86-Special ready with noise problem

[Tillydog]

I just looked at the schematic again, and I realised that the EF86 is the third stage, not the second. I think you're hitting it too hard. They sound bad (IMHO) if driven to cut-off - the EF86 in the schematic needs about 1V peak to drive it into cut-off, and is compressing heavily from a few 10s of mV at the grid. You can clean it up a little by increasing the cathode resistor (1k8, say), but I think you will either need to really attenuate the signal going into it, or to make it the second stage to keep it from cutting off.

[mat]

I just looked at the schematic again, and I realised that the EF86 is the third stage, not the second. I think you're hitting it too hard. They sound bad (IMHO) if driven to cut-off - the EF86 in the schematic needs about 1V peak to drive it into cut-off, and is compressing heavily from a few 10s of mV at the grid. You can clean it up a little by increasing the cathode resistor (1k8, say), but I think you will either need to really attenuate the signal going into it, or to make it the second stage to keep it from cutting off.

Thanks Tillydog, will try 1k8 at the cathode asap. I also noticed that I dont have the 1M to ground after drive pot (parallel to the 250k drive pot) that my
smaller build has. That might have something to do what you mentioned - slamming the EF86 too hard.

London was lovely btw. 21 celsius and sunny sky.. and British beer...mmm

[mat]

Ok, changed the cathode resistor to 1K8 one and did put a 1M across the drive pot. Better, less gain but too rubbery - kind of a big loud clean fender barking loud. I'll try to remove the smoothing caps one by one.

I hope to get more compression/chewing feeling on the drive channel. Clean is close to perfect.

Tweaking continues...

[mat]

Much, much better !

I had another 2k2 on EL84 loose. I tried the amp with and without my old Brown Sound In A Box 1 -build and now the amp sounds fantastic.

Sorry, this is just noodling iPhone recording: http://soundcloud.com/ef86special/recor ... ecording-2

[10thTx]

Mat,

I'd be real happy with an amp sounding like that!

Great tone. And as always .............. great playing!

Thanks for sharing what you've done.

If you're comfortable sharing it, I'd love to see what the final version of the schematic is?

With respect, 10thtx

[10thTx]

I went thru the thread and tried to change as much as I could follow from the posts in the schematic.

Mat, if you are comfortable with it ................... is this close?

1) added LNFB to V1b
2) 1M vol 2 pot to 500kb
3) smoothing caps to EF86
4) grid resistor for EF86 to 470k
5) cathode resistor for EF86 to 1.8k
6) 2.2k EL84 screen resistors
7) removed NFB from LTPI to speaker

With respect, 10thtx

EDIT: made more changes per Mat and reposted.

[da Geezer]

Much, much better !

I had another 2k2 on EL84 loose. I tried the amp with and without my old Brown Sound In A Box 1 -build and now the amp sounds fantastic.

Sorry, this is just noodling iPhone recording: http://soundcloud.com/ef86special/recor ... ecording-2

Mat, VERY NICE tone indeed!

Thanks for sharing your journey thru the development of this amp.

Geezer

[Tillydog]

Sorry, this is just noodling....

Sounding good (chops & amp).

Glad that you're making progress!

[mat]

Big thanks Tillydog and 10thTX for helping me on the amp

Yes Jeff, pretty close. OD tone pot cap to ground is .0022. Drive pot input is not on the viper, output is. Will have to check the 390p from plate to ground. High switch cap is 150pF. EF86 cathode cap is 10uF but will try 1uF. Poweramp input after the loop is 270k in to 22nF.

I will try the grid2 cap to cathode, not ground. Also will try 1M from g2 to ground/cathode

EF86 Vp= 150V, Vg2=100V, Vc=1.8V.

Lower string OD sound is pretty ugly still, will try lowering the cathode cap.

Too busy at work but will record something with SM57 asap.

[10thTx]

OK, thanks Mat!

I think I'll wait for final tweaks before editing it again.

Best regard, 10thtx

[mat]

Thanks Jeff for the schem. I tried today 2M2 on place of 820K g2 resistor. Did not like it (voltages were cV1.8-g2V60-pV200). Took the plate to ground cap of. Smoothed too much because I had 470p in there. This is good mod for the future. I'll buy a pot with a switch and try 220p-330p in there. I also connected the g2 cap to cathode instead to the ground. No help for the little noise on OD channel.
Did not have chance to play the amp loud enough but I hope I'll get something recorded in the end of the week.
The BSIAB1 pedal sounds and plays really, really good on this amp.

I would like to have a little more drive with the drive pot. Any suggestions that might not alter the frequency range too much ?

Btw. I also have the pre volume pot under side of the chassis. Easily adjustable from back of the amp.

[Tillydog]

I would like to have a little more drive with the drive pot. Any suggestions that might not alter the frequency range too much ?

You could try dropping that EF86 cathode resistor back down a little - somewhere between where you are now and the 820R (?) you started with. I suggested the 1.8k as it gives about as much headroom as possible, the 820R gave pretty much none!

When I was tuning my amp with an EF86 in the second stage, I found it difficult to make sense of changes to the screen & plate resistors. Changing one in isolation made a huge difference, rarely for the better and it seemed better to change both to change the operating point, keeping them somewhere near 5:1 ratio. I came to the conclusion that I don't understand pentodes!

I lost track of the number of iterations that I tried (literally dozens). I ended up with 100K plate, 470k screen (to B+, not a split plate resistor, but with with Merlin's 'morph' control), 1k cathode resistor + 2u2 bypass and a 560k grid stopper (B+ was about 230V IIRC). For reference only - YMMV.

Keep tweakin'

[mat]

Thanks Tillydog, I'll tweak again next week

[mat]

Ok, I ended up with 1k2 cathode resistor that gave me good overdrive sound (for it being a OD channel).

Sorry, I did not have chance to record the amp but will asap.

I think I'm in the final stage of tweaking it. I just ordered 1M and 250k pots with dpdt pull switches to make the plate cap an maybe cathode resistor switchable.

I played my Kinman equipped strat on the rehearsals and was very pleased with the sound.

Will get back..

[lord preset]

I decided to try Mat's design for my next build. I've got most all the parts and am going to stuff it all in a 12x7x2 chassis. While I'm sure the original continues to morph as i write I am using the latest schematic provided by 10thtx as the starting point. Going to use the PT and OT from a defunct Laney Cub and use solid state rectification. I will also omit the effects loop as I never ever use them.

Here is my crack at updating a much older version of the 86 Special Duet layout previously posted in another thread to the current schematic posted above. My layout and ExpressSCH skillz are feeble at best but I
think this is workable.