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    Building, troubleshooting and testing of these amplifiers should only be
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6-volt rectifier tube

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E

E.T.

Member
Joined 2009
Hi everybody,

I'm planning to build a van Waarde 6as7 headphone amp, and I'm looking for a full wave rectifier tube with 6-volt heaters. The spec sheets have me stumped.

Current draw:

2 x 3a5 10 mA
6922 20 mA
6as7 250 mA @ 250V plate (I'll have plate @ 110V)

Does such a creature exist? Would za half-wave rectifier do?

Also, can anyone steer me to a simple passive bass and treble tone control circuit? :eek: I know this marks me out as a philistine among purists, but I'm always trying to tweak the frequency balance of various albums.

My most recent project is a Pete Millett "Starving Student" headphone amp. It's remarkably fast and accurate (certainly for $40!) but still has a touch of transistorized glassy-brittleness. I can definitely recommend it for beginners. When I get access to a digital camera, I'll send in some audio porn plus the modified schematic.

Thanks in advance!
 
E

E.T.

Member
Joined 2009
After 150 hours it's certainly mellowed. I'm using Hexfreds, 12,000 uF of Denon "Audio" electrolytics in the p/s, and Elna Silmic II's as local caps. I just figured the brittleness would be the only transistors in the circuit. What else might it be?

Thx!
 
dsavitsk

dsavitsk

diyAudio Member
Joined 2005
diyaudio.ecp.cc
E.T. said:
After 150 hours it's certainly mellowed. I'm using Hexfreds, 12,000 uF of Denon "Audio" electrolytics in the p/s, and Elna Silmic II's as local caps. I just figured the brittleness would be the only transistors in the circuit. What else might it be?
Click to expand...

This is the Starving Student? Mosfet followers are about the least brittle sounding circuits you can build -- too mellow perhaps, but only brittle if something is not working.

Hard to know exactly what that might be. Could be oscillating mosfets or tubes -- did you use grid/gate stoppers fight on all the pins? It could also be that the PS is unpleasantly noisy (it is generally OK, but maybe yours is worse than most) and due to the poor PSRR of the circuit that noise is creeping in.

However, assuming that this is built as Pete designed it, or as Tom and I modified it, it is most likely that the PS is unable to work correctly into 12000uf and is stuttering and or ringing. Reduce that to a reasonable amount, maybe 470u, as per the schematic and see if this improves. With 12000u, I am frankly surprised it works at all.
 
dsavitsk

dsavitsk

diyAudio Member
Joined 2005
diyaudio.ecp.cc
E.T. said:
6as7 250 mA @ 250V plate (I'll have plate @ 110V)
Click to expand...


By the way, for headphone use the 6as7 is not going to do anything for you at 125mA per side that it won't do just as well, if not better, at 50mA. Moreover, building a high voltage high current PS for that application is not trivial. I'd tone it down a tad.

Oh, and get some shields for the 3A5's -- they are also not trivial to work with.
 
E

E.T.

Member
Joined 2009
It seemed obvious that most headphones wouldn't use more than 50mA, but I don't know how you set the current capacity of the circuit. Would a rectifier tube of 100 mA be adequate? I don't understand the relation of continuous current draw to signal draw. :confused:

The 12,000 uF supply caps are bypassed right at the mosfets and tube plates by 220u and 100u Silmics. Maybe that's why it's stable.

My output caps are 1000uF@35V Silmics. Could that be where the harshness came from? It's smoothed out a lot over time.

Thx!
 
dsavitsk

dsavitsk

diyAudio Member
Joined 2005
diyaudio.ecp.cc
E.T. said:
but I don't know how you set the current capacity of the circuit.
Click to expand...

Worth a read: http://www.tpub.com/content/neets/14178/

The 12,000 uF supply caps
Click to expand...

I really don't think the PS can work properly into this much capacitance -- it is probably stuttering on and off. Replace the 12000u with 470u. Less is more here

My output caps are 1000uF@35V Silmics. Could that be where the harshness came from? It's smoothed out a lot over time.
Click to expand...

Almost certainly not. But, more importantly, these caps should be 50V minimum, and really they should be 63V.
 
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