V76 tube preamp build

[dmp]

How did you implement the di?
Do you switch in a grid resistor with a relay while disconnecting the input transformer? 

I just used a switching 1/4" input jack. So the lead from the input transformer is connected to the 1/4" jack and it is interrupted if a plug is inserted going back to the tube grid.
This is simple and I've found it reliable and easy to use. The wire routing needs to be done right (shielded) since the tube grid wiring is extended. 
I like the DI to connect directly to the tube grid without a blocking cap or any added resistors.
I connected the "ground" of the DI jack to the 10uf/100k resistor (isolated from the chassis).  This keeps the tube bias correct when a DI is used.

 

[hop.sing]

I just used a switching 1/4" input jack. So the lead from the input transformer is connected to the 1/4" jack and it is interrupted if a plug is inserted going back to the tube grid.
This is simple and I've found it reliable and easy to use. The wire routing needs to be done right (shielded) since the tube grid wiring is extended. 
I like the DI to connect directly to the tube grid without a blocking cap or any added resistors.
I connected the "ground" of the DI jack to the 10uf/100k resistor (isolated from the chassis).  This keeps the tube bias correct when a DI is used.

I guess that works only if the di is connected to a floating ground device, like a guitar or bass or  battery powered effect device.. if you take care of that it seems an elegant solution.

 

[dmp]

One observation on your build.  I have found that toroids are best mounted vertically for lowest hum, but I realise that the one you used was too wide for your case, still the MuMetal shielding did its job.

Thanks for your input.

DaveP

Yes, a 2U case is 3.5" and these Antek transformers are a greater diameter than that.  I've read the point where the wires exit the toriod can have higher flux so should be pointed away from things.  I didn't try different alignments with it though.

 

[dmp]

I guess that works only if the di is connected to a floating ground device, like a guitar or bass or  battery powered effect device.. if you take care of that it seems an elegant solution.

I only tried it with a passive pickup so far, but didn't give it much thought before. Now that I'm looking at it, it's not going to work well with a lot of sources, most likely.  The input secondary in the v76 floats at 8.8v and the grid bias is 8.8-11=-2.2v
A lot of tube preamps ground the grid, which makes switching to a DI no problem. If the tube grid in the v76 is grounded, however, the tube bias is way off. Thanks for pointing that out.
The DI will need a blocking cap.

 

[dmp]

Found this post from Oliver from a few years back:
"Without going into technical details, in order to utilize the circuit for DI input, the entire front end needs to be changed, due to some minor problems of in-state feedback as RADD47 pointed out in a different post.
In the new V72 and V76 that features a DI jack, the inputstage gets switched, x-former, grid leak, feedback loop, etc. via a DPDT switch and an additional relay for the feedback loop. All signal (grid) wire runs are less than 2 inches...
In order to keep the amazing S/N ratio all wire runs need to be as short as possible, so you need to drill the face plate, otherwise you end up with approx. 12 inch of wire that sucks the high end and acts like an antenna loading down the signal with RF; you can not hear it directly but the sound gets mushy."

The DI input needs a capacitor before the grid to block the bias voltage.
If the input transformer is disconnected from the grid, a resistor needs to be switched in to provide the 8.8v bias.
I just sketched this out to try.
 
I don't see why the feedback would need to be changed, unless it is to make the gain steps correct without the pad.

 

[hop.sing]

you could use a single pole switch:
https://groupdiy.com/index.php?topic=38252.0
look at the schematics.

 

[dmp]

Cool, I'll check it out.

 

[dmp]

you could use a single pole switch:
https://groupdiy.com/index.php?topic=38252.0
look at the schematics.

I like that better - it is simpler with the SPDT switch, but it also leaves the bias voltage connected to the DI blocking cap all the time, so there is less likely to be a pop when switching. I started putting this in last night. Hopefully I can try it out by the weekend (having a toddler puts everything on a slower timeline  )

 

[dmp]

Added the DI switch. Will test out when I get back to the studio.

I found this interesting paragraph - again by Oliver - on the EF80xs. They have a very low grid current. Looks like using a 2.2M resistor will be no problem.

The EF806s is pretty much the same than an EF86, audiophile products wasn't the reason when Telefunken introduced the 800 series of tubes, 800/802s, 804s, 805s, 806s as an long life version *10k hours min* of the EF80, EF804, EF85 and EF86. Those tubes were made for broadcast, military and medical machines like EEG/EKG. The EF804/806s uses all the same mechanical parts like the EF86 only that the mica wafer were heat treaded and different cathode nickel was used. Soundwise EF86/804 and the 'S' are the same, were for the 'S' version a better noise figure is practical build in. The heat treaded mechanical parts allow a higher grid leak resistor due to virtually no sneak currents inside the tube. You can replace the EF86 for a EF806s without any problem. However, even Telefunken tubes are called the bench mark for electron tubes, the Valvo made E80F is the best version out of the EF86 family type of tubes. Telefunken as well as Valvo developed their versions out of the rimlock type EF40, that was the post war version of the EF12. Valvo did a better job with microphonics, S/N and random noise. Compared to the EF806s made by Telefunken, the Valvo E80F also features heat treaded mechanical parts plus a gold plated first grid that allows a higher grid resistor than the Telefunken tubes, this difference comes handy with grid leaks above 3G ohm and High SPL. Without the gold plated grids tubes with 3G ohm resistor and high input levels tents to change its start up current point easier.
One reason why Neumann original used the Valvo EF86 instead of the Telefunken EF86 was not only price, Valvo cost 1/3 less than Telefunken, it was that Valvo tubes worked better in tube mic circuits. A standard Valvo EF86 was about DM1.95 in 1962, just a bit under the technical performance of a Telefunken made EF806s that went for DM16.00 in 1962. The E80F was DM25.00.
The E80F was mostly used as an electrometer tube and circuit were high grid leaks were needed, like O-scopes, etc. not used in consumer electronics like the EF86. It is still easy to find and works great in all applications that ask for a EF86/806s?. One minor problem is the taller size than EF86, but it fits without any big problems in U67s?. E80F made by Valvo were sold here in the US under the Amperex brand as 6084.
Best regards,
Oliver

 

[mrevans]

Hi DMP,
it is a beautiful piece of work you did there! congratulations!
I am new around here and I have three questions about the implementation you did I hope you can elaborate on them. How did you incorporated the Pad section is it before the input transformer? 
and in the original schematic there is a RF Filter in the input before the IT As I read in one of your posts you just used shielded cable am I right? or what other solutions did you used besides the faraday cage for the transformers and the chokes?
and the third one, do you have some audios to listen to it? maybe compare it with another preamp?
Thanks  and great work!

 

[dmp]

Hi DMP,
it is a beautiful piece of work you did there! congratulations!
I am new around here and I have three questions about the implementation you did I hope you can elaborate on them. How did you incorporated the Pad section is it before the input transformer? 
and in the original schematic there is a RF Filter in the input before the IT As I read in one of your posts you just used shielded cable am I right? or what other solutions did you used besides the faraday cage for the transformers and the chokes?
and the third one, do you have some audios to listen to it? maybe compare it with another preamp?
Thanks  and great work!

Thanks!
The pad is a balanced type before the input transformer, like in the original schematic. I messed with the dB but kept the same 200 ohm R as in the original design. I split the pad out from the gain switch to make the preamp more flexible than the original combined setup. Also, the combined pad/gain would not work with a DI in (where the DI  is post input transformer and not affected by the pad)
I left out all the filters of the original, including the stuff on the input transformer. Just used shielding on the chokes, transformers, and cable.
No audio at the moment but maybe in the future. Have done some work with it in the studio on it and very happy so far.

 

[Squeaky]

Looks like you have done a really great job of it. Congratulations on the performance you achieved. I have some really long wire runs in my version that need to be sorted out sometime (yeah right, that'll happen). I should have thought about the case depth a bit more.  Mine is a bit more of a prototype build but I don't think I'll ever build another one. Like you, I also went with a turret board type arrangement.

Over the last couple of years I used my build of the v76 extensively on an album I was recording (a vanity project I think they are now called). I must admit I abused it a bit (clamped with a limiter).

The idea of a DI input is quite attractive but the gain switch as it is arranged in the original works so nicely.

I never properly tested the performance of my V76 but I have no doubt that it is probably wanting. Certainly good enough for rock and roll though.

I removed the capacitors from the front end as well but left in the other filter. My gain switch is wired as the original on a 4 deck switch. A few years ago I sent some choke specs through to Sowter and they made some up for me pretty close to what I asked for. These ended up on their website. I had a local (at that time) guitar amp manufacturer (BJ Amplifiers, Brisbane) build a power transformer for me. I think I purchased the 100H filter choke from Electra Print. Input and output transformers also Sowter.