Electro Harmonix 12AY7 Mic Pre Amp Modifications

As I have noted a fair number of comments on the Internet, on problems and noise with the Electro Harmonix 12AY7 mic pre amp, I thought some experiences with repairing and modifying one may be of interest to this forum.
Interestingly, the original designer admits that the design could have been better.
Tape Op Message Board :: View topic - stupid question regarding EH12AY7 or any other outboard pre

The first encounter with this pre amp was in repairing one for my son. The pre amp struggled to work at all, which was traced to the internal power transformer having shorted turns and not giving full voltage output.
Fitting a new transformer would quickly cure the problem …, how wrong I was.
Having not used the pre amp before it went faulty, I wasn’t sure what the outputs should sound like, but I was sure that there shouldn’t be a loud hum on the XLR (single ended pin 3 and pin 1 connected) or the jack output, even when used as a balanced output.
Note that the jack output is balanced but has three OPAMPS (TL074) effectively in series/cascade between the XLR (valve output) and this jack output. The fourth OPAMP in the TL074 package is used for the input over load circuit.

Tracing the problems, this is was what I found and observed.

Problems/Observations

(a) Very high (75mV peak) ripple (rectified mains hum, ie. 100Hz in UK) on XLR and jack outputs.
Investigation showed that the ripple on the jack output was due to the 12V voltage regulator (L7812) not having sufficient voltage difference between input and output. Only 2V was measured with the original AC-AC supply adator, when at least 3V difference is required to obtain even the minimum Supply Voltage Rejection ratio.
The hum on the XLR outputs was found to be due in part to the ripple on the –12V supply (see above) but mostly due to the poor smoothing of the HT to the valves.

(b) The power-on LED turned off when a microphone requiring phantom power was attached and phantom power switched on.
This was found to be due to the power-on LED being in series with the zener diodes/resistors stabilising the 48V phantom power voltage. This circuit was also used to set the voltage at the grids on the 12AY7 to about +17V.
With phantom power on, this grid voltage dropped also, changing the bias on the 12AY7. In extreme cases (high phantom power currents of greater than about 6mA), this drop was sufficient to produce severe distortion or even stop any output from the pre amp. In effect the 12AY7 had turned off, due to a very high negative bias (grid to cathode) voltage.

(c) The cathode voltage of the 12AU7 output valve (cathode follower configuration) was sitting at about 195V, having set the anode of the 12AY7 at 200V (the on-line EH manual talks about the valves operating at 200V, so I set the anode voltage at this level for a first test). However, since the heaters are supplied from the –12V DC, this puts the cathode to filament (heater) voltage (Vkf) at 207V. This is above the maximum specified for this valve (Vkf = +/- 180V or +/- 200V depending on which data sheet you read).
Under these conditions relatively high grid currents are flowing, since there is a grid bias of +5V and the grid series resistor is only 4.7k ohm (no decoupling capacitors, grid directly connected via this resistor to the 12AY7 anode). Anode current was also high at about 5mA, each triode. This type of circuit is more akin to getting distortion/harmonics in guitar valve amplifiers, although I suspect the valve life may suffer as a result.

(d) With reference to XLR pin 1, about -3V was measured on both XLR pins 2 and 3.
At least this was a component fault (rather than a design “feature”), the two 1uF DC blocking capacitors, feeding the signal to the OPAMP jack output circuit, had high leakage (about 4k ohm and 8k ohm measured with a multimeter).


This is getting to be rather a long post, but hopefully you will be interested in the modifications. Unfortunately, not having an original pre amp to test against, I don’t know how these mods may have altered the original’s sound.

Modifications

(1) Power-on LED reversed on circuit board and track cut to disconnect present feed from phantom power/grid bias circuit, then fed from –12V DC supply via a 2.2k ohm resistor.

(2) Phantom power circuit now completed by use of a red LED to 0V, soldered directly to PCB tracks.

(3) Voltage divider chain of zeners/resistors, used also for phantom power, disconnected (track cut) to grids of 12AY7. Grids of 12AY7 then tied to 0V (Gnd). Phantom power still uses original circuit.

(4) HT voltage smoothing greatly improved by cutting track links between last three smoothing capacitors and connecting via 10k ohm resistors. DC voltage at the end of this capacitor/resistor filter at about 250V.

(5) Tracks cut to anode resistors of 12AY7 and anodes of 12AU7 and links used to connect to last HT smoothing capacitor.

(6) Anode voltages of 12AY7 set at 150V, about 0.5mA anode currents. With a load line of 100k ohm (anode resistor) this sets the operating point at a reasonably linear point. Grid bias voltage at around –3V.

(7) Cathode resistors of 12AU7 changed to 330k ohm (from 33k ohm). Grid bias voltage now at about –5V and cathode voltage around 155V.

(8) Higher voltage AC-AC adaptor (wall-wart) (13.5V) for AC supply. Found one on Ebay, works well with switching for 12V-13.5V-15V-18V AC at 18VA. Higher VA rating (say 24VA) 12V AC may also work. Just needed to fit a 2-pin DIN plug for connecting to the pre amp power socket.

The present design uses a 230V (0-115V-115V) to 9V toroidal transformer, but used with the 9V winding as the primary. To reduce further the loading on the AC-AC adaptor, a 230V to 12V transformer is worth considering, I have already bought one to try. May need to reduce the value of the HT smoothing resistors to compensate for any reduction in voltage.

Testing now shows no ripple (hum) on the XLR and jack outputs, whether taken balanced or unbalanced (using 2mV/div setting on oscilloscope and of course listening through headphones).
Sounds are clean, testing on vocals and acoustic guitar, but as I have already said, I don’t know how different it is from the original.

I am just wondering if the design has changed from the unit I have (it’s a few years old). It’s hard to believe that the design “features” I have uncovered have not been improved in later models.

WARNING
If you decide to open up your pre amp box, then take care, although it is fed from a 12V AC adaptor, the internal step up transformer means there are high dangerous voltages inside.


Hope someone may find this instructive/useful/interesting.


Regards,
Dr F.

Pictures below show the original, faulty, transformer (note the odd wording printed on the PCB) and the modifications. The 12AU7 cathode resistors, changed to 330k ohm, are on the other side of the PCB. The power-on LED is on the other, output, PCB.

[IMG]file:///C:/DOCUME%7E1/RichardF/LOCALS%7E1/Temp/msoclip1/01/clip_image001.gif[/IMG]

Hows things man thanks very much for taking the time to post this. I have been using this pre for around a year now as an upgrade from a presonus tube pre and have found it to be a worthwhile purchase. Definitely more life, body and dimension and worth the extra cash paid. It gets used quite regularly and has not let me down....yet, touch wood!... but one thing that I have found is that it is extremely sensitive to what it is connected to, and if everything is not perfectly balanced there will be a very disturbing hum that renders it nearly unusable. Fortunately I have cracked the code and found what it needs to be happy by experimenting with different connections. I would say this will be unique to each users own particular rig.

Anyway Im upgrading now to a GAP pre 73 in the "studio" and will attempt to use the ehx unit for live vocals as it seems to make my nd 767 mic sound almost condenser like and I think it might make a decent foundation for a modular channel strip on our live rig. God only knows how "upset" this pre will get in a live environment in the hum and reliability departments so I am strongly considering your mods as an ounce of prevention if I am happy with its sonic performance. That is of course if my basic electronics skills are up to the challenge Thanks again and I will post any updates on my experience(S) trying out these mods. All the best, Anto

This is fantastic work. I just bought a noisy one on Ebay for a great price. I will definitely be implementing these mods.

FYI, your first pic didn't seem to attach to your post properly. You may want to fix that. I assume you meant to post three pics.

thanks,
Brad

Hi Brad,
hoped it would be of some use to someone. I only posted two images, the one with the old transformer removed and the one with the mods. However, if you need more information let me know via this post.

Cheers,
Dr F.

Okay sounds good...just wanted to make sure I wasn't missing something.

How accommodating are the mods to tube rolling? How would a 12AX7 with the load lines you have chosen?

thanks,
Brad

Update: I just finished doing all the mods. Well except for reversing the power LED, and I'm still using the stock 12VAC adapter.

Would it be too much trouble to get you to post the voltages you are measuring for each tube? I don't believe these are right, but I'm getting the following:

12AY7:
pin 1 = 44.6 V
pin 2 = 0 V
pin 3 = -0.13 V
pin 4 = 0 V
pin 5 = - 11.8 v
pin 6 = 42.9 V
pin 7 = 0 V
pin 8 = -0.12 V
pin 9 = -5.97 V

12AU7:
pin 1 = 171.6 V
pin 2 = 42.9 V
pin 3 = 51.5 V
pin 4 = 0 V
pin 5 = -11.8 v
pin 6 = 171.1 V
pin 7 = 44.0 V
pin 8 = 52.4 V
pin 9 = -5.99 V

I'm measuring 68 VDC from pin 1 to pin 2 (or 3) on the input XLR when phantom power is switched on. That seems wrong. Dumb question--the red LED...should it light up power is applied? I have the cathode of the LED measuring 23.5 V. The anode is at ground. Is that correct?

What am I missing?

thanks,
Brad

I vaguely remember there was a long thread of modding this at the Tape OP forums back when it was a new product

Matti

Hi Brad,
got the message, I will do the measurements as soon as possible, and reply.

The red LED should light up, the cathode should be at ground, so you have it the wrong way round. Note that the red LED my go out if you have phantom power on and a mic requiring relatively high phantom power current connected.

What voltage are you measuring at the positive output of the HT bridge rectifier (connects to R15, 51 ohm resistor) ?
It should be around 300-340 V DC. Also what is the AC transformer secondary voltage ?

Once the mods are done, the voltage at the anodes (pin 1 or pin 6) of the 12AY7 (set these to 150V) are adjusted by using the TRIM1 pot (there is a hole in the PCB for access to this when the PCB is mounted in the case). The design uses a constant current circuit to set the current through the 12AY7 and is adjusted using TRIM1.

Your measurement of grid bias -0.12V/-0.13V for the 12AY7 is too low, it should be around -3V when the anode is at about 150V. With this grid bias, the higher anode currents will introduce higher voltage drops in the HT filter resistors (at 10K ohm each this will be relatively high), and this may be your problem. Try adjusting TRIM1, as described above.

Regards,
Dr F.

Thanks! That seemed to do the trick. I had the LED reversed as you pointed out. Now my phantom voltage is about 46.3V.

That trim pot was way out of whack. I set it for 150V. All is good now. New voltages are as follows:

12AU7:
pin 1 = 201.9
pin 2 = 150.5
pin 3 = 154.3
pin 4 = 0
pin 5 = -11.84
pin 6 = 203.1
pin 7 = 150.3
pin 8 = 153. 8
pin 9 = -6.00


12AY7:
pin 1 = 150.0
pin 2 = 0
pin 3 = 3.17
pin 4 = 0
pin 5 = -11.84
pin 6 = 151.0
pin 7 = 0
pin 8 = 3.13
pin 9 = -5.97

Measuring the secondaries of the onboard transformer I get 6.56 VAC at the pad "B", and 7.35 VAC at pad "C".

The output of the HT bridge rectifier is 265.7 VDC. That is a bit lower than the 300-340V you are quoting. Thoughts? Can you provide any more details on the adapter you scored on Ebay? I looked briefly but couldn't find one that matched your description.

I haven't had a chance to trace out the circuit like you have. Have you identified which caps are coupling caps? I'd like to upgrade whatever caps are in the signal path.

Thanks for your help,
Brad

I just tried out the preamp with an MXL 604 and an acoustic guitar. It sounds really nice. It's dead quiet, there is absolutely no hum, and the sound is big and detailed. Oh yeah...I swapped out the stock EH tubes for a 6072 (maybe RCA?), and an NOS GE 5-star 5814A. Nice! thumbsup

Thanks Dr. F for a fairly painless mod.

I found the input and output coupling caps. They are the large 47uF/250V electrolytics bypassed with the 0.1uF film caps. I think I'm going to eventually replace those with some Nichicon and Wimas if I can find something that will fit. I'll probably swap out the phantom blocking caps as well. For now, it's a pretty nice sounding preamp. Now I just need a few more.

Brad

Well Brad, looks like you are there. I think you measured the input AC voltage (with respect to Ground ?), rather than the AC into the HT Bridge. Remember when measuring the AC voltages, it is across the transformer windings, not with respect to ground.
I used the original AC adaptor, for the following readings.

Voltages taken with original transformer (9V AC, 12VA), Phantom Power OFF.

12AY7 Voltages (V)
(a2) pin 1 = 150.8
(g2) pin 2 = 0.0
(k2) pin 3 = 3.3
(h) pin 4 = 0.0
(h) pin 5 = -11.91
(a1) pin 6 = 151.0
(g1) pin 7 = 0.0
(k1) pin 8 = 3.2
(hc) pin 9 = -6.06 (pin 9, heater center tap, not used)

12AU7 Voltages (V)
(a2) pin 1 = 221
(g2) pin 2 = 150.8
(k2) pin 3 = 155.1
(h) pin 4 = 0.0
(h) pin 5 = -11.91
(a1) pin 6 = 221
(g1) pin 7 = 151.0
(k1) pin 8 = 156.1
(hc) pin 9 = -5.97 (pin 9, heater center tap, not used)

HT Bridge Voltage = 295V
Transformer Secondary = 239V AC (ie. Input to HT Bridge)

With 13.3V, 18VA AC adaptor tube voltages similar, apart from 12AU7 anodes, since this is fed directly from the last smoothing capacitor.
12AU7 anodes = 249V
HT Bridge Voltage = 340V
Transformer Secondary = 272V AC (ie. Input to HT Bridge)

I can only guess at why the HT Bridge voltage was lower than my reading (see above, 295V), but may be due to the fact that I replaced the transformer and the regulation under load may be better than the original fitted by Electro Harmonix.

I will see about posting a picture of the valve part of the schematic I have deduced. There are capacitors on the input and output (these you have found), the rest is directly coupled, although the balanced input does go through a FET amplifier !! The input capacitors are 1uF in parallel with 0.1uF, on the other PCB, Low Cut out. The 1uF is switched out when the Low Cut is selected.


Below are photos of the AC adaptor (2-pin DIN had to be added) and the new transformer I fitted. If you decide to use a different AC adaptor, then re-adjust the anodes to 150V using TRIM 1.

Glad it all went well in the end,
Dr F.

Hello Brad,
the mods make tube swapping more predictable as to the bias point of the tube in the 12AY7 position.
Since the original design had the potential to change the grid bias when using Phantom Power, the operating point may not have been as expected from the load line on the Plate Characteristics. Even though the anode current is set by a constant current circuit.

With the mods applied and setting the 12AY7 anode at 150V, the current through each triode is around 1mA. To accommodate different tubes in the 12AY7 position, since the current is maintained constant, the change that occurs is in the cathode voltage.
I have checked the voltages and currents in the circuit with a 12AX7 (ECC83) and can confirm that the position on the load line is as predicted from the Plate Characteristic graphs. In this case the cathode voltage was about 1.1V (ie, grid bias at -1.1V), the anode current being maintained at about 1mA.

I have attached the Plate Characteristics for the 12AY7, 12AU7 and 12AX7 with the 100k ohm load line and the 150V anode point indicated. I would say all three tubes should operate over a reasonably linear region, considering the relatively low voltages even from a Phantom Powered microphone. There is some amplification in the FET balanced input part of the circuit, but this is about a gain of 5.
The 6072 tube you are using has similar characteristics to the 12AY7, but with lower noise spec (according to RCA data sheet), so should be a definite improvement.

Regards,
Dr F.

Dr. F, thank you so much for posting this! I have the EH 12AY7, too, and this thread has great info for modding this diamond-in-the-rough mic pre.

Attached is the schematic for the 12AY7 preamp.

That's great, thanks very much lostintime, I am now sure I worked out the circuit correctly.

I shall eventually get round to producing my modified schematic and posting here.

On my 12AY7, LAMP1 is shown on the PCB, but not fitted.
Also, if the 220V DC on the anodes of V2A/B is correct, that's a very large voltage drop across 291 ohm (51+240). I wonder where all the current is supposed to go, either that or the transformer I fitted has very much better regulation, meaning the voltage at the output of the HT bridge (BR2) is a lot less than in my 12AY7 pre amp.

Thanks again,
Dr F.

What a great post but I am limited in my electrical skills. How much should one charge for doing this kind of mod? I know a tech who works on tube amps. I can solder but I am afraid to mess up my 12ay7. I have 2 of these. One is noisy and one isn't. I was going to send the one to EH but this mod sounds like a much better option. The hum is extremely noticable with my dynamic mics. Would just adjusting the trim pot help?

It may do. The trimpot is the current source set but adjusting it in order to lower noise has been known to work. The way to try it out is to mark the trimpot's current position so that you can return it to that point if you need to.

Hi,
are you taking about hum on the XLR output or the 1/4" jack output ?

A higher rated (VA) power supply may help or better still a few more AC volts (see the pictures I posted), if the hum is on the 1/4" jack. If you are using balanced output connections, then as the hum tends to be common on the Live and Return signals, this will cancel at the amp/mixing desk.

Show your tech this post and see what he quotes. It's pretty straightforward if he has done this sort of thing before. The most awkward part I found was in turning the power-on LED round, but if you can live without a power-on LED, just leave it disconnected.

Dr F.

Thank you Dr. F. and Lostintime for your responses. My first response on Gearslutz heh I always use the XLR output. That was an excellent question. Prior, I had read about others using the 1/4 out and being disappointed so I never used it. I can give the trimpot a try and see what happens.

When originally I took the noisy one apart I accidently pulled the shaft out of the pot when just removing the knob (They must have hammered it down LOL!) Use caution during disassembly. I was able to replace it with a 200 ohm (original I think was 100 ohm). I may have been a little more willing to try the mod if that didn't happen at a earlier time LOL. I did swap some tubes at the time and still had hum.

I have had success using the EH 12ay7 with a Triton Fethead to boost my Shure SM7 but still desire getting the noisy one quieter, overall. I wonder if one of mine is an older version like Dr. F mentioned as a possibility with these preamps?

Hi TMG,
I hope you mean 200k ohm ? Both of the pots are 100k ohm. The input gain is a Log taper and the 1/4" output LVL a Linear taper.

Have you tried both power supplies on the noisy one ? Still the same problem ?

The 1/4" jack is a balanced output, if you use TRS to TRS (balanced) cable and appropriate input, but the signal does go "through" three OP AMPs .
The date on the schematic that lostintime posted is 25 Jun 2009 and it looks pretty much the same as my pre amp, so my guess is the design hasn't changed from my version.
I think one of the mod's biggest improvements on the original design, is the HT voltage smoothing, which is key to low noise in valve amps.
I have just finished repairing a Soundelux E47 microphone power supply (P99) and the HT smoothing had the following chain of components after the bridge rectifier:-
C-L/C-R/C-R/C-R/C-Zener(120V)-C-R/C, needless to say there wasn't much ripple on that supply !

If you successfully changed the pot, then the mods are not that much more difficult, but good soldering technique is required.

Cheers,
Dr F.

What exactly are the components we need? Do you have the time to make a small list of what ohm and what watt the resistors are that you used? A simple 1 2 3 4 list would be great. Regards.

Hello again,
As requested component list for the mod and more details.

Components
10k ohm, 1W, ......3 off (added components)
2.2k ohm, 1/2W, ..1 off (added component)
330k ohm, 1W, ....2 off (changed components, R12, R13)
LED(red), ............1 off (added component, 3mm convenient size)
Wire (links, 12AY7 grids to ground and filtered power)

Suggested Mod Sequence

(1) Remove the circuit boards from the casing, remove tubes (valves) and disconnect
the PCB connecting multi-cable.
Note: - The pot knobs should pull straight off, but may be a little tight.
- All nuts and screws from the various pots/switches/sockets will
need to be removed, including the power socket.

(2) Remove R12 and R13 (33k ohm, orange-orange-orange) and replace
with 330k ohm resistors.

(3) Cut the tracks (7 places), as shown in the picture.

(4) Solder the wire link from the grids of the 12AY7 to ground (see picture, black wire).
Solder the filtered HT wire links (see picture, red wires).

(5) Solder in the 10k ohm resistors (brown-black-orange), use sleeving on the leads to avoid any shorts (see picture).

(6) Solder in the 2.2k ohm resistor (red-red-red) and LED, using sleeving where appropriate (see picture).
Note: the LED cathode connects to ground.

(7) On the small PCB, remove the power-on LED (D3) and replace it in the reverse orientation.
This is a little awkward, but can be removed without having to first remove the XLR connector.
It maybe a good idea to have a spare LED (5mm) available in case of damage
- any colour will do it doesn’t need to be blue.

(8) Re-assemble everything back into the case and refit the PCB connecting cable and tubes, but leave the base off.

(9) Apply power.Take care, there are high (>300V DC) voltages on the PCB.
With access through the hole in the PCB, adjust TRIM1 to give 150V on the 12AY7 anodes (pins 1 and 6).
There is likely to be a slight difference between the two anode voltages, this is to be expected.
The 12AY7 cathode voltages should be at about 3.2V (pins 3 and 8).
Without any phantom power microphone connected, the LED soldered on to the PCB should light up.
Turn on phantom power and check the voltage at the XLR input (pins 2,3) is at 48V +/-2V, referenced to ground (pin 1).

(10) Leave the power connected and re-check the 12AY7 anode voltages after 30 minutes,
re-adjusting TRIM1 as necessary, for 150V on the anodes.

(11) Remove power and attach the base. Power up and check operation.
The chances are that even using the original power supply, the hum will have been
greatly reduced, since the demands on the power supply with these modifications is significantly less than in the original design.

I think that’s covered everything, apart from advising to try and obtain a higher rated (VA) power supply (Wall Wart, see previous posts).

If you are successful in making these mods, then it would be interesting to hear from you on the outcome (hum reduced/not reduced, etc), via this post.

Cheers,
Dr F.
Pictures:- Modified schematic and modified PCB.

Thank you SO MUcH for doing all this. I am really thankfull! I will order the parts and do the mod asap. Regards.

What are the cuts that don't overlap with other "lines" for? Regards

Hi,
could you be more specific I don't understand what you mean by "other lines".

Dr F.

Oh, it's for breaking the connection. When looking at the scratch up around the tubes I thought they were made to bridge traces. Regards.

Dr. F. or another member.. could you point me in the correct way to measure the pins. I read that Dr. F mentioned a HT Bridge? I am assuming DC on the tester? I apologize for my extremely limited knowledge of electronics. The additional information posted by Dr. F. is greatly appreciated. Anyone else try this mod out? My goal is to compare my clean sounding EH 12ay7 with the hopefully, newly modified no longer.... noisy one. heh I will report my findings. Have a great rest of the weekend everyone.

Hi,
that's right, after the bridge rectifiers you are measuring DC voltages.

There are two bridge rectifiers, one for the HT (High Tension) voltage (BR2) and one that feeds the -12V DC supply (BR1). The AC input to BR1 is connected directly to the input AC power (from the "wall-wart").
The voltage readings will probably be closer to the (2nd set) posted by Brad McGowan, if you are using the original wall-wart.
The voltage at the Filtered HT Supply point, should be around 200V to 250V, depending on the wall-wart used.

Dr F.

A different angle to show tracks to be cut.

Hi,

fantastic timing for this thread, i was going to replace my eh12ay7 because it was too noisy (ebay bargain not !)

Instead, it's time to sniff some solder.

stupid question 1 - the 12ay7 goes in the left or right when looking at the unit 'normally' from the front ? (the tubes had fallen out in the mail)

stupid question 2 - I assume I can not bother with the power on led mod if i don't mind the power-on led fading out ?

--- since I'll be poking about the the thing I wondered if I could make it more functional ---

silly mod question 1 - I guess if I want to input a passive guitar signal- I'll need to get a di box - this circuit is probably not easily switchable for a high input impedance ?

silly mod question 2 - could I jam in a transformer for providing a high-Z input somewhere in the box ?? basically, I just add the key component of a passive DI box and another jack socket ???

Thanks All !

Gareth.


PS - just looked for DI transformers - something like the Jensen JT-DB-E looks ideal - it's all in a shielded can ~1" by 1" - I hope this might fit somewhere ?
JENSEN TRANSFORMERS, INC. - DIRECT BOX TRANSFORMERS

Well fatgaz, that's a fair number of questions:-) I will answer what I can.
(1) The 12AY7 goes on the right hand side. (see picture).

(2) The power-on LED fading out is an indication that the 12AY7 grid bias could be changing (the original design has it in series with the grid bias setting zeners/resistors). Therefore you should implement this part of the mod.
You may decide not to swap round the power-on LED, in which case you don't need to fit the 2.2k ohm resistor, but you must still cut all of the tracks indicated on the PCB.

(3) For guitar input a DI box is the easiest option. Just ensure that it has an input impedance appropriate for a guitar (ie High Z, ideally 500k ohm or greater).

(4) There is probably enough room inside the box to implement your own High Z DI circuit and to fit a jack socket. You will also probably need a DPST or DPDT switch to disconnect the jack input circuit, when using the XLR mic input.
This could be the transformer that you indicated (never used this so can't comment on it's suitability), or you could implement a solid state single to balanced input, using three OPAMPS, taking the supply from the -12V on the PCB.

(5) There maybe a simpler way. After the Input Gain control, the signal is fed into the gate of the two JFETs. Replace the 220k with ,say, 1M ohm, to get the input impedance up, ground one input and feed the (guitar) signal into the other. Use capacitive coupling and completely disconnect the XLR input signal chain. Probably need a DPDT switch and there would be no input volume control.
Just speculating here, haven't tried it, but seems feasible if the wiring/switch is done appropriately.

Hope you get the "ebay bargain" working soon.

I have been trying to get my hands on a duff unit, to try some further ideas, but nothing available as yet. I have been looking at gain control (using feedback, not just a volume control), which I think would be beneficial when using condenser mics, which tend to have high output levels, compared to dynamic mics.

Dr F.