Pathescope Son projector amplifier

[EF80TVVALVE]

Well I've spent a bit more time with the projector this evening. I removed the resistor across the output, it measured 50ohm and I've measured the speaker and it is indeed 15ohms. I spent a bit of time adjusting the lens for the PEC and that has made things a little better, with all of this done I have a better amount of volume but still too quiet especially amongst the chatter of the projector. I tried a 4ohm speaker, it was actually quieter than the 15ohm but possibly the 4ohm isn't very sensitive and the 15ohm more so?

Next I may purchase a couple of 6SH7 and see if fitting those gives me more gain, possibly scratch my head a bit more looking at the schematic and making some comparisons, just to work out anything that can be done to give more gain from the stages of the amp.

[buggies]

Ok - please ignore my comments then as the 50 Ohm will not be making any difference. Perhaps it was added to protect the amplifier if the speaker became disconnected accidentally.

[EF80TVVALVE]

Yes I believe that was the resistors purpose. As it was likely that the amplifier was used only for the first stage which was then connected to a larger power amp then there would have been no requirement to connect a speaker to the output of the amplifier.

Once I know nothing more can be done to improve gain from a repair point of view then I will turn my attention to see what can be done to the first stage in order to improve the gain there, I'm sure some improvement can be made as from my calculations that valve isn't actually working very hard.

[EF80TVVALVE]

Hi all,

A little more time spent on the Son amplifier this evening just to evaluate where to focus my attention from here on. I'm going to look further at the second and final stages mainly because I feel that doing a buzz test through the gram input just doesn't cause enough buzz through the speaker when the volume control is at full. The only valve I haven't yet checked is the output 6V6 so I will do so, just to be sure that there is no fault there, from there on I'll pay closer attention to the circuitry and work out where I'm losing the gain from the amplifier. Other possibilities I'm thinking of is a faulty output tx that has a short between windings causing low anode resistance but I'll check this further down the line.

[buggies]

Hi again,

You mentioned re-capping in the first post - just checking if the cathode bypass electrolytics are ok as they could cause low gain.

[EF80TVVALVE]

Hi George, yes I replaced these capacitors and they are okay, in fact it's quite annoying as the amplifier is exact to the schematic now so in theory it should work fine. I do wonder if inadequate loading on the output stage has caused damage to the output transformer or output valve, as I explained its likely this amplifier was often used only for its first stage where the signal was essential tapped to a larger amplifier, with this I suppose it could be likely any of the two are damaged especially if the amplifier doesn't like operating without a load.

Cheers
Bren

[Station X]

Thread reopened at OP's request.

[EF80TVVALVE]

Thanks Graham.

I've been taking a look at the Son amplifier again today. I put it to one side a while ago as I'd drawn a blank and decided to work on some other projects. After some time looking it over today I've began to question why the amplifier is running the 1st stage valve with only a 5v heater supply when it's a 6.3v valve? Does anybody have any ideas as to why this may be done?

This has lead me to consider if this is the cause of the low gain fault. I've seen with some valves when the heater voltage is lower than it should be that the gain falls off a cliff edge. This isn't always so with a brand new strong valve but I've seen it with new valves that are a little below spec and aged valves can be much more prone to it. Then there's types, some power valves won't show a difference but pre-amp valves such as ECC83 etc can be quite bad as soon as the voltage starts dropping below 6v.

I am considering temporarily testing this first stage at 6.3v to see if this makes a difference. 1.3v from the heater of a valve is quite a bit. Just before I do anything I'm intrigued to hear of anyone's opinions as to why it's been designed in this way. Just seems a bit odd to me and I've never seen another amplifier purposely running the heater voltage of a valve lower than it should be.

[wd40addict]

In posts 10 and 17 I asked if your first stage was running in starvation mode - this is what the reduced heater voltage does. In my machine the resistor has been removed (or was never fitted) and the first stage heater runs at 6.3V. Starvation mode is supposed to give higher gains, but in this context with a lowish anode resistor and no cathode bypass cap it seems an odd choice.

[EF80TVVALVE]

Wd40addict, thanks for clarifying, I don't know why I didn't check this at the time and perhaps I was just expecting the fault to be elsewhere. My example still has the resistor fitted so I wonder if this was a fault experienced with some models and the resistor later removed or never fitted. I'll have a look today and see what results I get from it. It would be interesting to know if any supplement information was ever released from Pathe about this, if yours has been removed then it was obviously for a reason. I'll report back my findings a little later on and see how things improve.

[EF80TVVALVE]

I've now removed the resistor bringing the heater voltage up to 6.3v on the 1st stage 6SJ7. I've also fitted a cathode bypass capacitor and both of these modifications have improved the output though it's not exactly booming with volume yet. I'm at a point now where I can just about hear it okay over the chattering of the projector but it's still not really enough. I'll have a think about where to go next but things look to be heading in the right direction. I am tempted to try a 6SH7 in the first stage and see if that offers any gain improvement though again it'll probably be just a smidge more if anything.

[wd40addict]

If you have a 6SH7 available it's definitely worth a go. The amplifier allegedly was built by Trix, but having never worked on any Trix stuff I can't compare the standard of construction to see if it might be true. Starvation mode is also used in the EF86 on the Mullard 3-3, but not by dropping heater voltage.

The Mk2 Son amplifier was based on the E*40 series and I've never seen a schematic for it. Would be interesting to know if they still used starvation.

[EF80TVVALVE]

I may have one hanging around so I'll check through my spares boxes and see. I think realistically though I have done all that I can do to improve the gain of the current set up. Touching a screwdriver on the grid of the first stage gives me a nice loud buzz so I think it's likely that the photocell is probably worn out and past it's best.

I do have a spare housing for a photocell so I'm going to try a few modern versions out and see how things go there. The first I've ordered is a BPW34 miniature solar cell, this should easily fit inside the housing directly on top of the optic tube. Hopefully it'll be sensitive enough to drive the amplifier though I know others have used this cell on their 16mm equipment with good results.

[wd40addict]

Make sure the ends of the glass rod 'fibre optic' are nice and clean.

[Jez1234]

I'm intrigued by the PEC being damaged by daylight! A sensor designed to have a projector lamp shone into it, (I would guess that when film runs out there's serious illumination of the PEC), yet ruined by normal daylight.... (is there a small dim bulb for the soundtrack maybe?). Is this due to the ultra violet part of sunlight? All new to me so interesting.

[wd40addict]

The illumination on the PEC in the sound gate isn't actually that much. Back in the day there were three methods to reduce hum from the illumination source. The best was to run a small lamp from a high frequency valve oscillator. The other methods rely on thermal inertia - if the lamp has a thick enough filament it won't cool down much between 50Hz cycles. This is the method used on the Son with a 36W lamp. Finally projectors such as the Pathescope Vox and the GB -L516 split off a little bit of light from the main lamp. The relative failure of this last method can be observed by the tuned hum filter across the speaker (!) in the L516.

[Jez1234]

Originally Posted by wd40addict

The illumination on the PEC in the sound gate isn't actually that much. Back in the day there were three methods to reduce hum from the illumination source. The best was to run a small lamp from a high frequency valve oscillator. The other methods rely on thermal inertia - if the lamp has a thick enough filament it won't cool down much between 50Hz cycles. This is the method used on the Son with a 36W lamp. Finally projectors such as the Pathescope Vox and the GB -L516 split off a little bit of light from the main lamp. The relative failure of this last method can be observed by the tuned hum filter across the speaker (!) in the L516.

Everyday's a school day! Thanks that's really interesting I recollect seeing something about before the days of AC bias tape recording, optical film recording could give really impressively hi fi results? Is that for real or a bit exaggerated by time and myth? (or I recollected very wrongly of course!)

[kalee20]

Originally Posted by Jez1234

Everyday's a school day! Thanks that's really interesting I recollect seeing something about before the days of AC bias tape recording, optical film recording could give really impressively hi fi results? Is that for real or a bit exaggerated by time and myth? (or I recollected very wrongly of course!)

For a while, optical film recording was indeed the best quality sound recording.

Alan Blumlein's experiments and demonstrations of stereophonic sound were done using optical recording.

[peter_sol]

Hello.
The 50 ohm resistor across the output will have loaded it enough to save damage with no speaker connected so I would re fit it.
The heater dropper would not increase the gain by starvation, that was done by Mullard by increasing the anode resistor.
If you need more gain, you can put a pot in the anode of the EF86 and adjust it.
You will find the gain increases until the HT becomes too low then decreases. you could then fit a fixed value.

[Synchrodyne]

Originally Posted by kalee20

For a while, optical film recording was indeed the best quality sound recording.

Alan Blumlein's experiments and demonstrations of stereophonic sound were done using optical recording.

On optical recording, for example see:

http://www.douglas-self.com/MUSEUM/C...ac/duotrac.htm


Also, WW 1937 March 19, p.277.

WW 19370319 p.277.pdf




Cheers,