My Latest Amp Build, Low Power Tweed Twin

Rich Hlaves · From: Wildomar, California, USA

A few months ago I found a used Fender Tweed Twin cabinet complete gutted on Craigslist for cheap. It had tinted laquer applied. So I decided to build this in my spare time.

Though not a steel amp this thing is a tone monster with a six string. It is the most responsive amp I have ever played. From clean to dirt to clean right at your fingertips. This is the 5E8-A circuit.

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On man....let the smoke out of another one.

Leslie Ehrlich · From: Saskatoon, Saskatchewan, Canada

Cartwright Thompson · Posted 3 Mar 2012 8:35 am

The two on the left are 5U4's.
Nice job Rich, it looks sweet.

Tim Marcus · From: San Francisco, CA

No 1k5's and 470 1W on the output tube sockets?

Looks awesome! That's got to be a great amp for a split gig where you also play some guitar. Think that's the Clapton circuit of choice
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Rich Hlaves · From: Wildomar, California, USA

Tim, the 1.5Ks are there just above heater wires in the pic. The 5E8-A and most of the Fender 6L6 amps of the day didn't use the 470s.

I built it exactly (almost) per the Fender 5E8-A schematic. I used modern Heyboyer iron that compensate for todays higher line voltages. I'm within 1-2% of the Fender specs on voltage. This PT also has a center tap for the 6.3V heaters. I did not have to make a virtual ground to quiet the heaters, nice. The amp is very quiet.

I did not add a bias pot but jumpered in bias resistors until I got it right. The schematic calls for a 56K bias resistor. I ended up with 22K in there for a 60% idle.

The electrolytics are TAD branded with the exception of one 25/25 Sprague. Tone and coupling caps are Mallory 150s. I used EH rectifiers, JJ 6L6s and the preamp tubes are also JJs. Speakers are reconed Chicago Jensen C12N. They will get felt dust covers when I get a chance. They are just slightly brittle.

I think you're right, Clapton plays these. I will say that with a Strat plugged in the tone is very much like his with this baby cranked.
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On man....let the smoke out of another one.

Tim Marcus · From: San Francisco, CA

Fender added the 470's in to their later designs. I think they protect the amp in case of tube failure, or the tube in case of amp failure - I add them in to all of my tweed builds. (and princeton reverb's, for that matter) It makes no sonic difference, but its a nice way to add in another $10 of parts and peace of mind Smile

Modern tubes pull so much more current than the old production tubes do, thats why your bias measurements are different than the 50's design. It is what it is. On an amp with less than 4 output tubes I usually just cathode bias the damned thing and call it a day - plus, you get a nice voltage point for DC elevated heaters that way.

It looks great - and I bet it sounds great too! Its got that cathode follower in the tone circuit which always sounds awesome! Totally different than the blackface sound.

nice work!
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Chris Reesor · From: British Columbia, Canada

Nice amp, Rich. Is that a Weber chassis?

On the subject of the 470 ohm screen resistors, I doubt they make any difference to the sound, but they can act as a fuse and save your power transformer if you get a shorted screen. But you already knew that, didn't you?
Wonder what Leo and Forrest were thinking, re; the twin 5U4's. Weren't there any higher current tubes available then, or were they just trying to reduce voltage sag, or maybe to keep the amp playable in case of a sudden 5U4 failure?
Too bad we can't ask them.

Chris
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J Fletcher · From: London,Ont,Canada

Hi Rich, your workmanship is excellent! I'm in the process of putting together a similar amp, based on a '56 or so Pro. I'm curious about the EQ circuit, and how you like that? Same basic EQ was used in several Fender amps of that era, quite different from the 5F8, or 5F6 EQ that followed.
My intent is to use the amp with my D8 Stringmaster, so I do want clean headroom. Is the front end too hot for this, using 12ax7's? I have some 6SL7's and 6SN7's I was thinking about using instead.
Also, going to add a spring reverb circuit, right after the volume control. Gotta have that reverb.
Is your power supply the same as the 5E8? Choke ahead of the power tube plates? I was wondering about whether to build like that, or to modernize that part of the amp?...Jerry

Bill A. Moore · From: Silver City, New Mexico, USA

Really looks great, Rich! I have a couple more blackface amps to build before I start on the tweeds, but I'm looking forward to it.

Rich Hlaves · From: Wildomar, California, USA

Chris,
I may add the 470s. I thought about it during the build but elected to keep it Fenderish. The Twin 5U4 thing IMO was to keep sag to a minumum. Fender did this on a lot of the amps from this era IE, the Bassmans. The chassis is from Mojo.

It is interesting how similar this circuit is to the 5D6-A Bassman. I have a Ken Fox 4-10 and looked at it the other night. Damn near identical.

Jerry,
When testing this thing I subbed in 12AX7s in V1 & V2. This drives the power section too hard for resonalbe clean headroom. I've got 12AY7s in there as called for in the 5E8(A). The power section is stock as well, Choke in front of the plate of the power tubes.

I think the amp will sound good with a Stringmaster, It does well with my laps.
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On man....let the smoke out of another one.

Ken Fox · From: Nashville GA USA

I borrowed the dual triode input from the 55 Twin and incorporated it into the 5D6 circuit, hence the 5D6A designation. The tone stack is different for sure in the amp and the 5D6 used twin 5Y3 rectifiers, not 5U4. One 5u4 will handle the amp just fine, as they later changed to that for 2 x 6L6GC amps. A 5U4 can handle twice the current of the 5Y3. The new 5D6B I produced used one 5U4, used the smaller chassis like an original 5D6 and also used the dual triode unputs.

Great amp designs from that era for sure!

Ken Fox · From: Nashville GA USA

Another major diffference and also a reason for a warmer tone from the 5D6 is the inverter sections are different. The 5D6 used a paraphase inverter (1/2 of a 12Ax7 drives the power tubes). The Tweed era Twin used both halves of the 12AX7 for the inverter.

Marc Jenkins · From: Victoria, British Columbia, Canada

Nice one, Rich!

Rich Hlaves · From: Wildomar, California, USA

Thanks Ken, Great info. If I decide I want more clean warmth I may rework the PI. I have a great model here at home to examine!

Thanks again!
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On man....let the smoke out of another one.

Tim Marcus · From: San Francisco, CA

Hey Rich - just a heads up: I have been experiencing a large amount of failures from switchcraft switched input jacks as of late. I can't use them anymore.

I see you've got 4 of them installed - if the amp starts acting weird in about 2 months, you know what to look for! Confused

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Rich Hlaves · From: Wildomar, California, USA

J Fletcher · From: London,Ont,Canada

I think you could tighten things up somewhat by, moving the 6L6 plate supply to the high side of the choke, and using a single GZ34 rectifier instead of two 5U4's. The choke is likely a source of sag when current demand increases, like when you make sudden increase in volume. GZ34 is a more efficient rectifier, with better regulation than a 5U4, and less voltage drop.
However both changes will affect the character of the amp, and may not be to your, or my liking. I'm building my amp with the choke after the 6L6 plate supply, mainly because I already have the correct choke. I'm using up some existing parts. Also went with all 20uF/500v filter caps for the same reason.
FWIW...Jerry

Rich Hlaves · From: Wildomar, California, USA

Jerry,

I'm really liking what this amp is doing. I've haven't played a gig with it yet so it's hard to say if I will want to change it's character or not. Right now I'm thinking not. It's quite different than my other amps (Showman, SR,ProR, TR, 5D6A Bassman and some modern stuff). I built it mostly for Strat, LP & 335 use and wanted it to get a bit nasty when I dig in for blues work.

I may play with the rectifier a bit and see if I can move the sweet spot up a number or two on the volume control. The choke too as you suggest. I may need that for live gigs. Other than that I think I have a winner and I can always mic the thing. I've got a Weber WZ34 copper cap laying around and may pop that in to see what happens. I also have a copper cap with no voltage drop that might deserve a try. I'll try the tube as well. Right now I've got 405V on the plates and actually would like a bit more with modern power tubes.

Thanks for all your suggestions. I'll print all this stuff and save it the file I have for this build.
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On man....let the smoke out of another one.

Dan Tyack · From: Olympia, WA USA

I've never played a 'real'low power Twin, but I think the Fender custom shop version of this amp is by far the best sounding 'reissue' amp from any major amp manufacturer. I love it for pedal steel or lap steel.

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