Tube Amps / Music Electronics
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|8/14/1999 10:12 PM|
|KG||Update on the CCS and etc.|
Well, just came back in from the garage after successfully powering up and slamming a few chords around the BAGA.
The CCS stages are alive and well in the amp. Some side benefits I encountered...
Previously there was a rise in average DC value at the cathodes of the CF's when under signal conditions. This is because they were NOT operated in class A, far from it, and this caused significant increases in plate current under signal. This is NOT desired, since this will decrease the bias (ie make it less negative) that the KT90's rely on to prevent meltdown.
With the CCS's in place, this rise is MUCH smaller--I don't really know why at this point, I've got to think about it some more and ponder.
Another thing I threw in there was a current inrush limiter from Allied Electronics. I picked up two of them, of different ratings, to try to ameliorate this HUGE spike of current that occured when a secondary winding of 1.5 (yes one point five) ohms DCR charged up 1000 mics of caps to 620VDC in a couple of AC cycles.
Along the way here I learned why SOMETIMES I would open a _15A_ slo blo fuse when powering this unit on.. it has to do with the residual flux. Any of you pondering toroids, here's a snippet from an email I received from Steve Bench (one fine dood, BTW):
This happened at a gig once, in Matunick, RI, about 30 seconds before we were supposed to start playing. Of course, I didn't have a spare fuse, and I didn't know why on earth the frigging 15A primary fuse would blow out, but blow it did. I had to take my life (and my amp's) in my hands and run to the kitchen for--you guessed it--aluminum foil! Damn thing powered up, and I played the gig. I also became very religous that day
So I threw in a C.I.L of 47 ohms cold, .5 ohms hot, and with a current rating of 3A. It works like a charm. One thing I learned is that IDEALLY you'd put this device in the PRIMARY, so as to limit this saturation current that MAY occur because of residual flux. I didn't know that when I ordered the CIL, so I put mine in the secondary due to the insufficient steady state current rating.
As far as sound quality of the CCS stage, I think there is a bit more clarity as the amp is really pushed hard--it's a bit more in control. I'm sure the amp itself is a bit happier. I went with 15 mA per EL84, across 235V, which should keep them running a while.
Also the rails (+/-150VDC) which power the EL84's do not droop under load--since they always have the same current across them! This is very handy, since I can get a feel for how the voltages are doing while it's idling on the bench.
Checked out the front end of the BAGA too, and I got a 40 dB voltage gain to the output grids (that's 100x input voltage)... had a decently clean square wave @ 1 KHz swing 100Vpk on one side, which means I can push a decent 200Vpk-pk into the finals. That really makes the KT90's sing, let me tell you.
Snapped some pics of the above procedures while under the hood, and will UL and post the address ASAP.
Rebiased the KT90's to 20 mils per, which works out to about 12W Pa each. Cold, I know, but these are NOS tubes now.
Didn't get a chance to pentode connect the CF's though. Next time.
|8/14/1999 10:39 PM|
Pardon my ignorance, but what is a CCS? a cascode?
|8/15/1999 5:04 AM|
I think Ken is refering to a constant current source (CCS) installed in a cathode bias circuit.
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