Re: 5.8v for filaments and low B+? On topic!!!
- From: "Phil S." <psymonds_no_spam@xxxxxxxxxxx>
- Date: Fri, 30 Jun 2006 16:28:24 -0400
"Mike Schway" <mschway@xxxxxxx> wrote in message
news:mschway-0B6387.08191230062006@xxxxxxxxxxxxxxxxxxxxxxxxxxx
In article <Y6idnQaMgvIGpzjZnZ2dnUVZ_tmdnZ2d@xxxxxxxxxxx>,
"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
Sounds to me like the power tranny may be overloaded. What are the
heater voltages when the rectifier tube is out of circuit? A ss
rectifier may not be a bad idea here.
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
mschway@xxxxxxx |
--------------------------------------------------------------------
This was too obvious for me <grin>, so thanks for the suggestion. The
filament supply rises to 3.0 on each leg without the rectifier, which should
do very nicely.
I was reasonably sure this tranny would work. The Webcor 2110 donor had the
following tubes:
12AX7
12AT7
(2) 6V6
6X5
6E5
I get a total of 2.4A for all of those.
The tube compliment I selected is 2.5A. I guess, in this case, that 0.1 is
the straw that broke the camel's back.
Another wild card I didn't consider earlier in the day is that the choke is
of unknown value. It has plenty of heft to it, but that's all I really
know. I came out of a Heathkit TV for which I could find no documentation.
..
I'll build the diode rectifier and see what happens. I'm leaving for Maine
on the redeye tomorrow for a week of vacation. (Hi Dave, call me a tourist;
I'll be in Bar Harbor and exploring Acadia. Nothing like a good hike in the
park to get a mood shift. Then we can get a shower and eat at a nice
restaurant in the evening. Life promises to be good.) Further work on this
amp will be on hold until I return, but I think the project can be salvaged.
There was a choke in the donor amp, but it looks less beefy than the
Heathkit. There was a very weird OT on the donor, too. Maybe I can post
the donor schematic when I return from vacation
Thanks, Mike.
Regards,
Phil
.
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