Voltage Dividers vs Bias

[roberto]

Without any schematic we are guessing sex of angels.
I would tame highs on second stage, if you go towards the crunch, and tailor the frequencies if you keep it clean.

[Littlewyan]

Apologies for the incredibly delayed response!

I have been tweaking this amp over the past two weeks and have now got the Ultra channel (4 gain stages) perfected. I went through every part of that circuit and it's near enough ended up as a modded JVM, but that's just how things go sometimes. I didn't intentionally go in that direction and the amp started out quite different. However, I still cannot get the Classic Channel to sound right! Hopefully you can kind of work out from this 'schematic' that it's basically an SLO setup, where a Clean/Crunch switch switches in a 470K/39K divider before the 2nd stage and both channels get switched in and out at the 330K grid leak before the final gain stage.

In its current state, there just isn't enough low end or gain on the classic channel. My goals for this channel are to have a Plexi style crunch and the ability to switch to a completely clean tone. Modifying the first and final gain stages are out of the question as this affects the Ultra channel. So over the past two weeks I've been experimenting with different voltage dividers and biasing on the 2nd stage, but so far haven't achieved my goal. I'll update this thread if I make any breakthroughs.

[jml77]

I think you could increase the cathode bypass cap on the first stage, at the moment it's only partially bypassed and whilst this cuts the bass on the gain channel, it does the same for the clean. Some texts recommend fully bypassing the first stage, for best noise rejection.

I think you'll find that the gain channel won't suffer too much as subsequent stages have small bypass caps. You could always make the bypass switchable if you want to keep a small bypass for the gain stage, but I think you'll find the fully bypassed 1st stage satisfactory.

edit: to add: You might not need quite so much attenuation on the clean second stage.

[Littlewyan]

Thanks for the suggestion. I've tried this a few times, but never liked the result. I tried both increasing the Cathode Bypass Cap and the Coupling Cap (not at the same time), but unfortunately both affected the Ultra channel in a negative way. The sound starts to get a bit fuzzy where there's too much bass getting distorted. Whatever I do it will have to be with the divider after the 2nd stage or the 2nd stage bias itself.

It may seem like a lot of attenuation, but without it, when the channel is in clean mode, I can't get the gain up past 5 without the sound starting to break up! One idea I had earlier which seems to help, is to change the R33 resistor to 1Meg. This means I'll be able to up the gain of the crunch mode a bit more without losing too much clean headroom, either by tinkering with the 2nd stage or the voltage divider. More experimenting is required!

[jml77]

Why not put it on a switch.

for example the ampmaker nsx kit shows 2 stages with switched bypass caps on the first 2 stages.

http://www.ampmaker.com/images/ak01kit/n5xsch2.jpg

[jml77]

Something else you might like to try, if you like experimenting, is to use a second stage with feedback from the plate to the grid.

That way you can select the gain you want, without resorting to attenuators after the stage.
There are pros and cons, as always. The gain is approx = Rf/Rin, it can have low output impedance (good), but it can also have a low input impedance (maybe not so good) which can interact with the output impedance of the preceding stage, thus making the gain less than desired.
It's also best not to overdrive the stage, as that will break the feedback loop causing a sudden increase in gain and distortion.

Aiken has a good break down of how to design a feedback stage. As does Merlin valvewizard.
https://www.aikenamps.com/index.php/des ... amplifiers
http://www.valvewizard.co.uk/localfeedback.html

I've used a feedback stage as the 3rd stage with a gain of ~5, in a 4 stage preamp. It was not the best choice of stage, but it has worked quite well, as long as it isn't overdriven. I think you would find it less of a problem being the second stage.

I would also experiment in reducing the treble peaking/shelving between stages, that might be causing some fuzzyness on the clean stage.

[roberto]

Instead of switching the load on the gain pot, switch the stage in and out: à la Marshall 1959 clean or 2203 crunch.
As a starting point configure it à la Marshall, or check what Bogner did on his XTC.

[Littlewyan]

I have made progress! With inspiration from your suggestions jml77 and the Friedman Dirty Shirley. I swapped R49 and C30 for a 1Meg/250pF combo, then put a 470K resistor across the 1Meg Gain Pot. This gave me the cut I needed on the right frequencies.

For the divider after the 2nd stage I've just used 390K/330K and have got a great almost boosted JTM45 type tone. As luck would have it, the clean tone is great too! I need to experiment more, but I'm thinking of messing with the 2nd stage bias to give me more gain (220K Anode perhaps) and changing R33 to 1Meg to keep the clean tone safe.

I'll keep you updated.

jml77 - I've thought about the LNFB stage, but that 2nd stage does get overdriven. Would be good to experiment with to see how it sounds though.
roberto - I've considered this too, but with how the amp is setup this wouldn't be easy. I have looked at the Bogner XTC, very nice sounding amp. I'll create a new thread and post photos at some point.

[Littlewyan]

Here is the final schematic. For now .
Classic channel is great. Nice balanced clean tone, then when you flick it into crunch mode you get a good boosted JTM45 type tone. Admittedly it did sound better with a 470K in the divider switch instead of 1Meg, but with 470K I wasn't getting enough range on the gain control. So bit of a trade off there.
The Ultra channel is pretty good, but does have room for improvement. Pretty much a boosted Marshall type tone. Could MAYBE do with a tad more compression, maybe changing the 2nd stage Anode Resistor to 220K, but that's something I'll save for a later date. For now, the chassis is back in its cabinet for the first time since Christmas, so I'm just going to play!

Interesting note. On the Ultra channel I have a relay that grounded the channel at the point of the 220K/220K voltage divider on the 2nd stage, but with this setup I found I still had bleed into the Classic channel. I moved the ground to the top of the divider where the coupling cap is and this solved that issue. Not 100% sure why though.

[Littlewyan]

One note about the Ultra channel, it's pretty unusable with the gain on 10. I was thinking of perhaps trying a 1Meg or 470K in parallel to the gain pot to solve this.