CCS in Mellow OTL

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Ray P
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#1 CCS in Mellow OTL

Post by Ray P »

I'm still planning to one day build the Mellow OTL amp and have started to slowly collect the necessary parts. Below is a sketch of the schematic;
Scan001.JPG
I have some spare K&K Audio constant current source (CCS) kits and I'm thinking that it might be worthwhile having the option to swap them in to replace R5 and R6 as the anode loads for the ECC83s?

To take that further with R5 & R6 = 100K and dropping 46V Mr Ohm tells me that the ECC83 valves are only pulling 0.46mA, which seems low - am I missing something?

Any thoughts/advice? Ta.
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#2 Re: CCS in Mellow OTL

Post by Ray P »

Incidentally, as per the suggestion in the original AudioXpress article, I plan to use a balanced input connected, via C1 and C2, to the grids of the ECC83.
Capture.JPG
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Nick
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#3 Re: CCS in Mellow OTL

Post by Nick »

What size is the resistor in series with the input cap? I can see either (if its large) miller causing poor HF response, or (if its small) the level of feedback varying with the setting of the input volume control. Applying the input to the other half of the LTP looks like a much better solution. Using a balanced input will still have the problem of the input resistor and miller.
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#4 Re: CCS in Mellow OTL

Post by Ray P »

Thanks Nick, here's the full schematic, albeit in a less 'friendly' layout.
Schematic.JPG
The issue you mention with the variable feedback was discussed somewhere, DIY Audio I think - I'll see if i can track it down - IIRC the conclusion was 'not significant issue' but would be good for your opinion.
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#5 Re: CCS in Mellow OTL

Post by Ray P »

Here's a snip from the article, it immediately precedes the section I included above.
Capture2.JPG
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#6 Re: CCS in Mellow OTL

Post by Nick »

So the 32k resistor would I think give a -3db point of 30k, not ideal, but maybe ok, I am tempted to say an output transformer would be about as bad so maybe its a valid question.

The feedback will vary by I think about 6dB so not trivial, but maybe not a problem.
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#7 Re: CCS in Mellow OTL

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Thanks Nick, much appreciated. I'll look out that conversation on the feedback a bit later. As shown in the schematic, the feedback is 26dB, however Mellow says that by omitting C3 and C4 it is reduced to 11dB, which I guess will reduce the feedback issue proportionally?

Any thoughts on the original question about using CCS instead of the resistors for the anode loads of the input valves and the low current calculated?
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#8 Re: CCS in Mellow OTL

Post by Nick »

Mellow says that by omitting C3 and C4 it is reduced to 11dB
At DC, the caps are there to allow more feedback at DC. For AC it should be the same.

Re the CCS, yes will work, but 470k is a long tail anyway, so probably not much gained by using a CCS.
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#9 Re: CCS in Mellow OTL

Post by Ray P »

Cheers Nick, very helpful.
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