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BTH power amplifier type A1A (1950s, the BBC?): any documentation

Well, I will arrange a drive down to the seller to snap photos and talk about the sales arrangement. For now the seller will fix the mono blocks so they are in full working order. As he is quite busy with work, this may take some time during which I will have time to sell off my Audio Note DAC and transport to fund the purchase.

It will be interesting to see the mono power amps for myself. Of course the question is whether they have beeen restored with care for the original circuit, parts and chassis. I will make sure to bring my digital camera for good photos of everything. Then I will post them here. When this will be I do not know. I can wait as I have an Amcron/Crown power amp in the house.
 
Could one of the mods fix the wrong type description in the topic heading from "AIA" to A1A"? Much appreciated!

I have asked for photos of the underside so I can see in what condition the wiring, componens and the valve sockets are. I will post them here. Hopefully, I will be able to have a look myself next week.
 
Being an electronics engineer he did this himself and also added a feature where, when not used for music replay, the valves are kept warm with a low current so as not to wear out the valves

I agree with Jez that it sounds rather odd, and as with all classic kit my aim would be to faithfully restore it back to original spec and get it working and sounding exactly as it should. You will add value doing that too as long as you rebuild with appropriate components. I wonder if it is the equivalent of a guitar amp ‘standby’ switch? There are some advantages to bringing tube gear up gently for sure, e.g. I volunteer on the big SSEM valve computer replica at MOSI which has 550 odd valves in it and we have a multi-step start-up procedure that involves switching the heaters on at half voltage via a huge variac, taking them full, then turning the HT on etc. Only takes a few minutes, but it eases the initial power cycle shock. Some rectifiers soft-start anyway, e.g. the GZ34 used in the Leak Stereo 20 and I never feel the need to do anything but turn mine on when I want to listen to it. It is fully restored though so should be as good/reliable as a new one.
 
Maybe I am describing this the wrong way. I think that the switch lowers the electric current and thus the strain on the valves. It is like a standby switch so that the valves are kept warm but not at full throttle.
 
I was kind of assuming that the easiest way to achieve lower current would be to put an additional resistor in the cathode circuit of the output valves which could be switched in or out and would reduce the standing current at the expense of slightly lower power out and a small rise in distortion. This would improve valve life but why bother? If you want a PA amp use solid state.

Cathode poisoning as mentioned by Jez is a very real issue and under running heaters will encourage it. Back in the days of valve digital computers it was sometimes found that double triodes used in flip flop circuits would develop assymetric characteristics if left in one state for very long periods even though they were well within their ratings.

Soft switch on is a different issue, the current surge into a cold valve heater is very large, often this is mitigated by the regulation factor of a smallish transformer but if you have a large transformer / transformers, as you would on equipment using 550 valves the surge would not be much limited which doesn`t help the valve heaters and may well blow the mains circuit breaker.

All valve rectifiers soft start but indirectly heated types such as GZ32, GZ34, 5Z4 etc are good because they take about the same time to start operating as (most) power valves and so no large surge occurs.
With directly heated rectifiers such as 5U4, 5R4, U50, U52 the switch on is still soft but it happens much faster, before the other valves have warmed up which means there will be a voltage surge until the output valves begin to conduct and pull the voltage down. This ia why on older equipment you will often see caps marked with a surge rating, i.e. you can run them at this voltage briefly as the kit warms up.
 
I’ll quiz the more tech-savvy members of the SSEM team about cathode poisoning as it sounds interesting. The valves are mainly EF50 and EF55 pentodes and a load of little ‘peanut’ diodes. Being a computer it obviously spends most of its time flipping between binary states. The replica was built almost 20 years ago, runs roughly four times a week and the vast majority of tubes date from the initial build. We spend more time replacing off-spec resistors to be honest.
 
As I understand it was only a problem in a few cases where the flip flop was almost never used, not the majority of the stages.

I have a few EF50s and 55s in my loft, they all date back to wartime radar equipment and don`t look pristine although most have probably never been used. Not much call for EF 50s these days unless you have an old Cossor scope and I`ve never seen an EF55 used anywhere.
 
We are always looking for donations of EF50s and 55s if you don’t want them! We are actually pretty well stocked with NOS EF55s but now only have used equipment pulls for 50s. The is a rather temperamental AVO 163 tube tester to hand so we can test them.
 
I`ll see what I have - this won`t be a quick job as they will be well hidden having been up there for over 30 years.

If you ever run short of EF50s you could try making up an adaptor base to take an EF80 or EF91, they have quite similar characteristics.
 
Special valves were developed for use in later valved computers which were designed to resistant to cathode poisoning effects...
 
I think you lost me in those last couple of posts. But I will make sure to report back when I have actually listened and seen the BTH A1A mono blocks. I will pay special attention to the way the electric current is lowered by the flick of a switch.
 
Here are a few photos of the functioning mono block:
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I haven't seen this in the flesh, so can't say whether the photos show it in a worse or better light than they are. The wiring looks like a rat's nest and the seller has added an IEC outlet instead of the original one. There are two switches on the front - I supse that one is the on/off switch, the other must be the one that puts the amp/valves into low current "sleep mode".

Does this seem like a worthwhile beginning to an exciting restoration project, or should I best leave it alone?

EDIT: I should add that the seller will make both of them work before a sale is agreed.
 
I know I’m uber-picky and have more of a collector mentality than many, but they have been messed with far too much for me. There is a load of stuff that is obviously not original and they are so rare it is hard to know what of what’s left is how it should be. The additional holes in the chassis mean they can never be restored so they’ll never have anything much in value. I’d walk away unless they are *real* cheap and you fancy building tube amps.
 
The Stereo 20 to follow on eBay at present IMO is this one as it is stock and hasn’t been botched/altered at all. It will obviously need a rebuild but it would be easy to get it done properly with like-for-like parts. I’m always very suspicious of ones that have lost their green drop safety resistor for a start.
 
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I agree with Tony.
Too many extra holes, too much 'homebrew' and too little originality, what a shame. No disrespect to whoever modified them, but that's worse than a rat's nest. I would not trust them to be reliable without a full strip down and refurb. Unless they are very cheap and you want a project, I would walk away.
 
... and I have no idea if they ever made black / dark blue ones, I suspect not, but on a closer look at the pictures it does look like it has been resprayed?

... and the rear EF86 has been replaced with an ECC83?
 
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