Voltage questions on new "Merlin" Marshall type build

[sluckey]

When you connect a sig gen to the input set the level for 100mV_PPand freq to about 500Hz as seen at the input jack with your scope. Then signal trace forward through the amp watching for the sine wave to start squaring off.

[wittyjeff]

When you connect a sig gen to the input set the level for 100mV_PPand freq to about 500Hz as seen at the input jack with your scope. Then signal trace forward through the amp watching for the sine wave to start squaring off.

Got it, will do.  I did the stuff noted below before I saw this just now, I'll do this above suggestion next.

[wittyjeff]

I checked out the signal generator app on my phone and through the amp it produces a reasonable sine wave.  So, the first photo below shows what the signal looks like at the grid of an EL35 when both gain and volume are maxed.  It has a nice sine wave but starts to distort about about 4.5.  The Voltage/div was set on 5 so there's about a 5V swing on this signal here... but I didn't measure it at the actual input, so I suppose that info is useless.  I'll be redoing all of this below according to Steve's suggestions, but thought I'd throw this out since I did it before reading his post.
The second photo is of the output as measured at the 4 Ohm speaker output tap, with all volumes at max.  Note that the voltage/div is set to 5mV... so there's only about a 15-20mV swing on this output signal.  Again, probably useless since I didn't measure the original input signal... but still, seems kinda low.
The third photo is of the same output signal, but with the gain turned down to about 5.
Anyway, for what it's worth, there's that.  I'll try to get some readings that are a bit more useful next...

[sluckey]

YOU DON'T HAVE TO RUN ALL POTS AT MAX.

[wittyjeff]

Ok, here's another run at it...
Pict 1 is just of the input signal.  Turns out I can only get a 50mV signal from my phone (it swings 100mV + to -, but I figured you wanted a 100mV amplitude, right?).  Anyway, with that in mind...
Pict 2 is the signal after stage 1 (1st half of V1) beginning to distort with gain at less than 3, around 15V.
Pict 3 is the signal after stage 2 beginning to distort gain just less than 3, around 30V (probe set to x10)
Pict 4 is signal after stage 2 with gain cranked 100%, around 40V

Not sure if these are useful, but let me know what readings to take next.  Interesting note, this time I was monitoring the voltage across the current sensing resistor from the power tube cathode and it got up to 50mV with things cranked.

[sluckey]

Better looking pics.

[wittyjeff]

I wonder if this isn't part of the problem...
It seems that the output of stage 1 (1st half of V1 as measured from the grid of stage 2) is clipping no matter what the gain knob is set to.  Pict 1 attached is with the gain turned up a tiny bit (to around 1 ~5V), definitely clips here.  The point is that this first stage seems to be clipping even at rather low gain settings. Anything over 3V is clipped. Note that this stage is only producing a max Voltage of under 20V with gain at max, this is with a 50mV input.  How do we get it to stop clipping.  In comparing this "Ultra High Gain" circuit with Merlin's "High Gain circuit, I see that the High Gain circuit has a higher plate voltage (a 100K plate resistor instead of a 220K) and a slightly lower value cathode resistor (which increases the cathode voltage from 1V to 1.4V).  Do you think slapping in those changes would get rid of this first stage clipping?

[sluckey]

The gain pot does not control the first stage.

You can change that 220K plate resistor to 100K to reduce clipping or you can reduce the signal level you are putting into the amp.

[pdf64]

Are you sure it isn’t the 2nd stage grid clipping the signal, due to the positive peak exceeding the bias voltage?
eg if the signal level there is reduced a bit, does that reduce / eliminate the clipping?

[wittyjeff]

I've connected the probe at the grid just prior to stage 2 as indicated by the red X on the attached schematic.  Because I only have a phone app to produce the signal, it's only 50mV... so not equivalent to a 100mV guitar pickup signal.  Regardless, at that signal input with the probe connected at that location, when the gain pot is turned to around 2, such that the signal at that location is around 3V, it's clipped (it's unclipped when the gain pot is below that)... I'm guessing being clipped like this is bad... so I need to fix it so such that it's not clipping at this early point.  Am I thinking correctly on that?
What's causing this clipping and how to correct it is the part I'm not sure about.
My guess is that reducing that stage 1 plate resistor might help, but that's simply because I noticed that Merlin's similar but lesser gain pre-amp circuit uses a 100K resistor on the 1st stage plate (and because Steve mentioned it as well).  I can't really reduce the input signal any more, it's already at half of what a guitar pickup would be sending (well, I guess I could, but I don't know how that would result in beneficial information). Hopefully I'll get the signal generator from Amazon tomorrow and presumably it will be adjustable to whatever signal strength I set it at, so we can see what a 100mV signal will do.  Regardless, if it's clipping with a 50mV signal upon entry into the 2nd stage... that's a problem.  Right?   ...I just want to make sure I'm thinking correctly and any suggestions for how to diagnose or correct would be very much appreciated as always.  Thanks in advance.

[wittyjeff]

Oh, also note that I experimented with bypassing that gain pot to ground resistor to see if that would result in more clean room.  I guess it kinda did, but I didn't see any real benefit to it as far as reducing clipping at the 2 point in the gain pot rotation. So, while it maybe was able to go a bit lower, it still seemed to clip at the same point with our without that resistor bypassed.  Just wanted to note that for what it's worth.

[pdf64]

Assuming a bypassed cathode, as the Vpeak (ie Vp-p/2) signal level at the grid is raised from 0 and gets close to the cathode voltage, the positive going wave will start to clip.
So if the cathode is biased to 2V, the max unclipped signal at the grid will be about 4Vp-p, 2Vp, 1.4Vrms.
With an unbypassed cathode, it will be about twice that, and the clipping will be more rounded.
Partially bypassed will move between the above behaviours as frequency is altered.

I’m sure Merlin covers the 2 clipping mechanisms- grid clipping / conduction and anode current cut off, in the preamp book?

[wittyjeff]

Assuming a bypassed cathode, as the Vpeak (ie Vp-p/2) signal level at the grid is raised from 0 and gets close to the cathode voltage, the positive going wave will start to clip.
So if the cathode is biased to 2V, the max unclipped signal at the grid will be about 4Vp-p, 2Vp, 1.4Vrms.
With an unbypassed cathode, it will be about twice that, and the clipping will be more rounded.
Partially bypassed will move between the above behaviours as frequency is altered.

I’m sure Merlin covers the 2 clipping mechanisms- grid clipping / conduction and anode current cut off, in the preamp book?

Well... yes, Merlin does covers a lot of things in his book... doesn't mean I completely understand or remember all of them. ;)   Because of these issues and your help, I am forming (albeit slowly) an elementary understanding of how the signal is manipulated by the various stages and components. 
For example, I'm now seeing that despite "leaving" the first stage, and prior to "entering" the second stage (incorrect way to conceptualize it I now realize) the signal is affected AT the grid.  To demonstrate that for myself, I connected to the signal just after the capacitor after the first plate on V1 and then still at the grid of stage 2 of V1.  The signals are markedly different.  Even though the signal is "between" stages.  As stupid as that may be, it helps me understand that the clipping I'm seeing is due to the Second stage and not the first.  The signal after the first plate (i.e. "coming out of the first stage") is not clipped (at least not with the lousy 50mV signal input I have right now).  The first pict attached shows a comparison of the two signals at the point where the stage 2 grid signal (the one on top, probe 1) begins to clip on the top half of the wave.
Interestingly, when the gain pot is maxed (and probes set to x10) the stage 1 plate signal looks clipped at the bottom half of the wave, but the stage 2 grid signal is still clipped only at the top half of the wave.
Thoughts on this?

[wittyjeff]

Depending on what observations may be gleaned from my last post and the discussion so far, does any part the behavior of V1 look "off" or account for the "nothin but low volume mush" that I'm getting at the speaker?  If V1 seems to be behaving as Merlin intended or at least within reasonable expectations, perhaps I should move on to V2?  I need some specific ideas of where to look or what diagnostic procedure to perform next.

[PRR]

...20V with gain at max, this is with a 50mV input.....

That stage's gain is 50 or 55. So 50mV in makes 2.7V out. So something is wrong with your measurements.

Are there any other DC voltages in there? B+ supply? Cathode? (Grid??) 140V on plate is not wrong but without more data we can't be sure it is biased right.

[sluckey]

A sidebar for a few things about your scope...

First, which trace is CH1? That's always important for us to know.

Second, Alt/Chop appears to be depressed (in Chop mode). Chop is only useful when viewing very slow waveforms. About the only time you would use Chop mode in a guitar amp would be if you are looking a slow tremolo signals. Need to leave that in Alt mode for most viewing. (This button is disabled when viewing a single channel.)

Third, CH2 INV appears to be depressed. This inverts the channel 2 signal. Mostly should be left undepressed unless you have a specific reason to invert. I suspect you don't.

Fourth, TRIG. ALT appears to be depressed. This causes the scope to be triggered on CH1 for one sweep and CH2 on the next sweep, and so on and on. This happens even though you have selected CH1 as your source for triggering. This mode is rarely useful in guitar or audio amps. Leave it undepressed and then CH1 will supply all the triggers even when CH2 is being displayed. This allows you to see the phase inversion of CH2 in reference to CH1 display. For example, if triggering on CH1 with CH1 probe connected to the input jack, and CH2 is connected to the plate of V1, you will see that the plate is 180° inverted from the signal on the input jack. Viewing phase shifts as you trace through an amp can be very useful. This is a good reason to not use the CH2 INV button.

Bottom line, ALT/CHOP, CH2 INV, and TRIG ALT should be left in the undepressed position unless you know what they do and have a particular reason to use them. CH2 INV can be useful occasionally. One use would be viewing the two out of phase signals from the phase inverter. You can invert the CH2 display and overlay CH1 and CH2 traces to see how closely matched they are. Always keep in mind though, that you are only inverting the display, not the actual signal.

[wittyjeff]

...20V with gain at max, this is with a 50mV input.....

That stage's gain is 50 or 55. So 50mV in makes 2.7V out. So something is wrong with your measurements.

Are there any other DC voltages in there? B+ supply? Cathode? (Grid??) 140V on plate is not wrong but without more data we can't be sure it is biased right.

I got a new signal generator (albeit a cheap crappy one), so I believe I got it to be putting out a 100mV sine wave signal at 1000Hz (was a bit squirly at 500Hz for some reason).  I checked it with the scope.  So with the usual set-up (although with the various buttons un-pushed per Mr. S. Lukey's help) and the Probe 1 (at x1) and Voltage/div set at 5V per square, I'm seeing a 5V output at the V1 Plate 1 (but past the cap so there's no DC on it)... so I'm thinking I've got it set up right and I'm seeing/calculating things correctly now. (i.e. stage 1 appears to be providing a gain of x50) ...or am I?
Did you want me to also take the additional voltage readings at those various points and report them?

[wittyjeff]

A sidebar for a few things about your scope...
First, which trace is CH1? That's always important for us to know.

First, thank you for your patience above and beyond the call of duty.  I do technical support of various kinds in my line of work (disaster restoration and environmental remediation) and sometimes I think to myself, "where do I even start with this guy?"  I assure you that I wasn't nor am not completely high while working with my scope although in retrospect, the evidence does not support that.  Again, my thanks for the remedial scope training... I obviously need it.  I do think I was just pushing buttons until the scope made nice pretty traces, but didn't change them back... I see now that they were all jacked up.
So... CH1 is the top trace.

Bottom line, ALT/CHOP, CH2 INV, and TRIG ALT should be left in the undepressed position unless you know what they do and have a particular reason to use them. CH2 INV can be useful occasionally. One use would be viewing the two out of phase signals from the phase inverter. You can invert the CH2 display and overlay CH1 and CH2 traces to see how closely matched they are. Always keep in mind though, that you are only inverting the display, not the actual signal.

Got it, I think all the settings are correct now (check in photos below).

So, with things on the scope set up differently/correctly: CH1 is the top trace, set to x1 on the probe, Voltage/Div set to 5V, and connected to Plate 1 of V1 (but after the cap)... so basically just showing the stable signal flowing out of the first stage.  CH2 is the bottom trace, set to x10 on the probe, Voltage/Div set to 2 (so every square = 20V if I'm understanding how this all works); it is connected to Plate 2 of V1 (but after the cap)... so showing the signal after stage 2 and before the EQ stuff.  The 3 picts below are all set up this way and:
Pict 1 is the resulting traces with the gain set at 25%
Pict 2 is with the gain at 50%
Pict 3 is with the gain at 75%

So... It seems that V1 is just starting to send a clipped signal on to the next stage when the gain pot is turned to around 70%.  Doesn't all this seem to indicate that the first two stages (i.e. V1) is functioning within reason?  I haven't changed any components from the schematic, but I DO still have V2 bypassed completely.  Also I have that resistor to ground on the gain pot bypassed straight to ground, FYI.

[sluckey]

so I believe I got it to be putting out a 100mV sine wave signal at 1000Hz (was a bit squirly at 500Hz for some reason).  I checked it with the scope.

Show me a pic of your scope display with your probe connected to the input jack. Also want to know the VOLTS/DIV setting and whether 1X or 10X probe.

[wittyjeff]

so I believe I got it to be putting out a 100mV sine wave signal at 1000Hz (was a bit squirly at 500Hz for some reason).  I checked it with the scope.

Show me a pic of your scope display with your probe connected to the input jack. Also want to know the VOLTS/DIV setting and whether 1X or 10X probe.

Got it...  Pict 1 attached is of the signal generator probe (red) and the Scope probe (grey) attached to the tip terminal on the input jack (there's a dummy jack inserted so the jack doesn't go to ground).  Pict 2 is of the scope.  Probe is on x1 for sure and the Voltage/Div is on 50mV, so my read is that 2 squares should be 100mV.  Amp is not turned on.

[sluckey]

Pict 2 is of the scope.  Probe is on x1 for sure and the Voltage/Div is on 50mV, so my read is that 2 squares should be 100mV.

Except that your scope is displaying almost 4 divisions! That's 200mV_pp. That's twice as much signal as you think!!! This is why I've been questioning your scope skills.

[wittyjeff]

Pict 2 is of the scope.  Probe is on x1 for sure and the Voltage/Div is on 50mV, so my read is that 2 squares should be 100mV.

Except that your scope is displaying almost 4 divisions! That's 200mV_pp. That's twice as much signal as you think!!! This is why I've been questioning your scope skills.

No problem questioning my scope skills... since I have none.  So to calculate the voltage one counts from peak to peak, not from the baseline...?  So the entire voltage swing is what is considered "signal strength" then, ...correct?

[sluckey]

The entire voltage swing from one peak to the other peak is known as peak to peak voltage (V_pp)

[wittyjeff]

Got it.  So, pict 1 (below) is of a 100mV signal then.  Probe@ x1, Voltage/Div at 50mV and a swing of two squares.  Correct?
Pict 2 is of that 100mV signal with Probe 1 (set at x1) connected again at V1 Plate1 after the cap (Volt/Div set at 5V) and Probe 2 (set at x10) on V1 Plate 2 after the cap (Volt/Div set at 2V).  Gain turned up just until the beginning of clipping.  About 50%.
Thoughts?

[PRR]

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