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Old 28th October 2012, 04:14 PM   #1
Minx is offline Minx  United Kingdom
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Default If it ain't broke....

I'm a valve newbie and have a really basic understanding of the physics involved, although I am learning a lot from reading posts on this excellent forum. So I apologise, in advance, for the crassness and ignorance of my question, which is:

I have a Separo P88i UL amp with 4 KT88s. I was interested in trying different valves so bought both Electro Harmonix ("EH") and Genelec Gold
Lions ("GL") as upgrades. I bought matched quads and as the amp allows biasing by way of adjusters on the side, I rebiased the valves after replacing each brand of valve.

However, after listening using the same CD references (Holly Cole's Temptation and Mitsuko Uchida's Schubert) on my Eastsound CD player, I find that the GLs
and the EHs sound rather harsh and I think I find the amp fatiguing now after an hour or so. Also, I now have a hum on the left hand channel, which wasn't there before I started messing. Is there a period of burning in which will make the GLs and the EHs sound better?

I've read that some valve amps sound pretty bad until they've warmed up. Trouble is, I now wish I hadn't experimented and can't help concluding that I have altered things from sounding, to my ears, pretty good, to a harsher sound with a hum. As a dumb newbie, I also forgot to note what settings the supplied kt88s were set at so going back will also be trial and error.

Reviewing this rambling message reveals that it less of a question and more of statement of my stupidity giving a lesson. Don't mess if you're happy and don't be tempted to try valve rolling unless you know what you're doing and can put it right if you're not happy!
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Old 28th October 2012, 04:27 PM   #2
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I don't have any experience with valve amps, but if the bias can be manually set to allow for drifting valve specs, or the replacement of deteriorated valves, then surely the manufacturer should have provided you with the correct bias along with the amp?

One thing that I can think of with respect to the hum is that a higher bias setting will draw more current and if not choke filtered, or due to bad component placement/layout, this will increase the supply ripple and could very well be coupling into a sensitive area of the circuit. You could try turning down the bias and see if it helps.
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Old 28th October 2012, 04:56 PM   #3
Minx is offline Minx  United Kingdom
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@5th element-thanks for your helpful comments. The amp manufacturer gives a maximum voltage only not to be exceeded. It suggests that by varying the bias you can subjectively alter the sound. I will do as you suggest and reduce the bias to eliminate the hum.

Thanks.
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Old 28th October 2012, 04:57 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Hi,

What idle current are you running the tubes at?

If you don't know..the its the voltage across the cathode resistors divided by the value (in ohms) of the cathode resistor..
Check the value of the cathode resistor and divide the max voltage they say..lets see what current they say is the max value..

I would run at 50ma..thats 0.05 A and see how it sounds... so as an example....if you have a 10 ohm cathode resistor on each power tube then to get 50mA..you would have 0.5v across it.

So as a starting point look at the max set voltage value they quote find the value of the cathode resistor and work out the max curent they say you can run..compare this to the 50ma...if the 50ma is in the range try it.

Regards
M. Gregg
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Old 28th October 2012, 05:09 PM   #5
Minx is offline Minx  United Kingdom
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@M Gregg- will let you know when I can. The boss is listening at the moment but she'll be stopping at 7:20 to watch Strictly Come Dancing. May get a chance then; otherwise will let you know when I can. Thanks for your interest.
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Old 28th October 2012, 05:15 PM   #6
M Gregg is offline M Gregg  United Kingdom
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Its simple ohms law..

Remember there are high voltages in the amp, and it might not have discharge resistors so it could hold power after switch off..take care!
It can kill.. also dont exceed the max value they quote...

I don't want to worry you, however if an amp hums then the bias could be to high....watch out for OP Tx damage..also..never power the amp with no speakers connected or a dummy load..you could damage the OP Tx..

I'm sure someone will follow up with you...

hope this helps.

Regards
M. Gregg
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Old 28th October 2012, 05:57 PM   #7
rmyauck is online now rmyauck  Canada
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Try running it at the lowest bias settings it sounds good at for maximum tube life.

The new tubes may change as breaking in so keep checking bias, and also sonics may improve. I suspect it will sound better at lower settiings anyway.

Randy
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