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    Building, troubleshooting and testing of these amplifiers should only be
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Phase splitter

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Hojvaelde

Member
Joined 2005
Hey

I work at a PP 19BG6GT amplifier and looks a little on phase splitter. I have some 6N7 which I'll use for this because they are quite strong.
Input stage is based on the 6J5 tube

The diagram is here (hand drawn):



I'm slightly in doubt about voltage division (20:1), it is well understood.

I do not know whether I will use a fixed or self-biasing yet.

Regards

Benny
 

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Alastair E

Alastair E

Member
Joined 2004
doctordiesel.co.uk
Personally, I dont like that type phase-splitter....

Ive nearly always used a Split-Load or Concertina type, and providing you keep each output at a high impedance and follow a few simple rules, they work faultlessly....

Nice, Simple, reliable they are, but the downside is no gain out of them--Easily fixed though, Just add another bottle!....
 
Miles Prower

Miles Prower

Member
Joined 2005
dolphin-hsl.com
Hojvaelde said:
I work at a PP 19BG6GT amplifier and looks a little on phase splitter.
Click to expand...

YUCK!
An externally hosted image should be here but it was not working when we last tested it.


That's just about the worst phase splitter: it's strictly in the "quick 'n' dirty" category. Useless for anything where you're the least bit concerned with sonic quality. It requires o'scoping to put it in balance, but it won't stay in balance, and will require periodic realignment: not user friendly. Like all paraphase splitters, it won't have harmonic balance between phases.

I'm slightly in doubt about voltage division (20:1), it is well understood.
Click to expand...

More than "slightly", it would seem. With the resistors shown, your voltage division is 21 : 1, not 20 : 1. Getting Av= 21 from tubes with u= 35 is pushing it.

You'd be much better off using the 6N7 as an LTP inverter with an active tail load. That will be significantly better: better phase-to-phase balance, and equal distortion between phases. These paraphase type splitters all have the problem of uneven distortion between phases, and that means more distortion at the output. Might help with guitar amp tone (mostly distortion) but it's not so swell for music reproduction. The only downside to the LTP is losing half the gain per phase. Since you already have a pre ahead of the splitter, this shouldn't pose much of a problem.

A cathodyne would also be a better choice, since you have a 6J5 pre, you could use one half of the 6N7 as another pre gain stage, and the second half as the cathodyne. That might make up for the gain you'd lose from a cathodyne and its less-than-unity phase-to-phase gain. At least the cathodyne has Ac phase and distortion balance so long as the load doesn't start pulling grid current.

Since it looks like you intend to drive the finals from the splitter with R-C coupling to the finals, you definitely want cathode bias for the finals. R-C coupling and fixed bias do not play well together. Any overdrive at the finals that turns on the GK parasitic diode will put a negative charge on the coupling capacitors. While that excess bias leaks off, the finals will operate more towards cutoff, and distort more than they should. Even if you don't hear the actual clip, you will notice the sonic degradation that follows. Cathode bias helps mitigate that effect.
 
C

ChrisA

Member
Joined 2008
Hojvaelde said:
.... I'll take a look at the LTP, and I will come back. Do you have a website you would recommend.
Click to expand...

This covers the design of an LTP in enough detail and is easy to read
http://www.freewebs.com/valvewizard/acltp.html

In the above if you replace Rt (the tail resistor) with a constant-current sink the balance will be nearly perfect. A tube purist would use a CCS made from a pentode others would use a solid state device. But a simple resistor if it is big enough is pretty good.
 
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