Kit 6550

[Paul Stoneman]

Good evening gentlemen,
Can any one help please, I have a KIT 6550 recently changed from a KAT 6550 with the aid of help from the forum. The amp has developed a hum and when tapped on the side tends to get very load, it is volume dependent and on both channels, I have rebuilt the power supply apart from the heater rectifier and resoldered all the connections. It sound like a bad earth but could be a component failing. Does anyone have any more ideas please.

[Richard]

Hi Paul, to clarify,

1) was it a power amp only before and is it an integrated with volume control and input selector now?

2) if so are you using the relay board or the original type rotary selector?

3) at what stage did the hum start, after the conversion, or was it fine for a while then started out of the blue etc?

4) is it on both channels, on all inputs, have you replaced or moved valves across channels, any other investigations so far?

[Paul Stoneman]

Hi Richard,
Many thanks for your rely.
It was a KIT before now a KAT, the one input is wired straight to the input valves there is no selector and no volume control. (separate pre basic pre amp)
The hum which is on both channels and gets louder with more volume, started out of the blue about 300 hours after conversion. When you tap the side of the case the hum either increases or decreases as if some thing was loose. I have swapped all valves with different sets in rotation from good spares and the problem is the same. It started a few days ago and when the hum started I tapped the side or top and it went, the day before yesterday I tapped the side when the hum appeared and it got very load at listening levels, no crackling or poping just sounds like mains hum.

I have re soldered the connections and checked for any damage or burning and all looks in order.

Thank you for any advise.

Kind regards Paul.

[John Caswell]

Hi Paul,
Were the input phono sockets the "old" variety with a red fibre washer each side of the chassis (which were prone to working loose) or the later types with a plastic washer each side of the chassis?
If the earlier types it would be worth replacing with the later types.
It does sound (sorry) like a earth lift problem.

John

[Paul Stoneman]

Good evening John,
Thank you for your reply.
The inputs are the newer plastic type. I have ordered the bits to rebuild the power supply for the ECF80 heaters, the rest of the power supply was done a few months ago, one of the 4700uf caps that was used was a bit squashed, probably done in the post so hopefully that is the problem. if I find what the problem is, sorry when, never give up, never surrender unless it is your best interests, I will put the info on the forum, it may help someone else.
Kind regards Paul.

[bob orbell]

Hi Paul, you say in your last post, (one of the 4700uF caps was squashed ) there is only one, or have you added another in a Pi filter arrangement, no problem if you have, myself, if I was changing C6, I would use a 16volt capacitor rather than the 10volt and maybe go to 10,000uF. keep us posted. BOB

[bikerhifinut]

I'd like to add to bobs observation:
If you have used the Schottky diode bridge rectifier in the ECF80 supply, then you most certainly will need a pi filter arrangement as you'll need to drop the heater volts a bit due to the lower Vf of the Schottkies. Might as well stick another cap after the resistor and get a smoother DC feed. I had to do this on the KEL84 I got as the ECF80 heaters were running about a volt high and long term it wasn't going to do the ECF80's much good. In fact I did have a couple that went noisy on me before I did the mod. I aint saying that's what caused it but the replacements didn't go noisy.

Andy

[Paul Stoneman]

Hi Bob and Andy,
Thank you for your help, I have only one 4700uf in the heater supply but was looking to change to Shottkys and Pi filter, hence my error in comment.
I have replaced the slightly squashed cap and it seems that that was the problem. I thought that caps blew if they had problems. Thankfully we live and learn.
Have a good week end everyone.

[Paul Stoneman]

Hi Guys,

Well after five hours on Sunday changing to Shottkey diodes and a Pi filter I was getting 9 .5 volts on the heaters without the ECF80's in. I tried three meters, at least I know the cheap meter is fit for the bin. I replaced the original rectifier and with cap in circuit it was still over 9 volts
I also replaced some of the thin wire with some Neotech with more to go.
The bad news it the hum is still there and is intermittent from quite low through the speakers to very load, it can just start getting loud and then goes quiet. There is no hum from the transformers just through the speaker which are approx 92db efficient.
The good news is the more I play the more I learn.

[bikerhifinut]

9.5V off load is quite possible, you should measure the volts with the 2 ECF80 in circuit. Then if the volts are still a bit high it's a simple ohms law calculation for the pi filter resistor.
I don't know why you felt you should put the original rectifiers back?
Andy.