EF86 with morph control in PP gigging amp (finished)

[tubeswell]

So much did I enjoy the EF86/morph control in the SE amp that I built a couple of months back, that I thought I'd take the same concept into more powerful gigging amp. (Chassis done on my recently acquired bar-brake)

Finished the (Ef86 concept) version of this amp tonight. Didn't get a chance to really blast it yet, but it sounds very promising.

Happy to say no buzzes or rattles or hums. Tone is EF86tastic! (Need to do some soundbytes). Tone controls need tweaking to get the scoop/hump concept improved upon.

(Edited/corrected) schematic of as-built version with measured idle voltages attached below. I think it needs a voltage tweak in the power rail. I'll chuck a 5AR4 in and see how that goes (or maybe drop the first supply resistor to 4k7).

[Willabe]

Wow! Looks beautiful.

You do real nice work tubeswell, very nice work indeed.


       Brad         

[LooseChange]

Nice work!  What's the morph control?

[tubenit]

What's the morph control

In the schematic in his first post.  Looks to be a cap into a 1M pot into a cap into the grid of a CF after the EF86.
Interesting idea!

With respect, Tubenit

[Tone Junkie]

Nice tubswell you do good work, the morph control sounds cool.
Bill

[Tone Junkie]

I also like your use of the 12dw7 tube i think that is an under utilized tube in guitar amps. expecially those with a blues flaver.
Bill

[tubeswell]

What's the morph control

In the schematic in his first post.  Looks to be a cap into a 1M pot into a cap into the grid of a CF after the EF86.
Interesting idea!

With respect, Tubenit

Correct.  Its in Merlin Blencowe's 1st book. At one end of the pot rotation the screen resistor is bypassed to ground (in this case via the 100nF), and the stage functions like a normal EF86 pentode. At the other end of the pot rotation, the 100nF is coupled to the plate via a bigger 470nF (to keep bandwidths about right). The screen is still at the screen voltage but there is screen compression effect and since the screen and the plate become effectively AC-coupled, it acts like a triode, so there is less gain overall. Somewhere in between, you get something akin to ultralinear operation, which sounds quite nice too.

Having the DC-coupled CF stage there is just an evolution of the same thing I did in an SE amp a few months ago.

[Fresh_Start]

I also like your use of the 12dw7 tube i think that is an under utilized tube in guitar amps. expecially those with a blues flaver.
Bill

I understand using Section 2 of a 12DW7 as the cathode follower if you need to push a lossy tone stack.  Did you design the circuit with that tube from the start or was 1/2 of a 12AX7 not giving you something you get with the 12DW7?  Does it do the compression thing that tubenit likes so much better?

Thanks for sharing.  NICE looking build.

Chip

[tubeswell]

I also like your use of the 12dw7 tube i think that is an under utilized tube in guitar amps. expecially those with a blues flaver.
Bill

I understand using Section 2 of a 12DW7 as the cathode follower if you need to push a lossy tone stack.  Did you design the circuit with that tube from the start or was 1/2 of a 12AX7 not giving you something you get with the 12DW7?  Does it do the compression thing that tubenit likes so much better?

Thanks for sharing.  NICE looking build.

Chip

Hi Chip

I initially built a SE amp with this kind of EF86 stage and Big Muff tone stack, and it was very nice, but for this one I thought I'd try the DC-coupled CF with a 12AU7 to see how it sounded, and I'm liking it a lot.  The 12AT7 in the LTP makes it sound extra sweet as well (altho' the extra current draw does lower the HT voltage a wee bit, compared to if you plug a 12AX7 in there).

[jojokeo]

Very nice TW, that's one to be proud of and well thought out even w/ extra long ac chord. You've put a lot of things in there I like to see w/ good values too. I'm sure it sounds like it's designed and the speaker is one of my favorites too. It looks like you made the cab to go w/ that custom chassis? What, no finger joints? (just kidding you) I'd like to hear some sound bites of her.

[kagliostro]

Very Very Nice Job !  

which is the purpose of the double ganged pot ? have you a PPIMV ?

can you post a link to the schematic of the SE version amp ?

Thanks

Kagliostro

[tubeswell]

Very Very Nice Job !  

which is the purpose of the double ganged pot ? have you a PPIMV ?

can you post a link to the schematic you posted ?

Thanks

Kagliostro

Thanks Kagliostro

That pot is just a (rare) double-ganged 5MRA pot that I'm using in parallel to get 2M5RA for the trem speed pot (using the 5G9 trem circuit for that off-your-face slam).

link is here: http://www.el34world.com/Forum/index.php?action=dlattach;topic=11813.0;attach=24182

[tubeswell]

Very nice TW, that's one to be proud of and well thought out even w/ extra long ac chord. You've put a lot of things in there I like to see w/ good values too. I'm sure it sounds like it's designed and the speaker is one of my favorites too. It looks like you made the cab to go w/ that custom chassis? What, no finger joints? (just kidding you) I'd like to hear some sound bites of her.

Thanks jojokeo

As for the soundbytes, I'll get a man on it right away ;-)

[kagliostro]

Hi Tubeswell

That pot is just a (rare) double-ganged 5MRA pot that I'm using in parallel to get 2M5RA for the trem speed pot (using the 5G9 trem circuit for that off-your-face slam)

Oh ....... I see

thanks for the schematic

have you posted also the SE version ?

I'll repeat, really a well build amp

Kagliostro

[Fresh_Start]

Taking a second look at the schematic as a whole, I really like your design.  You obviously put a lot of thought into it.  The color coded ground points on the schematic is a nice touch.

I can tell you've read the "Immortal Amplifier" articles (solid state diodes before the rectifier tube).  Is the second fuse on the HT line before the Standby switch?  (not shown on schematic)

Cheers,

Chip

[DummyLoad]

very nice work. 

those bias test points are all going to read "0". hope it's just a schematic error.

--DL

[jojokeo]

very nice work. 

those bias test points are all going to read "0". hope it's just a schematic error.

--DL

  good eye. Funny how simple things like that can pass right by you but it is still understood. Like in an emial ocne wehre if the ltetres bceome jmubeld a sntence can sltil be itnrepetred.

[tubeswell]

very nice work.  

those bias test points are all going to read "0". hope it's just a schematic error.

--DL

Indeed - thanks for spotting that! (How could I make such a clanger?). Of course they are actually wired properly (and they work the way I wired them) ;-) (Edit - I have now fixed that)

[tubeswell]

Taking a second look at the schematic as a whole, I really like your design.  You obviously put a lot of thought into it.  The color coded ground points on the schematic is a nice touch.

I can tell you've read the "Immortal Amplifier" articles (solid state diodes before the rectifier tube).  Is the second fuse on the HT line before the Standby switch?  (not shown on schematic)

Cheers,

Chip

Thanks Chip

Yes the HT fuse is before the std-by switch

[tubeswell]

have you posted also the SE version ?

http://www.el34world.com/Forum/index.php?action=dlattach;topic=10746.0;attach=24205

[kagliostro]

Thanks for the SE link Tubeswell

Kagliostro

[tubeswell]

Wow! I've just tried Merlin's 'Bone Ray' tone stack in this (using 1M pots and 1/2 scaled caps). Feckin' awesome! I've got to get some soundbytes up

[zendragon63]

Very nice work tubeswell! Regards

dennis

[tubeswell]

Updated Schematic (with voltages from 5AR4)

[Tone Junkie]

Thats definatly interesting tubeswell, i like the names on the function tilt and middle if Im reading it right, cool looking tonestack cant wait to hear the clips.
Thanks Bill

[jojokeo]

Wow! I've just tried Merlin's 'Bone Ray' tone stack

I looked in the preamp book but it wasn't there. Where'd you find this please? Is this a combo of the tilt and shift controls? Thanks TW

[Fresh_Start]

Wow! I've just tried Merlin's 'Bone Ray' tone stack

I looked in the preamp book but it wasn't there. Where'd you find this please? Is this a combo of the tilt and shift controls? Thanks TW

Searching for "bone ray tone stack" on AX84 turned up a discussion from 16 months ago where Merlin presented this circuit.  I can't figure out how to link to a specific thread on that forum though.

Happy Independence Day!

Chip

[tubeswell]

FYI

[jojokeo]

Thanks FS & TW. I like how he came up w/ the new name, word playing the "other" stack's letters.

TW - what caused you to change the values that you came up with? How does it differ from Merlin's?

[tubeswell]

Thanks FS & TW. I like how he came up w/ the new name, word playing the "other" stack's letters.

TW - what caused you to change the values that you came up with? How does it differ from Merlin's?

I just scaled the resistances x 2 (and correspondingly reduced the capacitances by half) because I only had 1M pots on hand. (I could've gotten some 500k pots from the local shop but I couldn't be bothered). Same freq roll-off point, but smaller AC load in mine version (although probably quieter with Merlin's version - even so, mine doesn't have too much noise in the S:N ratio)

[kagliostro]

Hi Tubeswell

I've one other curiosity

Wich is the AC voltage of the bias tap in the transformer you used ?

Thanks

Kagliostro

[tubeswell]

Hi Tubeswell

I've one other curiosity

Wich is the AC voltage of the bias tap in the transformer you used ?

Thanks

Kagliostro

45 or 50VAC IIRC (a wee while since I measured it) The PT I'm using for this actually has a separate bias winding.

[kagliostro]

I asked because of that diode connected to the 82K resistor and 10uF without any indication about

as allways you were very kind

Many Thanks

Kagliostro

[Tone Junkie]

tubeswell how are you enjoying this amp now after it has had time to break in a bit.The morph control sounds very interesting would this work on a 5879 tube.
Thanks Bill

[jojokeo]

It will work w/ virtually any pentode set up correctly. All it does is transition from pentode to triode mode where a switch does one or the other.

[Tone Junkie]

Cool I will study more.
Bill

[tubeswell]

The screen bypass cap (going from the screen resistor to ground) needs to be big enough (in conjunction with the R value of the screen resistor) to bypass all the relevant frequencies of concern to you (i.e.: to ensure that those frequencies are boosted at the roll-off point desired). And the other cap needs to be about 5 x more capacitance than the bypass cap, since the two caps will end up in series when the control is in 'triode mode', and you don't want only high frequencies to be triode connected (which is what would happen if the two caps were close in capacitance value).  The screen stays at 'screen voltage' throughout, but in triode mode it is connected at AC to the plate and not bypassed. With the pot set at 'in-between' settings, you get ultra-linear operation, which can sound quite funky.

[terminalgs]

very cool.   

how did you select the  values of the 100K and the 15K resistors on the plate of the High-Mu section of the 12dw7 ?

Is it the idea that it forms a 115k plate resistor for the tube, and a voltage divider for the signal that cuts the signal to about 10%?

[tubeswell]

That stage, and the stages after it, are simply lifted from the 6G3.

[Tone Junkie]

I notice it has one of your favorite vibrato, is that a 5g9 type, and after some time playing how are you feeling about the bone ray tone stack.
Thanks Bill