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Old 22nd Sep 2010, 9:31 am   #21
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Default Re: Stereo Amplifier

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...ad.php?t=59446
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Old 22nd Sep 2010, 1:54 pm   #22
G8HQP Dave
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Yes, I think Langford-Smith is talking about the same thing as me - non-linear small grid current in the negative bias region where most people expect to see no grid current at all. See also Ch 2, sec 2.2(iii) - especially fig 2.9 on page 19 (curve D).

When you mention grid current to most people they think of Class C or AB2, where tens of mA of grid current can be quite normal. They know that a low impedance drive is needed. The region I am talking about is not so well known, despite being in Langford-Smith. I suspect, but have no evidence to back this up, that this effect is responsible for some of the subtle differences between various good amplifiers, and some of the changes which are heard when valves are swapped ("tube rolling"). The size and position of the grid current curve varies from valve to valve, and changes with age. It is affected by residual gas, contact potential (depends on exact metals used), cathode temperature. It is quite common to bias a small signal valve right in the middle of this region, especially high mu ones like ECC83. You need to be below about -1.5V before the probem can be ignored, or have a low impedance source.
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Old 22nd Sep 2010, 3:52 pm   #23
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Default Re: Stereo Amplifier

...or have a low impedance source.

There seems to be enough room for a couple of ECC8x valves on the chassis as cathode followers.
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Old 22nd Sep 2010, 8:27 pm   #24
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Default Re: Stereo Amplifier

I think cathode followers might be considered overkill, when a change of phase splitter would suffice.
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Old 22nd Sep 2010, 8:44 pm   #25
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Default Re: Stereo Amplifier

er hmm I may be wrong, but that 47 uF cap across the plates of the phase splitter dont look good. you will also need two more 1k5 grid stoppers in the topmost and bottom most 6L6

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Old 27th Sep 2010, 9:35 pm   #26
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Default Re: Stereo Amplifier

Quote:
Originally Posted by PJL View Post
The last guitar amp I played with had 4xEL34 fixed bias driven from an ECC83 but of course distortion was probably a good thing!

Of course I could be wrong....!
Quite right, I had a "Sound City" 120 watt guitar amp that used 6 EL34's in push - pull and they were driven from one ECC83.

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Old 28th Sep 2010, 11:42 am   #27
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Should the 47uF between the phase splitter anodes be 47pF?

The input arrangements as shown are clearly wrong. 150V (presumably intended as PS grid bias?) is applied to the input signal, then isolated from the PS by two capacitors and a 1M resistor in series. The 1M should be in parallel with the signal, not series. It should attach to the PS grid, then feed the signal in via a capacitor.
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