• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Feedback wanted on PSU design

fdegrove

diyAudio Senior Member
2002-08-21 1:20 am
Belgium
PSU+300B

Hi,

I don't see anything wrong with it really.
Personally I would use ~6 mF as C after the rectifier,use the bigger self as the first one.
And I think you do the right thing by opting for the valves as rectifiers.
They don't introduce the noise all diodes do and ramp up the voltage for the output tube nice and slow.
On the cost side:
I wouldn't play cheap on a 300B amp but since you ask:
Selfs aren't cheap and you could replace the second one by something like this:
schema_ripplekiller.gif


I think your fellow Dutchman got it out of a Philips book, anyway it works fine and I think it would cost a fraction of a self.

Happy DIY,:)
 
daatkins said 'I'm certainly not an expert on tube rectifiers but... are they really significantly superior to diode designs?"

One reason to use tube rectifaction is so the B+ doesn't come up right away, giving the tubes time to warm up. Applying B+ to a cold tube can cause cathode stripping and shorten the life of the tube. SS diodes can be used but then you'd need to delay the B+ turn on.....


Dave
 
Well I've got a radio that has a directly heated rectifier (5Y3), that heats up in about a second to 320V+, which then drops to about 260V (normal B+) as the tubes warm up. It has lasted for decades with all original tubes... (had to replace all the caps though.)
Tubes are not too delicate, they can handle short voltage spikes without too much trouble.

The indirectly heated rectifiers, like 5V4 would be a good choice for a slow B+ design without a standby switch.

There is also a way of calculating the value of that first cap so it minimises ripple in the power supply, does anyone have that formula?
 
DaveInVA said:
One reason to use tube rectifaction is so the B+ doesn't come up right away, giving the tubes time to warm up. Applying B+ to a cold tube can cause cathode stripping and shorten the life of the tube. SS diodes can be used but then you'd need to delay the B+ turn on.....


Dave

This kind of thing is a real problem with fluorescent tubes operating from a HF inverter, if the filaments are not preheated before the HV is applied, as you say, the cathode material is gradually torn from the filaments in this case and causes tube end blackening and premature failure. Same principle at work. I wonder how it would go to insert a resistor in series with the rectifier tube filament to start it up even slower and then switch it out with a relay after 10 - 15 seconds.

Actually, now I think some more about it, if the main tubes are alight first wouldn't the rectifier be subject to cathode material stripping as it is coming up to temperature with anode volage already applied and the dc rail sucking whatever current it can get?

GP.
 
Let's step back to the PSU-issue.

I'm using a 5687 as driver (180V @ 15mA). This is it's designed supply: http://www.homepages.hetnet.nl/~rjonkers/psu_5687.jpg . It indicated I have to use 180V as secondary.

This is the slightly modified 300B supply: http://www.homepages.hetnet.nl/~rjonkers/psu_300b.jpg . It needs 520V.

Do you have any further suggestions for both supplies? What do you think about the secundary voltage on the transformer (520V to get 500V and 180V to get 180v)?

Ralph

BTW Frank, the 5R4 can only have 4uF max after it.
 

fdegrove

diyAudio Senior Member
2002-08-21 1:20 am
Belgium
PSU

Hello Ralph,

BTW Frank, the 5R4 can only have 4uF max after it.

Yes sir,that's what the databooks say.
But you are using two of them in a Graetz bridge full wave configuration in your simulation.
Remember these are not absolute design values!:att'n:
So you can use twice as much on paper and even more depending what you put behind it IMO.
And I love these 5R4G's,especially those 5R4GYB types.
It is a valve with a rather low internal impedance and very robust to boot.
I think the problem you encounter simulating your PSU is due to the fact that this internal resistance is nowhere stated in the datasheets.
Hence your confusion:ust pick a higher then estimated voltage and bleed off the excess with a series R.

Rgds,:)
 

fdegrove

diyAudio Senior Member
2002-08-21 1:20 am
Belgium
PSU

Hi Ralph,

Looks fine to me.
Something to experiment with later on:

http://www.homepages.hetnet.nl/~rjonkers/300B_design.jpg

Try to do without the decoupling cap on the first stage's cathode resistor (R16/C16).You will have a bit less gain (local feedback).
IMO the cap slows down transient response.
I find the the 300B already a bit dull in the high frequency region.
A matter of taste,of course.;)


Cheers,
:)