Marshall 18 watt Lite IIb with asymmetrical power amp

[timbo_93631]

For those of you that dabble in the really big stompboxes we call amps:
I banged up this Lite IIb variant schem incorporating a paralleled triode preamp, ZaphodPhil's huge PA cathode cap trick, and Brownnote Moss' ideas on the dissimalar PA arrangement with an el84 in pp with a 6v6 found here:
http://www.18watt.com/modules.php?name= ... t=6v6+el84
http://www.brownnote.net/forum/viewtopi ... =1819#1819
Do you see any glaring issues or changes that might make this work better? Looking to build somthing a little different than what I have done so far in the 18 watt realm. BTW 18watt.com is back up and at em!

[timbo_93631]

Schematic above has a few values, that on review and consultation with guru types at 18watt.com, would not be ideal. Here is updated schematic. The values of the cathode resistors in the PA might have to be messed with to get the same current flowing through the tubes as well, still working on ideal values there. Mods feel free to remove schematic posted above, edit button is gone-ski....

[timbo_93631]

And a few more changes after a little more discussion:

[azrael]

Whoa, this seems like a very cool idea. what would it sound it like?

[johnnyg]

Whoa, this seems like a very cool idea. what would it sound it like?

+1
I was curious to know what the difference should/would be as well - but didn't like to ask earlier in case I looked ignorant and stupid (which I don't claim not to be - I just don't want to look like that ).

The project is too advanced for me at the moment - I haven't built a standard 18w Lite yet, but that is my big hairy plan for the summer... I'm at the start of trying to get familiar with the circuit (and more of the basics of tube amps if I'm completely honest).

I hope it's not off topic - but the EZ81 rectifier. How significant an improvement does that make to the performance of this amp? Is it worth using one instead of four cheap and cheerful diodes?

[timbo_93631]

The point of doing the mixed output section would be to blend the glassy chime, early breakup, and hard rock grind qualities of an EL84 with the big and greasy round bottom bluesy qualities of a 6V6. And also because no one else really does it! If you use the EZ81 recto in conjunction with the 1000uF cathode bypass caps, it really stiffens the amp up so it has some of the stiffness and drive of a SS rectifier amp with just enough sag, and a non flubby bottom end. You end up with a lower output capability so you can run the power tubes a bit harder (where the magic in an 18 watt is really at) and also have the gentle startup cycle that keeps your tubes good longer. I'll be knocking one together over the next few months, look here for a layout in the next few weeks. An 18watt lite is a good place to get started in amp building if you want to try and tackle something only a little more challenging than a Champ.

[phatt]

A waste of glass,,,,It will end up in the BIN!
That is the kindest way to put it. But if you insist then have fun,,,
Phil.

[azrael]

A waste of glass,,,,It will end up in the BIN!
That is the kindest way to put it. But if you insist then have fun,,,
Phil.

please explain

[DrNomis]

My understanding is that a 6V6 and an EL84 have different Plate-Load requirements, an EL84 requires an 8k Plate load, and a 6V6 requires a 10k plate load, so each output valve won't be able to deliver it's rated power into the speaker...

[phatt]

A waste of glass,,,,It will end up in the BIN!
That is the kindest way to put it. But if you insist then have fun,,,
Phil.

please explain

Hi azrael,,
IMO all the great circuits for Valves have already been built.
The further you go back the better they sound,, simple is better than complex.
HINT HINT.

If you have time to waste money to burn then fine try anything your heart desires but will it do something that delivers something new, something that also sounds fantastic??
No I doubt it.

You can turn off one output Valve rendering it psuodo class A/ single ended!!!
I think Peavy used the idea for one model???

Like what for? As it sounds friggin aweful.
**But novelty sells BS**

That said if some poor soul who has had little experience with Valve technoligy finds a way to turn off one power tube ,, He may (for a while at least) think he has Discovered a secret.

A lot of these tricks can be found in King TUT's books where he gives the reader a lot of different ways to extract a noise from Valves.

Some Imbalance in the output is a good thing but (like distortion) once you reach a certain point it is no longer useful and actually starts going down hill from some point.

LOL,, I remember the first time I understood how simple diodes in the feedback of a simple opamp circuit could create distortion I also acted like a complete born again believer.

Did not take very long for the novelty to wear off and realize that there was a lot lot lot more to learn. Winky.
Phil.

[timbo_93631]

Phil,
Most anyone who has built an 18 knows that having mismatched EL84's results in a more complex and pleasing poweramp distortion. This is an experiment in building in another level of mismatch that is based on the tonal characteristics of each tube. The point, and the fun, is not to try and make it operate as a SE amp in some other mode, but to try and balance a 6V6 and an EL84. In a cathode biased class AB amp without NFB you have a good chance to experiment with this. I have built 18's and have enjoyed them, and been proud to sell them, but I always come back to the fact that they have a boomy/flubby bottom end with 12's unless you make changes to compensate for that like a 2x10 cab, changes to coupling cap values, or large cathode bypass caps on the power tubes. I'd personally like to see how the bottom end reacts with the mixed output section. Moss from Brown-note amps has designed some nice sounding amps, this is just an interpretation of his design with a 12AX7 based preamp instead of the EF86 pre. I am going to build it over the next few months, but even at the worst if it is a total failure, I can change a few caps and resistors and one tube socket and have a good old 2x6V6 or 2xEL84 LiteIIb. Big wow. The only things that go in the bin in my shop are stripped wire insulation, dead PCB's, and solder sucker trash.

[tictac]

I had a Mesa Blue Angel that had a quad of EL84's and a pair of 6V6's that you could combine or operate seperately. The reality is when you combine the tubes you can't hear the 6V6's because they have higher voltage requirements to drive them than the 84's.

The only way I can see to be able to combine both tube types in a way that you could blend them is for each tube type to have it's own phase inverter with level control. Each P.I. could be optimised for it's own power tube drive requirements.

Not a completely new idea but doable...
TT

[phatt]

Phil,
Most anyone who has built an 18 knows that having mismatched EL84's results in a more complex and pleasing poweramp distortion. This is an experiment in building in another level of mismatch that is based on the tonal characteristics of each tube. The point, and the fun, is not to try and make it operate as a SE amp in some other mode, but to try and balance a 6V6 and an EL84. In a cathode biased class AB amp without NFB you have a good chance to experiment with this. I have built 18's and have enjoyed them, and been proud to sell them, but I always come back to the fact that they have a boomy/flubby bottom end with 12's unless you make changes to compensate for that like a 2x10 cab, changes to coupling cap values, or large cathode bypass caps on the power tubes. I'd personally like to see how the bottom end reacts with the mixed output section. Moss from Brown-note amps has designed some nice sounding amps, this is just an interpretation of his design with a 12AX7 based preamp instead of the EF86 pre. I am going to build it over the next few months, but even at the worst if it is a total failure, I can change a few caps and resistors and one tube socket and have a good old 2x6V6 or 2xEL84 LiteIIb. Big wow. The only things that go in the bin in my shop are stripped wire insulation, dead PCB's, and solder sucker trash.

Then go for it?
Who's to say you might find a secret but I really think it's a time waster,,, which is why I made the comment.

If you really want to make in roads then start swapping OT's as Oddball OT mismatching will reap far better results. **meaning tonal mojo and serous OD tone tricks** can be found by serious mismatching of OT'rs.
WINKY WINKY

Go workout the OT of Trainwrecks and such gear. there you will eventually find the trick is in the
**** PASSSIVE component **** the very misunderstood Output Transformer.

You have got one thing RIGHT,,,, That being Cathode biased and NO FB!!!
Good Now start dropping the screen Voltage on a pair of 6V6's ,,,,winky.
Phil.

[bajaman]

some good tips Phil
bajaman

[azrael]

If you really want to make in roads then start swapping OT's as Oddball OT mismatching will reap far better results. **meaning tonal mojo and serous OD tone tricks** can be found by serious mismatching of OT'rs.
WINKY WINKY

please explain

[phatt]

If you really want to make in roads then start swapping OT's as Oddball OT mismatching will reap far better results. **meaning tonal mojo and serous OD tone tricks** can be found by serious mismatching of OT'rs.
WINKY WINKY

please explain

Hi azrael,
Simple answer,
Run lower than ideal primary loads for the magic old fashioned Limited bandwidth nasal saturated sweet OD/compressed trick.
Note; This is not the same as (the often debated) *mismatching the Secondaries* trick.

My problem is that I've never found anyone clever enough to explain what goes on here as most tecky folk just use the *reflected load equations* to ez splain the Z matching thing but I have found a couple of comments that suggest the *Primary load* holds a lot of tone secrets that just don't happen by swapping the speaker load.
I never was able to find any definitive answer and I figured as a mug guitar play who had trouble with maths I should ask someone who new,, but heck who?

A Short version of a very long story; build, trash, build trash, Blah blah

In the end I sent all the specs and voltages of my rig to *King TUT* and asked if a lower primary would damage the Amp and his answer was very short and sweet;

"Most Any Valve can drive any OT primary, go for it you will have a lot of fun"

Also adding that anyone who says otherwise does not understand how transfomers work.
(Insert a big head scratching moment)

I thought *Ouch!* That comment would start a lot of debates on some forums I'd been
reading where the load has to be right for the Valve used,,, hum maybe not?

But I figured he would be quite mad to say such a thing if it was way wrong.

Needless to say I took Him at his word and changed a 6k6 OT down to 2k6 on a 6V6 build and I've never looked back. By far the smartest tone tweak I ever did.
Mismatching secondaries changes things by a small degree whereas primary load has a much bigger impact on the whole.

And just in case I was fooling myself I did it again to a completely different rig. This time a small old cathode biased rig which had a 8k OT. I put the (now unused) 6k6 into that amp with very similar results.
When time permits I have recently aquired a 4k OT so I will get to test this yet again.

I did eventually find an article written way back in the late 40's I think where this *Primary mismatching* and it's effects on the resulting tone/response was explained to some extent. It would seem that a lower than ideal primary will limit the banwidth but also strongly *Accentuate the second harmonic content* of the signal.
Also this might bring about an earlier compression.

Bingo!!!! ***The Very Thing that some Amps (now worth a Fortune) are known for?***

So whether this be right or wrong is now a moot point for me???
Heck don't shoot me I'm just a guitar player passing on what has worked for me.

All I know is I thrash the living daylights out of a very small little Valve Amp that was built from old radio parts,, runs the Wrong OT primary and runs the wrong Secondary. It actually runs a 30 ohm sand box and gets reamped via a bigger Amp and I'm in heaven. Any sound at any volume I choose to use, perfect outcome for me.

That Amp gets gigged every 2 weeks for at least 5 hours at a time and has done so without any issue for close to 5 years now.

Now that I've pulled off the EVH type sound and if I can just stop building circuits and learn all those fancy licks I'll be good to go. LOL

Have fun with it all,, Phil.

[DeanM]

Maybe check out the egnater rebel amplifier.

It uses el84s and 6v6s and has a Blend knob. heres a vid demo to hear the differences.

[ppluis0]

Hi there,

timbo, it's very interesting the connection of the first 12AX7 sharing the same plate resistor but with different RC combination at each cathode.

Is there any other thread at that forum about this approach ?

Continuing with the idea to have unbalanced the output stage... why bother to buy a transformer expressely designed for tubes ?

Instead to have an expensive output transformer with low dispersed inductance and equal resistances and distributed capacity at both halves of the primary, we can buy ANY transformer intended to be used at 50 or 60 Hz at our local electricity store.

I think that a transformer wich have a primary side with two identical windings to be connected in parallel or series at 120 or 240 volts, can be linked each end to each plate and solder the +B connection at the center tap.

Furthermore, if the secondary winding is of 9 volts, we have a transformer of 5600 to 8 ohms or 11300 to 16 ohms, despite the absence of the speaker impedance selector.

In the same way, a commercial transformer built to 120+120 / 12 volts can be considered equal to one of 3200 to 8 oms. (A nice number to put to work a 6L6 and a EL34 in push pull... )

So bear in mind that the physical size must be bigger that the intended audio power, in order to not have risk to work close to core saturation.

I guess that the schematic you posted need a transformer rated at 30 watts and, in the case to build something that employs the aforementioned couple of tubes, find a unit close to 50 watts.

Cheers,
Jose

[RnFR]

could this underspec'd transformer thing be the reason I like the silvertone 1484 so much, and the same reason they all tend to eventually blow their Xformers(be it 40-50 years after they are built)? or is that a whole 'nother deal?

[timbo_93631]

Hey Jose,
The shared plate resistor/dissimilar cathode RC combination is from this thread at 18watt.com:
http://www.18watt.com/modules.php?name= ... lel+triode

Here are the transformers I am using, they are pretty cost effective:
http://triodeelectronics.com/trbufor18wma.html