Silverface Fender Twin Reverb Low Volume before Distortion
BJT3
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3rd June 2006
Old 3rd June 2006
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Silverface Fender Twin Reverb Low Volume before Distortion

Hello all, I recently purchased a 100 watt 1977 Silverface Fender Twin Reverb amp. I bought it to play my Fender Rhodes through. Anyway, when I got it I fell in love with it's sound. I'm having a couple problems with it though. I can't get hardly any volume out of it before it starts to break up (distort). Any ideas on what could be causing this?The amp has all new tubes the Blackface mods and the Master Volume circuit removed according to some receipts (from a reputable local tech) that came with it . It still has the original filter caps. I was thinking about replacing them. Is there a certain brand that sounds better than the others? Also it came with 12AT7's where the 12AX7's usually go. I was having bad microphonic problems so I switched the preamp tube with a 12AX7 I had laying around. Fixed the microphonics but I kinda lost the tone I had originally fell in love with. Are all 12AT7's gonna have that same problem, or is there a certain brand I could buy to solve this? TIA,
-Ben-
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3rd June 2006
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Quote:
Originally Posted by BJT3
... Also it came with 12AT7's where the 12AX7's usually go. I was having bad microphonic problems so I switched the preamp tube with a 12AX7 I had laying around. Fixed the microphonics but I kinda lost the tone I had originally fell in love with. Are all 12AT7's gonna have that same problem, or is there a certain brand I could buy to solve this? TIA,
-Ben-
What do you mean by that? Fender Twins should have (2) 12AT7 tubes in it. The Reverb driver (V3) and the Phase Inverter (V6). The rest should be 12AX7's.

Why don't you start off with the tubes in the correct positions so you can hear what a Twin Reverb SHOULD sound like before you go swaping around types of tubes, which will chage the gain and character of the amp.

Currently the Russian made Tung-Sol 12AX7 is the best sounding New Production preamp tube you can use in a guitar amp today.

-Tony
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5th June 2006
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Replacing the electrolytic capacitors in an amp that old is a must do & a good starting point. Twin Reverbs are noted for being hard & clean almost all the way to the top. If yours is distorting at very low volume, thats a very good sign that the filter caps need to be replaced. Replace the old caps & screen grid resistors, verify that you do indeed have a good matched set of power tubes & you may find that no additionial changes are necessary. You should be able to get this done by your local reputable tech for $150.00 or so plus cost of tubes. As the previous poster stated, Re-tube with tube values according to the tube chart...those tubes were selected by Fender for a reason.
BJT3
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6th June 2006
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tmarra and bluesman, thanks for the replies. Sorry if I wasn't clear in my original post. According to the service receipt (that came with the amp) it says the following has been done to the amp: Blackfaced, master volume circuit removed, bias rewired and set, new caps in the vibrato section, 3 new pots, and new tubes. For some reason it looks like all the 12ax7's were substituted with 12AT7's. The rest of the tubes match the tube chart in the amp. I had some 12AX7's laying around (from my Hammond Organ) that I put in so everything matched the tube chart. While this did give me a bit more volume before distortion (maybe 20% more?), it still distorted at a very low volume. I also found I much prefer the sound of the 12AT7's to the AX7's. That's where the question about finding lower microphonic AT7's came from. I read up on it a bit on the internet, and from what I could gather there is no danger in using these tubes as substitutions (either to the amp or myself). Is this correct? I recently read about these 12AT7WC or WA's that are supposed to be better as far as microphonics go. Anybody tried these? BTW, I'm certain the problem is not the fact that I have a Fender Rhodes going through it since the Twin is basically the amp of choice for the Rhodes and most everybody's only complaint is they can't get it loud enough to distort. Bluesman, I did not know that a symptom of filter caps getting ready to die is distortion at low volumes. Thanks for letting me know. I've heard that if one of those babies goes it can be really messy (black stuff?), let alone it could damage my transformer, etc.... I will replace these before I turn on my amp again. Is there any certain brand that you would recommend? I usually get my electrolytics from tubesandmore.com. Thanks again for your input,
-Ben-
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6th June 2006
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Hi
Very early distortion, try screen grid resistors (mentioned earlier).
ANY valve can be microphonic so it is 'pot luck' in many cases to find the good ones or PAY to get ones that have been double checked, Groove Tubes and others make this selection and chuck out the large percentage of 'duff' ones.
Substituting AT7 for positions where AX7 is specified and vice versa can lead to the anode resistors failing as the bias is incorrect. Most amps survive if they have suitably overrated resistors but you get the 'odd ones'.
Matt S
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7th June 2006
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Matt, thanks for the post. I took out the amp chassis today and put it on my workbench. After draining the caps, I tested the screen grid resistors on the Power tubes with my meter. They all tested good (well within tolerance). I then unsoldered the negative end of each of the Filter caps. They averaged about 20-25% over what they are labeled. I'm gonna go ahead and change them once my order from Tubesandmore arrives. Any other things to check that could be causing the distortion at low levels? TIA,
-Ben-
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7th June 2006
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Quote:
Originally Posted by BJT3
Any other things to check that could be causing the distortion at low levels?

early speaker breakup maybe.

which speakers you got in there ?
BJT3
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7th June 2006
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Adam, It doesn't sound like that kind of distortion to me. I think the speakers are ok. Thanks for your input though.
-Ben-
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7th June 2006
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Hi
Try getting the schematic of the web then measure as many DC points as you can find. The output stage is balanced so expect similar voltages on same pins of the output tubes. The cathodes of the small tubes are mostly at about 1.5 - 2.2 volt range so look for any that are very low. The phase splitter (little one before the outputs) cathodes are at about 60 volts and anodes should have similar voltages.
Voltages normally within 20% and you have to consider the loading of the meter sometimes.
Loose screws in the speakers?
Matt S
BJT3
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7th June 2006
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Matt, thanks again for the suggestions. The speaker screws check out ok. The distortion I'm hearing sounds more like overdrive or amp distortion than rattling or a blown speaker. I'll get a schematic and go through and see if I can find anything out of wack. I'm gonna try and contact the guy who did the mods and see if he could shed any light on it as well.
-Ben-
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8th June 2006
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it could be your 6L6s?
if it's only running on one you'll get less volume before distortion. the ouput from a rhodes is pretty hot so also try backing off the output if your amp's still distorting with new power valves. I think the 77 had master volume and preamp controls and are fiercely loud so with the master full up you should be able to dil in the amount of drive you want with the preamp (far left volume) and back off the master if it's too loud. that's what i'd do at least. hope this helps! It might be components but if sound is coming out it's probably power tubes
BJT3
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8th June 2006
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Owl, when the amp was on, I looked at the power tubes and didn't see anything that looked out of the ordinary. The tubes were recently replaced as of 12/05 (before I got the amp). I tried adjusting the Rhodes volume as well as the amp to try and maximize volume before distortion and also just to see what sounded best. It seemed no matter how I had it set the amp would always break up at about the same level. The master volume circuit has been removed so I didn't have that to adjust. After I got the amp I brought it and the Rhodes over to a friends to jam. While the setup sounds incredible at low volumes, I could nowhere near keep up with just a bass player and drummer at very low living room levels. If I tried to turn it up it just got really distorted, and it wasn't a pleasant sounding distortion like when you just start to overdrive the tubes. Thanks again for everybodys input, I'm sure sooner or later I'll get to the bottom of it. My new filter caps will be here in 2 days. I'll see how it sounds after I put them in. I may just take it into the tech that did the mods after that if it still is having the problem. If nothing else I can show him what's going on and see if he has any ideas. He also has a tube tester so maybe I could get him to test the tubes and make sure I didn't have any that went bad. Meanwhile, if anyone has any other ideas I'm still open for suggestions.
-Ben-
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8th June 2006
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Hi
Swap the AT7 driver if you haven't already. Also try new output tubes even new ones can fail either because you bounced the amp or it was just a weak one. New does not necessarily mean good.
I very much doubt new supply caps have anything to do with it but maybe a little one elsewhere is 'leaky' upsetting the biassing too much. Have you got the schematic off the web yet? Most Fenders are similar enough to translate from one to another, accepting that yours used to work properly once.
Matt S
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9th June 2006
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9th June 2006
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Take it to the 'reliable local Tech' and get it looked at. You could fiddle for ages and get nowhere. OK you have done the obvious which is good.
Paying a tech to sort it has a benefit of keeping his mortgage paid which in turn will mean that he will be there when things really go wrong. The cheapskate culture only works if you accept that you throw faulty goods away (old Neves, etc.).
Matt S
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11th June 2006
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Thanks again for all the good suggestions. I put in the new filter caps. I didn't notice any difference in sound, but glad I got 'em anyway. Won't have to worry about them for another 20-30 years hopefully. I got the schematic (thanks for the link Paully). Matt, I hear you about supporting the techs. I've been trying to get better at fixing my own gear not only to save money but for me it is much more convenient. The tech I know of is usually pretty slammed and if I brought it to him it may be 2 or 3 weeks before I got it back. I am glad to know of a good tech in my area though, in case I run into an absolute dead end with this thing. Couple questions, if one of the power tubes was bad, would I be able to see a difference by looking at it? Also, I'm wondering if maybe one of the blackface mods has something to do with the problem. What are the common blackface mods? is there a website that lists them? I'm gonna compare the amp to the schematic but thought maybe that would help me hone in on a few key areas. As a newby to the group, I hope I'm not wearing out my welcome with too many questions, I do appreciate the help though. Regards,
-Ben-
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11th June 2006
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Hi
Obvious valve faults are ANY bits rolling around inside and the inside of the glass going 'milky white'. There are other signs but it is pretty difficult to tell reliably.
You have a spare tyre for your car, get a set of (matched) output valves and some input types.
Matt S
Try the Tube Amp book available from Tubesandmore.com (written by Groovetubes guy) it has many schematics in it and repair tips.
Matt S
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26th June 2006
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Found one of my problems last night. I was looking at the speaker wiring and realized someone had it wired in series! So all this time the speakers have been putting a 16 ohm load on the amp. I corrected this and did get a bit of a volume bump, as well as a "healthier" sound, for lack of better words. Anyway, I'm still getting distortion fairly early. Probably could keep up with a drummer if we weren't playing too loud. I guess I'll have to spring for some new output tubes and go through with the schematic and double check all the mods this "reputable" tech did, since he obviously doesn't even know how to hook the speakers up right!
-Ben-
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3rd July 2006
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Hi there
I have a twin reverb in for repair right now with exactly the same symtoms as yours!

This has ALL new valves, ALL new screen resistors.
The strange thing is that after re-tubing the amp and changing the screen resistors (as a matter of course) the amp was fine and singing very nicely!
The boost on the master volume wasn't having any affect, and while I was playing it the volume suddenly dropped and was accompanied by a burning smell!I then found a 20MFD Cap in the power supply that had semi-leaked after replacing this the volume is still low and distorted.

When I get to the bottom of this I'll let you know what I found, in the meantime if you or any of the other guys has an idea post it here.

Thanks

Ampguy-UK
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3rd July 2006
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Hi
Check the value of all resistors with a multimeter or perhaps better, replace them all with good quality new ones. The Fender I looked at a couple of weeks ago had resistors that should be 100K measuring 139K, a bit wild even for 10% tolerance!
The 820 ohm feedback was at about 1K2 as well giving far more distortion than it should. At 10 volts out (at speaker jack) distortion was over 2% when I started. Replacing all resistors in the output section put it at under 0.3%.
Matt S
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5th July 2006
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OK, so I took the blackface schematics and systematically went through and tested the value of each resistor, capacitor and diode to make sure they matched. I found many variations from the schematic so I changed them to match the schematic. The only thing I left alone was the reverb circuit since I had read that the Silverface era Twins ran the verb a bit hotter which may be more desirable. I'm waiting on a order from Tubesandmore.com so I can complete it (just needed a few resistors and caps). The only things that I haven't tested are the transformers and new power tubes. I'm keeping my fingers crossed though that the changes I've made will fix the problem and I won't have to spring for the new power tubes right away. I think I'll go ahead and check the transformers before I button it up though. The parts should get here tomorrow so I'll hopefully be able to put them in tomorrow night and post with the results.
-Ben-
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9th July 2006
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OK, so I finished putting everything to match blackface schematic. When I went to check bias, I noticed the negative voltage measured on pin 5 of the power tube would only go down to negative 2 instead of the between -45 to -60 it should be at. I also noticed that winding was putting out 62.5 volts instead of the 48 listed on the blackface schematic. I ended up having to change the 470ohm 1watt resistor coming right off of the blue and red winding of the transformer to a 3k in order to get this down to the right negative voltage. Is this normal? Unfortunately, after checking everything and replacing anything that varied from the blackface schematics, my amp still sounds the same. I noticed after having it on for awhile the power transformer was getting pretty hot. I could keep my hand on it without burning it, but barely. Is this right? I biased my tubes at 19 dissipation watts. Next thing I'm gonna try is new power tubes, as soon as I can scrape up the money. I sure hope that's it. Sure would be nice to hear how this thing sounds when it's working properly.

-Ben-
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9th July 2006
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Hi
It is beginning to sound as if you have a shorted urn or two on the mains transformer. To check, pull out ALL tubes and leave it on for an hour, the TX should be barely warm after this time.
If your bias was 2 volts then you have probably trashed the output tubes (which may have killed the mains TX as well). If the mains TX does not overheat with no tubes, get the bias on pin 5's to be in the correct range. A winding of 62 volts is OK but when in action the DC on pin 5's needs to be - 30 to - 60 kind of range although -48 is where it 'should' be. Put the screen resistors back to 470 ohms.
The Fender circuit works, they have built millions so if yours doesnt then you have not found the true fault yet.
Matt S
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10th July 2006
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Matt, I will try and check the transformer the way you described. I need to pull all tubes including the preamp tubes correct? Do, I need to have the speakers plugged in? So basically what I did after I got everything checked and to blackface specs I put the amp on standby and checked pin 5. As soon as I noticed the -2 volts I checked to see if the power tubes were glowing or anything. The filaments were glowing but obviously not the plates or anything. I immediatly shut the amp off and fixed it so now it's around -56. I was hoping since my amp was in standby, I didn't fry the tubes, but I wasn't sure exactly how that works. My amp is currently biased to about 19 dissipation watts. The screen resistors are still the original 470 ohms. Thanks again for the suggestions.
-Ben-
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10th July 2006
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OK, so I took out all the tubes and left the amp on for an hour (testing the heat of the transformer occasionally.) After one hour, it was warm, (maybe a bit more than body tempurature?) It was definately not hot enough to burn my hand or anything. So what do you think next, gotta be the power tubes?

-Ben-
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10th July 2006
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Hi
How do you know you are biassed to 19 watts dissipation?
If this is true then your power tubes are probably OK and you have a fault (shorted turns) on the output transformer.
Is the rectifier a valve or silicon diodes?
Time consuming but you could repeat the all tubes out for a couple of hours, then with all in and on standby, then again all in, on but NO signal through it. I would expect that in each scenario the transformer will get progressively warmer although not too hot to touch.
If it is reasonably hot, ON with NO SIGNAL, then it may well be shorted tirns on the output transformer, which would make it distort early and make the mains TX hot (possibly what you started with!). There are simple tests you could make to check for shorted output TX turns but I don't have a similar amp to hand to give you possible results for such tests. Basically putting say 5 volts AC from a small transformer via a resistor (say 10 ohms) into the SPEAKER jack with the amp OFF and possibly the O/P tubes out, measure the voltage across the 10 ohm resistor.
This would give an indication of how much power is lost into the transformer. Unfortunately I can't tell you what such a measurement 'should' be as I don't have a transformer. If anyone else would be prepared to make this test for you it would be good. You don't have to dismantle your amp at all for this test as you can leave the tubes in (evertyhing switched OFF and unplugged).
Try the Fender field guide site for possible hints or tips or ask them a direct question.
Matt S
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11th July 2006
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I set the bias using the cathode resistor method described here http://www.duncanamps.com/technical/lvbias.html
I have the silicon diodes. I checked them with the diode setting on my meter and they checked out ok. I will check out the Fender Field Guide. Thanks,
-Ben-
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11th July 2006
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Talking Success!

After checking everything for correct value, I have found my problem ......... 1 seized loudspeaker!. On the way I found some other stuff that I most likely would have missed because the amp would have worked, but needed replacing. Like out of value resistors 2 electrolytics that had no capacitance what so ever!

The speaker had electrical impedance, but little movement. looking back in hindsight I should have plugged it into another cab, but it was a good exercise and good experience. Failure may have had something to do with the fact that the owner was running it on 3 6L6's...... It was too loud with 4!!!
So it now has 2 new Taydens in and sounds great.
Thanks for your help guys.
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11th July 2006
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Hi
So many guitar amps suffer from loose nuts. Sometimes inside the unit, sometimes out, sometimes both.
Matt S
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11th July 2006
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next question

Dunno if I should start a new thread for this, but has any body had problems with a Fender "twin amp" (1990's) changing channel by itself, but only when being played?
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