Radio Shack Pot ???

[Bill Epstein]

Have had this classic cheap Alps pot forever and forget, I can just ignore the Loudness Control tabs?

[Grainger49]

Yes, Bill.  I have had no problems with it but it has a reputation for bad tracking at the lowest ranges.

[Bill Epstein]

My Foreplay choices are the RS, a Noble ARC used during the McKinley Administration and a Taiwanese (VALAB) SMD attenuator I was saving for an Aikido build. Except for the necessity of the transformer purchase, I'm trying to keep the Foreplay a parts stash build. I might even use the pair of Meany Greeny output caps I still have after all these years.

Low level tracking is an obvious concern 'cause I don't like padding the 30dB (!) output.

[Grainger49]

A pair of $4 resistors are nice for padding.  I have never needed them.  But now I am running my Paramours with 76 drivers which have much less gain than the stock 12AT7.

[Bill Epstein]

Yes, Bill.  I have had no problems with it but it has a reputation for bad tracking at the lowest ranges.

I just tested the RS and it's less than 0.1% off channel to channel. Lucky me!

76 driver tubes must have a nicely unique flavor.

[Paul Birkeland]

0.1% off where and from what? (the deviation of the total trace from 100K, for instance, tells nothing about tracking) .

Padding the pot is a solid plan, - 20dB of padding would be a good place to start. (10.5K across the outer lugs, 100K in series with the input to the pot)

-PB

[Bill Epstein]

I ohmed input to output at various wiper positions channel to channel: ex. 79.1 vs. 79.5 kOhms.  That's not valid? It needs to be scoped with a load?

Thanks for the padding recs.

[2wo]

I remember those RS pots, they were marked Alps on the back and tracked very well...John 

[Paul Joppa]

Low level gain is dominated by the small resistance from ground to output, not the large input to output resistance.

For fully accurate tracking readings, you want the ratio of the two resistances (equal to the gain), expressed in dB so you can see the difference between channels.

[Bill Epstein]

Low level gain is dominated by the small resistance from ground to output, not the large input to output resistance.

For fully accurate tracking readings, you want the ratio of the two resistances (equal to the gain), expressed in dB so you can see the difference between channels.

Thank you both. I'm trying to get this done strictly from the parts bins so I'll simply give the RS Alps a try, not that the circuit isn't deserving. 15 years ago I went from the Alphas to Sweet Whispers to Goldpoints while instituting power and filament snubbers that made a dead quiet Foreplay which sounded better than a Blue Circle unit of that time.

How about David and Goliath? Received the Allied transformer today and it's so cute next to 50 watt Push-Pull PT end bells.

[Bill Epstein]

0.1% off where and from what? (the deviation of the total trace from 100K, for instance, tells nothing about tracking) .

Padding the pot is a solid plan, - 20dB of padding would be a good place to start. (10.5K across the outer lugs, 100K in series with the input to the pot)

-PB

Just to be clear: 10K connecting 2 OUTPUT lugs and 100K, 2 of them, each making the L and R input connections? TIA.