Cheers for that absolute pearl 
And when you do, stand back.
Looks like a well thought out kit, with decent instructions - the build guide looks good on a quick skim. The circuit itself looks pretty standard fare, nothing adventurous, but fine.
However if you have to ask, I would question whether you are ready to play with high voltages (not that they’re so high that they’re likely to kill you, but could result in a nasty shock if you’re careless). As long as you understand how to take precautions with voltages that could hurt you, then you should be ok.
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Yes, it’s a problem. How did we all learn to work safely with high voltages ? By working with high voltages (sometimes when we were still schoolchildren). Did we get shocked ? Yes, sooner or later most of us did. We didn’t die because the contact we made was fleeting enough, and maybe we happened to do it at a convenient point in our cardiac cycle. How close did we come to serious harm ? We’ll probably never know … I’ve had quite a few shocks over the decades and I’m not dead. But at work once I saw an ‘ordinary’ mains shock throw a colleague across the room and as he fell he nearly stoved the back of his skull in on the corner of a cast iron surface table.
The unit is powered from the mains - 240V AC. Internally it generates more than 300V DC and the transformer it uses to do this is not feeble (190mA capability) so if you make sufficiently good contact with it then it is capable of killing you. The trick is not to make that contact.
The 300V power supply smoothing capacitors are also pretty large (over 800uF in total). This means they store nearly 40 joules. Typical defibrillators use more energy than that - 100-200 joules, say. But these capacitors aren’t a long way short. And this matters because capacitors keep their stored energy for a while after the unit is powered down and disconnected from the mains. It flows away through bleed down resistors and, normally, through the valves too as long as they’ve had time to warm up and some fault hasn’t disconnected them from the supply. In the worst case (bleed down resistors on their own) the discharge time constant is 320kohm x 800uF which is about 250 seconds, i.e. more than four minutes ! I’d want to be sure, by measuring it, that the high voltage had been drained away before I touched the insides of this unit. Otherwise I wouldn’t go near it for 10-15 minutes at least.
Also worth remembering that while electric shock can kill you, so can electrical fires. The irritating thing about these is that they can start after you and everyone else has gone to bed, and they can also kill more than one person.
All that said, as Pete says this circuit looks competently designed and if the kit people have put the same level of skill into the safety aspects then all may well be well.
VB
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Death at bedtime - The best metal band to never record (Because they died in bed)
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AI Second Audio advice wanted.
I’ve finally got them out of storage and want to do some sensible refurb work before I fire them up again. They are late model (ECC82 + 2A3) 2nds and totally original as far as I can tell.
I have some 4-pin 2A3s and matching bases, also replacement bridge rectifiers on David Wright’s advice - what I was wondering is if I need to replace the electrolytic caps as well. Given their age I imagine it would be good sense, but should I just do them all, or is there a hierarchy of need / performance benefit in the caps?
I was also wondering if there is a benefit in stepping up to some level of foo in the film caps or electrolytics (or any other components while I’m in there) - if so where would most benefit be found and does anyone have specific recommendations?
I’ve found a supplier for a decent range of components here (PartsConnexion), but would also be grateful for any other recommendations (@Professor_Chaos?).
Pics:
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I’m sure @murrayjohnson will be along, with the advice, shortly.
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Also meant to ask: are any value changes beneficial? I bought valves from Nick Bacon (Fourlegs) and he sent me a few pics of his Seconds - could see some different values of caps there.
Final pic: some local nastiness that will also need to be dealt with 
@murrayjohnson is your man for this.
In general, given their age:
Electrolytic caps may benefit from replacement.
Check Jensen pio caps aren’t passing dc.
Could big psu resistor be replaced with a big choke?
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Through the 1960s and 70s electrolytic cap technology improved. However there’s always been pressure on manufacturers to shrink their size. The result was that as they got better at making them they felt more confident about running closer to the edge. They are AFAIK the only passive component where the datasheet includes an expected lifetime. They’re not immortal.
That said, running them below their rated voltage and below their rated temperature will both extend their life significantly. In the case of temperature the rule-of-thumb is that for every 10 deg C (18 deg F) you are below the rated temp you will get a doubling of the lifetime. Datasheet lifetimes tend to start at 2,000 hrs and to increase, as you pay more money, up to 8,000 or 10,000 hrs or even more. The standard temp rating is 85 deg C. 105 deg C rated ones are quite common. 125 deg C are much rarer (sometimes used close to the cylinder block under car bonnets in hot countries). I try to use 105 deg C ones where I can.
VB
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Nice 
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Thanks for the warning re the Jensens; I wouldn’t have thought of that.
I’ve seen some AI amps that have had a choke added to the PSU. Would a choke or choke + smaller value resistor to the same dc resistance as the original resistor be appropriate?
Thank you - I’ll replace them all and try to find better temp and voltage ratings that will fit in (space is a little limited above and below the plate the valve bases are mounted on).
A case can be made for not changing the voltage rating greatly. Electrolytic capacitors work by having a thin insulating layer deposited, electrolytically (hence the name), on one of the electrode surfaces. Its thickness is one of the factors controlling the capacitance. This layer is maintained by the DC voltage which the circuit applies to the capacitor. Over time it is possible for the layer partially to re-dissolve into the electrolyte if the ‘right’ voltage isn’t applied to the component. In modern electrolytics this re-dissolution process is inhibited. But if a capacitor is operated very far from its nominal voltage then it’s possible that the layer thickness will change over time, and therefore the capacitance change too. This isn’t a very big problem. You’ll quite often see 16V rated capacitors running with just a few volts across them. Or 63V rated ones running at 25V. But I’d be a bit more concerned about the long-term stability of 350V rated ones running at 25V.
VB
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Add a resistor before the choke if the choke dcr is less than the existing resistor, unless you want the ht voltage to increase
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I have experienced a few of the old ones doing just that
As for replacements, probably stick with PIO if you do change them, a move to plastic is likely to change the character somewhat
But beware a lot of the modern foo caps are quite a size, and may not fit within the restricted confines of the case
Are there any PIO caps you’d recommend? As you’ve said, the right values in most of the ranges I’ve looked at are mostly too big (and also pricey).
I fear it’s a bit like modern valve manufacture. Back in the day PIO was a very standard technology. Great big corporations spent decades working out how to make them both compact and reliable. They were turning them out in large numbers and could therefore afford the quality control needed to maintain high production standards. Now they’re made in much, much smaller numbers by much smaller businesses and the expertise is hard to retain. If you make the paper nice and thick then they won’t go electrically leaky. But they will be huge.
VB
I can see some ingenuity may be needed; there’s only a limited amount of space above and below the steel plate in the ´shallow’ part of the case, with heater circuit smoothing caps above and coupling caps below. If my new heater circuit caps are a little smaller than the originals I may be able to steal a few mms by spacing the steel plate upwards, and the bottom cover plate could likewise move down a bit (bigger feet will help there).
In any event, capacitor case size is going to be key.