ef86 amp

What you'd be looking for, in a low powered amp, is a Vox AC-4. It used an EF86 preamp and an EL84 power tube. Take a look at my link below and check the blog section. I'm using a solid state MOSFET in place of the EF86, but the circuit is very similar to the AC-4. The AC-4 circuit is easily found out on the internet and is a very simple build.

Also check out http://www.ax84.com/. You'll likely find most of what you need there.

Good luck,

Bill

 

Easy enough. Just copy a Fender amp with trem and verb. The only difference is that you need an extra resistor and capacitor for the screen on the EF86 and that you would change a couple of part values in the PI and the output tube bias for the EL84's. The rest is just going to the proper pin on the tubes.

 

What printer2 said.

Although I'd change the word selection a touch and say it is going to be simple to implement - find a Fender with 2x6v6s, trem and verb, and then make component adjustments accordingly to match your changes (ef86 input tube, el84 output tubes.)

But making it all actually sound good might not prove quite so easy.

Even looking at Fender diagrams one of your choices is going to be where to attach the trem oscillator - do you hook it to a recovery tube in the pre-amp, or attach it to the output section.

If you are going cathode bias the choice is realy yours to make. If you are thinking fixed bias I'd suggest going with the trem in the pre-amp section.

 

Hoffman forum schematics/layouts have what your looking for. Not the collection of schematic but their .sch schematics the members have built.

 

From the 18 Watt crew when they change a LTP PI from EL84's to 6V6's they reduce the tail resistor from 47k to 22k in order to supply more drive signal to the 6V6's as they are less sensitive than a EL84. You could go the other way when taking a Fender amp as a template. The actual values in the circuit can be changed easy enough, the layout and board is the more important thing and using them is what I meant in that it was easy enough. You do not have to reinvent the wheel and worry too much about oscillations and crosstalk. But I could live with simple.

 

I did a couple of EF86 amps earlier this year, a parallel SE and a PP amp, both with trem. Schematics attached FYI

Don't know that I'd put reverb into one of these, they work well with stand alone reverb in front. If I was going to put 'verb in, I'd probably just copy a BF type reverb circuit.

 

I was thinking of trying this input circuit with the morph control in one of my amps but with a 6AU6.

 

Printer and tubeswell, in your playing with small signal pentodes, I strongly suggest you try a preamp with a pentode as V2 or 3. Keep the plate resistor low to control gain and microphonics, and put enough gain in front of it to push it hard when the amp is pushed hard.

Using a small signal pentode for maximum gain just invites microphonics and tubes are not what they use to be, Nos is picked over and new production is not fully up to the old standards. So I suggest keeping gain below 100. Being a pentode makes it different enough from the ubiquitous 12AX7 that even at the reduced gain it can be worth the effort. Being a pentode also means it brakes up differently then small signal triodes (read 12AX7) which is also worth getting it to do. Used as an input tube it will never get pushed hard enough to yield all is is capable of unless lyou have a booster pedal in front of it. In other words, I feel using a pentode as an input tube only makes since if you want to reduce the gain stages and needs its high gain, but the high gain comes at the cost of increased risk of microphonics. A pentode will also not do all the cool stuff it can do if its input is not pushed which you have to use a pedal to do if it is the input. Moving it back in the chain allows you to reduce its gain and to overdrive it from within the amp.

Give it a try if you are looking for something to play with, I think you will like what it does.

 

Thanks for the advice celeste, I will keep the gain down. I have about 50 said to be NOS 6AU6's that I bid on at a low price just for the heck of it and ended up getting them. I experimented with this tube in the first position and found it did have a different flavor than a 12AX7.

While I did want to make an amp at some time with a pentode further along in the signal chain I never though of it on the one I have on the bench right now. It is an amp that I built with a switchable PI stage, cathodyne, paraphase, and LTP. For some reason I kept missing the fact I was not tying the cathodes together and when I tried the LTP I got silence. Silly me. Anyway I am thinking of simplifying the PI section and have another hole to fill in the chassis. Therefor my renewed interest in the pentode (have a 6AV6 in the hole right now).

I was going to use a Tweed tone control on the 6AU6 but if I use the 12AX7 as the first stage I could do a bass-treble tone stack after it. Drew up a quick schematic. Tubeswell have you tried a linear 1M? I do not have any log pots other than 500k's. What you you guys and gal think?

 

The morph control works better with an audio pot IMHO