Critical resistors for s.e.x amp?

Jim R. · 30753

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Online Doc B.

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Reply #15 on: October 19, 2009, 08:10:43 AM
Yes if you look at the URL for the S.E.X. C4S instructions you can see that it is bottlehead.com, not audioasylum.com

Dan "Doc B." Schmalle
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Bottlehead Corp.


Offline JC

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Reply #16 on: October 20, 2009, 05:44:19 AM
Thanks, Doc,

Perhaps Wardsweb's excellent construction photos would be another good thing to have here, too; I found them extremely useful when I constructed my S.E.X. as a supplement to the manual photos.

Just a thought,

Jim C.


Offline jufelling

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Reply #17 on: November 25, 2022, 12:36:43 AM
Since you mention C4S, I will assume that you are building the kit with the upgrade.

You could spend some money on nicer grid stoppers.  Anything from 100-1000 ohms is just fine here.  When I built my first SEX amp, I also upgraded the quality of the 120 ohm series resistors mounted to the headphone jack.  Get six really good 120 ohm resistors and use the same part for the grid stoppers also. 

The 100k pot can also be upgraded with several different parts.  The TKD pot at Parts Connexion so far fits the best.

Hi Paul!

Since you're referring to the initial post here - do I get it right that it would make sense to swap the 499 ohm resistors that are part of the C4S mod for better ones, e.g. some Vishay CAR Z-Foil?

I'd also like to ask which resistor of the S.E.X. Amp the grid stoppers actually are - I checked the schematic of the amp but I have no clue which are grid stoppers and which are for other purposes.

Best
Julian



Offline Paul Joppa

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Reply #18 on: November 25, 2022, 03:41:45 AM
Lok at the dates of the posts - the one you are responding to was from 2009. There have been multiple revisions since then. Which version do you have? The date on the manual will be enough to identify it.

Paul Joppa


Offline jufelling

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Reply #19 on: November 25, 2022, 09:19:42 AM
Thanks for your fast feedback Paul!
The date on the manual is 2017, I bought the amp 1 year ago.



Offline Paul Joppa

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Reply #20 on: November 25, 2022, 10:34:22 AM
You have the version 3.0 S.E.X. amp.

The grid stoppers are the 220-ohm resistors attached to pins 3 and 10 of the 6FJ7, labelled "Rstop" on the circuit. They replaced the somewhat fragile carbon composition used in earlier versions and there is little to no benefit in changing them.

There are no 120-ohm resistors on the headphone socket in this version 3.0, so there's no point in trying to replace them  :^)

The 499-ohm resistors on the C4S board, labelled R1A and R1B on the PC board, set the current. Those resistors do potentially affect the sound, so bulk metal foil resistors can be considered an upgrade.  The board is a little tight there, so be sure they will fit before spending the big bucks!

Paul Joppa


Offline jufelling

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Reply #21 on: November 25, 2022, 11:05:40 AM
Alright, thanks for all the information.

I‘m really interested to swap those two resistors with something different.
How much ohm should those two resistors have?
Exactly the same or is there a span, e.g. from 300 - 600 ohm? If yes, how will different ohm values affect the sound?

I assume other factors of the resistors are probably more influential, right?



Offline Paul Birkeland

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Reply #22 on: November 25, 2022, 11:26:31 AM
Just to be completely transparent, the grid stopper resistors handle zero signal current ;)

Ideal resistor types for this position are types with minimal inductance, which is why we used carbon composition originally.  Since these were fragile, we transitioned to carbon film for higher reliability. 

If you go up too high in value, you could potentially introduce some audible treble roll off, but 200-1000 ohms is certainly a fine range and I wouldn't be able to hear the difference between those values.

The coupling caps, parallel feed caps, and cathode bypass caps are far more influential to the overall sound of the amp.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Paul Joppa

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Reply #23 on: November 25, 2022, 11:36:44 AM
PB is talking about the grid stoppers, which are not critical in value.

The 499-ohm current set resistors on the C4S board determine the driver stage operating point, and should be within +/- 5% of 500 ohms.

Paul Joppa