Other Amps

Blondish amp build

51

Loving all the nerd talk!

I'm very intrigued about how its going to sound!

52

Me too Jayce! Hopefully first fireup this weekend. Let's hope it's the notes that blossom, not the amp. ;-)

nerd talk

Haha, I forget sometimes. 8-)

53

Ok, preamp is wired up too now (sorry no pic at this time).

Tomorrow I'll swap that rheostat for something more sensible. And then it's time for 1st power-up!:D

54

The amp is ready for first power up!

I've finished wiring up the preamp. Today I spent some extra time, relocating the bias control and replacing the 5k sag pot with a 100ohm one. Also added a virtual ground to the heater wires to reduce hum.

Chassis complete!

Replaced the doomsday device with something a little bit more sensible

Preamp section

Rotary switch to select different coupling caps between preamp and poweramp Blue ajax curtesy of GDP member Johnyon. Russian PIO curtesy of G-T member 6strings. Thanks guys, I'll put 'em to good use!:)

55

First powerup, yeah! ... and nothing has vapourized yet. :)

First -ah well, second- powerup! At first the light didn't go on when I powered up with a low 40V mains voltage (to go easy on the amp and caps). I measured the heater voltaged and realized I'd made a mistake with the leads on the jewel light socket. Effectively it shorted the heater voltage. Fixed that and, on the second attempt, got a faint but very gratifying glow from the jewel light.

Gently breaking in the caps I increase the mains voltage with steps of 20-40 Volts and measure the voltage drop over the last PSU resistor. After 10-15 minutes, when this drops down to about 100mV, I increase the mains voltage another step. I use a variac to control the mains voltage.

57

DANG!!! Very cool stuff indeed! This sort of stuff is WAY-WAY-WAY over my head. Glad to see you're undertaking an individual project! Kudos! Nice pics too!

Johnyon

58

Whahahahahah! :D

Least all the blue smoke stayed in the resistors and caps!

59

Okay, caps are broken in, time to measure voltages and put tubes in.

I figure, since the PSU voltages only settle when loaded correctly, I work my way backwards from the power tubes to the preamp.

First off: no heater glow! Quickly found that onr of the heater wires had dislodged when I installed caps for a virtual ground tap. Fixed: nicely glowing tubes with 480V B+.

Next: biassing. Hooked up bias probes and measured... Nothing! I'd forgotten to hook up the screens. D'Oh! Installed wires and sure enough, I got an arctic 10mA/tube. Set the bias to 30mA and prodded the circuit with a probe. I am very satisfied by how absolutely silent that staged sounds. No audible hum or noise, while I do hear the rustling of the probe where I touch the circuit. Nice!:) BTW, I am using the minimass with headphones for now. Only when that appeara to be working, I'll get to hooking up speakers.

Next: measure PI voltages. Yikes! A hefty 470V where there should be around 415V. And about 450 where there should be... 300.:I

The PSU resistors are set for an amp with 6 preamp tubes. These will load the PSU and bring down those voltages. I had to recalculate the PSU resistors to get the voltages in the rigt ballpark, with only 2 preamp tubes in the circuit. The PI stage is now getting about 400V.

... And that's where I had to leave it at for now; kid woke up, daddy-duty is calling. More tonight after I tuck her in.:) I have high hopes I'll have it debugged and running by tonight.:)

60

Nice project Michiel I had used that flimsy yellow board stuff next to the tranny and I really hated that stuff Decided I never use it again. I like the idea of the sag pot.

Maybe you should bring it the next meeting at zurich :D

61

MUHAAHAAHAAHAA! IT'S ALIIIIVE!!!:8-):D

Fixed the PSU resistor that feeds the preamp and... Presto! The amp works like a charm. Voltage on the PI is a little over spec (but with the 20% margin Fender used to spec 8-o). The preamp is spot-on. Lovely!

Tone controls work nicely too, although the mid control behaves a bit weird. It has a sweet-spot in the middle of the pot and rolls off to either side. I may have to try a different pot-value there.

It gives lovely punchy cleans up to about 4-5 and gets dirty in a lovely way. Very creamy and smokey. The sag pot is great too, although it doesn't kick in much until the amp is well into the driven range. A bigger pot should help there (oh god, I may have to go back to that 5k monster!;-))

I had so much fun cranking it actually (through the minimass), that after 2 minutes fully dimed, the HT fuse blew! And not because of something shorting out, but it blew because the amp is TOO effin LOUD.8-)

And now the weird part: when I turn up the presence, I get a strange 'boing' sound from the amp, tuned at exactly a B-flat. (a fading oscillation) D'oh, that's exactly the key we play in most.:|

I tried swapping the OT leads, but that wasn't it. I have an idea though; this may be related to my choice of the NFB feedback-resistor (56k iso std 100k there). I may be feeding back too much right now.

62

BTW, for this first powerup I used some 'expendable' tubes; an old set of GT 6l6gcs, a sovtek 12ax7 in the PI and a JJ 12ax7 in the preamp. Already though, the amp blew me (and the HT fuse) away.:P

Once I've ironed out the last quircks, I may start swapping the tubes for more premium glass.

The current B+ (~485V) is too high for Tung Sol 5881s. I hope to bring it down into the 440V region with a couple of zeners.

63

Nice project Michiel I had used that flimsy yellow board stuff next to the tranny and I really hated that stuff Decided I never use it again. I like the idea of the sag pot.

Maybe you should bring it the next meeting at zurich :D

– MaBerY

Nice project Michiel I had used that flimsy yellow board stuff next to the tranny and I really hated that stuff Decided I never use it again. I like the idea of the sag pot.

Maybe you should bring it the next meeting at zurich :D

– MaBerY

Thanks Marcel! I'm glad you like it.:)

Indeed, I don't think I'll ever use that yellow board again. I'm dead scared of that stuff. It's so thin, you can cut it with a pair of scissors. And those contact ponts drop out of the board if you bend them inward a bit. Really nasty stuff.:|

And ofcourse I will bring it to Zurich!.:)

64

Michiel,

Showed Ilana (my wife) and Priya (our 8 year old daughter) the shots of the tulips and both of them were stunned.

Thanks. Utter gorgeousness.

65

Showed Ilana (my wife) and Priya (our 8 year old daughter) the shots of the tulips and both of them were >stunned.

The tulip fields near my town and work at Petten are blooming now I might drop a pic too :D or should we start a whole new thread :)

66

And ofcourse I will bring it to Zurich!.

great!

I hope that my Schaller KV50-ST, my resurrected Geloso Bassman clone and Dynacord 62 echo will be finished by then. The Schaller echo still needs some attention, but already has new magnetic heads mounted. The Geloso was totally stripped and rebuilded as a bassman clone.

I love the smell of solder in this thread :P

67

Strum, I'm glad you liked the pics. Those areal shots never bore me.:)

And good idea Mabery, although the only 'original' shots I'd be able to take will be taken at the florist's around the corner. ;-)

68

Cool! I'm curious after that tape echo. Never had the chance to try one.:)

BTW, I've been looking into that feedback thing. On the minimass I don't have the 'boing', only on a real speaker cab (and it's not acoustic feedback!).

I replaced the 56k with a 100k even so; tomorrow I'll see what that does for the that feedback thing. Not sure yet, but over the minimass I have the impression the amp has lost a tiny tiny bit of sparkle, plus the 'presence' control doesn't seem to do much anymore with 100k.

I'm curious how it's going to sound now over the real cab.

Marcel, do you have any ideas about what could cause that 'boing' thing?

69

Mmmm, I suspect the speaker load reaches a peak impedance at a certain frequency and somewhere you introduced a parasitic positive feedback coupling due to bad wiring layout. this can create oscillations. you can probably see it on the scope.

I had this in a SuperSix reverb when I added a presence control instead of the master volume. the parasitic capacitances around that feedback wire caused oscillation in that case. Try a shielded wire for the feedback loop to try to get rid of the parasites. !#$@%

70

Hey, thanks for the suggestion Marcel! I'll try shieling the FB wire.

On second though, I think I'll need to look at the secondaries of the OT; those run very close to the PI board (lower left-hand corner, black & green in the pic, blue=FB wire).

Better yet might be to run those secondaries through the same hole as the primaries (just above the PI socket, left in the pic), to get them out of the PI area completely. I hope the leads are long enough.

Don't mind the rest in that pic; the 5k rheostat has been replaced with a smaller 100 ohm pot (1.5W), the bias circuit (top left) and bias pot has moved to the hole right below the PSU board. And I'd already redressed the grey screen wires to the 6L6es so they don't run that far parallel to the OT primaries anymore (blue, brown & red wire middle left, squeezed between the rheostat and the 6l6 socket). This was before I noticed the oscillation.

71

Here's a quick shot of this morning of where I left off yesterday night. Sorry for the grainy quality. This amp certainly -isn't- a looker right now. But that wasn't the point of this build either.

I tried to stuff the amp in the cab quickly before bandpractice, only to find that the holes in the cab had been drilled all wrong by the previous owner. So THAT's why it was so cheap. :|

I'll need to widen them some (and the top ones in the chassis) to make things fit better.

72

Very cool project, my friend! Keep building!

73

I tried to stuff the amp in the cab quickly before bandpractice, only to find ....

that's a bugger! Next week. Coming along really nicely! with your usual super-sonic speed, I feel more like a tortoise every day ;) . .. about a half of the parts showed up today for my less ambitious projects.. lol, maybe I'll get started before you finish ;)...

Very curious to find out what (or combination of things) in the end was responsible for the oscillations. Didn't you have a similar issue with the BM when you were trying out different NFB values? You were finding reactive differences (and oscillation susceptibility while tuning the nfb resistor values as well) with a speaker connected as opposed via the mini-mass... it's a while ago and that thread is very long .. you have a multi-tap in the BM, so it may have just been the impedance settings and NFB tuning part that I'm [mis]-remembering.

cu! Henry

74

YES! Found the bug tonight!:):)

As Mabery predicted, the problem was related to the NFB, but it was a finnicky one. Moving the OT secondaries outside the chassis didn't help, but shielding the NFB wire to the feedback resistor did... Sort of...

After making that fix, the amp would oscillate inmediately and brutally when switching out of standby, which ofcouse meant that the OT primaries were reversed!

So, the crosstalk from the poorly routed NFB path actually kept the amp STABLE. Wow, nasty one.:D

After fixing those leads, I finally got a lovely crisp and clear sound from the amp. Very much in the ballpark of my blonde bandmaster.

Mind you, this is still through the minimass. I'll need to check with a cab w/o attenuator tomorrow. But I have high hopes that this is solved now.

After that I experimented with the NFB resistor value. 56k was stable, but didn't yet excite that punhy sparkle that it should add. With 33k I -did- get that sparkle and that's where I left it at.

And finally I re-did he coupling-cap selector. There's a 12-way rotary switch there now, but I limited it to 7 positions. The other 5 were too difficult to access and utilize. So, with the rheostat-doomsday-device removed, I now have a nutty professor switch back. MUHAHAHA.:P

The cap values I put in for now are:

  • OD .047u
  • Ajax .047u
  • Mallory .047u
  • POI .047u
  • POI .033u
  • POI .022u
  • POI .010u

Haven't decided yet which ones I like best. The most notable differences I could hear were OD/Ajax vs Mallory vs .010u POI.

Henry: I didn't have any issues with the BMR; that was first-time-right (or was it first-time-charm?;)). The blonde BM caused me some headaches. I thought the oscillations there came from the NFB resistor, but in the end that was just reversed OT primaries ... As usual.;)

JB: thanks!:)

75

Mandatory pics!

The (new) NFB resistor with the shielded feedback wire. I put it dangling like this to minimisw crosstalk towards the rest of the PI. Originally the NFB resistor was mounted between and parallel to the 0.1uF coupling caps from the PI to the power tubes. Apparently this is not a good idea!

The nutty professor switch! :P

The chassis in its current form.

Oh, and I made the holes in the cab to fit the chassis; it fits nicely now.

Tomorrow I'll redo the PSU resistors to make them more permanent.

After that I'm going to shift all the pots a bit and maybe move the sag pot to the front panel.

And while I'm at it, that may br a good time to draw a template for a faceplate design.

Does anyone know an address where I could get my custom faceplate designed?


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