Mystery amplifier

[Kokotoni Will]

I've recently acquired this valve amplifier at a local auction. I am unable to test it, since I'm unsure how to connect it up. Can anyone help me out?

[Radio Wrangler]

It appears to be incomplete. I can see output connections for two loudspeakers and screened coax connectors for two inputs. So it's stereo. Each channel looks to have two power valves almost certainly in a push-pull arrangement. The two large transformers couple the outputs from the anodes of the power valves to the speakers, transforming the impedance and isolating the speakers from the high voltage supply.

Mentioning high voltage supply brings us to the problem. There is no high voltage or low voltage supply. I think that must have come in from a separate large chassis with one or more large transformers on it, connected to this unit via the Jones plug mounted to the centre of the panel face of this unit. This amplifier will need several amps at 6.3volts (most likely) to heat the cathodes in the valves, It will need a few hundred volts at 60-100mA to power the actual amplification.

Was there another mystery chassis also in the auction? It might have been the missing power supply. If you're lucky you bought it as well as a pair.

What you have is very nicely built and those output transformers would have been very expensive even back in the day. Are they branded Gardners or Parmeko by chance?

David

[sortedradio]

You will also need a stereo pre amp to control levels, tone etc.

[FIXITNOW]

Looks to be missing its power unit, auctions often dispose of the mains power unit as they can not be PAT tested and therefore are deemed unsafe to sell, go back and check the bin/skip

[trobbins]

Can you identify any markings on the two output transformers - they appear to have UL PP primary windings, and a split secondary - which appears to be differently set up for each channel.

Can you identify any markings on any parts that may be date codes?

Are there any QC markings on the chassis anywhere?

[Richardgr]

Beautifully constructed, smacks of quality. For example, the holes for the base plate have brass inserts by the look of it.

The amount of screening is unusual, including the use of coaxial inputs. Some sort of broadcasting or recording studio pedigree?

[Nickthedentist]

Agree with that, Richard.

Parmeko transformers?

[Radio Wrangler]

Oil filled. If Parmeko, I think they were their 'Neptune series'

David

Unless I am mistaken, that looks like BNC connectors for the inputs - a bit unusual.

[Kokotoni Will]

Many thanks for your replies.

I did get another chassis in the lot - the power supply David mentioned, hopefully - and I've attached some photos of it to this post. I'm guessing the big, round 3-pin connector on the left is for connecting it to the mains, and the 24-pin Jones plug goes to the amplifier.

I did find some markings on the transformers, both on the amplifier and on the power supply - I'll attach them to my next post.

[Kokotoni Will]

Here are the transformer markings for the amplifier + power supply, plus the two larger power supply valves. The four smaller valves are in two sizes; one pair are marked 641 R7H D and 641 R7E N respectively, but the other pair have no visible markings.

[Techman]

Very good!

Hopefully you've got the interconnecting lead in your 'lot', otherwise you've got a lot of connection modification to work out and do, also the 'control' unit.

The mains connection is a standard 5 amp 3 pin Bulgin.

[Silicon]

The CV378 is the full wave rectifier and the other valves could be for a stabilised power supply.

The item marked '63' is the smoothing choke.

The resistors with the pink bands are 'high stability' types and may be OK.

The main amplifier seems to have mica and electrolytic capacitors.

If you have a multimeter you could start by measuring the resistances of the transformer and choke windings.

[Craig Sawyers]

Everything smacks of quality.

The only potential difficulties are the several Hunts paper capacitors in the amp and power supply. The initial insulation resistance for those was 200 ohms/farad measured at 50% greater than the working voltage for less than 30s. So a 1uF capacitor should be 200M-ohms under those voltage conditions. Tolerance is +/-25%

They might well be OK. The usual sign of difficulty is when they are heat challenged and the plastic sleeve goes brown. None of them in the amp and supply show that.

But it is worth bearing in mind that there may be problems with them.

Craig

[knobtwiddler]

I can only second what's been said. It looks to be of high quality, and the transformers appear to be Parmeko to me. In a working state, I think it could be quite valuable to a valve amp afficianado. You sometimes see amps that have quality parts, that have made schoolboy errors and look messy. Likewise, you sometimes see amps that have budget parts, that are beautifully made. You seem to have good parts, along with a construction by someone who took pride in their work. Do be careful when applying power. Note that some O/P transformers can be damaged if not correctly loaded when powered up. Well spotted!

[McMurdo]

Reminds me of Hiwatt amps, they used all right angles and twists in their chassis wiring.

[trobbins]

I have a Parmeko with a two digit stamped number in that central location, although most Parmeko's I've seen also have other QC markings and/or labels. That number is like the Neptune 6000/xx model number.

[turretslug]

That's a lovely find, someone obviously took considerable pride in their work- maybe someone with a background in military/professional electronics? I think I can make out a wire going up beside the steel screen/shield on the power supply, connected to an HT tie point under the chassis, maybe an anode connection to a series regulator valve? That HT also goes via a small wire-wound stopper on pin 4 of the same valve socket, typically screen grid on octal power valves so triode-connected. (EL38??) Perhaps for the early stages of the amp, or maybe an outboard pre-amp, tuner etc. Shouldn't be too hard to trace out the Jones plug connections, it's a nice big, clear connector.

Good luck with it, we're all ears here (as it were).

[Kokotoni Will]

Originally Posted by Radio Wrangler

Are they branded Gardners or Parmeko by chance?

There is what looks a bit like an encircled "P" on the left amplifier transformer. So possibly Parmeko?

Originally Posted by sortedradio

You will also need a stereo pre amp to control levels, tone etc.

The power supply has a 10k potentiometer, marked with 250, 270 and 290, and therefore seemingly to adjust voltage - I've attached the image to this post. Would this work as a volume control?

Originally Posted by trobbins

Are there any QC markings on the chassis anywhere?

Nothing I haven't already posted, unfortunately.

Originally Posted by Silicon

If you have a multimeter you could start by measuring the resistances of the transformer and choke windings.

I've made some resistance checks between all the pins of both the transformer and choke, and here's what I've got. I've numbered the pins as per the attached photo. All other connections measure open circuit.

1-2: 42.7 ohms
1-3: 82.8 ohms
1-4: 20 mega ohms and increasing
1-B1: 20 mega ohms and increasing
1-B2: 20 mega ohms and increasing
2-3: 38.8 ohms
2-4: 5 mega ohms and increasing
2-B1: 20 mega ohms and increasing
2-B2: 20 mega ohms and increasing
3-4: 25 mega ohms and increasing
3-B1: 20 mega ohms and increasing
3-B2: 20 mega ohms and increasing
7-J1: short
8-J2: short
E-J3: short

Originally Posted by Craig Sawyers

The only potential difficulties are the several Hunts paper capacitors in the amp and power supply. The initial insulation resistance for those was 200 ohms/farad measured at 50% greater than the working voltage for less than 30s. So a 1uF capacitor should be 200M-ohms under those voltage conditions. Tolerance is +/-25%.

Are those the big capacitors strapped to the chassis? My multimeter can't reach anywhere near 200M-ohms. I have a basic capacitor checker which measures capacitance only - would that be any use?

Originally Posted by knobtwiddler

Note that some O/P transformers can be damaged if not correctly loaded when powered up. Well spotted!

To avoid this, would it suffice to have the power supply connected to the amplifier? Or would I need to wire the amp to some appropriate speakers too?

Originally Posted by turretslug

Shouldn't be too hard to trace out the Jones plug connections, it's a nice big, clear connector.

To refer to the attached photo, I've found that transformer pins 7 and 8 are wired to Jones plug pins J1 and J2 respectively. J3 is the earth connection. Transformer pin 2 goes to J4, via the two capacitor legs I've circled. The middle pin of the 10k potentiometer (bottom right) goes to J5, via a big capacitor (marked with an X). As per the other attached photo, on the top of the chassis, this potentiometer is marked 250, 270, 290 (volts?), so J5 looks like the pin that provides high voltage to the amp.

I'll do some more tests tomorrow.

[Stevie342000]

The series regulator valve (Ediswan) looks like a 12E1 to me and the rectifier looks like GZ37 (I think it is or 5R4 has a CV code on not hard to find what it is).

It is very well made and has high quality parts in both the power supply and the stereo power amplifier.