Mystery amplifier

[knobtwiddler]

Originally Posted by McMurdo

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

It made me think of guitar amps as well. Check this particularly neat example:

https://reverb.com/uk/item/61426409-wallace-ac-5000-xt-mkii-...ube-amp-early-70s-soundgas-serviced

(long sold, so hope I'm not breaking any rules!)

[Techman]

Originally Posted by Kokotoni Will

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?

Absolutely not! If you adjust any of the pre-set adjustments on the power supply without fully understanding their function you are likely to do serious damage to the unit.

You need that extra control unit that should go with this amplifier. It's possible that this control unit could have been solid state (transistorised) with a separate power supply and not another valve unit. You need to show us everything that you got in this auction 'lot'.

No disrespect to your good self, but with your obvious lack of knowledge regarding the workings of this type of equipment I think that there's a good chance that if you continue as you are and attempt to apply power then there's a real risk of you doing serious damage to the amplifier and even yourself!

You need to take stock of how you intend to go on with this amplifier and its power supply before you possibly blow it and yourself up!

You still haven't told us whether you have the all important interconnecting cable to go between the two units.

And lastly, ALL valve amplifiers MUST be run with a load, ie, appropriate speakers - running without speakers or other appropriate resistive loads is likely to destroy both output transformers!

Sorry if I'm sounding a bit harsh, but it's the way I'm seeing it - and I've seen it a lot over the years!

[Kokotoni Will]

Originally Posted by Techman

You need to show us everything that you got in this auction 'lot'.

Unfortunately, I've shown you all I have.

Originally Posted by Techman

And lastly, ALL valve amplifiers MUST be run with a load, ie, appropriate speakers - running without speakers or other appropriate resistive loads is likely to destroy both output transformers!

I see. Thank you for this information.

Originally Posted by Techman

Sorry if I'm sounding a bit harsh, but it's the way I'm seeing it - and I've seen it a lot over the years!

Point taken. I appreciate your directness.

[Welsh Anorak]

Could we ask what you're hoping to do with the amp?

If you were hoping for a valve amplifier to use in your hi-fi system this looks like a good bet, BUT it needs both a preamp (not too hard to find) and the appropriate connecting leads (needs someone with experience to make these correctly and safely).

Again, we need to know your level of experience with this sort of equipment. There's quite a high voltage available at the Jones socket which needs caution. There are also several tests needed and a couple of components replacing before you go any further.

As others have said it's beautifully made with high quality components and, given the right attention I suspect it will sound great. Should you feel it's beyond your level of experience it would certainly be worth asking in the 'Repair services required' on the forum. I suspect someone in your area will be delighted to take this job on - working on something this well made is a pleasure.

[knobtwiddler]

Valve amps from this period usually have a lot of gain, so you may not need a fully functioning preamp if you listen to digital or line sources mainly. A passive level control, possibly with a switch to select sources, may be all that's needed. It's the sort of thing you could rustle up in your shed in an afternoon.

The tricky bit here will be getting the power amp up and running safely...(unless your understanding of valve amps is near professional level, I would tread very, very carefully - it's a nice example, so worth consulting a pro). Once you've done that, you can add the preamp / control amp sections in stages, i.e. a passive level control to start, and a phono preamp in a separate box (you can get phono amps from the likes of Pro-ject or NAD for peanuts secondhand - but note that you'll need a level control between that and power amp).

[Edward Huggins]

This is a unit of the highest quality. I suggest the OP does not attempt any more work or powering up until this can be assessed by a suitable Member from this Forum. There are many local to to the OP.

[knobtwiddler]

BNC are a devil to work with for the average hobbyist or hifi buff. This makes me wonder if the unit were built by an engineer, for personal use only (I fantasise about having BNC-everything on my audio systems - much nicer than awful RCAs). Ditto to what Edward said.

[Silicon]

From the visual and multimeter information you have provided, mains transformer terminals 1,2 &3 are the HT windings.

4a &4b is probably the 5V AC rectifier heater winding.

7a & b and 8a & b are probably 6.3V AC heater windings.

To do a thorough test the winding insulation should be tested at 500V.

Amplifiers are sometimes retired from use when the primary of the output transformers fail.

Can you test the resistance of those windings?

[Kokotoni Will]

Originally Posted by Welsh Anorak

Could we ask what you're hoping to do with the amp?

It would be great to have it working in my hi-fi system. While I have some experience repairing transistorized stuff (e.g. ZX Spectrum computers), valve-powered equipment is a new area for me.

[Herald1360]

The construction looks similar to the rather beefier pair of 30W amps I have.

https://www.vintage-radio.net/forum/showpost.php?p=287032&postcount=7

I wonder if it could be a pair of GEC 12-14W amplififiers as described in the GEC book "AN APPROACH TO AUDIO AMPLIFIER DESIGN" built onto a common chassis? Or perhaps a pair of Mullard 5-10s.

A listing of the valves in the amplifier could be informative....

Could those input connectors be MUSA type rather than BNC? I can't see any bayonet latch pegs on the sides.

[trobbins]

Are you keen enough to inspect and test this amp, given the learning curve required, and the tools that would normally be required for checking parts and operation?

I, and maybe others, would suggest you need to undertake a few activities before even thinking of applying power. Even the act of applying power needs to be done in a safety first manner and is a considerable risk, as you likely don't have tools like an insulation resistance meter, or variac, or suitable meters, or a detailed step-by-step procedure for such testing.

Some people would naively turn it on and check if it works before listing it on an auction site - that is certainly not the recommended path to take for such vintage valve equipment.

[knobtwiddler]

the tools that would normally be required for checking parts and operation?

This is the crux of the discussion. Unless you have certain tools (or intend to get them, along with the knowledge to use the tools, before powering the amp), your only option is to find a pro or serious hobbyist who can help.

In the very least, you will need:

i) Lightbulb tester
ii) Variac
iii) IR Tester
iv) Oscillator
v) Dummy Load ("loads" plural if it's stereo and can't be powered one channel at a time)
vi) Oscillscope and high voltage probes

If you don't have the above, don't power the amp. You really want a schematic as well - most engineers worth their salt will spend an afternoon tracing out a mystery circuit before going anywhere near mains with it.

[Richardgr]

A significant point not mentioned above, is the value of what you have and how you preserve that.

Judging by the pictures of this amplifier, there may be a couple of thousand pounds worth of transformers, potentially that could be damaged, and they are probably irreplaceable. Apart from the craftmanship itself, so this amplifier is unique. Do you know the provenance?

(I know there is a commandment that a person must not covet another person's amp, but I can't help it!)

[Kokotoni Will]

Originally Posted by Herald1360

A listing of the valves in the amplifier could be informative....

Could those input connectors be MUSA type rather than BNC? I can't see any bayonet latch pegs on the sides.

I've attached some photos. I've numbered the valves as they appear from left to right in my opening post. The input connectors do have latch pegs.

Originally Posted by trobbins

Are you keen enough to inspect and test this amp, given the learning curve required, and the tools that would normally be required for checking parts and operation?

I have an oscilloscope It is starting to look like my best bet will be to find someone local to assist, though. There used to be a guy called JonnyHarmonic who did amplifier repairs locally. If I can't get in touch with him I'll try reaching out to some forum members.

Originally Posted by Richardgr

Do you know the provenance?

Sadly not. I will ask at the auction house next time.

[Radio Wrangler]

BNC connectors, confirmed.

CV valve numbers show they were made for British military supplies. Some had improved specification, all had better process control and/or testing to some extent. Even though they weren't necessarily better than the commercially-numbered part, they were more expensive, so they were unlikely to have been used in a commercially made hifi amplifier.

It looks like your amp was built by someone professionally involved in electronics having a high standard of skills and workmanship, building something for himself, using the best quality parts available.

As has already been said, it's an item that will get some valve amp aficionados overly excited to the point where they may need a bucket of cold water chucked over them.

Back in the day, there was a finite number of valve amplifier designs on the go. The origins were mostly in circuits published by the valve manufacturers showcaseing what their new devices could do. Home builders built these as per the published articles. Commercial hifi firms did their own designs but they were strongly influenced by what the valve makers had published. Those valve makers did deals with transformer makers to get suitable transformers ready for sale when the circuits and valves became available to constructors.

So, one important step to take with your amplifier is to trace out its circuit and post it here. The known published designs will get recognised quickly, and any variations on their themes will be spotted. Only then will you really know what you've got.

David

[trobbins]

Finding a local mentor would be a great step forward to growing your awareness of what you have. I hope you can progress in that direction, and even with a mentor I'd anticipate that describing your progress on this thread, and using it as a sounding board, is well worthwhile.

Even preparing a valve amp schematic can require a steep learning curve, but at least you've had some exposure to solid-state schematics. If you get keen to go that path then this forum would enjoy helping if you can pencil up a sketch and upload a photo.

[dave cox]

This is a very nicely made amplifier, assembled with top quality parts.
It could be worth £££s

On the down side :
It could have a transformer fault that cannot, realistically, be repairable.
It's not so powerful (15-20w at best) which will restrict your choice of loudspeakers.
You will need to spend ££s getting it up and running, reliably, then more ££s when it burns through the output valves
Its architecture is, ATM, unknown and despite its construction quality it could perform badly if the design or layout is wrong ...

My choice would be to check the 3 transformers (chokes as well), then sell it AS-IS with some photos of the tests carried out.
On the other hand it could be a very good looking centre piece with performance as good as it gets (from valves) !!

dc

[knobtwiddler]

There is a chance that the design could be flawed and that the builder knew how to make things pretty, but didn't understand the finer points of physics, but I think that's unlikely, in light of the attention paid. If you go back to post 1, see the neatness of the metalwork and how symmetrical everything is. The BNC is insulated from the chassis, the banana sockets are aligned straight - and all sockets are a nice mirror-image in relation to the valve sockets and transformers. If you went to that much bother, I like to think you'd also choose a topology that were proven.

The wiring and component positioning underneath tells a similar story, with an unusual amount of attention to symmetry. Even the cable clamps in base of the PSU are nicely aligned.

It has an unusually high degree of artistry and dedication to the craft. It's almost as if it were built to display at a show, or enter a competition.

The transformers will be very hard to replace (did any firm make better ones in the period? Parmeko stuff is highly respected and seen in military kit, I believe?).

Rehabilitating an amp of this quality is a double-espresso / phone-on-mute job, even for a learned engineer with a boatload of test gear.

[Welsh Anorak]

Might it be an idea to pop to the auction house and have a chat.

They might be able to tell you more about it, or even find the connecting cable that's lying in a corner - and there might be a preamp of similar quality.

Worth a try?

[daveobuchanan]

The rectifier / regulator valves and some other parts inside are very similar to 50's military AWA 300 300 power supply I had.