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Thread: Suggestions for Oscillator to Amp project

  1. #1
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    Suggestions for Oscillator to Amp project

    Good afternoon all,

    I've been humming and hahing for a while now as to whether I should strip out an old valve oscillator and give it a new lease of life as a guitar amp. It's such a well made thing I've been loathe to put it under the knife - as it were - especially as it's still going strong. Trouble is, I have no use for it (my dreams of setting up my own BBC Radiophonics Workshop and becoming the new Delia Derbyshire having been shelved, for now...), so probably better it gets used for something, than gathers dust in a corner of the workshop.

    The question is, what sort of amp to build from it?

    Pertinent info includes, but is not limited to:

    -290VDC B+
    -230VDC B+1

    The heaters are wired for (left to right):

    -1xEF86
    -1xEL84
    -1xECC83
    -2xEL84

    Naturally, the order could be easily changed (the EL84 next to the EF86 will have to go), but it's so neatly wired it seems a shame to change more than I have to.

    It has a U709 rectifier, which I think is equivalent to an EZ81.

    Any suggestions? I'm leaning towards something Vox-like, the EF86 and EL84's being common in early Vox amps. The EF86 has a bad rep for reliability, but I'll not be touring with it - I'm hoping a carefully designed layout should minimise the risks?





    Thanks for looking,

    Matt.
    Last edited by FalderGuitars; 08-12-2017 at 09:24 PM.

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    More photos:






    Also, regarding the transformers, they're sealed units, with no markings. I'll keep the mains transformer, for convenience sake, and since it seems well made (it's a very heavy unit). I'm guessing the second, smaller grey lump is a choke, since there's a voltage drop across it. I doubt the output transformer will be of much use to me, without knowing the specs.

    Aside from that, there are some pretty cool parts. I'm particularly taken with the giant 1524pF cap, at +/- 1%. That's some pretty heavy-duty accuracy!

    Finally, I should mention that the super-sized dial lights up - a feature I'll definitely be keeping

    Matt.

    Edit: It just occurs to me now, I've been thinking for a while of a kind of two-in-one amp, with a low watt, single-ended stage and a higher watt push-pull amp in the same box.
    The current layout of EF86-EL84-ECC83-EL84-EL84 lends itself nicely to a preamp-output/preamp-phase inverter-output topology - in my head at least - with the single EF86 acting as preamp for two seperate output stages.

    Any thoughts?

    I'm thinking it works well on paper, but would present no small number of practical headaches...
    Last edited by FalderGuitars; 08-12-2017 at 06:41 PM.

  3. #3
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    Not familiar with UK test equipment but it could be push pull output.

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    Quote Originally Posted by mozz View Post
    Not familiar with UK test equipment but it could be push pull output.
    My knowledge of UK test equipment is probably less than yours to the power of 10 - notwithstanding the fact that I'm from the UK! Seriously though, everything post choke will be cut and pasted into my parts drawers, barring the turret boards, which I see no reason not to re-use.

    On a practical note - given that I'm something of a beginner - how would I test the output transformer for primary/secondary impedance? Do I need to measure it in situ, or should I remove it from the circuit and test it solo?

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    Quote Originally Posted by FalderGuitars View Post
    On a practical note - given that I'm something of a beginner - how would I test the output transformer for primary/secondary impedance? Do I need to measure it in situ, or should I remove it from the circuit and test it solo?
    You could measure it in situ. Put 6.3V AC from a filament transformer on the OT secondary and measure the voltage between the EL84 plates. Divide the p-p voltage by the measured 6.3V and square the the result. That gives you the impedance ratio. If the impedance ratio is 1000 then it's perfect for 2 x EL84s driving an 8 ohm speaker. 8 0hms x 1000 = 8k p-p which is the recommended p-p impedance in the EL84 data sheet.

    EF86, ECC83, 2 x EL84 would make a Matchless DC30 EF86 channel.
    EF86, EL84 could be an AC4
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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Dave H View Post
    You could measure it in situ. Put 6.3V AC from a filament transformer on the OT secondary and measure the voltage between the EL84 plates. Divide the p-p voltage by the measured 6.3V and square the the result. That gives you the impedance ratio. If the impedance ratio is 1000 then it's perfect for 2 x EL84s driving an 8 ohm speaker. 8 0hms x 1000 = 8k p-p which is the recommended p-p impedance in the EL84 data sheet.

    EF86, ECC83, 2 x EL84 would make a Matchless DC30 EF86 channel.
    EF86, EL84 could be an AC4
    As a beginner you might not be aware that when doing this you can get 100's of volts on the primary. Take care.
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  7. #7
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    Thanks nick.

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    It looks like a perfect fit for a Marshall 18 watt clone. The limiting factor would be the power supply with anything you build it seems to me. Oscillators don't generally have a huge amount of output. Repurposing may draw too much current.

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    Quote Originally Posted by Dave H View Post
    You could measure it in situ. Put 6.3V AC from a filament transformer on the OT secondary and measure the voltage between the EL84 plates. Divide the p-p voltage by the measured 6.3V and square the the result. That gives you the impedance ratio. If the impedance ratio is 1000 then it's perfect for 2 x EL84s driving an 8 ohm speaker. 8 0hms x 1000 = 8k p-p which is the recommended p-p impedance in the EL84 data sheet.
    Thanks for the advice. I've a quiet week at work so I hope to have time to move on with the project, I'll try what you suggest and see what results I get. Hopefully not a cloud of smoke and a trip to casualty

    Quote Originally Posted by Dave H View Post
    EF86, ECC83, 2 x EL84 would make a Matchless DC30 EF86 channel.
    EF86, EL84 could be an AC4
    My thoughts exactly. I suppose I'd technically be making a DC15 (or more likely an AC15, since I have the schematic). If I re-purpose the first EL84, I can probably squeeze in some reverb, which would be nice.

    Matt.
    Last edited by FalderGuitars; 08-14-2017 at 05:45 PM.

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    Quote Originally Posted by nickb View Post
    As a beginner you might not be aware that when doing this you can get 100's of volts on the primary. Take care.

    Good point, and thanks for the warning. I've been building amps from kits and schematics for a good few years now, and have had enough shocks to remind me to be on my best behaviour when the lid is off.
    More recently, I've been trying (with limited success) to get my head around the maths behind valve amplifier design, so I describe myself as a beginner in that sense. I'm thinking re-purposing the oscillator as a guitar amp will make a good halfway house between building amps from kits/schematics, and actually sitting down and designing one from scratch, which is my ultimate goal.
    Probably take me about a decade to reinvent an existing product, but the days are long and there's time to fill...

    Matt.

  11. #11
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    Quote Originally Posted by olddawg View Post
    It looks like a perfect fit for a Marshall 18 watt clone. The limiting factor would be the power supply with anything you build it seems to me. Oscillators don't generally have a huge amount of output. Repurposing may draw too much current.

    That would be an excellent choice, if I didn't already have one! Thanks for the advice on the transformer, I hadn't considered the possibility I'd be drawing more current than it had to offer.
    Without having access to any of the specs, I don't suppose there's any way I can find out how much current it'll safely deliver?

    Matt.

  12. #12
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    I've come across oscillators with push-pull output. I currently have the output transformer from an HP unit that uses 2x6L6GC outputs. The problem is the (rather beautiful) output transformer is not matched for speaker use. Sometimes high-output oscillators are used for line testing and are matched for 600 Ohm. Some units dispense with the PI and use a small transformer as a phase splitter. Count the wires - 2 means choke, 5 means transformer.

    The probability is if the oscillator has 2xEL84 and an EZ81 equivalent, then the transformer would provide enough current to drive the output pair. Otherwise there's little point in a manufacturer going to the trouble of installing them.

    PS, your pics come with the invite for a free F*ck. From the look of the wretched hag in my offering, her milkshakes won't bring all the boys to the yard.
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    Quote Originally Posted by Mick Bailey View Post

    PS, your pics come with the invite for a free F*ck. From the look of the wretched hag in my offering, her milkshakes won't bring all the boys to the yard.
    Thanks for pointing that out, my adblocker had been shielding me from her 'charms'. Having witnessed the horror, I'll take them down when I get a moment and re-up them as attachments.

    Aside from that, thanks for the advice. I'll take a closer look tomorrow and see what's what.

    Matt.

  14. #14
    Senior Member Enzo's Avatar
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    I got the love ads too, I thought I had been sleep surfing or something.

    Mick beat me to it, the output transformer won't likely match up to 4/8/16 ohms, more like his 600. MAybe 150 at the lowest. Also, the power transformer h=may have enough juice for all the heaters, but a pair of EL84s doesn;t mean it has the current capacity to produce 18 watts.
    Education is what you're left with after you have forgotten what you have learned.

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    A rough and ready estimate can be made by measuring the secondary DCR and the unloaded AC RMS voltage. Assuming 10% regulation, use Ohms law to calculate the secondary current.

    That is to say, take 10% of your secondary voltage and use V(voltage drop)/R (secondary DCR)= I (secondary current rating).
    Last edited by Mick Bailey; 08-16-2017 at 06:34 PM.

  16. #16
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    Here's an interesting article on power transformers. Pity the scan is poor, but you can work it out;
    Attached Thumbnails Attached Thumbnails transformer1.jpg   transformer2.jpg  

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    Morning all,

    It took a while, but I finally had some spare time last week to dedicate to my Oscillator-to-Amp project. Thanks again to all the advice given

    In the end, I kept it simple and opted for the classic Vox AC4 circuit (with the EF86), minus the tremolo but with the reverb circuit from a Vox AC30 shoe-horned in.

    Here's the chassis, stripped out and ready for a new lease of life:

    osc-29-.jpg osc-31-.jpg

    I'd hoped to recycle the mains transformer, but at some point along the way it gave up the ghost and was replaced with a Danbury unit from Barry at Ampmaker.
    It was a similar tale with the reservoir caps, which were (perhaps unsurprisingly, given their age) leaky as an old sieve.

    osc-28-.jpg

    Naturally, it didn't occur to me to test them until after first switch on, when the amp was emitted a ferocious wail that grew steadily louder until I was afeared for the health my test speaker, not to mention my ears!
    Only after a speedy switch-off, and a thorough review of the wiring and layout did it occur to me that - in spite of appearance - 50 year old Electrolytic capacitors may not be quite at factory spec
    After knocking together a sub assembly from the parts drawer:

    osc-37-.jpg

    I cautiously flipped the on switch again, and to my surprise (and relief) it worked perfectly. Happy days.

    Here then is the amp in it's full glory:

    dscf4240.jpg osc-32-.jpg osc-34-.jpg osc-35-.jpg

    I'm still waiting on a transformer for the reverb circuit, and the keen eyed amongst you will notice that in the end I added the tremolo circuit too. Sadly, it's not the most elegent addition. I'd been careful with my original layout to keep my wire runs as short and neat as practical, in keeping with the spirit of the original wiring, which was a work of art. By using screened wires I've kept the noise down to a minimum, but it still 'chugs' quietly when the circuit is turned on.
    It sounds great though, I'll upload some sound clips when I get around to making them. The speaker is a 6" Jensen, in an old hi-fi speaker box with the back taken off, and is a pretty good match for the amp.

    There's plenty more photos, some of which I'll upload when I get a chance.

    Matt.
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  18. #18
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    Gorgeous!
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