5G9 Build - scratchy trem after changing V1

[tubeswell]

With my recent 5G9 Build, I've noticed since the day I built it (but never wondered if it was odd until now) that the Vol-only channel is alot less bright and more muted than the vol + tone channel. This is the case no-matter what tube I run in V1 - the channel is quite noticably duller and lacks highs.

So today I changed from a GEJAN5751 to a GE12AT7WA in V1. This tube made the Vol + Tone channel sound really good (plenty of sparkle), but when I dimed the tone to treble with the trem depth about halfway up. I started getting staticky pulses out of the speaker This happened in either channel. If I backed-off the tone from treble just a tad, I can dime the depth pot all the way without getting any static problem, or if I cut the depth pot so theres no trem at all, I can wind the tone to max treble without a problem. Is this DC entering the signal path at some point?

Either way the vol-only channel is still quite a bit duller than the vol + tone channel. So I put the GE5751 back in V1, and the problem went away, but the amp sounded a bit less sparkley.

Then I tried another GEJAN5751 in V1 and the amp had plenty of sparkle and texture in the tone, but it also had more interfrence - a weird extraneous noise like unwanted coupling or oscillation - maybe it is a slightly higher-gain (or better-condition?) tube than the other 5751 (although they are the same type of tube)? I was thinking this because it seemed to give better freq response.

Anyhow, then I tried an EH12AU7 in V1 and while it wasn't as grunty or as rich-sounding as the others, it didn't give me any problem with the treb/and depth pot. So my question is - why was there a problem with running a 12AT7 in V1 when I could run a 5751 without a problem? Is it merely the particular tube, or does the vol-only channel dullness indicate a potential problem I hadn't picked up until now? (or it is normally duller than the vol + tone channel)?

My other thought was - Could it be a bad coupling cap on that channel maybe? I am using 22nF 400V mustard caps after both plates on V1. Could the one on the vol-only channel have been a dud?

[tubeswell]

Thinking about this a bit more, it could possibly be too much DC voltage on either the tone pot or the depth pot when I have a good tube in V1 and have the a bigger voltage on the grid load resistors. But it doesn't do this with eth trem switched off. Does that make sense? Only thing I wonder about this hypothesis is that surely these pots should be able to handle a higher gain tube in V1. A 12AT7 is usually regarded as a suitable upgrade for a 12AY7, is it not? Hmmm...

[MWJB]

What do you mean by "vol only" channel? On a 5G9 both channels connect to the tone pot, it's just that the darker channel (what you describe sounds perfectly normal) does not feature the 500pf bright cap, just the tone pot and the 0.005uf cap to ground (simple hi cut).

If you try Bruce's Mission 5E3 "tone & vol" mod this will let you tweak both channels so that you can get a usale tone from either.

A good 12AY is an upgrade for a 12AY, a 12AT is a different tube.

Many old amps with tremolo have quirks that present themselves when dialling in a lot of highs, or playing with gain (I have a cheap SE amp with trem, you can't turn the treble over 2/3 without the amp chirping & chirruping & ALL models of this amp do the same...loads of brown tolex Fenders with the 2 or 3 preamp tube tremolo circuit get squirrelly when turning up the treble & owners often think that the treble pot is to blame), so whilst I don't have personal experience of the 5G9 specifically, I bet many of them would behave in the same way as yours does...

[MWJB]

Try reducing the 0.02uf coupling caps to 0.01uf?

[tubeswell]

Thanks MWJB

I was wondering whether the extra gain from the 12AT7 was doing it, but I had a 5751 in there before, and it wasn't a problem then (although the 5751 I had in there was a 'darker' sounding specimen than the 12AT7 I tried, so maybe it is to do with the higher freqs that I was getting from the 12AT7?) - The 12AT7 I tried was a '57 GE 12AT7WA/6201 with triple spacers and black plates, and is a nice sounding tube for a 12AT7 giving an unusally well-balanced freq spectrum - so I'm quite keen on seeing if I can make it work. I thought that maybe I was hearing parasitics resulting from the higher gain and that if I put 4k7 grid stoppers on the V1 grids, that might help.

Anyway Bruce's 5E3 Vol and Tone mod sounds interesting. I'll look it up.