David Loader
Evening all.

Last night I completed what had become a several year long project to build a pair of the old-school Beast amplifiers from a pair of circuit boards and plans that Bruce had kindly supplied to me, shortly before the product was taken off the market.  Yes, it's been some time in the making.  I have configured it with 12 tubes per side.

Unfortunately, whilst one amplifier works perfectly (and sounds amazing), when I set the initial bias on the other one, the dummy load resistor got very, very hot.

Further measurement with an oscillosope indicated a significant high frequency oscillation - it looked like about 20 volts peak-to-peak.  I don't know the exact frequency as it was getting late in the day but I think it's in the kHz.

All the LEDs remain lit, so no fuses have blown.

I realise at this stage that the data points I have provided so far are pretty rubbish.  I've been working very hard over the last few weeks to complete the build so I just need a couple of days off.  However, I thought I'd start throwing the question out there - could this sort of thing be caused by a tube issue or is it more likely to be a component failure?  I triple checked the wiring from the circuit board to the output stage so a miswire seems unlikely, but of couse anything is possible.

I will post more later when I get the thing back on the bench and start measuring voltages etc.
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David,

I believe that your instability is a component failure or bad joint.  I am not familiar with the circuit of the Beast.  Is there a snubber built into it?  If so, check it.  I would check the power supply rail voltages, check all your capacitors for 1) functionality 2) proper connection.  This is my two cents.  Best of luck!
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David Loader
The voltages that I was able to measure look within spec.

The oscillation was measured at 20V peak-to-peak at 116.2kHz.

Interestingly, it stopped dead when the multimeter probe was placed on V1 pin 6.
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David Loader
Problem solved.  The solution was to connect the preamp - seems if nothing was connected to the inputs there was some interaction with the input transformer - first clue was the oscillation disappearing when pins 2 and 3 of the input XLR were shorted.
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David Loader
And.. ohhhh wow!  Believe everything Ed Shilling says.  These are oldies, but they're goodies!
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David Loader
[IMG_0169]

At this point I still had the scope connected to make sure there weren't any more issues.  They're still working perfectly.  These have been about 3 years in the making.
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Slick! I like your take on The Beast. The mirror image pair looks good.
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David Loader
Note the special boutique Power Line Conditioning (multi-way from B&Q, think it was about a tenner).
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Gary Kemp
David Loader wrote:
Note the special boutique Power Line Conditioning (multi-way from B&Q, think it was about a tenner).


Magnificent power conditioner! Does it add an indefinable something to sound? 
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David Loader
Yes.  It makes everything sound more orange.
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Wow David,...these are just beautiful! I know how much time and effort it takes to get custom Beasts built,....but once done it's years of nirvana! They have speed, delicacy, and control "to burn". Respect!
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Wow David,...these are just beautiful! I know how much time and effort it takes to get custom Beasts built,....but once done it's years of nirvana! They have speed, delicacy, and control "to burn". Respect!
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David Loader
Thanks Mark - hey are you the guy behind these amazing creations?

http://sharksphotos.yolasite.com/

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Gary Kemp
A stupid question. The soldering seems to have been done without (1) wrapping the wire or resistor leads etc around the terminals, and (2) soldering.  It seems to have been done without step (1), as if one had three hands - one for the soldering iron, one for the solder, and one to hold the wire the pliers.  But one doesn't have three hands. One has just two. 


David Loader wrote:
Thanks Mark - hey are you the guy behind these amazing creations?

http://sharksphotos.yolasite.com/

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"Thanks Mark - hey are you the guy behind these amazing creations?"

http://sharksphotos.yolasite.com/

Hi David,..Yep,..that would be me.

Hi Gary, Yes, there are a handful of wire joins on these particular Beasts that have the equivalent of thru hole join where the wire at one end only is pushed thru an eyelet and soldered without wrapping. You should note, the other end is always anchored(wrapped)! I considered the environment these units would be used in before going down this path, and the amps are well ventilated, and virtually never travel. If they were guitar amps chucked around vans regularly and sit in a high ambient temp, then I may have reconsidered these joins. As it turns out, these amps have been in service for almost 10 years and performed flawlessly over that time. A good indication of Bruce's remarkable design prowess! But seriously Gary, if your amps are melting solder joins you have some serious problems to chase.

The "3 hand" joins were performed with a simple "third hand" stand like this one. https://www.bunnings.com.au/tradeflame-helping-hand_p5910343. They are cheap and work a treat for those impossible to hold in place joins.

Hope this helps, Cheers, Mark.
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