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Fender Super Twin Bass Head

#381
tyb507 said:
Hauled out the 301 and Super Twin the other night. Killer. Who's gigging with theirs? Let's see some gig pics!
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Man, I just got my Super Twin Reverb back from a (much needed) full service (and new tubes, because I just bought it as a chassis), and now I see someone within three hours of me has one of those old 301 cabinets. That's a tempting buy to run with this thing once I actually put it in a box.
 
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#382
tyb507 said:
...
For extra credit, who were we opening up for? (We are set up completely within the headliner's guitarist's third of the stage, and there's one more 4X12 and head that was being serviced by their tech while we played)
View attachment 5342663
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I'm guessing Dinosaur Jr. (I'm not sure I've actually listened to their music, but my inner teenage-self is still drawn to photos of multiple Marshall stacks lined up as someone's backline.)
 
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#385
Eli Brockway said:
Man, I just got my Super Twin Reverb back from a (much needed) full service (and new tubes, because I just bought it as a chassis), and now I see someone within three hours of me has one of those old 301 cabinets. That's a tempting buy to run with this thing once I actually put it in a box.
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I'm really happy with the 301/Super Twin! And 301s are usually pretty cheap, so you can't go wrong!
 
#387
Looks like I'm going to have to do some more servicing work on my mid 70's non-reverb Fender Super Twin amp, it needs some replacement pots for the stock preamp volume, treble, mid, bass, and presence pots, I think I know where I can order a full set of replacements from online within Australia, going to do it this coming week.
 
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#391
rufus.K said:
what cabinet did you use?
(Digging Heavy Blanket!)
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The Acoustic 301, a folded horn 1X18". Probably could have used 2! I sometimes use an Acoustic 2x15 with E140-8s in it, but this one is easier to move around and louder!
301ST.jpeg
 
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#393
Hello,

I will stop polluting the Fende Studio Bass Club! https://www.talkbass.com/threads/fender-studio-bass-club.809269/page-39#post-28314190 :)

Got one yesterday (a Super Twin) and, so far, so good:

- brand new power supply electrolytics already replaced by precedent owner’s tech;
- got it with the 6.25A fuse dead so expected something really wrong but…;
- starts fine without the power tubes!

My first hypothesis would be that one of the tube is misbehaving under working voltage and triggers the fuse or:
Something is amiss in the bias circuit (didn’t measured voltages yet and the two 80uf are brand new) and obviously, the power section is drawing too much current.

The sextet of Mesa 6L6GC STR-440 test good (70 to 80 percent) on my Precision tube tester but as you know, it means nothing except that their emission is ok.

More to follow during Easter break!
 
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#394
Ok, got the time to work a bit on the amp today:

- I’ve checked that all diodes were testing ok, they’re good;
- Measured the six 1.5k tied to pin 5: 1.5 to 1.7, good;
- Measured the 470 ohms and they were all over the place, from 850 ohms to 470 ohms so I’ve replaced all of them with tightly matched 470 ohms@3w;
- I’ve noticed that the two 100uf in the bias circuit were at 63v! Replaced them with 100uf@100v instead.

Then, I’ve fired the amp with only two 6L6GC’s at a time and… one immediately fireworked and the fuse blew! We have a culprit, one of the Mesa 6L6GC is dead! The five remaining ones didn’t tripped the fuse but some have sign of heat as their plates looks a bit warped. No red plating though!

So, I had two Ruby’s I’ve removed from my V4 (two are now in my son’s Garnet bass combo and the V4 has a quad of Preferred Series 6L6GC’s) and put one on each side with four Mesa’s and all looks fine. I didn’t measured the bias current for now but once the hum and tube balance were adjusted, the Super Twin was dead silent through an home made small 1x15 with a 4 ohms Peavey Black Widow.

Now, should I replace all other electrolytics? There are three 5uf@50v, two 25uf@25, one 750@3v and two 4uf@450v. All big power supply caps were already new when I bought it two weeks ago.

It’s over for now as the total eclipse will soon take place!
 
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#395
Oh well, I’ve replaced the two 4uf@450 and one 5uf@50v… and noticed that all the little electrolytics (except the two 4uf’s) were more recent and appeared to had been replaced already. So, I stopped here for the electrolytics.

However, one 12AX7, the 12AU7 and the 12AT7 were testing low emission on my tester so they were swapped for good used ones (old production).

I also did the 2.2k to 1meg swap in the distortion circuit as recommended in the wiki and honestly, it’s quite useable!

As for the bias:

504 vdc on the plates and 28ma, 21 and 24 ma on one side. On the other one, 27, 17 (!) and 24. The two lower ones are the old Ruby’s… I think I’ll have to do some shuffling with what I have in my stash (old 6L6GC’s and a spare quad of Preferred Series 6L6GC’s). Probably that adding a bias adjustment pot is in my planning too.

Quite cold biased!
 
#396
Ah, those white plastic dipped Mallory electrolytics are originals to the amp! I was sure that all the bypass caps were metallized like the two 4uf’s, my mistake. They’ll have to go too and I already have the replacements ready.

Anyway, even « cold » biased and with only a small 1x15, the sound is massive! I even tried briefly the two big square magnets 12 inches (CTS I think) once back in the cabinet (at low volume, don’t worry) and it sounded great!
 
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#397
Update:

- All electrolytics (old white Mallory’s) replaced;
- suddenly, fuse blowing first with the set of Mesa’s and two Ruby’s… and another Mesa fireworked (arcing);
- took old ANOS in the same current range… and fuses blown one after another and beautiful firework… again. Hum and red plate too, huh!
- one of the new 470 ohms grid resistor was reading open, probably took the hit when the Mesa 6L6GC arced!
- I’ve replaced the open 470 ohms, all the 1.5k to pins 5, cleaned and retentioned the sockets.

Then, I reinstalled my six old ANOS tubes but I was always tweaking the DC balance pot after each startup. I suspect one of the tubes was unstable. The fuse didn’t burned anymore but I wasn’t confident with the old tubes in the long run.

Being tired of it, I ordered a sextet of JJ’s 6L6GC’s and installed them. With the 2.7k bias resistor to ground, they were barely hitting 20ma at 507vdc plate voltage, 34% dissipation, way cold.

I then proceed to install a 4.7k bias pot and replaced the 2.7k with an 1.6k@3w. I tied the wiper lug and the adjacent lug with the resistor to ground (the other lug connected to the wiper of the DC balance pot), understanding than in case the wiper would fail, the bias will be very cold but won’t damage the components, right?

Now set at 51-52% dissipation but I’ll probably back it off at around 40. The sextet appeared to be really tightly matched with a difference of 2ma between the hottest and coldest tube! Verified with my VHT bias probe and DVM.

Yesterday, I bought two 16" x 12" x 48" pieces of wood and a roll of thin aluminium sheet so I could build my own head cabinet. I’ll reuse the top handle and cut the front grill to the right dimension.

At first, I thought I would try to cut the combo but it’ll look better with stained wood, IMHO.
 
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#398
Hello again,

I know the question was already been addressed from post #373 to 378 but anyway:

My local electronics store didn’t had any 750uf low voltage cap, the only approaching value was 820uf so I used it instead. Do I understand that there’s no benefit for such high cathode bypass cap value? Not sure I could even hear the difference it made going from 750 to 820!

Paging @Wasnex , @Pbman or @SimonKormendy53 ?

Thanks,

Patrice
 
#399
Well, I got an answer (sort of) with this excerpt from: Did THe Cap Job on my SF PA 100.

« Fender used that 750 uf 3V bypass cap on a couple circuit designs. You see it used in the Fender Super Twin, Fender Studio bass and Fender 140 too. Lots of people wonder why that cap value is so high. The reason they used that was that the larger the cap, the more lows that are boosted. Any size capacitor will transfer high range and it turns out a larger size capacitor passes the same highs, but with a larger amount of capacitance, more of the lower freqs are bypassed for more gain in that stage at all frequencies above that -3dB frequency. So in effect this is a form of shelving tone filtering. The closest equivalent is probably a 680 uf 10v axial. I am unaware of any other electronics manufacture that used the 750 uf 3V cap other than Fender. It must have been a good electronic component salesman that slipped them that part!

Lots of companies make a 820 uf 6.3V, like Nichicon, Panasonic, United Chemi-Con, but all in radial mount. The closest axial is Vishay / Sprague Aluminum Electrolytic Capacitors 800uF 15V (part number: 601D807G015FJ7) or 820 uf 16V (601D827F016FE1) size = 0.625 x 1.125 »

**I think mine is 820uf@16v…
 

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