AC30 query

Cirrus

It's one of those things that's so simple... but the mind of the guy who came up with it first!

ecc83

Do it right!

http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0160354/

Dave.

ICBM

ecc83 said:

Do it right!

http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0160354/

Not on an old AC30. There's nowhere to mount it without drilling the chassis, and the existing double-strip tag board provides more than enough support for a conventional 10-watter.

I like those resistors but there's overkill and overoverkill .

ecc83

ICBM said:

ecc83 said:

Do it right!

http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0160354/

Not on an old AC30. There's nowhere to mount it without drilling the chassis, and the existing double-strip tag board provides more than enough support for a conventional 10-watter.

I like those resistors but there's overkill and overoverkill .

I was 1/2 joking IC!

BTW, now that the OP has flexed his Ohm's Law muscles a bit he might like to measure Vk, Va and Vg2 and work out how hard he is working those EL84s?

Dave.

knuckleberryfinn

ecc83 said:

BTW, now that the OP has flexed his Ohm's Law muscles a bit he might like to measure Vk, Va and Vg2 and work out how hard he is working those EL84s?

Dave.

Thanks for the homework Dave.  I'll do it right away sir...

knuckleberryfinn

Right, I have some voltage and resistance values, so should be able to calculate dissipation.  This is my first time, so please correct me, or tell if there's a quicker way of doing things.  (I'm only following someone else's method anyway, so it's not like I'm actually using my brain).

Vk = 10.6

Va = 335

Vg2 = 309

Rk = 56R

Cathode current = Vk/Rk = 10.6/56 = 0.19A

Screen current = 0.43/100 = 4.3mA

Anode current = Ik - Is = 0.19-0.0043 = 0.186

Dissipation = Va x Ia = 335 x 0.186 = 62.31.

So does this mean each valve is running at 15.5W dissipation?  Isn't that a bit high?!  Have I done it right?

Also, shouldn't the anode and screen voltages be closer to one another?

 

ICBM

That all looks about right. 16W at idle is typical for old AC30s - decent EL84s will take it. The lower screen voltage will help save them a bit too - did you say you have a dual 390-ohm resistor instead of a choke? That will lower the screen voltage.

martinw

knuckleberryfinn said:

Dissipation = Va x Ia = 335 x 0.186 = 62.31.

It's actually (Va - Vk) x Ia

 so (335 - 10.6) x 0.186 = 60.34W

/4 = 15.08W per valve.

Over the design spec of the valve, but typical for AC30s. Change them regularly.

Also, shouldn't the anode and screen voltages be closer to one another?

 

No, that's in the typical range.

knuckleberryfinn

Yay!  I did something right!

I have a choke and two 390-ohm resistors.  I don't suppose it matters does it?

I might cool it down a bit with a 68-ohm cathode bias resistor.  I'm going to sell it as soon as I fix the vib/trem issues, so I don't want to be running it hard.

ecc83

Does it redplate in the dark?

Oh and BTW 9/10 but where's me apple?

Dave.

knuckleberryfinn

ecc83 said:

Does it redplate in the dark?

Oh and BTW 9/10 but where's me apple?

Dave.

No it doesn't redplate, but one of the valves does when the amp is first turned on - it dies down immediately though.  Do I have a problem?  

EDIT: This only happens when it's cold - i.e. it doesn't happen when turned off and then turned back on again.

(Apple's in the post...)

ICBM

knuckleberryfinn said:

No it doesn't redplate, but one of the valves does when the amp is first turned on - it dies down immediately though.  Do I have a problem?  

EDIT: This only happens when it's cold - i.e. it doesn't happen when turned off and then turned back on again.

Sounds like that valve has a faster filament warm-up than the others. Until all four valves are conducting fully, there will be much less bias voltage developed so if one comes on faster than the others it will be very underbiased until the others start to conduct.

This is another reason why Standby switches are a good thing! (Although not the way Vox implement them on the modern AC30s unfortunately.)