Biasing the Rangemaster

[frankie5fingers]

6.4v too low?  I made it the other day and wasn't really excited about it.  Rebuilt it yesterday but it still doesn't sing.  It's the MB board, no mods and works perfectly.  I have some other trannies on the way but I hate to give up on the OC44.  I dunno the hFE or leakage (and not sure how to check them).  Any ideas?

[midwayfair]

http://www.geofex.com/article_folders/rangemaster/drm.htm

It's not that 6.4 is too low, but the cutoff will be lower than the effect was designed for.

If you aren't overdriving a dark amp (or maybe just looking to clean up fingerpicking), it's very likely to not sound inspiring.

[frankie5fingers]

Thanks Jon.  According to that info it looks like I ought to tweak R2 a bit.  I'll socket R1 and R2 and see what I can get bias-wise.  Thanks for the link.
Frank

[frankie5fingers]

I read that article, really informative, lots of other good stuff too - thanks again for that link Jon.
So I had to bump R2 UP to 5.1K and drop R down to 47K in order to get 6.9v.  Did I not understand Keen's article - that I should LOWER R2 to get the bias up?  I'm finding I need to do the same thing with an AC125 and a couple of others.  What am I missing? Thanks

[ch1naski]

I think that's right. The resistors should be adjusted relative to each other.  I find that the pot/trimmer method works best. I switch trannies in my Rangemaster(s) on a regular basis. AF105, OC 44, AC 125..... I use the rg article  info every time.

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[frankie5fingers]

Well, certainly that makes sense.  Is there best way to get there?  Since one set determines the base bias point is it better to more radically alter R1 or R2, or doesn't it matter as long as Im stable ~7v?  Thanks

[ch1naski]

I personally get r1 so that I get proper bias voltage on the collector when r2 is closer to around 50k. But like I said, I'm using trimmers mounted on the board.
From what I understand of it, that's the way it works. Alter r1 so that r2 does not have to be radically altered.

[frankie5fingers]

Now on the stock MB build, R2 is 3.9K, so you think it'd be best to move R1 even lower to keep R2 as close to 3.9 as possible?

[ch1naski]

Now on the stock MB build, R2 is 3.9K, so you think it'd be best to move R1 even lower to keep R2 as close to 3.9 as possible?

you know, I think my r2 and yours (mine are built on vero, not madbean pcb's) are switched.  I believe that keeping r1 (your r1,)  close to the original spec is more important than r2 (the 3.9k) . In the RG article, it says if you end up having to go too high to get biased, up the value of r2 (the lower if the two resistors in question), and bias it again. Repeat as needed.

That's the way I've done it, on all 4 that I've built (stock, pot-adjusted input caps, rotary-switched caps, and java boost).

[midwayfair]

Now on the stock MB build, R2 is 3.9K, so you think it'd be best to move R1 even lower to keep R2 as close to 3.9 as possible?

you know, I think my r2 and yours (mine are built on vero, not madbean pcb's) are switched.  I believe that keeping r1 (your r1,)  close to the original spec is more important than r2 (the 3.9k) . In the RG article, it says if you end up having to go too high to get biased, up the value of r2 (the lower if the two resistors in question), and bias it again. Repeat as needed.

That's the way I've done it, on all 4 that I've built (stock, pot-adjusted input caps, rotary-switched caps, and java boost).

The 3.9K resistor sets the gain of the op amp. The 68K resistor sets the "maximum" gain -- it's a connection to ground directly from the base. If the 3.9K is too low, the transistor will hit cutoff sooner because the gain will be higher. If the 68K is too high, it'll let too much signal in and overdrive the transistor. If the 68K is too low, you'll lose a lot of guitar signal. If the 3.9K is too high, you won't get as much boost.

The 3.9K is less important. In fact, it's variable in some versions, like the Catalinbread Naga Viper.

[ch1naski]

And also the java boost.
I keep my Rangemaster circuits that way for swapping transistors.

So if you end up straying too far on the bias resistors, the transistor in question is probably no good for the circuit.

Jon knows more than myself, so I would listen to him before me.

Sent from my Nexus 7 using Tapatalk HD

[frankie5fingers]

I haven't had a chance to listen, but using a 47K in R1 and an 8.2K in R2 I have a 6.9 bias.  Do I need to raise R1 back up , closer to 68K and tweak even higher than 8.2 K on the other?  I'm still waiting on some higher hFE transistors but so far I haven't cared for a couple that were around 100, but with leakage a little high.  The 6.9 biased OC44 sounds OK, but again, not great.  I know if I take R1 back to 68K, I'll need R2 around 10-12K to get in the ~7v sweet spot.  I wonder which would be more preferable.

[frankie5fingers]

I put the 68K back in, took a 10K to balance out at 6.95v.  I guess that's that.  Still not blown away by the OC44.  I used to have a Java Boost that I liked.  I guess I was thinking the RM would be less brassy and more airy/springy.  Oh well...
As always though, thanks much for the replies and assistance.

[ch1naski]

I don't think that changing those bias resistors any more will change too much of the sound at the point where you are.

I actually like a 2n1307 in mine, less stinging top end. More warm, less piercing.
And it is 83hfe. Go figure..
My favorite is a leaky Telefunken AF105. I would say the sound is warm with that one. But the hiss and temperature-sensitivity make it almost unusable.

Java boost is my main build I use, as far as rangemasters go. You may want to look into building the naga viper, it's a little bit like the java..

For my setup, any rangemaster-esque pedal needs the tone control options that some modified versions offer.
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[frankie5fingers]

Got a couple OC45s yesterday - one hFE 89, the other is 97.  The 89 is the ticket.  Got it biased at 6.97v, swaped C2 out for a 2% .027 and it's good to go.  I suppose my original OC44 isn't a keeper, I get it now.  Sounds very good.  Thanks again for the help.  Frank