KiT 6550 Hum

[Richard]

Hi Tim,

The voltages are also shown on the circuit drawing. All you really need to do is,

1) correctly identify the valve pin numbers (maybe mark pin 1 with a magic marker or tipex so you can count around from there) and then

2) check the voltages at the numbered pins following that given on the circuit.

The majority are checked wrt (with respect to) signal ground/earth. So clip your meter neg lead to signal ground on the tags shown as Star Earth, then use just one hand with the other meter probe to check the voltage at the pin you are testing. You should be looking for mostly DC volts in a suitable range on your meter.

Exceptions are the heater voltages, put the probes on heater pins 4 and 5 of the ECF80 looking for 6.3V DC and pins 2 and 7 of the 6550s looking for 6.3V AC.

[Greg.]

Further, just in case you’ve not seen it, on the circuit diagrams pages, below the schematics, are diagrams of the valve base pin layout viewed from the underside, so as you view them when checking voltages inside the chassis.

[TimN]

Ah yes, all becomes clearer now. Thanks very much.

[TimN]

Can anybody identify whether this is a 4 or 8 ohm output transformer in my 6550? If it is a 4 ohm as I suspect, does it have any impact on the speakers I use with it?

[John Caswell]

Hi Tim,
It is a 4ohm as stated on the label. I doubt that you will notice any difference driving 4/6/8 ohm loudspeakers

John

[bob orbell]

Greg wrote - I cannot find anything in the archives that suggest C5 should be reduced to 85uF. If you can evidence this, I’d be grateful for a copy.
Sorry Greg, I can not find what you were asking for, I said 85uF, but meant 82uF, I will post if I find it. Or am I getting confused with the KIT and KAT 34 ? Bob

[TimN]

So i've just switched on again briefly after replacing the mains caps C1-5 and all valves are glowing but the hum is still there although it is a lot quieter. The hum seemed to be coming from the area around the choke which is now rubber mounted and the 5U4G rectifier valves although I switched off again after a short period. Could these be the source of the hum? Could changing them out cure it? Is there any difference between a 5U4G and a 5U4GB? Do they need to be matched/balanced?

[John Caswell]

I am going to guess that this is mechanical hum coming from the choke/transformer area. There will always be a very small amount of hum from the power tx due to a physical phenomenon called magnetostrsiction. But you often get lamination buzz in both choke and power tx. Try tightening the power tx bolts - I cannot remember if the 6550 tx uses 2 or 4 lamination bolts., if it is 2 then 4 may be a better bet.
Try removing the choke from the chassis and run it externally with long leads to eliminate choke or transformer buzz
There is no difference between a 5U4G and a 5U4GB, although some "golden ears" swear they can hear a difference, and no you do not need matched/balanced 5u4s. Once again I refer you to the "golden ear" brigade.
The 6D22S mod that I developed may help with the hum/buzz as it does reduce the amount of heater current needed.

John

[TimN]

Thanks John. Sound advice.

[Richard]

Hi Tim,
Although they may not be the source of your noise I would fit balancing resistors to the rectifiers as per the FAQ
http://www.world-designs.co.uk/forum...ead.php?t=3859