5F2A-with-6L6 build

Keefy

It's been a few weeks since I finished my Princeton Reverb rebuild. I did a quick-and-dirty distortion pedal after that but I fancied another amp project. Weber did a kit amp called the Maggie - it looks to be based on the early 5F2A Fender Princeton circuit, which itself is basically a Champ with a tone control. The Weber circuit has some boost/voicing options that don't really interest me, but it runs a 6L6 instead of a 6V6. I used to own a Swart STR which could run either, and I much preferred the more open sound of the 6L6. I have some suitable transformers, so that's the way I'm headed.

I don't want to be bothered with woodwork, so I snagged a dead Line 6 Spider III on eBay. This is the first picture from the listing:

Keefy

Having harvested any useful parts from the Spider's guts I did a bit of metalwork. Note the odd pentagonal chassis cross-section!

The recycled PT mounts through the chassis, so I used a drill and my new had nibbler tool to make a rectangular hole - fortunately for me it's made of aluminium. The secondary leads will be inside the chassis, which would leave the primary connections exposed in the speaker cab, so I fabricated a cover from some sheet aluminium.

PeteC

Another great amp coming!    I have to say, you must try building a 5E5A circuit next - it’s a fantastic amp.  

Looking forward to seeing this one come along 

Keefy

PeteC said:

Another great amp coming!    I have to say, you must try building a 5E5A circuit next - it’s a fantastic amp.  

Looking forward to seeing this one come along 

Cheers, I'll look into that, although I have a couple of other projects lined up after this one!

Keefy

I took delivery of some nice 50mm M4 bolts today so I reckoned it was time to mount the PT. I connected a 2-conductor mains cable to the PT primary and secured it with strain relief grommets through the cover and the chassis. This cable will carry mains from the power switch to the PT. I parked all the secondaries in a chocolate block. Once I have worked out where is best to install the OT then I can plan where the rest of the components will go.

Keefy

I rigged up mains to the PT with a load on the HT secondary, and connected some headphones to one of the OT secondaries. Then I tried the OT in different positions on the chassis while listening for induced hum. 45 degree positions are noisy, but 0 degree or 90 degree orientations are fine as long as the transformers are a few cm apart.

I am thinking of building the circuit on 2-row tag board, or possibly two parallel tag strips.

The speaker will be a Jensen C12Q - should complement the tweedy circuit nicely.

Keefy

Just done the layout for this. I've added a 1k 2W screen grid resistor as per the Weber Maggie design. Most of the components will be mounted between two tag strips. A third tag strip will help organise the PT secondaries. All the secondary wires are quite short, and I need to drop the voltage for the filament supplies, so it makes sense to mount those resistors plus the 6.3V CT resistors in this way. I'm using an illuminated mains switch, so no need for a 6.3V jewel lamp.

I've just noticed I've missed out the HT fuse, which won't be accessible without pulling the chassis.

PhilKing

Not that I could do this, but it says 6V6 not 6L6, don't know if that affects anything that you have put on there.  I think it's great that you can do this.   I think it could be a really interesting design.  I have a Jet City Pico Valve that a friend 'corrected' for me (he really know about calve stuff and he took a look at the inside of it and said 'I can't believe they've done this', and spent about 10 minutes on it, and the sound really opened up.  I have no idea what he did, but it really worked.  I have been able to try all sorts of octal valves in it, and currently have an old Grove Tubes KT66 (which I believe are made by Sarotov).  I liked this sound best (I also use these in my Ceriatone JTM 18/45 custom).

Keefy

PhilKing said:

Not that I could do this, but it says 6V6 not 6L6, don't know if that affects anything that you have put on there.  I think it's great that you can do this.   I think it could be a really interesting design. 
...

Yes I’m aware of that - you can’t just stick a 6L6 in place of a 6V6 and hope for the best!

As far as I can see the Weber Maggie circuit is the same as the Fender 5F2A except for:
1. Voicing options via 3-way switch on preamp cathode - not using this;
2. 6L6 instead of 6V6;
3. Bigger transformers - my OT is rated 10W and my recycled PT seems to have plenty of power on tap; and
4. Screen grid resistor on output valve.

I intend to try the amp with both 6L6 and 6V6 valves. I have several single 6V6s to try and a TAD 6L6 on order. As well as listening to how they sound I will check voltages and currents to make sure nothing is getting damaged.

Keefy

Metalwork today. Below you can see the holes cut for the valve sockets.

Keefy

Bolted the OT and valve sockets into place. Quite a relaxing thing to do at the end of a day.

PeteC

Ive tried this same circuit with the hammond 125 but found that a smaller old Radiospares output tx was crunchier and broke up more smoothly.   If youre going for a cleaner tone though then the larger Tx is better.   

Keefy

Cheers Pete, I’m not really going for a particular sound, just a single ended amp that will run 6V6 or 6L6.

Keefy

Made up the tag for the HT and heaters and fitted it next to the PT.


Connected the OT primary wires and bundled up the unused secondary wires with heat shrink. Started fitting the wires to the main tag strips.


Wired up the input socket with screened signal cable. Was going to wire up the pots but discovered the 1M pots I had saved from a dead Carvin all read 1.3M to 1.4M! Ordered some Alpha pots and a pair of white cup-cake knobs. Mmm...


Made most of the wire connections from the tag strips. Decided to fit the first filter cap while I was at it as the mounting hole was getting a bit full. I hope this works first time as it is pig to work inside this chassis!

Keefy

Glued the plastic face plate material to the front panel and opened up the holes for the input jack, pots, and illuminated rocker switch. Used the holes to assemble the volume and tone controls externally before fitting inside the chassis.


Installed most of the components between the stag strips:


Then realised my grounding scheme was bollocks, belatedly remembered the Modulus layout I'd seen, and spent an hour wrangling leads in and out of place. There is now a copper bus bar joining the filter caps, and the grounds follow a multiple star layout.


My thoughts at this time:

1. Re-working Line 6 Spider chassis? Never again!
2. Kudos to anyone who can make a tag strip build look neat - my favourite method is definitely eyelet board, but it would have been an issue to fit it in this chassis.
3. Note to self: triple-check grounding scheme before building - what starts out tidy rapidly becomes very bodgy.
4. I'm nearly out of solder. Another reel of 60/40 ordered off eBay, and tomorrow I'll cycle to our local 'four candles' emporium to try and get some a bit sooner.

Keefy

I finally got round to (provisionally) finishing this build today. I first checked that my dropping resistors had produced the correct voltages for the 5V and 6.3V filament supplies and was delighted to find that I was getting 4.97V and 6.45V respectively with valves running - I'll take those figures! When I brought the whole thing up to voltage with the speaker connected I was greeted by a horrendous squeal. I temporarily disconnected the NFB lead and it went away, so I swapped the OT primary leads and re-connected the NFB lead and all was well.

The big cement 220R resistor is a temporary measure intended to bring the B+ voltages close to the Fender spec with a 6V6 fitted - they mostly are, but it's only dropping a few volts. With that extra resistor bypassed I get plate dissipations of:

• 6V6 (JJ) - 15.74W (112% max)
• 6L6 (TAD) 17.49W (58% max)

My plan is to remove the 220R resistor and just replace it with a wire as per the original Fender design (albeit I've added an HT fuse). I'll then install a slightly larger bias resistor to get the PD of a 6V6 down to about 70-80% of max, and fit a rear-panel switch to bring in another resistor in parallel with it to optimise bias for a 6L6.

I haven't actually tried a guitar through the amp yet, but the volume and tone controls operate as expected. There is a small amount of 50Hz (not 100Hz) background hum but I'm only noticing it because the speaker is up on my work bench.

I'm off to bed now and hope to do a bit more tomorrow.

Winny_Pooh

Nice and the 12inch is so much better than an 8

Keefy

Finished (more or less).

I replaced the extra 220R resistor with a wire and played about with the V2 cathode resistor, ending up with 1k (actually 989R) for running with a 6V6. I tried adding a 680R resistor in parallel with that for 6L6 operation and that gave a reasonable plate dissipation figure. I could probably have gone lower but you end up chasing your tail, as more current through the valve pulls B+ down and raises the cathode voltage, and the PD doesn’t move much. Besides which a valve doesn’t have to be pushed close to destruction to sound good!

I put the 680R on a back panel toggle switch directly above V2 as a reminder when changing valves. If the switch is in the 6L6 position with a 6V6 fitted it’s going to get very hot!

I then checked voltages and PD with 4 different 6V6s in place and got PDs from 70% to 77% of max. I also did a quick guitar test with each one to make sure none were noisy or otherwise playing up. Finally I installed the chassis in the cab with a Tung Sol 6V6 in V2.

I like it! There's not a great deal of clean headroom available - from about 4 on the amp volume it is starting to break up. But the magic happens when you back off the guitar volume and you get this lovely harmonic richness, particularly with a Les Paul. It brought to mind a video of Joe Bonamassa demonstrating all the sounds you could get from a Les Paul into a Champ - hardly surprising really! I've linked the vid below.

With a 6V6 there is a very low level of background hum. With a 6L6 on board the headroom and available volume increases substantially, as does the bass (and the hum). So far I think I prefer the response of a 6V6, so I am going to take my time comparing the 6V6s I have on hand - Tung Sol, Groove Tubes, JJ, and vintage Brimar.

Here's a couple of pics of the finished article. I realise I forgot to take a final gut shot before putting the chassis into the cab, but I plan to move the fuse holder to the back panel, so I'll take some more pics then. Also look out for a phone recording.

Oh yes, the name. It's Spanish for dead Spider!

vanlooy1

Looks nifty, love the name!

Keefy

vanlooy1 said:

Looks nifty, love the name!

Cheers! Just had another play through this with the GT 6V6 on board. The amp really lets the character of each guitar come through. Strat and Squier Tele sound particularly nice tonight. There is something I’m not keen on in the upper middle/treble range though and I wonder whether that’s down to the Jensen - I’m not sure the guy I bought it from played it enough to break it in. I might try a different speaker at some point, maybe an old square-magnet Eminence I have in my stash.

Keefy

I moved the HT fuse to a panel mounted holder. If it blows I will pop the chassis out rather than just stick another fuse in, but I didn’t have much confidence in the board-mounting one that I had originally used.

The amp is much brighter than I would expect a tweed Champ to be, to the extent that my cheap-and-cheerful Mosky Spring Reverb (actually based on 3 delay chips like many such pedals) produces a veritable miasma of noise. At some point I will try it with a different speaker.

As promised, gut shots of the finished (ha!) amp.