My perfect single ended amp

MikeM

Hi guys

Not going to carry on the previous amp thread as that idea has gone out the window. Instead I'll be working towards an awesome single ended baby. Here's my design so far

However the problem comes in with my super high B+ (+-475v) - Even with my redesigned power filtering, I am getting 469 on the B+. And this goes almost directly to my the plate? Now according to specs online, the max plate voltage on 5881s is 400v. And the max Screen Voltage is 250-270v. How much of a problem is it to go over these?

Also I see that my optimal impedence is 9.5k - my OT is 5k (469^2/23). I am guessing best solution is to get a 16ohm speaker or something?

Trying to wrap my brain around this http://www.freewebs.com/valvewizard1/se.html - only figured out that I want 50mA on the plate so far. Starting to fly over my head a bit :/

Mike

Karel-Mars

Nice one Mike ?

I have a weak spot for single ended amplifiers - I think their tone and sensitivity is unmatched by push-pull designs.

Let's look at your B+ figure. 470V on the Anode. With cathode current running at 50mA, your cathode should be 23V above ground. So the EL34 would actually see 470-23=447V. (Measured between pins 3 and 1 on the OP Tube)

The rule of thumb says you can exceed the the anode max voltage, but you should keep the screen voltage in check. From Duncan's TDSL I get ( http://tdsl.duncanamps.com/show.php?des=EL34 ) a G2 (screen grid) max of 500V. Your power supply looks safe to me.

You can also consider using a choke input filter on the amp - in your case, all you do is remove the first (22uF) smoothing cap. (or reduce it to 1-4uF with a polyprop cap) This should drop the voltage significantly, and might give a nice dynamic and smooth sound.

Thanks for the lovely schematic. There is a few errata on your drawing ? I am not going to comment on them all, but the cathode bypass cap on the second 12AX7 triode should be removed, otherwise the NFB loop will not work as intended.

MikeM

Karel Mars wroteLet's look at your B+ figure. 470V on the Anode. With cathode current running at 50mA, your cathode should be 23V above ground. So the EL34 would actually see 470-23=447V. (Measured between pins 3 and 1 on the OP Tube)

The rule of thumb says you can exceed the the anode max voltage, but you should keep the screen voltage in check.

First off I was silly enough to not point out that I'll be using a 5881 output tube :-[ So my bad there.

Ok I see! That makes things seem a little better. Will keep that in mind. The past few days have been a bit of an information overload, but in time this will all make sense.

Karel Mars wroteYou can also consider using a choke input filter on the amp - in your case, all you do is remove the first (22uF) smoothing cap. (or reduce it to 1-4uF with a polyprop cap) This should drop the voltage significantly, and might give a nice dynamic and smooth sound.

Fiddling with this now will see what I can do.

You mention polyprop, how do they differ tonally from electrolytic?

Karel Mars wroteThanks for the lovely schematic. There is a few errata on your drawing ? I am not going to comment on them all, but the cathode bypass cap on the second 12AX7 triode should be removed, otherwise the NFB loop will not work as intended.

Thanks Karel ? I'm really trying to understand these things better. I appreciate the fault checking - any you spot just point them out ? I know I can't get this things right first try..

Perhaps what I'll do - install a 47k NFB resistor and run a dpdt (on/off/on) to either put another 47k in parallel with that to get closer to the 22k, or ground the cathode bypass to completely disable the NFB.

Cheers
Mike

Last update for today - 1.2uf Solen Fast cap - 4h choke - 200h 500v can electrolytic (LCR - comments?) will post pics in a new post quick.

MikeM

1s delay, overall 3000ms - shows the 200uF cap filling steadily till a steady state.

4s delay - 500ms (Using results from 10 000ms) Min - 406.59, Max - 407.14, Diff - 548.34m

Anyone see any problems with this? I read the Solen caps sound tight and quite bright. Could be cool.

MikeM

Added a few annotations ?

TomCat

Like your design.....the basics are not that different from my TomCat 8W. It will be interesting to do an AB between your amp and mine when you have finished your amp and it's all sorted.

I'm loving the tone of my SE at the moment.

My brother-in-law played it at our church a few weeks ago and the tone was awesome....We put a Rat in front of the Cascaded channel and drove it hard.......tone heaven.. ? ?....huge compression and sustain......loads of overtones without it becoming brittle....

MikeM

? Awesome man.. I believe most designs SE designs are quite similar anyway, mainly down to the nitty gritty.

Messing around with the power filtering some more and it turns out using 32uF - 2k2 resistor - 32uF gets me a good voltage with a tiny tiny ripple (0.11v). What would the difference be between using that and a choke? Couldn't find much info on it ? I'm guessing resistors aren't common for filters as it lowers the voltage, but that's what I want ? However I'm not sure what it'd do to the current draw.

MikeM

Torn betwen the baby cap / big cap filtering and this new resistor based one.

Does anyone know how to calculate current draw etc? Would you mind checking up on my values etc? I think the easiest is going to be to buy a couple resistors in case, and then change them to the best value once assembled. That way I can be 100% sure the voltages are right.

And then lastly, impedence. This is confusing the butt out of me. How do I calculate this?

Mike

MikeM

Ok. Modded my amp, it's a bit quieter, WAY less hum, way better touch response, overall awesome.

Now I just need to figure out the whole impedance thing, and then get money to start working ?

Karel-Mars

The impedance thing is both complex and simple. My knowledge is limited on pentode/tetrode behaviour :'( but the rules are simple:

Check the datasheets. The Tung Sol 5881 datasheet has a graph for power output versus load impedance. 4200ohms is the lowest distortion/highest power spot. Use the recommended voltages/impedances.

Check here for ideas : http://tdsl.duncanamps.com/show.php?des=5881

I would guess the 4200ohm load is the way to go. I would then play with the sceengrid voltage till the sound hits the sweet spot ?

Some people go as low as 2500ohm, but this is usually on lower voltages like 250V. You can go higher, like 6000ohms etc. for higher voltages (450V etc.), but in this case I would use fixed bias to lower the idle current and increase the distortion.
Consider removing the first caps of you power supply, converting the power supply to a choke input type. This should give a voltage close to 250V.

MikeM

I see. Interesting.

Why would a lower voltage be better though? At the moment I've got my B+ down to 380 and it's lowered the volume and increased the drive a little. Also adjusted my bias to something more appropriate. I know lower voltages are usually used on EL34s and such but I usually see Fenders range from 300v upwards (To the DRs and Twins being crazy high).

Thanks

arjunmenon

MikeM wrote Why would a lower voltage be better though?

Van Halen / Brown Sound

MikeM

That's a marshall sound, hence why I mentioned el34. If I think 6l6 and variants I think Fender Deluxe. I'm guessing I'd just get lower volume and more drive?

Gearhead

I say keep the 380, you were looking for Fender headroom.

Karel Mars wrote I would then play with the sceengrid voltage till the sound hits the sweet spot ?

Hmmmmmm (looks at own schematic and ponders if experimentation is called for)

TomCat

Gearhead wrote I say keep the 380, you were looking for Fender headroom.

Karel Mars wrote I would then play with the sceengrid voltage till the sound hits the sweet spot ?
Hmmmmmm (looks at own schematic and ponders if experimentation is called for)

Where is that multimeter and soldering iron......??........ ?

guidothepimmp

how much give or take will a amp home build set one back?

my old man is an electrical eng and it may be a cool project todo with the old man..

h built me my 1st amp 17 years ago. a transistor mp at 100W RMS ???

?

MikeM

Keeping a super tight budget, I'd say as little as R2k...

Cool stuf man ? if he's an oldie then he must have some tube knowledge which would be awesome ?

TomCat

MikeM wrote Keeping a super tight budget, I'd say as little as R2k...

Cool stuf man ? if he's an oldie then he must have some tube knowledge which would be awesome ?

And that is a really tight budget.......My TC8W in total set me back about R 3.5K......but I did try to go high end on components and the loudspeaker.....the results were well worth the money spent.

MikeM

Aha.. Budgetting my next amp :? up to R2250 without chassis/transformer/speaker however does include a reverb tank :?

MikeM

Transformers come up to R1500 :/ Will have to put that project on the backburner for a while.

Think I need to get tweaking my amp though ?