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Newcomer, Melbourne, Australia

Started by nodz, March 22, 2013, 04:42:53 PM

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nodz

Hi
Just thought it was time to introduce myself.
My name is Chris but everyone calls me Nodz.
Been lurking around various guitar/amp and hifi/diy audio forums for a while.
Have four guitars (3 electrics and one acoustic) and two SS practice amps.
Have made some SS and valve audio equipment, thought is was about time I started skimming around the net and try and design my own guitar amp.
So after a bit of reading, designing, redesigning, borrowing circuits from others I've come up with the attached.
Due to constraints both time and financial, will probably build amp in modular fashion, that is step 1) preamp, 2) stand alone reverb 3) then the PA. 
Possibly will include an overdrive section but the idea is to have an organic modular amplifier, that can be altered as I go along so if I don't like certain developments, I can revert back to the previous state if desired.
I also haven't decided on the final PA.  At the moment, it uses a couple of PP 12AQ5 (just because that's what I found in my junk box) but I may use LM3875 and have 2 x 50W output channels, 2 x 6L6 (50W) or 4 x 6L6 (100W).  I just haven't established what kind of O/P I require yet.
Cheers
Nodz

phatt

Hi Chris and a big welcome to ya matey,,
Yep there are a few Aussies here. :tu:

Plenty of project ideas float around here, so plenty to keep you experimenting.
Yes modular is darn good idea and is the approach I've been using for a long time.

I assume you are wishing for a real spring Rev unit then hard to beat ESP circuits.

IME Outboard Reverbs tend to work better than inbuilt setups.
I have built a few stand alone Tank circuits with the aid of Rod's wonderful reverb ideas.
Phil.

Roly

Hi Chris, and welcome to SSguitar (from Creswick, Vic).

I suppose you've seen the Lamington (15W) and Lamington-II (50W) designs.  I designed the HT quadrupler in those because I wasn't all that impressed with back-to-back trannies for HT, particularly at higher power levels like 50 watts, and I think they would be a lot more practical at the 100 watt level.  Those 12V QI trannies are starting to get a bit thin on the ground now that they are being displaced by Switch Mode "solid state transformers", but then that whole QI lighting scene is about to get a monster shakeup with the arrival of plug-in replacement LED's.

I notice you've used a 100k pot right at the input.  This is fine for active guitar or always using a stomp, but for passive guitar straight in I find that even 1Meg input is a bit low and have been building FET buffers with 5Meg input resistance to go ahead of things like amps and stomps with lowish input resistance, and the difference is quite noticeable, the liberated pickup resonances really sing out.

I've just had a glowing report from someone who built a Lamington using the M1115 and replaced it with a M1120, gave it a stage test this week, and is very satisfied with the improvement.

A couple of little points around the standby switch; I'd personally put it in series with the first 100r resistor to limit the current surge through the switch contacts, and you don't have a bleeder on your HT line.  These days I put something like a 47r/10W right across the OPT secondary after I had to repair a Bandmaster with a boofed OPT because the lead hadn't been plugged in at the speaker end and a shorting jack on the amp didn't help.  Not that M1120's are expensive, but a dead amp at the off is a real pain.

@norv at AGGH has just done a build using Rod's s.s. reverb mated into a valve Lamington, the Watson Wipeout, and that seems to have worked out well.  We've also got a s.s. LDR trem add-in well advanced (no heater demand), boards done and test built.

That's quite a developed design you've got there.  :dbtu:  Be interested to see how it turns out.

If you say theory and practice don't agree you haven't applied enough theory.

Enzo

welcome to the forum.

A detail often overlooked in your area is that for proper operation, amplifier designs from the USA must be operated upside down.

nodz

#4
Hi Everyone, thanks for the words of welcome.

@Enzo
Yes everything from the US uses an 'inverted' input down under...lol

@phatt
As you said I think that modular is the way to go.  I think that it helps with the trouble shooting.  I have an amateur radio background and nothing worse than getting to the end of a project and you have to debug it from the beginning.  I learned early on that it is much better to get one section working before moving onto the next.  I was thinking modular too, from the point that, I have other pieces of equipment around the house for instance an SS Sansui audio amp, some SS guitar amps, SS preamps and it would be good to develop each of the valve pieces independently and see how they affect the sound/performance of the other items.  Also would be good to build something like the real spring reverb unit and then be able to use it with other amps etc

@Roly,
Yes I am familiar with the Lamington but hadn't seen the Lamington II.  Have been around the ozvalveamps and AX84 sites, so have seen various schematics but must have  missed the 50W version. 

Thanks for your helpful hints with respect to my circuit.  It is quite developed. I have been piecing the design together for about five years.  As I've done further reading and gained better understanding or seen a design feature I've liked, the circuit has been modified.

I agree totally with your statement about the back-to-back HT transformers. Originally, I was going to use this configuration for the whole amplifier (which was far simpler), as it only had to deliver power to 1 x 12AX7 and 2 x 12AQ5.  So about 600mA in heaters/filaments and then about 95mA max signal current.  So not really an issue for these transformers.  However, I have now added the second 12AX7 for the reverb unit and I was thinking of adding a 5879 pentode for overdrive, so these transformers will not be sufficient.

You are right about them getting thin on the ground though.  I bought mine for $7 for the pair when Dick Smith's got rid of all there components and became a computer/audio equipment store. 

I was going to maintain this back-to-back PS config, purely for the preamp stage.  As part of the modular design concept I will include separate power supplies, one for each of the PA, the tube reverb and tube overdrive sections, this may be overkill but each modular piece can be completely stand alone (including power supply and enclosure/housing) so that I can chop and change and use them with other gear.  I will probably put together the Lamington type supply for the PA.

In regards to the input, I don't always use stomps but I do sometimes use the Behringer GDI21 which is a DI/amp modeller.  Because I don't always use that would you suggest that for a simple improvement, I change that 100K pot to 1M?

The 12AQ5 are probably not the best output tube but i thought that they would give me about 10W for a nice practice amp and seeing I had them laying about, I thought, why not?   The M1120 was used to give 16K at primary for 8ohm speaker.  This would reduce the output somewhat as the 12AQ5 requires a load resistance of about 12K in PP config.  This was taken into account when doing the biasing calcs etc.  The M1115 and M1120 are a good value option, seeing that things like the Hammond OPTs are so expensive.

I can change the position of the standby switch.  I will put it in after the 100R resistor.

The bleeder resistor, put it in parallel with the 220uF or at the end of the filter chain?  Something like 150K/2W? Take about 5 seconds for V to drop from about 240V to 30V on the 220uF.

The 47R/10W across the secondary of the OPT is not a problem.  That's easily done.

Cheers Chris

Roly

Quote from: nodzIn regards to the input, I don't always use stomps but I do sometimes use the Behringer GDI21 which is a DI/amp modeller.  Because I don't always use that would you suggest that for a simple improvement, I change that 100K pot to 1M?

In a previous era I held VK3YFF.

Yes, at least.  I discuss the issues in some detail here.

Rule of thumb for bleeders is 1k/volt; 300v supply, 270k or 330k.  You're experienced, but I like to encourage builder to also put in a neon tell-tail on the HT supply, helps to avoid those "OUCH! ghoddamit!" moments we can all have.  Because the neon goes out at about 45-90 volts this should be separate from the bleeder.  Both generally on the first filter cap where the voltage is highest, but if you are going to move the standby switch you really need a bleeder each side

Nuthin' wrong with a 10 watt amp, these days with PA's generally available at venues it seems like the favorite power for gigging guitarists is around 10-15 watts; you can go deep into output stage clipping without blowing down a wall.

You should find the Lamington-II on Grant Will's site valveheaven.com.

Just out of curiosity, what Melb suburb are you in?
If you say theory and practice don't agree you haven't applied enough theory.

nodz

I hold what was the limited license VK3TUB, which was the frequencies above 50MHz, I was interested in mainly 6 and 2 metre bands and 70cm, but when they did away with the Morse code requirements, I got into homebrewing for the 80 and 40 metre bands.

I'm in Narre Warren on SE side of Melb.

Rightho, so bridge, then 220uF cap, then bleeder of 270k to 330k and neon in parallel to them, then 100 ohm resistor with standby switch in series, then another bleeder of 270k to 330k to gnd.  I'll redraw and post, to ensure that I've got it right. 
Cheers Chris.

nodz

Made the additions of the 2 x bleeder resistors and the neon lamp.  New schematic attached.
Cheers Chris

Roly

Quote from: nodzwhen they did away with the Morse code requirements

They did?  I'm so out of touch with that world now I didn't know.  All that time spent slaving away at Morse, and now I wouldn't need it.  Ho hum.

Yeah, Rabbit Warren, used to service industrial tramp metal detectors in a pit at Harkaway (among others).  Don't know if you've ever see a "shot" fired, but even a small one is pretty impressive.  Servicing electronics in quarries has to be one of my least favorite passtimes - hanging upside down on a bouncing conveyor (they don't stop anything for the likes of me), being pelted by rocks from the nearby crusher, trying to read a meter and make sense of what's going on, knee-deep mud just getting to the equipment - makes guitar amps seem like a doddle.   ;)

Yeah, that looks good, remembering that the neon either has to be a "240V" one with an integral limiting resistor, or you can use a naked NE2 with 1k/volt resistor in series.

It's an idea I picked up from working on Variable Speed drives that had one neon for the mains in and another to mimic the output to the motor; a useful reminder when working direct-on-mains.   :loco
If you say theory and practice don't agree you haven't applied enough theory.

nodz

I've just seen a nifty idea on one of the audio forums.  I can get a 16V, 1.25A AC wall wart and use this as the mains input, step down transformer.  I went rummaging through the junk box and found a transformer that had 2 x 15V, 0.67A secondaries.  I'll parallel the secondaries and use them for the step up transformer.  Voila 256V out.  I'll use 2 x 2A bridges for the HT and heaters and an LM317 for 12.6V out.  Preamp PS done using a simple wall wart and a 2.1mm socket for the mains input.

Roly

16 * 1.25 = 20 watts, less heater power, divide by two for output power, multiply by 0-point-something for second transformer inefficiency - doesn't leave a lot of margin...

I despise those co-axial DC connectors with a passion - 'orrible things.  Have a look at CB microphone connectors with locking rings.
If you say theory and practice don't agree you haven't applied enough theory.

nodz

Only talking about the PS for the one 12AX7 tube in the preamp, 20W should be plenty, shouldn't it?
From my reckoning (and I am probably wrong :-) )
Heaters 12.6V @ 0.15A gives approx, 2W.
Less than 1mA on each plate, call it 0.5W combined.  Even at 25% efficiency in second transformer should be enough, shouldn't it?

Roly

Quote from: nodzOnly talking about the PS for the one 12AX7 tube in the preamp, 20W should be plenty, shouldn't it?

Oh sorry, misunderstood, thought you meant the whole amp - yeah, should be ample.
If you say theory and practice don't agree you haven't applied enough theory.

nodz

Will definitely have a look at the microphone screw ring connectors and sockets, good idea that.  Won't take much to rewire the tip of the AC wall wart.  Have got a lot of the components already, will have to see what else I need including the hardware.  Getting the PS operarational might be a project for the Easter weekend.

nodz

Re-did the power supply.  Powered by the 16V AC wall wart and then transformed back to 256V, however, will be using 6.3V, elevated heaters to recuce hum etc.   Updated circuit now attached.