ecc99 low powered amp

[printer2]

Well this is timely. Just been playing with a mosfet in the cathodyne position. Figured a 12AX7 will handle the gain duties and since the cathodyne really does not add much to the sound of an amp, unless it messes it up, might as well use a SS device there.



The above looks much better than the board, just sort of winged it and found I could have used an extra inch.



I have to change the tone control section in the amp, the above schematic will be the completed one. I am adding the NFB switch, not in it yet but otherwise the amp works and behaves fine. Basically one of the Tweed Fenders (Harvard, Princeton, Vibrolux) although lower voltage and no tremolo. No issues found using the IRF820, used it as it is about the most common device of its type used by guitar amp tinkerers. This is my $100 amp design, came out in response to a diyAudio challenge from years back.

Hope you don't mind me adding this to your thread. Don't want to start a new thread just for this amp, just wanted to throw in a last picture. Chassis is a that was a cover for a box, just cut it and roughly bent it. Switching 12V wallwart to feed the heaters (12AQ5's). 120:240V transformer fpr the HV, 70V line transformer for the OT. Was designed for a $100 Challenge for an amp (excluding chassis, speaker, cabinet) with components available to all. Will try tweaking the values a bit yet, a lot of gain on tap and want to see if I can make the best use of it. As is it is an alternative to a SE Champ. Speaking of the Champ, going into a cabinet the same size of the original Champ.



Not sure where my camera focused on, oh well.

[Zozobra]

Not at all! Looks great. I'm actually thinking of folding my own chassis for this project as I have access to a CNC mill to cut all the holes with prior to bending.

[printer2]

Not at all! Looks great. I'm actually thinking of folding my own chassis for this project as I have access to a CNC mill to cut all the holes with prior to bending.

I actually have a brake and shear at work that I could use but didn't bother to take this to work. I did cut and bend a stainless cover piece to go over the top so the aluminum chassis will not be visible. I'll put it all together once I finish playing around with the circuit. I did a SE amp that had Blackface treble and bass controls and flicking the switch turned it into a Tweed tone control. I might look at adding a bass control to be switched in, will have to look at some early Fender schematics to see if it is feasible. Not sure how this one will end up in the end but I think it was a project worth doing.

Would love to see your amp.


Drew up a schematic. Will have to swiss cheese the chassis, SS plate no longer appropriate. Want the Fender two input jack, with the both triodes bypassed and no NFB there is more gain than suitable for the amp in Tweed tone control mode. With the EQ losses the top jack will be fine. Messed with the Fender values as found in the 5F4 Super and others of the era, some wacky responses as far as Tone Stack Calculator goes. With the bass and treble full up you get a BF dip in the response, a bunch of interesting ones with different settings. Not much for losses also. Probably should have started my own thread now that I am doing some major modifications. Hey, who knew two hours ago?

[Zozobra]

So a year on this little amp has been a number of things and it is currently wired up as a standalone poweramp using an LND150 as the input stage which feeds a bog standard LTPI and cathode biased ecc99 poweramp.

The whole thing is a little bit bulky and the tubes are on the outside which means I'd need to build a box for it if I want to use it anywhere other than home. This has got me thinking about miniaturisation and cutting everything back as far as possible leading to using MOSFETs as much as possible and housing the tube inside the chassis. I'm also thinking why not use a ZVS DC boost board for the B+ and use a 12V wallwart to boot and save the PT for something else?

So the basic topology would be:

LND150 input stage > MV > IRF820 cathodyne PI > ecc99 PP poweramp.

Now this is where I show my ignorance. For this to be useful to me I'd also like to retain a few common global NFB loop features , i.e., depth and presence controls. With a Cathodyne PI the NFB would normally be injected at the cathode of the driver stage, which in this case is the LND150. Would the same principle apply here whereby it would be injected into the source for the same effect or will this mess things up?

[PeanutNore]

Connecting the negative feedback to the source of the LND150 should work just fine. Negative feedback to the cathode (or source) works on the same principle as cathode (or source) bias - the plate (drain) current depends on Vgk (Vgs) and by altering Vk (Vs) you alter the plate current. For a given grid (gate) voltage, put an AC voltage on the cathode (source) and you get an AC current at the plate (drain). LND150 stages can have a ton of gain, so you may find you need a larger resistor than typical - maybe a 100k or 250k linear pot wired as a variable resistor?

[Zozobra]

I'm glad that it works out that way. I suspected that it would but it's always nice to know beforehand. This should really simplify things and I'll probably have a stab at making a PCB for this project to keep size down. Before I get too carried away I need to build up my rack preamp so that I have something other than a metal zone to run in to it

[PeanutNore]

You could make a pretty decent sounding preamp entirely out of LND150s. I use them in pedal circuits because a 9v supply puts them nicely in the saturation region where you get a sort of tube-like nonlinear response out of them. They work really well with a drain resistor that gives you 1 or 2 mA of current with the full supply voltage across it and a source resistor around 220 to 330 ohms. For source bypass capacitors, something like 220 or 330 uF will be needed if you want to boost all audible frequencies. 47-100uF seems good for guitar. On a 9v supply, one LND150 driving another one can give you a really nice mid-gain overdrive and a pretty large volume increase.

[Zozobra]

It's a pretty awesome FET for sure. I assume you've seen KMG's LND150 based JCM800? Sounds convinving to me!
Fet version of the JCM800 using LND150 mosfets