Mistake in my layout or not?

[dude]

I know I've asked several questions about the 6V6 Plexi but asking for some clarification, the only difference between the two channels is in the first pre-amp stage. The bright channel, 1A, 12Ax7 is biased 2K7 and .68uf, into .0022 coupling cap and bight caps of course and the Normal, 1B, is biased 820 and 220uf, .022 coupling cap, no bright caps. (original was 330uf) That's it. But all my Plexi's the Normal channel's volume (alone, turning down bright to 0) has a lot less volume then when I do the same to the Bright channel (can't really be used alone). Maybe this is the way the amp was meant to be?
I'm thinking the 220uf cathode cap is amplifying the bass in the normal channel's pre-amp, not much highs, and this is the main reason for the much darker tone on the normal.  The coupling caps difference is there but.022 shouldn't block highs much but if there're "no highs to block" cause of the big cathode cap then there's the answer. I would think lowering that 220uf cathode cap would let more highs go through and that channel would be louder too. Then maybe not a Plexi...?
Maybe this is the magic of the amp, blending the two as the schematic is written but the normal channel is probably half as loud as the bright, each alone (due to no highs being amplified?). Maybe someone can set me straight on my thinking here and I have no layout issues, this is how the amp is meant to be.  Amp sounds great blending the channels, not complaining just curious if I got it right on the normal channel.


Thanks, al

[sluckey]

You got it right.

[SnickSound]

I know I've asked several questions about the 6V6 Plexi but asking for some clarification, the only difference between the two channels is in the first pre-amp stage. The bright channel, 1A, 12Ax7 is biased 2K7 and .68uf, into .0022 coupling cap and bight caps of course and the Normal, 1B, is biased 820 and 220uf, .022 coupling cap, no bright caps. (original was 330uf) That's it. But all my Plexi's the Normal channel's volume (alone, turning down bright to 0) has a lot less volume then when I do the same to the Bright channel (can't really be used alone). Maybe this is the way the amp was meant to be?
I'm thinking the 220uf cathode cap is amplifying the bass in the normal channel's pre-amp, not much highs, and this is the main reason for the much darker tone on the normal.  The coupling caps difference is there but.022 shouldn't block highs much but if there're "no highs to block" cause of the big cathode cap then there's the answer. I would think lowering that 220uf cathode cap would let more highs go through and that channel would be louder too. Then maybe not a Plexi...?
Maybe this is the magic of the amp, blending the two as the schematic is written but the normal channel is probably half as loud as the bright, each alone (due to no highs being amplified?). Maybe someone can set me straight on my thinking here and I have no layout issues, this is how the amp is meant to be.  Amp sounds great blending the channels, not complaining just curious if I got it right on the normal channel.


Thanks, al

The 220uF cap does not lower gain for higher frequencies, it just lowers the frequency at which gain increases. A smaller cap sounds brighter not because there's more gain in the highs, but because there's more lows. (Of course, lows have wide signal swings so headroom will seem lower)

One thing to note: with one volume at 0, the mixing resistors become a voltage divider since the other leg basically gets routed straight to ground. Now, when the Bright volume is on 0, add to the the cap on the resistor shunting a bunch of mids and highs straight to ground.

I would bet if you remove that bright cap, the volume difference will lessen. Of course, then the bright channel would get darker. I prefer it in stock config, but in stock it's normal for the Normal channel to be a bit too tame on its own. If you increase the Bright volume with nothing plugged in, the Normal channel will perk up a little

[Colas LeGrippa]

Just to make sure, measure the voltage at the plate of the normal channel ( 200v or so ).  Or swap the 12ax7 for a new one if you don't want to open the amplifier. It might be a bad tube. Happened to me last week.


Colas

[dude]

You got it right.

Thanks Sluckey, sometimes I doubt myself..., half the posts I probably know the answer, 


SnickSound, thanks for reply, I hear ya on turning the Bright completely off effects the Normal channel.

Just to make sure, measure the voltage at the plate of the normal channel ( 200v or so ).  Or swap the 12ax7 for a new one if you don't want to open the amplifier. It might be a bad tube. Happened to me last week.

Bingo, Colas, you nailed it. I was checking voltages and PS voltages were fine but node C was dropping way more voltage then it should have, and node E was 60 volts below normal and the plates on 1A and 1B, were 70v and 47v. I was saying what the heck, all the R read were dead on. I was thinking maybe I had a solder blob touching the chassis (I had painted the chassis so I was thinking not a complete short). I changed the three rail R, but out of the amp they were good. Couldn't figure what was going on, then I pulled V1, voltages were back and higher than normal without V1. I figured bad pre-amp tube, replaced it, now all voltages dead on. I always though if you had a bad pre-amp tube you would have more issues then just a little weaker sound and volume.


I might not have found it as fast before I read your post, thanks. Checking voltages are one of the first thing one should do. I just build this amp a week ago, all checked out but I always fool around changing that first 12Ax, searching the magic one, ha. I never buy new tubes, seem to think old American tubes sound better but the down side is a lot are beat or on their last leg...


Colas