Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 06, 2019, 06:10:47 am
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I've find this SE amp schematic indicated to be in UL configuration but I'm really confused
There Is no UL tap and there Is a 6CG7
Thanks for any explanation about the circuit
Franco
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I would guess that the circuit simulates UL. Actual UL would be a primary tap, which reduces screen voltage, but also distributes load and signal to the screen. The result is to greatly reduce distortion while retaining pentode power.
Load distribution seems to be lacking here. But, the 6CG7 seem to be affecting screen voltage. Its cathode seems to be getting some signal feedback through the OT primary; and its G1 seems to be getting signal feedback, being DC coupled to the plate of the EL86. The EL86 cathode also seems to be getting feedback from the bottom of the OT primary.
So I'm guessing the voltage supply to the EL86 screen, and the signal feedback arrangements, result in lower distortion with nearly full power, similar to actual UL???
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thinking that 6CG7 is simply a voltage regulator. 200K divider to grid so output would be 1/2 of supply.
--pete
EDIT: after some more thought, the cathode is not sitting at ground, so the 6CG7 would be simply a series pass regulator. output V would be closer to supply V.
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Agreed. Though not sure how to calculate the divided voltage to the screen. Regulated voltage to the screen should be OK, as the SE configuration for the plate is more or less "regulated" anyway.
Though I mentioned it above, I'm baffled by the "feedback" to the EL86 cathode. It seems that feedback signal would bleed to ground through the large cathode bypass cap on the EL86. And DC to that cathode is blocked by another cap. So if the EL86 cathode is getting neither DC nor signal, what's the point of that connection?
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Thanks friends
I'm reading with interest
Franco
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It is NOT what we usually call "Ultra Linear".
Conceptually the added cathode-follower is just a unity gain buffer. (In practice we need to check if it can swing as expected.)
For UL we tap between B+ and plate.
This taps between plate and ground.
The idle DC voltage on G2 is much less.
The AC swing on G2 is much less positive.
Without asking the Idiot, it *looks* to me like it spoils the performance of the EL86, starving it of G2 voltage. It *may* improve linearity and reduce Zout, but I suspect the loss of power makes small benefits not worth while.
In principle we can get true UL mode with the buffer. But note that the buffer must swing far above the B+ that we give to the plate. Without other tricks, that means a small high-V supply, cost and complication.
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Many Thanks PRR
Franco
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I may not be as dumb as I look. Or rather, the Idiot says about like I figured, so I may be as dumb as a Pentium. Or a SPICE model.
I didn't have a small power pentode so I took a "big" small pentode. TU1 is wired like the plan you found, call it "not UL", plot is blue. TU2 is wired plain-pentode, red. TU3 is wired triode, green.
The red curves are classic Pentode. The green curves are a very ordinary triode. The blue, "not UL", plots look like a low-performance triode. Say we load as a power amp, yellow line and yellow circle a suggested idle point (185V 12mA). The pentode can swing past 24mA. As triode almost 20mA. As "not UL" it barely swings to 14mA! What it really is, is a "high voltage triode". We could get some power with extra high B+ and high load impedance. Over 400V? However this may exceed tube ratings.
I do not see any improvement in linearity.... it may be there but way out at illegal voltages.
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I was blocked trying to simulate true UL mode. The hasty way is to have a second "B+" which does not vary. A more elegant way would be a single fixed B+ plus a varying +/- voltage to swing the plates and dividers. Same thing either way.
TU6 red is true UL mode. It shows the classic soft-knee curves of UL. (Power tubes suitable for UL will rise relatively higher; 6AN8(p) is made for gain not wild signal swings.) Green is not-UL and still looks like a bad triode.
The 1 Ohm resistors are to trick the simulator. The one I use will not Probe the current in a wire, only in a part. 1 Ohms has "no effect" in a circuit that has hundreds of Volts and only dozens of mA.
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Again Many Thanks
Franco