NAD: CJ's Cavalcade of New Smallish Cheapish Amps - #2

[movinginslomo]

Thanks for posting these reviews, I'm in the market for a cheap low wattage small gig/open mic amp

[culturejam]

Okay, I'm going to need to walk back a comment I made about the 0.1W setting. When I first listened to it, I had just done 20 minutes of playing/listening in the 5W setting. So of course it sounded quite and like all the highs had been chopped off.

Listening again tonight with fresh ears, it's actually decent sounding. Loud enough for having a little solo fun, definitely. And without having just had your ears pummeled with full 5W volume, the highs are decent enough.

Also, I swapped out the JJ EL84 I had in there with a reissue Tung-Sol. I actually like it better.

[cooder]

Looks interesting, have fun modding it! Looking forward to hear what you come up with and find useful.
Here's a good general easy terms write up of modding amps with some suggestions, could be useful.
Attached pdf.

[culturejam]

Looks interesting, have fun modding it! Looking forward to hear what you come up with and find useful.
Here's a good general easy terms write up of modding amps with some suggestions, could be useful.
Attached pdf.

Awesome, thanks!

[cooder]

Another great site of wisdom is
https://robrobinette.com/Amp_Stuff.htm
with a bunch of mod tips like Marshall stuff:
https://robrobinette.com/How_the_Marshall_JCM800_Works.htm

and of course Merlin / Valvewizard:
http://www.valvewizard.co.uk/

[thomasha]

That is definitely tone-per-buck worthy! You can't build the same amp for this price.

I also build a small amp in a lunchbox using an ECL86, and it was loud enough. There was not that much distortion on tap though.

I built something with the PCF802 and a PCL82. Heaters in series and high voltage from a quintupler voltage multiplier using only caps and diodes. Really cool for what it is. The triode in front of the pentode overdrives it pretty nicely. The PCL82 is much cheaper than the ECL82 or the ECL86.

The matchless circuit would be interesting to give a try.

[jimilee]

As promised, I am becoming the JimiLee of small cheapish amps. 

I felt another disturbance in the force. It's nice to be able to do things like this isn't? Those look great man.
   I didn't tell you guys I bought a fender ash body tele, got a great deal on it. I also traded for another G&L tele and a red fender mustang. Good times!


Sent from my iPhone using Tapatalk

[culturejam]

I've done some more listening of this amp. I'm really liking it better and better. All cranked up, it's got gain in the range of what I would call "Classic AC/DC". There's some great breakup, but it's pretty dynamic and uncompressed. Sounds great with humbuckers, but I haven't tested it with singles yet (only coil-cut on my PRS).

There's not a lot I want to change about the tone, but I think I'm going to try two simple mods that will allow for both more and less clean headroom:

1) First is to toggle the series input resistor value. Stock is 68K, so my plan is to use a DPDT center-off and have  two parallel value options (say, like 33K and 10K as the final value). This gives stock, and two levels higher gain.

2) Then I want to change the bypass cap value on V1a. Also using a DPDT center-off, I will pull the stock 22uf from the board and move it to one throw of the switch, and have something like 680n or 1uf on the other side. So that would give two boosted options and no bypass cap for minimum gain.

Optional: I'm considering doing negative feedback as well, but I'll do that if I don't think the other two mods makes enough difference on the clean headroom front.

[cooder]

1) First is to toggle the series input resistor value. Stock is 68K, so my plan is to use a DPDT center-off and have  two parallel value options (say, like 33K and 10K as the final value). This gives stock, and two levels higher gain.

2) Then I want to change the bypass cap value on V1a. Also using a DPDT center-off, I will pull the stock 22uf from the board and move it to one throw of the switch, and have something like 680n or 1uf on the other side. So that would give two boosted options and no bypass cap for minimum gain.

Optional: I'm considering doing negative feedback as well, but I'll do that if I don't think the other two mods makes enough difference on the clean headroom front.

Just my two cents for this:
1) I don't think you will hear much if any difference changing the 68K resistor as it's just a grid stopper resistor for V1 to avoid blocking distortion happening. It won't have much effect on gain or input, me thinks. Before you commit to drilling a hole for that switch I would test it and see if it makes any difference at all. My bet is none that you could hear.

2) will have a much more profound effect and there's a cool Uncle Doug video and a neat calculator how it not only affects gain but most of all also bass response.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/
http://bmamps.com/CapCal.html

Negative feedback can have a very worthwhile and noticable difference, depends on amp. Worth a try I'd say.

[culturejam]

Thanks, Cooder.

I thought about doing the grid stopper mod because it actually made a difference in the Valve Jr circuit back when I modded mine. Or at least I think it did.    Maybe I'll table that one and focus on the other two.

That video is great! Thanks for sharing.

Do you have any preferences on implementing negative feedback on a circuit like this one? I've seen it set up a few different ways.

[cooder]

You could also consider changing plate resistor R 3 (in schem 220K).
For lower gain take a lower value, like 100K (very common) or maybe 150K.
You could trying upping that value for more and see what happens, sometimes it gives squealing and self oscilation as downside. In any case that will be a noticable difference...

I'll have a quick look what i can find re negative feedback.

[cooder]

Negative feedback: if you google 5F1 schematic it may be easiest to see what happens there and how to apply it in your amp.
It would be as far as I can tell in your amp a connection with some resistance (in Fender 5F1 its 22 K at 4ohm OT tap).
The amount of negative feedback depends also on which OT tap (you have 8 and 16ohm) you take it from and what resistance would work best.
As that's overall anyway a matter of taste you might want to add a variable negative feedback with a 50K pot and a 22k resistor in series. You always want some resistance in circuit otherwise the signal will be completely cancelled off.
You could do a 1M pot which basically gives you zero neg feedback to 22K neg feedback, downside would be a coarse dial in as the 1M pot is very fast changing.
Or make it switchable. Switch to disengage (which is stock) and switch position with a 22K and a 56 K resistor (spdt on/off/on).
There's calculators to figure out amount of neg feedback based on resistor value and which tap you take it from.
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#3-Way_Negative_Feedback_Switch

And this mod on V1 could be interesting too to tame and tune drive sound. I've seen it for example in Dumble and Soldano amps.
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Preamp_Local_Negative_Feedback

Here's a neat way to implement the Bias cap switching on V1:
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#3-Way_Preamp_Bias