Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: zfiles1701 on April 01, 2011, 08:13:17 pm

Title: AB763 NO MID POT Compared to having MId POT
Post by: zfiles1701 on April 01, 2011, 08:13:17 pm

Would having no mid pot be the same as having the mid set at 5 (what I do almost all the time anyway)?  Any noticable tone differences?

And what do some builders out there use for the first power supply cap value?  I have about 94uF total now, but had it according to schem at 44 uF (two 22uF in para).  Notice it's a little more bassy, and maybe a tad more headroom.

Title: Re: AB763 NO MID POT Compared to having MId POT
Post by: P Batty on April 01, 2011, 08:35:07 pm

It would be close- the equivalent is nominally at "7", it would depend on the exact taper of the 10kΩ pot.  The "normal" value for the mid-range resistor is 6800Ω, which would place it anywhere from about 6100Ω to about 7500Ω.  The mid pot is useful for an extreme "scoop" when set to "0", other wise many people find a midrange "warmth" they like then set it and forget it.  If you make the mid pot 25kΩ you get a wider range of tones ("raw" control), but it may be harder to dial in. 

If you have a SS rectifier you can go up to very high values, I did a Bandmaster style AB763 with 680x2 uf once, I don't think anything over 100 makes any difference in tone.