Motorboating - please need

[tasmaniandevil]

Finally had time put this together.......826/100TH >> IT >> 250TL. but it motorboating badly. Can only get rid of the problem by adding 2 more LC to separate drivers and power tubes. Or add a complete set of filter for drivers. Is it possible for both driver tubes and power tubes to use the same HT? or there's something I've done wrong ?

Thanks a lot

 

[tasmaniandevil]

oops..something omitted. there's a grid choke at the driver tube.

 

[ErikdeBest]

AFAIK, a(the) cure for motorboating is separate Power supplies, or at least a good decoupling of the PS going to the driver - as you already noticed.

and, any chance of seeing some pictures of your amp?

 

[DF96]

High impedance PSU with subsonic resonance (around 17Hz) feeding two stages is asking for trouble.

 

[Sorento]

I would connect HT of output tube one stage earlier in that long filter chain and see what happens ...

 

[PRR]

8uFd and 10H resonates at 18Hz.

Resistors ARE useful.

 

[rongon]

Try simulating your power supply in PSUD2.
PSUD2

Playing with that, I found that LCLCLC power supplies often have bad resonances. It's often better to use larger cap values and/or something like a LCRCRC topology. Possibly simplifying to just LCRC might actually work better, depending on the iron you're using.