Bias problem!

[thomasdj]

Hi!

I am building the 3ch dual rectifier and I am almost finished...I think!

Well..i have made a few modifications to the schematic..I added individual bias pots to each power tube.

I 've got two problems.

I got signal through each channel and so I assume that the preamps are working properly. However I've got a nasty high freqency sound coming out of the speaker. Another problem is that I only get low power even when turned at high gain and master.

I run the amp with 4xEL34 so i adjusted the bias negativ voltage to ca. -40v at each power tube at Grid 1 (pin 5). But when measuring cahtode voltage across 1 ohm inserted cathode resistor I only measure 10-12 mV!!! ..what is wrong with my circuit??

Is it the 4x220K that should be lower??

Thanks

Thomas

[scole]

hey thom, the noise sounds like negative feedback wich you can solve by switching the ot leads to the power tubes. the other problem im not expert enough to help.

[MWJB]

What kind of plate voltages do you have on the EL34's?

[thomasdj]

Thanks for your reply

What kind of plate voltages do you have on the EL34's?

The plate voltage is 475..pretty close to the schematic..

Btw..the tubes get really hot! ..so do the PT ..when I tap the input jack tip ..the tubes "blink" and the OT gnits at the plate voltage:s

[MWJB]

At 475 I'd suggest that you want more than -40v at the grids. Do you have 1x 1ohm resistor at each tube cathode? Are you sure that you are not measuring for 2 tubes & have the decimal point in the wrong place?

[thomasdj]

At 475 I'd suggest that you want more than -40v at the grids. Do you have 1x 1ohm resistor at each tube cathode? Are you sure that you are not measuring for 2 tubes & have the decimal point in the wrong place?

I attached my bias measure setup..
Don't think that there is anything wrong with it..

But as I mentioned, the tubes get really hot! and when I tap the input jack tip the tubes "blink" and the OT sparkles at the plate voltage:s

[thomasdj]

hey thom, the noise sounds like negative feedback wich you can solve by switching the ot leads to the power tubes. the other problem im not expert enough to help.

Thanks Scole

But I have to solve the bias problem before doing anything else

[Bruce / Mission Amps]

Thanks Scole

But I have to solve the bias problem before doing anything else

Sounds more like ultrasonic oscillations to me.
Do you have an O'scope?
Disconnect the NFB completely and see if that helps... also if you have a presence control wired in there, check your lead dress with respect to the phase inverter... move the wire around and try grounding it somewhere else.. etc.
I think you have a wire dressing problem and a higher gain circuit is leaking signal through capacitively coupled wires back to a prior stage at some very high frequency.
Many the times it is right at the phase inverter but it can be anywhere with respect to grid lead wires.

[thomasdj]

Sounds more like ultrasonic oscillations to me.
Do you have an O'scope?
Disconnect the NFB completely and see if that helps... also if you have a presence control wired in there, check your lead dress with respect to the phase inverter... move the wire around and try grounding it somewhere else.. etc.
I think you have a wire dressing problem and a higher gain circuit is leaking signal through capacitively coupled wires back to a prior stage at some very high frequency.
Many the times it is right at the phase inverter but it can be anywhere with respect to grid lead wires.

Thanks Bruce..I'll try to do that right now! and see if it helps

[thomasdj]

Sounds more like ultrasonic oscillations to me.
Do you have an O'scope?
Disconnect the NFB completely and see if that helps... also if you have a presence control wired in there, check your lead dress with respect to the phase inverter... move the wire around and try grounding it somewhere else.. etc.
I think you have a wire dressing problem and a higher gain circuit is leaking signal through capacitively coupled wires back to a prior stage at some very high frequency.
Many the times it is right at the phase inverter but it can be anywhere with respect to grid lead wires.

Well...Don't know if it did something...but found a new thing..! when I touch the PT on the iron..some of the noise disappear! ..and the sparkling also disappear when tapping the jack tip and touching the PT iron simultaneously..

No..don't have a O'scope

[Steve Conner]

Sparkling?! Are you seeing sparks from the wires on your output transformer? If so, you need to stop right now before you wreck it. This sounds like ultrasonic oscillations, sure enough.

Try running the amp with no tubes fitted except the rectifiers and power tubes. Then fit the PI tube and try again. Then add the next tube back in the signal path, try again, and so on.

It can help to measure the plate or cathode current draw. If the amp is working properly, the idle current should adjust easily with the bias pot. You should be able to set it to 84mA or whatever, and it will stay there, only increasing when you play loud. If there are oscillations, the adjustment will be nearly impossible, and the current will jump to some high value when the oscillations start. Try all the controls and see if they affect the current draw. Particularly the volume controls and the presence control. Even moving your hand around the chassis may affect it.

Did you try reversing the plate connections to your OT? You might have the phasing wrong, so your negative feedback is actually positive.

Also, you might need a different value of compensating capacitor than the Dual Recto schematic shows. The compensating capacitor is usually hooked between the PI plates. It is the 75pF cap in your schematic. The required value depends on the make and model of OT you're using. If the amp oscillates with only the PI tube, rectifiers and power tubes fitted, and you're sure you have the OT plate connections right, then try a bigger value for this cap, maybe 100 or 150pF.

This is just a brain dump: I'm not personally familiar with the Dual Recto, but I've debugged ultrasonic oscillations before. The amp would draw 7 times the normal idle current and redplate as soon as the dirty channel was engaged and the master volume was turned above 10 o'clock. The cure was a tube shield on the first preamp tube.

[thomasdj]

Sparkling?! Are you seeing sparks from the wires on your output transformer? If so, you need to stop right now before you wreck it. This sounds like ultrasonic oscillations, sure enough.

Try running the amp with no tubes fitted except the rectifiers and power tubes. Then fit the PI tube and try again. Then add the next tube back in the signal path, try again, and so on.

It can help to measure the plate or cathode current draw. If the amp is working properly, the idle current should adjust easily with the bias pot. You should be able to set it to 84mA or whatever, and it will stay there, only increasing when you play loud. If there are oscillations, the adjustment will be nearly impossible, and the current will jump to some high value when the oscillations start. Try all the controls and see if they affect the current draw. Particularly the volume controls and the presence control. Even moving your hand around the chassis may affect it.

Did you try reversing the plate connections to your OT? You might have the phasing wrong, so your negative feedback is actually positive.

Also, you might need a different value of compensating capacitor than the Dual Recto schematic shows. The compensating capacitor is usually hooked between the PI plates. It is the 75pF cap in your schematic. The required value depends on the make and model of OT you're using. If the amp oscillates with only the PI tube, rectifiers and power tubes fitted, and you're sure you have the OT plate connections right, then try a bigger value for this cap, maybe 100 or 150pF.

This is just a brain dump: I'm not personally familiar with the Dual Recto, but I've debugged ultrasonic oscillations before. The amp would draw 7 times the normal idle current and redplate as soon as the dirty channel was engaged and the master volume was turned above 10 o'clock. The cure was a tube shield on the first preamp tube.

Many thanks Steve!

I really think this is what happens! ..it all makes sence.. I'll try the amp without the preamp tubes and so on...

I didn't swap the plate connections but I'll do it now! but what will you suggest me to do first?? swap the plate connections or pull out the tubes??

Btw...here is a link to the OT I use..
http://www.welter-electronic.de/baut...ebertrager.htm

number Ü23 Fender Bassman 100

sorry it's in german..

[thomasdj]

Sparkling?! Are you seeing sparks from the wires on your output transformer? If so, you need to stop right now before you wreck it. This sounds like ultrasonic oscillations, sure enough.

Try running the amp with no tubes fitted except the rectifiers and power tubes. Then fit the PI tube and try again. Then add the next tube back in the signal path, try again, and so on.

It can help to measure the plate or cathode current draw. If the amp is working properly, the idle current should adjust easily with the bias pot. You should be able to set it to 84mA or whatever, and it will stay there, only increasing when you play loud. If there are oscillations, the adjustment will be nearly impossible, and the current will jump to some high value when the oscillations start. Try all the controls and see if they affect the current draw. Particularly the volume controls and the presence control. Even moving your hand around the chassis may affect it.

Did you try reversing the plate connections to your OT? You might have the phasing wrong, so your negative feedback is actually positive.

Also, you might need a different value of compensating capacitor than the Dual Recto schematic shows. The compensating capacitor is usually hooked between the PI plates. It is the 75pF cap in your schematic. The required value depends on the make and model of OT you're using. If the amp oscillates with only the PI tube, rectifiers and power tubes fitted, and you're sure you have the OT plate connections right, then try a bigger value for this cap, maybe 100 or 150pF.

This is just a brain dump: I'm not personally familiar with the Dual Recto, but I've debugged ultrasonic oscillations before. The amp would draw 7 times the normal idle current and redplate as soon as the dirty channel was engaged and the master volume was turned above 10 o'clock. The cure was a tube shield on the first preamp tube.

Hi again Steve

I am pretty new in measuring mV but anyway...I switched the multimeter to 200mV ..the display then shows 00.0

I measured the cathode current on each power tube with the PI plugged in. The meter shows:

00.8
04.1
01.9
00.2

The bias voltage is -40v on each tube. If i turn up the negative voltage the tube grids glows blue!

However..The oscillations starts when V1 is plugged in and the gain or master is turned slightly..

What to do now??

[Steve Conner]

Well, those mV readings suggest you just are biased too cold. You need to adjust the negative voltage to be less than -40V, say -38, -35, or whatever, and as you do this the mV readings will all increase. Adjust it so that you get roughly 30mV across each cathode resistor.

Blue glow inside the tubes is normal.

If the oscillation only starts with V1 plugged in, then it sounds like a layout problem, that might need shielding or changes to your wiring layout to solve it. Also, it suggests that the plate wires on your OT were the right way round to start with, so probably no need to bother changing them.

Does the oscillation happen on all three channels, or only on the highest gain one? (red channel or whatever Mesa Boogers calls it?) Can you post photos of your chassis so we can see the layout?

In particular, if V1 is near to the power tubes, then it might need a shield. For a test, try wrapping V1 in tin foil connected to the chassis, and see if this affects the oscillations.

[thomasdj]

Well, those mV readings suggest you just are biased too cold. You need to adjust the negative voltage to be less than -40V, say -38, -35, or whatever, and as you do this the mV readings will all increase. Adjust it so that you get roughly 30mV across each cathode resistor.

Blue glow inside the tubes is normal.

If the oscillation only starts with V1 plugged in, then it sounds like a layout problem, that might need shielding or changes to your wiring layout to solve it. Also, it suggests that the plate wires on your OT were the right way round to start with, so probably no need to bother changing them.

Does the oscillation happen on all three channels, or only on the highest gain one? (red channel or whatever Mesa Boogers calls it?) Can you post photos of your chassis so we can see the layout?

In particular, if V1 is near to the power tubes, then it might need a shield. For a test, try wrapping V1 in tin foil connected to the chassis, and see if this affects the oscillations.

Hi Steve thank you!

I hava only tried the clean channel..since my OT spark from the primary I didn't want to push it..you know..

You wrote that blue glow in tubes are normal...is it normal that the grids are blue?? because when I turn down the bias voltage the grids turn blue..is this normal??

I have some pics i can send you..can't post them since they are to big
I know I have a layout problem..see pics ..don't get confused..all the "loose" wires are related to the switching circuit. The heavy red and heavy blac wire are both high power wires.. The heavy blue wires are connected to the preamp tubes..Underneath the PCB's are the power section wired PTP.. Hope it help's