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Posted: Tue Nov 16, 2010 4:13 pm
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PostPosted: 16 Nov 2010 22:32 Post subject:
Sceen grid resistor: From The Ultimate Tone by Kevin O'Connor, pages 6-48, 6-49

" Most technician expect a screen resistor under stress to overheat, and to increase is value or open completely. The loss of screen current should "turn off" or at least limit anode current. Although our test results confirm this idea, it rarely happens. In practice, screen resistor only burn up when a tube actualy short. A bias faillure, although extremely hard on the tube and the output transformer, doe's not necessary stress the screen resistor

".....Fender use 2 Watts carbon type resistor...EL 34 powered Marshall amps typicaly use 1K ohms, 5 watts wirewound resistor...."

" The obvious quality difference between the carbon and wirewound resistor made the sceen resistor "fuse" theory seem plausble. Low quality devices might fail under the stress of a bias failure, to protect the tubes and transformer, while high quality resistors provide no protection at all.

Hey Stratele,
It really seems like we are saying the same thing, just coming at it from different directions. The low watt carbon resistors should fail to protect tubes and transformer. Someone had installed 5w composite resistors on the power tube grids of this amp-5x the size of amp's specs, thus the resistors didn't fail and bedlam occurred. Cordially, ...O.


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Posted: Tue Nov 16, 2010 4:26 pm
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Indeed.

The grid stoppers are there to backstop the power supply so that if an output tube shorts it doen't fry the whole enchilada. Leo was a pretty smart cookie.

Arjay

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Posted: Tue Nov 16, 2010 5:11 pm
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But don't forget, Kevin O'Connor write that is not sure that resistor will protect .


IMO the 5 amps fuse was the ONLY problem, The standard 2.5 amps fuse will blow before any big dammage.

Think about that; the 5 amps fuse bring 2.5 amps MORE, the DOUBLE of the amplifier need .


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Posted: Tue Nov 16, 2010 6:12 pm
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I'm never gonna presume to second guess my betters, but I think Leo was more than just a smart cookie. The man must have been a damn Dada genius. Just look at how many of his former employees went on to start their own guitar/amp companies, mostly based on his designs. Consider how he never stopped trying to improve electronically produced sound, even decades after he left the company he founded. I am in awe of this Godfather.

A couple of pithy quotes:
"Genius is 1% inspiration and 99% perspiration ..."- T.A. Edison
"His genius wasn't just that he invented the solid-body electric guitar, but that he invented the amplifier to go with it."- Keith Richards

Apropos, no?


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Posted: Tue Nov 16, 2010 6:19 pm
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nyquilcoma wrote:
Apropos, no?


ABSOLUTELY!

Both Saint Leo and Sir Lester Paul each deserve a postage stamp IMO.

Arjay

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Posted: Tue Nov 16, 2010 7:14 pm
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Leo was also working with some tried and true circuits,and never stopped trying to improve what others had done before,so he was always learning.

The guy who works on my amps at times once met Leo and George Fullerton when he was playing for Rex Allen Jr. and was playing in Cali.....he said Leo seemed quiet and was working on a small piece of metal on a lathe and would take out a mic from his shirt pocket protector,measure it and put it back in the lathe,and work on it some more....just piddlin'.
My friend was fascinated as he watched him.


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Posted: Tue Nov 16, 2010 8:54 pm
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I'm with stratele52, something doesn't quite make sense with what the tech said. The modifications didn't cause the problem, the amp worked before the speaker was unhooked. The screen grid resistors on the tubes have nothing to do with any kind of protection. The resistors are there to set the voltage on the screen grid to repel stray electrons, that's all. The higher value fuse is a problem, it IS supposed to protect the amp somewhat, but it didn't cause the OT or tubes to burn up. I still say that the unhooked speaker was the cause of the problem, and that would have happened in exactly the same way whether or not the screen grid resistors were 1 watt or 10 watts, it wouldn't matter. :idea:

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Posted: Tue Nov 16, 2010 10:37 pm
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Yeah, I'm inclined to suspect that the 5 amp main fuse and 5watt screen grid resistors may be indicators of a different problem. But even if I'm wrong, that PT was on borrowed time. Obviously, there is no excuse for 1/4watt resistors. It's common practice, now, to have a high tension fuse in the output section. I have flyback diodes on my power tubes, but should really put in an HT fuse. Cheap insurance and doesn't affect the tone. I'll agree that a 5watt screen resistor, alone, is not problematic. Shimmilou, I think you meant the suppressor grid repels the electrons back towards the plate. As far as I know, the screen grid accelerates the electrons towards the plate and screens the control grid from a higher capacitance effect. JMO Art

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Posted: Wed Nov 17, 2010 6:35 am
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aclempoppi wrote:
...Shimmilou, I think you meant the suppressor grid repels the electrons back towards the plate. As far as I know, the screen grid accelerates the electrons towards the plate and screens the control grid from a higher capacitance effect. JMO Art


You're on the right track. :) Suppressor grid, screen grid, same thing. The control grid is the one that accelerates the electrons towards the Plate (set bias with control grid voltage), that is it's purpose. The screen grid(s) (suppressor grids) are just that, they repel the stray electrons. Note, two or more screen grids, one control grid. :idea: But, I agree that the beefier components might indicate that the grid resistors have gone out before, probably caused by a bad output tube. Grid resistors just don't go out unless there is a problem with the output tube(s), and they certainly don't provide any protection for the tubes or OTs. :)

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Posted: Wed Nov 17, 2010 9:12 pm
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Hey Shimmilou, might be a good topic for a new thread. Can't say I agree with you on the function of the elements in a pentode tube. But yeah, it's hard to tell what was going on with the Showman, without knowing the history. Nyquil, I'm not saying your tech is wrong. After all, he had firsthand experience with the amp. Art

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Posted: Thu Nov 18, 2010 5:55 am
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Well, my, my Landy! I never knew an update on possible causes for damages to my newly repaired DSR was going to spark such debate and controversy. Some seem to agree with my tech's assessment of what went wrong, while others feel other issues could have been at play. Fascinatin' stuff. Like I've always said, I'm no expert, and I wanted to warn others about the dangers of not having old amps examined and serviced by a competent tech. The proof is gonna be in the puddin' and if something else (God forbid!) should happen, there is now a reference point. As ever,...O.


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Posted: Thu Nov 18, 2010 6:15 am
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Yes is not easy to have THE ONLY right answer.

First; is not easy to do a diagnostic by e-mail about what you said, whithout seeing by ourself, the amp.

Second : you could sometimes forget some valuable informations from your tech and we go to wrong path.

Third : here in the forum we have not the same experience and knowledge about tubes amps.


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Posted: Thu Nov 18, 2010 9:34 am
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stratele52 wrote:
Yes is not easy to have THE ONLY right answer.

First; is not easy to do a diagnostic by e-mail about what you said, whithout seeing by ourself, the amp.

Second : you could sometimes forget some valuable informations from your tech and we go to wrong path.

Third : here in the forum we have not the same experience and knowledge about tubes amps.


Oh I don't know about the "third" statement. I'd disagree based on what I've seen.

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Posted: Sat Nov 20, 2010 12:58 am
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That's an interesting observation, Tyronne. I never thought about the overall view that some of us must project. Like any village, everyone is different and our views, experiences, knowledge, and personalities will reflect that. But as a community, I guess we are more the same then we are different. Music is probably the strongest- unifying factor ! The techinical side is just the means to an end. JMO Art

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Posted: Sat Nov 20, 2010 1:06 am
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aclempoppi wrote:
The techinical side is just the means to an end. JMO Art


+1

Arjay

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