Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on June 22, 2014, 04:56:33 pm
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What do you think about an Herzog Champ that uses an adequate in value resistor instead of the OT ?
(http://img.photobucket.com/albums/v474/mhammer/GordieJohnsonHerzog.png)
Thanks
K
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>Is possible to omit the OT in an Herzog Champ ???
No.
A resistor equal to the OT primary impedance would knock down plate voltage so much the 6V6 would have to idle near cutoff.
But even if that wasn't a problem (say you used a plate choke instead of an OT, with a coupling cap for the output), the plate a.c. swing would be a couple-hundred volts and you need less than 1v for input to the amp after the Herzog. The OT is stepping down that a.c. voltage, though it still needs to be knocked down further with the adjustable output control.
"Could I use some tube smaller than a 6V6 to avoid using an OT?"
Sure.
But you should realize that you'll be designing a preamp pentode stage intended to distort. That's probably not so different from any preamp distortion circuit. The intent of the Herzog was to take a distorting Champ amp and boost its volume with a bigger amp.
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Yes, I advocated a pre-amp pentode, in lieu of a big bottle, in one of the recent threads on this (Herzog) topic; and then someone posted a schematic of some Garnet incarnation using one.
However, you could use a dummy load resistor on the secondary side of the OT. All this and more was covered in the other threads.
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Could you possibly drive your speaker off the cathode of the 6v6? :think1:
-Aaron
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The bottom line is that the Herzog is easier an easy way to adapt a small amp as a distortion pedal, or a really bad way to build a tube-based distortion pedal. The benefit comes if you already have something like a Champ to use in this way.
Could you possibly drive your speaker off the cathode of the 6v6? :think1:
This is a good idea, as the cathode output would be lower than the plate output. But the cathode bypass cap will shunt any a.c. to ground, so you'd want to eliminate that.
Spit-balling ideas
Taking the output from the cathode would lower the output signal to be about the same as the 6V6 input signal. This is still bigger than the voltage present at the speaker terminals. A typical Champ has 18-19v across the cathode resistor, and an output tube runs out of clean power when its grid is driven close to 0v momentarily. That implies an 18-19v input signal to the 6V6 and ~18-19v of peak a.c. output at the cathode.
19v peak = 19v * 0.7071 = 13.4v RMS at the cathode. Assuming the Herzog makes about 3w with its OT and 10Ω load resistor, then the voltage across the resistor is √(3w * 10Ω) = ~5.5v RMS.
You could split the cathode resistance into 2 resistors, one connected to ground about 1/3 the original cathode resistance and the other from this resistor to the 6V6 cathode of 2/3 the original resistance. Take the output from that tap point to knock the cathode output signal down to about the same level as the Herzog output (it will really be 13.4 / 3 = ~4.5v RMS).
You still have the same problem of having an output transformer as the plate load for the 6V6, or a suitable plate choke in its place (this wouldn't be a power supply choke). A Champ OT is $30, so I don't think you'll save much on cost or weight by using a plate choke for the 6V6.
And we're still left with the problem of having a power tube not called upon to deliver any power output. You could run the 6V6 at low supply voltage (like 50-75v) which would drop down the power output, the bias voltage, the input signal level to drive it to distortion and the plate/cathode output voltages. But if we go through all that to make a power tube not deliver power, it seems like we should be able to use a smaller tube instead.
It would seem if you want a "tube distortion pedal" the Herzog makes sense if you're also going to have a speaker output to use it as a low-watt practice amp, or if you already have a Champ or a Valve Jr or the like for conversion. If you are opposed to having the output transformer, maybe for budget or space/weight reasons, there are plenty of easier, cheaper and lighter ways to create distortion.
At least, this is what I think I see when thinking through everything.
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Have not really thought it out, just going to bed so I can blame stupidity on the mind already has been winding down.
Now if you replaced the OT with a resistor the same dc resistance as the output transformer and biased it up the tube should not know the difference. It is only the AC load line that would be messed up not having the nominal 8k impedance to load the plate. The load line would almost be a vertical line. So you would get a lot of current swing for a small input.
Sorry, that is as far as I got. :huh:
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A transformer approximates 200 Ohms for DC and 7,000 Ohms for audio.
But when you OVER-drive, the approximation is very bad.
Which was very good for American Woman.
The overdriven transformer is a LARGE part of the "sound".
Think: if the last stage is resistor-loaded, then it overloads like a preamp. Preamp distortion is A Sound. But it isn't the Sound of the Herzog.
If you must prove it to yourself.....
You "could" use a 7K resistor and a 700V supply to get about the same operating point as a Champ. That's obscene.
You do not need ~~300V audio output. The Herzog used about 42:1 step-down OT. So replace the OT with a 167 Ohm (or 100r or even 50r) resistor. The 7V voltage-drop is insignificant, so you can use stock B+. You get roughly the same signal level (though NOT the same flavor). You do need a GOOD output cap, and there will be a HUGE thump at turn-on.
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Many Thanks to ALL
The intention wasn't to use the output signal from plate as is
but to use (like in preamp stages) a decoupling cap
followed by a resistor divider to tame the signal to more reasonable level
However I've understand that this thing can't be done with results similar to those given by an OT
Again Thanks
K