High-Voltage Entertainment in the Home

The Tannoys are bi-amped by a quad set of SE 6080 amps (above, supplied by the scuzzy-looking power supplies, below) each of which produces about 9W of Class A triode audio power.  While not as sexy looking as, say a 211 or 845, the 6AS7/6080 has three big advantages:  it employs a high current, low-voltage (290 VDC) power supply, which is easier to build and safer to be around than the1kV unit a 211, say, would require; it has a low plate resistance, which means lower primary impedance on the output transformer, which is consequently wider in bandwith; it is orders of magnitude cheaper (NOS GE 6080s go for $10 apiece.  Compare that to a NOS GE 845).  In the version pictured above, each channel has a parallel 6SN7 VA stage with a constant current source load, which drives the identical woofer and tweeter power stages through parallel 0.33uF foil-in-oil caps.  Each power stage consists of three sections of 6080, for a total of 180 mA drawn through 1.5K output transformers. The original transformers were Audio Note models that are no longer made, but Brian Sowter makes an equivalent model (SE-15)--and Brian is so much easier to deal with.  90V of cathode bias is produced by 560R 50W Xicon resistors bypassed by 470uF 250V Nichicon caps.  I tried not bypassing the cathode resistors, and the result seemed harsh and fuzzy on the high end. There is no feedback of any sort in this amp.   The glow eminating from beneath the amps is caused by the red LEDs in the constant current sources.  

Here is a printable copy of the power amp schematic.  

After the experience with the V-caps in the pre-amp (go look at the pre-amp discussion), I started to see if I could replace the paper-in-oil caps throughout the rig.  I started by replacing the 470µF cathode bypass caps on the output stage with 24µF Aeon polypropylenes.  That worked OK.  Then, I started to think about the coupling caps after the first stage.  Although, of course, I would just love to spend $70 apiece for 0.22µF TFTFs, I decided to search for alternative solutions first, since, if Nancy decided to kill me, $70 coupling caps  might very well constitute justification for homicide.  I mean, would you blame her?  Anyhow, it turned out there was a pair of Lundahl LL1635 5mA interstage transformers in the basement.  I have an interesting basement.  Initial experiments showed that the 6SN7 had too high a plate resistance to drive the 1635 properly, but that the 6BX7 looked good on 20kHz square waves on the O-scope (it has been my experience that anything that looks bad on square waves sounds bad, but two configurations that both pass the square wave test can still sound different.  So the square wave test is necessary but not sufficient.  And, the square waves themselves sound terrible).  Listening tests demonstrated that the 6BX7 (µ=10) didn't have enough gain, so I swapped the 12B4s (µ=5) in the pre-amp for 5687s (µ=15) , and the 6BX7s for 6BL7s  (µ=15).  That provided enough gain, but the mid-range seemed congested.  I realized that two sides of a 6BL7 (which I hadn't inspected on the O-scope) was way too much DC for the Lundahls, so I disconnected one side.  That opened up the midrange.  The next step is to parallel the primaries of the Lundhals, and use a 6EM7. More details, and a schematic, as they become available.

Adventures in Higher Voltages:  The 813 Push-Pull Amps

Desirous of more power and the milk-white glow of thoriated tungsten, and somewhat humiliated in a bake-off between the 6080 amps and Larry Moore's 845 SE amp, I decided I needed to focus and get the transmitting tube push-pull amp that has been on the drawing board for six years fired up.  For the proof of principle amp, I simplified the power supply by reducing the B+ from 1200V to 600V, and replaced (for now) the SV572-10 A2 output stage for a pair of Class A1 813s, triode strapped.  This allowed a reduction in the size of the power supply, so it is now only as big as a Japanese pickup truck.  The front end was a long-tailed pair 6BL7, with the plates loaded by a Lundahl 1660PP (hooked up using the 2.25+2.25:2+2 M'' scheme), and the cathodes by a 20mA constant current sink.  B+ turned out to be 540V (lower than I expected) using an LC filter, at about 100 mA/tube.  Because I didn't have much B+ to spare, I used fixed bias, fed through the secondary of the 1660PP.  The heaters were dead-on at 10.0VDC, using a CLC filter.  The output transformer was a Lundahl 1620PP, wired for 11.5K primary impedance.  Since I don't really have the environment for clip-lead amps, they were fixed, if somewhat exposed, on 6x12" aluminum plates:

 
The gain was a little low, because I was taking advantage of the pre-amp's high output, but still, at maximum volume, it could have been louder. They sounded good.   But, the 1620 OPTs were a problem; in the older datasheets I have, Lundahl was coy about the amount of current they could actually handle, but the American distributior of Lundahl products, K&K Audio, pointed me to new datasheets that indicate the maximum total current the 1620PP can tolerate is 150mA, 75mA/tube, and, at 540VDC, I was unable to bias the tubes low enough to sufficiently reduce the current to keep the transformers from running hot.  So, the cute little 1620PPs are history, and I discovered in the basement (I have an interesting basement) a pair of Sowter  8963s, 10K PP transformers capable of 300mA.  I swapped these much larger transformers in, set the bias at -65V, and left everything else the same, resulting in the second, even uglier, proof of principle amps:

These also sound excellent, with a coherence to the soundstage (there, I said it) that is missing from the 6080 bi-amp set-up, the high-end  is OK (I was worried about this using the high Rp tubes), the tubes glow that wonderful color, and the transformers run cool to the touch.  I think this one's a keeper, but, given that the 813 is designed to run with up to 2kV on the plates, I think I really do need to replace the full-wave rectifier in the PS with a hexfred bridge, and thereby get the B+ up to 1100V (and, yes, I will enclose the chassis) to enjoy the full benefit of the 813s, yet remain in Deep Class A.  Also, maybe add a 6CK4 front end to pick up the gain, and it would be nice to put the transformer on the same chassis as the tubes, don't you think?  Here's the current schematic.  

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