Orange OR-120 clone. Adjusting Split load driver for 2 x EL-84?

[fiveightandten]

I've been thinking about another build, and I love my old Orange amp, but it's way too loud! I'm ignorant to the world of PI design (and design in general, as I've only cloned). Can anyone point me in the right direction to re-design this split load driver to drive a pair of EL-84s (I'm leaning towards fixed bias)?

They used a few different versions, so whatever might be the best starting point to alter:
http://www.orangefieldguide.com/OFG_...hem_post74.jpg
http://www.orangefieldguide.com/OFG_...20schem_72.gif

I'm looking at cannibalizing an Orange OR-15, for the chassis and transformers, and just popping a turret board in there.
http://i1291.photobucket.com/albums/...psusartvfq.png

PT secondary looks to be good for 250V, or 320V loaded after the rectifier. I'd like to get the pre-amp to run off the voltages the big OR-120 runs off, for it to be as close as possible to an OR-120 with 2 x EL-84 (fixed bias, NFB intact).

Pre-amp voltages at the plates should be something like (According to schematic, I haven't measured in my OR-120 yet):
PI - 275V
3rd stage - 275V
2nd stage - 170V
1st stage - 170V

Thanks in advance for any assistance!

-Nick

[J M Fahey]

I've been thinking about another build, and I love my old Orange amp, but it's way too loud! I'm ignorant to the world of PI design (and design in general, as I've only cloned). Can anyone point me in the right direction to re-design this split load driver to drive a pair of EL-84s (I'm leaning towards fixed bias)?

Pity you chose an Orange and not any other Guitar amp out there.
Main problem when building "EL84 versions of large amplifiers" is that EL84 are way easy to drive (so easier to over-drive) than 6L6 or EL34, so amp response changes ... a lot.
That can reasonably be compensated for in LTPI such as in Fender/Marshall (and most anybody else) by using slit plate load resistors, so if , say, you want to halve available drive signal you replace 100k plate resistor with 2 x 47k in series, the 82k one with 2 x 39 or 43k ones, and in both cases take signal out from the centerpoint of each series, effectively halving drive available to power tubes.

But Orange uses a cheesy Cathodine PI (you call it split load but thatīs the wrong name) which does not allow that.

Just as an experiment, I would suggest you attenuate signal reaching power tubes, build circuit as-is, but use properly biased and loaded EL84 as outputs and attenuate signal reaching grids 3:1 , which is about what you need.
You can add 470k resistors in series with power tubes coupling caps C24 and C23.
Then on top tube(s) you will have C24+470k > R22 to ground, and on the bottom tube(s) C23+470k > R23 to ground , both nets providing 3:1 signal attenuation but leaving biasing network as-is.
I am referring to the first schematic, just to avoid confusions.

They used a few different versions, so whatever might be the best starting point to alter:

PT secondary looks to be good for 250V AC, or 320V DC loaded after the rectifier.

Not the same, *always* make clear what are you talking about.

I'd like to get the pre-amp to run off the voltages the big OR-120 runs off, for it to be as close as possible to an OR-120 with 2 x EL-84 (fixed bias, NFB intact).

Pre-amp voltages at the plates should be something like (According to schematic, I haven't measured in my OR-120 yet):
PI - 275V
3rd stage - 275V
2nd stage - 170V
1st stage - 170V

Not too important, if at all, and you are starting with lower +V anyway, so donīt obsess with that , in any case sound will NOT audibly change unless you get to starvation voltages, say 90V or less, not the case here.

[Malcolm Irving]

...
But Orange uses a cheesy Cathodine PI (you call it split load but thatīs the wrong name) which does not allow that. ...

Probably I'm missing something, but I don't see why not. Can't you just split R21 and R20 each into two equal halves and take C24 and C23 from the mid-points?

[Mike Sulzer]

Probably I'm missing something, but I don't see why not. Can't you just split R21 and R20 each into two equal halves and take C24 and C23 from the mid-points?

I agree, but maybe take it down a bit more than half with unequal resistors. You might need to improve the ripple filtering on the cathodyne B+ when using a more sensitive output tube..

JMF: In any case if a cathodyne is cheesy (not what I think), then I suppose you want to clone the cheesiness along with the rest!

[Zozobra]

The cathodyne is perfectly fine and used with great success in a whole bunch of amps. I agree that the DC coupled setup in orange and matamp designs is going to flap around like crazy but that is where the tone comes from in these amps! It may not be your cup of tea but if you want that sound that's the way to get it. AC coupled is perfectly fine and in some high gain circuits you don't want the LTPI flavour in the tone.

I've played around with orange-esque circuits using a pair of 6V6s at ~300VDC which may be better suited to this circuit than EL84s as they don't immediately bounce off the rails when presented with a tickle on the grids. The first thing to try is to lower the cathodyne plate and cathode resistors to tame the signal swing. 47k is more reasonable. Are you dead set on EL84s?

[J M Fahey]

To tell a long story in few words: "Orange are cheesy *designs* built with impressive *components*"

And I am not talking fairy dust resistors or magic colour capacitors, but "mechanical" or "electromechanical" (to use a name) components where high quality *does* make a difference.

I am talking mainly speakers, both Orange and Hiwatt used impressive FANE speakers, way better than any Celestion, the *first* true 101dB Guitar speaker (leaving aside JBLD130 which was a PA speaker pressed into Guitar duty) , the first true 100W one (when Celestion had 20/25/30W tops and Jensen didnīt fare any better) , they invented the fiberglass former voice coil when all others used paper or , at best, Nomex (Kapton wasnīt even invented yet) AND both used impressive Partridge transformers.

Such high quality speakers and transformers were never used by Marshall, VOX, or any other classic British amp maker because of cost.

Applying the (very true) Enzoism : "the amplifier is that thingie between Power Supply and Speaker" , you see that in this particular case a meh design becomes acceptable, not because itīs "good" but because supply and speakers used were impressive, thatīs why you would see Orange (and Hiwatt, which *are* excellent designs) on the best stages.

But you build the plain schematic using common parts and speakers and itīs a shadow of the "famous sound".

I built many of them, and was dissappointed by results, compared to similar amps from other brands, simply playing them side by side, into same cabinets and using same iron.

Only special and useful feature is "F.A.C." , the rotary selectable Low Cut/High Pass (pick one) which lets you dial down the excessive Bass created by the Home Audio type Tone Control.

Cathodyne PI are acceptable in Hi Fi / Home Audio , because they work clean, but heavy overdrive makes power amps kink and shift output waveform in a nasty way, as well as block distortion the cathode driven half, compared to the plate driven one.

Yes, I mentioned splitting plate resistors or plate/cathode ones in halves, just as the most intuitive example, you scale that to whatever attenuation factor you want, but suggested series resistors instead, specially to correct the cathode driven half tendency to get into blocking distortion.

Of course, I must admit that Mojo by brand association with Guitar Gods and killer graphics and Tolex colour are a big factor in brand desirability.
But for the full Monty you should also use the original Transformers and Speakers.

[Malcolm Irving]

I think 6V6s is very good suggestion, but I don't think changing to 47k cathodyne loads would help much. My rough estimates (based on DC load-lines) are 125V swing for 100k loads and 110V swing for 47k loads. Either of which is enough to drive EL84s crazy.

[Zozobra]

I'll also add that I've built a few orange circuits and I've played a bunch of originals and a couple of Matamps GTs too; I like how they sound when other people use them but I just can't get them to work for me. I generally like that wet sock (tm) stoner doom tone too haha!

[Helmholtz]

If the main purpose of the project is to build a lower wattage OR, why change output tubes at all? Lower wattage comes from lower voltages not from smaller tubes. EL 34s work fine at 250-300V. Transconductance will be lower at lower screen voltage but probably still be higher than 6V6s'. The lower gain can probably be partly compensated by a higher load impedance. Lower voltages give more freedom to choose a load line.

[J M Fahey]

Yes, a couple EL34 at EL84 voltage and load will put out about what a couple EL84 will under same circunstance.

[PeanutNore]

For what it's worth, I've built an OR clone that runs a pair of 6V6s, not that different from EL84s, and didn't make any changes to the circuit to reduce the signal that drives the power tubes. All it does is shift the point on the volume knob where distortion takes hold, and affects the amount of power tube distortion that you can achieve.

I did however follow Merlin's recommendations for reducing the possibility of nasty distortion from the cathodyne and added a large grid stopper resistor to it.

It's also important to note that there are two main versions of the OR-120, with the main difference being the cathodyne. I feel that the AC coupled cathodyne is superior to the DC coupled version.

[Gingertube]

The cathodyne PI - a copy of something I posted elsewhere regarding an Aussie Goldentone (6DQ6 Output tubes).

The reason for the 100K anode and cathode load resistors on the cathodyne for big amps is purely for ease of swinging the required output tube grid drive voltage. The required voltage swing for the Goldentone is about the same as what is required in the ORANGE MATAMP to drive a quad of EL34. It uses 100K too. All of the EL34 and 6L6 Amps I've seen with cathodyne splitters use 100K or 82K.

Smaller Amps with EL84 or 6V6 which do not require the same voltage swing, typically use 56K. That is, they are swinging the same current in the cathodyne as the bigger amps but generating less voltage drive into the output tubes.

Calculating Overdrive Effects
If MATHS gets on your wick then skip to the SUMMARY below:

For equal loads on Anode and cathode on the Cathodyne

Zout = RL.ra/RL(u+2)+ra

The ra term on the bottom line is insignificant compared to RL(u+2) term so drop it. Then the RL terms top and bottom lines cancel leaving

Zout approx = ra/u+2

At typical values of u (>=20) u+2 approx = u, so simplify again

Zout approx = ra/u = 1/gm

So for equal loads at the anode and cathode the Zout at both is approximately 1/gm or about 650 Ohms for a 12AX7

If driving output stage directly The equal loads on Anode and Cathode will NOT be guaranteed if:
1) Output stage strays out of Class A (When a tube cuts off it has no gain so Miller capictance will change, particularly with Triode Mode Output, less so with Ultralinear and less so again in Pentode Mode)
BUT more importantly
2) Output Stage strays into grid current (trying to overdrive the output tube). When this happend the load resistance presented by the output tube grid drops significantly.


If the Anode load drops significantly then:

Zout cathode = RL+ra/(u+2) x ra/RL The ra/RL term insignigicant so

Zout cathode approx = RL+ra/u+2 At usual values of u

Zout cathode approx = RL/u + ra/u = RL/u + 1/gm

That is it increases by RL/u


If the cathode load drops significantly then:

Zout anode = RLxRL(u+1)+RL.ra / RL(u+2)+ra

RL squared (u+1) is much larger than RL.ra and RL(u+2) is much larger than ra so

Zout anode approx = RLxRL(u+1)/RL(u+2)

and at reasonable values of u

Zout anode approx = RL


SKIP TO HERE:
SUMMARY:
As the loads on Anode and cathode become unbalanced (as when overdriving the output tubes) then

Zout anode increases from 1/gm toward RL (from 650 Ohms toward 100 KOhms in the Goldentone)
Zout cathode increase from 1/gm by maximum factor of RL/u (from 650 Ohms toward 650 + 100K/100 = 1650 Ohms in the Goldentone)

The imbalance from an over driven output will therefore be worse with the 100K but the difference between the anode and cathode output impedance in overdrive is so dramatic it doesn't really matter if its between 650 Ohms and 100K or between 650 ohms and 56K, Also the effect of this will be asymetrical, affecting one side of the push pull and therefore introducing large gobs of 2nd harmonic distortion.

The trick here is to use large grid stops on the output tubes to reduce the loading effects on the cathodyne. Take those output tube grid stop resistors way up to say 47K. THis is effectively tuning the amount of 2nd harmonic distortion you get in overdrive.

Is this post useful or should I pick up my marbles (if I can find them) and go home.

Cheers,
Ian

[Helmholtz]

Sorry, I don't quite agree with your maths.

While the voltage gains for plate and cathode outputs are fairly equal, the output impedances are not. The reason is, the tube internal impedances at plate and cathode are vastly different. Detailled calculation shows that the plate output impedance can be approximated by the plate resistor Rp, while the cathode output impedance is (Rp+ra)/(ĩ+1) in parallel with the cathode resistor, resulting in a value of around 1k (1/Gm being a very rough approximation).

It is mainly this difference in output impedances which makes the cathodyne deliver asymmetric drive voltages as soon as (grid) load current flows.

[Malcolm Irving]

...
Smaller Amps with EL84 or 6V6 which do not require the same voltage swing, typically use 56K. That is, they are swinging the same current in the cathodyne as the bigger amps but generating less voltage drive into the output tubes.
...

Sorry, minor quibble with that bit. The cathodyne current doesn’t swing the same.
With 100k cathodyne loads the plate current varies between zero and 1.25mA.
With 56k cathodyne loads the plate current varies between zero and 1.9mA
Output voltage swings are therefore 125V and 106V respectively.

(I am assuming B+ = 300V in both cases. I suppose smaller amps probably have a lower B+ at the PI, which will reduced the output voltage swing of the cathodyne accordingly.)

[J M Fahey]

As the loads on Anode and cathode become unbalanced (as when overdriving the output tubes)

the output impedances are not. The reason is, the tube internal impedances at plate and cathode are vastly different.
.............................
It is mainly this difference in output impedances which makes the cathodyne deliver asymmetric drive voltages as soon as (grid) load current flows.

Thatīs what I have been saying from the beginning.
I couldnīt care less "cathodyne is used by many famous brands", they are perfectly acceptable in Hi Fi amps, which never clip, or sort of acceptable in guitar amps which are not "supposed" to clip, at least by Leo Fender intentions.

Once amp is driven hard, output waveform becomes īorrivel , there are many threads devoted to "correcting" the "broken amp" which is not, or to better say it, is doing what it can.

One such correction is adding series grid resistors, but which of course is NOT in the original schematic.

As said before, I built Orange clones, tested them live, measured and scoped them .... was dissappointed and eventually gutted them and built "something else" which always worked better.

Modern "Orange" are anything but, just share stunning graphics and colour.

I think original covering cloth was called "Rexine" or "Rexinite" and has not been made for decades ... and last production plant was in India, go figure.