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COBALT SET 45
Parafeed Dual Mono Amplifier

Kurt Strain's SET 45  Cobalt Parafeed
by Kurt Strain



This amp uses a 6H30pi driver with active constant current source (bipolar CCS) direct coupled to a 45 in parallel feed configuration. The input stage is direct coupled to the output stage. The bipolar transistor constant current source provides a steady high impedance 15mA, acting as the active load for the input stage. This is a very tight constant current source that maximizes the gain and linearity of this input stage (very high impedance load line) and has not shown any noticeable "transistory" quality to the sound.


Power Output: 1.3 W at class A1 clipping (low distortion), 2.5W at hard clipping

Frequency Response: 10 Hz - 60 kHz

Power Bandwidth: 15 Hz - 50 kHz

Input sensitivity: 3 Vrms (should use a preamp with some gain)

Noise and hum: < 1mVrms

Tubes: 6H30pi (1)  45 (2)


This amp works best with speakers rated an easy 8 ohms and at least 100 dB/1W/1m sensitivity. Solid state rectification is used via Hexfred diodes to an RCLC filter. The rather large high power input resistor combined with fast recovery rectifiers makes for a smooth, quiet HV supply of 450VDC. The input resistor limits the surge current through the diodes, which lowers the noise further. Also, this is a very linear supply because the diodes do not shift appreciably in dynamic impedance as would tube rectifiers. I think it sounds outstanding and I did not opt for tube rectifiers.

Click for full 1000 pixel view of wiringThe idea for this constant current source and direct coupling scheme can be attributed to Bottlehead Corporation's topology, with minor deviations. I use a Zener diode for a voltage reference bypassed by a 10 uF Tantalum electrolytic capacitor to reduce the Zener noise. Tantalum capacitors are quite good in this application since it is two levels removed from the direct signal path, are small and cheap, and does a good job in wideband noise suppression in the circuit.

The oil capacitors are used as bypasses for the Elna Cerafines, but I am not feeling they are necessary even though I left them in.

The hum nulling capacitor from B+ to 45 cathode acts to reduce residual hum, and even though AC heated, it is remarkably quiet even on 108 dB sensitive speakers. This hum nulling capacitor drops hum on the speaker outputs by 10 dB or so.

The parafeed transformer is not sent to ground, but instead to the cathode of the 45. This improved the sound, but I am not 100% sure why. My theory is that there's a little amount of positive feedback for any amount of signal that is not sent to ground through the cathode bypass capacitors, which enhances the sound quality in opposite effect as negative feedback. I use quality film caps for cathode bypassing and the parafeed caps. I also liked the bypassing I did to these caps using smaller caps but I can assure you good results without them.

Hammond transformers are made with a primary rated for 115VAC, which is low compared to US voltages. Therefore I had to find the correct voltage dropping resistors for all the supplies to get the correct voltages for heaters and HV. This came through trial and error.

Click here for schematicThere are power resistors on both sides of the 15mA CCS. This is necessary to not overvoltage the MPE350 transistor. It's best to use these between the plate of the 6H30pi and the collector of the MPE350. Here I have them split for build convenience. The MPE350 needs a fairly hefty heatsink to operate safely because it dissipates quite a bit of heat.

Powering on requires that the mains be turned on first, which activates the heaters. After a 45 second warmup the HV switch located at the center tap can then be thrown. Because of DC coupling, careful turn-on is essential to preserve the 45 tubes. Powering down can be accomplished in either order and without regard for timing.

Because the CCS produces a fairly large current (15mA) and it has large compliance (more than Vgk of the 45) it can pull positive grid current effectively, which increases the power output capability to a class A2 power rating of about 2.5W. This may be running the 45's a bit hard for some folks, but it sure sounds good and powerful.

I use the cobalt core EXO-45 output transformer and the EXO-04 plate transformer on the output. Both provide exceptional performance even though the EXO-04 is probably overkill and could be replaced with a BCP-15 plate choke instead.

I have used these amps on Avantgarde DUOs and Oris 150 / PM4AER speakers and it's the best amp I have heard on such speakers. Solid top to bottom with the best midrange I know. The cobalt core helped extend the frequency extremes to the point you can't recognize those "SET limitations" anymore.


Kurt Strain
May 30th 2001

NOTE: Of course, there had to be the usual disclaimer; if you are clumsy and do not know anything about electricity please stay away from playing with tubes circuits and high voltage without learning the basics and being cautious... carelessness may kill you!



Important links

- Kurt Strain SET45 schematic w/printable version
- Full 1000 pixel wiring view

NOTE: Please see schematic for parts list and ID




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