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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Jul 2021, 6:02 pm   #1
Graham_W
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Default PEN3520 Amplifier project

Hi All,

I'm about to start my next project, an amplifier with push pull PEN3520 output valves, AC/HL amplifier and MU14 rectifier. When I first saw it I thought it was an AC/DC unit but it has a proper isolating mains transformer. The dropper resistor on top of the chassis (reads 800 ohms) feeds the output valve G2's and output transformer CT from the B+.
The input stage is a bit confusing as it has a 3V battery (well past its best)! and
transformer that can be put in series with the input jack via the switch on the front panel. Can anyone tell me what this was for? That transformer reads O/C, presumably attacked by the leaky battery.
Does anyone recognise the chassis? it is earlier than what I am used to.
Thanks and regards
Graham
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Old 22nd Jul 2021, 6:41 pm   #2
G6Tanuki
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Default Re: PEN3520 Amplifier project

It looks like some sort of public-address amplifier; I'd imagine the 3V battery and associated transformer/switch were intended to be used with a carbon-mic.
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Old 22nd Jul 2021, 6:43 pm   #3
peterpiper
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Default Re: PEN3520 Amplifier project

Hi Graham, looks a bit like an early Quad amplifier before the Quad ii. I think it was called the QAP12 I think.It was usually paired with an long/medium wave tuner. I owned one many years ago but your is a bit different, i.e mine didnt have the the telephone keyswitch , dropper resistor or the dry battery. the knobs look correct though. A friend still owns the amp/tuner so I may be able to get a picture.
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Old 23rd Jul 2021, 6:43 am   #4
Graham_W
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Default Re: PEN3520 Amplifier project

Thank you for your comments. I would be interested to see a photo of something similar.
A quick read about carbon microphones would confirm you are right about the battery.

Regards
Graham
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Old 23rd Jul 2021, 8:03 am   #5
Edward Huggins
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Default Re: PEN3520 Amplifier project

What do you intend to use this for?
If as a good Mono amplifier, its likely you'll need to add some additional AF (EF37A?) amplification.
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Old 23rd Jul 2021, 8:27 am   #6
rontech
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Default Re: PEN3520 Amplifier project

Amplifiers of that general shape were used in some early 16 mm sound projectors. I used to have a Victor 40, the amp of which was similar. Output valves were KT66 though.
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Old 23rd Jul 2021, 8:40 am   #7
Herald1360
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Default Re: PEN3520 Amplifier project

Looks like a mouldering modified cycle lamp battery in there. An 800 with knobs on!

Does it have its own type number?
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Old 23rd Jul 2021, 10:04 am   #8
snowman_al
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Default Re: PEN3520 Amplifier project

I do not recognise the chassis, but the Acoustical QAP12 (Quad) does have a mirror image look of your amplifier. Though it uses KT66s like the Quad 1 and Quad II.

As the PEN3520 dates from the mid 1930s I would guess it is a pre WWII, or just after, amplifier. It looks as if it has been through multiple 'improvements' in its life...

PEN3520 needs very little drive so the gain will depend on the phase splitter type.

Alan
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Old 23rd Jul 2021, 6:57 pm   #9
Graham_W
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Default Re: PEN3520 Amplifier project

Thank you all for your comments, all very useful.
I intend the amplifier to be used with a turntable. I agree that some form of preamp will be necessary as apart from the PEN3520's there is only the AC/HL triode. The phase splitting is somehow done by the cylindrical transformer twixt the AC/HL and PEN3520's.
I can see the similarities with the QAP12 so maybe mine is an early Quad. Can't beat a pair of KT66's though!
The battery is of the '800' type and I remember the style from my youth. Not much left of it except to say it is an Exide and has the price of 1 shilling.
Yes, it has seen some 'improvements' at some time. One of them you can see is a pair of flying leads connected to the PEN3520 anodes, presumably at sometime connected to a meter of some sort. I'm hoping it can be sorted though and look forward to seeing how it performs, at a guess all 7 watts of it.

Regards
Graham
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Old 23rd Jul 2021, 7:31 pm   #10
G6Tanuki
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Default Re: PEN3520 Amplifier project

The cylindrical transformer as a phase-splitter/driver sort-of suggests that the amplifier was intended to operate in a mode where the push-pull valves could be driven into a hint of grid-current [Class-AB2 or Class-B] - the transformer secondary windings providing a low-DC-resistance so there would be less perturbation of the bias-conditions of the push-pull stage when grid-current-rectification occurred.

If you want it to be a 'hi-fi' amp rather than one optimised for maximum-loudness PA service then you'll be wanting to use a mode like Class-A or Class-AB1 in which case it would be possible to repurpose the AC/HL as a phase-splitter.

The PEN3520 pair, with 250V on the anodes and 40mA anode-current per valve should give 10 Watts or so of audio output.

Note that they're quite 'high slope' valves for their time - gm of around 8 - so you should include 'grid stopper' resistors to minimise the chance of the amplifier turning into a push-pull ultrasonic power-oscillator!
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Old 24th Jul 2021, 6:23 pm   #11
Graham_W
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Default Re: PEN3520 Amplifier project

Thank you for your input. It certainly confirms a previous reply that this was designed for PA service. I will have a rethink what to do with the amplifier once I have got it to work something like the manufacturer intended.
Currently no grid stopper resistors are fitted but will add them as advised. I will also aim for 40mA anode current.
As usual, all the capacitors have failed along with the higher value resistors that have skyrocketed.
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Old 24th Jul 2021, 7:57 pm   #12
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Default Re: PEN3520 Amplifier project

I tried restoring an old PA and recounted my experiences here:
Lorenz PA

Firstly, I was a bit hasty dismantling it after noting the positions of wires. I should have sketeched the circuit diagram whilst I still had all the wires in place. I had assumed it must be a simple circuit, but these designes from the 30's and 40's are deceptively complicated, leveraging off other things to simplify a circuit, or solving a problem in a cost effective way.

Secondly, I had no clue where I was going with it. If I had had two, then monoblocks migt have been an option, being faithful to the original tube lineup.

I managed to get it to work, and I learnt a lot from the process, but it is now on the 'round toit' pile. I think I will try and make it into an SE headphone amp, using ECL11's (same sockets).

So, following this thread with interest!
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