 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
75.168.141.235
In Reply to: RE: Class A push-pull posted by Triode_Kingdom on December 07, 2024 at 16:04:17
I've been refurbishing an Electro Voice A-20 amplifier, which uses a pair of 6V6s in class B and makes 20-25 Watts. It has a unique driver circuit which keeps the output tubes operating at the right point (so no bias or balance controls). The output transformer is conventional but its in the cathode circuit of the output tubes and the output section is wired as a Circlotron. The Circlotron operation prevents the usual spike that would otherwise occur when the tubes go into cutoff, so no distortion at low power levels. It has excellent bandwidth.
In addition there is a feedback control which is a 2-deck device, allowing you to balance between full Voltage feedback (so the amp can behave as a Voltage source) or full current feedback, allowing it to act as a current source. For speakers like we see often driven by SETs, you set the damping control to noon, as which point its a power source (similar output impedance to an SET, but with equal amounts of current and Voltage feedback).
BTW if you google the schematic, Electro Voice published it with an error. The screen resistor also serves as the plate resistor for the driver tube. The schematic shows something else.
At any rate, 6V6s are much easier to drive than 6L6s or any of the Kinkless Tetrodes. In proper UL mode or in Circlotron mode as described above this might make for an amp that could suit the bill.
Follow Ups:
Ralph, I don't suppose you could post a marked up version of what EV actually built, re. the driver/screen grid resistor, could you?
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The plate resistor of the driver is also the screen resistor of the associated output tube. The coupling cap coming off of the plate of the driver is driving G1 of the power tube.
The schematic shows the driver plate resistor as tied to the screen and the screens then tied to the plate of the opposite power tube. Both versions will work.
In my amp and also in the schematic the driver plate resistor is a 12K 2 Watt device. The schematic as shown was glued to the bottom panel.
Here's a PDF of the user manual that includes EV's description of the circuit and a schematic. It's a PDF, so it can be rotated by most browsers. What I found most interesting about this is the statement that the output transformer is 1/4 the usual impedance.
-With the lower turns ratio the output transformer has wider bandwidth due to lower distributed capacitance. You don't even need it to be a transformer- it can work as an autoformer too (but you have to be more careful about DC Offsets).
The original Circlotron patents employed a bootstrapped driver circuit like you see in the schematic. But I think that is higher distortion and you can get into issues with blocking distortion and preventing thermal runaway of the output tubes. EV ran into a problem with this and so reduced the output tube grid resistor value to 100K and also increased the value of the coupling cap to maintain the timing constant.
Definitely an interesting design!
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
