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Old 11th March 2012, 09:56 PM   #1
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Question Tweaking PP AMP Suggestions Please

Hi folks,

I would like to call for your knowledge and experience with tweaking schematic shown below. It was built almost as is, but 6L6GC were replaced with KT88/6550, thus, output power increased from 40W to 60W, and put everything on PCB.

There are few things:

1) Screen supply with resistor voltage divider (1K/15W + 15K/30W). It just dissipates too much heat. I don't see its necessity for KT88/6550 (original amp used 6L6GC), standard penthode connection should work.

2) Radio Gijutsu published an article with measurement data. This is what they find. THD increase with the rise of output power and frequency. For example, at low power THD at any frequency are below 0.1%. However, with the rise of output power and frequency THD also increase (in original amp at 30W / 10 KHz THD reaches 2.5%). I think source of this distortions is a phase shift caused by leakage inductance/stray capacitance of the output transformer and NFB loop. I tried phase shift RC (22K resistor + 47 - 510 pF capacitor, connected between pin 2 of upper triode 12BH7A and ground), seem to be no changes. RC 22K + 150 pF connected to 6AQ8 plate were disconnected.

What do you think about removing separate screen supply and lowering THD level at high frequencies?

Thanks in advance for any suggestion(s).
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Old 11th March 2012, 10:18 PM   #2
DF96 is offline DF96  England
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Phase shift does not cause distortion; valve curvature does. Phase shift affects stability, which can limit how much NFB can be applied.

The screen supply does seem to waste a lot of power, but it is important to maintain the screen voltage constant. Maybe use a cathode/emitter follower instead?
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Old 11th March 2012, 10:48 PM   #3
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Hi, DF86,

Thanks for suggestions !

Quote:
Originally Posted by DF96 View Post
Phase shift does not cause distortion; valve curvature does. Phase shift affects stability, which can limit how much NFB can be applied.
I'm thinking to replace 12BH7 with 6SN7 and adjust values of corresponding resistors in phase inverter circuit. May be this could help.

Quote:
Originally Posted by DF96 View Post
The screen supply does seem to waste a lot of power, but it is important to maintain the screen voltage constant. Maybe use a cathode/emitter follower instead?
Question is why the original designers choose separate screen supply, and not standard penthode connections?
6L6GC works fine with screen voltage up to 500V, although for class AB1 GE recommends 450V and 400V for plate and screen respectively, so my assumption this is only the reason of this power waste.
For KT88/6550 which I use recommended plate/grid voltage = 460V/453V, so separate screen supply doesn't seem to be necessary. Please correct me if I'm wrong.

Last edited by LinuksGuru; 11th March 2012 at 11:01 PM.
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Old 11th March 2012, 11:05 PM   #4
DF96 is offline DF96  England
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Fixing the screen voltage is standard pentode connection. It may be that they used low value resistors to give a low impedance screen supply down to DC so that it doesn't change much over the whole range of Class AB operation. The traditional screen supply of a resistor to HT+ and a cap to ground is just about OK for Class A, but not AB.
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Old 12th March 2012, 01:18 AM   #5
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by DF96 View Post
The traditional screen supply of a resistor to HT+ and a cap to ground is just about OK for Class A, but not AB.
That's a good tid bit, I hadn't realized or thought about that before.
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