Merlin's morphing Pentode/Ultralinear/Triode EF86

[LeftyStrat]

Has anyone built the triode/ultralinear/pentode circuit (p. 95) from Merlin's preamp book?

I was wondering if it was a worthwhile mod, or something you're likely to set once and never use.

[jelle]

I have and it is fun! It is a nice gain- and mostly character control.

[LeftyStrat]

Thanks for the info. I'm experimenting with a very minimalist amp that is sort of AC10 inspired, thought it might be interesting to try. I want to see what I can manage to squeeze out of an ef86 -> 12ax7 PI -> high cut -> 2xel84.

My only other concern is that the original AC10 ran the plates on the ef86 at 470k compared to Merlin going more conservative at 100k to reduce microphonics. So I'm not sure I will have sufficient gain.

Thought it would be an interesting platform to gauge the effects of different caps and resistors.

[gingertube]

Lefty,

Look at DR Z Route 66 Schematic here:
http://www.prowessamplifiers.com/schematics/misc/

Exactly what you describe but different output tubes and less the morph control.

Cheers,
Ian

[Dingleberry]

You guys read minds?

I have been planning a "Route 84" type amp for a long time and finally last week I started to prepare chassis for that build. I have few TFK EF86's that I have wanted to try, but I have no amp with EF86 pre.

Cool to hear that other people are experimenting with same tube compliment. Keep in touch!
-T

[jelle]

Yes, did it with el84s...works great. Have fun!

[Structo]

How about this scheme.

[M Fowler]

I built a Dr Z Route 66 first with JTM45 iron and then I replaced those with Hammond iron and the OT has UL taps.

Mark

[slimvince]

I gutted an Orange AD5, reused some parts (chassis, transformers, pots etc) and converted it to an EF86 single ended amp. I incorporated Merlin's triode/pentode morph. I run it through a 2x12 Celestion Blue/Gold or through the original AD5-speaker. No prices for guessing which is best .

I have modded it with a switch to bypass the tone control after I did the schematic.

The difference in gain between triode and pentode modes is big enough making it difficult to do a direct a/b-comparison. With a standard strat it needs a boost pedal in front of it if you want it to start to break up, but not with something with hotter pickups. The VOX-twang certainly is there.

Very likely I will hardwire it 100% pentode, I like the pentode early breakup more. For heavier gain a pedal (SHOD, TubeScreamer or Zehndrive) sounds better than driving the EF86 harder IMHO.

[briane]

ef86 -> 12ax7 PI -> high cut -> 2xel84.

this can be an awesome combination - just look at the AC30 or DC30.

I build an ef86 version of the 18 watt marshall that is real sweet - has the crunch the 12ax7 front end version lacks - schematic at 18watt.

[Colossal]

I build an ef86 version of the 18 watt marshall that is real sweet - has the crunch the 12ax7 front end version lacks - schematic at 18watt.

Was your amp something similar to the Foxy 18?

[212Mavguy]

ef86 -> 12ax7 PI -> high cut -> 2xel84.

this can be an awesome combination - just look at the AC30 or DC30.

I build an ef86 version of the 18 watt marshall that is real sweet - has the crunch the 12ax7 front end version lacks - schematic at 18watt.

...in the "I have an ef86 amp" peanut gallery, have a slightly modded Ampex A620 suitcase style amplified speaker, 8" JBL, Mullard circuit, 6ax5, pair of hot running cathode biased 6v6gt, ef86/12au7 PI front end. Nice fat Triad iron all flying PTP guts with nice parts. OS 6085 sounds godly in the PI spot.

I had a friend disconnect the tone control upon his advice a few years ago when I knew absolutely nothing, he set the gain factor for the ef86 at 40 and voiced the preamp for the speaker. When I stuck a personally tubed Peavey Rockmaster 3 channel guitar preamp in front of it the results were jaw dropping, so good that I later wired in a switching speaker jack so I could use the thing as an amp head. With nothing in front it's a clean, not very loud mellow sounding tube practice amp.

In this particular circuit, the ef86 loves to get spanked, and hard. And not having it in V1 (so to speak as the result of the 4 tube preamp in front) is good from the microphonics point of view, so it helps when using the RM in front to wake up and open up all the tones and get OD, and three channels on the footswitch worth of gain levels are nice. In addition, the RM has 5 separate fx loops built in, and they are dirt cheap used. It's tone knobs are fine to use instead of having any in the main amp, so no loss there. I can turn the Ampex's volume knob down to half way-ish, smack the front end with a lot of signal and that 45 year old old hifi amplified speaker will not only deliver a great clean, it will flat out roar, bellow, and scream like a drunken blues diva dimed or right on down to decent, smaller rooms volume if necessary.

[tubeswell]

I've done this on a couple of amps now where the EF86 was used for the input and then the 2nd stage.





As jelle says, it mostly works like a gain control. However at the pentode end of the pot rotation, the bandwidth is noticeably wider as well. And on the lower strings you tend to get interesting tonal shifts.

FWIW, I have also tried the other circuits in that chapter (just before the morph control) and they also produce noticeable tonal effect variation.

I also did a variation where the EF86 had a CF stage and where I took the morph control input from the CF cathode. This helps with the loading of the EF86.

[Colossal]

I also did a variation where the EF86 had a CF stage and where I took the morph control input from the CF cathode. This helps with the loading of the EF86.

Thanks for the comments. I've been wondering greatly about that circuit. Also thought about a CF post EF86, that seems like it would let the EF86 do more of what it does even better. How did you like the effect the CF had on the tone? I have been really interested in an EF86 front end into *something*, into a LTPPI, and into a 2xEL84 power stage. Where did you bias your amp? Vox style super hot Class AB or more towards true Class A?

[LeftyStrat]

I've done this on a couple of amps now where the EF86 was used for the input and then the 2nd stage.

Interesting. Especially the second schematic with the EF86 as the second stage. I've often wondered if you could slam the input of the ef86 to get most of the amps distortion prior to the PI/output.

Thanks for sharing.