Calling Central Scotland valve guru's..

[Gazjam]

Looking to upgrade my valve amp.
Been measured by someone in the know and it measured well, true 240v supply. Bias is fixed manually with a trim pot.

Have a list of components Im going to get for it, but my main question is about valve choice.

Its a KT88 push-pull amp, and I would like to know if I can fit KT120's in there?
I know its not a case of a straight swap, need to make sure amp can take them.
If not, a fresh set of Gold Lion KT88's.

Possibly relevant forum commment here?
http://www.audiokarma.org/forums/showthread.php?t=502990
"Transformer won't saturate. In common push-pull circuit current from each tube compensates another, so there won't be constant magnetic field other than from difference between tubes. But still all that current will flow through windings. Here is the problem - if manufacturer decided to save copper, transformer may overheat due to high resistance and thus dissipated power."

and here...

http://www.stereo.net.au/forums/index.php?/topic/34533-kt120-valves-in-a-kt90-se-amp/
"If you can find out the diameter of the winding wire used for the heater wind(s) I can figure out the maximum current available. Also if it has separate windings for the small signal tubes and output tubes?
Also will need to know the HT supply voltage and max current available.
From this I can figure out the bias current needed for best performance. I'm assuming the amplifier is fixed bias, meaning you manually set the bias."

Is there anyone central Scotland(ish) who can measure the amp and can confirm if the transformer can handle the extra heater current of the KT120's?

Ali?
Summit you can do?

thanks.
gaz.

[Gazjam]

Anyone? :scratch:

[Effem]

You could ask Guy at Puresound for his advice what to do Gary

He isn't Scottish of course :lol:

[Ali Tait]

Sorry mate, missed this first time around. Not sure how you would do this except by testing the mains tranny to destruction!

TBH, I'd stick with 88's. There's not really any easy way to measure what the max current output of the mains transformer is. Making a guess based on the gauge of the wire isn't really going to cut it. One way would be to remove the trannie, and send it away to have it copied, stipulating what the max current delivery you require is.

If you were confident of an honest answer, you could try contacting the amp manufacturer?

[Ali Tait]

You could ask Guy at Puresound for his advice what to do Gary

He isn't Scottish of course :lol:

No one's perfect.. :-)

[Gazjam]

Cheers Ali,
knew it wouldn't be straightforward!

Given the amp weighs the best part of 5 stone and it measured well, could that suggest its well put together and the ironwork is strong (enough to take extra heater current?)
Know it might be a case of try it and see.
Nowt wrong with the KT88's, sound great love 'em, but changing valves COULD be a significant upgrade and could drive the Edingdales even better?
Got that itch!

Bit of a risk mind, thinking if I'm getting a new set of valves at some point might just bite the bullet and get the KT120's.
If the amp doesn't sit well with them can sell them on and revert to the 88's.

That's my thinking anyway, bit of a risk but the amp wont go bang...should know if the higher rated valves are too much before any damage done?

Emailed the manufacturer a couple of times but no response unfortunately.

[John]

Speak to Anthony he has upgraded a few amps to use KT120 and 150s

[Gazjam]

Thanks John, forgot about Anthony.

[Gazjam]

Was just hoping there was someone nearer home who cold do the testing so I didn't have to post my amp off...
That said, wonder how my wee T-Amp sounds hooked up to the Eddingdale's? :D

[Ali Tait]

And?

[Naughty Nigel]

You would need to establish the mains transformer's internal impedance. That would allow you to calculate theoretical power dissipation in the transformer if given a higher HT load.

Don't forget that you would probably need to replace the main smoothing capacitors with others of sufficient ripple current rating. The rectifier(s) might also need to be replaced.

Supplying additional heater current is not a problem as you could easily add a second transformer with a 6.3 volt secondary winding. If you were to offload the LT current on to a second transformer that would increase the safe HT current available by a similar wattage.

However, I would be more concerned about the output transformer, which could saturate. You might also need to think about cooling as your proposed system would dissipate a lot of heat!

All in all I would stick with what you have. Second guessing the designers parameters is never easy, and there is a very real danger that you could destroy some perfectly good components in the process.

Nigel.