Need help... 1976 Fender Twin Reverb w/ Master volume content.

pdf64

Member
Messages
8,929
Yes, but I was addressing the "scope bias" proposal which wouldn't be possible either without first modifying the bias circuit
Sorry, I missed that.

The UL TR back panel has a humdinger heater balance at the far end and a bias balance between the speaker jack sockets and the footswitch RCA sockets.

As stock, there's no control underneath, unlike most BFs and earlier SFs.

The humdinger can get zapped when a power tube shorts, resulting in excessive buzz; the clue is often that the lowest buzz setting is at a rotation extreme, rather than somewhere on the track.

Pete
 

Smgbad

Member
Messages
2,270
Ok... I am completely in love with this amp. The static that was occuring in time with the vibrato seems to have subsided, and now there is just some very minor random background static thats hard to hear but defnitely there for the firsrt 5 mintes or so that the amp is on. I'm assuming this is likely due to the caps being nearly 40 years old and would be fixed by doing a cap job.

I am familiar enough with amp repair to be dangerous, and am fairly good with a soldering iron. I have a new cheaper Weller pencil-style iron and a lot of time on my hands currently. Thinking of doing it myself as I haver no income currently and am a firm believer in doing things yourself if you feel yourself to be competant and brave enough to try new things. Teach a man to fish... right?

Is the cap job on this thing fairly easy to do? Could someone be so kind as to make me a list of all the parts I'm going to need? I looked online and couldnt find a definitive list. Will my regular pencil style (not solder station) Weller soldering iron do this job ok?
 

pdf64

Member
Messages
8,929
From http://www.webphix.com/schematic he...n.com/fenderamps/twin_reverb_sf_135_schem.pdf

220uF 285V x 2 (Consider F&T 220uF 350V)
20uF 500V x 3 (Consider F&T 22uF 500V)
70uF 100V x 1 (Consider F&T 100uF 100V)
80uF 75V x 1 (Consider F&T 100uF 100V)
25uF 25V x 5
5uF 50V x 1

Replacing the above won't guarantee the minor noises you mention get fixed; I think that tubes / plate resistors are the most likely causes of this.
I suggest that all resistor values are checked; some won't measure correctly due to being in circuit, maybe you could note the measured values on a schematic and post it.
Pete
 

Smgbad

Member
Messages
2,270
I have a 100W Twin, which I'm seeing snippets in my research suggesting is uses different values for caps than listed above. The only kits I'm seeing online offer the following values:

2 x 100uf/350v
3 x 22uf/500v
7 x 25uf/25v
2 x 100uf/100v

Anyone know if this is correct before I try to source them? Also, if anyone has a set of the correct caps laying around or in a parts drawer, let me know as I may just buy them off of you to save some $.
 

HayekFan

Member
Messages
1,524
So the reverb and trem work on both channels? Definitely not normal. And the overdrive with the master volume sounds good? Again, not normal.

Normally with these amps you get very little overdrive with the master volume -- just a little bit of grit with the channel volumes dimed. Pulling the pull boost gives a fair amount of distortion but it sounds awful.

I agree with the other poster that this amp has been modded, and probably pretty extensively. Replacing the filter caps is a good basic maintenance step but in this case I'll bet the noise is coming from the modded circuitry.

Here's the schematic for your amp:

http://ampwares.com/schematics/100W_Mast_Vol_SF_Twin_Rev.pdf

The caps you listed in your last post are correct though some are slightly uprated from the schematic values which is okay. Also, the 25uF/25V caps are cathode bypass caps, not filter caps, but it makes sense to replace them too since they're electrolytics. However, if the amp has been modded their values may have been changed.

In fact, if the amp was modded extensively the filter cap values may have been changed as well. Your best bet might be to open the amp and see what their values are and just match them. The main filter caps will be under the sheetmetal cover on the underside of the amp.

Any chance you could post a pic of the circuit board? It would be interesting to see what's going on in there.
 

Smgbad

Member
Messages
2,270
I took everything apart and here's what I was able to deduce thus far.

1. All the caps and components "appear" to be original. I am assuming someone simply bridged the two channels as (after researching) I found that to be an extremely common mod to these amps. One mod site claimed it to be more common even than the all-to common blackface mods.

2. All of the tubes are original except V3 which is an older EHX 12AT7EH. I swapped that with a Phillips JAN 12AT7 and found I actually liked the reverb better with the EHX.

3. The vibrato static noise only pops up rarely, but mostly only in the first 10 minutes of playing. It literally follows the throb of the vibrato and cannot be heard when not playing. The tremolo on this is fairly quiet when not playing, so I lucked out there as these opto-bugs can be quite noisy. There is also some random minor background static when first turned on for the first 5-10 minutes or so. Don't know if this is the tubes, dirty sockets, or maybe the super old caps...

4. None of the resistors look fried. The first 100uf filter cap has one of those tiny bubbles formed on the positive end ring, but all the others look fairly nice.













 
Last edited:

HayekFan

Member
Messages
1,524
Wow, I was expecting to see some serious modding going on, but yeah, it looks original in there. My thought now is that either it's got a simple mod where you just move a few wires to bridge or cascade the two channels like you mentioned, or there's something funny going on with the filter caps. Sometimes bad filter caps can let the signal ride around on the supply rail and get into places where it shouldn't be. You could wind up with, say, the normal channel bleeding into the reverb driver so that you wind up with reverb on the normal channel. You might try tracing the circuit against the schematic to see if you can spot any mods, and I do think you're on the right track replacing the filter caps.
 

HayekFan

Member
Messages
1,524
Okay, I think I see the mod that gives you reverb on both channels:

ReverbModSmall_zps630eca67.jpg


That black wire that loops back is from the normal channel. Normally it would continue on as a twisted pair with the white wire and feed the input of the phase inverter through a 220k mix resistor, but here it probably connects to an eyelet (circled in green) that is the input to the reverb circuit. Also connected there should be a 3.3Meg resistor (orange-orange-green stripes) and a 10pF cap.

This seems to be the mod described in this thread:

http://www.tdpri.com/forum/amp-central-station/261427-fender-twin-reverb-both-channels.html

One of the posters mentions something interesting, which is that if you feed the reverb input directly with the normal channed signal rather than through a series 220k resistor, it will be louder. That might be part of why your amp (apparently) has a satisfying amount of overdrive: it's getting a hotter than normal signal from the normal channel.
 

Smgbad

Member
Messages
2,270
I think somewhere along the way it was inferred my overdrive is acceptable. To clarify, the overdrive is awful as seems to be the case with all of these amps. The only way I've been able to get overdrive really is to turn the channel volumes past about 7. More to the point,I am in love with the clean and barely overdriven tones of this amp. The tremolo and verb sound absolutely gorgeous in comparison with the BF and SF Princeton, Princeton Reverb, and DR models I've owned.

I see what you mean about that mod, and believe you are correct. I'm ordering the caps today or tomorrow, and am thinking of just buying the 5 filter caps and the two bias caps. All of the preassembled "kits" come with the caps for the main board as well. The videos I've watched on the cap change also mention to change the board caps. It has been mentioned that these won't go bad, so why would so many people say they should be replaced? I will gladly do it if it's truly necessary.

Also, I've read that the big navy blue blob caps could benefit from being replaced as they are widely regarded as being quite bad in comparison with their blue molded or orange 60's counterparts. In one or two cases in this amp (as barely viewable in the pics) there is some hardened clear goo on th blue blob caps. Is this a leak that hardened or is it some glue of some sort? If it is glue, it's a very strange placement for it.

Thx again for all the help you guys are giving! Special thx to Hayekfan!
 

smolder

Gold Supporting Member
Messages
14,773
I'm ordering the caps today or tomorrow, and am thinking of just buying the 5 filter caps and the two bias caps. All of the preassembled "kits" come with the caps for the main board as well. The videos I've watched on the cap change also mention to change the board caps. It has been mentioned that these won't go bad, so why would so many people say they should be replaced? I will gladly do it if it's truly necessary.

The white caps on the board are electrolytics and they do dry out, leak, and go bad. The blue caps on the board rarely go bad, and are pretty critical to the tone of that amp. If it were my amp, I'd be replacing ALL of the white ones, and wouldn't touch any of the blue ones unless they showed signs of leakage... and like I mentioned, that rarely happens. Hope that is helpful.
 

sector9

Member
Messages
573
If it were my amp, i would turn it on, wait 5 minutes for the static to stop, then play it. Problem solved!
 

Smgbad

Member
Messages
2,270
If it were my amp, i would turn it on, wait 5 minutes for the static to stop, then play it. Problem solved!

While this has been my solution thus far, I do know that the amp is living on borrowed time. If I let those filter caps go much further (especially since one of the 100ufs has a small bubble showing) I risk MUCH larger problems introducing themselves. From what I've read, some of those can be catastrophic. Not saying you are wrong... just saying the filter caps and bias caps should be changed asap.
 

HayekFan

Member
Messages
1,524
I think somewhere along the way it was inferred my overdrive is acceptable. To clarify, the overdrive is awful as seems to be the case with all of these amps. The only way I've been able to get overdrive really is to turn the channel volumes past about 7. More to the point,I am in love with the clean and barely overdriven tones of this amp. The tremolo and verb sound absolutely gorgeous in comparison with the BF and SF Princeton, Princeton Reverb, and DR models I've owned.

I see what you mean about that mod, and believe you are correct. I'm ordering the caps today or tomorrow, and am thinking of just buying the 5 filter caps and the two bias caps. All of the preassembled "kits" come with the caps for the main board as well. The videos I've watched on the cap change also mention to change the board caps. It has been mentioned that these won't go bad, so why would so many people say they should be replaced? I will gladly do it if it's truly necessary.

Also, I've read that the big navy blue blob caps could benefit from being replaced as they are widely regarded as being quite bad in comparison with their blue molded or orange 60's counterparts. In one or two cases in this amp (as barely viewable in the pics) there is some hardened clear goo on th blue blob caps. Is this a leak that hardened or is it some glue of some sort? If it is glue, it's a very strange placement for it.

Thx again for all the help you guys are giving! Special thx to Hayekfan!

Hey no sweat. It's interesting to fiddle with silverface Fenders from a distance in cyberspace. The '70s Fender amps have almost no collector value but they're good - actually great - sounding workhorses. And you see them everywhere. I spotted one just the other day in a Lenny Kravitz video (http://www.youtube.com/watch?v=TmENMZFUU_0). They're like these humble worker bees that get the job done.

Regarding the overdrive, that was definitely a presumption on my part. You compared your Twin to some modern amps and I figured you were comparing overdrive tones. If you're getting just a little bit of breakup then that's normal for your amp, which is consistent with the originality of the circuit that we're seeing. But then the question is why the pull boost only gives a slight change in tone. It should be pretty radical. At some point you might want to troubleshoot the pull boost. It's not a "good" sound per se, but it's an interesting one and possibly useful in some situations.

Regarding the cathode bypass caps, I'm going to disagree with Smolder and say leave them. This amp is so original that I would try to preserve the interior aesthetics. Those caps just look cool. They're as white and crisp as Gavin McLeod's captain's outfit in Love Boat. Unless there's a pressing reason to change them, I'd leave them. And I wouldn't worry about the other tone caps either. The molded caps look cooler than the blob caps but I doubt they sound substantially better. Plus the proof is in the pudding: this amp sounds good as it is.

I do think replacing the filter caps is a good move. But I would save them, and I would try to remove them without cutting their leads.
 

Smgbad

Member
Messages
2,270
Ok... thanks for bearing with me here guys. A few more questions before I start ordering the caps.

In the pic below, I am counting 8 electrolytic caps on the board, yet from previous research and posts I've only heard of 7 mentioned. Are the ones I've circled the correct ones to remove, and if so, is it really 8?



As mentioned before, a couple of the big navy blue caps have a hardened epoxy-like material on top of them (seemingly random). Heres the ones... Are these leaking or should I just leave them alone?

 

HayekFan

Member
Messages
1,524
The hardened epoxy material on the blue tone caps is normal. I've seen it on several silverface amps that were operating normally. I think they were like that from the factory. It's really only electrolytic caps that fail over time, and those aren't electrolytics.

The white caps you've circled, on the other hand, are electrolytics. However, I'll bet they're okay, and I would only replace them if there's a clear reason to do so. The ones in the bias supply (on the right hand side of the board as seen in the pics) would be the first ones to replace.
 
Last edited:

Smgbad

Member
Messages
2,270
I wonder what those two top circled electrolytics (coming from what appears to be the trem/reverb footswitch rca inputs) do...
 




Trending Topics

Top Bottom