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Shopping List Assistance Please
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:

- 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)


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


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.
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:
AMP-50 (includes PA66-50 + PSU-50)
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

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Come in where it's warm!
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
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