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In Reply to: RE: Garg0yle 2A3 DC posted by Al Noakes on January 20, 2015 at 09:14:31
"I would suggest the original poster try both methods and decide for themselves before committing to a chassis . "
You raise some valid points Al Noaks.
As you can see though, test flights have already commenced!
Cheers.
Follow Ups:
Very nice ! I think Rage is right about your pans :) Are these making noises ?
Al
Thanks guys.
Yes they are making all kinds of noises!
Beautiful amps! Best cake pans I've seen yet.
Rage had in another thread, what would happen to a B+ voltage over a certain amount of time. Based on the in those sims in that discussion.I did a sim that shows what happens in that context, but with this power supply.
This is a 30Hz sine showing a 10 second interval.
Once the B+ recovers from the attack, the voltage stabilizes fine.
Edits: 01/21/15 01/21/15
looks quite a bit different than the other supply.
the other one was essentially ringing after the transient, right?
I thought you interested in seeing the long term stability of the B+ in a stiffer power supply that has more capacitance.
"gargoyle -what happens if you extend out beyond 4 seconds and allow the 150uf cap to discharge?"
I may have misunderstood you.
no we're on the same page... sorry a little under the weather.
I had debated changing driver tubes, parallel 6SN7, different
configurations etc. Rather then speculate, I put the scope on the speaker
terminals to see what kind of voltage I actually need.
95% of my listening is well under 1W, more like 240-500mW. This tells me I
can get away with chopping of some power in the name of quality.At first I debated whether this would be a waste of a 2A3, but in reality
it's not as even if I could push 3.5 watts, I would never need it. 45s may
be an option, but I don't feel compelled to switch.The BB-Proof was running my 2A3s a little hot, due to my higher voltage as
well. and lower OPT voltage drop.So I had to rework the voltages to fall inline with my iron. I am now
running the 2A3s at ~52.5mA/275V. It's a juggle, lowering the current
raises the B+ etc. so this is where I settled.
The 25watt 2A3 cathode resitor is now 4Kohm instead of 3Kohm.Next the driver stage, I was tempted to abandon the BB-proof concept
(Applying negative bias to the 2A3 during start up.), just go a more
traditional route and connect the 6J5 to the regular B+, as it would have
let me use a bigger plate resistor for more swing.
However I persevered and after 20 or so versions later, I settled on a
configuration that works.
Basically it has similar operating points as the BB-Proof, 6mA/~130V on the
plate, but now uses an 8Kohm plate load and a higher B+ of 180V instead of
157V. As Paul Joppa said, this is a good operating point. It works with the
voltages of the amp at least.
I did throw around a few different choke values, but nothing jumps out as
even being close to acceptable, performance, they are garbage lol.The output transformers are now 3500K instead of 2500K.
B+ is now 465VBias on the 6J5 is now stabilized using a 1500uF/50V cathode bypass
capacitor. FWIW I do not consider this to be in directly in the signal
path.The 390 ohm partial bias resistor is now 100ohm to allow for the larger
plate resistor.
With the 6J5 removed in the BB-Proof, the 2A3 saw 191V @ 74mA, -28 bias
resulting in 14watts dissipation.
With the 100ohm, I have 110V @79mA, -8 bias 8.7 watts dissipation.
While I have a touch more current under this fault mode the voltage is less
resulting in a lower wattage, so I should be BB-Proof.The operating points were chosen to get the lowest distortion. Both tubes
have around 2.5% each of mostly 2nd harmonic at full output. the 2A3 bias
is around -52V.
The amps put out 1watt to the speaker terminals.These amplifiers are now stunners. They are now the upgrade I was truly
wanting. They go lower and have better bass then my RH84 (with smallish
OPTs had) They sound "right" as the phase is much much better, this made a
huge difference in the bass department. Transients are much better not
having that delayed bass.I have a square wave of the BB-proof as well, but I will post that after I
square wave test these amps for comparison.I will clean up my schematic and post it here shortly.
Audio Illuminati
Edits: 07/28/15 07/28/15
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