Hi all!
A little about my background: Building audio related things for last 30 years (on and off). Speakers, hi fi audio amps (tube mostly 6B4G SE / PP), a couple of small guitar amps based on schematics, and a couple based on just basic guitar amp building knowledge. I do have a full set of test equipment. signal generator, oscilloscope, etc. Books by Merlin Blencowe and Richard Kuehnel. Others include a copy of the RDH4, Dave Hunters guitar amp books etc.
I want to build a EF86 -> 12AX7 CF -> MId Control / vol -> EF86 -> EL34 guitar amp. There will switchable NFB from output back to cathode of v3 EF86, plus switchable cathode bypass cap on the EF86. The EF86's would be loaded with 47K. 140v plate, 120v screen, -1.5v bias. 1/2 of 12AX7 CF is loaded with 47K. May even add in a tube / SS rectifier switch.
I just want to get input if I am heading in the right direction. I do have 1/2 a 12AX7 that isn't used (thought about a switchable cold clipper, or a tremolo)... or just not use it.
Anything I should be concerned about?
I know i may need to adjust the gain structure of the amp.
The mid control is just a framus.. probably stick to the recommended values .. but may play a bit.
Should I adjust the EF86's so one is biased warm (-1), the other cold(-2)? or bias them symmetrical? (-1.5)
I will probably build it ptp with a few tag boards connected to each tube socket.
Thank you all!
Richard
Beginner amp and member
Moderators: pompeiisneaks, Colossal
Re: Beginner amp and member
Hi, Richard!
Have you played EF86 amps before? I messed with a Vox style build with EF86 as first gain stage, and found no value because the stage does not distort (so adds no character), and most EF86 tubes are noisy bastards, badly microphonic.
Have you played EF86 amps before? I messed with a Vox style build with EF86 as first gain stage, and found no value because the stage does not distort (so adds no character), and most EF86 tubes are noisy bastards, badly microphonic.
I build and repair tube amps. http://amps.monkeymatic.com
Re: Beginner amp and member
Welcome!
A lot of this will depend on your goals for the amp. How much overdrive are you aiming for? What kind of overdriven sound would you like? I haven’t had any personal experience with EF86 since they’re pretty expensive (even when new production) and 5879 are still available for pretty cheap. On top of that, noise issues will get worse in a higher gain tube like EF86 relative to something like a 5879.
To xtian’s point about the pentode, have you considered a build somewhat like the Matchless Clubman topology? Boosting the level prior to the EF86 would reduce the noise issue as well as get you more of what many people look for from those tubes, I.e., overdrive.
Biasing questions likely will be easier to answer with sonic goals in mind. In general if you want a cleaner amp, bias toward center. But I do want to caution that those bias voltages you mention would mean a lot more in context of your power supply and what kinds of sounds you want out of the amp. Have you picked transformers yet?
A lot of this will depend on your goals for the amp. How much overdrive are you aiming for? What kind of overdriven sound would you like? I haven’t had any personal experience with EF86 since they’re pretty expensive (even when new production) and 5879 are still available for pretty cheap. On top of that, noise issues will get worse in a higher gain tube like EF86 relative to something like a 5879.
To xtian’s point about the pentode, have you considered a build somewhat like the Matchless Clubman topology? Boosting the level prior to the EF86 would reduce the noise issue as well as get you more of what many people look for from those tubes, I.e., overdrive.
Biasing questions likely will be easier to answer with sonic goals in mind. In general if you want a cleaner amp, bias toward center. But I do want to caution that those bias voltages you mention would mean a lot more in context of your power supply and what kinds of sounds you want out of the amp. Have you picked transformers yet?
Re: Beginner amp and member
Thanks for reminding me! Here's the build thread on my Clubman style thing:
https://ampgarage.com/forum/viewtopic.php?t=20741
https://ampgarage.com/forum/viewtopic.php?t=20741
I build and repair tube amps. http://amps.monkeymatic.com
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Stevem
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Re: Beginner amp and member
If your dead set on building with a EF86 then instead get a 6BR7.
They are less microphonic and seem to last longer.
They are less microphonic and seem to last longer.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Beginner amp and member
I bought a number of 6J32P a while back when everyone was buying $2 Russki valves. They've been great in my 18 Watt with EF86 channel, reliable and not microphonic.
Re: Beginner amp and member
i have also looked at the 5879. I was "trying" to build with all current available tubes. But for sure ok with the 5879. I was planning on running the pentode with lower gain to reduce microphonics potential i.e. similar gain to 12AX7.
Would a standard 12ax7 front end be better? 12ax7 -> 12ax7 CF -> Mid control / Vol -> EF86/5879 -> EL34 ?
I have read the advantages of a second stage pentode and being able to drive the heck out of it. I was thinking of a pure pentode amp... but maybe the outcome will not be as good as how having a pure pentode amp sounds. Maybe I need to reduce the screen of the input tube even more to allow a little more potential of distortion.
As far as gain... not sure. lol. middle of the road? i do like the sound of smaller (lower power) amps.. Supro, Gibson etc.. GA5T, Super...
I have not picked transformers yet... but do have a few Hammonds left over from prior tube amp projects.
Thank you so far for your feedback and advice.
Richard
Would a standard 12ax7 front end be better? 12ax7 -> 12ax7 CF -> Mid control / Vol -> EF86/5879 -> EL34 ?
I have read the advantages of a second stage pentode and being able to drive the heck out of it. I was thinking of a pure pentode amp... but maybe the outcome will not be as good as how having a pure pentode amp sounds. Maybe I need to reduce the screen of the input tube even more to allow a little more potential of distortion.
As far as gain... not sure. lol. middle of the road? i do like the sound of smaller (lower power) amps.. Supro, Gibson etc.. GA5T, Super...
I have not picked transformers yet... but do have a few Hammonds left over from prior tube amp projects.
Thank you so far for your feedback and advice.
Richard