Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Xses on April 23, 2020, 12:55:40 pm

Title: Guitar amp presence/feedback RC network from 15om to 4om - need help
Post by: Xses on April 23, 2020, 12:55:40 pm

Hello,

I've made a replica of Selmer TV10 guitar amp recently, but, since I couldn't find any schematics of it, I based on TV15's one (attachment), because TV15 is just bigger TV10. One thing I didn't realize is that TV15 has 15om speaker whereas TV10 has 4om. As a consequence presence/feedback control (aka "Tone"; in red circle) is not working properly, due to lower voltage. Can someone help me to redesign that RC network to match the lower impedance and keep its original sounding?

Thanks for any response.

Title: Re: Guitar amp presence/feedback RC network from 15om to 4om - need help
Post by: Xses on April 23, 2020, 01:18:59 pm

Is it just a division of every resistance by 2 and multiplication of capacitance by 2?

Title: Re: Guitar amp presence/feedback RC network from 15om to 4om - need help
Post by: shooter on April 23, 2020, 01:33:22 pm

Quote

Is it just a division

thinking it's more like 1/2pirc but not sure.
as an experimenter though, doubling or halving the fixed R's or cap, then listen  :dontknow:

Title: Re: Guitar amp presence/feedback RC network from 15om to 4om - need help
Post by: tubeswell on April 23, 2020, 02:25:30 pm

So on the OT that you used, does it have multiple secondary taps? (and if so, which tap are you sourcing the NFB loop from?)

Title: Re: Guitar amp presence/feedback RC network from 15om to 4om - need help
Post by: Xses on April 23, 2020, 03:19:49 pm

Quote from: tubeswell on April 23, 2020, 02:25:30 pm

So on the OT that you used, does it have multiple secondary taps? (and if so, which tap are you sourcing the NFB loop from?)

Yeas it has multiple taps. I didn't realize that I can connect the loop just to the 16om (not necessarily 4om) tap without changing its parameters.

Thanks a lot.

Title: Re: Guitar amp presence/feedback RC network from 15om to 4om - need help
Post by: Bantam on April 24, 2020, 01:48:17 pm

I'm probably wrong but I remember on Marshall's they hook up the nfb to certain places and I think it's non respective of whatever 'main' tap... Like there's a 4 8 & 16R taps and they hook up nfb to one that I guess is deemed correct irrespective of whatever will be used by different people. I have no idea if that fits here but I'm bored and thinking I might be mr. Helpful guy.

Title: Re: Guitar amp presence/feedback RC network from 15om to 4om - need help
Post by: PRR on April 24, 2020, 02:27:28 pm

The 4-8-16 taps are 99.5% the same signal, just different voltages. For stages-amps you may connect NFB to one and speaker to another, no problem.

Even most Hi-Fi amps do that, even though "for best specs" the NFB should be connected AT the speaker tap and the NFB ratio figured for that tap. The original Williamson had a big table (and equation!) for wiring 7 (9?) part-secondaries and NFB for absolute best "Damping Factor". Of course 20 feet of speaker wire spoils such fussiness; and most stage amp speakers do NOT want "infinite damping".