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HeadHunter
DRAFT!! Page 1: December 22, 2006
It's
been 4 months after starting this project on December 22, 2007. Because I had
several other projects to finish, this project needed to wait until they were
finished.
So after finishing the Martian amp today, I browsed through my open project looking for a next victim. For the last months I had two 6AS7 standing on my desk, mounted with their sockets on an aluminum profile. They were not connected, and although nice to look at once a while, I figured that they would even be beter when used in the next headphone amp.
I know that I need about 3V/40mA for two channels in order to keep my Sennheiser headphones happy. Therefore I started looking for the right tube to power the next headphone amp. Again there are several choices to be made:
Ik will get a few tubes in-house that are candidates for output tube:
Of-course these tubes are demanding for the heaters as well, 6.3V and 2.5A each. So I expect a heavy amp. This time, having two tubes in parallel is not acceptable, and since I want to keep things a simple as possible a triode would be just fine. However connecting one halve of the tube as a cathode load for the top tube which is working as a cathode follower would be OK.
If possible I want to use a nice glowing tube for this project and put the tube in the front of the amplifier. Therefore, I want to work with the 6AS7 as this is a nice show-off tube and I did not work with this tube before. It will obviously influence the power supply as this tube is a high-current tube wrt both the heater and plate currents.
I have 4 RCA 6AS7 tubes and 2 Svetlana 6N13S/6H13C available, and these are the bottle shaped tubes with nice glow. So the 6AS7 it will be.
The driver might be either SRPP (using one driver tube per channel) or just common cathode when sharing one tube for two channels. I think that 6N1p o the 5670 will be a nice tube in that position.
The power tube will be a cathode follower configuration either with resistor or with other tube halve as cathode load. I think I will use the White Cathode Follower for this application. It has low output impedance.
I need to re-check whether the 6AS7 can be heated using 6.3AC. According to the datasheets this is possible but I want to read other peoples experience before making a final choice. In any case this tube runs on 6.3V/2.5A which makes it a rather difficult animal for me to feed. I did not find too many cheap transformers capable of providing 2.5A on 6V. But since I use two 6AS7 tubes in the second stage I will be able to use a 12V transformer with 2.5A current and wire the heaters in series.
Looking at a "standard" transformer for tube projects, I spotted an Amplimo transformer with 6.3V/4A and 300V/0.31A secondaries. This transformer could be used but initially I'm planning on a lower B+ rails for the 6AS7 tube. Also, this transformer is only usable for one (1) 6AS7.
On the other hand, it is possible to use a double 6AS7 setup when working with a standard primary transformer for 6V heaters and use a step-up for the B+ rails. Conrad sells a transformer (nr 518670) with 120VA rating on 2*12Vac, which will be 60Watts per 12V equals 5A. This may or may not be too much for the transformer. And Amplimo sells a standard transformer (item 38010) with two rails of 6Vac rated at 6.67A. Each secondary would provide enough power for one channel consisting of an 6AS7, a 6N1P and 200V B+ rails (through step-up transformer).
For a 6.3Vdc setup, the 9V Amplimo transformer of 80VA would probably a better choice. In that case, the voltage regulator need to be able to provide at least 5A per channel. For a 12V setup, I need a 15Vac transformer and a voltage regulator such as the LT1083 that is capable of regulating at least 2.5 A current.
Not that we normally have to worry about cabinets, but I'm not very skillful at woodwork so that limits a little the size of power supplies and amp itself. I've another aluminum box available just as I used for the hadline amp and I have a nice cabinet from an Italian supplier which would even be better for this purpose. (Unfortunately, the latter has since been used for the Martian amp).
The 6AS7 is still available. But there are similar tubes that are worth considering. Typical characteristics:
|
Tube
|
V_plate
|
mu
|
Gm
|
ra (R_plate)
|
Ip
|
Vh | Ih |
|
6AS7
|
5,500
|
280
|
130 mA
|
6.3
|
2.5A |
||
|
6080
|
6.3
|
||||||
|
6N13S
|
90
|
6.3
|
|||||
|
6336A
|
190
|
2.7
|
13,500
|
400 mA
|
6.3
|
5.0A
|
|
|
CK6528
|
100
|
9.0
|
37,000
|
300 mA
|
6.3
|
5.0A
|
|
|
5998
|
90/100
|
5.5
|
14,000
|
140 mA
|
6.3
|
2.4A
|
|
|
7236
|
120
|
12,500
|
100 mA
|
6.3
|
2.4 A
|
||
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EC360 (not pin compat)
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^^ Back to DIY Page ^^
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