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Fender tone stack/gain Question

2/24/1998 2:58 PM
Craig	Fender tone stack/gain Question I have a Silvertone 1484 that I wanted to try an experiment with; I replaced the stock tone circuit on ch. 1 (I seem to remember reading about this tone circuit - is it called a "Baxandall" design?) with a BF Princeton circuit. (I kept all the original caps to return it to original). My main interest in doing this was to hear the differences in the two designs, and to form some new questions. Basically, I pulled everything between the stage 1 (V1) plate to the volume control, and replaced it with everything between those same two points from the Princeton spec. The only other thing I did was separate the cathode resistors on V1/V2, (12AX7) using the common 1.5K/25mmf config for each. Here's my trouble: 1) The BF Bass control doesn't work. The BF Treble works great. I am pretty sure I've got it wired correctly - I double checked it several times. I haven't verified the bass caps are good yet, (I'm recycling some relatively new caps) but they were good a few weeks ago when I pulled them from another project. 2) The BF channel's gain is wierd now: seems like I hit full volume at around 4-5, with distortion happening at around 2-3. I thought that maybe I had screwed up the gain structure so that the bass control was rendered useless somehow, but that doesn't make complete sense, as the treb ch. is fully useful. I don't have any audible oscillation happening, which makes me think maybe it's not a gain problem. As far as I know, the taper of the vol. pot shouldn't be an issue, since it worked appropriately before I modded it to BF. I tried restoring the original (V1/V2-shared) cathode resistor, with no major difference. I tried adding a voltage divider - the Sears spec has a 220K res. from B+ to V1, pin 6. So I tried 2 - 100K in series, with the juncture going to pin 6, and the 100K's to B+ and to the tone stack, repectively. Better, but still didn't solve my problem. I also tried using just a straight 100K like in many Fender preamps, but still no success. I'm beginning to think the first thing I should've checked was my bass caps. I did verify that both the 100K (between the .1 and the .047), and the 6.8K 'mids' resistors were OK. Here's what I don't get: if I have a bad cap in the bass circuit, how is gain affected? I'm thinking this must be the root of my problem...does a bad cap basically become a resistor? And if so, what would the effect be in such a tone circuit? Another thought just occurred to me; I didn't think to verify that the bass control was working before modding to BF - maybe the bass pot is history. I don't recall noticing it NOT working though... Anyway, I'm hoping someone may have some good troubleshooting suggestions - Thanks in advance.

2/24/1998 2:58 PM

Craig

Fender tone stack/gain Question

I have a Silvertone 1484 that I wanted to try an experiment with; I replaced the stock tone circuit on ch. 1 (I seem to remember reading about this tone circuit - is it called a "Baxandall" design?) with a BF Princeton circuit. (I kept all the original caps to return it to original). My main interest in doing this was to hear the differences in the two designs, and to form some new questions.

Basically, I pulled everything between the stage 1 (V1) plate to the volume control, and replaced it with everything between those same two points from the Princeton spec. The only other thing I did was separate the cathode resistors on V1/V2, (12AX7) using the common 1.5K/25mmf config for each.

Here's my trouble:

1) The BF Bass control doesn't work. The BF Treble works great. I am pretty sure I've got it wired correctly - I double checked it several times. I haven't verified the bass caps are good yet, (I'm recycling some relatively new caps) but they were good a few weeks ago when I pulled them from another project.

2) The BF channel's gain is wierd now: seems like I hit full volume at around 4-5, with distortion happening at around 2-3. I thought that maybe I had screwed up the gain structure so that the bass control was rendered useless somehow, but that doesn't make complete sense, as the treb ch. is fully useful. I don't have any audible oscillation happening, which makes me think maybe it's not a gain problem. As far as I know, the taper of the vol. pot shouldn't be an issue, since it worked appropriately before I modded it to BF.

I tried restoring the original (V1/V2-shared) cathode resistor, with no major difference.

I tried adding a voltage divider - the Sears spec has a 220K res. from B+ to V1, pin 6. So I tried 2 - 100K in series, with the juncture going to pin 6, and the 100K's to B+ and to the tone stack, repectively. Better, but still didn't solve my problem. I also tried using just a straight 100K like in many Fender preamps, but still no success.

I'm beginning to think the first thing I should've checked was my bass caps. I did verify that both the 100K (between the .1 and the .047), and the 6.8K 'mids' resistors were OK. Here's what I don't get: if I have a bad cap in the bass circuit, how is gain affected? I'm thinking this must be the root of my problem...does a bad cap basically become a resistor? And if so, what would the effect be in such a tone circuit?

Another thought just occurred to me; I didn't think to verify that the bass control was working before modding to BF - maybe the bass pot is history. I don't recall noticing it NOT working though...

Anyway, I'm hoping someone may have some good troubleshooting suggestions - Thanks in advance.

Replies:

John Greene If I had to guess from here, I woul... -- 2/24/1998 5:41 PM