Took some preamp signal measurements

[Fischerman]

I took some crude preamp measurements and looked at it on my scope...I have a Word document (with tables and imbedded VISIO sketches) on it but don't know how to post it.

Anyway, one thing I found was that when I put in a 100mV 880 Hz signal with PAB on and mid-boost off, at a Volume setting of about 6 or so, the waveform at V2a's grid was just starting to pull grid current and clip the top of the waveform. Measuring the AC voltage on that grid it measured 1.7vac. The idle DC cathode voltage of V2a is 1.7vdc. This is right where the Volume control hits its 'sensitive' zone which I assume is normal. At Volume setting of say 5.5 or so...V1b's plate is totally clean and the signal at V2a's grid is totally clean. At say 6.5 or so, V1b's plate is ever so slightly 'slanted' at the bottom and V2a's grid is slightly clipped on top. I'm guessing that the slight 'slant' I see on the bottom of V1b's plate is due to V2a drawing that grid current? In other words, it's not really V1b that's distorting?

I measured the DC voltage of the grids and the 'other' side of the grid stoppers with Volume on 6.5 and Drive on 8: V2b is at -8.8vdc with that 100mV signal at the input and the other side of the 150k grid stopper is at -3.7vdc so it appears that some major grid current is happening there.

And FWIW, it seems I liked the tone best when I backed off the Volume to just before that clipping on V2a's grid occured...or maybe right at it. VERY easy to go just past the sweet spot.

[Normster]

Very interesting. I'd love to see the word doc. (Zip it first and it'll allow you to attach to your post.)

[Fischerman]

OK, let's see if this works.

[Fischerman]

That little note about the HRM bass control was strange to me. The tops and bottoms of the square-ish waveform on V2b's plate would flatten out when I zeroed the HRM bass control (and I would lose a lot of 'amplitude')...at any setting above zero it jumped to the slanted tops/bottoms.

[Fischerman]

Also, my OD entrance is a slightly altered low-gain network...it goes 470k/47p-180k-25k pot- 22k-ground. The pot is set at about 45%.

Another unrelated finding: I only have a 100k-L and a 500k-A that will fit in the little hole for the Send Level (only an Alpha will fit). I know I need 250k-A for this built-in Dumble-ator but none onhand will fit. So I tried both values and the 100k just castrated the amp. The 500k had way more clarity/headroom/sparkle/punch and was way more transparent. I assume this is due to the loading or something because it wasn't the 'setting' of the control...it was the total resistance that seemed to make the difference.

[Bob-I]

That little note about the HRM bass control was strange to me. The tops and bottoms of the square-ish waveform on V2b's plate would flatten out when I zeroed the HRM bass control (and I would lose a lot of 'amplitude')...at any setting above zero it jumped to the slanted tops/bottoms.

That's really intersting, and may explain why the bass control seems to "jump" as soon as you move it off the 0 setting.

[Bob-I]

Also, my OD entrance is a slightly altered low-gain network...it goes 470k/47p-180k-25k pot- 22k-ground. The pot is set at about 45%.

That 22k tail might explain the oscillation at higher settings. That's lifted too far above ground. If you can get that down a 4.7k you might have better response to the more normal settings.

Another unrelated finding: I only have a 100k-L and a 500k-A that will fit in the little hole for the Send Level (only an Alpha will fit). I know I need 250k-A for this built-in Dumble-ator but none onhand will fit. So I tried both values and the 100k just castrated the amp. The 500k had way more clarity/headroom/sparkle/punch and was way more transparent. I assume this is due to the loading or something because it wasn't the 'setting' of the control...it was the total resistance that seemed to make the difference.

Excellent observation. It might also have something to do with the Miller capacatance and the way the 500k reacts to this.

[Fischerman]

I don't think there is any oscillation...the amp doesn't behave like there is any. The 22k tail and the 180k resistor 'above' the pot just change the range of the pot while keeping the total resistance to ground of the entire string the same. So instead of 470k+200k+25k+4.7k I have 470k-180k-25k-22k (should be 24k but I don't have one). When dialing in the OD trimmer I was always at the upper end of the range so I decided to move that range up a little bit (this little mod was a suggestion from dogears). With the 'stock' values and OD trimmer maxed the wiper of the trimmer is at @ 25k+4.7k from ground and right now I'm at 22k+10k (I'm guesstimating that 10k...the 25k trimmer is at about 45%). I changed it to these new values in order to get better response at the more normal settings. It's not a huge increase...it's effectively just a 'stock' OD trimmer 'set on 11'.

I'm guessing that the thing with the HRM Bass control is due to the AC loading of the output of V2b. With the Bass control on zero the output of V2b is heavily loaded.

[Fischerman]

The schematic with actual measured voltages.

[img:800:612]http://i108.photobucket.com/albums/n4/m ... oPre-2.gif[/img]
[img:800:645]http://i108.photobucket.com/albums/n4/m ... oPower.gif[/img]

[Fischerman]

It might also have something to do with the Miller capacatance and the way the 500k reacts to this.

I don't follow you here. This pot is on the output (cathode) of the Send triode...how does Miller capacitance come into play here? I think of Miller capacitance as 'plate-to-grid' and usually in the context of the input signal on the grid.

[dogears]

Bob, that is not it. All Matt did is alter the range of the stock setup. I have a 24K in my amp fwiw.

Also, my OD entrance is a slightly altered low-gain network...it goes 470k/47p-180k-25k pot- 22k-ground. The pot is set at about 45%.

That 22k tail might explain the oscillation at higher settings. That's lifted too far above ground. If you can get that down a 4.7k you might have better response to the more normal settings.

[Bob-I]

It might also have something to do with the Miller capacatance and the way the 500k reacts to this.

I don't follow you here. This pot is on the output (cathode) of the Send triode...how does Miller capacitance come into play here? I think of Miller capacitance as 'plate-to-grid' and usually in the context of the input signal on the grid.

We're both lost now.

The trim pot is on the grid of V2A. Miller capacitance can affect the cutoff point of the stage. Higher input resistance raises that cutoff. Check out the Aiken article on this topic.

http://aikenamps.com/MillerCapacitance.html

[Fischerman]

OK...we're on two different pages Bob. In one case I'm talking about the OD entrance and in the other I'm talking about the Send Level pot for the FX loop (Dumbleator).