Alternative FETs for ROG britannia

[feddozz]

>This is how early (and some modern *copies*) amps were set up

I see... Thanks for the explanation.

>If you don't like no sound, don't do that.

The initial reaction was: "there must be something wrong with my build". 
In addition, I made a mistake and installed a linear gain pot so it was difficult to get a clean sound even at very low pot angles. Combined with the non expected behaviour of no-sound it was casting doubts on the godness of my build. You will all remember what being a newbie is like, there is no experience to rely on, no reference points. With the right pot it is another story.


I measured the lowest resistance I get with the log pot and it is easily lower (2 Ohm easy) than the smallest resistor I had available (10 ohm). So it's cleat that if I put the 10 Ohm in, I would never be able to get lower than that.
Now that I know more about it and I have experimented with the circuit I decided to leave it as is. Please do not take this as indeciveness, it is just part of the learning I am going through with this circuit.

Something interesting, at least for me, is that teh 100K Ohm log pot does not get higher than 85K Ohm. Tha looks a bit too much error to me.

So now I wonder if I am getting full gain from the circuit.

>However maybe the build is complex enough already?
It is complex. Believe me a further resistor would have gone unnitced...

Little OT. I am really enjoying the learning I am getting from this post, thanks everyone for the contributions.

[duck_arse]

feddozz - it's not the lowest resistance of the pot you're worried about. turn the pot down to the lowest usable volume level. switch off, measure the resistance from wiper to ground. that is the resistance, to the nearest standard value, you want to put under the pot, to ground. then, no matter how hard you turn the knob, the pot won't ever go below your min volume.

and, the pot value in the gain pot position doesn't really matter. it is wired as a voltage divider, so if you put a 10k pot in, the full signal volts from the preceeding stage appear at the top/hot/CW end. if you put 1M, the full volts appear at the top/hot. the slight difference in your pot may have a minor effect on the high-bypass cap response, or it might be the perfect invitation to stick 15k as your bottom stop resistor.

[PRR]

> 100K Ohm log pot does not get higher than 85K Ohm. Tha looks a bit too much error to me.

Pots are low-tolerance. Especially audio-taper pots. 15% is not unexpected. The wise circuit designer already considered this.

> So now I wonder if I am getting full gain from the circuit.

The 85K is loading a source (Q1 trim) which is less than 5K. The difference in gain is less than 1%, much-much less than you can hear (10% - 5%).

Anyway I _suspect_ this chain-of-gain has plenty to spare. That you would "never" run both Gain and Vol full-up. (Or if you did, ears would bleed.)

[feddozz]

Hi
I boxed the Britannia but I noticed it never reaches unity, even with all at max.

I am planning to use an audio probe  and report on the voltages.
Are there any obvious things I should check for this kind of problem? I'll try to post a pic of the circuit.

One other thing I noticed is that the biasing voltage of Q1 does not go below 6.7 volts. I replaced the transistor but the minimum was 7 with a full rotation of the trim. Is this suspicious?

Update:
I cheched the voltages of the TL072
1- 7.45
2- 1.28
3- 3.62
4- 0
5- 1.95
6- 3.98
7- 3.99
8- 8.00

I fount some voltages reported on tagboardefects as follows
1- 4.21
2- 4.21
3- 3.8
4- 0
5- 2.09
6- 4.21
7- 4.21
8- 8.43

I do not know what that means but they seem different than mine.

Thanks in advance

[bloxstompboxes]

At just a glance, the voltages on pins 1 and 2 are really the only ones that appear different, if you consider the difference related to battery power indicated by pin 8. I would check portions connected to pins 1 and 2.

[feddozz]

Found amissing link in that area, hope that's all. Thanks