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

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Old 20th Aug 2019, 7:35 am   #61
crackle
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Default Re: Shaftesbury 519 amplifier

It does not sound like the amp design was based on the Mullard 3-3 circuit which was popular at the time.
I restored an old Antoria amp 2 years ago, it had a single EL84 output, they can push out quite a volume on a decent speaker.
https://www.vintage-radio.net/forum/...d.php?t=138751.

Mike
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Old 20th Aug 2019, 8:27 am   #62
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Default Re: Shaftesbury 519 amplifier

Hi Mike
What a terrific little amp and I had no idea that Antoria branded equipment was produced that long ago. I was under the impression that Antoria was more of a 1970's Japanese branded manufacturer a.k.a Matsumoku made guitars ( I had a Kimbara Ripper bass from 1973).

Best Regards

Rob
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Old 23rd Aug 2019, 5:33 pm   #63
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Default Re: Shaftesbury 519 amplifier

I've now replaced all the old hunts caps and almost in a position to power it up for the first time BUT:
- should I replace the UCC caps? I am actually loath to doing in this unless absolutely necessary. Is the any way I can reform the caps (if necessary) in situ?
- should I replace the 200R red power resistor?
- with regard to extra safe operation is there any way to wire in a quick blow fuse? It the oldest amp I have and has no other safeguard than the mains fuse. I do have mini rcd's fitted to the mains fuse board

Best Regards

Rob
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Old 23rd Aug 2019, 6:47 pm   #64
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Default Re: Shaftesbury 519 amplifier

Quote:
Originally Posted by Guitarist28 View Post
- should I replace the 200R red power resistor?

Best Regards

Rob
Test it. If it measures around 200R it will be fine.
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Old 23rd Aug 2019, 8:26 pm   #65
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Default Re: Shaftesbury 519 amplifier

If you do not have an isolating transformer take all the valves out and put them in a padded box.
Power the amp up and check all the AC voltages from the transformer.
If the transformer is good add a 1N4008 diode in series with a 200k 2W resistor in place of one section of the rectifier valve and leave it powered for a few hours until the voltage climbs up on the capacitors.
If they turn out to be open circuit you will get hum when you later power it up with the valves in.
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Old 23rd Aug 2019, 10:30 pm   #66
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Default Re: Shaftesbury 519 amplifier

A 1N4008 does exist (1200V 1A) but good luck with finding one.

The jellybean 1N4007 will be a lot easier.....
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Old 24th Aug 2019, 9:46 am   #67
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Default Re: Shaftesbury 519 amplifier

With the valves removed do I have to put any load across the output transformer?
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Old 24th Aug 2019, 11:53 am   #68
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Default Re: Shaftesbury 519 amplifier

No need.
The transformer primary will be at a steady voltage that you will be measuring while the capacitors are reforming.
Output transformers only get damaged if the output stage is driven into distortion without a load.
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Old 30th Aug 2019, 9:03 pm   #69
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Default Re: Shaftesbury 519 amplifier

Just to double check on the EZ80 pinouts - those pins with the orange arrows I assume are the secondary power supply from the transformer (1 & 7). The pin marked with a blue arrow is the supply to the smoothing cap (through the 200R resistor). I can connect either pin 1 or 7 to pin 3 with the 200k 2 watt resistor in series with an 1N4007.
Is this indeed correct?

Many thanks
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Old 30th Aug 2019, 10:27 pm   #70
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Default Re: Shaftesbury 519 amplifier

That looks correct.
If you find any voltage at all on any of the G1 pins while the electrolytic capacitors are reforming replace the G1 coupling capacitor from the previous stage of the amplifier.
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Old 1st Sep 2019, 8:06 pm   #71
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Default Re: Shaftesbury 519 amplifier

The incoming transformer power measurements to the rectifier heater pins are 7.5v and across pins 1 & 7 is 570v. Does this appear reasonable?
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Old 1st Sep 2019, 10:18 pm   #72
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Default Re: Shaftesbury 519 amplifier

The diode and resistor goes from either 1 or 7 to pin 3.
The transformer will be marked something like 280 0 280.
The heater pins look a little high. It the primary of the transformer set to 220V instead of 240V?
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Old 1st Sep 2019, 10:37 pm   #73
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Default Re: Shaftesbury 519 amplifier

That’s interesting. I cannot see any means to adjust the voltage of the transformer primary? The mains connections come directly from the mains lead. Is there anything I can check?
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Old 1st Sep 2019, 10:51 pm   #74
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Default Re: Shaftesbury 519 amplifier

Unplug the mains lead and clip a meter probe on the neutral pin.
Then with the power switch in the on position measure the resistance to the various terminals on the transformer and find the highest one and then check that it goes to the live terminal.
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Old 2nd Sep 2019, 11:05 am   #75
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Default Re: Shaftesbury 519 amplifier

I've measured in with the transformer in situ (was this a good idea?). The readings are as follows:
- green/white to earth - O/L
- purple to mains switch - 0R
- purple to mains lead - 0R
- to rectifier pins 1 & 7 - O/L
- to rectifier heater pins - O/L
Best Regards
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Old 2nd Sep 2019, 11:13 am   #76
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Default Re: Shaftesbury 519 amplifier

What resistance range are you using on your meter? If it's a low range 'O/L' could indicate either an out of range or infinite reading.

You were asked to measure the resistance to the neutral pin of the mains lead, but it appears you may have measured to earth?

EDIT. Not so sure about that. Maybe you're defining where each lead from the mains transformer goes to? I would expect the resistance of the primary winding to show up on at least one tag.
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Old 2nd Sep 2019, 11:28 am   #77
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Default Re: Shaftesbury 519 amplifier

The two purple wires are the primary, so far as I can make out there are no primary taps that are visible.

The mains neutral is switched, the mains live isn't switched.

Lawrence.
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Old 2nd Sep 2019, 11:53 am   #78
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Default Re: Shaftesbury 519 amplifier

Yes, I did measure from the neutral pin of the mains lead and did adjust the multimeter range. I've attached some pics of the inside of the power transformer which I hope will be of help?

The nearest circuit diagram that has been somewhat helpful is the Vox Domino (very early AC4).

Thanks
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Old 2nd Sep 2019, 12:07 pm   #79
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Default Re: Shaftesbury 519 amplifier

I can see seven wires there. Two purple wires for the primary, two green wires for the LT secondary and two red wires and a white wire for the HT secondary (centre tapped). If this is correct there are no voltage taps on the primary.

The 7.5V measured at the heater terminals will drop when the valves are inserted.
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Old 2nd Sep 2019, 12:08 pm   #80
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Default Re: Shaftesbury 519 amplifier

Quote:
Originally Posted by Guitarist28 View Post
Yes, I did measure from the neutral pin of the mains lead and did adjust the multimeter range. I've attached some pics of the inside of the power transformer which I hope will be of help?

The nearest circuit diagram that has been somewhat helpful is the Vox Domino (very early AC4).

Thanks
There's no mains voltage selection tappings in there so far as I can make out.

Looks like the heater winding is centre tapped (Black wire)

EDIT: Post crossed.

Lawrence.
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