wiring

Willie, hope the recovery is going as well as you wish.

It is possible that the valves will change sound after some period of operation, it is also possible that the bias may need re-adjusting.  

Not all amps have adjustable bias and yours may not, however, if it does, it is fairly normal to adjust it when fitting new valves and then to re-adjust it after some period of running.

It might be worth checking this first.

KT88 and 6550 although generally  interchangeable, do have different ratings and performance. The Winged "C" have a good reputation with many, but so do the Gold Lion.  

Whether the change you are hearing is due to the change of valve or something else is difficult to say.

If you still have the 6550 that were changed out and they are still serviceable, it might be worth refitting them to see if that alleviates the problem.

regards,

Tony G

Continuing from the above...

The amount of bias/anode current/conduction depends on how negative the grid voltage is WRT the cathode voltage.

Increasing the value of cathode resistor makes the cathode more positive and hence the grid more negative and should reduce the conduction - reduce the quiescent current.

The KT88 is possibly 'turned on too hard' and that may have the effect of moving the tonal balance toward the bass. It does much the same with transistors.

Also watch out for 'motor boating' which is due to the transformer phase shift at low frequency. If that's experienced then the cathode resistor bypass capacitor could be reduced in value (by say half).

As valves are inverters just the same as transistors, then as the grid voltage goes positive the anode goes negative. As mentioned the mutual conductance of the KT88 is higher than the 6550 so the change of voltage on the grid will result in more anode current flow, and from this you'll realise that the KT88 has higher voltage gain.

I'm not an expert in valve amp tweaking but from what I read on the net I can see many others aren't. The difference being is that many think they are.

hgvmech wrote: dear all iv,e been away recuperating ,still feel like crap [but still alive] i asked a question about valve amp. it is now away getting repaired ,seemingly 1 valve gave up the ghost and damaged some parts inside.i sent kt88 valves back to supplier as they are still under warranty.i tested cusat cables for 2 weeks and they were quite a revelation haha.now has anybody any thoughts on bi-wiring .i,m terrible for fiddling with things and would like to know if there is any benefit in doing it. many thanks take care hgvmech willie

Sticking my head into the lions mouth........I've tried bi-wiring and didn't hear a difference