more headroom for my PI

[catalin gramada]

Hello,
friend of mine asked for a simple ampeg b-15 clone. we used el-34 as power tubes cause this was available, and thinking will be more easy to drive. the PI is very well balanced and ht supply arround 380v for PI. the signal can reach cca.60v peak before clipping in power grids point with power tubes pulled off but with power tubes in circuit 25v and bang the PI goes into clipping. we tried with 6ca7 with same result. I cannot understand what's going on. we have a very generous load 6k6 OT, and power tubes was biased for 60% dissipation arround 36v
Can someone explain how to fix that , please? Thanks
Cheers
Catalin

[J M Fahey]

It's normal.

You reach 60V Pk unloaded but then you add the clipping diodes and of course it will clip, what else did you expect?

Which clipping diodes?

The ones inside the EL34/6CA7/any tube: the grid(anode)/cathode one, which will conduct as soon as it is forward biased, say 1V positive or even less.

Say you have -38V grid bias.
As soon as you apply 39V peak (39*0.707=27V RMS) that anode is 1V more positive than the cathode and it will forward conduct, clipping the signal at the PI plate.

[catalin gramada]

have a lot of sense. hmm...didn't thought in term of RMS voltage and expected signal to be clipped way over my mistake . Thanks for answer.
Cheers
Catalin