EF 86 Microphone preamp circuit

[Toxophilite]

I'm looking for the best way to configure a single ef86 to produce the most gain for a microphone preamp.



There is a shure m67 type of input transformer prior to the EF86 which I believe is a 10-1 ration

Any ideas?

[glass54]

Hi Toxo,
Not really what you asked for BUT some ideas
http://solorb.com/elect/musiccirc/tubemicpre1/
https://tapeop.com/interviews/30/build-tube-mic-pre/
Note use of 1:10 Tx.
Kind Regards
Mirek

[PRR]

A pentode can give audio gain over 100 which with 1:10 transformer looks like way more gain than anyone needs before a gain control.

That's at infinite load. If you are driving the 10K of modern "interfaces", the pentode gain could be less than 10. Cathode follower, opamp chip, etc would be a fix.

[Toxophilite]

Some good ideas!, Thanks very much


This is going straight to tape. I seem to have lost the Jschem program I had for schematic drawing so I'm trying to download another so I can draw exactly what I currently have.
I guess I could %$&% draw it and take a picture! what an idea!


What I have is almost identical to this except no resistors on the input, just the transformer going to a 500k A pot, then into the EF86
Also  a 250k resistor on the plate instead of 220k, no 250pf cap and a .22 cap on the output .
The voltages
plate is 56V
cathode is 1.5V
Forgot the others and don't currently have the unit, will soon and will draw a proper schematic
They seemed in proper relation to the ones in this schematic considering the lower plate voltage


There is a redundant power tube I can pull on this unit which boosts the plate voltage about 18V

[Toxophilite]

OKay I think this is right..why I don't hand draw schematics. Done on the back of one of those real estate note pads.

[Toxophilite]

Correction. My suppressor grid is tied straight to ground rather than to the cathode as it seems to be in many circuits.
Important?

The other difference I'm seeing between the way I have it laid out and most datasheet etc. schematics is that I don't have a large resistor to ground on the control grid.
I was assuming the transformer and or the 500k pot does this job, no? Should I have a resistor to ground before the input pot?


Ammended badly drawn schematic added

[PRR]

.

[Toxophilite]

Thanks for lightening that up,  indeed clearer.
While I am familiar with brightness and contrast, I don't do a lot of photo editing on my phone


And thanks for correcting the schematic, I guess I drew it too quickly


Any insights as to increasing gain?  Not possible? Foolish question?

[Toxophilite]

Fixed again! It's an angry looking schematic

[PRR]

Any insights as to increasing gain?  Not possible? Foolish question?

https://el34world.com/Forum/index.php?topic=31132.msg343041#msg343041

[kagliostro]

Just curious

Is a commercial product (which one if it is) or is a custom build someone did ?

Franco

[Toxophilite]

Any insights as to increasing gain?  Not possible? Foolish question?

https://el34world.com/Forum/index.php?topic=31132.msg343041#msg343041

I guess no other tweaks within the existing circuit. This is a build based on a roberts reel to reel pre. I guess I will look into opamps. How they work and how to implement them. As said my understanding is these are being used straight to actual tape, not DAW which actually they suit better as you don't need a lot of signal.

[kagliostro]

Since I haven't yet understood exactly what could be useful to you, I post this diagram from Telefunken


Franco

[Toxophilite]

Well I was hoping not to add a whole new section to this existing unit. I don't have room for another tube,(maybe a minature like a 6au6 or a 6av6) and it uses a sort of turret board rather than a PCB setup.
I was wondering if there was a way I could tweak the schematic I posted, ie, increase/decrease component values, add components, Increase the B+ supply.
It is being used a s a microphone preamp straight to an actual 16 track tape machine and it has been found to be hard to get a usuable signal with vocals. Works well on louder sources, like drums.

[kagliostro]

I was looking (on my archive) for one other thing to post it here for you and I've find this (that I forgot to have)

Figure 1 is the classic connection for an EF86 as a maximum-gain audio preamp, and is derived from numerous technical sources, such as old professional-audio designs and the Mullard "Valve Circuits For Audio Amplifiers" textbook. With appropriate changes to the grid resistor, this circuit may be used anywhere a low-noise low-level preamp stage is needed. For example, making Rg=10 megohms allows the use of this stage in a condenser microphone. The output may be attached to a suitable plate-to-600-ohm transformer for driving a standard balanced line, or to a cathode follower for driving low-impedance unbalanced loads. If a preamp for use with a moving-magnet cartridge is needed, make Rg=47k ohms. If this circuit will be used as a pre-preamp with a moving-coil cartridge, Rg should be chosen to match the load required by the cartridge's manufacturer. This resistance is typically in the range of 10 to 1000 ohms. And for direct attachment to an unbalanced 600-ohm dynamic microphone, Rg=620 ohms. DC heater power is recommended for best noise performance. The internal shield (pins 2 or 7) may be connected either to the cathode or to signal ground.

and this is what I was looking for, thinking it can be of interest for you



---

Works well on louder sources, like drums.

Have you considered to add something like this before of your circuit ?

http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm



Franco

[PRR]

Again: the main problem is probably the very high output impedance of a pentode relative to any studio gear of the last 80 years. Fooling with the pentode's input grid resistor won't change that. Adding a stage in front probably changes the cherished flavor. However an LND150 (thanks for mentioning that) as source follower after can buffer low impedances with little change to pentode flavor.

[Toxophilite]

Hey thanks and Merry Christmas/Happy Holidays (if you're having some)


Much useful information here Looks like this is basically the circuit I have, though I think I could use a little more plate voltage.
Regarding a cathode follower, I've dug around and found some minature tubes(6Au6, 6aV6 ) I guess if there's room for a minature socket(which I only have one of) I could probably enlarge the hole with a step drill and drop in another twin triode and try a cathode follower.Time to search for a cathode follower circuit


Is there an advantage to using the transistor over a  cathode follower?. Those are schottky diodes?(had to look up the symbol)

[PRR]

...or a schematici with a LND150 source follower

See above (I edited).

[Toxophilite]

THanks very much!
 I edited my reply once I noticed the schematic. I had a few questions about it.

[sluckey]

6AN8 or 7199 is a pentode/triode that was used a lot by Ampeg and Sunn. Someone on this forum just recently built something using a 7199 in the preamp and he also said he had a bucket full of 7199s. Probably less than 2 months ago. Scroll back through the topics and you'll probably find it pretty quickly.

[sluckey]

Found it in less than a minute. Just what you need...

     https://el34world.com/Forum/index.php?topic=31020.msg341743#msg341743

[Toxophilite]

Thanks. I found these online. I was wondering though if it was possible/useful to just use one side of a 12au7, or a 6au6?
I certainly have many of those on hand. I can't find the LND150 at the local (usually very well supplied) electronics place though I'm sure they could get it. 7199s(when I was selling tubes) would be the ones I'd sell as they were very expensive and didn't show up that often. I don't have any and I'm sticking with the EF86 for now thanks



I think in the last one a couple of the grids are accidentally mixed up

[Toxophilite]

Found this and found it useful:
https://www.valvewizard.co.uk/accf.html


I was wondering if self biasing or fixed biasing of the cathode follower would be better in this application?
And can I use one of these tagged on after my ef86? Or should i be using both halves of the 12au7?

[kagliostro]

As Source Follower there are many examples on which was used an IRF820, here in the forum

Franco

[Toxophilite]

OKay my thick skull has finally grasped the idea of a mosfet or transistor cathode follower providing a more appropriate impedance which will result in better gain.
I could do a tube one but not having to add a tube socket, heaters etc. is obviously less intensive invasive etc. etc.
I appreciate the patience shown to impart this.



Here's one last nutty question before I venture out to the electronics store or order something here or elsewhere.


The original M67, M68 etc units had a transformer balanced low impedance out ,  the manual says 150-300 ohms for low impedance balanced ouput


 If I used one of these transformers post EF86 would I achieve a more reasonable impedance allowing me to achieve more gain or do I basically end up back where I started?


Thanks

[Toxophilite]

Nobody?

[shooter]

since it's been 40 years since I sat in class learn'n the virtues of inductance n impendence and and...


the "ideal" coupling transformer will produce the same "gain" that you feed it.  some couplers I've used as tube substitutes for the PI had a 2:1 gain factor, in the real world the 2:1 option yielded unity in to out, where the 1:1 ate about 5% of the signal

[jjasilli]

I respectfully submit that this thread is reinventing the wheel.  There are numerous how-to conversions of existing preamp conversions, such as from Akai, etc. tape recorders.  Why not just clone one of those?  For separate channels, there's conversions of a Bogen mic preamp/mixer > separate outputs.

[Toxophilite]

I'm not reinventing anything. I'm using Rod Childers conversion of the Akai rogers units, with some Akai rogers units. Building them for a friend who is going straight to tape. He is finding the Ef86 preamps to have too little gain for vocals and softer sources in that application. I have used these in front of a DAW and they actually are fine because with the DAW you can do a lot to boost a source, not so straight to tape.


PRR and Kagliostro and Shooter and helping me out with ways to sort out that particular problem which would seem to be about impedance


I think all the shure transformers are 10 to one. They all seem to measure 20 ohms on the primary and 2k on When used on the M68 output they are simply used in reverse. secondary hooked to circuit, primary hooked top XLR out


So I will  likely be going with the LND150 follower circuit suggest by PRR.


Looks like the local store can order some in for me. I'd probably get the ones that look most like a conventional transistor(little round top 3 long legs) because that seems the easiest with the lug board the Akai/Roberts has.
What would be a good current rating on the 18V Shottky diodes needed?

[Toxophilite]

Any advantage to using the LND150 over the IRF820 as a follower in this application?

[jjasilli]

Not a direct reply, but maybe some ideas here:  https://www.jensen-transformers.com/schematics/

[Toxophilite]

Not a direct reply, but maybe some ideas here:  https://www.jensen-transformers.com/schematics/

Thanks. Jensen makes great stuff and a some of those or some cinemags might go into the last set of these I build as input transformers for sonic variety.


However I need to keep the costs down for the fix to the EF86 gain issue to tape so I will either go with what I have transformer-wise if possible(I suspect not) or build a solid state follower as suggested which I think would suit as it's compact, requires less work and won't need the current another tube's heaters would.
Trying to decide between the suggestions but unfortunately I have no experience with  either

[Toxophilite]

I hope I'm not being a pest. I appreciate those who have show me some good options and I hope to implement one of them,
I just have to special order the parts and then drive about 45 minutes across town to pick them up so I wanted to make sure I'm picking the best and simplest option. I'm tryingto do further research online but am very unfamiliar with solid state stuff

[shaun]

I've been fooling around with some old Gates preamps lately that use EF86 tubes. A pal uses them in his hi-end studio - he loves old tube pres because they add characteristic tones that cannot be gotten any other way, not even through hi-end modern stuff; he says that's too clean.
Anyway, here is an old Gates pre schematic - very simple, but he loves them. Swears that it is the old signal transformers - in and out - that make all the difference. Schematic attached.
Happy Hols everyone.

ps Btw, he says this circuit offers something like 80dB of gain, which is getting up there.

[Toxophilite]

Cool, Looks like a cool unit, Uses 2 EF86 and a 12AU7, thanks.


For fun while I await the arrival of some LND150s I hooked up one of the Shure transformers on the output of the EF86 circuit and it gave me about 15db more gain. More bottom too but maybe a little top end attenuation. Probably that's another impedance thing,

[Toxophilite]

Again: the main problem is probably the very high output impedance of a pentode relative to any studio gear of the last 80 years. Fooling with the pentode's input grid resistor won't change that. Adding a stage in front probably changes the cherished flavor. However an LND150 (thanks for mentioning that) as source follower after can buffer low impedances with little change to pentode flavor.

I ordered and received some LDN150s, Still pick up the zeeners and other bits. Not sure why I didn't order the whole shebang in the same shipment. Probably because it was tagged onto an order of my brother inlaw's....maybe I really want to drive across town to buy bits??


Anywho I had a couple questions regarding it.


- Is the LDN150 supposed to be hooked straight to the plate of the EF86 or should there be a cap in between?


Also the B+ seems to be coming in from the right of the schematic with the LDN upstream of the EF86 with it's own filtercap and resistor. Is that what's intended?


thanks very much

[Toxophilite]

Where's Eric Carmen when you need him?

[Toxophilite]

I fully realize I'm not entitled to an answer but I would hugely appreciate one. I'm about o buy the rest of the parts for the add on follower and I want to make sure I have it right. Just need to know about the .22 cap at this point

[PRR]

> need to know about the .22 cap

I don't know any .22 cap???

[Toxophilite]

next! (couldn't delete)

[Toxophilite]

> need to know about the .22 cap

I don't know any .22 cap???

You removed it from the output of my initial schematic when you added the follower circuit

[Toxophilite]

Finally got all the bits...had to rebuild the brake light switch in my car first as it was stopping the car from going into gear. THat's another forum.


Implemented it and it worked great. Thanks!


Though I did leave the .22 cap between the EF86 plate and the LND150Gate,,,it was already attached to the plate of the EF86. Unnecessary? problematic?

[kagliostro]

Implemented it and it worked great

The SF or the brake light switch on the car ?

---

The cap acts to avoid DC cross, for the SF as far as I can know is unnecessary, problems ... don't know, surely it acts as a frequency cut

Franco

[Toxophilite]

It was suggested  the .22 cap might interfere with LDN150 Gate bias.


However it was aslo suggested  might want to have zeeners in anti-series across the output  to keep from blowing any gear connected to the output.
Putting a meter on the output shows a brief 126V before the preamp warms up

[shooter]

Putting a meter on the output shows a brief 126V before the preamp warms up

DCv??
shouldn't be any DCV on the right side of the 10uF cap

[Toxophilite]

SO far, both happily, Brake light switch controls whether car can go into gear from standstill.

Implemented it and it worked great

The SF or the brake light switch on the car ?

---

The cap acts to avoid DC cross, for the SF as far as I can know is unnecessary, problems ... don't know, surely it acts as a frequency cut

Franco