Ef86 12ax7 preamp

Hi there,

I am designing a single ended amp, with an ef86 followed by a 12ax7. The idea is to get a nice ef86 tone with a maximum headroom. I post layout of the preamp here. Please let me know your opinions!

You will get plenty of gain from the combination of the input transformer and the EF86. The output stage can therefore concentrate mostly on delivering power rather than gain. So you might like to consider changing the 12AX7 to something like an ECC88 or 12AU7.

Cheers

Ian

ruffrecords said:

You will get plenty of gain from the combination of the input transformer and the EF86. The output stage can therefore concentrate mostly on delivering power rather than gain. So you might like to consider changing the 12AX7 to something like an ECC88 or 12AU7.

Cheers

Ian

Click to expand...

From OP: "single ended amp". I'd assume it is either a guitar or hifi amp, leaning more towards guitar due to the mention of tone and a triode/pentode switch. Also I don't see any indication of an input transformer.


jaidn, the 220k grid resistor will reduce the gain of the pentode a fair bit. What exactly are you looking for, feedback on your layout? Then include the tube sockets, pots, input jacks etc.

volker said:

From OP: "single ended amp". I'd assume it is either a guitar or hifi amp, leaning more towards guitar due to the mention of tone and a triode/pentode switch. Also I don't see any indication of an input transformer.

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Single ended is a topology. The REDD 47 is single ended. I agree I assumed mic pre when all the OP said was preamp. I saw no mention of a triode/pentode switch?

Cheers

Ian

And if it is a guitar amp, do consider that triode/pentode switch. There aren't a lot of guitar amps with pentode preamps, for a reason....

Thanks!,

Let's add more info...

- It's a guitar amp.
-there is a pentode triode switch.
-better layout coming soon.

I am not sure about the resistor values. Designs I have checked use 220K for the grid, but Merlin's book suggest no more than 100k. Indeed, he states that 47k might be perfect. Another hot topic is the 470K resistor on pin 1 of the EF86, VOX has used 5.6M and 1M in its designs.

Merlin also says that the EF86 turns easily microphonic due to poor design. Choosing the right resistors could improve the performance of the tube.

Let me know what you think!!!

jaidn said:

Merlin also says that the EF86 turns easily microphonic due to poor design. Choosing the right resistors could improve the performance of the tube.

Let me know what you think!!!

Click to expand...

Resistor choice is not going to affect microphonics much, except that higher gain circuits (a function of both topology and resistor choices) will amplify microphonics in a tube prone to them more than lower gain circuits will. But microphony occurs due to the physical parts within the tube (e.g., the getter, plate or grid supports, etc) literally rattling when the tube is physically disturbed (speaker vibration, bumping into the head/amp, etc). Tubes were (are) mostly handmade, and there's variance in microphony from one tube to another and one tube type to another. EF86 is known for having more microphonic examples out there than some other preamp tube types.

The best way to deal with microphony that I know of is to mount the tube socket with damped supports of some kind. There are socket bases that are wider and heavier (heavier being better as weight resists vibration better than light mounts) and made to accommodate silicon or rubber washers where the mounting screws go, for example. Lots of ways to do this, so look up "electron tube damping mount" or "tube shock mount" or something like that on Google to get some ideas from the images.

You can also use the little rubber or urethane O rings and slide them onto the tube itself, but it's debatable how well those alone work. I definitely think shock mounts work better. But the two in combination can work very well. Adding weight to the tube just increases its inertia and makes putting it into motion harder, so they can help in that regard.

BT