The reason Alex is having all these issues is because he is using mains toroidals as outputs and interstage which makes this whole design approach marginal. Current balance has to be addressed and matched tubes will not be sufficient.. The interstage will have just about enough inductance if driven hard (which you found out by upping the drive current - good idea). I would suggest upping it some more. I run mine at 40mA of drive through the interstage - a bit over the top but it does the trick and it might solve your issues.
I am not an advocate of gNFB (and never use it) but in this particular case I would be.
As regards moving the interstage. We have a saying in Ireland when asking directions "If I was going there, I wouldn't start from here" and unfortunately that is the advise I give you. No you can't implement Plate to Plate feedback with the circuit as currently configured.
Can you configure your interstage as a phase splitting transformer ?
Next thing I would attempt is tunning the parafeed resonance to see if you can get any useful bass boost. In my 6080 amp I started out with a 1uf cap and it had a subtle bass boost. I didn't want this so I increased it to 4uf and it went away. This should tell you that you can get a useful result by trying something in the range of 0.47 - 2uf. This should be an easy experiment to try out and well worth a mess with.
Hope that helps.
Shoog
I am not an advocate of gNFB (and never use it) but in this particular case I would be.
As regards moving the interstage. We have a saying in Ireland when asking directions "If I was going there, I wouldn't start from here" and unfortunately that is the advise I give you. No you can't implement Plate to Plate feedback with the circuit as currently configured.
Can you configure your interstage as a phase splitting transformer ?
Next thing I would attempt is tunning the parafeed resonance to see if you can get any useful bass boost. In my 6080 amp I started out with a 1uf cap and it had a subtle bass boost. I didn't want this so I increased it to 4uf and it went away. This should tell you that you can get a useful result by trying something in the range of 0.47 - 2uf. This should be an easy experiment to try out and well worth a mess with.
Hope that helps.
Shoog
Thanks Shoog.
My phase inverter is an autoformer so no primary/secondary.
I will be increasing the current of the 6n1p to about 13 to 14ma each triode (it is supposed to be rated at 20ma). So paralleled it should be around 28ma.
I just bought a Lundahl interstage and I might try it on this circuit.
Thanks for the tips.
My phase inverter is an autoformer so no primary/secondary.
I will be increasing the current of the 6n1p to about 13 to 14ma each triode (it is supposed to be rated at 20ma). So paralleled it should be around 28ma.
I just bought a Lundahl interstage and I might try it on this circuit.
Thanks for the tips.
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