Class A 807/EL34 PP with transformer gain stage.
I have pretty much done all I intend to do with tube audio and decided to do a “left overs special” using some good parts I have on on hand including a very “bling” commercial chassis from an amp that had been dropped by Fedex. In particular, I have a pair of Sowter 9045 microphone transformers that are reputed to be excellent. They can provide a gain of 1:5+5 thus creating accurate push-pull phases. Not surprisingly, they can’t swing enough voltage to drive the output stage directly, even using easy to drive 807s and EL34s. However, the additional gain and swing needed is not great and I decided to use 6BX7s for a common cathode push-pull drive stage. As with the transformers, the 6BX7 is known for its audio quality. I also had a suitable One-Electron power transformer on hand, all that was missing was output transformers and I bought a pair of good-quality-for-the-money James 6225 HS units. At $275 for the pair, they do represent good value. The choice of tubes came about because fellow vintage oscilloscope custodian Volker Klocke gave me four good 807s in return for repairing a Tektronix HV transformer for him.
Here’s the bare chassis with the original sub-chassis:
It is basically a polished stainless steel cake pan which is not good for tubes, something mechanically dead is required so I placed strips of double sided foam thus:
And then having removed the backing tape replaced the original sub-chassis with a glass epoxy copper clad panel:
This is also secured by a multiplicity of screws that hold the tube socket bling rims in place. The resulting sandwich arrangement significantly damps the liveliness of the cake tin. I also made a cocobolo plinth for the transformers. (I took the precaution of wearing latex gloves and a face mask while working this wood.):
Very gaudy hence “Blamp” as in bling amp. Still it ends up looking quite attractive, here it is with a nice set of EL34s showing off the Sowter microphone transformers:
Here it is with the 807s, using 5 pin to octal adaptors that do make them rather excessively tall:
And so to the design narrative:
If you have read my paper that I published earlier on this blog, you will know that I am a proponent of cascode (depletion mode) mosfet current regulators, largely because of the near total isolation of the audio path from the power supply that they allow when used thoughtfully (Class A only of course). I refer to them as regulators since they are two terminal and can be used either as sinks or sources. They are available as kits from K&K Audio.
Starting at the front end, the first challenge was to come up with a neat way to drive the microphone transformers. I knew a cathode follower was the way to go and because there are not enough tube locations on the chassis, I decided to use 5840 sub-miniature pentodes. The use of a cascode current regulator in the cathode circuit was a given. The novel aspect is that I know that the way to make a pentode really perform as a cathode follower is to dc couple the screen grid to the cathode because this provides constant transconductance. Taking my lead from the tube instrumentation world, I decided to use NE-2 neons. Any noise that they introduce will be non measurable, at least with any equipment in my lab. For the plate current I wanted, 6mA, the screen needs to be at 100V or more (relative to the cathode of course) otherwise the plate voltage will rise towards the limit which I think (senility doesn’t help while writing this stuff) is 160V. So I used two NE-2s in series, fed by a J509 3mA current diode that having a high impedance, leaves the screen grid completely free to jive with the cathode. I also placed a 47V zener across the diode to prevent the 50V limit for the diode being reached in case of some kind of trouble. In normal operation, the zener is effectively out of the circuit. For good measure – and no better reason than that – I bypassed the two neons with a 1nF cap. After some screwing around with a signal generator, dB meter and oscilloscope, I determined that a 10μF capacitor would be correct for driving the paralleled primary of the transformer. I just happened to having some of those good sounding Russian pink paper in vodka caps, or is it paper in pink vodka? I used a low voltage PSU kit from K&K audio to provide a -20V return for the cathode current regulators. The cascode current regulators also come from K&K Audio.
So, now we have 20dB of gain (less 0.6 dB due to the cathode follower and transformer insertion loss) and do not need a great deal more so I chose a 6BX7 for the push-pull driver that has a mu of 10 providing a further 20dB (referred to the push pull output, not one phase). Since the 6BX7 is current fed and the load is due to the 220k grid resistor only, the measured gain actually is 20dB. I chose a bias of 20mA for the 6BX7 sections that with a common cathode resistor of 270Ω results in a plate voltage of around 170V for a new tube. The plates are fed by current regulators from a 230V choke input supply that, further filtered, also feeds the cathode follower. I think there is a real sonic benefit in using such a powerful drive stage even with easy to drive output tubes, I say more about this aspect at the end of this post.
The output stage is also somewhat unusual. Because I didn’t have room for lots of conventional high current B+ filtering, I chose to use, once again, current regulation feeding into a 40μF motor-run type bypass cap. The effective impedance of the cascode regulator is so high that there is no measurable ripple at this location, again using the fairly comprehensive test equipment that I have. This arrangement effectively limits operation to Class A and that too is intentional. To assist in good current sharing between the output tubes, I stood them on separate 1k resistors and this means that I could ac cathode couple the tubes which is a real sonic win for a Class A push pull output stage. The grid bias is variable and fed by a 75V stabilised point that also provides the positive heater supply bias that ensures hum free operation of the drive and output stages. The grid resistors are 220k in value and the output stage is coupled to the drive stage by 100nF Russian paper in oil caps, this time green though so perhaps we are onto Absinthe rather than Vodka? Since the cathode current is fixed, varying the grid bias voltage will set the plate voltages and with the EL34s, I set them at +350V relative to the cathodes. On replacing the EL34s with the 807s, the voltage dropped between 20 and 30V. As the tubes age, the plate voltage will rise to compensate for falling emission. With the tubes I have, the size of the cathode standoff resistors provides current equalisation better than 2mA. Yes, it’s a power hungry topology however the whole amp consumes less than 200W and I am happy with that. Once again, screwing around with a signal generator e.t.c. I arrived at a cathode coupling capacitor value of 4μF. Enter pink ruskies again, see the picture of the “works” below. The James 6225HS output transformer has a plate to plate reflected impedance of 5k however, I found that things got a LOT better when I loaded the 4Ω tap with 8Ω thus reflecting 10k plate to plate. To get the output Z below 1Ω, I added some negative feedback, introduced at the ground end of the microphone transformer primary. With the EL34s the output stage gain is -14.7dB and with 5.4dB negative feedback, the resulting loop gain is 19.4dB, perfect for my system requirements. With the 807s, the output stage gain falls to -16.7dB and the negative feedback to 4.4dB giving a loop gain of 18.3dB, still more than sufficient for me. I did experience a problem with unstable plate voltage on one channel. This turned out to be due to one of the green caps being leaky. I’ve got loads of them so finding a good replacement was not a problem, they do sound good……
Power with the EL34s is 1w at 0.05% THD, 12w at 2% THD, with the 807, the power is a bit less at 10W. Remember, it is Class A. Some commercial push pull tube amps (especially Chinese) that are claimed to be Class A are not.
The power supply uses a One-Electron BFT-1 transformer run well with in its rating. The highest voltage taps are used for the +500V supply for the output stages at 240mA while the lower voltage taps are used for a 100mA choke input 230V supply for the drive and input stages. I used the spare 5V and 6.3V windings in series into a voltage doubler and then a LT1085 configured as a 150mA current regulator to power the 5840 heaters that are connected in series. A 2A slo-blo fuse protects the whole thing.
Oh, the front end has a nice 20k switched attenuator. It doesn’t have to be 20k, I design for relatively low interface impedances because interconnect cables like that and it keeps noise down. This amp is silent using my 95dB/W/m Tannoys.
If you’ve made it this far, here is the schematic:
Talking of keeping noise down, I made a soldered tinplate box to shield the power switch that is bang in the middle of the front ends! I also connected the power switch using shielded wire.
Here’s a picture of the “works”:
Please take the time to click on it to see it properly!
As for the sound, I think it is a good as anything I have done and this being no time for false modesty, I design and build excellent sounding equipment. In particular, this amp has what I can only describe as kick, transients are extremely clear and since a lot of music is transient, this may explain to some extent the good sound. It resolves very deeply, and has marvelous clarity, it just gets out of the way. I think this may be, at least in part, due to the use of a power drive stage combined with easy to drive output tubes. Whilst on the subject of output tubes, good EL34s are excellent. I have built equipment using the much vaunted 845, 300B, 50, 2A3 and 45. Frankly the EL34 doesn’t fall far short if at all, and is MUCH less expensive. I haven’t listened to the 807s enough to form a good impression yet. I have them playing as I am writing this and at the least they make music that way I want to hear it, clear and in the room, not stuck to the speakers. Reproducing music well in a domestic environment is difficult and 100s of watts hinder more than help! Driving my Tannoys this amp is powerful both with the EL34s and the 807s, if loud is what you want. I can’t hear deeply into the music if it is loud.
I then got around to repairing my HP 3580A audio spectrum analyser and since the amp can accommodate both EL34s and 807s, I put el34s in one channel and 807s in the other then ran a spectrum comparison at 1W and 8W. The analyser can store one scan digitally then it can be restored once a second scan has been taken, allowing direct comparison.
Here’s the comparison at 1W:
The EL34 is showing 2nd only at -61dB. The THD was 0.055%
The 807 is showing 2nd -60, 3rd -62, 4th nil, 5th -78. The THD was 0.11%
Here is the comparison at 8W:
EL34: 2nd -50, 3rd -75, 4th -77, 5th -65, 6th -71, 7th -70, 8th -74, 9th -72, 10th -74, 11th -73. The THD was 0.256%
807: 2nd -49, 3rd -45, 4th -70, 5th -51, 6th -81, 7th -52, 8th -80, 9th -56, 10th -81, 11th -62. The THD was 0.72%
I “should” have repeated the tests after swapping the output tubes between the channels, however, I am not going to! The results are shown purely for the sake of interest though they do seem to reveal that the EL34 actually is cleaner than the 807 at low levels and much cleaner at high levels.
The HP 3580A can also perform a log frequency sweep over the range 20Hz to 43kHz. Here are the sweeps using the EL34s at 10W upper and 1W lower:
It reveals about a 2dB rise overall and notably, the HF performance does not drop off at the higher power. The frequency scaling is logarithmic starting at 20Hz, then 43, 98, 200, the same repeating for the next decades ending at 43kHz. So the dead flat response is roughly over the range 40Hz to 10kHz then rising gently to 43kHz.