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[irfrench]

TWEAKZ

So the sound samples didn't happen - I've been uber busy at work (I work from home most days, hence possibly squeezing in) and I didn't get chance.  I WILL do it though! :)

I also got chance to faff with some negative feedback... So the idea is to put some of the output back into the amp at an earlier stage - read HERE for info - the main purpose of this for me was to reduce distortion a little.  I had a spare 10k resistor and whacked it in.

SQQEEEEEEEEEEEEEEEEEEAAAALLLLLL

OK, I didn't luck out.  The negative feedback is only negative if it is out of phase with the original.  As it was wired it was a positive feedback loop so it was generating its own signal.  'Helpfully' I always keep my wiring quite tidy/tight so I had to reverse the secondary leads instead of the primary.

Much better - AND a much more usable range of gain.  Sweet.  I then thought I'd better do it properly and try a pot there.  I stuck a resistor in series (to maintain a minimum resistance/maximum level of feedback) with a 100k pot to see what varying it does (using the helpfully placed legacy IEC hole!):

After much faffing I found a spot I liked, disconnected the leads (with the amp off) and measured the resistance - 11k - so I whacked the 10k back in... happy days!

I've done some calcs too, it looks like the B+ is a little low (at about 200v - I was aiming for 220v) and the ECL82 dissipation is at about 4.6w.

As mentioned before - this is NOT a loud bass amp.  If you're sticking a mic on the amp for recording, or just practising at home it'll be fine.  Gigging? NO WAY.

Just thought I'd mention that again! :)

x

[irfrench]

So... Enough thinking. Lets build.

ITS ALIVE!!!

I did as much pre-planning as I could, I thought about it some more and then eventually broke.  Time to build:

Having received the last components from Tube-town - check component sizes on turret layout.

Cutting turret board down to size.  Barry at AMPMAKER sells these easy-drill & easy-cut boards.  DEFO recommended!

Pre-punch holes to stop the drill slipping.

Barry also sells these screw-in/nut-tighten turrets... Kinda cool idea so I thought I'd give it a go. 

He also sells a specific sized drill bit to allow the turret to fix into the board.  As I was stupid and used a dremel to drill the holes this precision was lost on me and the holes over-sized. I then resorted to plan-b where the turrets are fixed with a nut.  BTW when drilling these boards the plastic tends to gets stuck on the drill bit.  Hurts like hell with those things whipping around at 10,000 rpm!

Parts organised and ready to populate the chassis - which BTW used to be a broken MOSFET amp...

Once again - I cannot recommend using INKSCAPE enough.  It is SO useful being able to mock-up things to scale... The turret board for example, and the chassis... well.

Once the metalwork was done - which goes quickly if you don't ever have to actually do it! - and the dust settled, time to stuff.

Early days turret population.  I think I will re-work the dropping resistor to the ECL82.  I had to piggy-back two 8K2/5w to get the 4K1/10w I was aiming for.  I think I may drop this down to 3K9 to bump up the B+ a little and use one of those fancy chassis-mount aluminium 25w jobbies.

Finished populating the board and adding flying leads prior to installation.

Nearly there:

Did a few necessary safety checks first - grounding, continuity etc... Then fire it up WITHOUT the valves.  Check the filament voltage and the rectified B+...  All good.  Add the preamp valve only... All good...  Add a dummy load and then the ECL82... Power up and...  and... ANNNNDD.

HURRAY!  No fire, no smoke, no burning smell.  No electrocution.

Only one thing left to do.  Plug it in..

Works a TREAT!  Sounds great already.  There is a little bit of hum but I think that is down to the 6sl7 (the little pre-preamp had the same noise when fully cranked). It is a great volume for recording. NOT GIGGING! but I do feel it breaks up pretty quickly, as such I'm investigating/asking questions about the negative-feedback suggestion...

Hopefully at lunchtime I'll get chance to record a quick sound-bite... so far I am VERY HAPPY.

Once more, a quick thanks to all the kind and knowledgeable gentlemen at GroupDIY that have helped get this idea together.

x

[irfrench]

Ha ha, the best laid plans!

Oh well... The best laid plans...

I built the preamp and there were a few errors. In all fairness they weren't all my doing but I've learnt and am moving on.

First thing. The B15 preamp section is not a high output preamp.  It is followed by a number of additional gain stages which make it usable (in the way I wanted to).  It sounded fine going into my Ashdown effects return but thats not what I wanted!

Second.  The output impedance of the preamp (in this version) is such that it can be loaded down very easily...  I ran it via the bass-splitter and the signal became much more friendly... Again not how I wanted to use it.

Third.  This is the first time I have tried the back-to-back transformer method to generate power.  Due to the space limitations I used tiny transformers which are very lossy and easily get loaded down (causing the voltage to drop).  I used a spare alternative transformer to bring up the HT voltage but ultimately I wasn't happy with the bodge.

Now there is something that is my fault - the layout.  I am not sure if it is wiring or the proximity of the two PTs to the 6sl7 but on the extreme end of the volume level hum started to become audible...

So there we have it.  An experiment and a learning experience.

Very helpfully though the kind gentlemen at GroupDIY have helped steer me in the right direction and in particular PRR - who gave the cool idea of using an ECL82 valve.  This is a really cool little triode and pentode combo in one bottle.  The intention being to use the triode as a gain-stage and the pentode as a single-ended output using a Champ-esp OT (7k primary)

I have done a LOAD of reading over at ValveWizard and tried to make some loosely educated guesses at a 'good' circuit and run it by the gents at GroupDIY.  So far it looks workable and I've now drawn up a chassis layout.

I need to do some double/triple checking but hopefully it'll be a great sounding 3.5w bass amp.  I am NOT expecting this to be LOUD and CLEAN, but then that's why I'm building it!

When I know how this is going I'll go into more detail and properly share schematic/bom/layout/samples.  Which leads me to another thing to mention - I've had a couple of requests for layouts etc. for prjects I've done on here.  Sorry to those I've not got back to but until I'm 100% happy with the files/layout I'd rather not share (I usually build one version - find some tweaks, update the layout and never get around to verifying the tweaks...) sorry.

Hopefully this amp build will work 100% and be shareable immediately!

Fingers crossed.

Ian

x