the unstable sound from tube pre-amp !

[up4]

hello everyone...
have anyone suffered unstable sound coming from the tube pre-amp whenever you turned on it ?

I know somebody might say it's "procedure problem", turn on the tube pre-amp and then the power-amp.

isn't there any other solution to get rid of the irritating unstable current sound ?

usually I have to wait 20~ 30 secs to turn on my power-amp...
otherwise....

the sound is like killing a poor chicken ...

 

[SHiFTY]

You might have some sort of oscillation? Hmmm...

Tubes do take a while to warm up (longer than I thought!)

Otherwise... arrange a relay delay switch?

 

[up4]

not really oscillation...
I tested it with oscilloscope... it's ok...
well...it at least take 10 sec to be stable....
I think I'll get a delay circuit .... since I always forgot turn on my tube pre-amp first... @@

You might have some sort of oscillation? Hmmm...

Tubes do take a while to warm up (longer than I thought!)

Otherwise... arrange a relay delay switch?

 

[Gabevee]

Actually.. it is an oscillation.. of the power supply. It may not show in the scope. Perhaps adding some filter to the B+? Or maybe a bad filter?

If that doesn't help then swap out the tubes (do this one first. Easier). I have heard tubes do that. Generally I hear it in the 1 -3 volt tubes in my battery radios.

My 2 cents,
Gabe

 

[phil]

Are you using a solid state power amp?
If so this warm up instability is normal. You may want to try fitting a timer delay until your valves have reached their operating temp.
It sounds like your diong the right thing by delaying the turn on of your power amp.
Phil.

 

[Gabevee]

Phil,

Why?

If the inpedance of the power amp is low, while the output impedance is high, this might make sense.

But output impedances of any good preamp should either be universal (low) or adjustable/switchable, if not two separate outputs.

Why?

Thanks,
Gabe

 

[phil]

Hi Gabe

I just presumed that this is what may be happening here.

By the way Gabe I allways look for your interesting posts.
Great web site too, learned a lot from it. Thanks, keep it up.

Phil.

 

[richt]

salute folks

i have a tube pre & power and i suffered a lot of wierd sound
and coloration.

i installed a variac in my system (power & pre).
my stand by mode is 60 v & play mode 117v.

tubes dosnt like to be turned on & off.
plates suffers from temperatue changes.

i am a lot happier with my system & sound more even every
time that i hear music. ( i dont have to suffer the 2 hr wamup)

i hope this info can help!

thanks
richt

 

[up4]

hi hi ...

sorry... but what is "variac" ???

variable capacitor ???

 

[richt]

hi

variac means variable ac voltage.
it is a big rehostat that you can control the voltage from
0 to 120 vac. (it is some kind of dimmer for lamp but wirewound
fot technical use on repair shops)

this is great when you use black gate caps (very epensive)
& expensive tubes on your amp and you apply 117 vac
slowly to make some sort of "soft start"


thanks
richt

 

[Circlotron]

Lots of variac pictures.

http://images.google.com/images?q=variac&num=30&hl=en&lr=lang_en&ie=UTF-8&safe=active&sa=N&tab=wi

 

[Gabevee]

More like a variable inductor. (VARIable reACtor. Or is it simply Vary AC?)

It is a variable voltage source for AC. You pretty much vary the AC line voltage up to full manually, slowly, so that old components that may be susceptible to sudden full current and voltage power ons won't blow up. Instead of being a variable resistor, thoguh it is a variable inductance.

Hope this helps

Hope its right!

Gabe

 

[up4]

totally understand !!!

thanks folks ~

I think it's good for building a new amp, right?

 

[Gabevee]

phil,

thanks for the kind words, BTW.

richt,

While I agree that the sound of a tube amp will change a little over the time it is on, until it stablilizes heatwise, if you are suffering from noises and coloration more than just a little, then I would suspect that the circuit is probably designed to squeeze too much from the tubes.

I can think of two things that would cause that. One is using a fraction of the current that the tube was designed to handle. For instance, I have a couple of amps that bias the tubes at about 0.25mA. For a 12AX7 or 6EU7 that isn't much of a big deal (2 mA max). But for a 12AT7, with a max current handling of about 10mA, that is a bit extreme. Yet the 12AT7 is a medium mu tube with a relatively low transconductance (heretofore known as Gm). So slight current changes won't affect its sonic character.

OTOH the 12AX7 and 6EU7 will be a little more affected by the changes in current due to their high mu.

Now, there are some preamps that use small signal audio pentodes. Pentodes have very high Gm. As I presume you know, but I will explain it here for those who may not, Gm is the change in plate current for the change in grid voltage. So a pentode stage designed for high gain (can be up to 1000 per tube stage!!) will be susceptible to heat changes.

The other factor is the use of NFB. Some design circuits with the overall gain way up, so when they apply NFB the THD, etc., is way low. But, sometimes due to capacitances, microphonics, etc., the circuit can become unstable and oscillate. This can happen with a slight change in current, induced by heat.

BTW, the circuits I mentioned in the amps I play with (not my amps, but Magnavox) are extremely stable. I hear very little change from 10 minutes to 10 hours after power up.

Gabe

 

[fdegrove]

12B4

Hi Up4,

A full circuit diagram would help,include your psu.
Did you include a mute switch?
I adopted the habit of always muting the preamp before I turn on anything else.
All things considered a warm up time of 20 to 30 secs for tube gear is quite normal,you're probably spoiled by the almost instant on service of solid state gear?

Rgds,