 
|
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 |
97.95.43.235
The 55a uses GZ34s but can use 5V4s as well but also uses 6550/KT88s. Notice the .01uf cap going into the choke right off the rectifiers.Being it is so little,I don't think that can legitimately disqualify this from being a choke input circuit.I wonder its fundamental purpose because it's not going to remove any residual AC that would make any change in the DC voltage?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Follow Ups:
Mike, how does the triode half of 12AU7 (V1B?) serve as a bias voltage regulator?
Thanks!
Steve
If you look at V1B,it has a common element with the grounded plate.The neg bias rail feeds the control grid so I would assume the cathode that feeds the bias to the 6550s will change according whether or not the 6550s go into AB1 or AB2 where they start to draw a bit of grid current and that 12AU7 can help maintain the bias voltage..Maybe someone else can explain it better but that's the only way I see that regulator working because it is somewhat crude.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 08/30/16
What is the mfd of that first cap (prior to the choke) in the PSU?
Can't read schemo. Not clear on my screen (resolution-wise).
.01uf is that first cap which is there for voltage spike suppression.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Michael, that sure looks like a RF bypass to me. It's meant to keep RF from entering the amp through the B+ line. That amp also has a cap from one side of the AC mains to ground. That creates a risk of electrocution unless a 3-wire line cord has been installed.
--------------------------
Buy Chinese. Bury freedom.
It looks a little small to snub the voltage spikes generated by the choke at the zero crossing point of the mains where the rectifiers cut off. I'm thinking of the role of the condenser in the old school battery/coil /points ignition system.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I'm pretty sure it's a bypass for RF or any line noise that makes it through the transformer's rectifier filament winding. Immunity to RF from local ham ops and radio stations was a design factor when these amps were made. That cap can not serve any other purpose that I'm aware of.
--------------------------
Buy Chinese. Bury freedom.
Edits: 08/30/16
I still see it as a Choke Input because that cap doesn't effect regulation one bit.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Some old school choke input PS used a resistor and cap in series across the choke as a snubber. M.Jones point out that this arrangement does degrade the ability of the choke to filter higher frequencies.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
M.Jones point out that this arrangement does degrade the ability of the choke to filter higher frequencies.
Ok but that choke is only filtering 120hz and even if it had to filter far above the fundamental at the 10th harmonic,you are still only talking 1.2khz which is low.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
If you perform a Fourier analysis on the rectified output (the rectifiers perform a non-linear transformation of the mains AC 60Hz) you will see that there are many harmonics of 120Hz that need to be filtered. Also, HF noise from the AC mains (do a spectrum analysis of your incoming AC, it will not be pure 60Hz) can couple through parasitic capacitance (interwinding capacitance?). Also the voltage spikes generated by the choke if not snubbed will produce HF harmonics of the 120Hz fundamental. While the best of human ears can't hear past 20kHz, ultrasonic noise can and will degrade audio signals in ways that are unpleasant to the listening experience.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
They use dual rectifiers. I wonder if transient current supply is an issue. There is an article I read, about smoothing out current demands using an input cap (prior to choke). I suppose the amount of mfd is critical to what this input cap is doing.
Amps I've seen use a 1-8 mfd cap prior to the choke. With usual caps after the choke.
Maybe historical note. When higher current power transformers, plus GZ34 and better 5U4GB became more available and affordable... cap before choke wasn't needed (?).
Edits: 08/31/16
Steve
Keep in mind that the Mac Mc60s also use two rectifier tubes. When you are running the output tubes at 150ma plus for the pair, and then have 5 other tubes to feed as well,you are going to be on the edge of static current with just one rectifier in the case of the GZ34.The transient current you speak of is the reason for running two rectifiers as opposed to one.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
The 55a is using Pentode drivers which is the 6CL6 vs the 6S4 used in the 50a.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Those pentode drivers in the 55-AZ are wired in triode mode. :)
--------------------------
Buy Chinese. Bury freedom.
I did a power and distortion test last night and it makes 55 watts but the distortion at that level with a straight 1khz signal in is out of slight and it looks just like a SET amp when it clips..The sinewave basically turns into a sq wave the at 55w..Now,the amp is stock with old filter caps as are thee original 12AU7 tubes.I could wire this amp as a pentode pretty easily by using a second choke to feed G2 screens.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
~!
The Mind has No Firewall~ U.S. Army War College.
Post a Followup:
| 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 © 2024, Audio Asylum, all rights reserved ]
