- last updated 04/12/11 14:03:20
History - last updated 16/09/06 00:47:22
I now have an Accutronics reverb unit, a mains transformer, an output transformer, and a pair of Tesla EL84's to give it a kick. The amp design has been modelled, and should push out around 5W/20W before clipping depending on whether it's triode or pentode mode. It's going to be a single channel amp, and the preamp design from the DDS-2250V is going to be used due to it's versatility.
The chassis is going to hang Bassman style down the back of the cabinet with the controls on the top. 1.6mm brass sheet folded, then chromed - I hate painting things, so this is going to be a bit of an experiment. Also awaiting a quote for laser cutting to do the stainless steel badge for the front.
Now where can I get a Celestion Vintage 30, and some of that tweed covering material.........
Still no sign of a chassis, the search for "tweed" covering material has really pissed me off. Skip the tweed, I've got some nice dark green leather look alike material along with speaker cloth from those nice people at Trace Elliot. Also got a brand new 16 ohm Celestion Vintage 30. The wooden box should be completed in the next couple of days...
the box is now made and covered etc. The Vintage 30 has been bolted in and the whole rig
is plugged into my stereo for "testing". Have weighed the baby, looking at 33lbs
for the speaker and box - no amplifier yet. I reckon the whole rig will come to near 50lbs
- fairly bloated for a 20W combo. V30's must be pretty sensitive, I've measured 4 watts
RMS required to shake the house to its foundations. Better give it a break before the
police turn up.
Have now got a sheet of brass (courtesy of Kearsley Precision Engineering in Basildon) - some holes have been drilled, and the flat sheet has been sent off to have the control legends engraved. Also got confirmation that the people who are going to bend the chassis can deal with 1/2 metre wide 16 gauge brass sheet....
Total project cost to date, UKP £213 (USD $354). Still only a speaker in an empty box, and a load of components..... If I had some wonderful CNC gear from Ah-Ha! Design Group, the chassis would probably be finished by now...
Hmm.... No sign of my chassis back. I hope the guy hasn't screwed it up... 5 weeks on, and my patience (or what little patience I had) is running out.
The project started off as a "cheap" practice amp. I think I'm going to come up with the most expensive practice amp in history. Mind you, VOX AC-30 meets hot-rodded Fender Blues Deville sounds pretty good to me.
Just to maintain my interest, here's a render of what the chassis will look like when a few bits and pieces are bolted onto it. At the bottom right is the choke - the only bit of Marconi CR-100 which will end up in the amp. It's 55 years old, so I decided to give it a coat of hammerite. Should keep it going for another half century.
I hope to post a real picture of the chassis in the not too distant future...
Yes!!! Things are looking up. The chassis is back now, all engraved, but with not much in the way of holes drilled. I've been told that the turnround for chroming the chassis is a week, so I'm about a fortnight away from completing the metalwork (I hope).
Managed to get a day off work to go down to Go Karts 'R' Us to get the chassis drilled.
All the holes are in the right places now. Next step: get the thing folded up and off to the guys who are going to chrome plate it.
That "real" picture of the chassis is now a step closer...
The chassis is expected back around Friday 8th, all nicely chromed of course. I've made the effort to draw the circuit diagram up. You can grab it as an Adobe Acrobat PDF file [143k].
Got the chassis!!! Will order the remainder of the parts required today. This project could be finished in less than a fortnight (more famous last words).
Lots of good news. A PCB was made up for the triple low voltages supplies yesterday, and works fine. The power supplies are all built into the chassis now, as is the phase splitter and power amp. The input jack has been temporarily wired to the master volume control (no preamp or reverb PCB yet), so the rig can be tested.
Plugged the old Gibson in, and fired it up. There's enough output from the Gibson plugged into the phase splitter to push the power tubes into distortion - it's a nice rock sound, not harsh at all. It's plenty loud too....
Putting that 1940's choke in was a good move. There is *zero* hum coming out of the speaker, and a barely perceptible amount of hiss. On tickover, it sounds like a (quiet) solid state amp. In fact, it was so silent when I dropped the standby switch for the first time, I was convinced something was wrong. No click when the standby went down, no short hum, no thump. Then I hit a chord and got smacked by 20 watts of class A tone.
I like my old Gibson Sonex, if for no other reason that it has so much balls on that low E string, it will make poor amps and cheap speakers crap out straight away - it's a real acid test guitar. I'm pleased to report that the Blues 112 doesn't crap out like most small combos, it just shakes the house. It should have plenty of punch for live playing, although it would need to be miked up for larger gatherings.
Have to make up a reverb board now, and get that preamp wired up. It's looking good, and sounding good - reckon I'm around a week away from completion (even more famous last words).
Checked the output power at the onset of clipping (400Hz sinewave). 10.0W in pentode and 2.9W in triode. Hmmm..... That sounds a little light, so my previous claim of getting smacked by 20 watts of class A tone was way off the mark.
Have checked things through, and there are a few small changes I could make to boost the output to around 16W, like cut the power supply sag resistor, stick a 12V Zener across the EL84's cathode resistor, and... Hang on, I made this for a reason - it's a practice amp!!!
It's plenty loud already and I don't plan to sacrifice reliablity for 6 extra watts I will probably never use - so, looks like it's gonna get left like it is. Also looks like it might be finished by Sunday....
Bugger. Got three quarters of the preamp built, and ran out of solder. Looks like it might not be finished by Sunday. Bugger #002: the preamp is generating so much hiss, it almost swamps the guitar signal. Something is not right. I've swapped valves, checked voltages, everything.... Checked the output to the speaker. With the master volume at zero, there is 60mV p-p at bang on 10MHz. Something is definately not right. Where's my stock of ferrite beads......
|25/8/97 - It doesn't work
Mad Mick at Go Karts 'R' Us dug me out of trouble, and provided much solder yesterday morning. The 60mV at 10MHz still appears with the amp switched off, so that's not the problem.... Also, the preamp is overloading when using the Gibson. Back to the drawing board.....
|31/8/97 - It's getting quieter
It's get well time! The residual hum and hiss is down to 75uW (volume at full, gain and reverb at zero) - although this is still quite audible with the sensitive Vintage 30.
On preamp style 1 (Clean), with the gain at max and the volume at around 2 o'clock, clipping starts to occur with the bridge pickup of a Strat. The noise floor at these settings is 20uW. This represents 57dB Signal to Noise ratio. Although I can still hear some hiss from the speaker up close, it isn't worth me trying to beat this figure...
On preamp style 3 (Lets Rock), with the gain at max and the volume at 2 o'clock again, the amp kicks out a measured 10W RMS from a guitar signal. A nice buzz at 100Hz has been reduced from a ridiculous 250mW to 400uW. SNR is up from 16dB to 44dB. The culprit is a noisy power feed to the solid state board which houses the reverb drive/pickup and front-end preamp. The reverb circuit makes plenty noise too, which troubles me a little more....
A pair of back to back diodes in the solid state preamp have killed off all tendencies to overload and block the following valve stages.
To Do: Fix the solid state board to kill the buzzy reverb, and reduce the input preamp noise a little more. Also, make a new wooden back panel to fit the vents I've got hold of.
|25/9/97 - It's Finished!
Or at least it's about as finished as I'm going to get.... Killed the buzz on the reverb: the scope was showing 20mV pk-pk of mains hum on the supply lines. Doesn't sound like a lot, but you can certainly hear it... The cure? 150 ohm resistors in the supply line feed to the op-amps. All-tube reverb for me next time!
Got a few muso's to run through it last weekend and expected comments like "it sounds like an AC30, but....". Didn't get any of that. Couple of guys said it had a distinctive sound like the Music Man 112RP when cranked up. Suppose that's a compliment.....