two problems with tweed style build

[Bluefinger]

mwjb,

I have grounded the first filter cap to the tranny mounting bolt and everything else to the inpot jacks. This is a scheme that I have used on all of my builds lately and it's literally dead quiet. I have tried a couple of attempts previously including the one you described but this one definitely works best. Even with all volume controlls wide open there is almost zero hiss you really have to put your ears close to the speakers to hear it. Since the grounding is solid and everything else is working I seriously doubt that it's the cause for the motorboating ... or could a grounding scheme like this really be a reason for motorboating??? What do I know ...

yes, the power tubes are wired exactly like in a 5F6A Bassman (no grid stoppers). Would you think it's better to connect the screens directly to the power supply and use the grid stoppers instead ... or should I keep the 470R screen resistors as well? There is hardly a voltage drop across them so I have to admit that I have not quite understood the reason for both of them yet ...

[MWJB]

Keep the 470ohm screen resistors.

Everybody who commercially builds tweed Fenders installs 1.5K grid stoppers feeding pin 5.

First filter cap (main B+) and the screen supply cap should be grounded to the PT bolt, along with the B+ CT, bias supply ground & power tube cathodes. The long ground wires from the pots/control panel make me uneasy, as do the daisy chained grounds along the board, because there is potential for grounds to be out of phase with each other...you may not effectively have ground potential on the long ground wires. It's great that there is no hum or hiss, congratulations, but you have an issue and that is most likely down to layout/grounding...something is causing this, it's a question of eliminating exactly what.

[MWJB]

Can we have a better pic of your PI circuit please?

[Bluefinger]

ok, I'll try the the grid stoppers. I guess it's best to run the wires to pin 1 and put the resistor in between pin 1 and 5 ...

My center taps are grounded at the PT bolts as well of course. The only thing different is the screen supply cap. I had it connected to the PT once on a previous build and it created some hum so I never tried it again and was a happy camper ever since.

I'll try some different grounding schemes to see if that changes anything.

thanks a lot for your input!

... still I wonder why it only occurs in fixed bias mode if it's a preamp grounding issue ...

[Bluefinger]

Can we have a better pic of your PI circuit please?

I'll post one later or maybe tomorrow ...

[MWJB]

"ok, I'll try the the grid stoppers. I guess it's best to run the wires to pin 1 and put the resistor in between pin 1 and 5 ..." yes, or you can stand them up straight from pin 5 & connect the grid wire to the free end of the resistor. It's a good idea to keep these wires as short as you can.

[Bluefinger]

"ok, I'll try the the grid stoppers. I guess it's best to run the wires to pin 1 and put the resistor in between pin 1 and 5 ..." yes, or you can stand them up straight from pin 5 & connect the grid wire to the free end of the resistor. It's a good idea to keep these wires as short as you can.

by connecting them to pin 1 I should be able to save at least half an inch of wire and I can keep them close to the chassis so that will probably be the best way to go.

Can the choice of filter cap sizes be a problem ... I never considered this ...

[MWJB]

Filter caps will be fine up to 100uf total for the main B+, you could use up to 47uf everywhere else, but most guys would use 20-47uf for the screeen supply, 20uf for the PI supply & anywhere from 8 to 47uf for the preamp filter.

[tubeswell]

I have grounded the first filter cap to the tranny mounting bolt and everything else to the inpot jacks. This is a scheme that I have used on all of my builds lately and it's literally dead quiet. I have tried a couple of attempts previously including the one you described but this one definitely works best.

Hi there Bluefinger. What works very well for one amp may not work with another amp.

You have described your amp as some sort of hybrid, but I think it may help if you drew the schematic so we could see how many filter caps you have and which different parts of the circuit they supply, compared to how everything is grounded. If you are still getting hum, its quite possibly a grounding issue. Mixing up the ground-return points with different filter cap ground return nodes is likely to cause hum, i.e.: if the screen node parts of the circuit are grounded with the pre-amp, that could be causing hum.

[Bluefinger]

... no hum at all ... it seems to work in this circuit as well ...

[Bluefinger]

sounds good ... I have a 100 µF for the main filter cap, 47 µF for the screen supply and 8µF in the preamp ... only the PI is a 33 if I recall correctly but I'd have to check ...

edit: nope, it's a 22 at the PI ... fine as well ...

[tubeswell]

Okay - my bad

How's that schematic coming along?

[Bluefinger]

It seems like the grid stoppers really did the trick. I did a quick test run and the motorboating was gone completely.

It also turned out that the mid pot does work but with very little effect. I might try a different value pot to increase it's range. Tomorrow I might have time for some extended testing but it seems like I have a working amp and I can concentrate on the fine tuning now.

thanks for all the suggestions!!!!!

[Bluefinger]

How's that schematic coming along?

The motorboating seems to be cured so I don't think I'll draw one anytime soon ... all is looking and sounding good now since I installed grid stoppers. I will probably change around a couple of things before I'm really satisfied anyway. Especially the tone stack might need some modifying to be better suited for harp. Before I start with the fine tuning I need the actual cab and speakers because they might change it's characteristic again. Hooked up to my Tweed Super cab with Weber 10A125s it sounds really great so far ... even better than the Super itself which I already liked. So far, the trouble shooting seems to be done ... I have a feeling that this is going to be a great amp

[loudthud]

Your amp seems to be marginally stable, it's right on the hairy edge of oscillating. That's why it only oscillated in fixed bias mode. So, the oscillation might come back any time you make changes, move some wires, change bias or tubes. Keep this in mind as you make changes. Don't make too many changes at one time, try the amp often. Do you have the (typically) 47pf cap between the PI plates? Increasing that cap to 100pF might improve the stability.