Tube Amp Forum: The Ultimate Tone - London Power Kits & Mods

Z-Pre Quesitons

Mon, 30 Jun 2025 17:57:04 +0000

Hi I'm interested in knowing a bit more about the Z Pre-amp design. From the description it says it's Soldano inspired? I'm assuming that it does away with the cathode follower to lower the tube count and make the EQ more effective?

I'm looking to be build something with the gain level of SLO-100 but smoother and fuller and sweeter sounding---a little less fizzy than the SLOs I've heard. Will the Z-Pre amp work for that sort of thing? Are there any suggestions about places in the circuit to tweak? I'm interested to hear people's experience with it. Thanks everyone!

Kit Mounting

Thu, 05 Jun 2025 15:05:22 +0000

Hi Guys

Some of the kits will be mounted in a specific way, where others have a very usual way, and others can be mounted various ways.

The first group includes:
Power Scaling kits
QS kits
VCK

The second group includes:
PSU kits but not RBX
MICRO
PA50H1

The third group includes:
preamp kits
PA66
PX66
PAF
TREM which can be zip-tied to a wire harness

Card guides are typically used in computers where there are standardised board slots where the accessory cards are of a prescribed shape with edge connection "fingers" that fit into a connectors on the backplane card. usually the metal front of the card is mechanically secured to the computer case once in position.

In audio, such a scheme is never used.

The trickiest is mounting the preamp PCBs, PAF and the PA66 variants, which can be mounted parallel to the chassis or perpendicular to it. Parallel mounting uses spacers, bolts and lock nuts, or a spacer and two bolts per hole position. For the internal and external mounting of the PCB as shown in the info sheet supplied with the kits, these hardware approaches are used.

If you are mounting the preamp board in a 1U chassis, then an internal mounting requires the use of angle brackets. If you mount the board on the rear panel, then it can follow the internal/external info sheet.

As far as typical hardware goes, you can get standard 4-40 threaded spacers with 1/4" hex or round shape - hex is easier for tightening, and use 4-40 machine screws. The spacers are standard Keystone parts that you can obtain from Digikey or Mouser. If you want the tube to stick out maximally, then a 0.375" (2202) to 0.5" (2203) spacer will suffice; if you want a modest stick-out, then a 1" (2205) spacer is good leaving enough of the tube out to be easily removed while providing good protection.

Have fun

MICRO Cigar Box amplifier

Thu, 22 Aug 2024 14:43:12 +0000

Hi Guys

Our little cigar box kit amplifier called MICRO can be tricky to assemble. PLUS it has more gain than most players expect.

For the assembly, MICRO uses all through-hole components which are easier to handle than surface-mount components as they are more of a Human-scale size. This also means that component values can be changed in certain circuit areas to affect a custom sound. The circuit itself is pretty standard stuff from the 1970s - nothing clever or surprising - but it is all discreet with no ICs.

To make the unit small overall, we use cordwood construction, which is a common method used in compact electronic equipment - think "8-transistor radio" - haha. With this method, a resistor lead is folded back parallel to the body of the resistor and both leads go through holes in the PCB on a fairly tight spacing. It is important to install one component at a time to assure that the parts are straight and do not foul on each other. Soldering has to be done carefully.

The caps and other parts are selected to have a small footprint on the PCB and the final assembly can be installed in quite a small box.

There is a single jfet gain stage with variable gain (Edge) that can be frequency-focused if the player wishes, just by changing one capacitor value. This is followed by a standard volume control (Retro) allowing output to be dialled to zero. It also allows the input stage to be overdriven on its own if one desires. The power amp has variable gain (Blast) and may have too much gain for many players. So, you have to approach using MICRO in a specific way.

Remember that your electric guitar has a volume control. Set it low to begin with.
Set all of MICRO's controls to zero - fully CCW.
Power up and turn Retro up part way to hear the guitar.
Turn up the guitar volume as desired.
Adjust Edge to hear what it does, then turn it down.
Adjust Blast upward to hear what it does.

If your guitar does not have a level control, you will rely on Retro to control loudness and maybe rely on Edge for basic sensitivity.

Blast has a lot of gain on tap and this may need some taming.

If you flip the board over with controls facing you, the left-hand trace going to Blast can be slashed and a resistor tacked across the gap. Solder the leads to the solder pad for Blast at the front edge of the PCB and to the solder pad for R1, where the slashed trace used to go to. The new resistor works against R44=10k, the PA feedback resistor, to set the maximum gain when Blast is fully CW. The maximum gain is about 600 stock and we can dial this down to <2 with Blast.

Different resistor values for the new resistor yield different maximum gains. You may find you need something in the low range:
10k > 2
4k7 > 3
1k > 11
470 > 22
348 > 30
100 > 100

You may find that you do not need the gain of the input stage, in which case you can change R17=10k to 3k3. This provides unity gain (gain = 1) and Edge can still be used to bump it up. If you want to make Edge more of a treble boost, reduce C8=220n to a lower value.

The jfet input stage has a high input impedance which allows full tonal capture from a guitar with a raw piezo pickup.

MICRO can produce a couple of watts of output depending on the power supply used and whether the output transistors are heat sinked. Through a full-size cabinet this can sound massive. Being solid-state the clean sound can be quite clean and the distortion can be quite metallic if you desire.

Have fun

RBX Question

Tue, 18 Jul 2023 16:44:06 +0000

I'm planning on adding seperate power tube bias for a fixed bias amp. I have the BMK already. Would it be recommend I get a seperate bias supply (BMX) for a jcm800 50W type amp? In the notes if I read it correctly the power is supplied from Va which is doable. But I'd guess it is much better to have a seperate supply?

M-Pre partial success, some troubleshooting to go

Wed, 14 Jun 2023 02:26:42 +0000

Hi, I've recently built up a PSU-PRE and an M-Pre. I have partial success, but I'm having two issues and I've run out of things to check/know to troubleshoot.

1) In each channel, I get a good quality sound (clean in one, distortion in the other). However, the EQ doesn't work in either channel. The knobs make no difference as if the tone stack is completely defeated.

2) Also in each channel if I turn the volume up above 1 o'clock then it goes from normal-sounding to suddenly heavily clipped, fuzzy, lower volume. Playing harder makes it worse and tends to make it cut out entirely.

Of note:
This is a fully 'stock' 800 build, 12V heaters.
I've carefully triple-checked all my wiring.
I gently scratched between all joints, and then used alcohol and a plastic brush to clear any possible solder bridges.
I can power it up without tubes, through a PLSS with a 60W lamp and read 371 at the plate input, -12 at the heater input.
Changing tubes does not affect anything.

Does anyone with more experience have any suggestions about what else to check?

BMK2 used in cathode-biased amp

Fri, 26 May 2023 02:37:24 +0000

Hi Guys

A member of the forum brought up a question about adjusting idle in a cathode-biased amp and bought the BMK2 Bias Mod Kit for 2-tubes from us. The BMKs are intended for use with fixed-biased amps and the notes reflect that application. However, you can use BMK2 to hum-balance a cathode-biased amp. Each pot supplied will control one side of the push-pull circuit.

Where is the bias voltage coming from? It is the voltage across Rk. Each tube sees this voltage and adjusts its own current in a bit of a tug and war with the opposite tube(s). The two sides are unlikely to have equal currents, let alone end up at the more important hum-balance point for the OT. Being able to tap this voltage with the pots allows the effective bias voltage to one side to be REDUCED. This will make that side run hotter.

Since cathode-biased amps are notoriously poorly designed, lacking grid-stops and screen-stops, sharing one bias resistor for the entire output stage, and aiming to run each tube at its full dissipation, having a bias adjustment that makes things even hotter is not really a good idea, but it is what we can do. The fix is to increase the value of Rk to a safer operating point that gives leeway for making one side (or both) a little warmer. To do this, we can either add another resistor in series with the stock Rk, or change Rk outright with a higher value. The new net value should be about 20% higher or more. So, a 250R would change to 300R, maybe go to 330R to use a standard value. The 100R in an AC30 would change to 120R minimum. Higher is better as we can make things warmer at will using BMK2.

To limit the adjustment range of the pots, we use the 6k81s in series with the X end of the pot, just as in fixed-bias.

The drawing shows how BMK2 is incorporated into a 2-tube cathode-biased amp. The RED-1, RED-2 and BLK are the meter jacks, along with the 1R current-sense resistors. We can measure 1mV per 1mA through the tube and know how much heat it is dissipating.

If the amp is a type with independent cathode-bias resistors and bypass caps, the BMK2 can still be used with half the kit applied to each tube. You would need a second BLK meter jack at the top of the second Rk,Ck.

BMK2-in-cathode-biased-amp-1.pdf (Size: 14.63 KB / Downloads: 11)

Bleeder Resistor question

Wed, 03 May 2023 01:21:11 +0000

Hi all.

I've always put in a bleeder resistor across the main power filter cap but not knowing how hot it gets. In a jcm800 style build I had a 27k 5W resistor across the main cap. Then I tested it with a 110K 7W resistor. It gets really hot to the touch. Not glowing hot but hot enough you wouldn't want your finger on it too long. Any good way to discharge caps? It's for safety but I'm worried the resistor might overheat and cause more danger.

I got these new higher resistor values based on the formula to calculate the resistance value as a function of the time and voltage.

I've heard of an active bleeder circuit which shuts off through a relay when the amp is on but connects back when you turn the amp off. Basically senses the mains voltage when on and disconnects from the cap. But this seems like too much of a project with the limited space I have in this amp build.

Any ideas?

London Power TBS Tracking Bias Supply

Thu, 08 Jul 2021 16:18:52 +0000

Hi Guys

TBS Tracking Bias Supply is a slight misnomer inasmuch as it is not the supply itself; rather, TBS is a regulator for the bias supply that tracks the screen supply (Vs) or plate supply (Va,) as appropriate.

TBS requires a high-voltage input equal to about 16-20% of absolute Vs. For example, if there is 500Vs, then -Vb should be 0.16 x 500V = 80V, up to 0.2 x 500V = 100V.

The raw bias voltage should be generated to present a low- to medium-ompedance voltage source. We do this by using a separate winding with full bridge rectification. Filtering should be high-value capacitors with low-value resistors feeding them. A typical supply would use 100R feeding 47u to 100u, feeding a second identical RC section.

Remember to tie a 100k across the bridge output to protect it, and secondarily to drain the raw bias filter caps at power turn-off.

If the winding voltage is too low to generate the required voltage, a voltage doubler can be used. This simply places two cas across two of the diodes in the bridge. These extra caps perform the doubling function and the extra RCs are still needed.

Any kind of capacitively-coupled raw voltage source should be avoided. Same goes for deriving the raw bias supply from the end of a centre-tapped plate winding.

Once TBS is assembled and installed TEST WITHOUT TUBES.

Make sure that negative voltage of an appropriate range is present at the control grid pin of the power tube sockets.

TBS has a trimpot that can be used to set the maximum nagative output suitable to control the tubes to be used. This adjustment should turn the tubes off and is a set-and-forget adjustment. The trimpot is a convenience only. Ideally, R5 is increased to the next standard value as needed and the three holes for the trimpot are jumpered.

A panel-style pot is provided to make day-to-day bias adjustments, or when you change tubes. This pot should be mounted on the tube plane of the chassis near the tube it controls. This may be tricky to install but once in place is quite convenient. The bias pot is then as protected and as accessible as the tubes are, as TUT3 The Ultimate Tone vol.3 illustrates.

Using chassis-mounted sockets with the kits

Mon, 12 Apr 2021 19:25:41 +0000

Hi Guys

Hobbyists often ask if they can use a chassis-mounted tube socket with the kits, to which the answer is 'yes', but with caveats.

The PCB layout incorporates the tube sockets and this controls the physical form of the connections to the sockets. All the parasitic capacitances and inductances are then controlled and stability is assured for the assembly. There is a further benefit that unit-to-unit consistency is assured even though most hobbyists will only build one of a thing.

If you really must use chassis-mounted sockets but want to use London Power PCB project as the basis, then the grid-stop resistors should be mounted right at the socket pins and a wire link installed in those positions on the PCB. The wiring to each triode or pentode should just be #22 stranded wire, properly twisted. For example, each triode in a dual-triode needs three wires. You can solder the wires to the PCB end, say, then measure out more wire than the distance to the socket side to be wired; then cut the wires; then twist them starting at the anchored end and allow ALL 3 free ends to rotate as you twist; only then do you begin terminating at the socket. Of course, you can reverse this order and begin at the socket remembering to install the grid-stop between the grid wire and the socket pin. Each triode then has its twisted-triple.

Heater wires should be twisted unto themselves. London Power PCBs that carry dual-triodes accommodate both 6V and 12V wiring. 12V wiring is preferred for 12V-capable tubes, as TUT4 explains, and in that case you do not have to bring out the pin-9 wire. The PCB keeps the heater wiring away from the triodes but now you are changing the effective layout and need to do so with care.

DO NOT use multi-conductor shielded cable to the tube socket. The cable capacitance will kill the frequency response and you'll be replacing it with twisted hook-up wire, so save a step

The same guidance applies to octal tubes, pentodes and tetrodes. With the latter, especially with transformer connections, the transformer connections can still go to the PCB unless it is neater to have them go directly to the tube socket. Remember that the screen-stop should be tied to the screen-pin at the socket. The PCB achieves this, but now you have to place the resistor on the socket in the traditional manner. Where 300V wire is okay for everything else, the plate and possibly the screen wires for a power pentode - tetrode should be rated higher, at least 1kV.

Have fun

S-PRE-2

Thu, 08 Apr 2021 13:49:51 +0000

A few questions about the S-PRE-2 kit. Hopefully Kevin can help here.

I'm stuffing the board, and I have ended up with two capacitors that I'm not 100% on.

caps2.jpg (Size: 52.83 KB / Downloads: 40)

Markings are (as far as I can tell) 471J on the blue green one, and 0.47 J100 on the red one. The board has spots for a 470p (C7) and a 470n (C9). My assumption is that the 0.47 on the red one is 0.47 microfarads, which means 470Nanofarads, and it is C9, which leaves the blue green one for C7.

Also - I assume there's nothing keeping me from doing the tube sockets off board? Is 20AWG stranded good enough for that?

London Power's Power Supply for Preamps (PSU-PRE)

Tue, 14 Apr 2020 13:30:33 +0000

Have you tried the PSU-PRE Power Supply Kit for Preamps? Post your comments or questions here!

The PSU-PRE power supply kit for preamps is low-profile and will fit inside a 1U rack chassis. It uses a pair of DC, well-filtered, standard Hammond 229-series transformers to create the heater and plate supplies. The plate supply has two nodes that can be individually tapped with paired wiring. The heater supply can be wired for either 6V or 12V; 12Vr is preferred for use with London Power preamp kits and for using the ERK Electronic Relay Kit for channel switching. The heater supply also supports the switching circuits.

The regulated heater output can support up to two/four 300mA heaters at 6Vac or up to two/four 150mA heaters at 12Vac. There are three Links on-board that set the heater transformer wiring and regulator output to "6V" or "12V". The regulator keeps the heater voltage hum free.

There are two basic versions:
PSU-PRE-12 can support up to two 12A_7 tubes - enough for a 2-channel preamp
PSU-PRE-24 can support up to four 12A_7 tubes - enough for a 3-channel preamp

Read more about the PSU-PRE at https://londonpower.com/electronics/tube-preamp-power-supply/ .

If you're interested in London Power's tube preamp kits, see this page: https://londonpower.com/preamp-kit-selection/ .

London Power Electronic Relay Kit (ERK)

Tue, 14 Apr 2020 12:53:54 +0000

Have you tried this relay kit? Post your questions and comments here!

The Electronic Relay Kit (ERK) features:

A, B exclusive selection
ground-referenced jfet shunt switch elements
silent operation
can be used with push-on push-off remote switch (not included)
easy wiring for panel control (switch not included – order our PSW)
can control panel LEDs (LEDs not included)
6-poles included; infinitely expandable
use for channel muting or selection
use for gain selection via cathode-bypass caps
low current consumption of <2mA
operates from -12Vdc or high-voltage AC

This kit uses “shunt” switch elements for silent, reliable channel selection or gain stepping. Three transistors create and control two control lines that respond to a remote foot-switch. You can connect any required number of jfet switches to these lines.

For example, you can use the ERK to select the channels of the LP-PRE Preamp Kit or the FPM Fender Preamp Mod by using one jfet on the control line to mute the ‘Clean’ channel, and two jfets on the second control line to mute the ‘Lead’ channel. Because jfet gates create almost no loading on the control lines, there is no true limit on how many jfets you tie on.

ERK has a fourth BJT to control one or two LEDs, or a bicolour 3-lead LED, to indicate which channel is ‘on’. If you use LEDs, ERK should be supported by a proper -12Vdc supply rather than the high-AC voltage.

ERK also has wiring accommodation for a front panel switch, which will be over-ridden if a foot-switch cable is inserted into ERK’s 1/4″ jack.

More information here: https://londonpower.com/electronics/elec...relay-kit/

New PSU kits +/-12V

Fri, 15 Mar 2019 20:19:10 +0000

Hi Guys

We have two new power supply kits to support low-voltage circuitry, such as for switching and opamps. The kits provide symmetric voltage outputs aka "split rails" of +/-12Vdc at two different power levels.

PM12-6 provides +/-12V at 175mA

PM12-12 provides +/-12V at 350mA.

PM12 has universal mains fusing, and uses Hammond semi-toroidal PTs, metal-film resistrs, and 10,000-hour caps.

London Power - Power Scaling with SV1 and RBX (Fixed Bias)

Tue, 30 Oct 2018 17:00:29 +0000

Have you tried this Power Scaling solution? Post your questions or comments!

London Power's Power Scaling Kit for Fixed Bias (SV1) and their Raw Bias Auxiliary Supply Kit (RBX) are almost always used together to achieve Power Scaling in 99% of guitar amps. Check the SV1 product page to see if your amplifier is one of the 99%. If in doubt, contact Kevin O'Connor at London Power:
amps@londonpower.com.

The Raw Bias Auxiliary Supply Kit is typically needed because the amp's original supply is not robust enough to support Power Scaling. Does your amp need RBX? See this article to check: When Do You Need RBX Raw Bias Auxiliary Supply?

SV1 Power Scaling Kit

RBX Raw Bias Auxiliary Supply Kit

London Power's "PA"-series Tube Power Amplifier Kits

Sat, 27 Oct 2018 21:34:58 +0000

Have you tried one of these? Post your questions or comments!

These octal, push-pull power amp kits use London Power's full-audio-bandwidth OT-10, OT-25 or OT-50 toroidal output transformers. Performance is so good they can be used for hi-fi. The power amp can be used with any of the common octal-based output tubes that use the standard pin-out. Tubes can be mixed as well!

Switching between fixed-bias and cathode-bias can be "global" (both tubes) or independent.

Lots more info at:
10W Power Amp Kit (PA66-10)
25W Power Amp Kit (PA66-25)
50W Power Amp (PA66-50)