18Watt.com

I am just building one of my cage amps with the EF86 pre amp.
When I looked in the box I only have a few EF86 valves left.

But, I have a big stash of Mullard brand new EF85 still in their lovely cradle boxes.

I am not a valve tube expert and I have downloaded the spec sheets and searched these learned pages.
The question has arisen before but the poor guy was referred to Duncans Amp Pages.

Take pity on me and have a look at the sheets attached, or give me the benefit of your experience with EF85's.

I can deduce the pin out is slightly different, not a problem on a new build. The data differs slightly as well, but they are lovely looking valves a sort of in between 12AX7 and EL84 size.

Steve UK

www.valvepower.co.uk


Cannot add files to this Forum.
So some links instead.

http://tubedata.itchurch.org/sheets/010/e/EF85.pdf
http://tubedata.itchurch.org/sheets/010/e/EF86.pdf

Steve you have a nice product in your 18 watt amp. I believe that it would be a bad business decision to use the ef85 as a replacement for the ef86; you’re losing 1/3 of your gain right off the top. You should be able to get a decent price over in the UK on the Russian 6J32P, there equivalent of the ef86.

But hey if your iron is hot try it, you might like the lower gain and solid punch that the ef85 would deliver. But I believe that it would sound pretty lifeless.

It looks to me like the 12AT7 is to the 12AX7 what the EF85 is to the EF86, very roughly speaking - less gain, higher transconductance, etc.. You will probably want to use a different resistor combination on the anode, screen and cathode, unfortunately, there's no chart like there is for the EF86, but if you read the Valvewizard site enough, you can figure it out.

Some people find their EF86 18 watts to have a little too much gain, so maybe this would be a good substitute, it will also have a much lower output impedance, so it's a little "better" than the EF86 in that respect, in that it should work better with tone and volume controls.

But you never know how a particular valve will sound, you could say all the above about the 12AT7 vs. 12AX7 and it looks like an appealing option, but in practice, 12AT7's can just sound kind of yucky sometimes.

But I think it;s definitely worth a shot, we'd all be curious to hear the results!

It was another experiment just to investigate possibilities. Thanks for your input.
Just looking for constant improvement. I did notice the 'amplification factor' was lower on the EF85. I'll give it a try and rewire V1 and stick to EF86 or 12AX7 if it doesn't work out.
I am waiting for my new batch of chassis's to come from the factory. Slight changes, both laydown transformers, one IEC socket/fuse combined. Different finish in black textured powder coating. New faceplate with on off standby switch in one.
New PT to permit valve rectifier option.

Steve UK

www.valvepower.co.uk

Yes, do try it and let us know. It might turn out quite nice. Just watch out for any signs of microphony.

zaphod_phil wrote:Yes, do try it and let us know. It might turn out quite nice. Just watch out for any signs of microphony.

Yeah it's fun to play with unpopular pentodes. The lower gain of the EF85 should make it less prone to microphonics, I'd have thought. Looks like a 100k anode load would be the natural place to start, giving a gain of around 100, depending on screen voltage.

Here are some curves for you: