what mods to fender 135 watt twin?

Chris Erbacher · Post by **Chris Erbacher** » 27 Jan 2006 9:31 am

okay, i got a great deal on a 135 watt twin, and i stuck some e-120's in there, sings nicely, but still sort of mushy in the low end and not enough detail overall. i know about the healy mods and changing the caps, is there anything else that would make this amp really sing with my zb? i love the warmth that this amp has, my guess that the bias being set at 32 has something to do with that. i also have the option of changing the output transformer to that of a blackface twin for 75-100 bucks, is that a good deal? thanks in advance for your ideas...

Ben Jones · Post by **Ben Jones** » 27 Jan 2006 10:55 am

Chris , the plexi palace forums would be a good place to ask this question as well. Those people know their stuff. Here is a link to the fender section: http://www.vintageamps.com/plexiboard/viewforum.php?f=8&sid=08decaa56a04d1931986dd69bddcba52

Post by **Dave Mudgett** » 27 Jan 2006 11:25 am

If this amp is strictly for pedal steel, or perhaps real clean jazz or country guitar, I personally wouldn't change that output transformer unless I thought there was something wrong with it. The ultralinear transformer is a big part of what gives it the extra clean headroom, which is great for steel, IMO.

As has been said many times in many threads, if the filter caps aren't fairly fresh, that probably will make the most difference. I usually do one thing at a time to really see what the effect of each change is. If you take the 'shotgun' approach, you may not really know what the issue was.

Interesting to see so much interest in old Fender Twin-style amps right now.

Chris Erbacher · Post by **Chris Erbacher** » 27 Jan 2006 11:34 am

my thing is that i like the best of both types of sounds and yes this is just for steel. i like the brumley type of sound from a blackface, but i'm also a fan of the sound of garcia's zb thru his rig, which was clean. i just want sweetness and definition, but i like the growl also, so if there is a way to get that done, then i'd be super happy.

David Doggett · Post by **David Doggett** » 27 Jan 2006 12:39 pm

I agree with Dave M. I usually hate to see people mod amps to do things they weren't designed for. The silverface Fenders, and especially the later, more powerful ones with the ultralinear transformers, were designed to have lots of clean headroom, for loud venues in a time when amps weren't miked, and heavy distortion was not desirable. This makes them ideal for steelers, who need the clean headroom for volume pedal sustain, not sheer loudness. At the same time they have beautiful tube tone, but that stay clean even when pushed. Jazz guitarists also like that. It seems a shame to mess that up by going back to the staticky, crunchier, earlier breakup of the blackface amps. If you want that, and can't afford a genuine blackface, there are gazillions of guitar amps on the market at all price levels that create that for 6-stringers. There is nothing quite like the rich all-tube harmonics, but clean all the way to the top, that you get with a silverface Fender. I don't know if anyone is recreating that in modern amps. The whole world seems have gone to either sterily clean solid state amps, or dirty all-tube amps. Even Fender's '90s Vibrasonic Custom with the steel channel, and the new reissue Custom 15 Twin are both based on the black-face Twin.

You can get some grit in your Twin by cranking up the channel volume (preamp gain) and keeping the master volume (power amp output) low. But this is not great distortion, and seems to be what gives the "master volume" silverfaces a bad name among 6-stringers. You get the best distortion when both the preamp tubes and power amp tubes are driven. However, a Twin would be too loud at that point for any practical purposes in this age of miked amps. Cutting the output with a smaller transformer and altering the amp for early distortion just seems like such a waste, when so many guitar amps are designed from the start for that. The 6-stringer world, which outnumbers the steel world by a million to 1, has all the crunchy distorting amps it needs. While they disdain them, and insist on butchering them, I consider the late model silverface Fenders to be masterpieces of beautiful, clean all-tube tone. However, I agree that adding D120s or K120s or some other good modern speakers would be a good move.[This message was edited by David Doggett on 27 January 2006 at 12:40 PM.]

David Rich · Post by **David Rich** » 27 Jan 2006 1:24 pm

A simple, easily reversible mod for these UL amps is to disable the negative feedback loop. The easiest way to do this is to locate the 100 ohm resistor in the feedback loop and short it to ground. This resistor is connected to one end of the master volume. You can use the bright switch to flip between the mod and the stock circuit. Also, it's a good idea to remove the .01 cap in the feedback loop or it will be shunted across your speaker.

Ken Fox · Post by **Ken Fox** » 27 Jan 2006 1:28 pm

Change out all the electrolytic caps, if it has not been done! Both the cathode bypass and filters. I have seen so many of the cathode bypass caps drift from 25uf uppwards to 70-100uf! That pass way too much low end thru the preamp stages.

Also the older filters start to pass some DC and load down the plate voltage supply.

Check the bias. If possible change to adjustable. It can be in a range of 30-45 ma for this amp with a plate voltage of 490 volts. Most of these amps plates are at 490-500 volts. Takes a really tough 6L6GC to stand up to that plate voltage. I use JJ Telsa, but you need an adjustable bias with these tubes. I find they tend to run a bit cold and need to be cranked up a bit!

If you replace the OT with a 100 watter you will have to add a choke for the screen grid voltage! The UL taps were the choke, feeding the screens from the taps on the transformer.

These are great amps, just replace the old worn parts and truck on!
[This message was edited by Ken Fox on 27 January 2006 at 01:31 PM.][This message was edited by Ken Fox on 27 January 2006 at 01:34 PM.][This message was edited by Ken Fox on 27 January 2006 at 01:35 PM.]

Michael Brebes · Post by **Michael Brebes** » 27 Jan 2006 4:43 pm

Remove the capacitor that is across the wiper and high side of the Master Volume pot. This makes the pot actually usable.

Donny Hinson · Post by **Donny Hinson** » 27 Jan 2006 5:36 pm

Michael's suggestion is a good one, that cap boosts the highs (which this amp doesn't need). make sure you've got the right tubes in the preamp (12AX7A or 7025), and the phase inverter (12AT7). Many times, lead players will jockey tubes around in this amp to try to get distortion.

For a clean sound with good bottom and "bite", keep the mid control very low (3 or less), and the bass control fairly high (6 or higher), then try adjusting the tone with just the treble control (from 2-4).

Tony Prior · Post by **Tony Prior** » 28 Jan 2006 2:04 am

wheels..

James Morehead · Post by **James Morehead** » 28 Jan 2006 5:35 am

Trailer hitch.

Chris Erbacher · Post by **Chris Erbacher** » 28 Jan 2006 10:59 am

thanks guys, i spent some time with it yesterday and seem to have it dialed, but i am still going to change the caps and see if i can get some sort of half-power switch put on this thing, even on 2 it is way too loud with the jbl's in there, but the tone is f'ing killer. seems that the settings were the opposite of what i expected to set them to. at first, i was using settings that were akin to what donnie reccomended, but is sounded like a blanket was on the amp, really closed off and no definition except in the highs, then yesterday i just basically ignored the numbers and went for the sound, treble is set to 5, mids 10, and bass 3 and it is a lot more open and tighter in the low end than before. now to play with the band and see what happens. is there a good book on doing basic amp repairs/changes that i can order? i'm pretty good with a soldering iron and handy with all sorts of tools. now if i can just move the thing...

Jon Light · Post by **Jon Light** » 28 Jan 2006 11:26 am

Amp Repair Video

This video could be better in a lot of ways but it was excellent for me for one reason---it clearly showed how to discharge the caps so I wouldn't die (that's good) and it showed how to change the filter caps in the doghouse on a Fender amp. Watching the guy do it (even if his haircut is weird) is worth a bunch of pages of book or forum instruction. Three thumbs up.

James Morehead · Post by **James Morehead** » 28 Jan 2006 8:12 pm

Chris, before you mod your amp to cut power, you might just remove the two outer power tubes, and cut your power nearly in half. You can do that with the older twins, I've heard, and it should be the same for the 135 amp ultralinear you have. Maybe some of the techs can jump in here and confirm the yes or no of this idea??

James Morehead · Post by **James Morehead** » 28 Jan 2006 8:25 pm

On second thought Chris, I'll just save you the troble. You should just load that twin up and I'll meet you in Dallas in March and trade you out of it! HA!! (I do kinda have one eye open looking for a 135 watt twin , my wife closed the other eye for looking at another shobud to buy!

)[This message was edited by James Morehead on 28 January 2006 at 08:25 PM.]

Post by **Dave Mudgett** » 28 Jan 2006 8:39 pm

Yes, you can remove any push-pull pair. It matters which pair you pull. On BF/SF Fenders, the outer two and inner two power tubes are push-pull pairs, you should be able to pull either pair. All this assumes nobody has messed with the amp.

When one push-pull pair is removed, the correct speaker load doubles. Assuming the speakers are each 8 ohms, connected in parallel to give 4 ohms, and the output transformer is the original, wound for a 4 ohm load, I would disconnect one of the speakers to give the correct load. It probably won't hurt to leave them both connected, but I prefer to run Twins with the correct load.

On the other hand, if the speakers are each 4 ohms, connected in parallel for 2 ohms, you definitely shouldn't leave them like that. Again assuming the output transformer is original, then it would make sense to rewire two 4-ohm speakers in series, to give the correct 8 ohm load.

David Wren · Post by **David Wren** » 29 Jan 2006 5:43 am

Just a thought, but adding Brad's black box to your setup might give you the added tube warmth you're looking for, and with the new added variable impedance, you really have a lot of room for dialing in just what you want. If you are going to be playing in the Sacramento area any time soon, I'd be glad to drive down and let you test drive my SGBB.

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Dave Wren
'95Carter S12-E9/B6,7X7; Twin Session 500s; Hilton Pedal; Black Box
www.ameechapman.com