KiT6550 5ode/3ode voltages

[Matthew Snell]

If anyone can help me on this....?

Recently completed my amp. Checked and checked again, fired up and (apart from a few blown fuses - it doesn't like being switched on hot) it sounds amazing - everything works - but......

Voltages on the the ECF80s are significantly at variance to those advised by WAD. They are as follows:

aT - 180V
g1P - 0V
g2P - 80V
aP - 145(R) / 165 (L)
c3, g3P - 1.6V
cT - 5.3V

All other voltages - everywhere - are within tolerance.

Can anyone advise me a) whether the amp is OK to use or will damage any components (or catch fire) or b) which components might be damaged or off spec. to cause this problem (WAD clearly had a few issues with typos on circuit diagrams/instruction manuals and they certainly mis-supplied several components with the kit)

enormously grateful

Matthew

[John Caswell]

Hi Matthew,
These voltages are all in line with the variance between ECF80s. I am sure you will find that if you swap over the ECF80s the voltage differences will follow the valves. You will probably also find that if you substitute the 2 ECF80s for different ones the volts will also be at variance, although of course now I have said that you will change a valve and the volts will be near right (Sods law!) Frankly I don't think you need to worry
However for peace of mind and safety's sake I suggest that you check the wiring and component values around the ECF80s, particularly R15/16, R11/12, R17/18. Look for dry joints and/or wrong component values.

Regards

John aka Dr John

[Mike G]

Hi Matthew,

The fuse blowing problem is also common, and I'm sure a number of people on this BB have had experience of this. Some have slightly increased the fuse size, but this is not best practice, and can compromise safety.

Dr John has done some work on this problem I believe, and sugested a current limiting device that reduces the inrush current at turn on. I carried out that mod on my KAT6550, and it works fine!

Maybe as it's quite a common problem, it could be posted on this BB with the Mods and tweaks also pasted in from the old BB?

I also suffered with a mechanical clonk during turn on, so a couple of other things you may wish to consider.

- Check your transformer fixing bolts after a week or so, these do work quite loose.

- Also the base plate of my amp rattled on turn on. (due to inrush and loose transformers) As the fixing screws are at the corners, extra centre screws have now been fitted. Rattles, clonks, and blown fuses are now a thing of the past!!

I'm sure that both these topics are still on the old WAD BB, so might be worth a trawl through.



Mike.

[John Caswell]

Hi all,
Oops! Sorry missed replying to the fuse blowing problem.
Yes had a few problems with people on high mains especially so try these in the live mains input, all from Farnell:-
606-777...12R>0R22
606-765...7R>0R15
606-753... 10R>0R2
As you can see they all high cold R Low hot R.

This permanently cured a friends 6550 amp which often had 253Vac on it in the evening.
Simplest way is to mount it on the switched IEC if you have one, from the fuse output tag to the mains switch input tag, with the wires wrapped around the tags if possible to mechanically hold it in place.

John aka Dr John

[Richard]

Hi Matthew

Switching on and off will sometimes catch the mains on it's AC peak and give a big inrush. You may have found this when switching with the amp hot as the rect would have been heated and so conducted the HT almost immediately, rather than after heating slowly from cold.

The mains fuse is really there to protect the supply, ie the mains lead up to the amp, not specifically the equipment. So long as the fuse blows before the mains lead catches fire it's done it's job. Trying to make the fuse do more than this, ie protecting part of the amp, is a different issue and may or may not be successful. ( I use a 5A T fuse but that's my personal choice. )

If you fancy taming the inrush Maplin may be easier to buy if you just want one,

http://www.maplin.co.uk/Module.aspx?...ce=14&doy=28m2

N99BT looks OK to me but I've no exp of using them, maybe John or others will have a look,

http://www.maplin.co.uk/Media/PDFs/n99bt.pdf

Looks like they can run very hot so I'd keep the leads long and mount in free air.

Rich

[Matthew Snell]

Dear Dr John

Further to my WAD queries re KiT6550 pentode/triode voltages:

Thanks eversomuch for your ideas.

I’ve checked everything again and it all looks OK. Given the symmetry and portability of the iffy voltages (you were right - the voltages do follow the valves) , my reckoning is that it’s most unlikely I’ve made the same mistakes on both sides of the amp and equally unlikely that there are any damaged components. This leads me to think that either WAD have got their specs wrong (unlikely I think because I can’t see any other references to this either on the WAD or the WD BBs) or that you were right in your original reply to me in that I shouldn’t worry too much!

But I’m still a bit concerned - particularly about the aT voltage which at 180V is 45% up on the WAD "instructions" of 125V +/- 15%. Would it be a good idea to replace the ECF80s? (and if so is any manufacturer better than any other) Or is 180V perfectly acceptable (and safe)? I don’t understand why, if these voltages are ok, the WAD instructions don’t give a wider acceptable margin of error............

Re the fuses issue - I wasn’t particularly bothered about this but your suggestion of a current limiter sounds good and I would like to pop one in. You suggested 606-777; 606-765 and 606-753 from Farnell and Richard suggested a N99BT from Maplin. Is any of these as likely to do the trick for me as any other (I already have an account at Maplins so that one would be easiest) or would you point me at one in particular.

Many many thanks

Matthew

[John Caswell]

Hi Matthew
If you are absolutely sure that all the components are the correct values and are wired correctly the trying different ECF80s is the next step.
Have a look for some E80CFs which are the SQ versions and are much more likely to be closer to spec, and see how it goes. Here Mullard are probably the only maker. Do not pay through the nose for them about £4-7 is reasonable
ECF80/PCF80s in my TV servicing days were pretty notorious valves and often wouldn't work at all in some TV tuners, or would throw the tuning so far off that you hadn't a snowballs chance of retuning to a specific channel.

The Maplin surge protectors are rated at too high a current for this application so probably will not get hot enough to drop in value, so I suggest the Farnell types. If you pm me with an address and I will drop one in the post to you then you can report back on the BB

Regards

John aka Dr John

[Richard]

The Maplin surge protectors are rated at too high a current for this application so probably will not get hot enough to drop in value, so I suggest the Farnell types.

John

They are suggested for audio power supply use but only that 1 of the 4 Maplin ones looks suitable.

At 1A steady running from it's starting resistance of 10R @ 25C it will have to dissipate 10W so it will get very hot and will soon drop it's resistance to meet a Temp/R equilibrium it can handle

Now, whether that ends up being a lower or higher R than a smaller current device we don't know as there isn't a graph.....ho hum

Rich

[Matthew Snell]

Bingo!

I've swapped the Chelmer ECF80s for a couple of Mullard E80CFs and suddenly and miraculously all my voltages have fallen into line. Thank you all I'm now a happy chappie!

Further question (mostly curiosity): Voltages, while now comfortably within WAD stated tolerances, are still a bit asymmetric. Would buying a few more valves (they're cheap as chips) and finding the best "matched" pair give me a better performing amp or would it be a complete waste of time and money?

Thanks everyone

Matthew

[John Caswell]

Hi Matthew,
Don't get paranoid about voltage accuracy ±15% is generally fine. In my opinion it would be a complete waste of time and money to buy more ECF80s, you are starting to chase the "Holy Grail" but if you have nothing better to do ........
If you decide that you are going to go this route have a look for some Mullard/Philips E80CFs as they are the Special Quality types selected for Military/Instrumentation purposes and are therefore closer specced.

Regards

John aka Dr John