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general loud applause....

Posted by keto (A) on May 13, 2004 at 15:06:18

In Reply to: Criticism please!! posted by Bas Horneman on May 13, 2004 at 13:35:33:

Looks great! Just a couple suggestions.

The 100K may be a little large. You might try a lower value, say above 10K. Allen Wright suggested to me 1K, and he builds a lot of amps. You want that R to be at least 10X the output impedance of the preceding stage. I've tried a pseudo grid choke (primary of a cobalt B7--about 1/10 the inductance of the real thing) in that position and its fabulous.

The coupling cap should be about 3-5uF and >200V. With 0.47uF it will work, but you'll have some strange things going on around 30hz. You want those strange things at around 3hz, instead. Maybe try that 3.3uF PSU cap, which may not be of such vital importance to the sound (if the stuff is on hand already). I've got a chart someone made for me that shows the interaction, which I'll try to find and post.

One way of balancing the current 60mA-60mA between the OPT primary is to measure the DCR of the two halves, and make them equal by adding a precision resistor. Michael Percy has a little multi-turn Vishay trimmer, which I use in its 100R form in a constant current source to do a similar thing. A value of 50R would give you the right amount of flexibility and fine adjustment. If you can up end your amp (I C-clamp mine to the bench) so that the filament pins are in a vertical plane and heat-generating elements wont otherwise damage some other part, THEN, set up the little trimmer in some sort of stable module, and you can do the fine tuning on the run. I've found this fine tuning to be really stable, once its done and though perhaps independent of true zero DC offset, but at least close to it, humm should null to about 0.5mV. I've also found that even between TJs, the two tubes want to be in a particular order.

I'm not familiar with the 6AX4 function.

A single 220uF or 470uF instead of your 47uF+40uF might help. The bleeder draws less than 1mA and may take a while to drain voltage; especially if you go to a bigger cap, maybe change it to a 250K.

The 100K dropping R is ingenious, but I bet it doesn't look as good as an OB3!

As described, it should work fine; you might want to try my suggestions if convenient. I look forward to hearing your impressions.

--keto

Edits: 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/11/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20 08/12/20

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