guitar goes dull

andrew · Post by **andrew** » Sun Oct 28, 2012 6:25 pm

Hi I have two amps with the same pot, v1 and PI values. One is a 4- el84 with a 270v PT and the other is a 2-KT66 with a 310 v PT. They are very clean sounding amps. When I turn down the guitar volume on the el84 amp the clarity is retained but on the KT66 amp it sounds dull. Has anyone had this problem? Thanks

Structo · Post by **Structo** » Sun Oct 28, 2012 9:06 pm

What value grid leak resistor does each amp have?

What value grid stopper resistor on the input grid?

andrew · Post by **andrew** » Mon Oct 29, 2012 12:00 am

Hi Tom, the grid stoppers on each amp are 27k ( parallel wired 12ax7 with 27k on each grid). The grid leak resistor is a 1 Meg on the input jack. I forgot to mention that even with a 6sl7 in the V1 of the el84 amp I can still get the clarity when I turn the guitar volume down. The cap and pot brands are the same in both amps.Thanks.

Structo · Post by **Structo** » Mon Oct 29, 2012 1:41 pm

It may seem obvious but did you try different tubes in V1?

tubeswell · Post by **tubeswell** » Mon Oct 29, 2012 6:32 pm

What about trying a different cable? or a different guitar?

(Sounds like you have an issue with the output impedance from that guitar and the input impedance on V1 of the KT66 amp - and apart from the grid stopper at V1, I'm trying to think what else could affect this).

Another possible fixit for HF roll-off on your guitar vol pot is a bright/bypass cap between the pot input and wiper.

andrew · Post by **andrew** » Tue Oct 30, 2012 12:30 am

I found the problem. I used a piece of guitar cord for the input to V1 on the el84 amp. On the KT66 input to V1 I used a piece of shielded interconnect cable (Mogami W2330 Interconnect Wire)

that had a very thin signal wire. I did away with this interconnect cable and used plain hook up wire. Now it retains clarity. Thanks for helping me think through this.

tubeswell · Post by **tubeswell** » Tue Oct 30, 2012 5:15 am

Sounds about right

Icetech · Post by **Icetech** » Tue Oct 30, 2012 12:31 pm

Thanks for posting the fix andrew.. i would have never thought of that..

Man this is gonna be a stupid question but i have to.... i have been using 24ga teflon wire in my amps.. Is there actually too small of a gauge that will affect sound?

Structo · Post by **Structo** » Tue Oct 30, 2012 3:21 pm

I would not use that in a tube amp.

Just about everything in my amp is 20ga Teflon.

18ga for power tube heater wiring & 20ga for preamp tube heaters.

Some guys say 22ga is OK but I like a fat pipe for my electrons.

Icetech · Post by **Icetech** » Tue Oct 30, 2012 5:58 pm

Thanks struc.. ill have to check what exactly im using.. its been about 2 years since i touched the inside of a amp, just going off my bad memory:)

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