Pentode (EF86) pre-amps and combo's?

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Pentode (EF86) pre-amps and combo's?

Post by mark0614 »

I recently heard a EF-86 in a 2 X EL-84 amp and I really liked it. I have heard there are a lot of hassles using pentodes in combo amps.

Have any of you guys experimented with various pentodes and what are the results you have had?

Basically, I do like the sound but I don't like the idea of an unreliable amp and I don't want to have to spend a fortune of NOS valves which go microphonic every couple of months.

In closing thanks for your reply, look forward to hearing from you.
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Post by unchained »

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Post by mark0614 »

Dear Unchained

That is good but I'm not sure it gives definite answers.

Thanks for your assistance.
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Post by LooseChange »

The EF86 causes problems because many are microphonic. I think the tube sounds great in the front end of an 18W.
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Post by zaphod_phil »

There probably aren't any definite answers, Mark. However, here is what I think is a fair summary of the situation with pentodes, including EF86s.

For completeness, here's the response I posted into the other similar thread about using EF86s:

1 - Preamp pentodes generally have extremely high gain levels, and especially the EF86. This makes them very prone to amplifying the slightest vibration of their electrodes. Also EF86s aren't generally that good at handling vibration anyway and many are microphonic. Pentodes are therefore better used later in the preamp, where the signal levels are high compared to the microphonic noise.

2 - Alternatively consider using low-microphonic pentodes, such as EF83 or 6BR7s, which will also sound every bit as juicy as an EF86. Unfortunately, everyone IMO has gotten over-fixated on EF86s. 5879s, may also do better than EF86s, even if only because of having less gain. 6SJ7s can also be microphonic, but I'm starting to get the feeling they have a somewhat better track record than EF86s. However 6SJ7s have the peculiarity that they often need a coupling cap at the grid, or you end up with DC on the guitar pots. I have some 6SJ7s I plan to experiment with.

3 - It's a good idea to shock mount preamp pentodes. The attached picture shows one scheme, similar to that described by Dan Boul, below. This is another similar idea by HBamp in Holland - http://www.18watt.com/modules.php?name= ... ad&id=7545 (excel) or http://www.18watt.com/modules.php?name= ... ad&id=7547 (pdf)

4 - Cascode-connected triodes can also be used as a low-microphony substitutes for pentodes. They sound quite similar to real pentodes. GT used to do a "Substi-tube" convertor with the cascode wiring inside a socket assembly, allowing you to plug a 12AX7 into an EF86 socket. At one time 65 Amps also appeared to be using cascodes as a substitute - perhaps they were short of good EF86s for a while.

Finally some very helpful advice from Dan Boul of 65 Amps.
dboul wrote:I too build an EF86 variant of the 18 watter and I've seen and done a few easy things to reduce microphonics.

1. Enlarge the tube socket hole slightly to mildly separate the socket from the chassis and put a large fiber washer between the tube socket and chassis.

2. Put rubber washers above and below the tube socket mounting screws.

3. Use Micalex tube sockets for the EF86 (they seem to be better at absorbing vibration)

4. Use a hi-fi style dampening ring on the tube, works great.

This all creates a "shock mount" that should help.

I've found that the new Sovteks and Svetlana's from Mike Matthews (Newsensor) are either great or soft right away. I've also seen plenty of NOS stuff be soft also. NOS doesn't guarantee low microphonics at all.

I'm sure there are many ways to mount the chassis, etc. to reduce vibrations. The EF86 is so sensitive that they will always fail a "tap-test", but that doesn't mean they are necessarily bad.
dboul wrote:I've experimented with very very complex shock mounting schemes and I've found out that it's really about the tubes. Certain brands are much more solid than others.

Second, it's important that you let the tube get hot for a long time the first time you fire it up. Don't turn it on and off often when you first use it. A lot of EF86 microphonics is caused by the spacers cooling off and shrinking. After they've shrunk, there's no hope. They will rattle. So make sure you burn the tube in gently and slowly before you run a lot of signal in to it.

Lastly, the most effective thing we've found at 65 is to use neoprene rubber washers on the tube socket to reduce vibration, we use 3 on the bottom and 2 on the top.

Also really helps to orient the tube horizontally instead of vertically. The regular 18watt chassis puts the tube about 2 inches from the back of the speaker. The concussion from the speaker just beats the **** out of the tube. Our chassis has the tubes horizontal like a JTM45 and that REALLY helped with the EF86.

But really, it's still about the tubes. Dr Z uses a standard tube socket, not shock mounted. Same with Matchless. They just have to do what we do . . . buy 100 at a time and weed them out. Strangely enough, I've noticed that Mullard's and Amperex's are really cool high gain tubes, but about a 60% reject rate, especially in a combo.
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Post by clobo »

Hi,

I got a EF806 (modern) Tung-sol from Lord Valve (he's really serious about quality control).

Tube works very well, even with standard chassis mounting. Microphonics level is about the same as my selected 12AX7's (means good enough for the first position in my other Marshall/Fender style amps). Hopefully it will pass the test of time.

I initially ordered it for my next project build, but then put it in a friend's Dr-Z Stangray that needed a replacement. Tube also works very well in there, and the 2 original orange rings don't seem to do any difference in microphonics level.

Claude
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Post by mark0614 »

Dear Phil and Claude

Thanks for the reply. Much appreciated.

By the way, what ever happened to Dan?

I miss reading his posts.
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Post by zaphod_phil »

Dan's still a member here, but like Gabi, he's become a victim of his own success. :) Basically, he's just too busy building amplifiers and running the business with Peter Stroud, to post here very often. Still it's nice to think that all those "65" amplifiers have ingredients from 18watt.com inside them. :D
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ef-86 tubes

Post by jtmjtm45 »

I have some russian EF-86 tubes could they be used insted of 12ax7 or 12at7s in a amp with 1 or 2 el84s also what about the for mentioned EF83 or 6BR7 tubes. also Are you refering to v1? or??
would there be coponet changes in the amp needed? love the shock mount idea :lol:
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Post by sub »

I've few NOS Tungsram EF86 (currently i haven't EF86 amp).
Anybody tried these tubes?
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Re: ef-86 tubes

Post by rjgtr »

jtmjtm45 wrote:I have some russian EF-86 tubes could they be used insted of 12ax7 or 12at7s in a amp with 1 or 2 el84s also what about the for mentioned EF83 or 6BR7 tubes. also Are you refering to v1? or??
would there be coponet changes in the amp needed? love the shock mount idea :lol:
You'd definitely need to make changes to go from 12ax7 to EF86. The tube socket would need to be rewired and you'd have a much different set of 'support' circuitry for it. You could do it on an existing turretboard, but it would take some effort to do. It's definitely not as simple as plugging the EF86 in!
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Post by billv »

clobo wrote:I got a EF806 (modern) Tung-sol from Lord Valve (he's really serious about quality control).
What do you think about it, tonally? I've tried a couple of new Tung-Sol 12ax7s and they sounded real good IMHO. Can you compare them to other/older EF86?

BTW, I was just in NBS Electronics picking up some Caig. It is either a disaster area or a gold mine, depending on your outlook. :lol: LV is a good chap and I highly recommend him for new production stuff.
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Re: ef-86 tubes

Post by zaphod_phil »

rjgtr wrote:You'd definitely need to make changes to go from 12ax7 to EF86. The tube socket would need to be rewired and you'd have a much different set of 'support' circuitry for it.
Richard is right. It becomes a somewhat different preamp design when you put in a pentode, whether EF86, 6SJ7, 6BR7, EF83, etc....
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Post by clobo »

billv wrote:
clobo wrote:I got a EF806 (modern) Tung-sol from Lord Valve (he's really serious about quality control).
What do you think about it, tonally? I've tried a couple of new Tung-Sol 12ax7s and they sounded real good IMHO. Can you compare them to other/older EF86?

BTW, I was just in NBS Electronics picking up some Caig. It is either a disaster area or a gold mine, depending on your outlook. :lol: LV is a good chap and I highly recommend him for new production stuff.
Hi again,

I've been LV customer for many years, and his tube were always very reliable. He also has a good ear and can give wise advice on sound.

I only heard the EF806 in a Stangray. Original EF86 had already failed so I cannot compare its sound to others. To my ear, the EF86 seems to respond much faster than other typical dual-triode designs based on 12AX7. It has lots of clean sparkle but when you push it, it overly twangs while remains very dry and bright (not creamy). But then maybe i'm biased by the Stangray pretty strange eq.

I will probably order another EF806, plus an 12DW7 and build a vajra based preamp, with a 36W power section.

Claude
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Post by zaphod_phil »

Forget about the Stangray then, and try listening to an 18W, especially with distortion. You get richer, chimier harmonics in the tone with a pentode in the signal chain. If you listen to Gabi's Voxer 18 sound clips, you can clearly hear this difference between the EF86 ("Bright" used to be called "VOXISH") and 12AX7 ("MARSHALLISH") channels - the latter one almost sounds a little dull by comparison.

http://www.gabtone.com/V18.htm" onclick="window.open(this.href);return false;
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Re: Pentode (EF86) pre-amps and combo's?

Post by StarGeezers »

Just a little more on pentodes .... We’ve had excellent results (Thanks to ZP ! ) with the “ 6SJ7 Pentode 18 watt Lite...Formerly the ABSOLUTE Minimum , and had no troubles with microphonics ... Nice Sweet Tone , loves pedals , LOUD !!! 8)
Also have had Great success with the 6U8 , triode pentode tube in the “Croaker “ series at Wattkins.com ... Again chime and sweetness of Tone .... been called “ Jimi in a box” ... :mrgreen:

Lots of alternative pentodes to choose from ... just sayin’ ... and we gig with ours , so it’s been well tested ... :wink:
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Re: Pentode (EF86) pre-amps and combo's?

Post by DaveW »

I have used EF83 tubes, they are cheap, pin compatible with EF86 and can be got quite cheaply. They can be found in some Grundig reel to reel recorders where they were used used for automatic level control.
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Re: Pentode (EF86) pre-amps and combo's?

Post by zaphod_phil »

Yes, there is some evidence that the degree of microphony is dependent on the type of tube, with EF86s possibly being more sensitive to microphony than other types. I've also had reports that Amperex Bugle Boy EF86s are pretty good with microphony, and shouldn't normally need shock mounting.
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Re: Pentode (EF86) pre-amps and combo's?

Post by DaveW »

I have recently got a bunch of 5725 (6AS6W) pentodes, which are 7 pin valves used in military airborne applications and are stated as able to withstand severe mechanical vibration.

Has anyone tried these as a preamp?

http://www.r-type.org/pdfs/5725.pdf" onclick="window.open(this.href);return false;
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Re: Pentode (EF86) pre-amps and combo's?

Post by StarGeezers »

Dave , that’s Very Interesting !!! Thanks !!! 8)
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