The Vamp
A Champ amp in a Vox box
This amp was meant to be a 5F1 Champ/5F2 Princeton style amp, housed inside the cabinet of my broken Vox Cambridge hybrid amp. Hence the name, Vamp. The idea is to have a simple SE 6V6 amp which can produce somewhat more volume if needed, by using larger octal tubes such as the 6L6, EL34 or KT66. It also had to be portable to lug it to band practice; also, it had to be loud enough. Replacing the 8" Celestion speaker with a 12" Weber (which would barely fit) has helped a lot!

Initially, it was housed in the former P1 prototype chassis. Eventually, a new chassis has been made which fits, tweed-style (ie vertically) in the back of the cab with controls at the top, transformers at the front inside the cab and tubes dangling from the bottom.

Major electronic components are as follows:

• Hammond 369JX power transformer, 250-0-250 at 60mA
• Hammond 125ESE output transformer, 15W SE.
• 1x 6V6 (EL34, 6L6) power tube
• 2 cascades preamp stages. I tried the following:
  1. ECC83, as per stock 5F1 style. This worked in the proto chassis, but I got problems when I moved it into the Vox chassis.
  2. 2 6SJ7 pentodes. It worked, but not as planned. Next time I'll try a voltage divider, play with bias values and bypass caps.
  3. 6K11 12-pin compactron. This worked OK-ish, needed some tweaking, but it was too much gain from one tube socket: I could not stop the squeal when mounted in the Vox box. Not enough space for a board, either.
  4. Final setup: a 6SL7 octal triode. This is what I stuck with as it works a charm!
• 2x 1N4007 silicon diodes

The PT should grudgingly accept the 6L6 and EL34. Some people run Hammond PTs at over 1.5x the rated current, and it barely gets warm. Well, my PT does get rather warm but I can still keep my hand on it, so I guess it's OK then. The problem isn't so much the heater current draw, but the anode current draw. I guess these big tubes draw about 90mA, while the PT is spec'd for 60mA... It's OK for a short while but I'd rather not try to run it for long periods of time (like band practice).

The 125ESE is a perfect fit for the new chassis it hangs in. Initial plans of replacing it with a Heyboer are cancelled, as the Heyboer doesn't have the right shape. (But apparently the full-size 18W Radio Spares made for SE, with bluish M27 steel core does sound really good, at least according to this thread. Maybe for a next project...).

The 1N4007s were used because I did not have space on the final chassis for a tube recto, nor did the PT have a 5V tap for the recto heater. Besides, it's a class A amp with constant current draw. There are no advantages to using a tube recto; only disadvantages. With silicon diodes, I got very close to the desired 340V B+ the amp was originally designed for. When the utility company's midgets try their best, it's even more than that!

ECC83
At first, the prototype build with ECC83. I followed the stock layout of the 5F1 as much as possible. Still, at first the amp would squeal at all volume control settings, with a slight dent in the squeal when the volume pot was opened half way (there was no master volume pot at the time). Thanks to the kind folk over at the AX84 BBS, the problem has now been fully sorted. (And of course, it turned out to be so easy) This is the first time I've made an amp with negative feedback, and if you connect the wires of the OT primary the wrong way around, you get... positive feedback! Swapped wires, problem cured.

Which allowed me to play with some extra's (see schematic below), to make this a 5F1++:

• Added a 10µF/63V cap over the first stage's cathode resistor to increase gain a bit. This sounded better already; before, it was too clean for my taste before.
• Later I added a similar cap over the 2nd stage's resistor (not yet in the schematic). Even more grind. Your grandmother won't be amused!
• Added an MV. This meant doubling the 220K grid-to-ground-reference resistor for the power tube, and putting a 500K log pot parallel to it, to ground (wiper to resistor to grid). Now it can crunch when the neighbours are in. In fact, it crunched rather a lot when I replace the 470k resistor + 500k-A pot with a 1M resistor + 1M-A pot, very much reducing the gain that's dropped before hitting the power tube. This thing cooks!
• Added a cut switch for the NFB. It simply opens the wire. This gives more grind, but only when the MV was wide open. There's apparently not much feedback at low (output) volume, regardless of preamp volume; so no big difference at low preamp settings. The NFB allows me to play loud and fairly clean; cutting it allows me to play soft and distorted. I put the DPDT switch "around" the presence control.
• Added a Presence control. I used a 5K lin pot for this, and a .1µF-200V cap. Copied it from some old Fender's schematic, but I think it'll need some tweaking, since the 5k pot will have some influence on the preamp bias and also on the 22k resistor.

With the gain increase that resulted from all this, your in-laws will lock up their daughter! I was going to add a 1-pot tone control, as described in the Gar Gillies book; but I decided to scrap it due to a lack of space. Also it would reduce the gain of the 2nd stage, and I liked the tone too much. Sadly, moving it into the Vox box caused the ol' dreaded "DC on the grids" problem, which I could only partially cure. It occurred regardless of which preamp tubes were in there; even an ECC82 would suffer from this problem. So I let go of the original setup.

6SJ7
Since by now, I had quite a varied tube stash, I tried something else: the 6SJ7 pentode. Wired it up without a volume control between it and the power amp, and it was desperately short on volume. This was obviously not the way to go. So I added a 2nd 6SJ7, in front of the first (now second) one. Holy mother of rock! This thing was loud! I forgot to use a voltage divider between the preamp stages so the 2nd pentode would clip horribly! A 470k series from R6, with 470k to ground might do fine, also an MV was used in the prototype. And R6/R8 were 250k-A / 250k instead. Still had too much distortion (which farted out upon decay) so I took it apart again. Maybe I wouldn't have if I hadn't forgotten the voltage divider! Oh well. Also, someone on the BBS mentioned that removing the bypass caps would have helped. I'll keep it in mind for next time. Also I'll have to dig into the load line theory, to optimize the operating point.

One more thing about the cascaded 6SJ7: the amp hummed horribly! I couldn't crank it, either with or without guitar plugged in! So I added some heavy filtering: four 47µF caps, separated by low value resistors (typically 51 ohm), before the B+ gets sent to the OT. In other words, this little module came before the regular (and inadequate) 5F1 filter section (which I had already beefed up). After this, I could crank it all I want and only hear hiss! So this setup is probably overkill, but I'd reccommend to anyone to filter the B+ with one stage (or a pi-filter), before the signal gets sent to the output transformer and power tube.

Compactron
I had bought some tubes from a fellow AX84 board member, and among them were 6K11 compactrons carrying 3 triodes. I tried those, and this was when I found out about the voltage divider, as I had the same bad distortion as before! I went along with the compactron anyway; no need to keep swapping back. I did my best to adjust the bias, using pots for cathode resistors; I got it nearly right, with the last triode running one more than 12k. This gave the best sound, with just a little bit of fart-out on full tilt; but keeping the volume control 2 notches below that kept the sound acceptible. Sadly, I could not succesfully transfer this setup into the Vox box, as there was not enough room for the wiring not to interfere with itself. It kept squealing. I gave up on this setup too, but with some tweaking and a more spacious chassis (and suitable board layout, rather than PTP), maybe it will go back into a future project?

6SL7
This is the final setup. I went back to octal sockets, as I seemed to have a bit more luck with them; also the 6SL7 is similar to the ECC83 as it's a high-gain dual triode. The 6SL7 has a µ of about 70, so it's akin to an ECC81, really. I installed the socket in the hole for the 12-pin compactron; both sockets are the same size, fortunately. This saved me from having to commit major chassis surgery again. It's wired point to point, but no squeal problems this time! It needs a bit more tone tweaking and some wires are still hanging loose, but I'm not touching not until I've built a spare amp! I won't tempt my luck on this one, now that I've got a working rehearsal amp.

A short list of the work that remains to be done:

• Swap cathode bypass caps, it now has 10µF on stage 1 and 1µF on stage 2. This could be the reason for the flabby bass when driven. Maybe I'll use 1µF on both stages. Either way, right now it's too bassy to use the neck pickups on my guitars.
• Add "high-frequency" bypass cap. Or find another trick to make the amp a bit brighter. So long as I don't have to buy a new speaker to achieve that...
• Hook up NFB and switch, so I can play loud and clean, or soft and distorted. It sounds great cranked, but some passages have to be played loud and clear.
• Connect the Presence control. I have to find a way to keep this from upsetting the bias, which it seemde to do in the ECC83 prototype.

That's about it, really. I'm happy with it because it fulfills all the purposes I set out to fulfill: it's relatively light; compact; loud enough for rehearsals; sounds good; and it even looks good. It looks like a Vox, but it doesn't sound like one!

I use it for band practice which works great, but to keep it doing that, I made an amp cover from some sail-material I bought at a yacht supply store. See pictures below. I didn't exactly drew out design plans for cutting up the material (which is made of plastic, BTW, not cotton as you might've expected) since that would leave too big a margin for error.. I simply made up sketches and folded the material over the amp to see where I'd have to cut. Oh, and the opening for the handle: I wanted to put a flap there, which would cover the hole there, to prevent rain from seeping in. I didn't do it, because the material would be too sturdy and not tuck under the handle nicely. So far it's never rained, so no test, but the amp doesn't look any worse for wear and tear from being transported in my car (I'm a gentle driver, BTW.) because the cover shielded it nicely. So it [i]does[/i] serve a purpose. I did paste some strips of material along the opening for the handle to keep the cover from fraying, so far no complaints about that.

Some images of the Vamp:

29-jun-2008
My DIY amp cover	950kb My DIY amp covered by my DIY amp cover Side view of the amp with cover - to give you an idea of how it looks.
cover's inside	inside view of the amp cover, note the lining along the opening for the handle.
corners	Side view of the whole thing. It's corners aren't done so neatly, but should be waterproof
09-jan-2006
Hey Joe 1	950kb I'm "a bit" rusty on the guitar at the moment, but here's my take on Hey Joe, played on the Epiphone Firebird's neck and mid pickups (the mid is "always on", unless you turn down it's volume control). Control settings: preamp volume at 6, master volume wide open at 12. This recording seems to capture the impressive soudn the best of all. In real live, it sounded even more impressive than on the recording. Maybe that's because of the 12" speaker pushing so much air. I haven't been able to capture that sound in this recording.
Hey Joe 2	900kb Take 2 of Hey Joe, played on the Epiphone Firebird's neck, mid and bridge pickups. Apart from the lower recording volume, the actual sound really hasn't been captured on this recording but it gives an idea. The Firebird pickups are about as powerful as a good Strat, with a slightly warmer/darker tone.
Hey Joe 3	900kb Take 3 off Hey Joe, played on the Epiphone Firebird's mid and bridge pickup. Again, my hat's off to the professional sound engineers. And to good gutiarists, too.... Maybe I should stop messing with amps and work on my chops instead?
Sexy	650kb An abbreviated performance of Sexy by Marius Müller-Westernhagen. To showcase the immense gain and distortion that even a "medium-gain" preamp tube like the 6SL7 can give you (actually a mu of 70 counts as high gain). Helped by the lack of gain-draining tone stack, obviously.
16-dec-2005
Schematic for 6SL7	20kb This design is almost completed. I might change R8 to 680 to get rid of some of the nasty farting-out of that stage when gain is maxed and a guitar with hot output is used (my hacked Strat with Duncan Distortion SH-6 'buckers). Other than that, this seems like a keeper. An extremely hot amp! There's a lot of poweramp distortion in this amp, so R7 might even be reduced to 150k or even 120k, depending on the power of the guitar signal.
6SL7 with NFB	22kb The same schem, but with (untried) negative feedback loop, presence control and switch to move the whole NFB-electronics out of the circuit. The NFB should allow me to play loud and (relatively) clean, which is totally impossible without NFB.
11-november-2005
Front view	175kb 12" Weber 12A125S speaker hidden behind Vox cloth, with white trim, from Verduin Electronics. Looks almost like a real Vox.
6SJ7	70kb This is the tube that saved my amp! Sadly this particular tube went microphonic after about one hour of playing.... but I had a spare, and these tubes are still being made by Sovtek so I can carry cheap spares to band practice.
Gutshot with 6SL7	95kb Point to point preamp: the power amp has it's cathode bias resistors + cap mounted on the filter cap board. BTW can you imagine that this amp does not hum an awful lot?
19-september-2005
With 2 6SJ7s	10kb One 6SJ7 on it's own was too weak, so I cascaded two. Put a 1M-A || 1M volume + ground ref resistor in between. Holy smokes! Waaay too much gain and distortion! There's still some work to be done... Hum is too loud as well, but extra filtering will cure that.
No voltage divider	10kb This is the sound you get without a voltage divider to dump some of the gain, as well as a poorly-biased 2nd stage. Biasing that stage a lot hotter will do miracles for the distortion upon fade-out. Notice the fartiness on the chord's decay.
17-august-2005
Schematic v002	25kb OK here's the latest ECC83-type schematic, in convenient PNG format. Tone control is gone, NFB circuitry is now as per reality, but you'll have to excuse the lack of input and output jacks! :-) The only difference is that the MV is spec'd as 500k-A with 470k to ground in parallel; in reality this is 1M-A with 1M to ground in parallel, but just for testing I've inserted a 470k in there as well so now the resistance to ground is ± 250k, almost as per the original schem.
14-august-2005
Almost finished...	75kb I wanted to see how it would look with both backpanels on (top and bottom), all it needs now (looks-wise) is the small nameplate which will come to the left of the impedance selector switch + I/O jacks. It will also have a red tube shield for the preamp, to match the panels' colour.
Top view	75kb I think this turned out quite nicely. Controls, from left to right: input, gain, master volume, raw/Hi-Fi switch (NFB cut), Presence control, pilot light (it's red but the camera didn't catch that properly), power and standby switches (On 1 and On 2). In honour of the original 5F1 Champ, I've numbered the controls through to 12. Except I've just found out that Fender started at '1' and not at '0'! Oh well.
Rear view inside the Vox box	120kb Rear view of the amp in the old Vox cabinet, showing the tight fit of the 12" speaker, tubes and OT. The PT is hidden from view behind the chassis plate. There are 2 fuses because in my country, we have symmetrical power cords, so you can't tell which wire will be the hot lead. Note rudimentary but effective construction. This is an amp that I'll put together only once unless really necessary!
2nd rear view inside the Vox box	105kb A slightly better view of the controls: from left to right, input, gain, MV, NFB switch, presence, pilot light, power and standby switches.
Rear panel	105kb Note impedance selector and auxiliary in-/outputs on rear panel. The long wires and protruding bits are a bit of a concern when putting the panel on the back, but first I've got to get it working! The short yellow stubby connects speaker negative to common ground; green connects speaker negative to aux input negative; red connects OT secondary positive to speaker positive. Note the blue bundle of OT secondary wires leading to the impedance selector, from where the wire goes to the jacks and speaker.
13-may-2005
Schematic with changes	55kb A redrawn schematic with changes, which turn this into a 5F1++. Shows bypass caps, tone control (from Gar Gillies' book), master volume, NFB cut switch, presence control. I've taken the artistic liberty to leave out certain components due to them not being available in the schematic software's library, or me being too lazy (or both). I used Circuit Maker Student to draw this (free but limited to 50 components). As stated above, this circuit is now obsolete but I leave it here for prosperity. I have yet to draw an updated version.
24-feb-2005
Top view of the amp	150kb First time it's put together (prototype chassis). Still no MV, NFB cut, etc. The power tube sits rather close to the OT, but that's OK. In the final product, the transformers will be on a different plane to the tubes, so even heat transfer won't be a problem. Electric radiation already isn't.
Inside view of electronics	120kb It's a bit cobbled up to one side of this big chassis, but the final chassis will be a lot smaller so I got some fitment practice. There's a volume knob, and the ground bus sits at the back. No shielded wire anywhere, but that didn't cause the squeal. I don't like running wires underneath, regardless of what the "official" Fender layout says. You can't reach them and after a while you forget which wire goes where. I've put a 10µF/63V bypass cap on the leftmost resistor, to add a touch more gain.
Inside view of electronics	160kb A nasty tangle of wires, but I didn't want to cut them too short for the prototype. At least the heater wires stand proud of the rest. These octal sockets are a dream to solder to, much more room than on the noval ones. Got them from Tube Town, type sk02 (so not the Fender style!)