Another PRR Vari-Mu question...

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bluebird

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I know this is supposed to be a cheap project but I would like to know what effect a 6386 would have on this circuit?

I have one I was going to try and build a Sta-Level with, but I just don't have the time for that now.

So I thought I'd try to make a high end PRR VM.

I have some nice 10K:7K input transformers with center tap and a couple Opamp Labs 1:1 line transformers I can use for the plates.

So would a 6386 be able to handle a bit more input level? would the compression sound better?
 
the 6386's are traditionally running at very high negative gate voltages - below the dent in the mu curve. This is why several are used in parallel, to get the current up to a useable level. The ECC82/12AU7 is much more linear overall, so decent start-out currents can be used.

The benefit of using 6386's is lower distortion at very large gain reductions. But you rarely need that really.

The opamp labs line transformer might have too low a primary inductance for a tube driving low-end through it - specially if it's optimized for a job as an output transformer for opamp circuits. But you'll need to try it to find out (unless you have primary-inductance specifications for it)

Jakob E.
 
[quote author="bluebird"]I know this is supposed to be a cheap project [/quote]

Pardon my ignorance, but howmuch would this costs in total?

Tony
 
Tony, I think PRR or Kent said you could make it for under $100. Look on the meta. Thats using very cheap transformers.

Thanks Jakob, Ok I'll save the 6386 for something else. maybe I'll get to the Sta-Level some time before I die.

The sta level does use one 6386...I guess it is biased through the cathode somehow.

sta.JPG



How much primary inductance would one need for the plate TX?

Does the TX have to be gapped? I have a couple good sized MCI 600:600 line TX's I got from Butta...maybe those would work?
 
How much primary inductance would one need for the plate TX?

As much as it takes to drive your wanted low-end corner frequency through it at the tube's output impedance. The formula is somewhere on the transformer pages.

Does the TX have to be gapped?

No. The driving stage is push-pull - you will have no DC current through the interstage transformer - it's blocked by C1 and C2.

Jakob E.
 
sorry :oops: i was too lazy to search .

Thanks for the info, (less then) 100 $ is really cheap!

Tony
 
Yes in the sta level the DC is blocked but in the PRR circuit the TX is linked directly to the plates.
Comp5.gif


So would a typical 600:600 line transformer work here? PRR suggests the ground isolator TX from raidio shack. I would like to use something with a better freq. response.
 
> PRR suggests the ground isolator TX from raidio shack. I would like to use something with a better freq. response.

Have you tried it?
 
No, I have not tried it...Have you?

I already have two or three different 1:1 output transformers laying around that MIGHT be a bit better than a radio shack transformer.

Maybe not...but if one of mine will work I won't have to buy a RS one.


Is it impossible for this circuit to get any better?

No I have not made it... but from the review I read from Kent it seemed it worked well for light compression. I thought maybe some better TX's or a 6386 would give it a little push into "this thing rocks" or "it sounds really cool when overdriven" or something.

So if the word is this circuit cannot sound any better and must use cheap parts and better transformers will not sound better, I'll just shut up about it then.
 
But... One thing I can´t understand. Won´t it have any DC running through the interstage windings? Unless you have a PERFECT matched tube, as I see, you will have some DC there...
 
Bluebird,

If at all possible, PLEASE try the radio shack ones AND a better spec ones. I haven't had the time but am dying to find out if there's a big improvement.

cheers,
kent
 
OK. great!

But I also don´t understand the 1k resistor across the windings. Wouldn´t the tubes be "seeing" a 1k load with it?

But offcoarse, I trust PRR. I´m just curious about it.
 
> Won´t it have any DC running through the interstage windings?

Yes. That is why you use a medium-Z winding: they take more DC than a winding with more turns.

> Wouldn´t the tubes be "seeing" a 1k load with it?

Yes. The tubes and load need to be NOT "matched". If load and plate resistance are similar, then changing tube current has little effect on gain. The load must be greater than the maximum (low-gain) plate resistance, or less than the minimum (hi-gain) plate resistance. In this case, either over 300KΩ, or under about 2KΩ. Fairchild went hi-Z by using a bunch of tubes to lower the plate resistance (and still it needs a pretty fancy winding). I went low-Z for low-price: $100 versus $10,000. The disadvantage of the low-Z loading is low tube gain, but gain is cheap today: that 5532 output stage costs $2.

If you have a healthy 600:600 or 2K:2K, use that, don't spend the $18 for a pair of RS-brand cores. OTOH, if you don't have a pair of $95 trannies handy, try the $18 dual-trans gizmo first. I have used it a LOT to break ground-loops, and never noticed much "color".

In any limiter building, you need a good signal generator and good AC voltmeter. Levels through the tube-stage are moderately critical. Comments like "good for light compression" may be inherent in using a 12AU7, or may be levels very different from I simulated. (And of course "light compression" is a matter of personal taste: I whacked those organ-pipes 2dB and wished I hadn't; in other works I've slapped-down a runaway singer by 15dB and maybe could have hit her harder.)

Have I built it? My building these days is limited by time, eyesight, and especially now by back-spasms. I'm going to need a walk around the building after this short note. No way I can hunch over a solder-iron this month. But Kent and others have the enthusiasm I once had, and I'm trying to trick you into doing my solder-work for me.
 
Thank you for all the answers. I have 4 different transformers. I will try them all.

I have built or owned all compressor types except Vari Mu. I look forward to trying this.

PRR sorry to hear about your back problems. I hope you feel better.
 
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