What would cause this?

[daz]

No schematic, but i can edit one if absolutly necassary. But same old 18 watt i been asking about forever now. This time with 2 6V6's tho, which has helped it sound a lot better. however, when i turn the post tone stack master up, the tone changes radically bu the time it's at about 1/3 of full volume. So i'm reading a post elsewhere where someone is describing the difference between various master volumes. He says the one i have changes the tone a lot. Agreed. then he says the cross line post PI master does not. I have wired these up before but always left the other master wired up and just turned it to 10. But this time i tried it and removed the pre PI master out of circuit. To my suprise the tone because horrible ratty/fuzzy and thin as he|| at any volume and even with the pregain turned way down.

This tells me there is a design flaw in the amp somewhere between the input and the crossline master. any idea what could cause this? My amp goes as follows....

input>68k grid stop>.022>1meg pot>100k stopper>.022>220k voltage divider>V2A>cathode follower>tone stack

basically same amp i've been posting schematics of in other threads with changes that won't make any differnce. (i figure same guys like Bruce and others wil be the ones replying )

[Jag]

The input of the PI still needs a ground reference. Did you just remove the pot? If so, you need to put a resistor equivalent to the max value of the pot in it's place. Most likely 1M

[daz]

Thing is, the guy said to lift that if you put in a crossline pot so i did. Plus i figured the grid is grounded thru the 470 and 56k resistors in the tail section.

[Jag]

Right, I don't know what I was thinking.

[d95err]

Did I understand correct that you have a conventional master volume after the tonestack, and then a crossline (Matchless style?) master after the PI?

How does the tone change?

[daz]

if i use the master thats between the stack and the PI with no crossline master at the power tube grids, the tone is normal. If i remove the master and put a crossline in, the tone is thin and ratty and loose in a very flawed way. Like theres something radically wrong.

[Alex R]

- needs to be 1 meg, just wired as a variable resistor, no ground, grid to grid after the coupling caps & before the stoppers. But you knew that...

It's a cathode biased amp I'm assuming, so no bias voltage on the pot?

[daz]

Yes, i know. Thats how i wired it.

[d95err]

So, the tone gets thin and rattly when you turn down the crossline master? If that's the case it's quite normal. When you lower the crossline master, you loose more and more bass the lower you go. This is because the RC time constant created by the coupling caps after the PI and the master is lowered.

[daz]

No.

pre PI master and no crossline master....normal sound

post PI crossline and NO pre PI master...thin ratty and useless at ANY volume or any gain level.....anywhere

when i say thin and ratty, i don't just mean a horrible sound, i mean literally unusable like if you used a fuzzface in front of the amp and turned the treble on the amp to 10 and the mid and bass to zero. No, worse than that...unusable...flawed....broken

[d95err]

What happens if you disconnect the crossline master? (No master at all)
If the problem persists, then it has something to do with the pre PI stuff.

[daz]

Good idea. I tried it and with NO master at all the tone stays normal. So something about the crossline master is funky i guess. I don't see why, tho maybe i'm missing something. I connected an outer lug to one power tube grid AFTER the cap from the PI of course, and before the grid stopper, and the wiper to the same place on the other power tube. Anything wrong with that setup?

[Alex R]

when 'on 10' does it actually measure 1 meg?

[Jag]

Is your amp cathode biased? If not, the pot will be messing with the bias.

[daz]

Yes, cathode biased