Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: lkrasner on January 06, 2015, 10:17:29 am
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I'm thinking of building a 2 channel amp based around a P1ex. I built a P1ex already, and loved the tone. My electronics skills are good, but I am still a little new to tube amp theory so I need some guidance.
Here's what I want to do:
1. Add a clean preamp channel. I assume this shouldn't be that hard, but I need a few pointers to get me going. I want it fairly clean, but maybe some bluesy breakup when cranked. I assume this will have a fixed gain right?
2. Add an effects loop. This should be trivial, right?
3. Add a parallel octal/ noval setup for the poweramp. basically put 2 master volume controls, each feeding 1 tube, with each tube having it's own bias circuitry and such, but both leading into the same OT. I've seen this done before, so it should work.
I have a few questions for #3 though. Will this change my transformer needs for the primary impedance, and if so, how? The B+ voltage in a P1ex is way too high for an el84, so what do I need to change to knock it down for that tube? can I tap into the part of the power supply running the preamp tubes and beef up the power supply resistors for the extra load?
Some links: P1ex docs (http://www.ax84.com/p1x.html)
Parallel output done in an HO build (lower B+) (http://ax84.com/bbs/dm.php?thread=284057)
Thanks for the help in advance. Any pointers are welcome. this should be a cool build.
{EDIT} fix munged URL links --PRR}
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> change my transformer needs for the primary impedance,
What is your goal? Fill-the-stadium POWER?
There is no right-answer for 1 *and* 2 tubes in the same load.
EL34 is run at 25W Pdiss and ~~10W output; EL84 is a much smaller bottle and good for half the power.
The "logical" thing to do is get a PT for two EL84 (the universal "18Watt" design) and work the EL34 down around EL84 conditions. Let the power fall where it may at whatever impedance-- won't rock the stadium but sure will fill the house.
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1) You can build the preamp section twice & use a 12AX7 in one and an almost identical 12AY7 preamp in another preamp section. With the 12AY7 one, maybe lose the V1-b cathode cap and use a higher value resistor like a 100k/plate & a 2.7k/cathode.
Use a relay to switch between each of those. Look in ARCHIVES and there is a ton of information on relay switching and Hoffman has the parts you need to make that easy.
2) Look in ARCHIVES, there is a ton of info about effects loops and how to install those. You will have to decide whether you want the effects look in series or in parallel. In series, will possibly add more gain.
3) IF you used a EL84 and a 6V6 ............ you'd need a PT with maybe 100ma or more and enough heater power also.
with respect, Tubenit
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> change my transformer needs for the primary impedance,
What is your goal? Fill-the-stadium POWER?
There is no right-answer for 1 *and* 2 tubes in the same load.
EL34 is run at 25W Pdiss and ~~10W output; EL84 is a much smaller bottle and good for half the power.
The "logical" thing to do is get a PT for two EL84 (the universal "18Watt" design) and work the EL34 down around EL84 conditions. Let the power fall where it may at whatever impedance-- won't rock the stadium but sure will fill the house.
Definitely not looking for ridiculous power, more trying to get some more interesting tone. Does it hurt to have an over powered PT in any way?
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One More Question:
Can I have both channels share a tone stack? I assume as long as it is after all the tubes, not in between like some designs, I can just switch before the tone controls.
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Can I have both channels share a tone stack?
I don't see why not. However, I would prefer to have two tone stack so I can mid scoop the clean one and have more mids on the overdriven one.
With respect, Tubenit