Valve life ?

C

Chris Davies

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Hi Guys me again

My wad kat6550 is making a scratching sound on the right channel when it does so one of the kt88 valves flashes blue so i'm guessing its valve degradation. These are new from hot rox about 8 months ago i use them most nights for about 4-5 hours mostly at about 70% volume.

Worn valves ? or possible an amp problem ?

Thanks for any help

Chris
 
Let us start with something simple before you go crazy and start spending lots of money. Plug out the valves (and the amp from the mains :D!), clean the sockets and put the valves back into place again. Eventually swap places (right/left), and don't be afraid of pressing them down a bit. Sometimes they get loose, kinda like a normal lamp bulb...

Don't know your particular model, whether it's a fixed bias or autobias circuit. If not, check bias with a voltimeter for peace of mind as you swap valves. Identify socket and valve, and check whether the problem appears again and where.

If there's something wrong with a particular valve, you'll realise by swapping them. If not, it might be something down the chain. Is your amp hardwired? That'd make repairs much easier -and hopefully cheaper too.

Good luck ;)!

Regards,

Vincula
 
Hi Vincula

Thanks for the help. As far as i know its a self bias type amp this is what i was told at purchase. Hard wired, I have chased the valve around and it swaps channels following the valve. Oddly though this is the second one to go in as many weeks. Having said that i purchased them all at the same time.

I have dropped a spare Chinese one in for now (YUK :-( )

Working ok on both channels so i guess its that.

Chris
 
4-5 hours every day is quite heavy use and this amp does run the tubes quite hard (40wpc all Class A).

You can drop the plate current by changing out just four resistors to reduce the max output (class A) but that depends on the speaker load, sensitivity and how loudly you listen.

I used to run my 6550 with max output reduced to about 28-30w which gives the output tubes an easier ride and was plenty of output for the Impulse H2 horns I used at the time.
There used to be plenty over on the WD forum about this so well worth checking.
If not, ask Dr John for a suitable resistor value - he's the WAD & WD guru.
 
Hi Rob

Thanks for that i will go have a ganders. I guess in the mean time i should order some sovtek kt88s apparently abit hardier

Chris
 
Hi Chris,

I liked Stevlana Winged 'C' best in that amp but they cost a little more than the Sovteks.

You can use KT90s too - those are effectively a beefed-up KT88.
 
Hmm interesting

thank you google ahoy

Think i might try the Winged C matched quad from hotrox

Buying a backup set of kt90s
 
Last edited by a moderator:
Just found my notes on this.

Some versions of this amp came as standard with the cathode resistors R35/36/37/38 set at 560R which drives the tubes flat out.
Better to use 680R or even 750R for long life.

You want 7w resistors if you change them.

Remember the stories of the old Quad II running for many years on a set of GEC KT66 tubes?
That's because they were given an easy life delivering 12w from a tube capable of twice that.
 
scrathcy sound is usually dirty valve pins or valve bases ...give them a good clean ...

I've used brasso wadding and contact

as long as you remove any residue and polish up the pins things might return to normal ...but it may be there reaching the end of there life too !
 
Chris Davies said:
Thanks Zanash i will give it a try nothing to loose
Click to expand...

And a lot to be gained ;)! Give this a try:

http://www.audiolincs.co.uk/wtsnew.php?cat_id=4&prd_id=52

I've got a Qt. of EL34 Winged C, and a Qt. of Sovtek Reflector as back-up (the ones my Audio Innovations 500 came with). No issues so far -and the AI500 is not known for its reliabily, so I cross my fingers :rolleyes:. Smooth and detailed sound without any agressive tendencies. Suit my tastes well.

Regards,

Vincula
 
Chris Davies said:
Hi Guys me again

My wad kat6550 is making a scratching sound on the right channel when it does so one of the kt88 valves flashes blue so i'm guessing its valve degradation. These are new from hot rox about 8 months ago i use them most nights for about 4-5 hours mostly at about 70% volume.

Worn valves ? or possible an amp problem ?

Thanks for any help

Chris
Click to expand...

70% volume? I have never had any pre set for more than 50%
of it's maximum turn on the volume pot. Unless I am misunderstanding you, the must be something wrong if you are having to push the system so hard.

A good KT88 will last for 1000-2000 hours or more if not abused.
 
at 7 on the volume dial too!!, mind you I've got KT66s with at
least 4000 hours on them.

And my curreny Winged Svet KT88s have 2000hrs on them with no
sign of anything going wrong. Mind you my volume is set between
2 and 4.
 
kt66 said:
at 7 on the volume dial too!!, mind you I've got KT66s with at
least 4000 hours on them.

And my curreny Winged Svet KT88s have 2000hrs on them with no
sign of anything going wrong. Mind you my volume is set between
2 and 4.
Click to expand...

if the amp is heavily biased in to class A it will still eat output valves regardless of volume.
Chris has a WAD amp that runs to full output (around 40w) in Class A so a new set of tubes annually is not unreasonable.
Reducing the Class A working will lengthen tube life with no adverse effect if the reduced output in still sufficient.
 
Yeah Kt66 I do like the volume way up :-)

I will try that Rob thank you. Enjoying the vinyl i keep buying to much to dive in and start soldering yet. (i have a used record store not 40 yards away) I I started off 2 weeks ago with no vinyl i have about 200 now and building by the day..

Having said that my cartridge still isn't set up correctly as it skips at anything under 2 grams. I'm awaiting alignment tools from thebay.

Chris

I guess im lucky in that my brother is my neighbor and he like my music :-)

Oh and i don't have a TV
 
Hi Rob - A project Experience 2 comfort in olive - Standard Carbon Arm 9cc i think. and as we know the DL160 cart
 
More importantly what amp and speakers are you using.?

No amp I've ever known is meant to be pushed that hard for so long. You must need either more powerful amp or more sensitive speakers.
 

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