NAD Recycled old amp build - Fender 5C5 Pro circuit (long detailed post, photos at end!)

ESchap

After having some success building an AC4 circuit out of an old reel to reel tape recorder, I was looking for a slightly more challenging project.

I've always wanted to try an early 50's Fender tweed, a circuit with octal preamp valves. This chassis with the right arrangement of preamp octals, power valves and rectifier, along with transformers came up on eBay, got bidding ... got it for a song (£35!).  I did a bit of research into the British manufacturer (Romec Audio) and they went out of business in 1951.  So that most likely dates this to the late '40's, early 50's.

I decided I'd use it to house a 5C5 circuit Fender Pro build.  The circuit uses grid leak bias on the 2 x 6SC7 preamp valves a 6SC7 wired as a paraphase phase inverter, two 6L6G in cathode Bias and a 5U4G rectifier.  Fender made these using a tag board.  Read a lot of experienced amp builder's tales of woe when trying to build this circuit on a tag board and how they generally had major issues with "Hum" to the point that some had only cured the problem by rebuilding it point to point; I decided I'd go straight to point to point. 

For parts I used Jupiter (Astron red and yellow repro's) caps, carbon comp resistors, Jensen electrolytics and period correct valves.  50's Tungsol 5U4G and 40's Sylvania branded RCA 6L6G's.   Got 50's Tungsol 6SC7's from Langrex, bought 4 at a great price, they are genuine NOS and none are microphonic, which is apparently a big problem with these valves.

The unloaded output voltage of the mains transformer was quite a bit higher than I was ideally looking for at 360V out; yielding around 450V after the rectifier.  The Fender 5C5 seems to have had a B+ of 360-390V on the anodes of the 6L6's which is a bit over the manufacturer's spec anyway and they would certainly not be happy up at 450V.  I looked at various ways to bring the post rectifier voltage down, doesn't seem to be too many eloquent solutions for that problem, so in the end I decided to sod it and build it and see what I got.  I could always use later, higher spec 6L6's if I needed to, or change the transformer, though not really what I wanted to do.  Need not have worried in the end, with everything wired up and the full circuit load on the Transformer, it's putting out 387V on the first node of B+ .. perfect!  The voltages round the amp at key points are also pretty much perfect too.  If my physics is right, I think that the mains transformer is probably not particularly high (mA) spec and being pushed by the current draw of the circuit, thus the voltage dropping as it works to keep up?  That said, the transformer stays reasonably cool in use so it's not obviously being over worked.

I had no visual reference when "designing" how I'd lay out the components.  Had to work entirely from the Fender circuit diagrams.  I also knew that the main enemy in this circuit would be parasitic capacitance, so I endeavoured to make sure that wherever possible, components were at right angles or separated by distance or height.   Then I worked through a plan for the order of soldering the components, to prevent "painting" myself into a corner where I couldn't get at joints needing soldered or was burning the expensive caps.  Once I'd got a plan, it was pretty easy and quick to put together.

With the circuit built, I fired it up using my variac, watching for runaway current at the same time.  No problems at all and at mains voltage there was no smoke, flames or horrible noises ...too good to be true! ......  So what problems did I have ... I'd very low output and a great deal of hum ... bugger!  Checked everything over and couldn't see what I'd done wrong.  SOS call to @ICBM and a quick visit sorted the stupid mistakes I'd made.  First; I'd used "Cliff" plastic jacks for the input, I'd earthed them together and through the pots, but nowhere did I have that earth properly to ground ... doh!  Second; which accounted for the low output, I'd used two grounding jack sockets for the outputs at 16 ohm and 8 ohm.  Yup, so when I had a speaker in one output, the other was still grounding the output transformer ... double doh!   However, that was it, 10 mins to fix those two issues and it was working. 

So how does this primitive old circuit sound? .....  I expected it to be jazz in character, dark with knarly break up.   Well it's not!  It's bright and chimy clean and has fairly late glorious break-up.  Its very touch sensitive and when pushed, it compresses and sags in very a distinctive manner which if you hold the note yields great controllable harmonic feedback as the voltage recovers.  Through an open back Jensen 15" Alnico it sounds very "big" possibly more "Vox" than Fender cleans, real chime to it, lots of harmonics.  When it breaks up and compresses it definitely sounds old Fender tweed, like Deluxe grunge with a fair helping of JTM45 type harmonic feedback.  Through my old WEM closed back 1 x 12, shod with a Celestion Gold, it has the same chime, though is a little more scooped in the mids, but the bottom end is fantastic, would make a great bass amp at low volume!  I've tried Fender single coils, P90's and PAF type humbuckers and I think for me, the best pickup / speaker combination is a humbucker with the 15" Jensen and single coils with the Celestion.  Shouldn't that be the other way round? 

Still learning to use it.  The single volume and tone control are really interactive and you can change the dynamics of the amp a lot just with those two controls.  Interestingly, the tone control almost acts like a master volume while the volume is more like a gain control.

I've still got a little work to do on it, firstly the noise floor is higher than I'd like, most likely down to not having the earth positioning in the circuit optimised, so will do some moving around to see if I can improve it.  The 6L6's are also running biased a little cold (~70%) just now, so I'll get some more high wattage cathode resistors and taste test, seems I should be targeting 85-90%.

 

Looks like a Radio out of a Lancaster bomber!

 

Mmmm ... those coke bottles

 

Transformers and heaters wired

 

The circuit wired

 

"3D" Structure

 

1 x 12 "Rig"

 

ThePrettyDamned

Mmm... Tasty!

Sound clips or it doesn't work

ecc83

Well done! 

The rat's nest appearance is I know a consequence of P2P but I think I would have had a crack at a turret board!

Is there a remnant hum even with the pots at zero? If so that choke could be coupling AC into the op traff secondary! I would also have turned the op traff thru 90dgrs. I know the mains traff is wound in the "other sense" but another swing of the OP transformer would have cut induction even more. (but then I am a bit anal about hum. I modded son's HT-20 and got it 10dB or so better. I have never heard anyone else complain mind!).

I would not run the 6L6s any hotter myself. Those beauties are never going to get LESS expensive/harder to get.

Dave.

Misterg

Nice job!

ESchap

ThePrettyDamned said:

Mmm... Tasty!

Sound clips or it doesn't work

 My embarrasment5 at my own playing stops me, also best I've got is an iPhone to record, so its not going to be very representative.  Might give it a try though. 

 ecc83 said:

Well done! 

The rat's nest appearance is I know a consequence of P2P but I think I would have had a crack at a turret board!

Is there a remnant hum even with the pots at zero? If so that choke could be coupling AC into the op traff secondary! I would also have turned the op traff thru 90dgrs. I know the mains traff is wound in the "other sense" but another swing of the OP transformer would have cut induction even more. (but then I am a bit anal about hum. I modded son's HT-20 and got it 10dB or so better. I have never heard anyone else complain mind!).

I would not run the 6L6s any hotter myself. Those beauties are never going to get LESS expensive/harder to get.

Dave.

Thanks Dave.   It's silent with the controls at zero.  The nose floor I mentioned isn't really hum now, that got fixed with the proper earthing of the input jacks.  It's more hiss and higher pitched "electrical noise".  However, it's very dependent on how much the tone control is on and that pot does crackle badly from time to time.  Going to try a new pot in case the old pot simply has a knackered track.

Having said all that, V1 which is the closest to the choke is humming (though V2 isn't).  Do you think that could be related to its proximity to the choke?  The choke was on the chassis and though Fender didn't use one in that circuit, I wired it in, firstly because it was there and secondly because it drops the B+ about 10V and I was paranoid about being too high on that value for the 6L6's anodes.

martinw

I've never known a choke couple to an OT. At least not measurably.

surfguy13

Just amazing Gordon......I am VERY impressed.  I also love the way it looks, very cool indeed.

My favourite control layout on an amp is a single volume and tone.....no messing about, just very basic but very effective.  You tend to put far more emphasis on the volume/tone on the guitar which is a far better way of getting a decent tone out of an amp, well for me anyway.

Very interested to see that you bought some 6Sc7s from Langrex......they keep popping up everywhere and nice to know that these valves worked out.

Dave_Mc

Oh wow, that's awesome I really like the cosmetics of it, too.