EF86 preamplifier suggestions

[Giaime]

Hello all,

I'm in the process of designing a new amp for a friend of mine. I already built a similar one in head format: KT77 no-nfb power stage, 12AT7 LTP, post-pi master volume, GZ34 and a 2 stage preamplifier like this

http://users.telenet.be/orangefg/OFG...Cmainboard.pdf

the upper one (slightly less gainy than the lower one).

The owner likes the sound VERY much, so asked me to build another one in combo (1x12") format. My idea was to replicate identical electronics as the other amp (maybe with a little lower B+ and little less beefier transformers, for weight and early breakup), BUT I would like to add another "channel" based on a pentode (maybe EF86) that goes to the other (unusued) input of the LTP to mix the two channels (and to allow "bridging" of the two). This "other" pre will share only the master volume with the standard one, so I would like it to have gain and a single knob "tone" (or a rotary selector maybe?).

So, what would be a good circuit to obtain a VERY DIFFERENT sound from the 2 stage 12AX7 preamplifier of the normal channel? I would like something very compressed, fizzy, nasty, to be different than the relative "cleaness" of the 12AX7 based pre...

And also, a little bit OT, with those circuits in mind, what would be a good speaker? My friend likes the Fender Blues Junior tone, dislikes his G12H based closed back cabinet, sort of likes the Vintage 30 but think it's too bright. I'm oriented toward an Eminence Red Coat series.

[clyde1]

I haven't had good luck with the EF86 in a combo. Perhaps someone here has a method to overcome the microphonic problem.

[Steve Conner]

How do you like the sound clips of my Ninja Deluxe?
scopeblog Ninja Deluxe

The dirty channel uses an EF86.

I overcame the microphonics problem by using a rubber mounted tube chassis, and an original Mullard EF86.

[jrfrond]

This might be a ways off of your beaten path, but if you want a killer amp that is simple with an EF86 front-end, try something like an AC-15 design with two 6L6's, 6550's, EL34's, etc. with no NFB and cathode-biased.

A single EF86 has adequate gain for a front-end by itself. Run it into a LTP with a post-PI MV and a BRILLIANCE control after that, and you will have all the great-sounding amp you need with just three knobs.

I used to build and sell amps that were exactly that. I lifted the EF86 preamp component values from the Matchless DC30. You won't have the "gainiest" amp around, but the tone will be awesome, and the low-end will be tight when you turn it up, thanks to the pentode design. I used 50W Bassman replacement transformers (New Sensor) with a 250ohm/25W cathode-bias resistor on the power tubes, and can freely swap the aforementioned tubes for different flavors, but the 6550's actually rule.

Just my .02....

P.S.- I squelch the EF86 microphonics by placing a couple of bands of heat-shrink around the tube, about 1/4" wide.

[mooreamps]

P.S.- I squelch the EF86 microphonics by placing a couple of bands of heat-shrink around the tube, about 1/4" wide.

this is good engineering...


-g

[olddawg]

How do you like the sound clips of my Ninja Deluxe?
scopeblog Ninja Deluxe

The dirty channel uses an EF86.

I overcame the microphonics problem by using a rubber mounted tube chassis, and an original Mullard EF86.

That seems to be the standard proceedure. Use a good NOS tube and a shock mounted floating tube socket for the EF86. There are a number of builds discused a 18 watt.com for EF86 preamps and a number of schematics. I have considered building one but wonder if it is more trouble than it is worth. If I did, it would definitely be a 2 channel amp with the EF86 only used in one channel. Frankly, I have had great results just using a NOS 12AX7 for V1 and a good OD pedal in a single channel configuration.

[Giaime]

Thank you all for the replies.

Steve, I like very much the sound of your Ninja amplifier! I think I will pursue this road: I'll take inspiration from the AC15 and the various Matchless schematics around.

I'm still open on suggestions, of course, also about the speaker. Ah, I also have to add that the amp will mainly be used with Epiphone Dot guitars (335 copy) in a very '90s british-flavoured rock band.

[clyde1]

Unfortunately I only have a few (2) Mullards left. I also mount the socket with rubber o-rings, one between the screw head and the socket tab and one between the tab and chassis. I also make the mounting hole bigger than normal so that the bottom of the socket never touches the chassis. I tried a JJ, which made weird sounds when wiggled in the socket and a NOS Soviet-era
Svetlana, the Russky being better, but still a no-go. BTW, nice tone Steve.

[Giaime]

Is there a lesser famous alternative to EF86? I'm willing to experiment. I've heard of E80F, and maybe octal pentodes are more immune to microphonics due to the larger contact area.

[octal]

Well, you could try the 6sj7. NOS American ones are pretty cheap. I never built a combo with one, but I did build a head (placed directly on top of a 1X12 cabinet) and microphonics didn't seem to be an issue.

Nathan

[Giaime]

I can get european tubes more easily: I was thinking 6AU6 and 6BA6 (that seems close) and ECF82 (not so close, maybe I can use the triode section as a cathode follower after the pentode stage)...

[mooreamps]

Is there a lesser famous alternative to EF86? I'm willing to experiment. I've heard of E80F, and maybe octal pentodes are more immune to microphonics due to the larger contact area.

If you want to go the octal / plexi route, you could do this.



6SL7 > 6SJ7 > C/F > volume ( 100 K ) > bax or gmm tonestack > P/I using 6SL7 > two 6V6.. So, with this layout, you can do it with 5 tubes......

I picked a couple of 12SJ7 recently ; they were only like 5 dollars each. Not as much gain as the EF-86, and they need the extra gain stage in front to drive them.


-g

[octal]

I can get european tubes more easily: I was thinking 6AU6 and 6BA6 (that seems close) and ECF82 (not so close, maybe I can use the triode section as a cathode follower after the pentode stage)...

I have heard that both of those pentodes "sound bad"-whatever that means. I have never tried either myself, so I wouldn't take internet hearsay on them too seriously. I believe one or two really low end guitar amps (Kays, perhaps?) used a 6AU6 as an amp.

Those 6sj7s are cheap enough ($4.51 for NOS from tubesandmore.com) that shipping several to Europe would probably still be cheaper than an EF86.

I cribbed my 6sj7 circuit from the Ampeg Dolphin II schematic:

http://www.el34world.com/charts/Sche...te_Dolphin.pdf

I left off the tone stacks as they were missing values and it seemed like it would load the pentode down too much...

Good luck!

[octal]

One more thought. If you're going to use the "unused" other input of the LTP, I believe both plate resistors should be equal value. If you keep it "as is" one input will be in balance and the other input will be doubly out of balance.

Nathan.

[Giaime]

Thank you for your replies.

About the 6 way tone selector switch on the Matchless DC30, do you think it's a keeper or it is better to use a "tone" control knob? I think that the basic function of low-pass filtering is already embedded in the guitar tone knob, so it's useless to duplicate it in the amp: better to have an high-pass tone control like the 6 way switch.

About the speaker, and the possibility to have an "ext. speaker" jack: would you use an 8Ohm or 16Ohm internal speaker? I think that 16Ohm should be better (less Z ratio in the OT = better sound?), as there are more 16Ohm external speaker enclosures than 8Ohm ones. Or the "ext. speaker" jack could keep the internal speaker active... What do you think?