New Design - Feedback welcome

[lkrasner]

Designed this a while ago as a somewhat champ-like build with some extra options. Just thought of going back to it, curious for any feedback anyone has before I start getting more into the project. Anything welcome. Thinking of changing the power supply caps to be values that I can get in a multi section can cap to save space.

Side note, does anyone have a good source for getting finished heads or combo cabs to put amps in. I like the Mojotone ones, but they are a bit more expensive than I'd like to spend.

[PRR]

No heater wiring shown.

If you want NFB off the speaker, you gonna hafta ground the other end of the speaker winding.

That series-pot NFB adjustment won't dial to zero. IMHO, a sweet Champ tried with a variety of speakers may want a lot to no NFB. The attached hack will do that, no extra parts.

[drgonzonm]

Not sure why you are using filament wiring to feed cathode on power tube.  (fed through R18 and 19)
If you are creating artificial center tap, with this design, and you have wired heater wires upstream of the two resistors, then I wonder if you are not creating a ground loop.

At R14, you are throwing away a lot of power.   

[PRR]

Tying heater loop to a small positive voltage (such as power cathodes) is a common trick to reduce buzz from cathode leakage.

> At R14, you are throwing away a lot of power.

More than I would pick, if only because 220K 5W is not a common value, and I am somewhat fearless (or highly cautious) about sticking my hand in a recently-ON chassis.

But 87uFd times 220K is like 20 seconds to bleed to 63%. Assuming 350V, that is 220V after 20 sec, 140V in 40 sec, 87V in a minute. That's still a nasty sting.

Also the actual dissipation, even at 400V, in 220K is only 0.7 Watts. That's like 5% of B+ power and a couple % of total power, not a big waste.

[lkrasner]

No heater wiring shown.

I usually just leave it out because I'm lazy. I'll add it in.

If you want NFB off the speaker, you gonna hafta ground the other end of the speaker winding.

That series-pot NFB adjustment won't dial to zero. IMHO, a sweet Champ tried with a variety of speakers may want a lot to no NFB. The attached hack will do that, no extra parts.

Noted, I'll update that, thanks.

Not sure why you are using filament wiring to feed cathode on power tube.  (fed through R18 and 19)
If you are creating artificial center tap, with this design, and you have wired heater wires upstream of the two resistors, then I wonder if you are not creating a ground loop.

That is meant to be a center tap, using the voltage off the cathode to raise the heater voltage and reduce noise.


I'll update the schematic with a few changes, thanks for the input.

[lkrasner]

Alright made a few changes. Updated power transformer to be the MOJO759 sold at Hoffman, which is a bit higher voltage (290 vs 270), should I be concerned about this in any way or are my other values still good based off that?

Changed bleeder to 3W due to availability. Seems most every one goes with 220K, that's what I've used before, and I'm usually pretty careful to wait and check voltage before touching stuff.

Changed power supply caps to match 40-20-20-20 JJ can cap to save space and cost. Close enough that it shouldn't really effect anything?

Changed R15 and R16 to 2.2K and 10K respectively due to availability, once again shouldn't be a problem?


Changed C4 to .0047 uF, and coupling caps to .022 uF, once again, really shouldn't change anything.

[sluckey]

Heaters must connect directly to the PT, not at those resistors as shown.

Still need to ground the bottom side of the speaker for NFB to work.

[PRR]

> I usually just leave it out because I'm lazy.

Sure, me too.

But since you were asking, I pointed, just in case you didn't know (all sorts of folks here). Didn't want you to yellow-marker the build against the diagram no-error, then still not light-up.

(I have found it is often the things I "know to do" which I forget when hot and sweaty with the solder and axe.)

[lkrasner]

> I usually just leave it out because I'm lazy.

Sure, me too.

But since you were asking, I pointed, just in case you didn't know (all sorts of folks here). Didn't want you to yellow-marker the build against the diagram no-error, then still not light-up.

(I have found it is often the things I "know to do" which I forget when hot and sweaty with the solder and axe.)

Believe me, I've been stuck due to much stupider problems than that before  , thanks for pointing it out.




Believe I fixed the NFB and changed the transformer to have a center tapped heater winding (eliminating the artificial one) to match the new one I'm planning to use.

[drgonzonm]

You guys have more experience than I do regarding wiring a center tap on the filaments leads to the slightly elevated voltage on the power tubes cathode.  Is this going to be a problem, or are you better off using an artificial CT at the slightly elevated voltage? 

PRR, thanks for the info on wiring to a slightly elevated voltage. 

Steve, I am not sure if I understand your comment regarding putting the NFB on the bottom side of the speaker.  I did a quick perusal of the 5F1 schematic, and it is wired in a similar fashion to the proposed schematic. 

[lkrasner]

I'm thinking of adding a bit to this. I want to keep the tweed style circuit, but I'm going to build it as a combo, probably in a 5e3 style 1x12 cab so I have some room to add on.

How do you think this would respond to adding a reverb circuit in? Maybe something like this? https://robrobinette.com/Compact_Reverb.htm#Add-a-circuit or this: http://el34world.com/Forum/index.php?topic=7957.0.

If I went with the first, anyone have a good idea for what to do with the other half 12ax7

[sluckey]

Steve, I am not sure if I understand your comment regarding putting the NFB on the bottom side of the speaker.

That's not what I said. Read it again. Maybe you'll get it.

[drgonzonm]

Steve, I am not sure if I understand your comment regarding putting the NFB on the bottom side of the speaker.

That's not what I said. Read it again. Maybe you'll get it.

Steve,  As I stated, I did not really understand your comment.  However, more importantly, apparently lkrasner took care of your recommendation in an updated schematic.