V76 Build (Pics)

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Squeaky

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Hello Everyone,

Here are a few pictures of my attempt at a V76. Includes custom Sowter plate inductors, which I requested be wound, as well as the usual Sowter input and output transformers. I had the power transformer wound by a local guitar amp builder who had previously done some stuff for me. The preamp is up and running, passes audio, sounds good to me and seems quiet. No noticeable artifacts running it up at max gain - just some clipping.  I haven't had time lately to fully check everything, although I did check the output plate voltage and this is running a little high. I'll definitely have to get a couple of cooler knobs.

Cheers,

Nick
 

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Nice, we don't see diy V76 that often and it has pad with nfb control on one switch. I like front plate color too.
You wrote there are no artifacts on max gain, how about noise with tubes sticking out of chasis? 
Not long ago someone here had problems with his V76 build not having correct voltages. Don't remember what it was, it seemed to me this design is a bit critical about that...
 
Thanks for the comments.

I love the 4 deck switch.

I was expecting the worst in respect of noise/motorboating, after reading about the challenges associated with this pre. I have only tested it with an MD421 so far (which I'd expect to be quiet), but I was surprised that it fired up straight off the bat and really did sound very quiet (and very loud). I'll plug in a valve condenser and stick an STA Level on the back end of it and see how quiet it is then! I've tried to float the input valves but I don't have a lot of faith in what I did. I hope they don't go microphonic too quickly.

I really did think that I'd have to rewire this pre a fair bit. I don't have a scope so I don't really know what is going on up top but so far it sounds good to me. Full testing will have to wait for a while as the studio is currently decommissioned.

It is pretty close to original spec I think (V76m?). I left off the filter at the input transformer and the small choke and two little caps after the HPF. It has a 100H inductor for the HPF.

I don't think I am that far off spec with E83F plate voltage (maybe 180V when it should be 170V?), I think the DC resistance of the plate choke might be a little low. I'll stick another voltage dropping resistor in when I get around to it.
 
Seems like most of work is done, why would you rewire or lift heaters if it works well? My first thought was that some wires (from circuit to tubes, etc.) are a bit long, although i've done the same with another tube preamp and it wasn't a problem at all.
The only thing i can think of are tubes mounted on the back, as you say some real tests in rack will show if that is ok at high gains..
Add rubber grommets to exposed holes even if they are only for signal trafos. A friend burned power transformer by shorting primary to chasis this way and wire insulation was pretty thick. It made a lot of sparks, wire cut about 2mm of Al, room smelled of something burned  :eek:  ;D
M version looks a bit different than what you have, wanted frequency response and sound is probably what tells you which version you want.
There is one tube that seems like a Telefunken, never had much luck with them, don't know why. So now i only use nickel pin EH EF86s to replace EF804s and other tubes of this type. They last at least a few years at ussual pentode mode at low current as in V series and most other designs, or wired as triodes up to 3-4mA. No microphonics, noise or any other troubles, we even comapred them to several known good NOS and couldn't hear any differences in sound. Often 1 in 3 is too noisy (hiss, not microphonics) for input stage which is ok with me. Checked well two of them after about year and a half of heavy abuse, they were like new. Only one NOS RFT had higher gain which of course sounded different, didn't try them in audiophile gear, maybe should :)
If using EF804s you could rewire it for EF86 to avoid buying expensive tubes in the long run.
 
There is no reason tubes on the outside would cause any problems.    Most tube equipment ever built is that way. 
 
I tend to prefer tubes on the outside.  They run cooler, helping them last longer.  Can sometimes make microphonics less of an issue.  And, it can also help keep the gear from sounding different over the first few hours of use as the tubes slowly heat up the inside of the enclosure.

  Regarding the EF86 vs EF804, if you plan on using quality tubes, I recommend going for the EF804.  People are often under the impression that the EF804 is more expensive, but when the quality of the tubes is equal, it's far less expensive.  You can get Telefunken EF804s for around 1/3 the price of a Telefunken EF86 and the EF804s haven't been picked through as badly so you're far less likely to end up with used or reject tubes (which is a serious problem with supposedly NOS EF86s these days). 
If you are looking for the cheapest parts possible, you can get modern EF86s for around $20 but the quality isn't anywhere near the Tele EF804, which can commonly be found for under $50.

  Thanks for posting pics of your build!  I'm loving my V72 and have been strongly considering a p2p V76.  This is nudging me much closer in that direction!
 
emrr said:
There is no reason tubes on the outside would cause any problems.    Most tube equipment ever built is that way. 

Bowie said:
I tend to prefer tubes on the outside.  They run cooler, helping them last longer.  Can sometimes make microphonics less of an issue.  And, it can also help keep the gear from sounding different over the first few hours of use as the tubes slowly heat up the inside of the enclosure.

Regarding the EF86 vs EF804, if you plan on using quality tubes, I recommend going for the EF804.  People are often under the impression that the EF804 is more expensive, but when the quality of the tubes is equal, it's far less expensive.  You can get Telefunken EF804s for around 1/3 the price of a Telefunken EF86 and the EF804s haven't been picked through as badly so you're far less likely to end up with used or reject tubes (which is a serious problem with supposedly NOS EF86s these days). 
If you are looking for the cheapest parts possible, you can get modern EF86s for around $20 but the quality isn't anywhere near the Tele EF804, which can commonly be found for under $50.

Thanks for posting pics of your build!  I'm loving my V72 and have been strongly considering a p2p V76.  This is nudging me much closer in that direction!

Thanks for both information, looking at new quality tube mic preamps made me think tubes need to be inside chasis because of possible interferences. From computers, almost saturated power transformers inside cheap new gear in close proximity to mic amp in question, other gear in rack, etc.  Beside what runs at line level, compressors and similar, i don't think i've seen any new factory made gear with tubes protruding from the back plate.
This post shows some RCA gear with tubes out of chasis which is not closed from all sides:
http://groupdiy.com/index.php?topic=41032.msg513738#msg513738
Those are metal tubes, some have sockets with metal shield. EF86 type has internal shield, does this also work well for mic input stages with glass tubes without internal shield? If so it is easier to build it, although never tried to measure difference assuming this practice isn't used in new gear  because of interferences.
I like using Belton micalex sockets which hold tubes much better than ussual ceramic ones. They don't have option to mount shield over tube and lack ground pin in the middle of the socket which can be handy. Probably have to use sockets with metal shield because of safety if going with tubes from the back.

My tests also show that tubes inside chasis needs hours for warming up everything to max temperature, while it takes maybe half hour when they are on metal plate while doing prototypes. I don't think this is reason we couldn't hear any real differences between new EH and known good NOS tubes because tube itself warms up very fast compared to the rest. I like to run regulators at max 70 degrees C, transformers cool, only some tubes are warmer than this. So gear should last a very long time, regarding sound heat doesn't influence the rest of the circuit much after about half hour, it shouldn't drift so much that could be heard. At least it seems logical, might be wrong.

Tube prices for EF804s seem to be a bit lower than a few years back, am i correct about it or just know better sources? It is not a type certain people "need", at times it worries me more that stocks are not endless, what is opinion on this?
Current prices have advantages over new EF86s, can we expect sorting will happen to them as for collectable types?
Btw, factories are changing materials and design of new tubes, i think all EH EF86s from my vendor have 06 08 codes, others used more often have 2nd number up to 12. Could second number mean year of production?
 
Tube shielding in broadcast gear has more to do with being parked near a transmitter RF field than any other reason.  You can put glass tubes in my RCA's with no difference under normal conditions.  Spend some time looking at vintage pics. Everything is on the outside.  Now everything is on a PCB and on the inside.  Saves $.  That's the only argument for it.  Transformer fields/pickups are of much more concern.  Audio and power transformers should never be inside the same box, but today they usually are in commercial products.    Many metal tube 'shields' on 9 pin tubes are there for temperature control more than any other reason, if you read the manuals. 
 
My3gger said:
My tests also show that tubes inside chasis needs hours for warming up everything to max temperature, while it takes maybe half hour when they are on metal plate while doing prototypes. I don't think this is reason we couldn't hear any real differences between new EH and known good NOS tubes because tube itself warms up very fast compared to the rest. I like to run regulators at max 70 degrees C, transformers cool, only some tubes are warmer than this. So gear should last a very long time, regarding sound heat doesn't influence the rest of the circuit much after about half hour, it shouldn't drift so much that could be heard. At least it seems logical, might be wrong.

What Emrr said is absolutely true, IMO.  It's so much cheaper for companies to slap it all on a PCB.  Regarding power, my limited experience is that each torroid has a different magnetic field and I've learned not to bolt them down until I have the circuit running and can move them around to minimize hum.  I've come to prefer a PT in an can, bolted on the outside.  One other reason I can think for tubes being in the enclosure is that it creates fewer problems in shipping gear/warranty costs.

  Regarding heat, it would seem as if the tube heats up within half an hour and everything is stable but in practice it doesn't always work out that way.  I deal in tubes and have heard endless comments from mix engineers as to their tube gear sounding different over the first few hours of use.  Granted, we're talking minutia but for a professional who is who is splicing takes, making comparisons, etc, it can be a clear difference.  Some get so irritated by it that they might leave the gear on all week during a mix session.  Usually, the comments are related to specific pieces of gear and most of them happen to have the tubes in the enclosure.
  In my own observations, and in a compilation of observations from others, the general consensus seems to be that tubes tend to go through most of their "warm-up" over the first 90min, regardless of being inside a chassis or out.  But, when inside, it's slowly bringing up the temp of other components as well.  In modern gear with good parts, this difference is smaller than in vintage gear or modern gear using vintage parts, where the values can certainly drift and change the operating points and thus sound.  I don't think I've heard anyone mention that they needed more than 3 hours for the gear to stabilize but some are pretty insistent that they can can hear the tone change over those first 3 hours and with their credentials in the recording field, I'm not going to question them. 
I've always heard it said that solid state gear does the same though I've not personally found it to be the case (not that it doesn't happen in SS gear, just that I've never noticed). 

  As to the price of the EF804, I believe you.  With tubes there are wild variances in price so it's completely feasible that you saw higher prices a few years ago, depending where you were looking.  There is a lot of B-stock out there now so places like ebay tend to fetch lower prices because the trust factor isn't there due to the high number of scammers and poor quality tubes being sold as good.
  Regarding why you didn't hear a difference in modern vs known good NOS, there could be a lot of reasons.  I get a lot of emails that are very passionate about the differences in modern and vintage EF86s but I don't feel that it's just a vintage vs modern thing because there are also a lot of low-quality NOS EF86s out there.  In fact, most remaining NOS EF86s are not much better, if any better, than current production.  Companies like Siemens, RCA, Amperex, etc even put their names on those lower quality tubes once they stopped producing them in their own factories (in fact, RCA, Tung Sol, and GE never even made their own EF86 but they sold a a variety of imports under their names).  So, it can get very confusing to those who are not intimately familiar with all the various productions.  Even sellers get confused and sell them with incorrect descriptions.  This is why sellers that know what they're doing and have the good stuff tend to make sure they get a good price for it.  It's also unfortunate because buyers often get their first experience with NOS tubes in the form of an RFT or other moderate quality tube and end up thinking all NOS tubes are overpriced/under-performing.  Not saying this is what you experienced, I just have a tendency to ramble on when it comes to tubes.

  Regarding your EH codes, there's a good chance that's true.  It's usually easier to produce batches when it's not a high-volume tube.  For example, in the 60's, Phillips would often have one of their Euro factories produce a large volume of moderately popular designs for one year and distribute them to the other brands under the Phillips umbrella (Amperex, Valvo, Mullard, Miniwatt) during their downtime for that specific tube.
 
An engineer friend in Nashville kept a spreadsheet on a particular piece of tube gear he kept on his mix bus at all times, to calibrate for gain changes.  Absolute gain stabilization was over 24 hrs, though the changes after a few hours were in the 1/10th dB range.  This was also gear with no NFB. 
 
I agree that the leads look a bit long running from the board to back panel, and even worse over to the front panel, and there should be a grommet protecting the output transformer wires. I only noticed that I overlooked the grommet when I saw it in the photo!

I'm pretty wary of EF86 valves from using them in AC30/4-type circuits, which is where my only experience with that valve lies. I have a few Mullard-branded and Amperex EF86 valves, and they are all microphonic to some extent (were all apparently NOS but were probably really TAD – tested and discarded). My experience with the new production ones is also not good (although I don’t think I tried enough of them).

All of that said, the EF86 sounds amazing as a V1 in a guitar amp, but they just don't seem to survive that long in combos. I’ve had more success in a head I built though. Unusably microphonic in a combo doesn’t seem to mean that it won’t be ok (for a while at least) in a head. I tried an E80F as a sub for an EF86 in an AC15-type amp, and it sounded lacking in mids or something. Richness you might call it. So I went back to using EF86s. Not sure if there is an electronic/materials of construction basis to that, I seem to recall that they are very similar tubes. I really wanted the E80F to work as well. I think there was a guitar amp builder (maybe Dumble?) who used to test all his valves and only use those that were on the edge of microphony, claimed they sounded better. That might be pure hearsay and unsubstantiated rumour though, and has nothing to do with V76s.

If I ever get round to building a U67 or one of those well-known British valve-pres, then I might try and get hold of a really decent EF86 or two, or alternatively, I might just throw in the ones I have to start with, and see what happens.

I’m a big fan of hanging valves out the back of stuff (so pretty at night as well). It’s actually the only way I know. Doing it this way has definitely been influenced by the equipment I’ve decided to build (mainly guitar amps and vari-mu comps). Probably the same reasons I also like can caps.

I kind of approached the V76 the same way as the other stuff I’d built (which are all turret board adaptions of the generally p2p originals). Whereas with the other stuff I tried to follow the original component layouts as best I could, I couldn’t do that with the V76, so as far as I know the layout I posted is new. Pretty obvious, but new. Hence the post. All in all it’s a bit agricultural but seems to work.

There might have been a misunderstanding before when I wrote “floating”. I didn’t mean electronically floating, I meant the V1 socket physically floating to avoid vibrations from the chassis. Buffered from the chassis with rubber or silicon or something. Not that easy to do (without still having screw connections to the chassis, which would probably defeat the whole point of it).
 

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I haven't found much troubles with microphonics in preamps, combos are much more susceptible to it. Try Svetlana EF86 in them, for some reason many of us find they have lower microphonics compared to many other tubes of the same type. I used to sort them anyway because many were out of tolerance regarding parameters and also sound. Under 50$ is cheap for real NOS EF804, i agree that it is cheaper to buy one of those instead of two from new production. I use EH because situation from good cheap Western and Eastern EF86 type to expensive and bad changed so fast.
E80F should be very good replacement where space allows for their height, you can find out more about them here:
http://www.neumann.com/forums/view.php?bn=neumann_archive&key=1015336865&v=f
Seems like a good one for mics (don't have experiences in this application), in preamps it sounds as good EF86 and i never got one that would be bad in any way. Tubes that are too noisy for V1 or V2 (like in V76 with 3 of them) can be perfectly ok for later stages. I just had situation where both were wired as triodes and hiss was pretty high, when i exchange V1 fo V2 preamplifier got much more quiet than all other quite good solid state preamps in the studio. Of course grounding was properly made and i oredered toroid to my specifications, so it didn't have much external field, guess you can imagine how happy i was :) Even "normal" toroids enclosed in chasis never caused me problems, in cases of buzz it was always grounding problem created by me.
I floated tube socket by drilling a bit larger holes, put rubber grommets inside and up to the "head" and top of the screw. This way screw never touches metal directly, it is also well damped from top and bottom when tighten. There are other floatig mountings like plate with socket mounted on springs, etc. Maybe this is needed in some situations, i don't feel need to do this when mounting sockets on L profiles, don't know why...
 
Thanks for the comments, I'll look into the Svetlana brand tubes.

Thanks also for the E80F link, although I think that might have been the same series of posts that alerted me to the E80F in the first place! A year or so ago I was almost, very nearly, just about going to build a U67. It looked a bit finicky though. I love the comment in that thread by the poster Klaus Heyne:

"Obtain the best EF86 you can find on the market; then don't assume that it will work in your U67 until you have plugged it in, set it up (more below), left it on for 48 hrs. minimum, and then test for noise and functionality. Every other assumption or promise of functionality and noise-free operation is illusionary in the practical world. Sometimes you can luck out with a whole batch of flawlessly performing cheapos, other times you curse the seller for hawking $60.- NOS junk ('Tele with diamond...')".
 
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