Tube pedal preamp one-off project but with EF86 and 12AX7

[jmccanna]

Hello,

I am intrigued by making my own tube pedal preamp like the one Mr. Hoffman did. However, I would like to simplify it and put one EF86 on the initial stage and a 12AX7 on the next.  I am not feeling the need for extensive tone control.  I have purchased one of the little transformers and a 7812.  My question is since the 7812 outputs 12vdc that gets split between the two sides of the 12AX 7 would that result in too much voltage for the one side of the EF86?  The EF86 used 6.3 v for its pentode  circuit.  A reducing resistor in series? Would that be appropriate?

[kagliostro]

A reducing resistor in series?

May be it works well for a single preamp tube

other options

. a 7805 with a diode to elevate the ground reference

or

. in series connections of the heaters (12ax7 connected for 6.3v supply) (as the ef86 and 12ax7 has different heater consumption you must use an appropriate resistor connected in parallel with the ef86 heaters)

Franco

[pdf64]

I think that the use of a suitable series resistor, to drop 12V down to 6V for the EF86, would be fine.

[kagliostro]

If I'm not wrong, for the drop resistor to supply the ef86 from 12.6V

give a try to a 33R 5W resistor

Franco

[jjasilli]

Poor design concept.  Too much wasted heat especially for a pedal sized enclosure.  Think Easy Bake Oven.

[shooter]

maybe he lives up north were heat is a much sought commodity 

[PRR]

UF86 is 12V. It is $28, opposed to $19 for a new-made EF86. However it appears that there are no new UF86, so you get Golden Age production instead of modern production. (Golden-Age EF86 sells for $50-$100+.)

12AU6 is a $6 alternative. Does need 50 more mA heater current.

http://www.r-type.org/pdfs/ef86.pdf
http://www.r-type.org/pdfs/6au6.pdf

[jjasilli]

Other options:


* separate 6.3V power supply for the pentode.  Maybe OK if part of a pedal board.
* pentode/triode combo tube
* Doug's preamp pedal: 2 X 12ax7's @ 12V filaments total 300mA X 6.3V = 1.89W.  Maybe drill ventilation holes if you exceed that a bit.

[DummyLoad]

bah humbug! - 1.15W ballast R isn't going to melt the GD pedal or turn it into ez-bake. use a 27Ω 2W R in series with EF86 filaments strapped across the 12V rail. separate 6.3V supply is going to waste off more heat than 1.15W dropping with diode drops, cap ESR, regulator, etc..


--pete

[jmccanna]

Hi,

Thanks so much for the replies. I will take a look at all of them.  I was wondering if a variable resistor would be an option?

James

[HotBluePlates]

Not wanting to discourage, but offering FWIW.

I love the sound of the EF86, and the lone amp I have that uses one.  Be warned they can be prone to microphonics, and I would hesitate using one in a pedal on that basis.  I think the one in my AC10 is starting to ring a bit, though I haven't been playing it loudly enough to really cause problems (good thing I've got a good stock of EF86s).

Given the option, I'd choose to run an EF86 in a head, and even then consider ways to shock-mount the socket.

[Williamblake]

As mentioned in the post above you need 30 Ohm to drop 6V at 200mA. Maybe you will need 25 or 35 Ohm depending on your power supply. You would see that without a vr.

[jmccanna]

Not wanting to discourage, but offering FWIW.

I love the sound of the EF86, and the lone amp I have that uses one.  Be warned they can be prone to microphonics, and I would hesitate using one in a pedal on that basis.  I think the one in my AC10 is starting to ring a bit, though I haven't been playing it loudly enough to really cause problems (good thing I've got a good stock of EF86s).

Given the option, I'd choose to run an EF86 in a head, and even then consider ways to shock-mount the socket.

Thanks for the comment. It is very well taken. The EF86 is a little finicky but the tone makes it worth it.

I have an amp head with an EF86 in it of sorts and love its sound. I rewired/modified an Akai M-8 single-ended tube amp from its reel to reel. After I heavily modified it the signal goes through the EF86 and into the 1st half of a 12ax7 then to the pre-out.  Lovely tone but the set up is awkward because it is housed in this little amp. 

In essence I am wanting to duplicate that circuit in a more manageable format.  I am using the left over reverb box from a Hammond organ reverb circuit as the case. 

Your point about shock mounting is spot on.  Hmmm. I have also been concerned about protecting the tubes.  I might go back to the drawing board.  Thanks,  James

[DummyLoad]

you could take another approach and build into a shallow rack mount chassis. you'd have more space and it would be easier to shock mount the  EF86.

one could also experiment with the sub-miniature 8 pin 6788 - the only sub-miniature pentode purpose build as an AF amp. purportedly high G rated too. sourcing the sockets can be an issue. 

respectfully,

--pete

[jmccanna]

Hi,

Thanks, that is an interesting idea.

[airguitarist]

What about subbing the ef86 for a cascode?

[jmccanna]

Hi,

Thanks. I admit it I do not know how to do that but it sounds intriguing?  Can you explain a bit more?

I hope all you tube guys are staying safe.