Power Supply Help...

[Rustyroadie]

Hi,

Greetings! new memeber here, glad to have found this forum...

Anyway, could anyone shed a bit of light on this?

I am constructing a guitar amp from some old schematics, and a few half built pcbs...Im having a bit of a problem with the HT power (used for the tube). I have an AC supply of about 80V, when I connect up the following circuit, i only get about 40v DC out of it, and when i connect it up to my preamp, it drops to about 15v DC. Looking at the scope, the wave on the AC input changes to pretty much a square wave with a voltage of about 20V AC. I should be getting a pretty high voltage at the output....what is the purpose of the transistor? and what could be causing the low output?

Cheers!

 

[MAGNETRON]

4K7 resistor(high value!),100k high value,check these resistors for correct one's!

4K7..............try 4.7 Ohm high wattage type.

 

[Rustyroadie]

Thanks for that, I'll try lower values, the schematic I have uses those values, but is quite possibly wrong

 

[MAGNETRON]

Leave the 100k,I think the 4K7 should read 4R7ie 4.7 ohm

 

[MAGNETRON]

Try different values for the 100k resistor,this will bias the transistor on/off more or less....ie less voltage/current out or more voltage/current out.
depends on what voltage/current your equipment needs.

 

[Rustyroadie]

Thanks Magnetron, thats a great help. I'll let you know how it goes...

 

[MAGNETRON]

A few lower voltage/current circuits.

 

[MikeMl]

How many volts do you want on the plate of your preamp?

How many mA does the preamp pull?

 

[Rustyroadie]

Magnetron, I have taken the 4k7 resistor out all together, it doesn't seem to make any difference. And if I lower the 100k value, the voltage drops right down to a few volts, and if I put in a higher value, it doesn't really have an affect. Leaving out the 4k7, and using the 100k bias resistor, the output voltage climbs very slowly to about 60V dc...as if the cap is charging or something. I know I'm prob doing something daft, but what's bugging me is the fact that its a pre built pcb, so the components that I'm using SHOULD be the correct ones...hmmm.

MikeML, the voltage should be quite high...about 100 - 180 V dc going onto the anodes of a 12ax7 tube...only about 200 mA I guess

 

[mneary]

MikeML, the voltage should be quite high...about 100 - 180 V dc going onto the anodes of a 12ax7 tube...only about 200 mA I guess

Do some quick arithmetic... 180V at "only" 200mA is 36 watts. Is your 80V transformer capable of providing 36W?

 

[JimB]

200mA anode current in a 12AX7 is way too much!
Try 1 or 2 mA.

Your description of what is happening with the 80v psu suggests that something is very wrong.
If you have 80v AC, rectify it and smooth it, there should be about 110volts across the left hand 33uF capacitor.

Where is the 80v supply coming from? It should come from a floating winding on the transformer.
If the 80v winding is connected to anything else you will get some strange effects.
Until the rectifier/capacitor part is working correctly, the rest of the circuit is irrelevant.

JimB

 

[Rustyroadie]

My mistake...it's 200mA for the power supply of the whole pre-amp, not the HT for the valve.

The 80V is a floating winding on the transformer, and the ground is tied to the lo-voltage ground after rectification and smoothing.

Thanks for the advice, I'll start right from scratch and work my way up and let you know...

 

[Rustyroadie]

Gents, I seem to have come right...a leaky cap!!

Thanks for all the help, it's much appreciated.

I'm wiser (and slightly balder) now.

TA