misbias issue??

[Jazz P Bass]

After all of this, have you tried another speaker?
You want to rule out coil rub.

[yunger]

I all of this, have you tried another speaker?
You want to rule out coil rub.

i wondered about that too. i tried it and the speaker is fine. thanks.

[MWJB]

470ohm common cathode resistor will only bias the 12AT7 to ~78% at your B+, I wouldn't bias any cooler than that, probably tend towards hotter bias. I would only bias cooler if you were anticipating running a 12AU7/12BH7 in the power tube socket.

Are you sure you need to bypass the 12AT7 AND the the 12AX7 input triode? That's a lot of gain from the 2x 12AX7 triodes and fairly low cathode resistor values. I'd try bypassing the 12AT and lifting the bypass cap at the 12AX7 input &/or using larger cathode resistor values at the 12AX7, like 1.5K & 1.2K to lift the preamp plate voltage a little. I'd also consider trying 12AY7 or a 12DW7 in the preamp.

Your grounds appear to be daisy chained all the way along the board, I'd separate the preamp grounds (12AX7 cathodes, 22uf filter cap) from the rest of the amp grounds, ground the power amp cathodes & filter cap at the other end of the chassis.

I notice that you have used 250K pots. If the 250K volume pot is being used fully CW or thereabouts, there will be no series resistance between the wiper & input stage, this might be increasing highs in the circuit, perhaps try a 1MA pot or try a 330K, or 470K resistor in series from the input stage coupling cap to pot input?

Following on from Chuck's observations, you might be able to replace the IEC socket with a combined IEC & fuse holder for a 20mm fuse, thus freeing up the current fuse holder position for you speaker jack, getting it away from the input?

[Chuck H]

you might be able to replace the IEC socket with a combined IEC & fuse holder for a 20mm fuse, thus freeing up the current fuse holder position for you speaker jack, getting it away from the input?

Brilliant. I was trying to figure out a way to get the output jack into the fuse holder position. But I've never used an integral IEC/fuse holder so I didn't think of that.

Also: You can try this first. But you'll still have your input triode right next to the output jack. Better to move it to the other side (and carefully route the leads from the OT). I would like to see a shot of the inside back chassis wall and maybe a top shot so we can see the OT location.

[yunger]

470ohm common cathode resistor will only bias the 12AT7 to ~78% at your B+, I wouldn't bias any cooler than that, probably tend towards hotter bias. I would only bias cooler if you were anticipating running a 12AU7/12BH7 in the power tube socket.

i suspected this based on how muffled things got with a larger cathode resistor. i was thinking 270 or 330 there,,unbypassed.

Are you sure you need to bypass the 12AT7 AND the the 12AX7 input triode? That's a lot of gain from the 2x 12AX7 triodes and fairly low cathode resistor values. I'd try bypassing the 12AT and lifting the bypass cap at the 12AX7 input &/or using larger cathode resistor values at the 12AX7, like 1.5K & 1.2K to lift the preamp plate voltage a little. I'd also consider trying 12AY7 or a 12DW7 in the preamp.

i liked the sound of this configuration in my last build. would i still get a hint of preamp crunch if i lose the 1st stage bypass cap?

Your grounds appear to be daisy chained all the way along the board, I'd separate the preamp grounds (12AX7 cathodes, 22uf filter cap) from the rest of the amp grounds, ground the power amp cathodes & filter cap at the other end of the chassis.

they are daisy chained starting from input through r11(power tube grid leak) then to 22uf filter cap. seperately, the power tube cathode and heater 100 ohm center taps go to 47uf filter cap. from there, all connects to a single point on the chassis.

I notice that you have used 250K pots. If the 250K volume pot is being used fully CW or thereabouts, there will be no series resistance between the wiper & input stage, this might be increasing highs in the circuit, perhaps try a 1MA pot or try a 330K, or 470K resistor in series from the input stage coupling cap to pot input?

doesn't the 220k "grid stopper" take care of that?

Following on from Chuck's observations, you might be able to replace the IEC socket with a combined IEC & fuse holder for a 20mm fuse, thus freeing up the current fuse holder position for you speaker jack, getting it away from the input?

good idea!!

[yunger]

here you go chuck

[J M Fahey]

I would add 2 parts: a 4k7 to 22k resistor between R3 and R6 to better decouple the first triode, and a second 10 to 20uF filtering the node R6 / R13 .
It would also have the added benefit of making our friend jmaf happy
Don´t think it´s a biasing problem at all, but grounding, layout and decoupling, all 3 very tricky stuff.
jm2c

[yunger]

i put a 270 ohm resistor unbypassed as rk of the power tube and now i have much more significant output. again, the amp motorboats without any input from the guitar when the volume is turned up pretty high. i can touch the grid resistor of the power tube or the 2nd stage triode with my finger and it stops, at least until i pluck a note. makes me think bad things are floating around inside the chassis...or maybe decoupling between triodes would help as jm suggests.
i did move the speaker jack outside of the chassis away from the input temporarily with no change.
i'm curious, can transformer orientation cause these symptoms? grounding? is the aluminum chassis not sheilding?
i feel like i have a lot to check.

[Chuck H]

It may be that the aluminum chassis isn't shielding. You could unbolt the OT and that center box and cover the OT end of the chassis with copper. I think that will reduce any EMF on the input and preamp. The extra decoupling and ground modification can also help. I'm surprised that moving the output jack made no difference. But perhaps it would once the amp is making more watts so don't rule out the possibility of moving it.

[MWJB]

Thanks Chuck, but I've just thought of an even simpler solution: there's no NFB loop, so no need to run the OTsecondaries inside the chassis at all. Run the 8ohm tap direct to the speaker "+", then ground the common tap to the chassis at the lower, most inboard OT mounting & run anoher length of wire from there to the speaker "-".

[yunger]

that's a nice solution MWJB..eliminate the output wires from inside the chassis altogether.
i also plan on drilling a hole in the chassis next to the power tube socket and running the blue o.t. wire directly to the socket like enzo suggested.
i could rotate the o.t. 90 degrees and place it at the bottom corner as seen in the picture to reduce transformer coupling. the negative of doing that is it would be closer to the preamp tube. not sure if it is necessary either. i haven't read of transformer coupling causing motorboating, just hum. either way i'll pick up some copper to lay underneath.
regarding grounding, i have 3 wires connected to the "star" point on the chassis. one from each filter cap and one from the diode bridge/drain resistor. is there any mileage to be gained by manipulating that?
just to cover all possibilities, would grid resitors soldered directly to the tube sockets be worth my time?
does the fact that turning up the volume reveals motorboating mean that it is happening before the volume pot?
hopefully i'll have a solution soon that helps someone else with the same probem.

[MWJB]

You shouldn't need a 3rd filtering stage...I haven't needed to do so on my similar builds.

Grid stoppers of significant value in a design like this smack of a "band-aid", a temporary fix, rather than a solution...a small value grid stopper on the power tube grid should be satisfactory.

Use 2 ground points, one for the preamp at the input jack, another for the power amp grounds at the other end of the chassis.

[yunger]

i just tried moving the o.t. all over the place in all different orientations while the amp roared with no change. even holding the o.t. speaker as far as the east is from the west didn't make a difference. i don't think that is the single contributing culprit.
motorboat.mp3
if you listen to my recording you will hear me adjusting volume for the first half. adjusting tone control for the second half of the clip. sound familiar to anyone?
i'll try a different grounding configuration to verify but i thought the star was dummy proof....discouraging.

[yunger]

i tried bypassing the 2nd stage, volume pot and tone pot and everything is perfectly clean (no motorboating). as seen in the picture i jumpered from after the first stage coupling cap to the grid of the power tube.
i hope this means i've narrowed the problem to in between the gator clips. does this reinforce the additional decoupling of the first two stages theory? if so, why do tons of amplifiers not have this problem cause it seems pretty common for the first two stages to share a node? plus the two stages are out of phase.
if you listen to the clip i posted, this is not your typical hum.

[Jazz P Bass]

That is not "just" a wire.
There is probably somewhere around 50pf of capacitance with the clips & the connections.