proper voltage at PI input ?

[daz]

On an amp like mine, which no doubt everyone here is familiar with if not downright sick of hearing about, is there a AC voltage range i should be looking at at the input of the pre PI master and then at the PI input depending on where the master is? I just want to verify that i don't have way too high a signal because my amp is insanely loud and it gets there by the time i would say the master is at 10:00. So whether at the points i mentioned or wherever, what should i look for?

On a side note, do you guys use the scope to arrange lead dress? I been chopsticking the amp while looking at the waveform and really noticing where certain wires should be move. The worse offender being the input. Good lord, move a power wire within about an inch of it and the waveform falls apart completely ! Makes me wonder if it would be a good idea to shrink wrap the 68k grid R and have shielding over it right up to just shy of the socket pin. Can't help but wonder if some sort of nastiness might be finding it's way to the input before the waveform even starts changing.

[J M Fahey]

What does an amp like mine mean?

post schematic, at least of power stage.

[Alan0354]

I do not use the scope for arranging the wire. If you do the layout correctly, there shouldn't be any power wire close to the input area. I keep referring back to the good old Fender BF and SF. Their placement of components and power distribution works like a champ. They don't do one point ground, they ground the preamp area mostly onto the chassis close by, power away from the input area. The circuit is progressive from most sensitive input on one end of the chassis and propagates down to power amp on the other end of the chassis. Power wires are away from the input and tone stack on the other side of the fiber board. It's a good layout.

I just did a high gain amp using chassis as ground plane and follow a lot of concept of Fender, not any hum to report, I intentionally not using rectified DC filament supply to make a point. When I crank the amp up, I can only hear the hiss. AND it's still lower than my distortion pedal. Noise is not even a hint of issue. I spent time writing the post in this thread about noise, avoid loop and everything will be better:

http://music-electronics-forum.com/t35745/

[daz]

What does an amp like mine mean?

post schematic, at least of power stage.

This is it except the 820R on the 3rd stage cathode is a 1.5k and the 250 pf on the gain pot is a 500pf

[J M Fahey]

I just want to verify that i don't have way too high a signal because my amp is insanely loud and it gets there by the time i would say the master is at 10:00.

Well, you *do* have a ton of signal at that PI input ... for the very good reason that you have way too much gain ..... unless you want a Metal amp, that is.
A classic Plexi or Tweed Bassman would have input stage > volume > cathode follower tone stack driver.
You add a full gain stage , some 50X real gain in the path.
And start with Marshall style NFB , which is quite low, (compared to typical Fender) and lower it even more with a 250K series pot.

So, it´s doing exactly what you built it to do.

[daz]

So, it´s doing exactly what you built it to do.

Well, no, not really. That wasn't my intention. My intention was to create preamp distortion that sounded liek i wished my marshalls had sounded like, The power section wasn't meant to be over loaded with signal, but that was the result. So it's like this....i cannot change the signal level to the PI by modding the preamp because the tone will change radically. I've tried everything known to man, but the fact is that whether right or wrong in theory, the way the preamp is configured is giving me what i want a a tiny change will change that.

So my job then is as follows. Reduce the signal into the PI retaining the preamp tone i'm getting now while rendering the PA more linear and clean. At the moment i put a 1M/100k (1M is series) voltage divider before the master, but it seems like having two voltage diviers one after the other is not a proper design. I have done that before but it i think it affected the tone. The amp was different then tho. I think now due to recent changes it may work, but i'm not sure it's ideal. Any suggestions from the tone stack on are welcome. On the scope it's worlds better with that VD there. The waveform stays about the same all the way to max on the master. I think what i will do is put a pot as variable R in place of the ground resistor in the voltage divider and find a value that allows the waveform to change as it maxes out. Not sure what to look for tho. I know it should change or it's not compressing, and i would like it to begin to compress some if not totally distort. Before the wave would turn squarish looking by the time it got 1/3 up.

If someone could post a wav of a maxed marshall i'd love to see what the output should look like on 10 to give me a reference to shoot for.

[Dave H]

So my job then is as follows. Reduce the signal into the PI retaining the preamp tone i'm getting now while rendering the PA more linear and clean.

Just turn the master down then because that's its job That's what it is there to do. It doesn't matter that it's set to 10:00 or even 8:00 as the pre-amp is giving you the tone you want. Make sure it's a (10%) log pot not a linear one so it will give 40dB or more attenuation and still have control.

[daz]

Just turn the master down then because that's its job That's what it is there to do. It doesn't matter that it's set to 10:00 or even 8:00 as the pre-mp is giving you the tone you want. Make sure it's a (10%) log pot not a linear one so it will give 40dB or more attenuation and still have control.

thats not the point. It's like on a mixer where you have a channel volume AND a channel gain that sets the range of the volume depending on the input signal. If the input signal is too hot the channel will distort even at the volume slider's lowest setting if the gain is too high. So you reduce the gain and now the volume slider's range is proper and no distortion along it's range. Thats whats happening here. the input to the PI is much too hot and it's distorting the PI in a way that hurts the sweet preamp distortion tone

This appears to work well even tho with a second divider the treble is cut down. But thats a good thing in this case except maybe at low levels. Not sure yet, as i don't have my speaker here and plugged into a very unfamiliar one.

[Alan0354]

I was actually trying to calculate the at what signal the power amp clips as the gain is not defined by NFB because your loop gain is too low and using NFB is not accurate.

From reading this morning, your concern is you don't want the sound of the power amp and PI distortion come into play to your sound. You want the sound to be from the preamp. Since the PI is part of the feed back circuit, it's better to look at the the PI/power amp as one big opamp with negative feedback.

From my understanding, there is an easy way. This is a NFB, the input voltage range of 12AT7/12AX7 tube is not that important. Treat the PI and power amp as one big opamp and just look at the point when the power amp clips. You want to map the input range to the power amp clipping range but setting the max input signal from the preamp when the power amp just starting to clip to ensure the power amp is not being driven to clipping and produce the clipping sound of the power amp instead.

Use a 2 channel scope and hook to both input of the PI ( +ve and -ve). Increase the master volume and find the point where the -ve input ( NFB summing point) stop tracking the +ve ( from preamp). That is the point the power amp saturates. You need to put resistor divider to match that point when volume is at full.


Another thing, you set the closed loop gain very high, you use over 100K to feed back and the gain setting resistor is only 4.7K with the presence control that tries to up the gain at high frequency even more. Another way to match is to reduce the feedback resistor to lower the close loop gain a little, that will increase the linear input range of the PI/power amp section. Adjust the feedback resistor so the Power amp clips when the master volume is max.

[Dave H]

Thats whats happening here. the input to the PI is much too hot and it's distorting the PI in a way that hurts the sweet preamp distortion tone

I don’t understand. I thought you were playing at low volume. There is no post PI master. If the PI input is being overdriven the power amp will be an almost flat out square wave. The PI input can’t be overdrive at any volume below full power output. If all you are worried about is that the position of the master vol knob is at 8:00 but it’s still too loud then use your attenuator before the master or better still change the master to 100k and put a 900k resistor in series between its CW end and the tone stack. The impedance will then be unchanged from the circuit you posted above.

[daz]

I don’t understand. I thought you were playing at low volume. There is no post PI master. If the PI input is being overdriven the power amp will be an almost flat out square wave. The PI input can’t be overdrive at any volume below full power output. If all you are worried about is that the position of the master vol knob is at 8:00 but it’s still too loud then use your attenuator before the master or better still change the master to 100k and put a 900k resistor in series between its CW end and the tone stack. The impedance will then be unchanged from the circuit you posted above.

Well, all I can tell you is at the same playing volume with the attenuation in place before the master, the tone is farbetter, as tho there is oscillation happening or some such thing. So whatever you wanna call it, there is a nastiness happening in the PI without that attenuation that isn't there with it, and thats while playing at the same volume. If i play at very low volume with either scenario its fine. But i don't want an amp that only sounds good at bedroom volume and not even a little above that. I don't gig anymore, but on rare occasion i do and i need an amp i can use on those occasions.

[silverfox]

Curious Fox Says:

I would be interested in hearing what this amp sounds like. Do you have a link for it somewhere? I ask only as an interested party. Not a critic.

Regards,

Silverfox.

[Alan0354]

Well, all I can tell you is at the same playing volume with the attenuation in place before the master, the tone is farbetter, as tho there is oscillation happening or some such thing. So whatever you wanna call it, there is a nastiness happening in the PI without that attenuation that isn't there with it, and thats while playing at the same volume. If i play at very low volume with either scenario its fine. But i don't want an amp that only sounds good at bedroom volume and not even a little above that. I don't gig anymore, but on rare occasion i do and i need an amp i can use on those occasions.

More reason to scope the signal. I think you might have more than clipping of the PI or power amp. Yes, the sound change a little when cranked up, but usually don't get ugly!!! I experience with tube microphonics with cascaded gain stage as if the whole circuit is feeding back. Never been able to pin point the problem, ended up putting small caps from signal to ground to stop that.

[daz]

More reason to scope the signal. I think you might have more than clipping of the PI or power amp. Yes, the sound change a little when cranked up, but usually don't get ugly!!! I experience with tube microphonics with cascaded gain stage as if the whole circuit is feeding back. Never been able to pin point the problem, ended up putting small caps from signal to ground to stop that.

I have scoped it. But i just got the thing recently and all i know of the waveforms i see is whats dead obvious. I can see that when i turn the master up the wave becomes square. But with attenuation in front of the master i can adjust it to have just enough gain to where the wave looks a little more exaggerated than it is which i assume is good and stops there when maxed. Yet even then it's plenty loud.


No, no clips

[Alan0354]

I have scoped it. But i just got the thing recently and all i know of the waveforms i see is whats dead obvious. I can see that when i turn the master up the wave becomes square. But with attenuation in front of the master i can adjust it to have just enough gain to where the wave looks a little more exaggerated than it is which i assume is good and stops there when maxed. Yet even then it's plenty loud.


No, no clips

Where it become square? At the input of the PI, or at the output of the power amp?

You need to match the input dynamic range of the power amp to the output dynamic range of the master volume:

1) Input dynamic range is the range where the output of the power amp is not clipping. It is defined as the max input to the power amp( to the PI stage in your case) where the output of the power amp just enter into clipping mode. It is the range that the power amp response in a LINEAR way with the input.

2) Output dynamic range of at the master volume is the range of output at the master volume wiper out with the proper load( input impedance of the power amp).

I know this sounds very redundant like it is so simple. But this is a standard method of mapping multiple stages to get the most dynamic range of the system( your amp). We have to recheck this all the time in electronics because if you are not careful, it is easy to get off the optimal matching and creating bottle neck and limits the system dynamic range.

You said you don't want to over drive the power stage that the distortion of the power amp taking over. You want to have the distortion totally comes from the preamp. This is THE way to do this. You want to either

1) lower the gain of the power amp by changing the closed loop gain of the power amp so the power amp starts to clip at the input level where your master volume is at 10. OR

2) Use voltage divider before the master volume to reduce the output of the master volume so you barely drive the power amp to clipping when you turn the master to 10.

This is called matching the input and output dynamic range of the different stages. This is so simple and obvious that a lot of people over look it. This even remind me I have to do this myself!!!! I can talk a good game, I notice the sound of my amp change quite a bit when I crank it up. I need to do it myself!!!! Hope this help. Sure help me!!!