5F6-A - Solid State Rectification?

[Wakculloch]

Just come across this thread again, and remembered that the man from Marshall who i spoke to about the bias coupling cap recommended using one that was designed to fail with a short circuit (class Y ?), so that if it did stuff up, bias would increase rather than disappearing.
I don't know if they said this as a precautionary measure or if they'd had problems, so i can't put any weight on his comments, but he was pretty insistant.

[j-stylez]

Allright, I'll see if I can find a cap of that type.

Biased to 28mA, plate voltage is at 490V.

The amp sounds great. I replaced the 2 inputs with shielded wire, as well as the pin 2 cable on the second preamp tube. It is now completely quiet, almost dead silent.

Controls all work fine, only one problem I still need to fix: At high volumes I get crackling/popping noises. There is none of that at low to medium volume, as soon as I turn the volume up past a certain point, the popping occurs when I hit a powerful chord.
Then turn up the voume some further, and the popping/crackling starts even when I hit the strings only a little harder.
Any ideas how I can try to trace the source of this problem?

[MWJB]

"Controls all work fine, only one problem I still need to fix: At high volumes I get crackling/popping noises. There is none of that at low to medium volume, as soon as I turn the volume up past a certain point, the popping occurs when I hit a powerful chord.
Then turn up the voume some further, and the popping/crackling starts even when I hit the strings only a little harder.
Any ideas how I can try to trace the source of this problem?" Shorten all grid wires, shield them if necessary (even to the power tubes). What grounding scheme have you used? Any pics so we can see your layout?

[j-stylez]

Here is what my grounding looks like at the moment:

Pots are grounded to the chassis, same is true for the speaker jack.

Black dots means connection to chassis.



I first had the turretboard grounded to the point on the right side, but then changed it to the drawn diagram (its connected to the chassis close to the input jack now).

I started to read a document on grounding on the valvewizard site, and came to the conclusion that my grounding isnt ideal yet

I tried to copy the grounding of the jtm-45, but I'm not sure where to connect all the star grounds. If i run them all to one point, my wires would be quite long and going all over the place is what i thought.

Should I ground the turretboard on the right side, to the star ground? And if so, should I leave the connectin of turretboard to input jack ground? I thought this would make a groundloop.

But when I still had the turretboard grounded at the right side, it was much quieter.

Also, is it a good idea to run a cable from the speaker jack to the star ground on the right side? Its currently grounded to the chassis only.

[MWJB]

Your grounds from the board should not be daisy chained, run each wire separately to the input jack ground.

Preamp and PI filter caps ground to the input jack ground. Screen & main filters to main power amp ground.

Why not ground the OT at the speaker jack? It works well enough on ALL 5F6As.

Do you have a buss wire for the control pots?

I can't tell from this how long things like your grid wires are, or how well components on the board line up with threir respective tube pins etc.

[j-stylez]

This is what it looks like. I changed my grounding a bit.

The right side still looks ugly, because I havent attached the wires neatly yet, I will do that once I have everything figured out! Also I have to cover the unnused wire ends with shrink tubing and arrange them nicely, and attach the caps better than they are right now. Also the chord will be changed, so it cannot be ripped out accidentally.

There is no noise at all now, not even the slightest hum or hiss. Very silent.

The only persisting problem is the one i mentioned before, once a certain volume is reached, there is crackling/popping noise. It sounds like someone is switching the instrument cable while the amp is turned up. Very ugly sounding thing, i bet the speakers dont like it.

The preamp tubes and pi are lined up quite well, but I drilled the power tube sockets a bit too far to the right. I shoul dhave changed their position with the speaker jack maybe...but thats too late now. The Grid wire of the right power tube is a bit longer than that of the left as you can see.

[MWJB]

Your OT secondaries would be better running through a hole in the chassis, by the speaker jack, out to the OT, rather than running accross your board.

Move the 27K NFB resistor to the left and rotate your PI coupling caps, plate resistors & bias feed resistor thru 90degrees to shorten the wires to your power tube grids.

I can't see what is going on with your filter caps, you seem to have a 22uf 500 PI cap, a 22uf 500 screen cap, then 2x22uf 500 in parallel (I don't know what these connect to), 2x 100uf 40 in series but no 220K resistors bypassing them?

12A#7 plate wires can be long & sweep around the tube socket close to the chassis floor, keep them away from grid wires.

[j-stylez]

The parallel caps are wired exactly as in the 5f6a schematic. There are no 220k bypass resistors, so I didnt put any in...
The 2x100 in series are part of the voltage doubler.

I will try to reshape the layout as you recommended, but it will be difficult... i would have to take the whole thing apart again. I should have thought of htis before hand...well, i learned something again

looking at this picture, it doesnt seem like my power tubes are that far away...are there other things that could be the cause for my stated problem?
http://www.classictubeamps.com/gut%20shot.JPG
http://web.comhem.se/~u70623527/Fend...0and%20CBS.jpg
http://www.fydamps.com/images/my_bassman.jpg
both ahve their tubes in similar positions as I do.
Could using shielded wire for grids help? Or maybe using a heavier gauge wire for the grids? Or maybe make both the same length by making the short one longer? I still dont understand what causes the problem.

I listened to the sound again. Its a stuttering type of noise. I can turn the volume all the way up and play silently, and it wont matter, but once i play a bit harder, its like a stuttering-crack noise. Like an oscillation type of sound.

Something else I just remembered, but I dont think this matters. The two diodes in the bias supply circuit are not identical. One is a regular 1n4007, and the other one is an ultrafast one.

[MWJB]

Your voltages are too high for 500v rated caps at screens & main B+, your voltage doubler caps replace the 2x20uf 500 in the obsolete Fender schem (WHICH YOU SHOULD NOT BE COPYING).

"looking at this picture, it doesnt seem like my power tubes are that far away...are there other things that could be the cause for my stated problem?" Here's the thing, your amp doesn't work...this is because your layout and/or your grounding is bad. Shielding the grid wires (as I already suggested) may help, heavier guage won't help, shorter the better. Grid wires act like ariels, picking up all sorts of unwanted effects.

[j-stylez]

Yes, I noticed that. I use 350v rated caps in series now, and using 47uf instead of the 20uF.