Fewest gain stages.....

[jrfrond]

EF86 also gives me an idea re. the original post/fewest stages - EF86 input, straight into a self-split 4xEL84 output.

This would be the old, moldy Philco PI design, specifically to save a tube in a table radio. Never was well-received, but worth a shot I guess. I don't know how it will work with a quad of EL84's though.

[MWJB]

Gibson & Magnatone had self-split designs, twin triode preamps, 2x6V6 are good for about 10W, if it'll drive a quad of any power tubes, it'll drive the EL84. I've built a couple of 6SN7 powered, living room amps, doubling up on the output triodes, doubles the power, no reason to think it won't work scaled up.

How it sounds....ha, ha...anyone's guess.

[jrfrond]

How it sounds....ha, ha...anyone's guess.

That's the beauty of amp design. Sometimes "good on paper" is as good as it gets! And sometimes not.

[redelephant]

How'bout a medium power triode driving a step up interstage transformer with a PP output driving a pair of power tubes in PP? Hmmm, I've always wanted to try something like that, and perhaps it's time...

[kg]

you might try this out:



the IT is a VERY CHEAP 1:1 that is bridged via a cap to give fantastic frequency response.

Allied - 6W68HF - Transformers - Power Products - Allied Electronics

if you don't care so much about that, you can use it in the intended sep config.

[jrfrond]

KG, that's an interesting take on the IT design, combining grid and cathode drive on the outputs, rather than splitting them at the IT, not too unlike a LTP PI. I have just two questions:

- Considering the equally-split plate load at the OT, which cannot be adjusted, how is the symmetry affected?

- Most important, how does it SOUND?

[kg]

i used this as a hi fi amp for a few years. it sounded DAMNED good in that role. couple of friends with the typical speaker component systems couldn't get over how "low power" it was. speakers were nothing special, just two-way sealed boxes with decent drivers.. 6" woofers iirc. never hooked up a MI to it though. then i had kid(s) and moved and my listening space hasn't been redefined yet. eventually going to be in a basement HT/hifi room.

the symmetry isn't perfect, obviously, but what is there is even order. no coupling cap and very low impedance grid leak makes for very very quick recovery from overdriving.

i had wanted to build a totally SE design, but i had these PP outputs hanging around... from a peavey classic 60/60. i had considered fancy stuff like parallel feed for the final stage but settled on this, basically because i had all the parts on hand.

[MWJB]

JRF "- Considering the equally-split plate load at the OT, which cannot be adjusted, how is the symmetry affected? - Most important, how does it SOUND?" KGs design apart for the moment, just about any cathode-biased amp can easily be switched to self-split PP, you just need to ground out the 'out of phase' power tube grid, whilst disconnecting the power tube bypass cap (if fitted). Obviously, the lack of bypass cap at the power tubes reduces gain overall, but you might get "enough" earlier in the circuit to negate this to some degree? Disconnecting any NFB loop may be an idea too? Magnatone coupled the tubes via the screen grids, allowing the use of a power tube cathode bypass cap....never heard the M109 myself though.

KG, I hate generalisations & quoting "vox pops" but I have heard hi fi builders eschew PP because a phase inverter invariably "screws up the sound", I have found my self split experiments to be rather natural & open sounding (if on the lower gain side), especially compared to the amp that I can switch in/out of PI driven.

[Ken Moon]

Here's an LND150-based input booster, designed by Alex Kenis over at AX84, that doesn't require a separate power rail - with this circuit, plus one 12Ax7 and a single-ended 6L6/EL34/KT77power amp, you can get about 12 loud watts with nice crunch:



There's a whoe thread over at AX84 from 3 years ago where Alex explains the circuit - I can't get the thread to link for some reason, but search for author AletheianAlex and keyword 'heretical', and the thread will be the first one listed.

The LND150 is very handy for all sorts of uses in a tube amp

[imaradiostar]

I love this thread. I've had a few thoughts about this sort of thing recently. You could go even simpler/less tubes- use a pair of 6bm8's for push-pull output and either a LTPI or a gain stage+cathodyne with an LND150 out front. Two bottles, AC15 like simplicity and tone.

I built an amp last summer that used a single 12ax7 for gain with a traynor/dr z style tone control between the triodes and an LND150 phase inverter after them. With a proper voltage divider between the preceeding triode and the solid state cathodyne you can ensure that the cathodyne will never clip, meanwhile it'll run right up to the rails and drive the output tubes well into clipping.

I have some PT's that are a little low on filament current. I'm working on something blackface sounding with a normal fender pre up front, vox style PI, 6v6 outputs and a merlin style SRPP reverb tank driver. Verb will be recovered with an LND150 and blended using a 1m pot/220k mix resistor before the PI. 5 bottles, hopefully all the clean Fendery tones I'm looking for.

I've been toying with the idea of a plexi-ish thing with a plexi preamp driving a single fixed-bias LND150 cathodyne and a pair of EL84's- again, proper voltage divider and it removes the mosfet from the sonic equation and allows preamp and power tone to shine through. If you don't like that circuit without a LTPI just use one or a pair of mosfets or jfets out front with a normal marshall circuit from there out.

My favorite amp is an EF86 into a two knob tone stack, vox type LTPI, EH 7591's and a triode electronics ultralinear 6k6 output TF. It's running ultralinear with no other negative feedback. I takes pedals really well and can make the harshest of guitars sound musical in a mix.

jamie

[mooreamps]

Here's an LND150-based input booster, designed by Alex Kenis over at AX84, that doesn't require a separate power rail - with this circuit, plus one 12Ax7 and a single-ended 6L6/EL34/KT77power amp, you can get about 12 loud watts with nice crunch:

So, everyone is doing the fixed bias thing on their preamps these days ? ? I thought the thing over there was their ability to run a "green" led in parallel with a "red" led... :|

-g

[Steve Conner]

I like the implication that the LND150 doesn't count as a gain stage because it's not a tube. Why not? Equal rights for silicon!

Or to paraphrase the Monty Python sketch: "Unfair to MOS"

[imaradiostar]

But mosfets don't count! :wink: In all seriousness- if they're used in places where they don't really contribute tonal change through distortion then they're not part of the overall tone.

Gary, fixed bias was invented before you were born and you know it. You just showed us all the light, right?

Another simplified scheme I like it a 6GN8 or similar triode/pentode setup with the pentode driving a simple tone control and volume and the triode driving a phase splitter transformer and a pair of output tubes. Once again- three bottles, plenty of gain.

jamie

[mooreamps]

Gary, fixed bias was invented before you were born and you know it. You just showed us all the light, right?


jamie

I was flamed badly when I first started talking about.. or don't you people reminder all of the crap I was taking from UR12 and others about my "bite me" lamps ???

-g

[imaradiostar]

I was flamed badly when I first started talking about.. or don't you people reminder all of the crap I was taking from UR12 and others about my "bite me" lamps ???

-g

Yeah, I was just trying to be lighthearted about it. In the end I figure each person builds what they like for the reasons they choose. What works for you may not be someone else's way of doing things. I think that the assertion that it is a new idea is what got some people torqued up. Even if you came upon the idea independantly you hopefully can see why others were giving you crap.

jamie