Pathescope Son projector amplifier

[peter_sol]

I hope this is not off he subject but in the days before tape recording, Radio Luxembourg
recorded their London programmes in on film. the BBC wondered at the quality as they were still recording on disc.

[EF80TVVALVE]

Hi everyone,

A little update on the Son amplifier. The BPW34 cell arrived yesterday and I worked late into the night testing it and mounting it inside the spare cell housing that I have. The results from the cell are a little better than that of the original PEC but the gain still seems low. It was getting very late by this point so I haven't actually film tested it yet but I will do so today and compare the results. I'm also going to try the same cell on one of my 16mm projectors and see how it compares in gain on that amplifier in comparison to the Son.

I still need to look for a 6SH7 and try one out in the first stage but from there forward I suppose I will have to begin looking at what can be done to the amplifier to increase it's sensitivity. I know there's many folk who just replace older valve amplifiers on cine projectors but I'd rather keep this projector as original as possible without having to hack it up in any way.

[wd40addict]

It should be possible to get the volume up. The speaker should be a nice efficient 4R Rola so I do wonder about the substitution someone has made. Assuming a perfect match (unlikely) a 4R speaker should give you 4 times the power compared to the 16R unit.

I keep meaning to get my Son on the bench to give you some benchmarks, but unfortunately other stuff keeps intervening. I may be able to do it in a week or so.

[EF80TVVALVE]

Yes I've been on the look out for a suitable 4r replacement but I've had nothing turn up as of yet. If you could get your Son on the bench for a comparison then that would be great, there's no rush though as this is one of those projects where I'm quite happy to put to one side for any amount of time

[wd40addict]

OK, here we go: back in the day 'undistorted' meant 10% THD+N (yeah I know!). Using this as a yardstick the amplifier gives 3.5W into 4R from 850mv RMS @ 1kHz fed into the gram socket. Power falls to 3W into 8R and 2W into 16R. Distortion into 8 & 16 shoots up due to the poor match and no feedback.

[Nickthedentist]

A 10” 3R speaker of the period, on a big baffle, would have filled a small hall driven by 3.5 W, I would have thought.

[wd40addict]

Sorry, should've gone to Specsavers it was 270mv RMS in, not 850.

Yes Nick, it's pretty loud!

[wd40addict]

@EF80 how do these figures compare to yours?

[EF80TVVALVE]

Thanks for the figures Wd40addict, my apologies as I hadn't seen your reply to the thread. I'll get a look in this weekend and let you know the results.