High frecuency oscillation when moving volume pot

[pingfloid]

I forgot to mention:

I did check the output with a scope, and there is no trace of high frequency oscillation anymore.

Cheers,

Manuel

[pingfloid]

Before buying a new transformer for the power supply, I am thinking to re-calculate the curves to adapt the amplifier to the supply of +203 V that I already have.

If I drop the value of the cathode resistor from 360 Ohm to 200 Ohm, then the current will be about 100 mA and the idle working point -20,6 Volt.

¿What is your opinion?

[fdegrove]

Hi,

Before buying a new transformer for the power supply, I am thinking to re-calculate the curves to adapt the amplifier to the supply of +203 V that I already have.

Miguel,

There are a few points in the way you constructed the amp where you're bound to lose some volts:

1/ As said before, the value of the first filter cap is important to obtain the B+ you need.

2/ As you use a different OPT you may also lose some voltage there.

3/ The filter choke may have a different DCR as well.

4/ The 15K dropping resistor could be lowered considerably without affecting ripple too much if needed.

5/ Setting the OP of the KT88 too low you risk to overdrive it causing distortion.

Cheers,

[pingfloid]

Hi Dr. Frank,

I don't know how to calculate the value for a new transformer to compensate this voltage drop and get the desired +375V.

I've been messing with PSUD but I am not even able to simulate the working conditions.

This are the values that I introduce:

T1 = 300V / 159Ohm
C1 = 50 uF
L1 = 10H / 63 Ohm
C2 = 200 uF
I1 = 2x104 mA (In my amp I measure 2x70mA)
R1 = 15 K
C3 = 50 uF
I2 = 1 mA

With this values, PSUD gives me V(C2) = 250 V.

To obtain 375V I have to raise T1 to almost 400V! I know I'm doing something wrong. How do I know the new value for the transformer resistance?

Thank you,

Manuel (not Miguel)

[fdegrove]

Hi,

Unfortunately I'm not familiar with PSU at all but could it be the transformer rating is just introduced into the soft the wrong way?

Could you try rerunning it with a bigger filter cap after the rectifier? Say,68µF to 100µF for starters.

To obtain 375V I have to raise T1 to almost 400V! I know I'm doing something wrong. How do I know the new value for the transformer resistance?

Are you sure it's seen by the soft as a 300V-CT-300V and not 150V-CT-150 ?

The copper losses of the xformer aren't a big issue, if it delivers the voltage at the current that's specified you'd be O.K.

Cheers,

[pingfloid]

Hola Paco,

Could you try rerunning it with a bigger filter cap after the rectifier? Say,68µF to 100µF for starters

It doesn't make much difference at the final result to change the value of this cap from 50 to 100 uF at PSUD

The copper losses of the xformer aren't a big issue, if it delivers the voltage at the current that's specified you'd be O.K.

Thanks, I feel better now

Now I am going to measure V and I connecting the components one after the other, so I will now when the V drop occurs.

Cheers,

[pedroskova]

Originally posted by pingfloid


To obtain 375V I have to raise T1 to almost 400V! I know I'm doing something wrong. How do I know the new value for the transformer resistance?

a couple of observations:

your transformer in PSUD is 300vct with 159ohm source resistance. If you use the edit function for the transformer in PSUD, this works out to a transformer with 16% regulation(probably too high). Assuming 5% regulation, change the transformer values to 315V @ 50 ohms. This will get you up to ~ 325V at the first cap.

To get the rest, you need to switch to SS rectification or a more efficient tube rectifier. PSUD is calculating a large voltage drop for the 5U4G.

Hope this helps.

pete

[thomasholley]

In PSUD you can right click the transformer and enter the voltage, current and regulation values and the output voltage and resistance are calculated. Don't worry too much about the program. It is just something to help set up your circuit.

I think you should leave the first filter cap at 50uF to avoid stressing the rectifier. You have the actual measured voltages so you can go from there to set your circuit parameters. You should follow your instinct and lower the cathode resistor of the KT88 to bring up the current a bit. Try a couple of different values and see how it operates electrically, but more importantly how it SOUNDS.

You may loose some dynamic range with a lower voltage on the plate so you have to experiment a little to see where you can ballance things out. Varying the cathode resistor to change the bias point is about the only thing you can do at this point without a major, costly redesign. Your power supply transformer is not giving you what you asked for but what you have may not be so bad. Listen to it awhile and decide.

I don't know if it will matter but you may try a higher value than 68k for the grid resistor on the KT88.

Thomas

[pingfloid]

your transformer in PSUD is 300vct with 159ohm source resistance. If you use the edit function for the transformer in PSUD, this works out to a transformer with 16% regulation(probably too high).

159 Ohm is the result from the following actual measures:

Primary Supply Voltage: 220V
Primary resistance: 8,4 Ohm
Secondary off-load voltage: 312V
Secondary resistance: 143 Ohm.

Then I introduce this values at the "Source Impedance Calculator" editor of PSUD and I have the result of 159,89 Ohm. You are right: this correspond to a 16% regulation, but that are the real values that I measure from the transformer. So maybe my transformer really have this high regulation tolerance. (I didn't specify that when I did order it). Is it usual to tell the manufacturer the desired regulation of the transformer, or maybe the expected impedance?

you need to switch to SS rectification or a more efficient tube rectifier

Ok, I'll experiment with a couple of 1N4007

Try a couple of different values and see how it operates electrically, but more importantly how it SOUNDS.

Yeah, It sound good to me from the beginning! Even when I had the little oscillation at some positions of the volume pot the sound was not bad Now I just want to balance the electrical behaviour and see if it can improve the sound.

Thank you very much for your help

[audiobot]

Hi,

I kind of hate to bring it up, but if you go for SS rectification you could try a bridge instead of a full wave. With choke input that should give you about 425V after the filter. Or you could go with a hybrid bridge with two SS diodes and two tubes (or maybe one with separate cathodes?) for something a bit lower. Just something to kick around

Michael