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We were discussing the Fisher 50a which is pretty similar to the 55a

97.95.43.235
Posted on August 29, 2016 at 21:12:51
Michael Samra
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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

 

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.67 is the demarcation line for choke loading... above .68 and is no longer choke input~nT, posted on August 30, 2016 at 07:54:00
Cleantimestream
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 ~!
The Mind has No Firewall~ U.S. Army War College.

 

RF Bypass, posted on August 30, 2016 at 08:45:43
Triode_Kingdom
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 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.


 

one 12AU7 triode as bias voltage regulator, posted on August 30, 2016 at 09:42:55
FenderLover
 Mike, how does the triode half of 12AU7 (V1B?) serve as a bias voltage regulator?

Thanks!

 

THere is a difference in the 50a to 55a, posted on August 30, 2016 at 13:16:50
Michael Samra
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 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

 

RE: one 12AU7 triode as bias voltage regulator, posted on August 30, 2016 at 13:41:56
Michael Samra
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 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

 

Not as different as they might seem..., posted on August 30, 2016 at 14:02:30
Triode_Kingdom
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 Those pentode drivers in the 55-AZ are wired in triode mode. :)


 

RE: RF Bypass, posted on August 30, 2016 at 16:49:48
JKT
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 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

 

RE: Not as different as they might seem..., posted on August 30, 2016 at 16:51:13
Michael Samra
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 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

 

That what my guess in that it was for spike suppression., posted on August 30, 2016 at 17:23:07
Michael Samra
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 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

 

RE: That what my guess in that it was for spike suppression., posted on August 30, 2016 at 18:17:38
JKT
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 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

 

RE: RF Bypass, posted on August 30, 2016 at 19:39:45
Triode_Kingdom
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 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.


 

RE: That what my guess in that it was for spike suppression., posted on August 30, 2016 at 19:51:16
Michael Samra
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 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

 

RE: That what my guess in that it was for spike suppression., posted on August 30, 2016 at 20:17:55
JKT
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 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

 

Thanks!, posted on August 30, 2016 at 20:35:50
FenderLover
 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).

 

I noticed on these amps..., posted on August 30, 2016 at 21:04:39
FenderLover

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.

 

This other article., posted on August 30, 2016 at 21:06:24
FenderLover
 Maybe historical note. When higher current power transformers, plus GZ34 and better 5U4GB became more available and affordable... cap before choke wasn't needed (?).

 

RE: This other article., posted on August 31, 2016 at 11:22:52
Michael Samra
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 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

 

RE: Thanks!, posted on August 31, 2016 at 11:27:33
Michael Samra
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Posts: 36118
Location: saginaw michigan
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 .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

 

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