1W woes - anyone spent time on this before?

[Milkmansound]

I have been futzing around with a 1W amp design for a few months now and I can not get it to sound right. The idea was to do a class A mini tweed princeton using a 12AU7 and Fender reverb transformer.

The amp sounds great clean - but the onset of distortion is kind of fizzy. Tried a lot of things to smooth it out - grid stops, plate stops, different gain and voltages.

So I scrapped the 12AU7, dropped the voltage down to 180V and re-wired it to accept a 6AK6 mini pentode with a different output transformer. Again, clean tone is awesome - distortion is awesome - but the transition still has that fizz.

No change in layout or corrective filtering seems to stop it. I even loaded the sucker up with caps to nuke oscillations and the fizz is still there.

Has anyone spent time on this kind of thing? Should I just bail and move to a push pull design?

I need a pep talk because I have 4 cabinets on the way and a pile of chassis and transformers here

[da Geezer]

Since you've tried everything else, I suspect the OT primary is not to the liking of the tube(s) you're using.

Get one of those Hammond OT's with the endless selection of primary imps (125 series, I think), swap the wires around until you find the right combo, then either use that trannie (if its a one off) or get Edcor to wind the correct units for you (if for production)

$.02

G

[Milkmansound]

here is a very basic version of the schematic for those interested

I was thinking it has to have something to do with the transformer and coupling it to a reactive load like a speaker - as compared with a reverb spring. It sounds like a reflection to me.

[Milkmansound]

you mean this output transformer?

http://www.mouser.com/Search/ProductDet ... 4ukA%3D%3D

[martin manning]

That is probably the OT Geezer is referring to.

For another take on this genera, have a look and a listen here: http://music-electronics-forum.com/t4563/

[EtherealWidow]

Maybe a different brand of OT with the same primary impedance would suit it better. Some people don't like the Hammond trannies for the AX84 Firefly (self-split PP 12AU7). I can't find the thread, it was on a different site, but some dude suggested another tranny for the same application. I realize yours is SE, just trying to say that it might be worth experimenting with different OT's in super low wattage designs, as much of a pain as it is.

[xtian]

Coincidentally, I was just revisiting my AX84 Firefly build (my first build). It sounds pretty great. Only 1.5 watts clean, with ECC99 acting as push-pull. Check it out for inspiration.

https://tubeamparchive.com/viewtopic.php?t=11248

[Tillydog]

here is a very basic version of the schematic for those interested

What is your B+ ? (looks like ~350V which is above the Va max for 12AU7, I think... If it's ~175V, then I think the paralleled 12AU7 will be biased very cold.)

I guess that all your distortion will be coming from the output stage, so I think you will need a grid-stopper there, or you will get instant blocking distortion. With the mini pentode, you should be able to use huge values here.

Your coupling caps into stage 2 and 3 are very big (IMHO) try a factor of 10 or so less, particularly the one into the output stage.

As you turn the gain up, you will lose the grid-stopping effect of the top half of the gain pot into the 2nd stage, so I think you should fit a grid stopper there, too (may not be your problem).

The 25u / 1k in the cathode might not help, as it ends up having a roll-off in the 'buzzy' range (6Hz) - You can try reducing this (2-5uf?? or remove it?) to get the roll off frequency up, or make it bigger (200uF??) to take it out of the equation all together.

If you *are* getting some distortion from the 2nd stage, you might want to try biasing it a bit hotter to make sure that it isn't cold-clipping.

Just my £0.02 - take it or leave it!

[Milkmansound]

Yeah, I've played around with all of those values and added grid stoppers. I cut out the bypass caps and tried a few values. The fizz remained even as the amp changed character.

I played around with bias on the power tube as well.

Sounds like I have some more futzing to do. I think it's worth it because the amp does sound great when it's clean and during break up. Just gotta smooth out the middle!

It's not even that quiet - hard to believe that a single 12au7 can make that much sound!

[Structo]

I've been wanting to build one of these so I will be following your progress.

[Phil_S]

Take one triode off line and see how it sounds into the 22K reverb transformer. For parallel SE, I think you want a lower primary. I see someone else already suggested a 10K primary from Hammond.

[EtherealWidow]

It's not even that quiet - hard to believe that a single 12au7 can make that much sound!

1/4 as loud as a 50w amp!

[Tillydog]

Sounds like I have some more futzing to do...

In fairness, I should point out that I gave up on triodes for output stages as I couldn't get rid of that 'buzz' as the distortion fades out, no matter what I did....

[Phil_S]

If you want a clean triode amp that isn't real loud, try something like this. Preamp of your choice, just not high gain. You might want to back down the PT voltage a bit. I've got mine running on the edge of 450V, the max for 6SN7. I'm thinking 400-ish B+ would be good. I built this because I wanted to find a use the that 460-460 PT I had on the bone pile.

[romberg]

Sounds like I have some more futzing to do...

In fairness, I should point out that I gave up on triodes for output stages as I couldn't get rid of that 'buzz' as the distortion fades out, no matter what I did....

Did you try the Ruby zener clamp? My 4-4-0 did this swirly buzz thing on fading notes. Adding a 12v zener clamp fixed it right up.

Mike