EF86 pre & parallel SE build Pics

[tubeswell]

Just finished the chassis for this at 11pm-ish tonight. Took me about 4 days and 3 nights from scratch (chassis bent on my new bar brake).

PT is a spare Weber W025130EU that I had lying around, Choke is a 100mA 50R 3H Mojo 777 I got specially for the job, OT is a custom-built SE (8k Pr 4R, 8R 16R Sec) that I had lying around. Curiously the outside of the PT (including the copper band) has no continuity with the chassis, even tho the PT bolts have continuity. The bolts are mounted with nylon grommets on this PT and that was stock from the factory. I guess that is normal for this PT.

The pre-amp has an EF86 slot in V1 with Merlin's Triode/Pentode Morph control (labelled "Mojo" on top, in case it doesn't work and I have to think of something else to change it to). This is followed by a 5F2A tone stack and Vol and driver stage.

The output stage has two parallel cathode-biased octal sockets. The cathodes have a common 470R/10uF that has a second common, switchable, 470R in parallel (with that mini toggle switch on the bottom). I am aiming to run 2 x 6V6 in there or 1 x 6L6/KT66. I probably could run a KT88 with that PT and choke. I went with a tube rectifier for flexibility in adjusting the B+ for this reason. (Why didn't I go with PP? Well I have wanted to try this parallel SE thing for a year or two now, altho' I've got heaps of PP iron, so I could do that later, but I want to get this going first).

The speaker taps are on a rotary selector switch. (The other socket is for the trem footswitch)

The trem circuit has an IRF820 source follower, and is set to wiggle the common 220k grid load resistor. I took the overall idea from the Gibson GA17RVT and adapted it, but I haven't tried it out yet to see if it works. More on that later. (I can't wait for Merlin's next book to come out on designing trem circuits ;-) ) I'm interested to hear any opinions about the trem circuit idea (see the schematic).

The grounding system is based on Merlin's article.

I'm too tired to fire it up now - so tomorrow. I have a small cab 1/2 finished in the garage, I'll try and finish that tomorrow as well. I have a Greenback G10 to try in it.

[mooreamps]

I think you should try it with the 2 6V6 first..

-g

[Steve Conner]

Very nice! Let's hear how it sounds.

Just one thing, you will probably have to ground the PT somehow for safety.

[tubeswell]

Very nice! Let's hear how it sounds.

Just one thing, you will probably have to ground the PT somehow for safety.

Thanks Steve. Yes I'm curious as to why the iron has no continuity with the chassis. Another amp tech I know locally (Clarry Schollum in Auckland) said that it isn't unusual for some PT cases not to have continuity with the chassis, and he had a Mesa on his bench yesterday when I was talking to him on the phone and he tested it while we were yacking, and it also didn't have continuity with the chassis. The Weber PT came stock with Nylon grommets between the PT bolts and the PT case, so AFAICT, it was deliberately designed that way.

[tubeswell]

Well after a mild kerfuffle this morning, (insanity break caused by troubleshooting mis-wiring and extending the height of the board off the chassis to avoid unwanted shorts), I figured out that the way I have the trem set up must be interferring with the 6V6 bias something wicked, so back to the drawing board for that part - looking for ideas please (maybe pre-amp driver tube trem wiggle??).

Anyhow I'm getting past myself.

I disconnected the trem and sorted out the NFB polarity and decided to swap over to the 'low power' HT taps on the PT. Still at this stage with one 6V6 plugged in running off a 5Y3GT. I also 'upped the 22k resistor to 27k, but I might put that 22k back again.

Anyway my first impression are that the 'Mojo' control is really effective at going from pentode to triode mode, and it has a wider range of tones and sounds than I wasn't expecting from a tweed princetony style amp - what with the 'Mojo' control and the volume and tone, there's quite a lot of scope with this little baby. And its loud - I couldn't turn it up past 4 with the little 15W alnico something speaker I found in the wardrobe. Scoops first prize. I will wire up the Greenback G10 later and try out the 2nd 6V6, although I have a hunch I'l be going to a 5U4G or GZ34 or going to those high taps.

I must get some soundbytes going.

PT
HT 558VAC (279-0-279)
Heater – 6.79VAC
Rectifier – 5.27VAC


Loaded
5Y3GT, 6V6S (8K Pr Z), 12AX7, EF86

HT voltages
B+ = 326 VDC at reservoir cap
10k node = 275 VDC
27k node = 209 VDC

6V6S
Plate = 321 VDC
Screen = 272 VDC 560R g2 resistor dropping 3V = 0.005A
Cathode = 16.4 VDC (across 472R measured) = .0347A

.0347A Ia+Ig2 x 304.6V a-k = 10.6W (minus 5.3mA Ig2 = 9.5W plate dissipation)

12AX7

Driver Stage (209 HT)
Plate = 127 VDC
Cathode = 1.23 VDC

LFO Stage (343 HT)
Plate = 155 VDC
Cathode = 1.75 VDC

EF86
(209 HT)
Plate = 92 VDC
Screen = 69 VDC
Cathode = 0.99 VDC


Voltages on the 'high-power" HT tap:


PT
HT 719VAC (359.5-0-359.5)
Heater – 6.79VAC
Rectifier – 5.27VAC

Loaded
5Y3GT, 6V6S (8K Pr Z), 12AX7, EF86

HT voltages
B+ = 415 VDC at reservoir cap (470 peak at startup)
10k node = 343 VDC (436 peak at startup)
22k node = 280 VDC (369 peak at startup)

6V6S
Plate = 406 VDC
Screen = 340 VDC 560R g2 resistor dropping 3V = 0.005A
Cathode = 19.8 VDC (across 472R measured) = .0419A

.0419A Ia+Ig2 x 386.2V a-k = 16W (minus 5.3mA Ig2 = 14W plate dissipation)

12AX7
Driver Stage (280 HT)
Plate = 182 VDC
Cathode = 1.31 VDC

LFO Stage (343 HT)
Plate = 208 VDC
Cathode = 1.94VDC

EF86
Plate = 120 VDC
Screen = 91 VDC
Cathode = 1.3 VDC

[tubeswell]

I must get some soundbytes going.

Errr... 2 bits of monotony FWIW. (Me crudely trying to explain the different combinations of sound with only 1 x 6V6 going into a Weber Sig 12S. Not mic'd at all well sorry, just with the internal mic on my emac and garbageband. I promise to set up a proper recording of it when I find the optimum speaker and tube setting etc.)

Sorry about the large file size:quality equation tboy

[tubeswell]

Well I've tried the 1 x 6V6 into another old 12" 15W alnico (Jensen maybe?) I had and it sound pretty bluesy into that. Then I tried 2 x 6V6S in parallel into the Sig12S and it makes that speaker sounds bluesier (if I say so myself). The extra tube somehow seems to make the Sig12S work a bit harder or something. I have to get some properly recorded soundbytes when I have time.

Edit - I just tried a 1 x 6L6 (in '6L6 mode" of course) and it sounds even better into the Sig12S - much bigger sound and much more rocky. I also note that the 6L6 can take more gain than the 6V6s without breaking up, and that translates into less mush at higher vols. (I wonder how a 1 x KT88 would go in there? - okay with the 100mA supply choke?)

With the 5Y3GT off the 270-0-270 HT tap, the B+ is 308 and screen supply node is 253 with the 6L6 (probably similar with the 2 x 6V6s) - plate is 301, screen is 251, cathode is 14.2 through 236R (.060A) = 16.1W (after the 3.5mA screen current is deducted).

With a 5U4GB and a 6L6 off the 270-0-270 tap the B+ is 333 and screen supply node is 285, and so everything is a little higher throughout. Biased differently (with 472R) the cathode voltage is 20.6, and the plate and screen voltages are 332 and 288 respectively.

[mooreamps]

I ran a KT-88 inside one of my 36 watt SE amps driven by a 6SL7. That combination gives you a really big sounding tone with lots of low end.. Maybe too much low end, I don't know...

-g

[tubeswell]

Cab done

Now I can get on with sorting out the trem.

The fabric definitely helps to eliminate vibrations. I found some O-rings to go around the EF86, so it can be dimed now quite happily. Has a great blues tone in pentode mode with the vol on about 1/3 running a 5U4GB and 2 x 6V6s driving the G10. I got to get some proper sound-bytes done next w/e.

[tubeswell]

Trem sorted - turned out to be not enough voltage swing to the grid(s).

[redelephant]

Beautiful man. Did u make the cab yourself too? Awesome!

[GregS]

Nice build!

I found some O-rings to go around the EF86, so it can be dimed now quite happily.

Were you actually having problems with microphonics before you did that? I just did a build with an EF86 in it and have done nothing special to shield it from vibarations and I don't have any problems at all. Mind you mine is a head and speaker cabinet rather than a combo and I don't have the EF86 stage set up for really high gain.

[tubeswell]

Were you actually having problems with microphonics before you did that? .

Nah it was more of a precaution really. The EF86 isn't set up for that much gain in this circuit either.

[tubeswell]

Beautiful man. Did u make the cab yourself too? Awesome!

Yup. I've got my system for making these cabs now, and I'm sticking to it.

[paggerman]

Very nice & thank you

Hi Tubes - I was in your part of the world last week (south island) and what a beautiful place it is. Thanks for posting your schematic here - it's exactly what I've been looking for the last few weeks. I have bought a hybrid PA with a SS preamp and 4 EL84 output tubes. I wanted to try Merlin's morph control as well so I was just wondering if I needed a triode as well (before or after EF86?). I think that you have answered my question with your diagram. Many thanks. BTW what is the 'ball and socket' in the top left hand corner of the trem circuit?