PT and Rectification Question

[ericnuke]

Replaced the cathode resistor on V2a with 2 820's in series that were testing low so I'm just about 1k5, didn't have any 1k5's and no difference.

In that other thread about biasing a triode I used that newly acquired knowledge to get the first gain stage right bias wise so I'm assuming this problem is in V2 gain stages.

Upon more listening it sounds kind of like it you run multiple overdrive pedals into each other and it gets that real squashed overgained super compressed kind of sound. While there is a good bit of gain it still isn't over the top like a messa or a soldano preamp.

This problem is definitely above my current tube intelligence level.

[martin manning]

Have you tried changing V2? Notice the voltages you reported for the cathode follower side (pins 6-7-8 ): plate 251, grid 129, cathode 133. It isn't bahaving properly, or maybe there is something wrong with your measurements or wiring. The grid is 4 volts below the cathode so it should be nearly in cut-off. But, with the cathode at 133V there is nearly 2.4mA flowing through the 56k cathode resistor. Sorry, should have looked more closely at your revised voltage table.

Regarding rebiasing one or both of those stages with 1k5 instead of 820, the 820 is part of the sauce here. That puts the bias point closer to grid current limiting, and adds some distortion flavor a little earlier.

[ericnuke]

Tried a new tube in V2 and it is just slightly better but still doing it. With the new tube I got some different voltages. Grid is 159v and Cathode is 164 and Plate at 256v.

I've been reading the Valve Wizard website for the last hour or 2 under the DC coupled cathode follower section trying to get all this to make sense in my head. Now in the example he gives on the site he's got the same value (100k) for both the plate resistor for the first triode stage and for the cathode resistor to ground on the cathode follower stage. I'm gonna try this in the morning when I can make noise again to see if that helps. The article also talks about how lowering the value of the cathode follower cathode resistor will "warm up" the sound and reduce fizzyness which sounds kind of like what I'm hearing but way way way to the extreme.

I'm still not totally understanding this cathode follower business so I'll keep researching it but based on my current (pun intended) understanding it looks like my cathode voltage is still too high as I think it needs to be just a couple volts higher than the grid, correct?

[martin manning]

...With the new tube I got some different voltages. Grid is 159v and Cathode is 164 and Plate at 256v.
...it looks like my cathode voltage is still too high as I think it needs to be just a couple volts higher than the grid, correct?

Your thinking is correct. You can try changing the value of the cathode resistor to see what happens, but the cathode should come up to ~1 volt above the grid either way, so I don't understand why yours is 4-5V above. You've got 2.9mA flowing through the thing now, and you are at 92 volts Vg-k. Check the plate curves- at that Vg-k a typical 12AX7 Vg=0 line shows about 2.1mA. Is the present cathode resistor really 56k?

[ericnuke]

Yep the plate resistor is really 56k. I've been about pulling my hair out checking and rechecking the values and wiring of everything. I put a 100k on the cathode and no change. I tried several different 12AX7's in all the different positions and even a 12AT7 just to see and they all behaved the same. I've been reading all the threads I can find and a few people solved voltage issues with a new tube socket. So is this the kind of situation where the socket might have some continuity in in somewhere either through a wayward solder wick or some deposits or something? I've checked the soldering work a few times and can't see any problems and every joint checks out good with an ohm meter.

Also my voltages aren't spot on with the Trinity schematic that posts voltages so i'm thinking that may be an issue. I'm gonna see about getting a handful of the metal oxide resistors to play with some values in the power string to get those sorted as well. I've tried searching about how to calculate voltage drop over a resistor but I don't believe ohms law works for that so is there any easy way to figure that? I'll keep researching but a google search didn't turn up much.

Edit: found aiken's site and the section on voltage divider rule so I'm good with that now!

[ericnuke]

Crazy conclusion here. I had to leave town for a few weeks this morning, I'm a touring musician, so last night I thought I'd just put the chassis in the head shell while I was away. So I put it in the shell and plugged it into my cabinet and played it for a bit and the problem was gone. Gain knob sounded great all through out it's range and that weird, overcompressed distortion was gone. WTF? Either way I don't have to worry about it now for a while so when I get back home I'll look into it with fresh eyes I guess. Thanks for all the help so far!

[martin manning]

This is good, you have a clue now! I suspect a bad connection somewhere was restored (temporarily, most likely) by handling and mounting the chassis. Check the CF's grid connection, since that triode didn't seem to be paying any attention to it's grid voltage. ...and I might add that this is all part of learning to get beyond the "paint by numbers" stage.

[martin manning]

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