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Post subject: A Valuable(read Expensive!) Lesson Learned...
Posted: Tue Nov 16, 2010 10:10 am
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Just got my '68 SF Dual Showman Reverb back from the amp tech last night- this is a follow up to my "Did I do Something Really, Really Stupid to my DSR?" post of some weeks back. My DSR went up during a speaker demo. Here's what the tech said happened: someone had attempted some mods to this amp (bias control pot, blackface stuff) but in so doing had added oversize 5w resistors on power tube grids, undersize 1/4w resistors on the board and a massive 5 amp fuse in place of the 2.5 amp required (fuse was not marked). Result? A power tube short doesn't blow fuse or take out grid resistors, but the surge continues-burning up the board, taking out rest of power and pre-amp tubes and frying power tranny. A perfect storm. Worst part is he says,"If I had seen this before it would have been a cheap fix, less than $75." Now I've got a new Hammond PT, all new power and pre-amp tubes, a rebuilt board and spec small components. He also biased the amp, and cleaned all pots, sockets, jacks and heater wires. Cost? 400 bucks! Yow!
How's it sound now? Different. I've owned several Fender tube amps and never knew they could run this quietly-no hums, no crackling pots, no clips with reverb or trem- just dead quiet until noise signal is supplied. Highs are sparkling clean and chimey. However those fat, thick Fender bass and mid tones are not all there yet. I know new power tubes take awhile to break in and I only got about an hour to wang on it last night as it was the middle of the evening when I got home and my wife doesn't understand my penchant for loud noises at all hours, so I'm hoping with repeated usage its full tonal range will reveal itself. I've never had to replace pre-amp tubes in anything, and certainly never a PT, but I've read on this forum that new trannys can take many, many hours to open up. Any experiences with new pre-amp tubes and trannys please chime in here.
So to answer my own question, "Did I do Something Really, Really Stupid to my DSR?" Yeah, I did. I didn't have a prized piece of 42 year-old electrical equipment checked out by someone who knew what he was doing. If only I'd listened to Arjay and Art and Rebel and Johnny and, and, and....Live and Learn. Cordially yours, O...


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Posted: Tue Nov 16, 2010 10:43 am
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You said it man,live and learn...usually expensive lessons.We all have screwed up.
The person who had done those things probably or could have read somebody online passing bad information along that some "guru" had told somebody.
Best thing is,you know it's right now and will take some time to settle in,I hope you'll be happy.
Maybe somebody will learn a valuable lesson from reading this...at a smaller expense.


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Posted: Tue Nov 16, 2010 11:02 am
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"Preventative medicine" is the best treatment for any vintage amp.

I'm glad your DSR is back up and running to spec.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Posted: Tue Nov 16, 2010 11:25 am
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Unfortunately, these things happen because amp techs may not understand the philosophy and topology of why grid stopper & screen resistors, and other "stabilizing" devices are applied. If you talk to a good RF Ham tech they can tell you exactly what devices are needed to help stop parasitic oscillations and other forms of instability. They deal with bad things like this all the time... with the MHz frequencies of RF.

Amp techs are often hunting for tone, only. Not knowing that they maybe compromising safety too much, for that ultimate tone. I remember on a Marshall amp tech site... 5 watt grid stoppers and screen resistors were touted by many as the ULTIMATE. Some ppl even put 12 watt Mills resistors to get even "better" tone. Imagine using high-end, $$$ parts to blow up precious NOS Mullard EL34's?

These resistors on the power tube act not only to limit instabilities and supersonic oscillations... but they act as FUSES. Any current flow consistently above 1/2-watt through these resistors may indicate parasitic resonance, crazy positive feedback, or a tube starting to short out. Prolly better to have them pop & silence the amp, than the power tubes and OPT.

Also, if one of these resistor pops, the tech HAS TO FIGURE OUT WHY THEY POPPED. Before replacing the tubes and other fried parts. Or risk doing it all over again. Often times, the cause is an improper tweak. Like an impedance mis-match of a new tranny. Wrong speaker impedance load. Or even putting in modern hi-end parts that increase bandwidth, but also increase the chances of inducing instabilities.

Good luck with your fix. These old Fenders are tough, but even a Cummins diesel can be fried by improper maintenace and "tweaks."


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Posted: Tue Nov 16, 2010 11:55 am
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I'm nothing more than a "piddler" when it comes to these things,but I know people,like Arjay and BMW here,who do know the amps like the back of their hands,and like BMW said,the reasons why the components are the values they are,and why they do what they do...it's way more than just replacing caps and resistors,
Anyone who has ever looked at an electronics book on tube amps,I mean a REAL electronics textbook,can easily see it ain't simple. Even the RCA tube manual is an in-depth textbook on tubes,not just a cross reference of tube types to replace other tubes.
Before you even talk to an "amp tech" about your repairing or modding your amp,try to arm yourself with as much knowledge as you can muster from real books about it....and then make a judgement if he is worthy of even opening up your valuable amp.
I know my limitations,and only trust a very few to do the work and when I do trust them,I listen and learn.


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Posted: Tue Nov 16, 2010 12:08 pm
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nyquilcoma, I try to understand what's was wrong with your amp that blow transformer and circuit board.

The only thing I see is:
1- The wrong size fuse ( 5 Amp) that cause this big mess .
2- And some bad luck.

The 5 watts resistor on the grid, (I suppose you talk about the screen grid 470 ohms) ? Is not there to protect your transformer or your circuit board if tube come short. This is not the problem.

It you are badlucky, you could have a tube short any time and the only way to protect your amp's expensive parts is to have a fuse on B+, like some Marhall have or fuse on power tube's plate .

But no Fender amps have that. The right input power fuse do the job almost all time.

So I desagree whith what your tech said.


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Posted: Tue Nov 16, 2010 1:23 pm
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Ouch!! Ouch! Ouch!!!

Sorry to hear about that experience. I for one will remember that.
Been looking at used DRRI myself on Craigslist and been wondering and asking all those I find, what mods have been done. Just in case something like this were to happen. Sort of scares me away from the used amp market.

Expensive learning but the furure sounds like it will be better with the new parts in there. (pun intended) :)

Thanks for sharing that with us. I'm sure it will help out more than just myself.

-T

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"You can't spend what you ain't got, you can't lose what you ain't never had" ~ McKinley Morganfield


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Posted: Tue Nov 16, 2010 1:48 pm
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When buying used amp, always check for proper fuse size. Like your home, you won't put a 30 amp fuse or braker when the house wiring need 15 amps .

If you do : house could burn, like the amp


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Posted: Tue Nov 16, 2010 2:00 pm
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I always test-drive a vintage piece before writing the owner a check. If it survives my rigorous criteria and I bring it home, the first place it goes is into the shop for a thorough examination and full bench-check. Any non-approved mods or field-expedient fixes are removed and the amp brought back to original spec before proceeding further. I'm not a big fan of mods, especially those performed solely for the sake of modding (by and large, Leo got it right the first time). Any necessary safety and/or reliabilty upgrades not already extant are installed then I can take some liberties "tuning" the chassis via hand-selected tubes. A output stage bias check (and adjustment if needed) is performed coincident with this phase. My procedure has never failed me and I've yet to own a Fender amp that left me "high and dry" at a critical moment.

There are many members here who offer some of the finest technical advice available anywhere and at absolutely no cost. Those who ignore it do so to their own detriment and at their gear's own peril.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Posted: Tue Nov 16, 2010 2:22 pm
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Hey Gang, thanks for all your positive feedback and encouragement. I'm gonna stop beating myself and my wallet up over this incident. Hopefully some other fairly new members will learn something from my experience and take their prized Fender tube amps in for a check up every so often. Like many, I'm more of a player than an amp expert. My Fenders have always performed like champs (and Twins, and Broncos, and Showmans, sorry...), so I sorta got that invincible thing going. Also, I'm a big "if it ain't broke..." kinda guy who will generally drive something into the ground, and then take it to the shop. This experience has taught me that can be a much more expensive option where 30-40-50- year old electronics are concerned.
Stratele- I enjoy your posts, one of my best friends in high school was Quebecois; however, BMW essentially re-iterated what my amp tech told me, that being that the resistors on the tube grids also act as fuses part of the amp's fail safe mechanisms in the event of shorts and power surges. This amp calls for 1w resistors there, a previous owner had installed 5w resistors, so instead of the resistor failing and stopping things there and then, current was allowed to flow throughout the circuit. This combined with the oversize fuse and 1/4w resistors on the board caused the meltdown. Please, anyone with experience about how long new pre-amp tubes and PTs can take to break in let me know. Also, to Arjay concerning bench rates-here in Baltimore, c. $50/hr, there in Tombstone c. $35/hr, have seen posts in other forums for rates in Philly/New Yawk/Boston megalopoli from $60-$100 per!


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Posted: Tue Nov 16, 2010 2:37 pm
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Retroverbial wrote:
I always test-drive a vintage piece before writing the owner a check. If it survives my rigorous criteria and I bring it home, the first place it goes is into the shop for a thorough examination and full bench-check. Any non-approved mods or field-expedient fixes are removed and the amp brought back to original spec before proceeding further. I'm not a big fan of mods, especially those performed solely for the sake of modding (by and large, Leo got it right the first time). Any necessary safety and/or reliabilty upgrades not already extant are installed then I can take some liberties "tuning" the chassis via hand-selected tubes. A output stage bias check (and adjustment if needed) is performed coincident with this phase. My procedure has never failed me and I've yet to own a Fender amp that left me "high and dry" at a critical moment.

There are many members here who offer some of the finest technical advice available anywhere and at absolutely no cost. Those who ignore it do so to their own detriment and at their gear's own peril.

HTH

Arjay


You Sir, are one of the few I listen to and enjoy reading posts from.
There are others too! I'm not good at remembering names but I agree that I see many fine folks on these forums that certainly have knowledge that shouldn't be ignored.

All the best,
-T

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"You can't spend what you ain't got, you can't lose what you ain't never had" ~ McKinley Morganfield


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Posted: Tue Nov 16, 2010 2:38 pm
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nyquilcoma wrote:
Also, to Arjay concerning bench rates-here in Baltimore, c. $50/hr, there in Tombstone c. $35/hr, have seen posts in other forums for rates in Philly/New Yawk/Boston megalopoli from $60-$100 per!


Indeed, you are correct.

But I know you're aware of the adage which holds that "A ounce of prevention......".

And as you become more aware of these things and develop some curiosity, some of the procedures needed to keep an amp operating correctly you'll be able to perform yourself. As well you'll learn to do some basic trouble-shooting if/when symptoms of a problem occur. Thus, the learning never ceases.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Posted: Tue Nov 16, 2010 2:41 pm
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BMW and Rebel post was not there when I write mine. They gave good informations and I have to check mine


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Posted: Tue Nov 16, 2010 3:15 pm
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As ever, thanks Gentlemen. I have very much learned my lesson, and my head is getting smaller by the hour-I need to know more about this subject and not just take "Fender Quality" for granted. Actually the tech I took my amp to (Joe R. at Invisible Sound) told me they are thinking about starting a Tube Amp 101-style seminar there at the shop/studio-"Sign me up", I said. If you're ever in the area look them up-they are the home of the North American Amp Museum-over 170 vintage amps from the '40's thru the '70's and a treat for any gear head. '49 Harvard? Sure got two of 'em. Marshall Plexi? Only got about a dozen of those left. Every model Twin ever made? Yeah, got a bunch of those. Browns/Tweeds/Blacks/Silvers, Supros, Gibsons, Gretschs, Sunns, Hiwatts, DIYs, and stuff no-one ever heard of. I was pretty starry-eyed. Makes me realize there is a LOT more to learn. Cordially, ...O.


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Posted: Tue Nov 16, 2010 3:32 pm
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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.
Marshall's compensation for this was to add a real fuse, but this is not effective against bias failure - only tube shorts. Unfortunatly bias failure are much more frequent"


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