Shopping List Assistance Please

[galejt]

Greetings Kevin and all y'all.

I want to build a 50W combo amp around the LP-PRE. I 
would appreciate your help in populating my "To Buy" list.

Expectations are as follows:

LP-PRE
- foot and panel channel switch
- possible FET input jack (Grainger?)

PA-66 + PSU (50W)
- individual bias points
- pentode/triode panel switch
- Body control
- feedback loop panel switch
- Presence control
- independent fixed/cathode bias panel switch
- Power Scaling (Drive control vice MV)

SV1 + RBX

BFX
- parallel/serial panel switch
- in/out foot and panel switch

QS-LV

Switching options - here is where I really need help. I am looking for a three button foot switch that
- switches channels on the LP-PRE
- switches the BFX in and out
- switches an FET input jack circuit (3rd party vendor)

I am going to Hoffman in N.C. to make me up a chassis and a guy in N.H. does nice cabinets. I'm harboring 90W ET-90 here for the loudspeaker position. Yup. My dream amp. I will have the Soma84 and a D-PRE under my belt beforehand so I'm hopeful all will go well.

Any comments or corrections are greatly appreciated! For real.
Thanks,
Jim

[K O'Connor]

Hi Jim

Four notes first:

Note that combo amps are the worst environment for tubes. A head-style amp is better for tube life and for the ability to use less mechanically robust tubes.

Note that it is better to have fewer amps than more amps as far as electrolytic capacitor life goes. Eleectrolytics require regular application of voltage to remain healthy.

Note that if you use our PSU-50 kit, you do not need QS-LV as the PSU has active fum filtering in addition to massive raw filters.

Note that our kits use 16mm pots which fit into a 0.2812" dia hole, plus a 0.125" anti-rotation hole 0.3" from centre. You may want to get all the kits first to do some "dry" layouts so the chassis can be specified accurately.

For the 3-button switching you envision, it is best to use mechanical relays for the audio interface. In this context, one relay will suffice for selecting between the Clean and Dirty paths of LP-PRE; one relay to bypass the front-end boost you want to add; and one or two relays to switch BFX.

For BFX, if you wire it to be a mixing loop all the time, then only one relay is needed to switch the loop on/off. If the loop is wired to be series all the time, again it only takes one relay to turn the loop on/off. If you wish to retain the series-mix capability, then it takes two relays to control the loop.

If you are using London Power kits to build the entire amp, then you would need:
LP-PRE
BFX
AMP-50 (includes PA66-50 + PSU-50)
GND
SV1 for Power Scaling
3x RLY
3x PSW (panel switch)
3x FSW (foot switch)

Note that PSU-50 has a regulated auxiliary supply of +/-12V which you can use to power the relays and the gain boost circuit.

LP-PRE has a lot of latitude for distortion voicing and saturation and you may not need the extra gain boost. On the other hand, having excess gain is always good as it makes the sound more "effortless" and "liquid", to the point where it can feel like the guitar is playing itself. In my case, that would be a good thing - hehe.

Of course, if you have some or all of the TUTs, you can see various hand-wiring circuit blocks and use chassis-mounted jacks, pots and tube sockets. For example, The Standard in TUT5 has most of the features you listed. The 17" chassis width is a constraint there, but the project uses off-he-shelf Hammond transformers and chassis. All the PA features can be added to any of the TUT3 or TUT5 projects. The TUT3 800 makes a particularly good foundation for customisation and mods.

Remember too, that the bulk of wiring only needs to be #22, even for power tube heaters if you use the PSU-50 approach.

Have fun
Kevin

[uzerman]

To build your 50W combo amp around the LP-PRE, you'll need several components: a DPDT footswitch and high-quality FETs (like 2N5457 or J201) for channel switching and input buffering, preamp tubes (12AX7 or equivalent), potentiometers, capacitors, and resistors for tone controls. For the PA-66 power amp, gather parts for individual bias points (trimpots, test points), a pentode/triode switch (high-voltage rated), components for body and presence controls, feedback loop and bias mode switches, and a power scaling kit (like London Power). You'll also need parts for the SV1 reverb and RBX modules, and switching options for the BFX loop, including a parallel/serial switch and in/out bypass. For footswitch control, get a custom three-button footswitch to manage channel switching, FX loop, and FET input, possibly using relays or a controller from companies like Voodoo Lab. Ensure Hoffman in NC makes your chassis with all necessary cutouts, and coordinate with your cabinet maker for compatibility with the ET-90 speaker. Don’t forget miscellaneous items like high-quality jacks, wiring, fuses, and test equipment (multimeter, soldering tools). Your experience with the Soma84 and D-PRE will serve you well—this build will be challenging but rewarding, bringing your dream amp to life!





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[K O'Connor]

Hi Guys

Uzerman, I would say you should review the LP-PRE and PA66 product descriptions more carefully, and maybe look at "About Our Kits".

LP-PRE includes all the components that comprises the audio path, including: Rs, Cs, pots, jacks, tube sockets, PCB to carry main circuitry, small PCBs for each pot and jack.

PA66 includes all of the same, as well as bias test jacks and individual bias pots per tube onboard.

Your suggestion of needing a DPDT switch for channel switching assumes one would do the switching incorrectly, i.e. sending audio through the switch even if it is remote. We don't do that. Since there is a requirement for remote switching, we use remote-controllable muting/switching elements placed where the audio already is, then control those elements using DC voltages that can then be themselves controlled however is appropriate. In our own products, even if remote switching were not fitted, the audio control would be handled the same way to maintain short signal paths and reduce possible noise intrusion.

BFX switching can be easily handled using a new kit called BFX-RLY, which has two relays mounted on a small PCB, allowing proper on/off and series/mix functions.

Note that LP-PRE has dedicated wire blocks for the muting element connections. Typically LP-PRE is mated with ERK, which uses shunt switching via jfets. You definitely would NOT use 2N5457 as a muting element as its Rds-on is way too high.; it is fine in the audio path as a buffer or gain stage.

Note that the input impedance of LP-PRE is high and there is no need for any other buffering unless you wish to add a Thru jack to link inputs of other preamps/amps/tuner to the input jack.

Regarding the triode-pentode switch: Historically we used the C&K 7000-series for all switching in our amps as the specs allow for use as T/P and even mains, for the latter depending on the power of the amp. STUDIO has T-U-P switching, which offers ultra-linear in addition to triode and pentode modes. Jim, considering your amp history, you need the U option. C&K 7211 allows wiring as a 3-way switch and accommodates one tube, or bank of tubes - 'push' or 'pull' side. The C&Ks are a bit pricey and you can get identical switches from Carling for about 2/3 the price.

I forgot to mention in my previous post that PSU-50 has a proper raw bias supply so RBX is not required. This assumes you use our PT or a combination of PTs to take advantage of the PSU PCB.

Have fun