Chapter 5 - Design a Simple power AMP.

I was thinking what should be use as a tour on this chapter. I hope this tour’s project is cheap and easy available, also easy to build.

On my mind, a 2A3 single ended is easier. But the power is too small for most speakers. 300B? It was too common. I think you can find the information on other web site easy. Pentode like KT88/6550/EL34? Yes, they are good and easy too, but design is a bit more complicated. So, I choose 845?

845 need high voltage?

845 can be work from 420V to 1250V.
Here is some working point for your reference:

Plate voltage Plate current Grid voltage Power output Output transformer
440V 50mA -50V 4.4W 3.5k
540V 60mA -69V 9.1W 3.5k
750V 72mA -100V 15.2W 5k
900V 80mA -125V 20W 7k
1250V 58mA -200V 29W 12k

As a tour, I want to recommend the lowest voltage 440V. Including the grid voltage, for self-bais, we need 490V B+. But the power really too small for most speaker. So, I choose the 540V and –70V bais. Beware, 609V is very high voltage. I recommend you try the pre-amp project in earlier chapter, before try this Power AMP.

Circuit design: -

We want the easier way to have a good sounding tube AMP. So, the circuit only has 2 stages. I used 6C45 as driver. RC coupling is most simple and cheap. Circuit as below:

Figure 5.1 - 6C45 RC 845 Amplifier circuit

Why use 610V? Because Most Capacitor are only available up to 630V. So 610V is a limited point!

R7 used to drop voltage to around 370V for 6c45, it need at 4.5W, so we need 20W resistor. C2 provide additional filtering ability and is the reservoir for 6C45. R6 is the loading resistor for 6C45. This resistor direct affect the sound quality. Choose the best wire wound resistor. But it need 20W, so quite limited choose. R2 is used to bais the 6C45 to work at about 20mA. C1 with R2 make a network to cut the F3 frequency at about 5Hz. R1 is loading for pre-amp. R8 is loading for 6C45.

C3 is coupling capacitor to 845. Choose the best capacitor here. Recommended PP, Oil, or Teflon type. R8 and C3 form a network to cut the F3 frequency at about 7Hz.

R3 and R4 need matched with 1%. It can be omitted if your filament transformer has center tap. Then change R5 is 11.51k (11k + 510ohm serial connection). C4 with R5 make a network to cut the F3 frequency at about 7Hz. T2 is Output transformer, it could be 3.5k or 5k. More linear on 5k, but a bit lower power!
All resistor is 0.5W, unless otherwise stated.

PSU design: -

On Power amp, the PSU no need some much filtering as in pre-amp. One stage pi filtering is good enough. We want more tube taste, so this PSU used a tube 5AR4 and two 1N4007 as a bridge. That make the power transformer can be easier obtaining.

Figure 5.2 - PSU of Low voltage 845 SE AMP

Power transformer: -
For 610 voltage B+, we need a transformer with 230V bridge rectifier.
Here is a list of the winding we needed:

1. 260-230-0-230-260, 200mA (For further upgrade)
2. 2.5-0-2.5, 4A
3. 0-6.3V, 2A
4. 5-0-5V, 4A

I found a R-core transformer which quite cheap and easy placed inside my chassis. Winding 1-3 could be common found there. But winding 4 may need a custom make. Filament transformer. Separated filament transformer even sound better!

Choke: -
This project is not a small one. For easier construction, I recommended a Mono block design. So, we need a filtering choke of 10H 100mA 150ohm or better.

Capacitor: -
I choose Solen Fast capacitor 51uF 630V. It is non-polar MKP type. 1000 times faster than any electrolysis capacitor. It could be the best Size, value and easy available. R1 and R2 is bleeding resistors. Choose general grade is OK.

Output transformer: -
There are many brands. From HK$300 to 6000 (US$40-750) each. For best result, you could choose Tango, Tamura. For prototype, many China made also sound good! I choose the Taiwan make James JS-6123HS. Which is a multi-tap 2.5/3.5/5k 20W output transformer. The sound is details very balance and natural. It don’t as details as the Japans but the Price Just HK$800 (US$103) each. Good for Beginner.

Construction: -
Most tube amp output transformer has 2 to 3 winding for 4/8/16-ohm speaker. So, you need make 4 diameter 10mm hole for the most speakers terminal. You also need 1 extra 10mm hole for RCA signal input. An IEC socket hole for power cord. A ON/OFF switch. May be an LED hole if you like!

Drill the large holes: -
You need a power drill, better mounted on a stand. You need several drill bit with saw teeth; 20mm for 9-pin socket for 6C45; 53mm for 845 socket and output transformer. That bit is quite expansive! So, we got some cheaper one selling on the online shop.

Fuse, You have to use 1.2A slow action type or 2A normal type.
ON/OFF switch, need 250V 1A type, please put a capacitor to absorb the pulse when switching it.

Wiring, I love Teflon silver plate wire on power circuit and pure silver with Teflon wire for signal path.

Soldering, some solder has some silver. But I don’t feel that is necessary. You better use the type without lead. That can be found in some large metal-shop. That soldering is design for drinking water system. So, need avoid the lead, it is toxic. If you can’t found it, just use any good quality for easier working! The solder not affect the sound much!

Testing, please test the voltage after the transformer (230VAC) after you connected it. After you finished the PSU, test the voltage on the B+. It should be a bit higher than 610V, since no loading there. So, switch off fast, otherwise it will fire the 630V Solen easy. And then work on the power tube parts circuit. Don’t forgot the install the 47k R8 before you apply the B+ onto it. Power on and Measure the voltage across R5, it should be around 70V. If a big different here, turn-off the amp ASAP. You have to check anything wrong there!
And then test voltage across R2. IT should be 2V around. That means it running at 20mA. Finally, test the AC voltage at the speaker terminal. It should be below 20mV for acceptable noise level. Beware!! if there is any DC, that mean a problem on your Output transformer.

If every thing go fine, Plug a cheap speaker and let it sound for several minutes. Then that should be safe to plug it into your main speaker!

Enjoy the sound!