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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 9:50 pm
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BMW2002Ti wrote:
Arjay, please check your email. I believe that these screen resistors really don't act like fuses.

Thanks!

:D


Parkside Drive, no, I believe wire-wound are fine here. If you want to go back to original --- use metal-oxide that are heat-resistant. As they are usually placed between socket tangs UNDER the 6L6GCs. 1-watt will do fine.


When the screen grid of V5 in my BDRI shorted, it took out the 1 watt, 470 ohm carbon film screen grid resistor. I replaced both with 2 watt, 470 ohm metal film resistors.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 10:14 pm
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Bluesky, my experience is when screen resistors pop, usually it's an unstable amp & not just a bad output tube. Grid stoppers, fuses, even OPTs ( :cry: ) go South, too.

Anyhow, a stable output section depends not only on components, but layout and basic design. Short leads and uncluttered layouts usually = long happy life. As well as GOOD TUBES!

Kinda like this amp's layout (note the 5-watt screen grid resistors on the output tubes):


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 10:55 pm
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BMW2002Ti wrote:
Bluesky, my experience is when screen resistors pop, usually it's an unstable amp & not just a bad output tube. Grid stoppers, fuses, even OPTs ( :cry: ) go South, too.

Anyhow, a stable output section depends not only on components, but layout and basic design. Short leads and uncluttered layouts usually = long happy life. As well as GOOD TUBES!

Kinda like this amp's layout (note the 5-watt screen grid resistors on the output tubes):


Nope. It was a bad tube. I wrote about it here:

viewtopic.php?f=13&t=74557

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 30, 2012 6:32 am
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Bluesky, ok thanks! Yeah, excessive current draw will pop the screen resistors. But, should not have an effect on the bad tube. It's still going to draw excessive current.

With some output tube types (esp big ones, like KT-88 or ones that are really pushed hard, like EL34 amps) --- it not uncommon to go to 1.5k-ohm of bigger screen resistors.

ROT (Rule of Thumb) --- for 425-450VDC on the plates. A 470-ohm resistor drops the VDC about 5 volts. The more drop = more stable tube, when it's pushed. But, the more effect on the tone (esp OD). There are some output tubes that really do not like being driven into OD. :(


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 30, 2012 10:05 am
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Alright...so it can be an issue, but it's not my immediate issue.

So what's my next move guys? Those Filter caps have been 100% mapped out. Should I test the 2x100k resistors? Anything else causing this great tone I need to look at?

Thanks

Bmw that amp looks amazing, so clean.


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 30, 2012 10:41 am
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:D Parkside, test ALL resistors in both the MAIN POWER SUPPLY (B+) and the BIAS POWER SUPPLY.

Be sure there is proper continuity to the components. Test for bad solder points and funky wiring. Once all the caps are replaced and any out-of-spec resistors. Pull all tubes. Have the speakers plugged in. No volume. No input.

REMEMBER: High DC Voltage can kill you. No lie. It's no sin to think about an amp tech doing this work.

If you decide to do the work--- use ONLY ONE HAND to probe the circuitry. DO NOT put the other hand on the chassis (it's an unconscious thing, that most ppl have to avoid). NO DRINKS! I know it sounds stupid, but there are a lot of stories of fried amps and techs and cold beer. Use a GOOD DVM. One with pro quality clips and probes. I like Flukes.

Have the schemo and your drawings posted near the amp. So you can easily see them without having to fumble around. A solid amp cradle of some sort is a must. Good lighting and magnification. I like Luxo.

One last thing before firing up. Check the sockets tangs (from the chassis side). They should be tight and not worn. Replace any hinky ones, now. While you have the chassis out and the caps drained.

Use the lamp current limiter. And the series of light bulbs (range of wattage, as in article). All ok?

Now, start measuring voltages. At each test point laid out in the schematic. they should be within 10-20% of rated values. Remember that your outlet voltage maybe a bit higher than the voltages, when the amp was made. So, it's not uncommon to have higher readings, even with the fact that there are no tubes to draw current.

These must be a steady voltage readings. Even if you tap on sloder points and components with a WOODEN chopstick. Both B+ points and the BIAS SUPPLY.

Good luck & keep me informed! :D


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 30, 2012 11:09 am
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Parkside, test ALL resistors in both the MAIN POWER SUPPLY (B+) and the BIAS POWER SUPPLY.

OK. Basically I will test every resistor that exists in the drawings I made (or in other words, every resistor directly connected to the Filter cap

Be sure there is proper continuity to the components. Test for bad solder points and funky wiring.

I will have a better look but I have looked, and probed and everything seems solid.

Once all the caps are replaced and any out-of-spec resistors. Pull all tubes. Have the speakers plugged in. No volume. No input.

Once I have my dim bulb meter built and connected, its safe to power up the amp without tubes? (no input cable, volumes turned off?)

REMEMBER: High DC Voltage can kill you. No lie. It's no sin to think about an amp tech doing this work.

Got it.

If you decide to do the work--- use ONLY ONE HAND to probe the circuitry. DO NOT put the other hand on the chassis (it's an unconscious thing, that most ppl have to avoid). NO DRINKS! I know it sounds stupid, but there are a lot of stories of fried amps and techs and cold beer. Use a GOOD DVM. One with pro quality clips and probes. I like Flukes.Have the schemo and your drawings posted near the amp. So you can easily see them without having to fumble around. A solid amp cradle of some sort is a must. Good lighting and magnification. I like Luxo.

Got it, I take work like this very seriously.

One last thing before firing up. Check the sockets tangs (from the chassis side). They should be tight and not worn. Replace any hinky ones, now. While you have the chassis out and the caps drained.


How do you test them? I know they were very loose before, which Is why one socket was replaced, and I now have metal tube harnesses on the Power tubes.

Use the lamp current limiter. And the series of light bulbs (range of wattage, as in article). All ok? Now, start measuring voltages. At each test point laid out in the schematic. they should be within 10-20% of rated values. Remember that your outlet voltage maybe a bit higher than the voltages, when the amp was made. So, it's not uncommon to have higher readings, even with the fact that there are no tubes to draw current.


OK.

These must be a steady voltage readings. Even if you tap on sloder points and components with a WOODEN chopstick. Both B+ points and the BIAS SUPPLY.

OK.

Im going to start with the resistors and get back to you before anything gets plugged in.

Thanks


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 30, 2012 11:28 am
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" How do you test them (sockets)? I know they were very loose before, which Is why one socket was replaced, and I now have metal tube harnesses on the Power tubes. "

Get a magnifying glass and inspect the tangs. Look for spread tangs and worn marks where the pins insert. Bent and loose tangs should be tightened. I like using a stainless steel Dental probe:

http://www.widgetsupply.com/category/dental-probe.html

If the tubes are still loose even after "tightening" or you have noise when slightly rocking the tube, you may want to think about replacing the socket.

" Once I have my dim bulb meter built and connected, its safe to power up the amp without tubes? (no input cable, volumes turned off?) "

Volume(s) set to zero.

Yes! Have no tubes in, when first firing-up, as per instruction on website. READ THIS CAREFULLY! It describes the step-by-step process of using a limiter.


http://antiqueradio.org/dimbulb.htm

Good luck!


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Nov 04, 2012 9:17 am
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Parkside, I sure hope all is going well with that amp. We are kinda worried about using this model as a test bed, for learning high voltage receiver circuitry. (at least I am) :)

Well, it is a bit safer than learning electronics on an old Hallicrafter transmitter. :lol:

Be safe & good luck!


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Nov 05, 2012 6:43 pm
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Hey BMW, guys

Sorry it's been a while since I last posted, but I guess I was defeated,and had to get back to gigs.

I ended up swapping out the 2x46uf caps in the power rail for a couple F&T 20ufs (to match the OEM spec).

I lifted the legs of all the resistors in the B+, all they were all to spec. I changed the 1K half watt resistor to 1 watt to match the schem.

Built a dim light bulb tester, put it all back together and tested it, no change in sound.

...I figured at that point it was time to bring it back to my good tech, and let a pro take over. Whatever it is, I don't get it....

I submitted it for repair with the proper schem, in the hopes he will be able to figure it out.

I'll keep you posted.

Thanks for everyone's help, I learned a ton about amplifier repair, and feel I will be much better at troubleshooting amps

Cheers guys


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Nov 05, 2012 6:53 pm
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Parkside, good to hear from you. The more you know about these amps, the better. Even if you don't do the work. Just like cars.

If you become more interested in working on amps, maybe look for a Princeton Reverb to restore. The general concepts of input stage, gain/EFX/tone control stages, phase inverter function, and push-pull output stages are basically the same. And lot easier to work on. New replacement parts are readily available.

Minus the Ultra-linear setup. And a different style of phase inversion (but principle is the same).

Even simpler, is the single-ended Champ amp.

Anyhow, I hope the tech can bring the amp into spec. And you can get that thing to wail. When you look up sound projection, you see a photo of a TR. :D


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Nov 05, 2012 9:36 pm
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ParksideDrive wrote:
Thanks for everyone's help, I learned a ton about amplifier repair, and feel I will be much better at troubleshooting amps

Cheers guys


I think you made a wise choice.

Echoing what BMW had to say, you might want to consider buying a small, cheap amp to work and learn on. I converted a modern Champion 600 to a 5F1 clone with a few simple mods:

viewtopic.php?f=13&t=63259

You might also want to consider building an amp. I built a 5F6A Bassman clone:

viewtopic.php?f=13&t=65668

but there are many smaller amp kits available.

Either way would be a great way to learn about amp design and construction.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Nov 17, 2012 8:02 am
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YANLI wrote:
It is not known what function those capacitors are performing in the circuit without tracing things out and comparing the schematic and amp. Without that knowledge, replacing them with different value caps MIGHT make the amp sound better or it could make the amp sound worse or even damage the amp. It is not wise to randomly replace parts without knowing what those parts do.


https://www.youtube.com/watch?v=PTv56Ef ... ature=plcp


You are way late to the party as that issue has already been thoroughly discussed by much more knowledgable people than you.

Oh, and spamming the forum with your stupid shoe advertisments is not appreciated. Your post has been reported to the mods.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Mar 10, 2013 3:13 pm
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well guys.... Thought I'd post an update, but first a review.

Bought the amp, knowing that it had a few minor issues, but it sounded good. (though I never gigged with it )

Tech #1 Changed some pre amp tubes, and did some minor repairs. I noticed some more issues and took it to tech #2

Tech #2 showed me a few other problems, and told me to take it back to tech #1 (under warranty)

Tech #1 Changed a loose power tube socket, changed all the filter caps. It still sounded bad

Tech #2 worked on it. Repaired some bad tack soldering done by tech #1. Gave it back...it still sounded bad

I spent about a month learning everything I could on this forum, tried to troubleshoot as much as possible. But, the prevailing wisdom was "this amp is beyond you, take it to a qualified tech"

Back to tech #2. it sat in his shop for 3 months. I got it back at Christmas and took it to Tech #3

Tech 3 listened to it a bit and said..."its the speakers" I took it home and hooked it up to a separate 2x12, it still sounded bad.

Took it back to tech #3. He said: "the speakers aren't great, but, theres something else.." He then changed both phase inverters, and gave it back. This was yesterday.

Took it to a gig. sounded terrible as always on the low notes. This was a big room with a big crowd, and the amp was at a healthy, loud, club volume. Putting distortion pedals in front of it, even worse. My mind set at the time "This amp can't handle intense current, from either low strings, or dirt pedals, or both" higher notes still sound pretty sweet.

....Then it got really quiet, the pilot light flickered a bit, then it turned off. I had another amp handy and finished the gig. Checked the fuse, it was fine. Pilot light still turned on, tubes still lit. but no sound. Got home and hooked it up to my 2x12 cab, sounds back (still sounds a bit dirty to me, but at least theres sound)

I think I blew the speakers? But why did the amp shut itself down?

I have to say, this has been an epic disaster. I bought this thing in August, its been in my possession for maybe 3 months since then, and no tech seems to be able to really comprehensively solve the problem. A lot of trial and error. I would sell it, but I feel it really dishonest to just hand this mess to another local player.

I am not a tech, and don't know the technical information necessary, and, the tech's arent really cutting in either (3 over 7 months for those keeping score)...anyone who wants to look at the symptoms and say "its probably this or that:" would be appreciated.

Thanks


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Mar 10, 2013 9:01 pm
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Parkside, when you say the amp " turned off", do you mean that the sound was off, but the pilot lamp was still lit? (yes, I noted the lamp flicker) Art

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