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Old 23rd May 2024, 10:48 am   #1
EF80TVVALVE
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Default Mullard amp build for Pathe projector

Hi everyone,

Some may remember a while back I was mulling over the idea of building a valve amplifier from scratch to use with one of my Pathe Vox projectors. The project took a bit of a turn when I managed to get hold of an original Pathe amplifier and the project was shelved. By coincidence I picked up another Pathe Vox projector from a local carboot sale, this one also missing the original amplifier. Whilst I do have a few spare amps now for these projectors they have been heavily modified and bodged, after some thinking I decided to resurrect my amp build project and got to work.

I decided to base the amp circuit on the Mullard 5-10 design with of course relevant changes in order to make the amp suitable for the projector, this involved fitting in a circuit to supply an original photocell and I also made a DC supply for the EF86 and ECC83 heaters. The HT is also rectified with a bridge rectifier to save an extra valve socket and get more volts out of that 140-0-140v winding. It was quite a job to get it all onto the original amplifier chassis which measures only around 6x6" but I've been quite successful in that respect.

I now have the amplifier working relatively well but there are still some issues which need to be ironed out. The biggest issue being the output transformer which I had to nick from a scrap Pathe chassis, this is originally for a push pull 6F6 circuit and is a puney little thing, you'd have thought it would be a single ended type from a valve radio judging by the size alone. Whilst the 6F6 and EL84 aren't miles away in terms of anode loading they aren't identical either and the original transformer is very different to one that would be considered suitable for a normal 5-10 build.

The amplifier works very well without feedback but begins to oscillate when any is fed back to the EF86. I need to investigate further but first thoughts lead me to suspect that the output transformer has a role in this, the problem is that there is just no space on the chassis to fit a conventional transformer so this was the most suitable type I had to hand.

Another option would be to run the amp without feedback but this would involve further mods to reduce the gain. The standard 5-10 circuit only requires 40mV for full output and without feedback becomes more sensitive. Triode strapping the EF86 would be the obvious starting point but I won't explore such options until I know for certain that this is the route I'll have to take, I'm sure with some investigation I can find the exact cause and hopefully adjust the circuit as necessary to rectify the issue.

So far I'm quite pleased with the project, my first proper scratch build amp! I've attached a couple of photos, some parts are still a bit messy as there's still a few things to be finalised circuit wise but I'm sure I'll have it all sorted soon!
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Old 23rd May 2024, 10:59 am   #2
Gabe001
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Default Re: Mullard amp build for Pathe projector

Are you sure you've got the feedback the correct way round? You may be providing positive feedback and may need to flip the secondaries of the opt (i.e ground the other wire) to switch the phase and change it to negative

If this makes no difference, my guess is that the output transformer can't take 20db of feedback. Have you tried it with 10db ie with a larger feedback resistor?

You will also have to adjust lead and lag compensation to cater for the transformer (to stop the ringing) and you won't have a great stability margin, and you may end up with -5db or so at 20kHz......

You may be better off with a simple ecc81 -> concertina phase splitter -> el84 push pull and minimal feedback.

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Old 23rd May 2024, 11:50 am   #3
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Default Re: Mullard amp build for Pathe projector

Thanks for the suggestions Gabe001, yes I have reversed the secondary but with little improvement. I have also tried reducing the feedback again with no improvement but I need to look into this further as I haven't given it much attention yet. HF isn't so much an issue with this particular build as the sound strip on 9.5mm is by no means HiFi though I have considered finding a better suited OP TX as this realistically seems the easiest answer without having to go too far into circuit changes. The closest I could find new are made by OEP but still just a little too large for the chassis.

I will keep in mind your suggestion for circuit changes at the bottom of your post as the amp doesn't need to stick to the 5-10 design but it does need to remain sensitive as the Photocells require a high gain stage in order to drive the amp well. Firstly I want to see how poorly the amp performs with no feedback, investigate lead/lag and see if there is a level of feedback which it is happy with. From there I may consider triode strapping the EF86 which seems to often be met with success by many project builders of the 5-10 though depending on how sensitive the amp is. If I'm not successful with this then it looks like a more heavier redesign will be required or a way to fit a better TX.
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Old 23rd May 2024, 12:00 pm   #4
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Default Re: Mullard amp build for Pathe projector

I did triode strap my EF86 in the Mullard 5-10 I've built and it works very well.

I have a smallish spare ultra linear push pull output transformer for EL84 valves. It isn't a Sowter by any means but it doesn't oscillate with 26db of feedback. You're welcome to it if you're stuck.
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Old 23rd May 2024, 12:32 pm   #5
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Default Re: Mullard amp build for Pathe projector

When you say 'any feedback' how far have you tried reducing it? Try to find the point where it stops oscillating. As above you'll have to keep reversing the windings to establish phase (or scope it).

Have a play with increasing C1 and/or C6 this will help too. Obviously this is a bit empirical - the proper way to do it is to plot gain and phase.

As a high feedback design the 5-10 is a bit touchy. I built mine with a high quality output transformer, but still needed to be very careful with the routing of the wiring to it to keep it stable.

It should be possible to find a nice compromise where amp is stable with a bit of feedback and gain is increased (which should be useful for optical sound).

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Old 23rd May 2024, 3:44 pm   #6
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Default Re: Mullard amp build for Pathe projector

Problem with the standard 5-10 circuit it has far too much open loop gain for most applications. Add to that an OPT out of the parts bin... As Gabriel said, try reducing the gain, do you need that much gain?

Second what is the frequency of the oscillation? You could try adding two LP RC filters from anodes to ground to roll off the HF response above 30khz, something like a 1k & 4n7, the 1k R needs to be a few watts with a similar RC filter across the speaker terminals with the same HF roll off response, it's a bodge though. Either that or connect a cap from anode tap to anode tap on the OPT. The later needs a high V spec though, 1kv ish.

Lastly when I apply FB for the first time I used a 10 turn 20k pot whilst monitoring the OP into a dummy load on a scope & THD meter or speaker: stop when the amp starts freaking out, back off some. Bit more too it than that, but it can be done by ear which is the final arbiter.

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Old 23rd May 2024, 4:06 pm   #7
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Default Re: Mullard amp build for Pathe projector

I am wondering what the ideal input sensitivity is for a pathe projector? Im afraid I don't know much about these.

I'm assuming around 300mv RMS?
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Old 23rd May 2024, 6:24 pm   #8
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Default Re: Mullard amp build for Pathe projector

Thanks everyone for the replies, you've made some good points to investigate further, I'll try and reply in order:

Gabe, thanks for offer of providing an output transformer, if I don't get very far then I will take you up on that. As for sensitivity, I'm not certain exactly on the ideal input sensitivity, I suppose it would depend on the exact PEC used and also if it's lost sensitivity over the years. I would (from experience of working on various projector amps) expect the input sensitivity to be measured in tens of mv rather than hundreds hence why the 5-10 circuit looked quite appealing in terms of sensitivity.

WD40, I tried a few different resistors in series with the 33k + 100pF currently fitted to the circuit, in the end I was up into hundreds of KOhms and still with oscillation. You have made a very good point of reversing the polarity again and I will try this tonight and perhaps instead fit a pot as Andy (DA) notes to find a suitable point and go from there if I see any improvement.

Andy, yes it is a shame about the OP transformer but there's not really a lot that can be done being limited to the space available on a very small chassis, it also pre-dates a lot of HiFi theory and even common use of feedback in audio amplifiers but for compactness and anode loading it's really the best I could muster up. I haven't measured the frequency (I only got the amp test-ready yesterday evening) but it is lower-mid range and definitely sub 1kHz, I will measure it tonight and update a little better with my findings. I will attempt, in a similar fashion to yourself, to find a level of feedback that the amplifier is happy with.
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Old 24th May 2024, 7:21 am   #9
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Default Re: Mullard amp build for Pathe projector

Ahh, sub 1khz, that's a different fettle of kish. Have a play with the FB first but check your grounds, de-coupling and power supply connections.

Andy.
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Old 24th May 2024, 8:42 am   #10
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Default Re: Mullard amp build for Pathe projector

Will definitely check the circuit and earth connections again. I did have a play with the feedback last night, found a 2.2meg pot to put in series with the 33k resistor, the oscillation does start low but increases in frequency the more the pot is turned back, allowing more feedback to pass through. I also reversed the secondary windings but was met with a high frequency oscillation regardless of pot positioning. Without feedback the amp works well but is way too sensitive. I am inclined to begin with subbing the OP transformer for something better suited, it seems almost illogical to mod the circuit to accept a 'not quite right' transformer and I'm betting that it's the likely culprit. I'll just have to get a bit more creative on where a replacement one can live.
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Old 24th May 2024, 9:22 am   #11
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Default Re: Mullard amp build for Pathe projector

Can you fit this? It's ultralinear 8k. Good for around 6-7 watts. I had this in my Armstrong 220 before I replaced with the primarywindings ones, and it didn't oscillate. It'll deliver quality watts as long as it's not too loud. You may need to adjust HF compensation a bit if ringing on square wave with 20db fb, or back off the feedback slightly.

Also, it likes to enthusiastically pick up magnetic coupling from the mains transformer, so ensure its oriented at 90degrees and give it some space (or do the headphone test).
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Old 24th May 2024, 11:35 am   #12
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Default Re: Mullard amp build for Pathe projector

Hi Gabe, I'll measure up tonight and see if it'll fit, thanks for digging it out and taking some measurements. I'll also see what we have here at work but I presume any EL84 transformers here will be way to big to fit onto the chassis. It may also be that the new transformer will have to be fitted on the underside, I'll have to put the chassis back into the casing to see how much free space is underneath and whether a larger replacement can be fitted in that way.
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Old 24th May 2024, 12:15 pm   #13
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Default Re: Mullard amp build for Pathe projector

If you're tight for space you could possibly do the 3-3 instead. Using solid state rectification will save you a valve space
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Old 25th May 2024, 11:12 am   #14
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Default Re: Mullard amp build for Pathe projector

After some searching yesterday we managed to find a P-P EL84 transformer at work. Slightly bigger than the OEP ones but luckily small enough to fit under the chassis once it's back in the metal casing. I strung it in yesterday evening and was finally met with success, no horrible oscillation and the amp now working very well. Hum levels are good and the input sensitivity seems just about perfect but I'll give this a good test once it's mated with the projector. For now I have changed the feedback components with those given for a 3.75Ohm speaker as this transformer has 4 & 8 ohm outputs but I'll have a proper look with a sig gen and scope to see if it's happy with the given values. I have a few older 3 Ohm speakers hanging around so will likely keep along this route.

I did check the size of that transformer Gabe but it is just a little too big. Once it's all back together I'll upload some pictures so you can see the tiny size of this chassis, I'm still surprised that I've managed to get a 5-10 circuit built on there without much trouble.
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Old 25th May 2024, 11:29 am   #15
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Default Re: Mullard amp build for Pathe projector

Great job.

One final thing. Feed it a 1khz square wave to make sure it isn't ringing, even if there is no oscillation. If it is, which is very common if your transformer's HF response isn't as good as the ones mullard recommended, you may need to change some RC figures to fit your opt.
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Old 25th May 2024, 5:51 pm   #16
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Default Re: Mullard amp build for Pathe projector

Will do just that, I'm looking forward to seeing if it offers any improvement on reproduction compared to the original amps, either way it's been a very interesting project. I also have a box containing an almost complete stereo 5-20 in the loft. It's given me some inspiration to get that finished now too!
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Old 26th May 2024, 8:02 am   #17
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Default Re: Mullard amp build for Pathe projector

You've caught the bug. I know the feeling. Have fun
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