new custom poweramp idea!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by spaceboy, Jul 18, 2004.


  1. spaceboy

    spaceboy Member

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    ok, so the previous idea (4 El84, 2 6L6, 2 EL34 and 2 KT88 for ultimate versatility) was gonna be very difficult with transformers and such, but what about my new idea:

    A tone generating power amp. probably been done to some extent, but 1 of each kind of power tube - doesn't necessarily matter which ones - and a built in attenuator (sp?). now - a few issues do arrise.

    the practicality issue is the same as before - how much weight and complication would be involved in having, say, three different tube types, or even four? considering it'll just be one of each, does that help the problem, or not really change it?

    attenuated tone is something that's widely disputed, but I saw a thread ages ago (might've been one that was sticky'd on the HC amp forum, or maybe here in the archives...) comparing two attenuators (hotplate and something else) with the amp recorded at volume 3 and volume 6 or something. but the hotplate came out very favourbly, and though pure valve tone would be favourable, the idea is of having power tube distortion at, essentially, the turn of a gain knob - so it's a compromise that should still sound great. i really just need to buy a hotplate and experiment.

    having a gain knob - it seems not-too-difficult, but i would like the "volume" knob to be a "gain" knob by linking it to the built-in attenuator somehow so that as the volume is increased, so is the attenuation, so the output stays at line-level. possible? simple? hope so...

    oh, and of course, as before, it'll have to be Class A, because it'll only be using one tube at a time, so how does that affect the tones of tubes like an EL34? famous for it's use in marshalls, but would it still sound marshally outside of the AB setup? and what about other kinds of tubes? any idea how it would affect their tone compared to their "typical" sound?

    ok, well, a lot of ideas that have been all jumbled and mumbled but see if you can make sense of it... so once again - is it possible/practical, and also is it worth it? would power tubes attenuated all the way down to line level sound any better than preamp tubes? they'd atleast sound different though right? so worth having...

    anyhoo, cheers!
     
  2. loverocker

    loverocker Member

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    whoh! Millions of questions there. :)

    Have a look at London Power's Studio amp: it lets you run pairs of the most popular octal power valves in any combination (6V6+EL34, etc), and lets you run anywhere from single-ended to push-pull. Plus you can switch from fixed to cathode bias. And you don't need an attenuator because it has Power Scaling (way smarter). But low volume and handmade = expensive.

    But I'm not sure if you're after switching power valves on-the-fly (like a channel-switching power amp)? It's feasible but would make the amp design extremely complex. It'd take a very determined homebrewer to build and debug it all. It'd be a ton simpler to A/B switch amps+cabs - which is what many players do - even the ones with deeeeeep pockets ;)

    But if you just want to be able to switch from one type of valve to another from time to time (switching the amp off and opening it up to change valves), there's the Chameleon version of the AX84 P1 project - very simple to build - or the THD Univalve - simple to buy :p
     
  3. spaceboy

    spaceboy Member

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    hmmm... many more ideas forming...

    I've come across power scaling before and it looks great - but complicated...

    I think by far the best idea would indeed be to have one power tube that can be swapped with a simple bias change...

    but could you, say, go from an EL84 all the way to a KT88? or is that too far?... i'll ask in another thread i think

    cheers!
     
  4. loverocker

    loverocker Member

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    Yes you could go from an EL84 to a KT88 (and all points between) in one amp. The AX84 Chameleon is a version of a very popular DIY project over at www.ax84.com - The Chameleon nicjname comes from its ability to run EL84/6L6/EL34.

    You'd just need to do the maths to get the right output transformer and power transformer (to cover the different heater currents) for other output valves.
     
  5. malabarmusic

    malabarmusic Member

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    Isn't that the raison d'etre of the THD Univalve? And with one power tube in Class A biasing shouldn't be issue, unless I'm way off base.

    - DB
     
  6. drbob1

    drbob1 Silver Supporting Member

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    Many of the single ended, cathode biased amps allow the use of ANY tube (at least with the Yellowjacket adapter for EL84s). There are drawbacks with that plan-you shoot for a bias voltage that works for everything, and none of the tubes are likely in their sweet spot, so the sound is a little more generic than you might expect. That said, my Superbaby sounds great and very different with different types of tubes (big bottle 6L6 for clean and Siemens 6CA7 for distorted are my current favorites).

    The London power has a few other tricks. You can bias the two power tubes independently, so you can have your output as symmetrical or asymmetrical as you want. By putting a clean tube in the back socket and a low power tube in the front (KT88 and 6K6 for example) you can get a pretty clean/hard tone one the push/pull setting and a pretty early breakup on the single ended (which just uses the front socket tube) and anywhere in between. The cool thing about the LP stuff is that you can also switch from fixed to cathode bias and triode to pentode mode, and vary the OT winding in use-so you can match the power amp topology to the sound you're trying to get. Imagine running a Sansamp PSA1 (with the various amp sounds) into a power amp that you can reconfigure to match the power amp topology of the amp you're emulating-cool concept. Just that none of the functions are midi or footswitchable, so you can't do it in real time (gotta throw those switches and twist those knobs!).
     
  7. spaceboy

    spaceboy Member

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    hold on, are you saying that you can't have an adjustable bias in a class A, single ended amp?
     

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