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Thread: (Slow) Build log for Vox JMI-era AC15 OA-031 circuit

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    (Slow) Build log for Vox JMI-era AC15 OA-031 circuit

    You guys can ignore this thread for a while. I'll post progress on my Vox JMI-era AC15 OA-031 circuit build incrementally as work progresses, so it will be a long while before there is anything interesting to see in here. The end goal is a mostly-period-correct repro of a 1963-64 fawn 1x12 combo with copper faceplate, 12" Celestial Alnico Blue speaker, ClassicTone transformers and choke, and NCM licensed replacement cabinet. Current total cost of all parts is just under $1500. The most expensive single item is the ~$500 combo cabinet.

    To start, here are photos of tagstrip boards made in England from paxolin. They are made and sold on eBay by Steve in the UK (expensive, as this is the only source out there) and started out as 42-lug (for AC30s). I cut them down to 36-lug with a Japanese-style fine-tooth pull saw (better fine motor control with a pull saw) and started populating them with Dale 1/2W mil spec resistors and CDE WMF-series 400V foil-and-polyester-film capacitors as well as CDE 500V silver mica capacitors and a handful of Sprague Atom electrolytic capacitors.

    A couple of capacitor values were not available for sale in the CDE WMF-series, so I had to use CDE's Mallory 150 series instead or step up to 630V WMF-series (white colored instead of yellow). I don't think these substitutions are in the signal path, but I didn't check closely.

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    The lower tagstrip board (further away from the controls) is raised by 3/8" standoffs (installed). The upper board will eventually get 3/16" standoffs when they arrive. I'll put the remaining components on that board after installing the standoffs.

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    Last edited by dchang0; 11-10-2015 at 09:16 AM.

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    The NCM 1x12 repro combo cabinet includes a slider board that hasn't been drilled.

    I had to assemble the empty chassis halves with faceplate and fit it into the cabinet to determine where the holes will be drilled.

    It ended up being off-center by about 8mm to the left (when facing the back of the amp) in order to get the faceplate to be centered in the cutout at the top.

    Photo shows the lower chassis half just before drilling (you can't see the pencil lines on the slider board).

    The two holes in the rear cover that attach to the top chassis half need to be moved to the left too (not shown).

    FYI, the chassis halves and faceplate are made and sold by the same Steve from the UK on ebay that made the paxolin tagstrip boards. When shipping, ask him to pack it extra carefully. The one I ordered got damaged in shipping (one corner of the soft aluminum chassis half bent inwards). I'll have to figure out how to flatten/re-bend the dent out.

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    Next photo is of the Classictone transformers and choke installed in the lower chassis half, along with rubber grommets.

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    I originally planned to install the transformers flush against the chassis, but due to the copper shield curving a bit too far away from the power transformer's windings, I would either have to widen the rectangular hole in the chassis or raise the transformer off of the chassis to get it to fit. I decided to raise it a tiny bit using the plastic bushings that came with the bell cover.

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    The copper shield on the output transformer fit perfectly flush, but I raised it too to match the power transformer.

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    Last edited by dchang0; 11-10-2015 at 11:48 PM.

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    Quote Originally Posted by dchang0 View Post
    The NCM 1x12 repro combo cabinet includes a slider board that hasn't been drilled.

    I had to assemble the empty chassis halves with faceplate and fit it into the cabinet to determine where the holes will be drilled.

    It ended up being off-center by about 8mm to the left (when facing the back of the amp) in order to get the faceplate to be centered in the cutout at the top.
    ...
    The copper shield on the output transformer fit flush, but I raised it too to match the power transformer.
    I built a head cab for my 1964 JMI AC30, and that 'off centre' thing surprised me, nearly caused a problem.


    I've raised its Woden PT up from the chassis slightly with some washers, as the windings were pressing down on the slider board. The tape over the windings had worn through, and I was worried that the winding insulation would be next. It looks like the heaters are the outermost windings, and the rectifier heater will be at full VHT. Unlikely to cause an actual problem in use, but may be an unexpected hazard when on the bench.

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    Thanks! So it seems I'm not the only one who has had to deal with these fitment issues... Good to know! Luckily, the ClassicTone transformer windings are not so "fat" as to strike the slider board as in your case.

    Next up: slowly installing the fixtures like capacitor clamps, terminal lug strips, barrier strips, and fuse block onto the chassis halves. Sadly, I didn't order enough 4-40 Keps nuts, so I can't finish the install of all the tube sockets nor the 3 standoffs on the upper tagstrip board. That'll hold up the build for a week while I wait for parts to arrive.

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    The capacitor clamp on the steel chassis didn't align with the holes. Turns out the chassis is out of spec--one of the mounting screw holes is very far from the center of the largest hole. You can see from the photo that it "pulls" the capacitor off-center. I had to use a Dremel with tungsten carbide carving bit to egg out the hole just to get the 6-32 screws through.

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    I finished installing the hardware components to the steel chassis (anything bolted down directly to the chassis). No soldering yet.

    FYI, the 16uF+16uF 450V capacitor can is an F&T, and the 32uF 450V electrolytic is a CDE. Both were chosen mainly because they match the correct clamp sizes. Finding these physically-large caps is difficult.

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    Last edited by dchang0; 11-11-2015 at 08:35 AM.

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    I ran into a problem with the fuse block placement:

    I tried to use an existing #6 hole in the aluminum chassis for the fuse block--only had to drill a small hole for the alignment pin (pictured).

    Turns out this was a mistake. The existing #6 hole caused the fuse block's mounting screw/nut to intersect with the power transformer. The proper way to handle this is to drill a new #6 hole out towards the right edge of the chassis (when the power transformer is to the right side and the output transformer is to the left) along with a matching alignment pin hole.

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    (Photo is rotated--right edge of chassis is at bottom of photo.)

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    Next up is drilling the holes to mount the Schurter rotary voltage selector switch (p/n 0033.1109) under the hole where the vintage selector is meant to go. The two holes are for 4-40 x 1/2" flat head screws. The copper faceplate will cover the screws. A countersink bit will be necessary in order to make the flat head screw fit flush under the faceplate. You can't "poor-man's countersink" this with a larger drill bit.

    Schurter is pretty much the only maker of high-end voltage selector switches. You can get them from European suppliers (ebay) for about $20 each. It is possible to select a six-position voltage selector that has the voltage markings that fairly closely matches your power transformer--I got four out of six to match here. (The ClassicTone power transformer doesn't have 115V but does have 230V--the other four markings match. I will assign 230V to the blank 6th position.) Most of this effort is actually useless--how many people actually fly around the world enough with their amp to need all these different voltages? But the fake non-functional reproductions of vintage JMI voltage selectors are so expensive and rare, and the working original vintage JMI voltage selectors are unsafe and illegal and even more rare, and the huge hole needs to be filled somehow, and the power transformer has all these extra voltage taps, so it is worth installing the Schurter.

    Note also that the Schurter switch is meant for through-hole chassis mounting through a 30mm hole, but the aluminum chassis' hole is not that large, and the switch is mounted under instead of through the chassis instead.

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    Last edited by dchang0; 11-11-2015 at 08:27 PM.

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    The 4-40 fasteners I ordered arrived, so I finished mounting the tube sockets and populating the upper tagstrip board. This board sits very low using 3/16" standoffs, so I had to replace the 3/8" 4-40 screws on the V1 socket (turned 90 degrees relative to the other sockets) with much shorter ones (see picture--the upper screw has been replaced with a 3/16" 4-40 pan head screw while the lower one has yet to be replaced with a 1/4" 4-40 pan head screw). Otherwise the tagstrips would short to ground.

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    I'm temporarily installing the front panel controls to see if there will be any fitment issues. The biggest problem is that all but one of the Bourns pots came with a locking washer or second hex nut, so they all stick up far too much out of the panel. I will have to order 6 locking washers and 5 hex nuts before soldering the bus wire to the backs of the pots. Ugh.

    If I had to do this all over again, I'd choose smaller pots too, so there's more room inside the chassis.

    The second problem is that the square cage nut at the left side of the aluminum chassis is very close to the nearest guitar input jack--if the machine screw for the cabinet is very long, it WILL intersect with the jack and may short out some of the solder tabs. I will have to use heat shrink tubing on them to protect them and/or select exactly the right length of machine screw.

    Note: the pilot lamp is actually an expensive Dialight unit that includes a through-panel socket and a removable incandescent light cartridge. What you see is the empty socket.

    Also, I plan on using pots for the 3-way switches originally meant for the Bright, Tremolo Speed and Tremolo Depth (with push-pull on/off). Obviously not period-correct, but way more useful. Most people went with the 6-way Brilliance switch, but I will run a bass-cut pot instead (from pages 233-4 of Merlin Blencowe's "Designing Tube Preamps for Guitar and Bass" book, figure 10.7b). I did order the 6-way Brilliance switch in case it turns out to sound bad when combined with the stock treble-cut control. The use of pots for the Tremolo controls are validated by Ted Weber's 6V30 and 6V15 mods--they are known to sound good and work well.

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    Last edited by dchang0; 11-18-2015 at 08:11 PM.

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    I suggest that you consider adding a B+ fuse.
    A F200mA in the B+ winding CT would offer a good degree of protection, especially combined with series diodes on the rectifier plates.
    If you've not got one, make a light bulb limiter asap.

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    Thanks--by F200mA, do you mean "Fast-Blo" fuses at 200mA rating?

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    Yes, F is fast / quick blow, T is time delay / slo-blo

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    Interesting--I've never seen that notation before. Thanks for the tips! I've got the spare fuse block but no diodes. Will have to order those--might as well order them at the same time as the missing hex nuts and lock washers for the pots.

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    See http://www.fmicassets.com/Damroot/Or...schematics.pdf
    That shows the F and T fuse thing in use, and also an implementation of the protection diodes, in series with the rectifier plates.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by pdf64 View Post
    See http://www.fmicassets.com/Damroot/Or...schematics.pdf
    That shows the F and T fuse thing in use, and also an implementation of the protection diodes, in series with the rectifier plates.
    It is interesting that the linked schematic shows that the heater AC supplies are fused but the B+ is not fused.

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    Yes, I've posted before that I think it's strange Fender don't have a more consistent approach to the issue of tube amp circuit protection.
    I'm aware of one Fender, the EC Vibro Champ, that has line, heater and B+ fusing, as well as the tube rectifier protection diodes.
    The TRRI may be fitted with fuses that aren't shown on the publicly available documentation.
    The only common, non-statutory protection strategy seems to be the line NTC surge limiting thermistor, which may allow F type fusing of the mains circuit.

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    Nice! Thanks for the link and tips!

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    This may be an easier-to-read drawing of the B+ on PT center tap fuse and diodes-on-rectifier mods:

    https://robrobinette.com/5F6A_Modifi...rotection_Mods

    A question: Merlin's recommendation on B+ PT center tap fusing is to put TWO fuses in, one on each half, rather than one on the center tap that they share like in the last schematic on this page:

    The Valve Wizard

    From what little I have read so far about the difference, there seems to be some situations where the one fuse in the center tap doesn't protect against certain failure situations where two fuses on the legs does. Is this correct? Is it worth it to put two fuses as Merlin suggests? (Probably not, in my case, as finding a place to put the two fuses where they are accessible is probably too challenging. I notice that modern-day JMI Amplification puts one B+ fuse in a new fifth hole drilled between the two chassis halves. Pretty clever location.)

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    Last edited by dchang0; 11-21-2015 at 08:34 PM.

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    The fuse on the CT doesn't protect the B+ winding if the rectifier shorts plate to plate.
    Adding the diodes between the winding and tube rectifier plates should prevent such a failure damaging the PT.

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    If I understand Merlin's first diagram on the aforementioned page (with the unhappy face) correctly, then if BOTH diodes fail AND the rectifier fails, the center-tap-fuse would not protect the PT. Is that correct? (An ultra-rare failure event, to be sure. This hypothetical is posed out of curiousity.)

    If it helps others, here are the two diagrams from Merlin's website:

    Fuse on each half/leg:
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    Fuse on center tap:
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    Last edited by dchang0; 11-21-2015 at 02:03 AM.

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    Had to drill two holes at the edge of the steel chassis for the B+ fuse block, to the right of the PT. This fuse will be visible and accessible from the side if the slider board is pulled out far enough.

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    Quote Originally Posted by dchang0 View Post
    If ... BOTH diodes fail AND the rectifier fails, the center-tap-fuse would not protect the PT. Is that correct? (An ultra-rare failure event, to be sure. This hypothetical is posed out of curiousity.
    I think that if either Si diode and the tube rectifier failed, then the CT fuse wouldn't help; so a slightly higher likelihood but still hopefully low
    So it would be a good idea to use Si diodes with a sufficient voltage rating; for this amp, 1kV should be fine.
    Though if the PT B+ winding voltage may exceed 350-0-350 (consider unloaded at power up, high mains) then a higher voltage rating is required.

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    Ah! Good point--thanks for the precise explanation. I will be ordering UF5408 diodes rated at 1kV and 3A. I heard that the 1400V-rated diodes Fender uses are quite "noisy in audio applications." More important, they are hard to source and cost ~$5 each, ten times as much as the UF5408.

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    As the Si diodes are in series with the tube diodes in this application, we don't need to be concerned about switching noise, ultra fast types etc; the smooth switching characteristics of the tube will control the current flow.
    A single 1N4007 in series with each plate would be fine, make it a series pair on each plate if you really want to have belt, braces and another belt.

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    Cool. I'm not at that level of caution, but two diodes per leg is still ridiculously cheap insurance...

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    Made a little progress on the steel chassis. Added the passive components in roughly the same manner as the vintage layout researched and drawn by Turret over at Amp Garage forum (sort of point-to-point). It was a pain getting the Z-axis arrangement down--I probably will follow the current-day JMI-15 approach next time (using longer terminal strips), although it is pretty clear that current-day JMI has their chassis drilled in much different locations than the vintage holes.

    I didn't go crazy and install 12W wirewound resistors, though. Just used a 3W 22.1K metal film for R33, 5W 100R wirewound for R24 and R25, and 5W 130R wirewound for R22. R21 and R23 are the usual 0.5W 1.5K.

    NOTE: On V4, I had to use a 4-40 0.25" screw on the side of the socket under the criss-crossed resistors to minimize the chance of shorting. The other tube socket screws are all 3/8" long.

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    Not shown: the extra potentiometer flat-nuts arrived, so I was able to get the proper height on all the pots.

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    Last edited by dchang0; 12-03-2015 at 08:04 AM.

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    Old Timer Tom Phillips's Avatar
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    A suggestion:
    You should allow for some physical spacing between the power amp cathode resistor and the cathode bypass cap. The goal is to minimize heating of the cathode bypass electrolytic cap by the heat given off by the resistor. Less heating = longer cap life.

    You are referencing designation numbers for the components in your narrative. It would be helpful if you would post the version of the schematic that uses those exact designation numbers.

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    Thanks, Tom, for the good advice. I pried them apart a bit--maybe 1/8". Each component is now airgapped in all directions.

    The schematic is attached here. It is the original OA-031 that I relettered a few months ago with help from members of this forum.

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    This is the layout that Turret over at Amp Garage drew up. I can't find the original he posted (turretboard.org is gone), so I am attaching it here. It is drawn from an actual vintage JMI-era AC-15 and is true to the schematic except for R38 (the amp he examined uses a 2k2 resistor, while the schematic says to use a 1k5).

    voxac15layoutrev4_112.pdf

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    Last edited by dchang0; 12-03-2015 at 06:03 PM.

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    Question about using tube socket pins for the "insurance" diodes...

    Hi, guys--

    I remember reading a long time ago an exchange between an amp builder and an known-expert builder. The first builder was using unused pins on a tube socket as a convenient terminal to solder other wires/components, and the expert said this was a very bad idea because a guitarist could accidentally plug in the wrong tube into the wrong socket and blow something up. I had a hard time envisioning exactly what was being argued, but the gist of it was that inserting the wrong tube would cause a new circuit path to appear, which would then burn some valuable thing up.

    So, I'm asking for your help in laying out how to do the double-diode or single-diode "insurance policy" on the EZ81 rectifier as described by pdf64.

    I've attached two diagrams with my proposed diode layout--are these dangerous/bad if a wrong tube (most likely an EL84 but also possibly some other 9-pin rectifier) is inserted? I am assuming yes, since I'm unidirectionally-shorting an awful lot of pins out. If they are, what I'll do is fall back to the safe method of not putting the diodes across any pins at all (just go with single diodes soldered inline with the wire with heat shrink to protect the solder joint between the wire and diode).

    Of course, it may be very bad to insert an EL84 in the EZ81 socket even without any diodes in place, in which case the answer becomes, "DON'T INSERT AN EL84 IN THE EZ81 SOCKET!!!" But the diodes probably make things worse.

    Fuse on center tap of PT secondary is not shown.

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    Thanks in advance!

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  29. #29
    Old Timer Leo_Gnardo's Avatar
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    Having "insurance" rectifiers is an excellent idea. My practice is to attach them to a separate tie strip, not the tube socket. I'm more worried about hazard of an arc with components crowded so closely together on a socket. Breakdown voltage of air is 1000V across 1/8 inch or 3mm. Remember that's instantaneous voltage, and under "ideal" conditions, 0% humidity at sea level pressure, & no dust accumulation on your parts. Give your hi voltage parts some elbow room, more cheap insurance, that's my tuppence worth.

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    Advice taken--I'll forgo using the socket pins. I don't particularly feel like drilling another hole in the chassis for attaching a terminal strip, so I'll use the inline approach and single diodes. Thanks very much!

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  31. #31
    Don't forget the joker g1's Avatar
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    Trying to keep all circuitry safe from wrong tube types being installed would be a never ending quest. Then you have to consider, what about correct type tubes with the guide pins broken off?
    I don't think it's a legitimate concern.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Well, I figure I shouldn't make it worse than factory-stock if a person inserts the wrong tube type, if you know what I mean.

    That is to say, if it is "X amount of bad" if a guitarist inserts the wrong tube (most likely an EL84 or a 12AX7, since it is likely the guitarist will at least know that an AC15 has those tubes), then if I add the diodes to the tube socket pins, does it make it "2 times X amount of bad" if the guitarist screwed up OR does it increase the probability of something going bad by Y amount?

    So far, out of the few amps I've been asked to repair, a surprising number (to me at least) have been "broken" by the guitarist either inserting the wrong tube or breaking the guide key/pins off and inserting the tube rotated one or two positions. In one case, it did result in a scary poof as the main fuse would vaporize immediately upon flicking the switch, so some serious damage was probably averted by the fuse doing its job. I replaced the tube with a brand new one, correctly-inserted, and the amp seemed to be undamaged and worked fine after that--lucky, lucky.

    I know I can't prevent buzzed guitarists from impatiently and forcefully shoving tubes into sockets in the dark of a club's stage, and I won't try, but I don't want to make things worse with my wiring choices, if that makes sense...

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  33. #33
    Don't forget the joker g1's Avatar
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    My point is you can't design it to be safe from people putting in tubes with broken guide pins incorrectly. And that is probably more likely than incorrect tube type. So I don't think you need to worry about the other scenario either.
    But yes, if it's just a matter of not using the unused socket pins as tie-points, then that is easy enough to implement.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Ah, I see what you were saying. True, true. Thanks!

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    A little more progress. Heater wires are in for the steel chassis--I had to add yet another 3-lug terminal strip (not shown). On the plus side, the terminal strip will make installing a virtual center tap for the heater wires ridiculously easy.

    The upper leg of the choke has not been soldered to the positive terminal of cap can half C41 yet. I haven't figured out how to get all the wires at that point onto the cap can's terminal.

    Output transformer is not soldered to anything yet, either.

    Waiting on diodes to arrive to finish the rectifier wiring.

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