EF86-type Sonic Differences.....

[jrfrond]

Just an FYI really.

I build guitar amps with pentode front-ends. Pentodes, as the more experienced will know, have issues with microphonics and hum, the former being magnified when used in combos at medium to high gain settings. The latter was much better when I decided to float the heaters on a DC bias (top of the power tubes' cathode resistor, about 38V).

I did a roundup of new EF86-type tubes, which includes the "upscale" EF806. The EH EF86 is pretty balanced tonally with some chime and good lows, but seems to be susceptible to microphonics and has an acceptable hum level.

Conversely, the J-J EF806, while having very good residual hum level and near-zero microphonics, sounds like a bandpass filter, with a distinct lack of chime and "woofier" lows, which yields a kind of midrangey, fatter tone, rather than a jangly bite. Not a BAD thing mind you, but it's a way different flavor than my design calls for (think more along the lines of Matchless' DC30 pentode channel.

Lastly, the tube I originally put in there, the TungSol EF806 Gold (gold-plated pins), has the nice chime and bite, plus clean lows. It CAN be a bit microphonic, but the hum levels are acceptable. Ironically, the "edge of madness" microphonics are what probably contributes to the somewhat raw edge in the overall tone.

Once again, as we all know, all tubes of a given type are NOT created equal.

[big_teee]

I don't want you to give up any trade secrets, but can you explain how you float the heaters?
Also where do you get the dc bias?
Does it require a separate DC supply?
Thanks in advance!
Terry

[jrfrond]

There are no trade secrets with this. You can float the heaters on almost any DC voltage in the amp, as long as you pay attention to the heater-cathode voltage differential ratings. In this case, the amp is cathode-biased, and I went right to the cathodes of the power tubes, which are elevated above ground by a 250 ohm cathode resistor, and sit at about 38VDC.

In order to do this, either the heater winding center-tap lead from the power transformer (IF the heater winding has one) or the center of the heater balancing resistors (usually 100 ohms) or wiper of the hum balance pot is connected to the DC voltage source rather than ground. Other voltage sources would be the bias supply, or a voltage divider across the lowest B+ in the power supply. It's just another reference point OTHER than 0V. There are some amps which employ shielded cabling that float the shielding at a point other than ground as well.

There's actually no right or wrong, and the process of floating heaters or shields is just another debugging tool in a designer's or repairperson's arsenal. You try it, and if it reduces the hum or noise you are experiencing, then it is correct. If it doesn't work, then it's not correct. Pretty simple actually.

[big_teee]

Great Info.
I was thinking of my next build using a power supply that is equipped with 5V for unused rectifier tube.
Taking that unused 5 Volts and rectifying that for heater supply for the first preamp tube.
Do you also shock mount V1 on some of your builds?
Getting off topic for tube section, but great info.
Tell us more about the traits of EF86s!
I don't really understand when you would want to use a EF86 vs 12AX7!
Terry

[jrfrond]

My penchant for the EF86 is two-fold: first, I like the sound. It's tighter and chimier than a 12AX7, making it an excellent guitar preamp tube. The hum issues and microphonics are really the only drawbacks. And although it is readily-available, it's use isn't as widespread as the common 12AX7. It's also pricier.

My goal with amp design has always been the shortest signal path possible, and the use of a single EF86 in the front end, rather than cascaded 12AX7's eliminates a few extra signal path components. The end result is an amp that has more sparkle, faster response, better feel and more actual guitar tone. With a short signal path, you really hear the guitar tone, the wood, everything. This is why so many players love the old Tweed designs. To my ears, simpler is always better.

[big_teee]

Do you use the EF806 also or just the plain ef86?
I notice there are Tungsol versions of the 806!
T

[soundmasterg]

I noticed in a build that I did that using DC filaments for the EF86 greatly reduced hum. I built another amp recently using a 5879 with elevated AC filaments and it is a bit noisier than I would like. You may also consider DC filaments in your build for the EF86.

Greg

[jrfrond]

I noticed in a build that I did that using DC filaments for the EF86 greatly reduced hum. I built another amp recently using a 5879 with elevated AC filaments and it is a bit noisier than I would like. You may also consider DC filaments in your build for the EF86.

Greg

A secondary DC heater supply for the EF86 would have been my next plan of attack, had the floating method not worked out so well. Though there is still some hum, it is at very acceptable levels. As an experiment, I disconnected the EF86 heaters and powered them off of my DC bench supply, and it was dead-quiet, so that became my noise floor reference. I might still do it at some point. The PT still has an unused bias winding that I could chop down to a regulated heater supply with a few components, IF I can find the space in the chassis.

Terry, the tube I wound up using is a TungSol EF806.

[soundmasterg]

I modified a Sovtek Mig 100 into a clone of a Vox AC100, but I added a second channel with an EF86 for more gain. The power transformer had a 12v supply stock as they were using a DC filament on the 12AX7, and I decided to use that also. I used a regulated DC heater for the EF86 and the first 12AX7 in the AC100 stock channel and this amp is dead quiet. I'm a believer now after doing that. Actually I think I added a seperate transformer to do that because the stock supply didn't have enough capability to supply both, and I needed a seperate transformer to power some relays. I like the DC heaters though...works good.

Greg

[greekie]

The Tung-Sol tube does sound great. But my main concern is, that I can get NOS Mullard EF86 at the same price as my distributor sells the Tung-Sols... So right now, it's not my preffered choice as you might guess!

Jake

[imaradiostar]

Thanks for the informative thread. My favorite home-built amp has an Amperex EF86 out front, as Vox-style 12ax7 phase inverter and a cathode biased pair of EH 7591's running at pretty ridiculous idle current. There's some sort of magic to it that I just can't put my finger on! I guess I'm saying I agree with you about amps like this.

Would you mind sharing your preferred plate, screen and cathode bias resistor values for the EF86? I suppose to make sense of those values we'd also need to know the supply voltage.

Have you messed with other readily available pentodes? I've had some good results with 6au6's, uninspiring results with 6ak5's, and I have an 6GH8 based build in progress.

jamie

[imaradiostar]

bump?

Funny how you never see "bump" with this forum.

jamie

[imaradiostar]

I just realized we bumped an ancient thread...either way, I'm still interested in the topic.