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Old 16th Aug 2013, 8:03 pm   #41
Heatercathodeshort
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Default Re: A Gramophone amplifier from 'JUNK'.

This is the circuit for the Gramophone amplifier constructed from scrap parts. It is entirely conventional and I built this from memory as it is such a common design encountered in record players and the amplifier stage of slightly 'better' broadcast receivers.

The components are easily available from scrap radio chassis but if you intend to use a 6L6 valve for output it is well worth hunting for a 'better' output transformer. It will be worth the search but don't waste money on transformers that are claimed to have 'magical powers'. They may well be thus blessed but alas will not sound any better than their brothers from the wrong side of town..The all metal American types are good quality but often have surface rust and appear very scruffy. They are therefore shunned and can be purchased cheaply. Do not dismiss them, they are very reliable valves. Almost any mains transformer can be employed providing it will deliver the correct heater voltages for the valves and rectifier and around 290v of smoothed H.T.
The aim is to get a working amplifier and I can assure you that when you hear your first record playing through it, you will want to build on your success.
I have used a very old record deck and pick up just for the fun of it but of course you can employ any later record deck or changer and virtually any cartridge other than the very low output types used with genuine better quality amplifiers. The end. John.
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Old 16th Aug 2013, 8:52 pm   #42
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Default Re: A Gramophone amplifier from 'JUNK'.

A few nitty picky points......

1. Won't the increasing load on the pickup from the "reversed" volume control upset the frequency response at low volume settings?

2. The 25uF cathode bypass on V1 is also bypassing any negative feedback. The bottom part of the cathode R should be unbypassed with only the top part bypassed by the cap. Which resistor goes where will depend on how much feedback is required.

3. If the tone control C fails short it could take out the output TX primary and/or the tone control pot. The tone control is also inside the feedback loop which is a bit undesirable on stability grounds.

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Old 16th Aug 2013, 9:13 pm   #43
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Default Re: A Gramophone amplifier from 'JUNK'.

Yes a mistake in the diagram! I just jotted it down and made an error in the connection.
I have used a 1000v .047uf of super quality for the tone control. It should be OK.
I've tried all configurations for the volume control and with the old type pick up used I can only say that the results are excellent. I've been playing it this evening for about two hours and the quality is smooth and crisp. It will probably be played at quite high volume settings and this is what I aimed for.
I totally agree with your comments but it works well so will probably leave it alone.
I must admit I omitted a grid stopper to the output valve and thought I might have to add one but so far all is calm! Thanks, John.

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Old 17th Aug 2013, 8:40 am   #44
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Default Re: A Gramophone amplifier from 'JUNK'.

I have corrected the circuit of the NF and added the circuit mods that Chris has suggested. The negative feedback circuit was incorrectly drawn in my original circuit. I played about with the feedback when constructing the amp. I fitted a pot in place of the 12K resistor and adjusted this for a slight dip in sound quality, then backed it off. It read very close to 12K so substituted the pot for a fixed resistor.
The technically correct wiring of the volume control tended to have a rather harsh operation and reconfiguring it to my original circuit has considerably improved the smoothness giving good quality over the entire range of the track.
The tone control or rather 'top cut' modification is shown in the third section of the scan.
Just out of curiosity I flicked through a number of broadcast receiver circuits of the 40's/50's and was surprised to see how many of the correction caps were wired to earth and that was back in the days of the wax caps. Saying that I cannot remember encountering any that had actually gone short circuit damaging the transformer. Thanks again for your comments Chris and all FEEDBACK is of course very welcome. [I was going to say NEGATIVE but that would have been a yawn too far] Regards, John.
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Old 17th Aug 2013, 2:25 pm   #45
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Default Re: A Gramophone amplifier from 'JUNK'.

Ah, the joys of English, where positive feedback is desirable from a personal viewpoint but undesirable from an amplifier's one

TBH from a personal safety point of view, connecting the top cut pot one end and wiper to chassis with the other end to anode via a decent capacitor is better, but from the viewpoint of a (possibly expensive) output transformer, it isn't. In any case, for a junk box special, if it works, it'll do!
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Old 17th Aug 2013, 2:38 pm   #46
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Default Re: A Gramophone amplifier from 'JUNK'.

If across-the-primary connection was considered safe in live-chassis record players (and just think how many machines were built with just a UL84 and a UY85 .....) then it'll be fine here. The impedance of the power supply is much lower than that of the tone control network, is what's important.

With across-the-valve connection, if the cap went leaky, the potentiometer track would probably act as a fuse and protect the transformer; or else limit the fault current to a safe value.
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Old 17th Aug 2013, 2:55 pm   #47
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Default Re: A Gramophone Amplifier from 'JUNK'.

A bit late in the day, but this thread really belongs in the (Proudly) Homebrew section.
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Old 17th Aug 2013, 3:46 pm   #48
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Default Re: A Gramophone Amplifier from 'JUNK'.

This is what Pamphonic sold in 1932
Senior Pamphonic. Sorry can't do it as an attachment as it's too big.
Look at pages 9 onwards from this 1932 catalogue.
Two decks and cross-fade, for use in Theatres. Price, probably about £150-£200 with amp and speaker baffles(s) - pretty much the cost of a house back then!
(Sorry if a bit off-topic. Just responding to the post earlier in teh thread about early twin-decks)
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Old 17th Aug 2013, 5:34 pm   #49
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Default Re: A Gramophone Amplifier from 'JUNK'.

Thanks Bill. It started off as just any old amp and like all home brew projects, it grew!
Thanks agin for your comments Chris. This is what the Forum is all about and little bits and pieces here and there are probably interesting to the present generation who have never had the chance to build anything but are very curious about doing so. John.
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Old 29th May 2014, 1:15 pm   #50
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Default Re: A Gramophone Amplifier from 'JUNK'.

For those commenting on the Mullard 3-3 and the price of EF86's:
Fifty pounds each for NOS Telefunken EF86 & EL84 is nothing compared to the price of really good transformers. At the time I made my 3-3 I saw Sowter charged 135 pounds for a o/p transformer and about the same for the power transformer
I had a recycled power transformer & even the aluminium chassis was re-cycled. There's not a bit of hum as everything is well spaced out. O/p was wound by Ed Dinning.

BTW I used Russian valves: Svetlana EF86 & Reflektor EL84
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Old 29th May 2014, 9:46 pm   #51
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Default Re: A Gramophone Amplifier from 'JUNK'.

Hi Neil, glad you liked it. The 3-3 is an underrated amp and is a good domestic performer, I've since wound some stereo pairs for another forum member.

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