Hi-Gain Circuit - Affect of a lower value gain pot

[Darkbluemurder]

The amp in question has a 500kA gain pot after the first gain stage (the amp has four gain stages in total). There is a 470k resistor going from the pot input to ground. There is a 1200 pf cap going from the pot output to ground (maybe the designer wanted to balance the increased highs with the gain pot turned up full?).

I am thinking of swapping the 500kA pot for a 250kA pot. I believe this would have the following effects:

1. The low frequency bypass created by the 1200 pf cap would shift upwards.
2. The series resistance would significantly decrease therefore making the amp clearer as well. This would especially be the case at lower gain settings.

What do you think? If my thinking is correct this would be a worthwile change since the amp is a bit too midrangey and lacks the openness I would like to hear.

Thanks for your input.

[Firestorm]

If I understand you correctly, it sounds like the amp has a rudimentary band-pass filter, with the high-pass turnover set by the interaction of the coupling cap from the first gain stage with the parallel 470K resistor and 500K pot, and the low-pass turnover set by the interaction of the resistor and pot with the 1200pf cap. Just looking at the low-pass side (since you didn't mention what the coupling cap was), it would seem that the filter is designed to attenuate higher and higher frequencies as the gain is turned up (decreasing the resistance in front of the 1200p cap). Decreasing the pot value would likely cut even higher frequencies. The series resistance could be smaller (depending on the pot setting), but not by a whole lot. With the 470K resistor in parallel with the pot, a 500K pot is already essentially a 242K pot and a 250K pot would become a 163K pot. It's a little more complex if it's a log pot, but the principle is the same.

[John_P_WI]

Without a schematic or knowing the following stage values, tone stack location and the interstage attenuation setup etc, it is difficult to to say how much would be gained.

I have the feeling that you would not get that much change. Basically the 470k r and 500K pot paralleled is 250K. You possibly would sense a more accurate feeling in the "taper" using the new pot.

I don't understand the cap to ground from pot output. If you study the rc frequency responce it changes wildly with the wiper position, which does not seem like a good idea. I'd either remove the cap or rewire it as a "bright" cap, give it a go even though it is 10 times higher value than a standard "bright" cap value.

If this is the only volume or drive control in a 4 gain stage setup, you may be better served moving it between the 2nd and 3rd gain stage. Once again more information would help.

John

[Darkbluemurder]

More info on the circuit:

Coupling cap from the first stage is a 1500pf. Plate resistor is a 220k.

Output from gain pot goes to a 470k grid stopper into the grid of the second gain stage. This stage also has a 220k plate resistor.

The amp has a level control between the third stage and the fourth stage. After the fourth stage there is a cathode follower, then the tone stack.

I calculated the low pass frequency knee as follows:

1. 500kA pot turned up full

242k in parallel with 1200pf = 548 Hz

2. 250kA pot turned up full

163k in parallel with 1200pf = 814 Hz

Series resistance is the same in both cases (zero resp. 470k to the next gain stage).

3. 500kA turned up halfways (450k series and 50k to ground)

45k (50k in parallel with 470k) in parallel with 1200pf = 2.95 kHz
450 k series resistance (2.3 kHz roll off)
If you take the 470k grid stopper into account, then 920k = 1.15 kHz (considered a miller capacitance of 150pf in the tube).

4. 250kA turned up halfways (225k series and 25k ground)

24k (25k in parallel with 470k) in parallel with 1200pf = 5.5 kHz
225 k series resistance (4.7 kHz roll off)
With 470k grid stopper = 695k = 1.5 kHz.

If my calculations are not flawed then I agree there would not be much benefit with the pot turned up full. 810 Hz appears to be hardly better than 550 Hz. The benefit would increase with the pot turned up halfways because then the R/C roll off and the series R roll off rise. 5.5 kHz should make a significant difference compared to 2.95 kHz, and 1.5kHz compared to 1.15kHz may also make a difference. Of course the low pass filter will make a bigger effect with the pot turned up.

I have seen a schematic of that particular amp that specifies a 250kA pot together with a 1500pf cap but my amp has a 500kA pot and a 1200pf cap. I am a bit reluctant to mention the model's name because I am not sure whether it is OK to discuss this particular amp.

[ampdoc1]

Gosh,

Are discussions censored now? I know there are some schematics that Omar doesn't want posted because they are current models, and nobody wants to turn this into a negative rant about particular models. But a limit on discussing component values/modifications on any amp, new or old? I thought exchanging information like that is exactly what this site is for.

ampdoc

[groovtubin]

The amp in question has a 500kA gain pot after the first gain stage (the amp has four gain stages in total). There is a 470k resistor going from the pot input to ground. There is a 1200 pf cap going from the pot output to ground (maybe the designer wanted to balance the increased highs with the gain pot turned up full?).

I am thinking of swapping the 500kA pot for a 250kA pot. I believe this would have the following effects:

1. The low frequency bypass created by the 1200 pf cap would shift upwards.
2. The series resistance would significantly decrease therefore making the amp clearer as well. This would especially be the case at lower gain settings.

What do you think? If my thinking is correct this would be a worthwile change since the amp is a bit too midrangey and lacks the openness I would like to hear.

Thanks for your input.

Using a PF cap to ground at ANY wiper to ground, is a FALSE way of getting INCREASED bandwidth, hi-end roll-off and compression, properly designed, it would be much better w/o that cap, i ASSURE you!

jim

[Darkbluemurder]

The amp in question has a 500kA gain pot after the first gain stage (the amp has four gain stages in total). There is a 470k resistor going from the pot input to ground. There is a 1200 pf cap going from the pot output to ground (maybe the designer wanted to balance the increased highs with the gain pot turned up full?).

I am thinking of swapping the 500kA pot for a 250kA pot. I believe this would have the following effects:

1. The low frequency bypass created by the 1200 pf cap would shift upwards.
2. The series resistance would significantly decrease therefore making the amp clearer as well. This would especially be the case at lower gain settings.

What do you think? If my thinking is correct this would be a worthwile change since the amp is a bit too midrangey and lacks the openness I would like to hear.

Thanks for your input.

Using a PF cap to ground at ANY wiper to ground, is a FALSE way of getting INCREASED bandwidth, hi-end roll-off and compression, properly designed, it would be much better w/o that cap, i ASSURE you!

jim

That's what I thought as well but maybe the designer put it in for a reason. One thing confuses me though. From my experience I can understand that putting a cap to ground imparts a high freq roll-off and compression but increased bandwith? I thought a frequency roll-off decreases bandwith.
It will be probably the easiest way to check if I just desolder one leg of the cap and isolate it. Then I can put it back in if the amp gets too bright.
Thanks for all your help and input.

[roberto]

The cap to ground on gain pots is a way to have multiple positive effects:
at lower gain settings it has quite no influence on the sound, on middle to higher settings it stops squealing highs like a grid stopper, but without the hard-sound of stoppers at low gain settings.

PS
freq rolls-off on halfway pots are wrong if the 470k is // to the pot. For the 500kA is 50k//(450k+470k), not 50k//470k.

PPS
pay attention to the fact that if 250kA may have less noise than 500k, the load of the first stage could be too low, and the low load can bring to more highs in the next stage.