Saturday, May 05, 2012

In the days since I updated last I have brought this beast on line.

It has not been without struggle mind you.

Phase 1: rebuilt bias dropping circuit to reflect values close to those originally published for the 7027A configuration of the amplifier (130K dropping with a 15K straddling the bias filtering caps, 10K trim on top of a 56.1K to ground). This is pretty much where I was at end of April. Netted a nominal 4 ma idle current on three of the tubes, whereas one read no current. I was busy checking voltages on the no current tube when my meter gave up the ghost (shunting enough voltage to ground to increase at the wall current consumption by a half amp). Meter no longer trusted for voltage work, everything is shelved in await for a replacement.

Phase 2: knowing that 4 ma idle current is indicative of an over biased situation, regardless of potential wiring error on my part, I remove and replace the 130K with a 150K per later revision. In doing this work the tubes are removed and replaced. All four tubes JJ E34L, one pair had been in a red plate runaway situation and was easily identifiable by the darkened paint on the envelope; darkened pair inner, non-abused pair outer - as I've always been installing them.

Phase 3: armed now with a new meter I ease the amp up and read nominal idle current in the 20 ma range, except at the one outer socket - which has switched sides......

Break out tube tester and confirm that one of the non-abused E34L is in fact useless. Dig through the stacks to come up with a handful of used EL34 & 6CA7.

Phase 4: the next value of 2 watt resistor I have is banded 170K and reads at 200K, all three 170K resistors read similar. Fire the amp up and read in the 50s for the E34L and lower 60s for a pair of Sovtek EL34WXT I have sitting in the outer sockets. No red plate at idle just yet, though I realize this is ranging them a bit hot. I plug my Tektronix RM503 into the extension cab socket (modified to be wired in parallel to the main output, not a switched series stack) to have a peek at waveform while the 4 ohm dummy load dissipates power and observe a tinge of orange on the Sovteks. Pull the meter out and read well over 100 ma. Shut down the amp and pull the Sovteks, which tested high on transconductance compared to the E34L I already have installed (which have served as the baseline changes to the amp over all).

Phase 5: find a 174K 1% of sufficient girth and swap that in to main bias dropping resistor slot. Test out the pile of tubes for something closer to the level of the E34L. Sovteks test high, the Sylvania 6CA7 is off the charts, a pair of Matsushita 6CA7 weigh in a bit lower. These are installed as outer pair. I've observed a roughly 7-10% travel in current adjustment with my 10K bias supply pots, enough to trim in an already similar quartet, but proving insufficient to balance out dissimilar tubes, which remains one of the goals.

Anyhow, with the 174K I can level the E34L out at 42 ma, while the Matsushita 6CA7 are stuck in the upper 20s. I'd rather err on the side of too cold for the moment and finish up some adjustments so I can house the amp and actually get to listening tests. This entails adjusting this phase inverter balance pot so I plumb the dummy load and o-scope into the speaker jacks and run a sine wave oscillator into the input, grab the knob and give it a twist while watching the trace on the screen.

This process is not incredibly helpful; as the adjustments only seem to have bearing on amplitude as opposed to balance of waveform, though to be fair the waveform could be out of balance and I'm just missing the baseline. I crank the volume dial up to a point where I can hear the frequency rattling the guts of the amp/dummy load (which is getting warm) and observe a clipped output. Though clipped the output still looks balanced, so I ramp the amplitude up to the point of ringing the waveform, adjust the balance pot to smooth that out - have a glance at the amp which is groaning a little under load to observe the quartet of outputs well into the orange.

Flick the power switch, which does nothing. Kill power at the variac, walk away and eat lunch.

Fire the amp up without adjusting anything, read low 40s & high 20s at idle. Plug 4 ohm speaker cab in and play a little while watching current draw. Normal use has draw ranging from the 50s to the 80s, though the amp is in no way being run wide open.

In the end the phase inverter balance was adjusted by ear (I reached for the loudest position, extreme swing on either side fell into cut-off) & the amp is buttoned up for observation while playing with standard sources. Being able to read current while playing will allow me to avoid running the amp into self destruction.

So, still not through the hazard territory, but well on the way. I need to bring B+ down to original values (my wall voltage is a touch higher than 1969s) which should push the point of runaway out even farther, and I think upscaling my adjustment pots to 20K might be worthwhile, but lower on the priority scale.

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