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Very far from ideal. And poor design.

For me it would be extremely annoying, as one input I do leave running is often my tuner. Thus if listening to another source, the quiet passages would be audibly injected with tuner material.

Obviously being fastidious in turning them off will largely cure ...

I have been in extensive correspondence with Chris since coming across this issue, and he is most engaging and supportive, and is looking into enhancing his testing. He, and my supplier, have tested others in their stock and not found further errors.

It would seem that the relatively large values ...

They're neither 'small' nor 'glass'!

They're ceramic, industrial, high voltage (600V) High Breaking Capacity (10,000A) fuses.

The MV-55 was specified with Bussmann BBS-3/4 (i.e. 750mA fast blow).

An alternative is the Littelfuse BLS.750 - same rating and behaviour.

They're both 0.406" dia. by 1.375" long.

Looking at the Prem 12 schematic, my immediate choices for caps to upgrade would be:
1. The 0µ15 600V in each grid of the 6550s
2. The 1µ 400V with 0µ15 400V bypass in the 6FQ7 grids to ground

These are arguably the 'most' audible. I would suggest Solen FEP (Tin/PTFE).

In my MV-55 I did the No.1 ...

Agree fully re matching, and note that this means both sections as well as both tubes. Remember you have no balance control...

I've had my PV-10AL for 20 years and it remains a fine-sounding preamp. The only 'bad' tubes have been brand new chinese tubes - I've had several failures, poor performers ...

Subjectively: those resistors are life-expired. Replace.

Objectively: you've done well measuring the temps. Assuming correct, the burnt pair (which you've confirmed are approximately the same value to the lesser damaged ones) are some 7˚C hotter, which assuming an ambient of about 20˚C is about 20 ...

Very odd. I am now able to, after temporarily pausing, then re-enabling, my VPN connection.

I wonder if the original VPN connection was possibly showing up as a 'blacklisted' one?

Anyway, sorry for the excursion, clearly an issue at my end.

If I click on a member name, I get routed to an error page.

"Not Acceptable

An appropriate representation of the requested resource /memberlist.php could not be found on this server."

My MV-55 sprang a hum, which was traced to a failed heater rectifier, or possibly a dry joint on one due to the heat.

I took the opportunity to do some replacements and upgrades:

1. Replaced heater rectifiers with Vishay SBYV28-100-E3/54 which are faster and softer recovery, plus lower dissipation ...

Let me post a separate thread on that.

(Sorry - bit late to this conversation but only just spotted it).

Having owned a MV-55 for nearly 30 years I can attest to it being a very sweet performer, and so much so that this month I chose to perform a few minor upgrades to assure its future.

in terms of what I have observed, accepting I am ...

Anyone?

I'd be really interested to learn of the circuitry, purpose and any other details.

I buy only best quality caps for all HV and instrumentation work and they are randomly labelled - they tumble down the production line and the print ends up where it ends up.

These days, with semiconductor circuits a bit less susceptible to pickup due to lower impedance maybe, a general lack of AC ...

Yes, you may well be right.

It does emphasise that as modders we need to get this right when we install them, else large sums of money may well end up making the equipment far worse.

The lesser Solen PPE FC polypropylene range do not purport to be marked for polarity, and my experience is that they are truly random.

The vastly expensive FEP Tin/Teflon ones were stated on my supplier's website as:

"S3Q series have the outer foil on the right of the marking. The Outer foil end ...

No contact required: (a) as I get close to the cap you see noise increase, and (b) it's insulated!

It's a metal can which acts as an aerial. Although harder to see you may be able to spot that the residual noise level is higher with the 'backwards' cap.

I thought this practical test might be informative, and hopefully persuasive for anyone who considers orientation unimportant.

I have knocked together a simple 12AU7 two-stage cathode-follower amplifier with about 20dB of gain, running at 200V plate.

First video shows the input capacitor (which ...

An alternative is with your 'scope.

See with red end to the 'scope sense connection (red croc clip) we have outer foil 'exposed' and it picks up the 50Hz in my case.

With red end to black croc clip ('scope gnd) we are now 'shielding' the red input and it picks up almost nothing.

I use a bespoke foil side tester - proprietary design to Mr. Carlson's Lab on Patreon.

This unit is a very sensitive amplifier with a front end that electronically switches the two capacitor connections between input and gnd in the sensing amplifier.

LED bar graph indicates the received 'noise ...