My Ampeg SVT preamplifier build : documentation available

[vibrolax]

I have recently completed a turret board layout for an Ampeg SVT bass preamplifier, and built it into a 1U rack chassis.  It's derived from Ampeg's 1U rack SVP-CL preamp from about 10 years ago.

During my research for the project, I found a few undocumented DIY builds, but nothing of much use.  I basically started with just the factory schematic.  I laid out the turret board Doug Hoffman style.  The other crux for the project was reverse engineering the multi-tapped toroidal inductor used in the midrange tone control.

I published the detailed design and build documentation on my personal website:

http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifier.html

I will be happy to answer questions about it.
Happy building,

[kagliostro]

Very Cool 

I like it very much

 

K

[sluckey]

Very nice indeed! And the documentation is excellent. Hope to see lot's more of your work.

[jjasilli]

 

[HotBluePlates]

That's a pretty excellent build! Great job!!

[DummyLoad]

nice work and documentation. 


did you try cinemag? they will wind custom tapped inductors.


--pete

[g-man]

Deleted

[vibrolax]

nice work and documentation. 


did you try cinemag? they will wind custom tapped inductors.


--pete

No, I specifically wanted to be able to make the inductor myself, and to publish the winding recipe for the DIY community.  It was relatively easy to make: it was the first "real" inductor I ever wound!

I did order a couple Cinemag CM-2810-J's with wire leads for the balanced line output transformer.  I wasn't sure how the OEP A262 would work out.  Since the line out cathode follower is driven by a wimpy 12AX7, the OEP was fine (it's got high-nickel laminations as well).  The Cinemag would have been overkill.  I'm sure I'll use them when I need a high current balanced out.

[John]

That is excellent documentation, thanks so much for taking the time!

[vibrolax]

That is excellent documentation, thanks so much for taking the time!

Thank you all for the compliments.  The documentation means I can build another one without having to remember precisely how I did it the first time. 

Higher-res pictures are available in Google+ if/when someone wants them.

[TubeGeek]

Holy cow!


Nice job on this project.


I know a couple clients of mine that would love to have one of these.  They keep asking me to build something like this for them.


 

[vibrolax]

Holy cow!


Nice job on this project.


I know a couple clients of mine that would love to have one of these.  They keep asking me to build something like this for them.


 

Thanks.  My intention with the documentation was to put the SVT preamp into the DIY repertoire like all the infinitely cloned Marshall, Vox, Fender, Hiwatt, Trainwreck, etc. amps.  The best way to test the documentation is for someone else to build from it.  Go for it!

[kagliostro]

Just curious, how did you fix the PT to the chassis ?

Is it inserted floating on a plastic cylinder and stopped by the top of the rack unit ?


K

[jazbo8]

Great build and documentation, I like the DIY inductor and front panel design process in particular, thanks for sharing.

[vibrolax]

Just curious, how did you fix the PT to the chassis ?

Is it inserted floating on a plastic cylinder and stopped by the top of the rack unit ?


K

Yes, the delrin rod held by a screw below the bottom plate of the chassis, and the PT is slipped over it.  I wish Antek produced a slightly thinner 35 VA toroid that would fit better in a 1U, as 90 mA HV current is 4x what the circuit requires.   It does stay very cool though, and is very quiet mechanically.  You definitely don't want a bolt going through the center of the toroid and touching he bottom and top plate, as that would create a magnetic circuit.

I did find a manufacturer in England who made suitable toroids for a 1U chassis, but the cost and lead time would not have been as favorable.  A custom part with ~7 VAC filament windings would also be advantageous, as it wouldn't require low forward drop diodes and a low dropout regulator to stay in regulation.

I would consider organizing a group buy for such a thing if there were sufficient interest.  Does anyone else here know of any suitable off-the-shelf 1U toroids?

[vibrolax]

Great build and documentation, I like the DIY inductor and front panel design process in particular, thanks for sharing.

Thank you.  I initially tried to estimate the wire size and core size from an image of the guts of an Ampeg SVP-CL, but I felt that was too uncertain.  It was not difficult to figure out once I obtained an actual Ampeg part.  Now anybody can make one.

As I explained in my documentation, I wanted to do the panels with toner transfer and copper sulfate etching.  Others have been successful with this and the equipment costs can be quite low.   It is possible to get very artistic with it, but making a good dense toner transfer is the key.  I am going to get back on that horse eventually.

Front Panel Express did a great job with my panel, though, and it comes out of the box ready to use.  It was nice using blind holes for the pot/switch anti rotation tabs.  If you value your labor time, FPE is a decent value.

[kagliostro]

If you are on the other side of the pond I doubt you can find interesting this items, however

http://www.musikding.de/Toroidal-transformer-230V-270V-14V

http://www.musikding.de/Toroidal-transformer-230V-185V-63V

http://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-30VA::2192.html

http://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-27VA-Preamp-Engl-520::782.html

http://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-47VA::5391.html

http://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-50VA::527.html

http://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-56VA::4908.html

http://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-81VA-Power-Ultra-Flat::4786.html

As you can see prices on the other side of the pond are quite lower

I remember that in the USA or however near it (may be Canada) there was a brand that build toroid for tube circuits at a very good price, but unfortunately I didn't remember any more the company name

---

Thanks for the answer about the PT mounting

K

[vibrolax]

I'm in the USA on the north coast. The toroid manufacturer you are thinking of is probably Plitron in toronto.  If they were inexpensive in the distant past, they are now relatively expensive.

I had seen the various German offerings, but with the euro and shipping, they are not a good deal in the USA.

I like antekinc a great deal: USA made, high quality,and the least expensive. Just not perfect for 1U.  Very diy friendly, too.

In a future post I will present my hack for using step-down transformers with dual primary windings for both filament and b+ supplies.

[rzenc]

congrats Vibrolax!!! Outstanding build!!!!


Best Regards


R.

[DummyLoad]

perhaps an alternative for future build. filament windings assume you'll be using DC w/ linear Vreg. 110 bux is probably 3x the antecinc price tho, but much better quality and fits in a 1U

http://www.toroid.com/standard_transformers/rectifier_transformers/DataSheets/205.5076.pdf


--pete

[terminalgs]

awesome build.  great tech write up.  thank you for sharing.

[vibrolax]

perhaps an alternative for future build. filament windings assume you'll be using DC w/ linear Vreg. 110 bux is probably 3x the antecinc price tho, but much better quality and fits in a 1U

http://www.toroid.com/standard_transformers/rectifier_transformers/DataSheets/205.5076.pdf

--pete

The Toroids of Maryland height is a little better, so it wouldn't be quite the squeeze.  Only a single primary winding, so no 120/230 VAC.  It ought to be gold plated for $110.  Antek is $28, qty 1, and I haven't found any lack of quality.

The Antek's marginal AC voltage for regulated 12 VDC was easily dealt with using MUR-410G diodes and the LT1085.  It stays in regulation below 110 VAC.  There are plenty of 1A regulators with even lower drop out voltage.  3000 uF for the filter caps is also plenty for 0.75 A,  though more capacitance would increase the dropout headroom as well.    The advantage of having a low DC margin is the excess voltage does not get dissipated as heat.

There's a much better selection of 105C radial / snap-in caps these days, so I will try to use them in the future.  I bought some low profile Nichicon 100 uF 450V ones to try for the B+.  I have no qualms about putting the power supply boards on PCB's.  I think it also might be possible to mount snap-ins securely on 0.125 turret board using eyelets of the proper size.

[sean_thornton]

Gorgeous build, very very clean layout.

[vibrolax]

Gorgeous build, very very clean layout.

Thanks.  I had just finished my first tube scratch build in 2003 (a Champ-like pre with a SE 6S4 triode output stage) when I discovered Doug Hoffman's website, with his unique turret board layout style.  It made perfect sense to me the first time I saw it, and I have used that style ever since.  You can't help but make clean layouts if you do things that way.

It is always a challenge to make the off-board wiring as clean as the on-board.  I know that below-board connections are not as serviceable as above-board, but maybe that's the tradeoff for "clean".  Some builders do very nice control wiring above board -  I will have to find and study some inspiring examples so I can add it to my builder's palette.

[V1nce69]

Beautiful build, congrats Jon !

After having read all your very well documented notes about how you built this preamp, I decided I also wanted to build one for me.

It will be quite a challenge as I'm used to design, etch and build stomp box effects, but I haven't attempted to build a tube preamp yet.

Despite the reading I still have a few newbie questions regarding the filaments wiring:

- on the low voltage power supply, you specified F+ and F-, but on the main board, it's only written F1 and F2. Does F+ goes to F1 or F2 ?
- The filaments are wired in series, right ? Each F1 linked to the other F1; each F2 linked to the other F2.
- Is Fc used ? If yes, where does it go to

Regarding the toroidal power transformer, I chose the Antek AS-05T240, as the one you used wasn't available anymore. I will use the 240V output.
Do you think it will make a big difference in the power supply ?

Midrange inductor: in your notes you say that you wound it down and clockwise.
If I understand this correctly, it means your common wire goes from under the toroid towards the inside and then round in a clockwise turn. Right ?

Like in the image shown below:


Thank you for having made this preamp available to diyers.

Vincent

[vibrolax]

Thank you, and you are welcome.  At least 3 others have successfully completed building this design from my docs, so I'm confident that it's correct.

1) The filaments are 12V, and the filaments are connected in parallel.  Pins 4 & 5 (I tagged them F1 and F2 in my layout) on the 12AX7's and 12AU7's are connected to the F+ and F- outputs of the regulator.  The filament is not sensitive to polarity.  However, it is standard practice to keep the same polarity at each tube.  For example, connect all F- to pin 4, all F+ to pin 5.

2) The negative filament line, F-, is referenced to 1/4 the B+ voltage.

3) Using the 240V output of the AS-05T240 will give a lower B+ voltage that I used, but it will work fine.

4) It doesn't matter whether you wind the inductor clockwise or counterclockwise, inside or outside.  You just need to wind it in one direction.

5) The toroid is wound with a single continuous strand of wire.  After 129 turns, you make a  little 2" loop of wire outside the core for the 1st tap, then you continue winding until turn 223.  Make another 2" loop outside the core for the second tap, then continue winding until you have a total of 364 turns.  The coil start wire is the common, the first tap is 100 mH, the second tap is 300 mH, and the coil end wire is 800 mH.  Twist the tap wires as shown in the picture.  Scrape off the varnish on the start, end, and tap wires.  Do not cut the loop on the taps!  The toroid winding is not that technical.  You want a nice tight wind, but you don't want to stretch or break the wire.  As I wrote in the doc, the tension is supposed to be 2 lbs.  My wind was not as tight as the factory part, but it measured and worked fine.  You don't need to measure it if you use the right core size, material (FT-87J),  and wire gauge.  It's simple physics.

[V1nce69]

Thank you for your answer Jon. It's a lot clearer now.

1) I will follow your advice for the F1 and F2 filaments' wiring.

2) If I correctly understood your high voltage power supply,  the voltage divider on the B+ is at the junction of the 100k 1W and the 33k 1W (lower left turret).
 I assume each F- filament is wired separately to the voltage divider.

3) Power transformer. OK

4&5) For the inductor, I ordered exactly the same material (both inductor and wire from the Xtal Society) so there should be no problem.
While winding the inductor, did you note each 10 turns, so as not to get lost in the counting ? 380 turns is a lot.

I'll report as I progress in my build.

Vincent

[vibrolax]

The filaments are wired as a pair of wires from the regulator to the first tube, then from the first tube to the second tube, and so forth.

I just sang my winding counts to myself out loud.  It's a pain to put down the toroid and pick it back up while you're​ winding it, because you will lose tension on the wire.  You can use a tapered dowel or pen barrel in the hole to hold it if you need to put it down.  If you do 1 turn per 10 seconds, it will take one hour to finish.

[V1nce69]

Hi Jon,

My build is now complete, but unfortunately not working. On the first start, the led quickly lit up and immediately stopped. The fuse has blown.
The hasn't been any fumes or burnt resistors.

Am I'm right when I say that some high current must be creating a short to ground ?

I measured VDC on the capacitors on the two power supply boards using my multimeter one pole on each side. The answer was they didn't get current.

Then I unsoldered B+ from the main power supply board to take it out of the circuit and replaced to fuse. I measured voltage before powering the amp up.
I got 230V on the power pack line entrance, same on the fuse and after the fuse on the voltage selector switch. The fuse blew right away when I powered the amp up.

I took the ac out and made some other continuity controls. I noticed something strange: I have continuity between F+ and F-, as on each F poles of the sockets (with tubes in).

Is this continuity normal ?

What would you advise to check first, and the in which order ?

Here is a picture of my completed build

[vibrolax]

Hi Vince,

Your build looks nice, I'm sure you'll be able to figure it out.  I'm on the road today, so I can't write down a thorough/correct list of the steps I'd do to find the problem right now.  That will have to wait until tomorrow.

I would expect that you would get continuity between the F+ and F- with the tubes in.  The filaments are low resistance, and there are 4 or them in parallel.

I think you need to start back the the transformer wiring.  Are you sure you wired the voltage selector switch correctly, along with the transformer primaries?
The filament secondaries are wired in series, and have to be the proper phase so the voltages add.  Check that.

I gotta get on the road now.  I'll check back tomorrow.

Your picture is pretty good: I will have to check it against the pictures of my build.
A google photo album containing all the pictures I took during the build is here:
https://goo.gl/photos/MRyUN4xA1vA1ZJYT8

One proper practice violation I saw immediately: The safety ground (green/yellow) is supposed to go to its own bolt to the chassis.  You seem to have wired a few signal grounds to that bolt as well.  I don't think this is the cause of your problem, but it is against safety code for all electrical equipment.

Jon

[V1nce69]

Hi Jon,

Thanks for your quick answer!

After having taken out both B+ and F1&F2 filaments form the power supply boards, I still blew another fuse...
I'm am pretty sure, one error lies in the selector switch wiring.

I made my wiring as per your drawing.

I took one detailed pic of the power supply wiring. One thing I'm sure it's correct is the wiring on the ac entrance. I checked by measuring voltage on the power cord and found out line is on the right here in Switzerland. Maybe this detailed pic can help you.

[V1nce69]

Found out one error in the selector wiring: I wired the wrong mains in series. Unsoldered the selector switch. And re-soldered like this:

upper left pole : 1st winding red wire and red wire from power switch (lower pole)
upper middle pole: 2nd winding red wire
upper right pole: Jumper to lower right pole,
lower left pole: 2nd winding black wire and white wire from power switch (middle pole)
lower middle pole: 1st winding black wire.

On power up, the led lit normal and fuse didn't blow !

First progress.

Now on to voltage measuring.

[V1nce69]

Main power supply:
240 VAC between the white wire (240V transformer out) and the yellow wire (transformer out)

grey wire (220V transformer out) is tapped and unused.


Low voltage power supply:
6.2 VAC on the blue and orange wires.

At first sight it looks good.


Regarding your previous comment on the chassis ground, I did as electromik did. I didn't know it was against grounding rules. I will check my build and correct what is necessary according to your wiring schematic. For instance i noticed that the shielding wire on the balanced output transformer doesn't seem to go to chassis ground as it should.

[PRR]

The two "filament" (heater) leads are not only jumped by four low-R heaters (probably a couple Ohms), but by the PT heater winding (probably under a half-Ohm). Since most bench meters get dubious when close to 1 Ohm, a low-Ohm reading is expected and does not prove much.

Before buying-out all the fuses in the shop, *build a lamp limiter!!* (While you can still buy 100 Watt *Incandescent/Halogen* lamps!) A dead short makes a bright bulb, no damage. A heavy load makes a half-bright bulb, and 5-40W tube amps won't usually do this (after a start-up surge), so be wary. A happy amp makes a dim glow, won't play "right", but will play enough to verify it isn't real sick. Take the limiter off before you ask about power or tone.

Production gear inspection demands a *dedicated* wall-ground screw. So that later repair techs do not un-do it for other work and forget to re-install the wall-ground wire. In DIY self-repair work, you will hopefully remember what you did and do it again at every repair. OTOH I can't remember if I took my pills today, so build details will be very fuzzy 5 or 10 years down the road. If I can, I like to build so that in panic or dementia I will probably do a right thing. 

[vibrolax]

Of course I concur with PRR's suggestion to make a lamp current limiter to help prevent excessive profit of the fuse companies.

Your description of the re-wired voltage selector sounds correct.

You can test whether the voltage selector switch is wired correctly with the mains voltage removed.  Measure the resistance of the primaries at the IEC mains connector when the switch is in the 230V and in the 115V positions.  In the 115V position (primaries in parallel), the resistance should be 1/2 the resistance of a single primary winding. In the 230V position (primaries in series), the resistance should be 2X the resistance of a single primary winding.

The AC voltage from the HV secondary that you measured (240 VAC) appears low.  Did you use an Antek AN05-T280, or did you use a transformer with a lower voltage for the HV secondary?

I'm not sure what you think is not correct about the shield ground on the output transformer.  It appears the shield is wired to chassis ground, and chassis ground is wired to XLR pin 1 via an SPST toggle switch to provide a ground lift.

In my correspondence with Electromik during his build, he did not report finding any errors in my documentation.  It doesn't mean that errors don't exist in there, but I know 3 other builds that have been successfully completed.  Since it appears that you have closely followed my (and Electromik's) build, I think you will also be successful.  I made a few mistakes while putting it together.

It turns out that I will be traveling for the next several days.  It is possible, but not certain that I will be able to respond to questions during that time.

Jon

[PRR]

> test whether the voltage selector switch is wired correctly

Not totally fool-proof. You can get the indicated DC resistance, but if the windings are out-of-phase, the transformer acts as a near-short.

Lamp Limiter is just something you should have when messing with wall power.

There's other ways. A 6VAC winding through a 10 Ohm resistor will bring-up a happy amp to nearly 6V (too low to work, but enough to suggest no big fault), while a mis-wired near-short amplifier will show <1V (and a hot resistor). This is of course just a variant on 120 (or 240) through a large lamp (cheap power resistor). Possible advantage is you can stick your fingers on probes with 6V going in, something you should NOT do with 120/240 going in.

[vibrolax]

That's a great point about the resistance being insensitive to the phase of the windings.  I keep a 9 vac adapter handy for exactly that propose. 

[V1nce69]

>to make a lamp current limiter to help prevent excessive profit of the fuse companies.
I built one. It was useful. I had a few mistakes in my grounds. The main error was a chassis ground on the input jack that I had put on the wrong site (opposite to the link to the dpdt switch. It made a short circuit.

>The AC voltage from the HV secondary that you measured (240 VAC) appears low.  Did you use an Antek AN05-T280, or did you use a transformer with a lower voltage for the HV secondary?
I used an Antek AN05-T240, thus my voltages.

>Chassis ground: non respect of the good practice.
I modified my chassis ground according to your remark. It's my first amp build and I still have a lot to learn...


After verifying all my wires to ground, it appeared the one from output jack sleeve to the main board was missing. So I added it.
The amp now powers up normally. I have a normal clear sound on the tuner out. But no sound at all on the output jacks.

I measured voltages at different places to let you know and see what is wrong.

Main supply board
234 VAC between AC1 and AC2.
287.6 VDC between B+ and GND.

Low voltage board
12.17 VAC between AC-F1 and AC-F2
12.0 VDC between F+ and F-

Main board
281.9 VDC after 1st 1k5 3W resistor
277.5 VDC after 2nd 1k5 3W resistor
271.3 VDC after 6k81 resistor

Valve 1
p1 83.8 VDC
g1 1.48 VDC
k1 2.81 VDC
f1 84.4 VDC
f2 72.8 VDC
p2 277.6 VDC
g2 0 VDC
k2 0 VDC
fc 72.6 VDC

Valve 2
p1 277.8 VDC
g1 85.6 VDC
k1 97.1 VDC
f1 84.5 VDC
f2 72.7 VDC
p2 275.2 VDC
g2 0 VDC
k2 0 VDC
fc 72.6 VDC

Valve 3
p1 75.1 VDC
g1 6.02 VDC
k1 7.2 VDC
f1 84.8 VDC
f2 72.8 VDC
p2 266.4 VDC
g2 0.5 mVDC
k2 0 VDC
fc 72.8 VDC

Valve 4
p1 288.0 VDC
g1 135.4 VDC
k1 174.0 VDC
f1 84.8 VDC
f2 72.8 VDC
p2 287.6 VDC
g2 266.0 VDC
k2 3.47 VDC
fc 72.9 VDC

Audio probe results:
There's sound on the input until G1.  Sound on Tuner out is also OK.

No sound on G2.


My build looks now like the following pictures.


Vincent

[tubeswell]

Nice job on the toroidal inductor! Thank you for the detective work and the documentation.

[vibrolax]

I agree that the audio probe should be your next move, teaching the signal through each stage.

I will run the simulation with your measured b+ voltage, and see how it compares with your measurements.  I'm still traveling a lot, so it may take a few days until I can do it.

Jon

[V1nce69]

>I agree that the audio probe should be your next move, teaching the signal through each stage.
I don't have sound at V1G2, though I assume I should. Can this be related to the fact that the measured voltages on G2 and K2 of the V1,V2 and V3. Only V4 seems to have correct voltages to my newbie eyes.

>I will run the simulation with your measured b+ voltage, and see how it compares with your measurements.  I'm still traveling a lot, so it may take a few days until I can do it.

Jon
Don't worry. Take your time. I really appreciate all your comments. I have no deadline for this build. I only want it to work, when is not a question. ;-))

In the meanwhile, I'll try to take out the tubes and see if I get different voltages. I will also try to change the two 12AX7 with some spares I have around.

Thanks
Vincent

[bnwitt]

Jon,
Let me add my words of admiration for your project.  Top notch work from the planning, to sourcing parts, the build and all the way to sharing it with the tube community.  Fantastic!  And my, my,  I've never seen so many 1% tolerance resistors in one amplifier   I predict that preamp will be around for many many years.  Cudos.  By the way, I love your catch phrase: "In a vacuum tube, no one can hear you scream".  This is true, but they might here you wail.
Barry

[vibrolax]

Thanks for your compliments.  I use 1% rn-60d's most of the time, as they don't significantly increase the project cost.

I try to contribute back to the community in return for all that I have received from them.

I have a mostly complete set of parts for another build of this project.  Waiting for the day when I need to build another one.

So far the owner hasn't had any issues with it in three years.

Jon

[PRR]

> no sound at all on the output jacks. I measured voltages at different place

Many of those voltages are unlikely. Is there a known-good voltage chart for this plan which you can work against?

[V1nce69]

>Many of those voltages are unlikely. Is there a known-good voltage chart for this plan which you can work against?
I don't have one. Hopefully Jon has one.


The above mentioned voltages were with the tubes in.

Now I have taken them out and measured the voltages directly on the sockets.

Socket 1
p1 314.7 VDC
g1 0.3 mVDC
k1 0 VDC
f1 92.7 VDC
f2 80.8 VDC
p2 324.2 VDC
g2 0.2 mVDC
k2 0 VDC
fc 80.8 VDC

Socket 2
p1 324.6 VDC
g1 316.0 VDC
k1 0.1 mVDC
f1 92.6 VDC
f2 80-8 VDC
p2 322.8 VDC
g2 0.3 mVDC
k2 0 VDC
fc 80.8 VDC

Socket 3
p1 317.3 VDC
g1 0 VDC
k1 0 VDC
f1 93.0 VDC
f2 80.8 VDC
p2 307.7 VDC
g2 0 VDC
k2 0 VDC
fc 80.5 VDC

Socket 4
p1 325.0 VDC
g1 0 VDC
k1 0 VDC
f1 92.5 VDC
f2 80.7 VDC
p2 325.0 VDC
g2 307.2 VDC
k2 0 VDC
fc 80.8 VDC

[vibrolax]

I will have to produce the voltage chart from my simulation.  My build matched the simulation pretty well.  I published the ltspice model on my website, if anyone wants to run it themselves.

[tubeswell]

FWIW The SVT CL pre-amp schematics have voltages noted on them

[PRR]

These are just wrong:
Socket 2 ... g1 316.0 VDC
Socket 4 ... g2 307.2 VDC

> SVT CL pre-amp schematics have voltages noted on them

Plate only (unless I'm blind); I don't like the numbers he found on other pins.

[sluckey]

Socket 2 ... g1 316.0 VDC
Socket 4 ... g2 307.2 VDC

Socket 2-g1 is direct coupled to the previous stage plate. It's a cathode follower. Same deal for socket 4-g2.

Here's the schematic (actually layout)...
 
     http://www.frontiernet.net/~jff/SonOfSVPCL/Images/Visio-AmpegSVTPreamp_MainBoard_07Jul2014.pdf

[vibrolax]

I ran the LTSpice sim with B+ = 277 VDC, as reported by Vince, and have attached the voltage chart.
I have a vague memory during the build that there was one measured cathode voltage that didn't match the sim,
but I couldn't find anything operationally wrong.

Jon