scope settings

[mat]

Ok, it's time for me to try the scope first time. What would be the required settings on the front panel

[img:800:618]http://www.pichotel.com/pic/16028B31e/155815.jpg[/img]

The probe has 1x, 10x and REF position switch on it. I'll put it at 10x and feed the amp with PC soundgenerator with 1khz and 10khz signal one at time with the 8ohm dummyload on the speaker connection.... and then just follow the signal path with the probe... right?

What should be the amp settings ? (in this case dumble HRM 50w).

mat

[d2camero]

Looks like a pretty decent scope.

On 10X you can follow the signal path EXCEPT for the power tube outputs. This it best done with a high voltage output probe. Also on preamp tube outputs, connect the lead AFTER the coupling cap, to (hopefully) eliminate harmful HV DC.

When I scope test, I do the following:

1. I set the signal generator to the lowerst possible output voltage

2. I set the scope to something like 5V/graduation

3. I connect the scope right to the signal generator, and adjust the voltage down so this signal fills the screen

4. I then adjust the timescale to get 1 or two cycles of the waveform on the screen.


The each time I test the circuit, I put the scope voltage up to its highest, connect the probe and then adjust down. This way I don't screw up the scope (even though mine has overvoltage protection- it just somehow bugs me).

Good luck

Don

[mat]

Looks like a pretty decent scope.

On 10X you can follow the signal path EXCEPT for the power tube outputs. This it best done with a high voltage output probe. Also on preamp tube outputs, connect the lead AFTER the coupling cap, to (hopefully) eliminate harmful HV DC.

When I scope test, I do the following:

1. I set the signal generator to the lowerst possible output voltage

2. I set the scope to something like 5V/graduation

3. I connect the scope right to the signal generator, and adjust the voltage down so this signal fills the screen

4. I then adjust the timescale to get 1 or two cycles of the waveform on the screen.


The each time I test the circuit, I put the scope voltage up to its highest, connect the probe and then adjust down. This way I don't screw up the scope (even though mine has overvoltage protection- it just somehow bugs me).

Good luck

Don

Thanks Don, I'll follow Your instructions ... maybe I try the scope first with a overdrive pedal...

[mat]

OK, I tried (with 1khz signal output from a PC soundgenerator) the scope for the first time. I can see the waveform changing as tweaking the knobs of my OD pedal I tested it with.

Now I hooked the scope to the 8ohm speaker jack of my dumble OD hrm style build. When I turn the drive and drive master pots up or when on the clean channel I turn the clean master up the bias is gettin very high ? The DMM shows about 200mA+!! on the test points if I crank the pots. What is going on in here ??

Here is how it sees on the scope with PAB on and OD on:

http://www.putfile.com/pic.php?img=7279300

Here are the knobs (seperate clean and OD masters).

http://www.putfile.com/pic.php?img=7279301

Why does the bias get up with soundgenerator on amps input but not when guitar plugged in ?

[d2camero]

I am not familiar with the circuit of your amp so I am not so sure why the bias current is getting high. I would guess that the signal from your sound generator might be pretty high. I heavy guitar signal might peak around 150-200mV - what is your signal generator putting out?

[mlp-mx6]

Mat,

I'm not sure I'm tracking with you.

Are you saying that the current through the power tubes goes up as you crank the knobs? Your use of the term "bias" is confusing to me. Does the bias voltage itself change, or does the current through the tubes change? I would expect the current to increase as the output of the amp increases.

Also - Is this with input, or with no input? I would suggest you test on-scope with no amp input also - if the current jumps with nothing plugged in, it would suggest you have an ultrasonic oscillation going on.

Keep us posted.

[mat]

Ok, little bit more information. Please keep in mind that I'm not very good at this

The amp is based on the schem https://tubeamparchive.com/download/file.php?id=1179 with built in dumbleator. The PI section is changed to BluesMaster style.

I started from this layout: http://bad-domain/97HRM/97_ODS_HRM_Loop.JPG

I have a PC based signal generator that is outputted from soundcard to my stereo system and from there out of the phones jack to the amp input jack.

I have on back of my amps chassis 3 testpoints (one ground + 1 for each power tubes) and it is based on the 1ohm resistor method.

My DMM shows 40mV on each side with guitar or nothing connected to the amp input. As soon as I connect the 'soundgenerator' with ordinary 1/4 inch mono guitar cable to the amp input and open the clean master volume or OD master volume (when OD switch on) the reading on the DMM (in the test points) jumps up without tubes redplating?

The preamp volume does not affect to the DMM reading.

When cranking each of the volumes over 50% there is a faint high pitch whine eaven there is no speaker connected. The whine goes away when adjusting the bias pots.

I have 8ohm dummyload on the 8ohm speaker jack.

The PT gets hot when the amp is on for a while, so hot it is barely touchable.

Could the whine be the PT coil oscillation ?

[mlp-mx6]

OK - so you're sending signal through the amp.

Please run this test - with no input to the amp, turn everything full up and check the current through the power tubes. (meaning how many mv are measured across your 1 ohm resistors at the test points) If this goes up much at all then you likely have an oscillation. (it WILL go up a little, but not much)

Otherwise - it sounds like you're hearing the output xfmr vibrate with the input signal. Is the "whine" the same tone as your input sound? Also - what do you mean "the sound goes away when adjusting the bias pots"? What adjustment? Are you effectively turning off the output section by turning the bias voltage more negative?

Again - when you're actually generating output power you would expect the current through the output tubes to go up. Do not be concerned about that - be glad about it!

[Tonegeek]

When cranking each of the volumes over 50% there is a faint high pitch whine eaven there is no speaker connected. The whine goes away when adjusting the bias pots.

I have 8ohm dummyload on the 8ohm speaker jack.

The PT gets hot when the amp is on for a while, so hot it is barely touchable.

Could the whine be the PT coil oscillation ?

The PT will heat up when you are driving the amp hard. The whine could be normal. My amp does the same thing when on a dummy load. I can hear the signal from the signal generator as I drive the amp harder. I guess it is the OT but not sure. Still you should check for oscillations. It is normal for the bias to change with a load, but some one more experienced can answer how much as I don't know. Also it makes sense that the whine would change as you change the bias becuase raising the bias voltage would make the amp work less (and vice/versa) If you raise it enough (up to say -70 or -80 volts measured NOT at your test point but at the output of your bias pot) it would begin to cut the tube off. Keep the lights low so you can look for red plates.

[d2camero]

Hey Mat,

If the bias is good when idle, you are good to go. Don't sweat it when running a signal through.

If you are using a signal generator, dummy load and crank up the amp, all amps make themselves heard - this is normal.

OTs get hot, fry an egg hot? That might be a bit much. With loud guitar playing does it get this hot?


The other posts have a good point - you may have oscillation going on - since you have a scope you can check! Put your scope on the speaker output, crank it and see if there is any part of the waveform that has really compact little frequencies rammed in there.

[mat]

Please run this test - with no input to the amp, turn everything full up and check the current through the power tubes. (meaning how many mv are measured across your 1 ohm resistors at the test points) If this goes up much at all then you likely have an oscillation. (it WILL go up a little, but not much)

I did as You mentioned and there is almost no difference between every pot at 0 or full.

Otherwise - it sounds like you're hearing the output xfmr vibrate with the input signal. Is the "whine" the same tone as your input sound? Also - what do you mean "the sound goes away when adjusting the bias pots"? What adjustment? Are you effectively turning off the output section by turning the bias voltage more negative?

It is a little hard for me to tell if it is exactly same sound but it reminds the sound of the soundgenerator.
Yes, when I first saw the biastestpoints voltage go up I tried to adjust the two biaspots to get lower voltage. Then the whine went away also (tube was shutted down maybe).

Again - when you're actually generating output power you would expect the current through the output tubes to go up. Do not be concerned about that - be glad about it!

Is it normal the bias 'voltage' to go from 40mV to 220mV when feeding the amp with 1khz sinewave ?

How about the stereo jack of the stereo system ? Doesnt the ring be grounded at the amps input jack, or does it matter anyway ?

The PT will heat up when you are driving the amp hard. The whine could be normal. My amp does the same thing when on a dummy load. I can hear the signal from the signal generator as I drive the amp harder. I guess it is the OT but not sure.

Ok.

Still you should check for oscillations.

Do You mean to see from the scope if there is ... something wrong..

How do You think the scope wave looks I posted ? Is it normal looking ?

It is normal for the bias to change with a load, but some one more experienced can answer how much as I don't know. Also it makes sense that the whine would change as you change the bias becuase raising the bias voltage would make the amp work less (and vice/versa) If you raise it enough (up to say -70 or -80 volts measured NOT at your test point but at the output of your bias pot) it would begin to cut the tube off. Keep the lights low so you can look for red plates.

Ok, btw. anyone seen a picture of a redplating tube on the net ?

In the middle of the tube there is this orange glowing bar but the bigger metal plates keep looking grey..

[Fischerman]

Hi Mat,
I have a scope but I'm as new to this as you...so I probably won't be much help. But I have a few questions about your post. Where do you have the Send Level and Return Level controls set? What is the AC voltage of your input signal? When you say 'open the Clean master volume or OD master volume (when OD switch ON)'...do you mean when you just barely turn them up or when you crank them all the way or what?

By the way...I looked at your other picture and I couldn't really tell if you used any shielded cable on your D-lator. The built-in D'lator can oscillate... I think of the Return stage as similar to the input stage except it doesn't get amplified prior to the PI as much...but it's over there much closer to large-signal-carrying wires. Those FX loop grid wires are very sensitive.

[mlp-mx6]

Assuming the tubes are running at a normal quiescent current - If you turn all knobs full-up and the current through the tubes does not move substantially you do not have oscillation.

You keep using the term "bias voltage" when you're referring to the voltage measured across the 1-ohm resistor representing the quiescent current. Please be clear - the "bias voltage" is the negative voltage applied to the power tube grids. The voltage you are measuring is really an analog of the current through the tube - millivolts convert directly to milliamps if the 1-ohm resistor is accurate.

I do not think there is anything wrong with your amp based on what you've said. The current through the output section goes up when it is generating power - and this is exactly as it should be. What is your concern? Most important - Does it sound good?

[mat]

I have a few questions about your post. Where do you have the Send Level and Return Level controls set?

They are about 10-11 o'clock - slight boost in there.

What is the AC voltage of your input signal?

I don't know. Should I hook the DMM with AC setting between ground and tip ?

When you say 'open the Clean master volume or OD master volume (when OD switch ON)'...do you mean when you just barely turn them up or when you crank them all the way or what?

The bias ... current rises linearily when turning the pot up. And the input volume also puts the bias current up but not as much as the masters.

By the way...I looked at your other picture and I couldn't really tell if you used any shielded cable on your D-lator. The built-in D'lator can oscillate... I think of the Return stage as similar to the input stage except it doesn't get amplified prior to the PI as much...but it's over there much closer to large-signal-carrying wires. Those FX loop grid wires are very sensitive.

You are right ! There isn't shielded wire on the d-lator section. I'll put shielded on the grids ASAP. Thanks for the tip !

[mat]

Assuming the tubes are running at a normal quiescent current - If you turn all knobs full-up and the current through the tubes does not move substantially you do not have oscillation.

Ok, good to know.

You keep using the term "bias voltage" when you're referring to the voltage measured across the 1-ohm resistor representing the quiescent current. Please be clear - the "bias voltage" is the negative voltage applied to the power tube grids. The voltage you are measuring is really an analog of the current through the tube - millivolts convert directly to milliamps if the 1-ohm resistor is accurate.

I'm sorry I'll start to use 'bias is XXmA'.

I do not think there is anything wrong with your amp based on what you've said. The current through the output section goes up when it is generating power - and this is exactly as it should be. What is your concern? Most important - Does it sound good?

No it does not sound good Actually it sounds quite horrible. I'm having same kind of symptoms as Fisherman talks on his thread: https://tubeamparchive.com/viewtopic.php?t=4096

Barking ugly notes when putting more pressure on the pick and the OD is harsh and not very musical.

I've listened a lots of HRM BM clips lately and I've come to the conclusion that BM is not my cup of tea. The amp actually sounds good when playing very softly with a light touch. Many here likes it alot but if I am being honest it is not the sound I'm after. I like more middle honk and more compression. I'll put back the 'original' PI when I have the time.