Single Ended Amplifier Concept, Part 3

Hi!

The circuit of the amp his been detailed in the first two parts down to each component. What has not been covered yet is the power supply. So let's concentrate on this now. The requirements have already been set. The B+ is about 425VDC. It has to supply both channels. Each output draws 70-75mA. The drivers take 6-7mA each. The bleeder resistors swallow another 15mA. That sums up to almost 200mA.

The heaters are all connected in parallel. The 6CB5A draws 2.5A per tube, the 6N7 0.8A. That is 6.3V/6.6A for the heater supply.

Basically any power supply which can deliver these voltages at the given currents will do. But of course we will also propose a good solution to go with the circuit as presented in the first two parts.

Some deviation from the targetted 425V is not critical. This can move up a bit, as long as the max plate dissipation is not exceeded. The plates of the 6CB5A will start to glow red if they are beyond their maximum. In that case reduce the B+ voltage until the glow disapears. If you cannot reduce the voltage in the supply, you can dial down the current by increasing the cathode resistor.

The heater voltage should be as close as possible. I'd rather have the voltage a bit below than above the nominal 6.3V. Anything between 6 and 6.3V is ok.

As has been mentioned in the first part, we don't want the average cheap PSU, but something nice, yet affordable. So let's go for a classic choke input filtered supply with tube rectification. The choke input supply has some advantages. Voltage regulation is better compared to cap input. Current draw is spread over the whole conduction angle of the diodes rather than in pulses as with cap input. This means less current spikes with the potential to creep into our amp circuit. Also the rectifier and transformer have to deliver smaller current amplitudes and get less stress. But nothing comes for free, there are also disadvantages. The resulting DC voltage is much smaller at a given secondary voltage, about 0.9 times the secondary voltage versus the theoretical maximum of 1.4 times secondary voltage for cap input. The first choke needs to be rated for choke input duty, otherwise it can exhibit mechanical buzz.

Since we have chosen unusual and low cost tubes for the amp circuit, we will do the same for the rectifier. So we stay clear of the typical rectifiers seen in audio amps like GZ34, 5AR4, 5U4, etc. Looking into the TV tube arsenal we find the so called TV damper tubes. These have been developed to damp oscillations in the electron beam defelection system of TVs during the fly back of the beam. For this application they need to be capable of high current pulses and very high peak inverse voltages. There is an abundance of TV damper diodes with various bases, like 6AX4, 6AU4, 6CJ3, 6CG3. Unfortunately they are all single diodes, so two are needed for a full wave rectifer. But there is one exception. The 6BY5 contains 2 diodes in one tube. It has an octal base, so all tubes in the amp will use the same socket, which is a nice touch. Another good point is it's low cost. It lists for a few bucks in the catalogs of all major tube dealers. It's specs indicate more than enough current capability and peak inverse voltage rating. So let's use it.

The schematic above shows the complete power supply. Let's go through it component for component, starting with the power transformer. The secondary voltage can be estimated as follows. We want 425V DC out. There will be some voltage drop in the rectifier and in the chokes. Let's assume 25V. So the actual DC voltage we need out of the rectifier is 450. To get the secondary voltage we need to divide this value by 0.9 which gives 500V per leg of the secondary. that is 1000V across the entire secondary. Depending on the DC resistance of the windings in the chokes and power transformer the actual voltage might differ. But as mentioned it is not necessary to reach the exact 425V Anything between 400 and 450 will be pretty much ok. There is a nice freeware tool available on the web which does all those PSU calculations for you: The power supply designer PSUD2. Download it and play around with it. Very useful and educational tool!

As determined above, the supply needs to deliver 200mA. The choke input supply draws current continuoulsy during the conduction angle of the rectifier tube, but only from one half of the secondary winding at a time. The textbooks say that with choke input the current capability of the secondary winding should be about 10% higher than the DC current drawn. That would translate into 110mA across the whole secondary (220mA divided by 2 since each half only needs to deliver current 50% of the time). But it doesn't hurt to over specify the power transformer. This yields better voltage regulation and less heating of the transformer. So let's pick at least 150mA. For the first batch of power transformers I got wound for this project I spec'ed even 200mA.

There are also the heaters of the tubes which need to be supplied with 6,3VAC. For AC heating there is no benefit in having these supplied from separate transformers, of course it would not hurt either. I got the heater voltages wound on the B+ transfomer as well. We need separate heater windings for the rectifier and for the amplifier tubes. The 6BY5 does not withstand a very high voltage difference between heater and cathode as other TV dampers, so best to have the heater on the cathode potential. This is achieved by simply connecting one end of the heater directly to the cathodes as indicated in the schematic. Of course that means we need separate windings since we want to reference the other heaters to ground. The requirements for the heater windings are 1,6A for the 6BY5 and a total of 6.6A for the signal tube heater winding. Both heater windings need to have sufficient isolation between them.

Another important feature of the power transformer is the screen between primary and secondaries. Without a screen winding, there is capacitive coupling between the windings. This would allow high frequency crap from the mains to pass straight through. With the screen winding in between both primary and secondary now have a coupling capacitance to ground rather than between them. As indicated in the schematic I got my transformers wound with two independent screens. One is connected to protective earth and chassis and the second to signal ground.

Not indicated in the schematic is a feature which I get on all my power transformers. Taps on the primary for some fine adjustment of roughly + or - 5%. Instead of 0-230V the primary actually is 0-220-230-240V. Also not shown are mains on/off switch and the mains fuse. The average DIYer should be able to sort out these details himself.

The schematic shows two LC sections, each with a 10Hy choke and 40uF cap. As mentioned in the first parts, amp circuits with ultrapath connection and no cathode bypass cap need very well filtered B+. You might get away with just a single LC section. Then it is advisable to increase the capacitance and use cathode bypass caps at least on the driver tubes if there is still some hum. With a supply as drawn, there will be no hum even with very sensitive speakers.

The first inductor needs to be wound for choke input duty. Otherwise it can exhibit mechanical buzz due to the large AC voltage across it. You might get away with a choke for cap input service if it is sufficiently derated. A small input cap of 100nF can help if you have some mechanical buzz. This cap should be rated for at least 1000V since it can see high voltage spikes. This optional cap is shown in grey in the schematic. I use Lundahl LL1673 chokes. They are made for use in choke input supplies and work excellently in this PSU.

The heater supply for the signal tubes is referenced to ground via two 100 Ohm resistors. 2W types are sufficient for this. The third resistor is a bleeder. It ensures that the minimum current is drawn from the supply to maintain proper choke input operation. As a rough guideline 1kOhm load is needed per Henry inductance of the first choke. The first choke is 10Hy, so a load of roughly 10kOhm is needed. Since there are already bleeder circuits in the amp (22k and 6,8k in series per channel) we only need a 33k which in parallel with the other resistors gives about about 10k. Since this resistor dissipates quite some power, a 20W type is needed. This ensures that even with tubes unplugged or failing tubes, the current will not fall below the critical value. This resistor can be left out if the capacitors have sufficient voltage rating. If sub critical current is drawn the filter will stop working as a choke input but behave like a cap input and the voltage will rise by up to 50%. To be safe in a fault condition or when the power supply is tested without amp circuit attached we don't want the caps to blow. So either overrate them or install the bleeder circuit, or both to be on the save side. I selected ASC X386 440VAC caps. The DC rating of these caps is 630VDC minimum.

With this post all details have been laid out about this amplifier. In the next parts I will first show the cost down version in detail including it's power supply. After that I plan an article about an improved version of the power supply using a full wave Graetz-bridge with tubes. And after that we will see how this concept can be adapted for directly heated tubes as well.

Let's close this post with some photos. The first row shows the interior of 6CB5A amps based on the same circuit, built by different people

Here some 6CB5A amps with different chassis styles and output transformers:

The top row shows implementations with Tango, James and Hashimoto transformers. The second row shows a pair of monoblocks using Lundahl tranformers and the blue one is a Push Pull version, again with Tango.

The last picture shows an amplifier based on the very same circuit but with the power supply separated into an external chassis. There is a minor difference in the power supply though, instead of a 6BY5, two 6AX4s are used as rectifier. Both interstage and output transformers are Tango. NC20F interstage and FC30-3.5S output transformer:

Best regards

Thomas