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Minimalist PL519 OTL?

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CCS: Constant Current Source. The best CCSs are solid state: either ICs or discreet BJTs in cascode.

I'm pretty sure that that is not true. Take a look at the CCS in that schematic again. There is a bipolar supply, +/-300V.

The CCS has 2 stages. It rejects noise in the supply, sets the current for the differential cascode circuit, and makes sure its constant. It does it very well; with an unregulated supply and AC wall power from 100V to 130v, there is no measurable nor audible change in the performance of the voltage amplifier.

Now if you did not have the bipolar supply (which IMO is a compromise) then a solid state CCS would be indicated, as a tube circuit needs the extra voltage to work...

I've seen a lot of 'CCS' circuits in amps over the years, most of the amps being solid state. Frankly, the CCS designs I see in the vast majority of them are terrible. You have to have at least 2 stages to be effective, and even then it has to be designed right, otherwise you are barely better off than using a resistor. That's why I used the single quotes at the beginning of this paragraph.
 
But without a specialist guitar amp OPT, you'll never get that sound favoured by most guitarists of a saturating OPT. The actual OPT make a huge difference to the sound - try fitting a Fender OPT to a Marshall, or vice versa - you'll get a completely different sound.

Very hard to fake overloaded iron with an OTL.

Regards, Allen

First of all, those amps almost NEVER overload/saturate their OPT, since they simply go too loud before that happens.

Second, that was my greatest worry for a long time, that OPT saturation was key to good guitar tone. It is NOT.

Third, I don't believe those guys who make those claims have actually tried. I have, and was dissapointed that the OPT had so little to do with the tone.

That's my say on the OPT issue...
 
That would be a simple circuit, but it will not make power nearly as effectively as the Circlotron.

When you get rid of the output transformer, you are also rid of any reason for going single-ended; an OTL is not subject to crossover or notch distortion issues like push-pull transformer coupled amps can be. So the distortion character is more like single-ended, sans the 2nd and 4th harmonics (you get mostly 3rd, and not very much of it) if you keep the circuit simple as seen in the prior schematic.

Very interesting. I have not tried a circlotron yet, and came up with my own version of an OTL b/c I was convinced I'd sell heaps and didn't want a copyright issue...Mohaha...that was my big head doing a mission on me;)

My version is a PP totempole and if bias is too low it certainly has x-over dist. Some guitarists actually find that cool in some tunes, and therefore have a readilly available bias adjust knob to shape the 'color' of the output stage.

I have a funky drive arragement and can adjust the symmetry of the phase splitter to drive the output either assymetric for loads of even harmonics, or symmetric for mostly odd harmonics.

I find it interesting that no guitarist has yet to discover the difference in less than 1% to over 10% 2nd harmonics. So I have actually stopped including that feature in my amps.
 
How are you changing the symmetry of the phase splitter? That's a pretty cool idea but I thought overloaded valves generate their own even harmonics anyway

Well, in the case of the totempole circuit, the bottom tube acts like a common cathode (with some gain) and the top tube acts like a cathode follower (with neg gain). With 6AS7s this isn't as bad as it may seem, since those tubes have so low gain to begin with. But anyway, for minimal dist the drive signals must compensate for the totempole's weakness...(actually for guitar, or any instrument, it's usually not a weakness, since it only introduces a little 'character' to the tone). So the two out of phase drive signals can be adjusted in magnitude to produce an output that is very low in dist, or very high in even order dist. This of course means zero loop negative feedback, since the point is not to let the gainstage correct for this.

One quick and easy way to test this is simply parallel (with a capacitor) a potentiometer to one of the phase's gain resistor and adjust the potentiometer till you get mucho harmonics.

My last vintage amp with 6V6 (or 6L6) in PP I use a phase splitting transformer, interstage tranny, and it was so clean even without NFB, which of course I don't use at all, that I wanted som flavour. I used different value screen grid resistors, and voila, loads of nice sweet second order harmonies, and no reduction in power;)

The main reason I started building instrument amps is b/c in this area it's ok to play around like this and do crazy stuff that wouldn't be accepted in HiFi type circuits.
 
Keep it up dude! Its the goofy weird stuff that is what is essential and IMO makes life interesting and fun.

I just got a Circlotron-based amp running that uses a driver input transformer. The primary of the transformer is the plate load of a single 6SN7, and the other half of the 6SN7 is a gain stage. So overall there are 5 tubes in the amp- one 6SN7 and four 6AS7Gs. I'm still fiddling with it but I'll report on the sound soon :)
 
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