Building my 5F2A Chassis - progress report

[Casey4s]

Very nice Casey4s - thanks. How do you make sure they get everything (holes etc) lined up with your chassis?

I drew a 1:1 layout with what I wanted where. I also predrilled all the holes to fit and made the little "divits" for the numbers on the 5F6A in advance so it was a lot easier for the engraver to get it right the first time.

In my case I actually drilled the pilot holes through the chassis and the faceplate blank at the same time for perfect alignment.

BUT, if you get the cut to size engraving stock from the engraver, you can pull the contols out, place the stock on the panel and trace the hole locations onto the faceplate stock from the inside, then drill them out and get it engraved. Your engraver can make the marks for the numbers as either dots or dashes.

There are some photos of making the faceplate and chassis set-up at this link: (CH-11 to CH-17)
http://s238.photobucket.com/albums/f...-up/?start=all


EDIT: CH-9 shows the layout sheet I gave the engraver.

[tubeswell]

Ginghamy Thingamy

Tackled this today after much preliminary anxiety. Still didn't quite mange to get the fabric lined up straight, but I'm going to let it be.

The top and sides is a single piece.

Excuse the back shot, the glue wasn't quite dry on the bottom bit when I took the pic.

The fabric is a Gingham Tweed (of some nameless colour) that sort of goes with the Hessian grille. Now for a handle (and maybe a brown formica dashboard) (and possibly some square metal corners) - and this saga is nearly complete.

[Paul P]

Hey tubeswell, have you had time to get to know your amp a bit more ?
What do you think of it sonically ? I know you were happy initially, I'm
wondering if you have a bit more to say about the tone and whether or
not you intend to tinker with it.

Paul P

[tubeswell]

yeah I have taken it to a couple of practices now and played on it a bit for the last couple of weeks (has it been that long?). I have noticed that with the NFB switched out, it really comes alive and this lifts the low gain input. But on the high gain input, with the NFB switch cut, there is a faint but noticeable intermittent mid-to-high-end electronic buzzing noise when I have been thrashing the amp, but it gradually stops after I have stopped playing it for a while. It sounds similar to a ground buzz, but I don't think I can be, because the intermittentness doesn't seem to be related to whether I touch the strings or chassis or not, and it also stops immediately if I kick the NFB loop in. So I'm guessing that maybe its something to do with the power supply reservoir. So I was wondering maybe I should add a second Sprague Atom 20uF in parallel with the first filter cap (the tweed narrowpanel princeton had two 16uF in parallel for the first filtering stage).

Also at present while its very nice on low vols, it breaks up quickly when cranked and loses a bit of high end definition, I thought that maybe also lowering the 2nd and 3rd filter caps from 22uF that I have presently got in there to 10uF each (which is closer to the orignal 8uF value - and is also what MWJB suggested for tone) would do the trick. I did an A-B comparison side by with the new Champion 600 yesterday, and the Champion can't touch my thingamy for subtlety, although the Champion was obviously less bassy, it also had brighter mids (if my ears weren't deceiving me). So I'm guessing that has something to do with what MWJB was talking about.

Although the 10uF 450V caps I have are quite piddly in size compared to the Sprague Atoms, I don't think that should matter, especially if I have two of those big 20UF caps in parallel for the first filter stage.

I was also toying with the idea of maybe putting a lower value resistor in at the first supply resistor, say 4k7 instead of the 11k I have in there, in order to get a bit more headroom and chime. I've got plenty of leeway in the screen and plate voltages at present (55 volts difference at idle).

Having a look in the back of the chassis again, I also see possibilities with improving the lead dress, particularly where I have the blue wire going from the coupling cap to the output tube grid (see pic), as this is crossing the turret board right next to the two 22uF filter caps and also crosses the other 2 or 3 inch length of blue wire I have going from the NFB switch to the cathode of v1B. (This part of the disconnected NFB loop remains 'active' in as far as it is still attached to the cathode of the 2nd preamp gain stage when the switch is disconnected, and may be acting as some sort of antenna coupling between the aforementioned pre-amp plate to output-grid coupling wire and the cathode of v1B. - How's that for a hair-brained hyothesis?)

Anyway I thought I could move the aforementioned plate to output grid coupling wire to be located behind the turret board, thereby reducing that potential RF coupling source.

After that I'll do one tweek at a time, starting with the 20uF cap.

Another thought I had was that in completely disconnecting the NFB, that the 2 or 3 inches of little blue wire that remains connected to the cathode of v1B (that I was just talking about) could be acting as some sort of RF antenna without the NFB signal going through it.

Therefore another possibility I thought of instead, is that it might pay that instead of making the NFB loop switch completely disconnect, I could make the 'off' position of a DPDT switch connect back to the top of the OT secondary via a 1M or 2M2 resistor (i.e. the DPDT switch would switch the NFB loop between a 22k stock resistor and (say) a 2M2 resistor). I wonder in theory whether this would effectively provide 'infinite' resistance thereby mimicking the disconnect function of the current NFB switch setup, but without leaving any little RF antennas lying around. Is that too bone-headed or might it just work?

I also liked the (Holland made) NOS Philips 12AX7 I recently acquired in there. Its very sweet.

[Paul P]

Great report. I bet having your amp working gives you a much better
understanding of what all the different parts are doing (or might be doing).

But on the high gain input, with the NFB switch cut, there is a faint but noticeable intermittent mid-to-high-end electronic buzzing noise when I have been thrashing the amp, but it gradually stops after I have stopped playing it for a while.

This must be some sort of distortion, just the thing that the NFB is there to
cancel out. Could the fact that it happens under heavy playing and goes
away after having stopped playing be some heat-related thing ? It sounds
like a resistor (or a tube ?) is heating up too much when you're playing
hard and then it gets a chance to cool down when you stop.

Why would you play through both low and high inputs ? Isn't one better
suited to guitar than the other ?

Paul P

[tubeswell]

Thanks for the feedback and suggestons Paul P.

Yep the intermittent buzzing could be related to dissipating some sort of energy I guess. I have a 2W 11k (2 x 22k 1W in parallel) resistor at the first supply (from plate to screen) node. If I try a 4k7 5W in there, I guess it might:

a) reduce the resistance load (and thence and heat dissipation) on that first resistor, but also it would do something to the tone by bringing the screen and plate voltages closer (I don't quite know what to expect in that regard)

b) give more headroom and chime

On the Hi/Lo input question, the Hi Gain input is definitely brighter, but when I play my strat through it, it can be too bright in some applications - I can get nice muted jazzy tones through the Lo Gain input, and the combination of the tone control and the NFB cutout switch makes the amp very versatile in this regard. The 1MA tone control pot works very well with the mica caps. (I used 'Alpha' brand pots)

[tubeswell]

Well I added another 20uF to the first filter stage today (bringing it to 40uF).

The voltages all went up about 8V.

So I am getting about 364 on the Plate and 303 on the screen. Ran it for 15 or so mins, taking readings and didn't see any smoke yet. (Will update my report in this regard on the New Sensor Tung Sol 6V6GT in the Tubes forum).

I also twiddled a bit with the lead dress by re-routing the coupling wire between the pre-amp and the output stage so that it no longer crosses over the filter caps or the NFB switch wires.

Have yet to road test it with these refinements.

[tubeswell]

Trivia

I put a strap handle on it a couple of days ago so its alot easier to carry around (Duhhh!)

$40NZ for a Fender strap handle! (Pushes the price of the amp over $500)

[Paul P]

Nice !

$40NZ for a Fender strap handle! (Pushes the price of the amp over $500)

Who said you could save money building your own . My wife told me
today, with maybe a touch of exasperation in her voice, that I was getting
close to 3000$ with my amp. This does include quite a bit for shipping
charges, an extra part or two, and a lot of extra wire, but still. Quality
parts aren't cheap. It better sound good !

And I still need grill cloth...

So how's the tone of your amp coming along ?

Paul P

[tubeswell]

By and large I like what I'm hearing.

At this point I don't know if I did needed the extra 20uF in the first filter stage to be honest. It seems to make some difference when I cut the NFB loop. Otherwise I don't think it takes out much in the way of hum. The hum was pretty low to begin with. It does lift the Plate voltage about 8V. I'm thinking that maybe the amp loses a bit of it champiness with a plate voltage of 364

The way the cab is built, the amp is very bassy on the neck pickups. I haven't tried putting in the 10uF filter caps in the other positions yet. I have conflicting stories about whether they will make any difference in tone or not. I might try putting the switch in at the first cathode bypass cap to see if that takes any bass out. I don't want to have to change the coupling caps yet.

I wish I had a spare $3k to spend on building amps

[Paul P]

I wish I had a spare $3k to spend on building amps

So do I. I initially thought 500-1000$. At one point, somewhere in the
middle of it all, I began to realize that the thing was going to cost a
fortune. I also realized that I was pretty much past any point of no return
so I just said to myself ah, to hell with it, and plowed on. What's sort of
kept some sense in the picture is that it would have cost me about as
much to buy an equivalent amp (though I don't think one exists).

The way the cab is built, the amp is very bassy on the neck pickups.

Have you tried moving your amp up off the floor ? That'll reduce bass for
sure.

Paul P

[tubeswell]

Beam Blocker and other tweeks

Well after a couple of weeks jammin' and taking it to the regular band night practices, I saw another opportunity for improvement. Last night at band practice there were a few comments about the "ear-piercing highs" from the other guitarist who was standing next to his own amp 10-20 feet away across the room, and would I please tilt it back to face the ceiling mate! (Sure thing mate, but I was nervous about the angle it was left dangling at...) Well I never had taken that into account because I was always standing near the amp when i was playing it before!

So today in my spare time I made a beam blocker out of 3/4" ply and a bit of aluminium. Ist one was 120mm (or 4.8") diameter. I tried that out for a while and decided it cut slightly too much from the top, so I put a smaller one in 85mm (3.25") diam. The beam blocker appears to stop the dreaded ear piercing high freq intensity beam manifesting itself across the room and looks kind of retro. You can just see the disc and the aluminium mounting bracket through the grille cloth if you squint.

Following a hint a while back from MWJB, I also put 10uF caps in the 2nd and 3rd filter cap positions. I am happy to report that the 10uF caps have (to my ears) indeed reduced the bassiness slightly. Fancy that! Thing sounds a bit lighter and brighter in the mids.

I also adjusted the internal wiring a bit and re-soldered what appeared to be one or two dirtier joints, to improve the lead dress a bit more and limit potential coupling sources and re-tensioned the socket pins to limit other potential noises. It sounds pretty good now

[Paul P]

Just a thought on the treble response. Your speaker may not be the best
match for your amp. It looks like it's seen better days, is that a repaired
tear in the cone that you can see in the picture ? You must have another
amp lying around, can you try playing your amp through its speaker, or
vice versa ?

Paul P

[tubeswell]

I've noticed a phasey warble (like a phaser effect unit would produce) at high volumes that is quite apparent on certain notes esp when I hve the guitar vol knocked back a bit to clean up the signal. I'm not sure whether its more pronounced when the amp warms up.

Trying to find reading about this to see whether its related to higher than expected B+ and Plate voltage.

When I soldered in the 2nd 20uF filter cap I put the leads onto the first filter cap right at the point where the lead comes off the cap. I wonder whether I over-heated something in the soldering?, or whether perhaps the phasey sound was always there and I hadn't noticed before, and it might possibly be something to do with a defective coupling cap maybe? (I'm using old-style NOS 400V mustard caps).

[tubeswell]

On second thoughts, after re-evaluating it a bit and fiddling around tonight, I have decided the phasey warble is normal for when the amp is cranked with the guitar vol nearly cut, and that it is probably just the pre-amp harmonic distortion showing through. So I am coming to terms with it.