Re: 5.8v for filaments and low B+? On topic!!!
- From: Gilbert Bates <gilbert@xxxxxxxxxxxx>
- Date: Fri, 30 Jun 2006 14:52:07 -0600
On Fri, 30 Jun 2006 10:52:07 -0400, "Phil S."
<psymonds_no_spam@xxxxxxxxxxx> wrote:
I've built the Frankenvox AC15. I turned it on for some test readings, but
haven't had a chance to play it. I did put a cable in the input, so I know
it is passing signal. Noise level on the test speaker (a small hifi
speaker) is not an issue.
Heaters are running 2.8-2.9v per side, which is right about 90% of 6.3v.
Any thoughs on whether this will be OK for light use?
Also, B+ did not rise to the level called for in the schematic (more on this
in a bit). I think this tranny is a bit wimpy. It is quite warm to the
touch, but not so hot as to burn. I thought maybe that was as a result of
excessive heater draw. The tube line up is 6AU6, 12AX7, two 6AQ5, and 6CA4.
If I swap the tube rectifier for SS, I think I will get two benefits: 1) it
releases 1.0A for the filament circuit and that should allow heater voltage
to rise; also less stress on the tranny and 2) it should increase the B+ a
bit.
The HT secondary puts out ~255-0-255 without a load and ~227-0-227 with the
tube in the recto socket. It is rated at 220-0-220 on the Sams Photofact
for the donor amp. B+ at the first filter cap should be about 320v, but it
is more like 250v. This is not what I estimated for B+ (255*1.4*.85 ~=
300). Maybe I can squeeze out a little more B+ with the SS recto? Does the
rectifier tube drop B+ by that much?
Thanks for any input or comments.
Phil
As you tinkering around, you might check backing down on the idle
current a little and monitor your B+, the series resistance of the
little 6CA4 (EZ81) is probably a lot higher that something like the
5AR4 (GZ34)???
.
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