understanding in the last day than the last few months combined.

If reducing inputs and working on them already it's the space already being worked on so that's where they'll be added in mine.

Last question...for now. What are the purpose of the little yellow caps at the intake jack grounds? Assuming I should be leaving them in when installing the 100k resistor from input RCA hot to ground? Would an output jack be able to use the yellow .01uf cap if I swapped a couple of the input jacks to output? Or would a different resistor or just bare RCA ground to ground for output jacks? As I'm rewiring the inputs and outputs, they're already tied to a ground and looking appealing to reuse if tieing all the grounds together, input and output, at the jacks.

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Moreplay / Re: Can balance knob be bypassed

« Last post by Paul Joppa on Yesterday at 06:59:23 PM »

Very helpful and reading through the lingo as someone explains my exact scenario does help me understand more than just reading. So thank you very much.

Are these terminal #s correct then? A5 to 2, and B5 to 12. Grid stopper is at the end of the chain to ground at 2 & 12 with the other end everything hot starts from there out and the voltage can't pass through the stopper from the hot side is from what it sounds. And it's just this 100k resistor being installed, no other components need to be added?

Again, i greatly appreciate your guidance and time on this.

No, the 100K goes from A6 to terminal 2 and B6 to terminal 12. The grid stopper is already wired A5-A6 and B5-B6. A6 and B6 are not used by the tube so it's effectively just another terminal. Remember, "a grid stopper only works if it is the only thing physically connected to the actual grid; all other "grid" connections are actually made to the other end of the grid stopper."

The photo in the manual of the completed preamp shows red/black twisted pairs, with reds going to A6 and B6 and blacks going to 2 and 12.  The other end of the twisted pairs is connected to the balance controls. Unsolder those twisted pairs from the balance control and connect them to whichever input RCA jacks you want to use. (Obviously you'll have to remove the existing wires from those chosen RCAs!) You can then put the 100K resistors at either end of the twisted pairs, whichever is easiest.

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Moreplay / Re: Can balance knob be bypassed

« Last post by Paul Birkeland on Yesterday at 06:15:39 PM »

wouldn't I need to wire the grounds of the input and output jacks together at the jacks?

I don't really recommend doing this.  You have better control over the signal current running through the ground wires if all that is separated, and you may end up inadvertently creating ground loops if you deviate too far from the stock layout (which is tested).

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Moreplay / Re: Can balance knob be bypassed

« Last post by Finkle on Yesterday at 05:50:09 PM »

I'm doing work with the inputs, how would I go about running the resistor across the input jacks? One end to hot and one ground on each RCA input?

Also relooking at the volume pot wiring, am I seeing the input grounds running to the output grounds? Connecting on the same pin of the of the chassis side terminal of the volume pot? Without the selector and balance pot, wouldn't I need to wire the grounds of the input and output jacks together at the jacks? Any chance I could use the 4 input jacks that are tied 4 grounds together at the same L bracket with little yellow resistors or diodes? Use 2 jacks for input 2 for output?

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Moreplay / Re: Can balance knob be bypassed

« Last post by Paul Birkeland on Yesterday at 04:24:46 PM »

You may find it's a lot easier to just put the 100K resistors across the input jacks.

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Moreplay / Re: Can balance knob be bypassed

« Last post by Finkle on Yesterday at 04:09:26 PM »

Very helpful and reading through the lingo as someone explains my exact scenario does help me understand more than just reading. So thank you very much.

Are these terminal #s correct then? A5 to 2, and B5 to 12. Grid stopper is at the end of the chain to ground at 2 & 12 with the other end everything hot starts from there out and the voltage can't pass through the stopper from the hot side is from what it sounds. And it's just this 100k resistor being installed, no other components need to be added?

Again, i greatly appreciate your guidance and time on this.

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Stereomour II / Re: Finished building and the sounds is amazing, out of one channel [resolved]

« Last post by Paul Birkeland on Yesterday at 03:56:35 PM »

I would wait till you have a meter and can double check on those DC voltages.

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Moreplay / Re: Can balance knob be bypassed

« Last post by Paul Joppa on Yesterday at 02:29:32 PM »

On the circuit diagram, there is a resistor (Rgs, 220 ohms ) connected to the grid, pin 5. The other end of that resistor would be connected to the "hot" end of the input jack (the center pin) and to one end of the new 100K resistor.

The other end of the 100K resistor would be connected to the signal ground and to the "ground" of the RCA jack.

----

The resistor Rgs is called a grid stopper. In some special circumstances, a tube can oscillate at very high frequencies (think FM or television signals); the grid stopper reduces the chances of that happening. A grid stopper only works if it is the only thing physically connected to the actual grid; all other "grid" connections are actually made to the other end of the grid stopper.

Pin 8 is the cathode of the 6V6. The cathode resistor Rk (619 ohms) drops 8 volts due to the current through the tube, which sets the bias voltage  - the grid is 8v below the cathode. The capacitor Ck bypasses (shorts out) Rk at audio frequencies.

These two resistors are sometimes omitted in introductory discussions of how tubes work, because they have no effect at normal audio frequencies. But in the real world both are usually included.

I hope that helps?

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Stereomour II / Re: Finished building and the sounds is amazing, out of one channel

« Last post by heartlessnomad12 on Yesterday at 01:48:36 PM »

Sigh, after years of flawless service the left channel now takes a few minutes before it starts working. Swapped tubes and problem persists. Waiting on a new multimeter to be delivered, any thoughts on where to start reflowing joints while I wait? I figure where I had the issue last time around terminal 14 would be a good start.