spaghetti bowl PRR Vari Mu!!

Hey guys - happy as a pig in mud this arvo. Got Kents layout of PRR vari mu and got it up and running this afternoon with a pair of telefunken valves that I dont think have seen daylight in decades...

Sounds really nice and I am amazed for such a simple cheap build can sound so good!!

Still playing with transformer options - and hunting for some others...

Anyone who has built this - how much does the controls impact on the sound? I am yet to measure the attack / release and the "threshold" but they seem very subtle controls - is this how yours behaves??

All beers to the all knowledgeable PRR and to Kent for his layout and work on this project cheers!! another great DIY project...

;D  ;D Mac

I used a bread board and some 6bc8?? (used in 176) type tubes and it does sound quite good. I found the attack and release noticeable and usable but not extreme.



btw what tubes are you using?

Awesome Mac!  Well done.  I was literally looking at the schematic and then when I came back to the forum you have this post.

Enjoy.

Ruairi

. . . . and I just found my un-used board in the kitchen at home . . .

Hey Abe, The tubes are 12au7 telefunk's the old made in W Germany logo...

heres a closeup of the board..

Thanks Ruairi.  In the spirit of the original design -  I didn't buy anything and re-used other parts I had like the pots and even salvaged the tube sockets from an old radio chassis...

I really like it!

Mac

Mac, looking forward to hearing your impression on these. i'm planning to do two stereo versions pretty much stock then go into change/mod/swap/compare mode...

Are you considering the options for adding an output buffer?

Looks like oyu have some nice transformers in there.

Best,
j

Hi Mac,

what are you using as your power supply?  The powersupply board is hard to implement with aus power.

M

Tks j. I am going to live with it stock for a while before I get too excited about the mods.

Miles, I have a couple of power trafo options at the moment from old two track machines, but I will probably design my own board. Why do you say that kents board is hard with au power? As long as you have a transformer which can deliver ac in the range, his design can be spec'ed to suit.
Mac

mac said:

Anyone who has built this - how much does the controls impact on the sound? I am yet to measure the attack / release and the "threshold" but they seem very subtle controls - is this how yours behaves??

Click to expand...

Nice to hear that there's yet another happy PRR vari-mu user  8)

Yes, control knobs are all somewhat subtle, but it can get a bit aggressive if you hit the input harder (feed it a louder signal). I too love this unit. It's such a simple design, or, rather, I understand it as vari mu stripped down to the essential. What I did, and still do, is to study all vari-mu schematics that I can get hold of and then experiment. Don't get me wrong, this comp sounds wonderful as is and there's no real need to change anything. I just love adding stuff, then listen closely to how this changes the sound or imaging and then decide on what I want to keep and what I do not want to keep. Half of components in my unit are off-board now. See, I have enough time and this comp is the ideal platform for me to learn. BTW, changing the opamps too affects the sound. It's all in this forum, but not in PRR vari-mu related threads only

Hey Script,
Yes thank you for your post.

From looking at other schemos and reading others experimentation on here, I suppose basic first step future "investigations" that I shall do to play with this circuit could include;
- experiment with different tubes (starting with different 12au7 types);
- swap out 5532 with drop in replacements - OPA etc
- change signal path to the input (experiment with different gain stages to hit the input - external line amp, use my dual 1176 with compression off etc)
- different input and output transformers - slightly different ratios
- I have a spare 9k balancing board which I was considering strapping across the output as well!!


Please post some pictures of your build - photos of other peoples builds always inspires me!!

Mac

P.S. we monitor the tragedy unfolding in your country each day with hope and prayers - stay safe...

the power supply works well for 110v transformer but it seems to me that you need a transformer with a 110v primary and a 12v primary as well to use the power supply as designed.  IS that what you are using or are you building your own supply for the needed voltages on the main pcb?

M

Hey miles, I built kents board for testing because I have a trafo with similar specs.
240 primary: 0 6.3, 6.3, 15, 0 134 secondaries.
I have decided to do my own board for a few reasons with a doubler for the heaters now, and a linear crcr network to bring the 134ac (184dc dc rectified) down to filtered 100v.

mac

Hey mac,

as promised, here's a picture, quite old though and pretty bad (both resolution and content ). A dangerous sphaghetti bowl indeed, passing audio for listening tests...

*Picture didn't show* Here's the link:
http://img859.imageshack.us/i/1007141639141.jpg/

Whoever sees this, please do not imitate under any circumstance. And please always remember to discharge the PSU caps before sticking your iron or a single finger into it!!

It is beautiful - the way diy should be! ;D
Mac

I did not know the 12AU7 tube was a vari-mu type.

Cheers

Ian

Hi Ian,



    nor did we, til PRR was so kind as to post this puppy . . . It is only good for 10dB of compression or so. it will run deeper, but only with (lovely crunchy) distortion.

  It is no fairchild, but is very useful, especially considering how simple it is to make.



    Kindest regards,


    ANdyP

ruffrecords said:

I did not know the 12AU7 tube was a vari-mu type.

Click to expand...

well any triode is a "vari-mu" tube to an extent, but the curve might not be awfully suitable for compression, at least not without very careful compensation. There was a great discussion on this in the past and PRR certainly has elaborated on his design choice in detail. If I only could find the right keywords for the search. I think it was linked in META as well but I can't find them now.

Kingston said:

ruffrecords said:

I did not know the 12AU7 tube was a vari-mu type.

Click to expand...

well any triode is a "vari-mu" tube to an extent, but the curve might not be awfully suitable for compression, at least not without very careful compensation. There was a great discussion on this in the past and PRR certainly has elaborated on his design choice in detail. If I only could find the right keywords for the search. I think it was linked in META as well but I can't find them now.

Click to expand...

Except in any sharp cut off triode the designers try to keep mu as constant as possible in order to minimise distortion but this is not always possible at the extremes of operating conditions. The 12AU7 is renowned for it poor distortion performance and this is clearly reflected in is steadily downward curving mu curve. So maybe this is what Kent is exploiting rather that using a true variable mu tube?

Cheers

Ian

ruffrecords said:

but this is not always possible at the extremes of operating conditions.

Click to expand...

that's what I meant. they are all "variable mu" after a point. Like when NYD was experimenting with 12AV7 as a vari mu tube. He admitted it wasn't working well.