Hi all,
TL/DR - on a recent thread here, someone suggested that it would be better to have a higher capacitance at each step of the voltage multiplier of a quad 57 EHT block - 20nf, 47nf etc. Is this worth trying out before I go and pot the board again? 35,000 quad speakers can hardly be wrong with 10nf?
Longer version:
So I have just started on a long overdue recondition of my sets of 57s - having completed work on some 63s last year. I need to redo the EHT blocks on one set of 57s as the EHT voltage looks low (using one of those 1000m HV probes). Much like another poster here, I'm getting lower than you'd maybe expect at the 6kv tab, but its likely the probe pulling it down:
From rebuilt EHT unit with panels connected (1n4007 diodes/10nf 2kV caps replaced, 330k resistor was measuring high at 450k, so it was replaced, 2.2m measured fine)
AC after transformer - 620V
AC after voltage divider - 570V
DC from 1.5kV tab - 1.3kV
DC from 6kV tab - 4.5kV
In particular the 6kV measurement was a good bit lower before the rebuild - 3.2kV measured with the same probe.
Anyway, like another poster here, I think the HV probe is probably dragging down the 6kV measurement, and this is also suggested on the older quadesl site.
fran
TL/DR - on a recent thread here, someone suggested that it would be better to have a higher capacitance at each step of the voltage multiplier of a quad 57 EHT block - 20nf, 47nf etc. Is this worth trying out before I go and pot the board again? 35,000 quad speakers can hardly be wrong with 10nf?
Longer version:
So I have just started on a long overdue recondition of my sets of 57s - having completed work on some 63s last year. I need to redo the EHT blocks on one set of 57s as the EHT voltage looks low (using one of those 1000m HV probes). Much like another poster here, I'm getting lower than you'd maybe expect at the 6kv tab, but its likely the probe pulling it down:
From rebuilt EHT unit with panels connected (1n4007 diodes/10nf 2kV caps replaced, 330k resistor was measuring high at 450k, so it was replaced, 2.2m measured fine)
AC after transformer - 620V
AC after voltage divider - 570V
DC from 1.5kV tab - 1.3kV
DC from 6kV tab - 4.5kV
In particular the 6kV measurement was a good bit lower before the rebuild - 3.2kV measured with the same probe.
Anyway, like another poster here, I think the HV probe is probably dragging down the 6kV measurement, and this is also suggested on the older quadesl site.
fran
As I understand the higher is the capacitance in the voltage multiplier the lower is its internal impedance. There is always a natural leak due to humidity in the air, etc. that pulls down the HV if the internal impedance is too high. It should not be too low either, to keep constant charge condition. The 10nF is a good compromise, I believe.
Thanks. I thought about it some more, saw that it had never been changed by Quad all through the years etc. Anyway long story short, the wax is now cooling in the remade EHT blocks.
So here is another question. I can see a tiny bit of corrosion on some of the rivets on one of the bass panels. Its not on the stators, its just a little powdery deposit on the rivets themselves. There is no sign of anything else wrong, apart from the dust covers looking a bit tatty. There was a previous packing tape repair on one, but they are all functional. Is it worth opening them up to clean that little bit of powdery material on the rivets?
The general consensus is to do as little as possible to them.... before doing the EHT blocks I had them powered, and they were a bit dull sounding but working. No odd noises etc.
So here is another question. I can see a tiny bit of corrosion on some of the rivets on one of the bass panels. Its not on the stators, its just a little powdery deposit on the rivets themselves. There is no sign of anything else wrong, apart from the dust covers looking a bit tatty. There was a previous packing tape repair on one, but they are all functional. Is it worth opening them up to clean that little bit of powdery material on the rivets?
The general consensus is to do as little as possible to them.... before doing the EHT blocks I had them powered, and they were a bit dull sounding but working. No odd noises etc.
I would leave the corroded rivets as-is. You could do more damage from dust or filings.
Larger value capacitors of the same brand/type may have more leakage, so check the specs first.
Good luck.
Larger value capacitors of the same brand/type may have more leakage, so check the specs first.
Good luck.
@rayma - yes, I think I will obey all the advice seen around - Rob in ER audio said the same thing last time I was talking to him. Basically don't do anything unless you absolutely have to.
Anyway, I decided to keep with the 10nF caps, and both EHT boards are potted in wax now. I've just powered them on about 30 mins ago and the tonal balance seems right, but I'll wait for a day and then assess how things are.
These are a very early set, 1959 from the serial numbers, but I don't know if they have ever seen any maintenance before. There was a mountain of dust in there anyway and the front dust covers are painted that silvery Quad amp colour. My later set have a light blackish coloured coating (1975).
Anyway, I decided to keep with the 10nF caps, and both EHT boards are potted in wax now. I've just powered them on about 30 mins ago and the tonal balance seems right, but I'll wait for a day and then assess how things are.
These are a very early set, 1959 from the serial numbers, but I don't know if they have ever seen any maintenance before. There was a mountain of dust in there anyway and the front dust covers are painted that silvery Quad amp colour. My later set have a light blackish coloured coating (1975).
Yes, I've been lucky to pick up ESL57s in good nick. Once I get them tidied up, the plan is to slot them back into the stacked frames.
Thanks to all here for the generous sharing of info and experience.....
Thanks to all here for the generous sharing of info and experience.....