EF86 tmb 18 watt build - low ef86 voltages?

[Bluemeany]

OK, so I've just finished a new 18 watt ef86 tmb. All's fine on the tmb channel and power section, all the voltages are correct to within 10%. However , on the ef86 side i've got low screen (74v) and plate (68v) voltages. It's sounding good, I'm just wondering if I need to tweak any values to really tune the tube in or if I'm good as it is. I've got the standard 220k plate and 1M screen resistors in there ATM as well as the standard 2k2 cathode resistor...

[Geezer]

No experience with EF86, but lots with 5879......those voltages do seem low (but if it sounds good....)

It seems off to me that the screen (with 1M) is higher than the 220k plate

What's the B+ volts at the node you're tapping?

Do you have another EF86 to try?

EDIT: Well, I looked at Merlin's book for ref voltages....he shows 81v plate and 99v screen with the typical Voxy setup, so looks like you're close....maybe try lowering the dropping resistor into the EF86 node to raise the source voltages a bit, but of you like it, you can stay right there, I suppose

[Geezer]

Well, I just waded out to the shop (we've got 14" of snow so far, and still falling) to measure an amp with a 5879 and the same 220k/1M/2.2k as yours....
~290v B+
~140v plate
~95v screen

So it looks like the 5879 behaves differently than the EF86....I knew there was some reason those voltages looked strange to me.

For a "cleaner clean", I have used much lower plate and cathode values (56k/470R), but the grind is much better with the Vox standards. The lower values give a bit of a rough tone when overdriven, but the 220k/2.2k is super smooth when the volume pot is pushed. However, it never really gets clean (until to volume pot is really low), but that doesn't bother me too much.

Also, be sure to try different screen cap values....from 0.1 down to 0.01 give different low end response and also shift the low mids, so experimentation is good here.

Electricity just went off, so I'm signing off to conserve battery power  

G

[sluckey]

Here are my freshly measured numbers. My circuit uses an EF86 and is probably identical to your circuit. 220K/1M/2.2K...

B+ = 266V
Plate = 72V
Screen = 87V
Cathode = 2V

[Bluemeany]

It seems off to me too but sounds good so I was confused. I was wondering if the tolerances that the ef86 is made to would allow this sort of variation? I don't have another ef86 but the one I have is a tested TAD one. I'm getting 245v b+ for the ef86. Without a doubt i 'm enjoying the pentode tone, it's got a lot more life than the standard triode preamp!

I'm just looking at the ceriatone ef86 18 watt voltage chart and one of his has plate at 83v and screen at 98v. I've checked I'm getting the right resistances and the layout is correct. I guess it's just the tube!

Good luck with the snow, we've managed to get rid of ours :D

Cheers sluckey - that reassures me, I'm pretty sure its just differences in the tube.

[Geezer]

The ratios look about right, since your B+ is slightly lower than Sluckey's....so it looks like you're good.

I really like pentodes....some say they can't tell much difference between a pentode or triode V1, but I can sure hear (and feel) the difference, and I like it.

G

Signing off again...power is still out 

[Bluemeany]

Pentodes certainly are different to triodes, the whole feel of them is different - more edge , more shimmer and more compression as you turn the gain up. They're real fun!

One issue that has just turned up - as I play through the ef86 channel if I turn the tmb channel master volume to (near) zero I get an ugly 'bad ground' sort of oscillation. This doesn't happen on the tmb channel when I'm playing through it on its own. Hmmm. Strange my layout is basically the same as the ceriatone ef86 18 watt; http://www.ceriatone.com/images/layoutPic/marshallLayout/TMBEFceriatone.jpg

Turning the master volume should just ground the phase inverters input, pretty standard on most other amp designs... Hmmm, what do you reckon; bad phase inverter tube, bad master volume pot or grounding? I didn't get chance to check this evening - the old lady looked about ready to kill me so I packed my stuff up  

[tubeswell]

FWIW using this circuit



I have measured these idle voltages:

(293 HT)
Plate = 126
Screen = 96
Cathode = 1.37

(280 HT)
Plate = 120 VDC
Screen = 91 VDC
Cathode = 1.3 VDC

(241 HT)
Plate = 106 VDC
Screen = 80 VDC
Cathode = 1.11 VDC

(222 HT)
Plate = 98 VDC
Screen = 73 VDC
Cathode = 1.02 VDC

(209 HT)
Plate = 92 VDC
Screen = 69 VDC
Cathode = 0.99 VDC

(200 HT)
Plate = 89 VDC
Screen = 66 VDC
Cathode = 0.92 VDC

(189 HT)
Plate = 84 VDC
Screen = 62 VDC
Cathode = 0.87 VDC

Hope that helps

[DummyLoad]

on a matchless clone we built:

B+=327V
va=140V
vg2=70V
Ra=270K
Rg2=2M2
Rk=2K2

a 6BM8 lafayette PA w/ EF86 1st stage.

B+=288V
va=130V
vg2=60V
Ra=270K
Rg2=2M2
Rk=2K2

RicharD's lil borwn one.

B+=328V
va=190V
vg2=75
Ra=220K
Rg2=2M2
Rk=2K2

TeeVee Tube amp I designed and built. EF-86 first stage.

B+=290V
va=110V
vg2=60V
Ra=220K
Rg2=2M2
Rk=2K2

Have a MASCO with 6SJ7/6J7 tubes that ran grid-leak bias - this amp has killer compressed overdrive.
B+=110V
va=19.2V
vg2=17.7V
Ra=270K
Rg2=1M
Rk=0 (grid-leak)
Rg1=15M


HTH

--pete

[Bluemeany]

Wow, cheers guys, that's a pretty comprehensive list of voltages

Now I just need to figure out this master volume issue and I'm done!

[Bluemeany]

Master volume issue solved - bad pi tube. Replaced the JJ ecc83 with a sovtek 12ax7wa (which I quite like the tone of; a little more gritty than the JJ) and there is no problem.

One thing I will say about tmb builds - lead dress and shielded cable is crucial!

[sluckey]

Can you post some pics?

[Bluemeany]

Sure can, I'll go find my camera. I'll outline the mods I performed on the basic ef86 tmb 18 watt circuit too (most is standard);

Bass cap - 10n,
Bass pot - 500kA,
470k resistor into v3 pin 2 (2nd tmb gain stage) bypassed with 1n mica cap,
Mullard ecc82 for v3 - nicer shimmer on the cleans without too much compression,
Paul Ruby Zeners added to eliminate just a little of the crossover distortion (13 volt zeners),
16uf power supply cap upped to 22uf,
150 ohm cathode bias resistor,
Picked around with the caps on the ef86 tone switch,
1uf cathode bypass cap on pin 3 of v3,

I may add a switchable solid state rectifier in the future - anyone have experience with this? I like the added 'solidness' they add but I don't want to have to go fiddling around with correcting too many voltages after...

[Bluemeany]

Here's a crappy shot from my nexus - more shots (and the all important gut shots) when I find my digicam!

[Bluemeany]

pictures!

[Bluemeany]

more pictures!