Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: OrganicEffects on May 28, 2018, 07:15:30 pm
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Hello Everybody!
Strange journey on this one, but bare with me!
I really like the sound of a cranked Princeton Reverb, as I think that's the sound the Faces got on songs like 'Stay With Me'. I've seen posts here about the Herzog, being a champ with a load resistor put on the end and run into another amp. I've also seen what Rob Robinette is doing with his micro series of champ/deluxe/bassman using preamp triodes for the power amp. Finally, I've got a 12.6V 25W transformer and a 12VAC 24W wall wart. Put this all together, what do you have? Attached are schematics of the preamp, power amp and the power supply. Is this all crazy?
Transformer:
https://www.mouser.com/datasheet/2/410/PL12-2000-345827.pdf
I've put in a 'starve' control for the power amp as demonstrated on the 'Fuzznikator':
http://solorb.com/elect/musiccirc/fuzznikator1/
So basically it's the preamp section and phase inverter of the Princeton Reverb (with the reverb makeup gain stage turned into a regular one), the 12AU7 poweramp from Rob's micro bassman, and the starve control from Solorb's fuzznikator. On one level, is this going to sound ANYTHING like a cranked Princeton Reverb, and on a grander level, is this even going to work?
Gratze,
Matt
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Looks OK.
I wonder about the 1Meg pot in the NFB. I suspect only the first 10% does anything, relative to the 2k7 and 47r resistors. But you can tweak that on test.
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I'm guessing that there's too much signal voltage into the 12AU7's. The actual Princeton preamp is designed to drive 6V6's. That's 15- 20VAC signal volts which will swamp the 12AU7's. Note: Robinette's micro amps have no tone recovery stage.
My suggestion is to build a proven design. Still, Robinette's micro Bassman has a split PI, like a Princeton. If you put a Princeton preamp in front of that, minus the tone recovery stage, that might work. I would use his PS.
Robinette also reports poor experience with NFB in his micro Bassman.
Lastly, while triode (12AU7) overdrive may be fine, it's not clear that triode overdrive = pentode overdrive. That seems to argue for preamp size pentode tubes in the micro "power amp". Or a full-size Princeton with VVR. I can see the appeal of a micro amp. But a full size amp with VVR is more versatile.
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My suggestion is to build a proven design.
Give a look here and see if you find something of interesting among this proven schematics (there are some with small output tubes)
https://www.tube-town.net/ttstore/Kits/Verstaerker:::64_436.html (https://www.tube-town.net/ttstore/Kits/Verstaerker:::64_436.html)
here a pair of examples of what I mean
https://www.tube-town.net/cms/userfiles/media/low-emma/low-emma-schematic.pdf (https://www.tube-town.net/cms/userfiles/media/low-emma/low-emma-schematic.pdf)
(https://www.tube-town.net/cms/userfiles/media/low-lumex/low-lummerland.jpg)
https://www.tube-town.net/cms/userfiles/media/low-molly/low-molly.pdf (https://www.tube-town.net/cms/userfiles/media/low-molly/low-molly.pdf)
(https://www.tube-town.net/cms/userfiles/media/low-molly/low-molly.JPG)
Franco
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6AU6s are a small pentode option. Triodes won't sound the same-- that doesn't mean they're bad, but you're trying to clone the sound of a particular (pentode) amp here.
Also, a big part of tube sound is interaction with the speaker. Due to high output impedance ("current-source" behavior), pentodes will push more power into high impedances compared to a constant-voltage signal... until they hit power supply (voltage) limits, then a high load impedance limits power.
Because speakers are nowhere close to having constant or as-labeled impedance, this different from how SS amps act and there are some weird changes as you transition from clean to overdrive. You'll lose some of this by running a poweramp into a simple resistor load, and "cleanly" amplifying/sending that signal to a speaker.
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Other small pentode is the EL95 (used insome project also as ELL80 that is a pair of EL95 on the same envelope)
but the EL95 is a bit powerfull for your purpose (I suppose)
http://www.r-type.org/exhib/aaa0406.htm (http://www.r-type.org/exhib/aaa0406.htm)
http://www.r-type.org/exhib/aad0124.htm (http://www.r-type.org/exhib/aad0124.htm)
here the schematic of an old italian amp that used those tubes
(https://i.imgur.com/jCp3o9N.jpg)
Franco
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You could also use an ECL82 (combined triode-pentode). Covers both the PI/output with fewer tubes... keeps heater current, cost, and size manageable. It "could" make more power than you need but you can limit that with bias and load impedance. And it's already small, 5-7w plate dissipation limit depending on voltage.
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There's clips out there that show Ron Wood using a Fender Champ live with Rod Stewart/Faces.
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Thanks everybody.
There's clips out there that show Ron Wood using a Fender Champ live with Rod Stewart/Faces.
Oof.. I think you're right. Back to the drawing board!
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I realize that you may gone down another road at this point ……………. however ……………..
Somewhere down the line, IF you (or someone following this thread) want to make a low wattage push/pull amp, we have a "Gallery" of low watt push/pull amps in our SCH library of schematics (thanks to Pete/ Dummy Load for his contributions to that thread) :
http://el34world.com/Forum/index.php?topic=12526.0
In reply #5, there is a "SoLow Watt 12A_7" design that I built for a friend years ago. He says it's still his main practice amp at home. He also has a Tweed Overdrive Special and a Plexi 50w. There is both a schematic and layout for the SoLow Watt. A guy on the Amp Garage built one and made the same comment that I did that it's got remarkable bass response for a 3-4 watt amp.
It uses ECL84 power tubes which has a pentode and a triode in each tube.
And a guy on this forum built one that really turned out nice! http://el34world.com/Forum/index.php?topic=13151.msg122821#msg122821
This is the one my friend has: http://el34world.com/Forum/index.php?topic=13438.msg127145#msg127145
With respect, Tubenit
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For these small watt amps, why not use a 20 watt speaker as the load, tap off that, or a 20 watt inductor out of a cross over network?
silverfox
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Now that I think of it, this may be instructive. The only problem I had at the time was, it sounded to clean. Note the inductive load on the line out.
silverfox.
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There's clips out there that show Ron Wood using a Fender Champ live with Rod Stewart/Faces.
Ha! I remember that thread! :sad2: :dontknow: :laugh:
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12au7's simply don't have enough power or tone - too small and weak for triode push-pull. Better to use 12bh7a and for even a tick more output than that use ecc99. These can be swapped in & out and can use the same biasing. Biasing smaller tubes like this is much less critical than biasing larger tubes.
Consider 6AK6 or 6au6 for smaller wattage pentode push-pull at a bit more power and tone that 12au7 also.
EL95s are harder to find and more expensive giving about the same output as ecc99 but are a true pentode at least at about 3 watts SE.
ECL82/6bm8 are closer to 7 - 10 watts push-pull and in current production. This would be better used in SE for a lower power amp.