SF Twin breaks up too early - help

John Phinney · Post by **John Phinney** » 14 Mar 2013 10:37 am

The amp is a 1971 Fender Super Six Reverb that has been transplanted into a head cab. Recently replaced the filter caps, all the electrolytic caps and the brown turd caps (left the ceramics in), swapped the plate resistors to MOX, got new power and preamp tubes (JJ fender twin set from tubes and more), had the death cap removed and 3 prong power cord fixed, added the bias control mod, and had a local amp tech bias it. I told the amp tech to bias it "colder" because I wanted as much clean headroom as I could get.

My signal chain is GFI (w/GFI-II pickups) --> Sarno Freeloader --> Telonics VP --> Fender Super Six (vibrato channel input 1/channel volume 6-7/master 6-7) --> 1x15 cab with a ceramic magnet Black Widow

I've been playing it for a couple months at gigs and rehearsals and it breaks up too early for my taste. I was hoping it needed some burn in time, but I'm past 40 hours of playing time and it's not getting better. When I was strictly a guitar player I had a SF Twin and I hated that I couldn't get it to break up, now I've got one that won't stay clean. My kingdom for a time machine!

I swapped out v2 for an old RCA 12AT7 and v4 for an old RCA 7025 for a gig last night, but it was no help and the amp actually seemed to be more gritty, annoyingly gritty with the volume pedal opened up.

I did try plugging into input 2 on the vibrato channel at a rehearsal and it did help a little, but not enough. I will run the PSG through input 2 at my next gig.

What can I do to increase this amp's clean headroom?
Is this a matter of having the tubes re-biased "colder"? Maybe a different set of power tubes?

Tim Marcus · Post by **Tim Marcus** » 14 Mar 2013 10:52 am

you may be blowing out the first preamp stage with an impedance mismatch or too much signal from the volume pedal.

If you clip the 25/25 cathode bypass cap you will get much more headroom from that first gain stage. Theoretically a 12AX7 with an 820R resistor and no cap will pass up to 1V peak to peak with no distortion. Dave Funk recommends that configuration for steel guitar players. However, you can probably leave the 1k5 resistor in there and only remove the cap. If the cathodes are shared, you may want to separate them out and control the gains that way.

It might also be as simple as a worn out input jack or corroded wire to the tube grid. You should have clean headroom for days with a Fender with a 12AX7 in V1 and V2

Stephen Cowell · Post by **Stephen Cowell** » 14 Mar 2013 12:32 pm

Turn the channel volume down to 4/5 and dime the master... there's no reason not to. Fender preamps start to clip above 4, even for normal (non-steel) pickups.

John Phinney · Post by **John Phinney** » 14 Mar 2013 2:01 pm

Thanks guys. I will try turning the vibrato channel down to 4-5 and diming the master first.

Tim - Which cathode cap should be clipped? Section 1 or section 2 off v2? Here's a pic of the part of the board I believe you're talking about.

Edit: Whoops in my haste to get this image done I circled the cathode bypass cap off v1 and labelled it "section 2". I realize its the cap right next to it. Unfortunately its hidden under the Orange Drop.

Ken Metcalf · Post by **Ken Metcalf** » 14 Mar 2013 3:32 pm

I am not an amp wizz but I own a 67 Twin Reverb, 72 Deluxe Reverb and a Fox 4-10 Bassman.
Most guitar players like tubes after they wear in a little. But for me playing steel or tele... tubes lose the edge after a couple years of regular use.
Also amps I have owned that are over 30 years old benefit from new speakers.
The Deluxe was sort of dead sounding and a couple new JJ 6V6s really added sparkle, presence and clarity.
A new Weber 12f150 also added more sparkle, presence and clarity making the amp really shine with a Telecaster.
I can hear a loss of volume and clarity when tubes start to wear that seems fairly obvious to me.
Others well say those tubes are fine but I like amps that cut and will part your hair.
Even if you have a flat top.

This type of mod... will give much more bang for the buck than new pick-ups or effects!

Ken Metcalf · Post by **Ken Metcalf** » 14 Mar 2013 3:38 pm

P.S.
In a really loud band most amps are not loud / clear enough turned up past 5-6.
This is why I have a rack with a Stewart world 1.2
Silly to dime a session or twin and be lost in the mix...
Huh what did you say...

Tim Marcus · Post by **Tim Marcus** » 14 Mar 2013 4:02 pm

follow pin 3 of V2 - thats the cathode of the first triode, and the initial preamp of that channel

if its a shared cathode ground, better leave it alone or go through and separate it from V1 - hard to see in that picture

John Phinney · Post by **John Phinney** » 14 Mar 2013 6:05 pm

Ken - I have thought about changing speakers or even going back to my NV400. I like that I can run my tele, uke, banjo thru the Super Six. I used to bring a 2nd amp before and def don't miss leaving it at home. Compliments on your amp stable btw.

Tim - Both 25 25 caps do share the same socket going to ground. Both 1500 resistors also share the same socket going to ground. Looks just like the standard layout on the Fender Model "Twin Reverb -- Amp 100W RMS" layout. Are you saying I need to run the 1500 res and 25 25 cap connected to v1 cathode to ground separate from the ones connected to v2 cathode?

Dan Beller-McKenna · Post by **Dan Beller-McKenna** » 15 Mar 2013 3:06 am

Not to be discouraging, because this issue is often solved by one or a combination of the suggestions above, but I had a silver face dual showman reverb (basically the same amp as yours) last year that was in and out of the shop and we were never able to tame the breakup. Made it a dynamite rock and roll amp for a six-stringer, but pretty useless to me for steel. Had to sell it.

Never did nail down the reason; could be one of the transformers, in which case it might not be worth addressing. Hopefully your story turns out differently and better.

Dan

Bob Carlucci · Post by **Bob Carlucci** » 15 Mar 2013 5:33 am

I have used Fender tube amps of every type for steel and guitar since I was a young kid.. Name the amp, and I have had at least 2 or 3 of them in the past severaldecades... They almost all broke up too early when pushed with a loud band.. The best [cleanest]were the fender 140 head[Rivera era], a Super Twin Reverb, and a Fender 75 [Rivera era] and a a Quad Reverb... These amps handled the input of a pedal steel the best of the dozens of Fender amps I have used with pedal steel.
Twins, Showman, Showman Reverb, Bandmaster, Bandmaster Reverb, Super, Bassman, Tremolux, Bassman 70, all broke up too early depending on the tubes, band, volume .. Lower volume gigs were ok, but IMHO, Fender preamps weren't designed to handle the input signal a modern steel can generate... Lightly wound pickups such as I had on my Carter and Fender steels did better than lets say the 18 K pickups I have had on Buds, MSA, and Williams guitars.

I had a little better luck staying clean with 100 watt Traynor all tube heads, and even with Peavey all tube heads in the Classic series.. Classic 50, Classic 100, and Classic Blues[ very rare] amps stayed clean better than their Fender counterparts, .. I can't say why that is, but they did.
However, the Peavey tube amps never sounded all that great for steel, so I always went back to Fender as far as tube amps went.
Some steel players swear they can keep their Fender amps clean at any volume, but I never really could. These days, I am not playing as loud, but still have concerns about "staying clean".. I use an old Peavey SS head, and it seems very able to stay clean even at volumes where my various Fenders would be into full blown overdrive... I love the sound of Fenders, but never could keep the majority of them as clean as I wanted to.. At least at the volumes I used to play at, which were ... well,, lets just say, substantial. bob

Tim Marcus · Post by **Tim Marcus** » 15 Mar 2013 8:23 am

John Phinney wrote:Tim - Both 25 25 caps do share the same socket going to ground. Both 1500 resistors also share the same socket going to ground. Looks just like the standard layout on the Fender Model "Twin Reverb -- Amp 100W RMS" layout. Are you saying I need to run the 1500 res and 25 25 cap connected to v1 cathode to ground separate from the ones connected to v2 cathode?

as an experiment - I would just lift the positive side of the 25/25 cap going to pin 3 of both tubes and see if that takes you in the direction you want to go

John Phinney · Post by **John Phinney** » 15 Mar 2013 8:35 am

I'm okay with a little grit at the top when I'm pushing the amp. I've recently played through a couple different 65 Twins, one was OG the other an RI, at a couple different rehearsal studios and both those amps gave me the sound I was expecting. I'm hoping I can get a similar sound from my amp, but I'm starting to gather that mine just might be a bad apple for psg. I'm hoping switching back to the new preamp tubes and using Stephen Cowell's suggested settings will help. I'll test it out at my gig tonight.

Thanks for your reply everyone.

Rich Santucci · Post by **Rich Santucci** » 17 Mar 2013 1:54 pm

John,

Has anyone measured the plate resistor values? They should all be 100K or close to it. I notice in that picture that one resistor has a blue line on it, I can not really see the other colors very well, but that should be a 100K resistor as well, like the others. If that resistor is to low of a value, it may be your problem. Plus those resistors all look original and should probably be replace as they tend to absorb moisture and get noisey.

Ken Fox · Post by **Ken Fox** » 17 Mar 2013 2:02 pm

The resistor on channel 2 with the blue line is a 56K. A common sub for moving the mid slope to around 750hz. At 100K the mid slope is around 450hz area.

John Phinney · Post by **John Phinney** » 17 Mar 2013 3:00 pm

Rich - Good eye. It is a 56k resistor that I swapped out on the vibrato channel to get the mid slope that Ken Fox is talking about. I found that particular resistor swap mentioned in a post in the forum archives about Twin mods. I believe it was in a post by Brad Sarno.

The good news about this problem is that going into input 2 on the vibrato channel and running the channel volume at 4 with the master dimed was close to as clean as my NV400, but at the gig Friday after the 1st set I was told I needed to turn up because I wasn't cutting through. With the vibrato channel volume at 5 and the master dimed I was able to cut through and found the grit to be acceptable. I also put the new JJ 12AX7 tubes back into v2 and v4. Those old RCA's were definitely a part of the problem.

I played another gig last night and those settings worked great. Maybe the new caps are finally burned in too, but the amp is finally starting to sound the way I thought it should.

Thanks to Stephen Cowell and everyone else for their help.

Rich Santucci · Post by **Rich Santucci** » 17 Mar 2013 4:27 pm

Ah yes, 56K resistor the change the slope of the tone circuit. Makes a big difference! I wasn't thinking of that.

William Hughes · Post by **William Hughes** » 17 Mar 2013 5:02 pm

Bias set to cold can cause crossover distortion.

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SF Twin breaks up too early - help

SF Twin breaks up too early - help

Early break up