audio-talk

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;
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.

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.

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.

Thanks Nick, here's the full schematic, albeit in a less 'friendly' layout.
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.

Here's a snip from the article, it immediately precedes the section I included above.

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.

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?

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.

Cheers Nick, very helpful.