Non-CF Clipper?

[sepulchre]

Martin, do you think the reservoir should be upped? I thought I had read that 10uF was the max. But that was some months ago so I cannot recall exactly.

I decided to do away with the tremolo. As I said, I'm not wild about it and it would probably get use once to see if it works. And the added gain stage is far more desirable IMO. This was originally going to be a copy of a Univox U45b which was my very first amp. But it didn't really sound all that great so I'm taking the opportunity to make something a little different.

I wanted to see how it would look when finished before tearing into it so I put it in the cab for a photo-op.

When I get it to where we cam hear it I'll post more. Thanks for all the advice and ideas everybody. Martin, I'm going to read Merlin's piece there and maybe modify the PS a bit.

Thanks again, guys!

[martin manning]

Looks very nice! I would run through the calculations to get the equivalent series resistance of your PT and see if it meets the requirement on the data sheet, and add some if necessary. Then, try upping the reservoir and see how it affects the sound. You can easily clip another cap across the existing one to try 20uF or 30uF, and choose the one you like.

[sepulchre]

Okay. Got it together, sort of. I guess I had been lucky up to now; for the first time in my amp building experience this one has All kinds of problems. If I turn the Volume control up to about 3 o'clock it starts motor boating. But that's the least of my woes. It sounds absolutely Terrible! It's blatty and frankly sounds like there's a bad tube in the woodpile. I swapped out the AX7 with no effect at all. Plus there's a Huge 60 cycle hum, even with both knobs down to below 9 o'clock.

Some of this is, I'm sure, is because of the long runs of signal wire across the board/chassis. I tried not using coax except for from the input to the first preamp tube. Here's the schematic and a gut shot.

[sluckey]

I see two issues on your schematic. V1-7 needs a 1M resistor connected to ground. The junction of R12 and R13 needs to be connected to ground.

Is the amp wired with these two errors?

[sepulchre]

Yes it is. And in fact there are some errors in the schematic as well as in my wiring. Note: there is no cap in the input signal anymore, just a 47K resistor for RF blocking.

The original design had no grid leak resistor (!). It was my oversight to leave it out of my new design. The other problem you mentioned is another oversight on my part - I simply forgot to ground the junction of R12 & R13, both on the schematic and in the actual amp.

Thank you for providing a very astute set of eyes. Aint this place great?

(I may not be as Duh as I appear, but a load of distractions and a lack of sleep make that a moot point; the results are the same.)

Thanks again!

[matt h]

I would absolutely expect a lot of motorboating and hum in that amp. The power supply is woefully, woefully underfiltered (and I like'em loose!) You need 4 B+ nodes with proper decoupling (often called "dropping" resistors). At least 1k, 2k better to properly decouple the preamp from the PI node. I'd want at least a 20uf (plate), 20uF (screen), 10uF (PI and driver), 10uF (preamp stages). You might get away with using that 4.7uF for the preamp node that late in the game, though. You'll want to reduce the 22k resistor though without voltages on the schematic it's hard to say.

as for the splattyawfulness of the distortion... basically, you're overdriving your bass first, that's almost never a sound. (fartyfartyfarty).

Were this my amp, I'd do the following:

-200k anode resistor on input stage, not second gain stage
-reduce value of 1st stage coupling cap (optional but strongly suggested)
increase value of 1st stage bias resistor (sure, use the 2.7k) and reduce cap value (sure, steal that 4.7uF from second stage, though I'd be tempted to go smaller like 1uF if you're not reducing the coupling cap)
-Change "tone" to a bridged-t "mid" control and leave in the current place. More mid scoop sounds better clean, more mids sounds better cranked *and* gives you more signal to distort the second stage. This also makes the "gain" knob optional)
-Add a "cut" control for your high end roll-off. (which you just freed up by removing the "gain" control)
-use the "paul C mod" style fixed-bias concertina, it handles distortion better. If not, at the very least get a grid stopper on that concertina.

I know you were told to increase the Rk on the driver stage, that's the opposite of what I'd do. go warm here. I don't know appropriate 6bm8 values, but if it were a 12ax7 stage, I'd be looking UNDER 700ohms (for comparison) Consider not using a Ck for improving tone when cranked. I can't stand the sound of more than two cascaded cold (or even center) biased stages.

You'll want to use your volume knob on the guitar more by not having the "gain" pot in there, but that'll give you a hugely flexible.

[sepulchre]

Okay, THANK YOU sluckey! Those two simple oversight were the lion's share of my problems. I still have some hum issue but I think at least some of that can be attributed to the fact that my speaker jack is not isolated from the chassis so I'm getting some ground loop hum.

I haven't checked the motor boating problem because my wife's asleep and this little sucker is Way louder than I expected so I don't dare crank the volume up to 3 o'clock. If it is still motor boating that would surely rock the foundations here.

Thank you again for your sharp eye. My troubles are 99% solved.

[sepulchre]

Matt, you were posting at he same time I was so I missed your post until now.

First, the 6X4 rectifier is somewhat limited on how much filtering I can use. Yes, the 10Uf are probably low but that's what was in the original design. I'm hoping 20uF for the reservoir will not push that little guy too far.

I'll try moving some of that 22K dropping resistor to between the PI and second stage. I wish I had better figures on the 6BM8 triode section - the data sheet doesn't provide all I'd like, but it's certain not like a 12AX7. The voltage gain is quoted as 70 but I have little data beyond that.

The splattyness is gone - adding a grid leak took care of that, along with grounding the junction where the tremolo used to be. (meant to do that, forgot.. duh)

So, you think I should basically swap the first and second stage values? 200K Ra, 2.7K (3K would be better) Rk and 4.7 Ck? I know a lot of designs take the preamp Ck on down to even .68uF but I like to preserve some lows as long as they're not causing problems.

I'm looking at concertina designs to see what a common value would be for the grid stopper but I'm really not seeing any stoppers at all, just coupling caps. So what do you think, 10K? 47K? 470K?

I'm think maybe 820R (yeah, it's above 700 but it's what I've got handy) for the PI driver, un-bypassed. I'll see how that sounds and adjust if needed.

Thanks for the pointers, matt! I think I'm getting there.

Btw, oyu saw the gut shot, right? Think I need some coax on the longer signal runs? (the blues wires)

edit: I'll try to get a schematic with voltages on it posted

[matt h]

I saw that there was a gut-shot, I didn't look at it. I know a lot of people here obsess about gut-shots, but I mostly think that if there are actual design problems, whether or not the gut shots look pretty doesn't really matter.

For the most part, "reversing" the values for the first two gain stages is a start. I don't know what sort of guitar you typically play, but I rarely find that values greater than 1uF are really necessary for input stages unless you've got a thin sounding guitar (i.e., weak single coils). Bass, in early stages, is the enemy for distortion. Remember that scene about amplifier design from the third Indiana Jones movie? "Only the flatulent note shall pass!" But my mantra is "EQ is all relative", you've probably seen me say it before. Need more bass? Well, if you use the bridged-T I was talking about, just moderately scoop your mids a lil more and you'll get more (relative) bass. Again, since you'll mostly want a pronounced low end when playing cleaner, it double-duties. If you use the gibson style T, rather than the Framus style, you can stick a fixed resistor underneath the variable-resistor (pot as rheostat) to limit the amount of cut. Something like 1/4-1/2 of the value of the pot would probably work out. But you might light the extreme scoop. (if you're someone who always wants mid-scoop, you could also use a carmen ghia style tone control in that position)

For a grid stopper value on the pi, the sorta standard array from 100ohms to 100k is your playground. If you've got a 10k handy, give it a whirl, that's a great starting point. Check out Merlin's page on http://valvewizard.co.uk/cathodyne.html I gotta say, there's a certain appeal have DC coupling the final (driver) stage into the concertina. Avoids a coupling cap (place where blocking distortion can occur).


As for things like needing shielded cable? Pfff. Plenty of people have managed to get, for instance, things like the AX84 HO built with no shielded cable.

Apparently I deleted the section I just wrote about "hey, ditch your tube recto if it's seriously limiting! free up a socket for amplification!" but this http://www.mif.pg.gda.pl/homepages/fran ... /6/6X4.gif makes me think you're absolutely fine with a 20uF filter cap.

[sepulchre]

Yeah, I built an AX84 SEL with no coax. Sounds great.

I put a 2K dropper between the driver/PI with a 10uF filter there. That should decouple it. And I swapped the values on the first & second stage so I'm on the way. I'll have to look into the tone control issue and figure what to do.

Thanks, Matt.

[matt h]

Definitely keep us posted! Power supplies are tricky beasts, depending on your design goals. Balancing noise, stability, and stiffness.

[martin manning]

Note Gibson GA-5T uses 6X4 with 20uF for plate, screen, and preamp filters, with 1k between the plate and screen nodes, 22k between the screen node and the preamp. What do you have for the supply voltages now?

Here's a data sheet for Svetlana 6BM8 that shows the triode curves: http://www.drtube.com/datasheets/6bm8-sed1996.pdf For reference it looks like with a 100k plate load a 2k7 cathode resistor would center the bias point pretty well, 4k7 for 200k.

Note the RCA data sheet lists mu of 70, but that's at 100V Va and 3.5mA. The Svetlana plate curves indicate that for an operating point like you would get with a 200k load and 4k7 Rk, or 100k and 2k7, mu will be more like 45-48.

[sepulchre]

Thanks Martin, and Matt. I've redone a good part of the circuit. I just checked the voltages and put them on the chart.

It sounds Way better! But there's still a lot of hum. It's not as bad as it was but still not acceptable.

[ampgeek]

Maybe try grounding the filament CT at the output tube cathodes/bias resistor junction just to see if the hum situation improves?

Just thinking out loud.

Good luck,
DAve O.

[sepulchre]

Hey, that's not bad. It worked for my single ended project. I'll give it a try tomorrow after work.

Thanks!