Fred's SET Amp
li_gangyi
I have just tested out Fred's amp...I am getting high-pitch
whining..maybe I should reverse some of my transformer's primary windings??
Thanks in advance.
Regards,
li_gangyi
li_gangyi
In addition to the last post...I have noticed that the 2
transformers also emit a hum...or vibrates very fast...wonder why...??
Reversing the primaries also do not help in this case...
Regards,
li_gangyi
"li_gangyi" <li_g...@yahoo.co.uk> wrote in message
news:bag3l3$8nd$1...@mawar.singnet.com.sg...
Fred Nachbaur
li_gangyi wrote:
> Hiyee,
> In addition to the last post...I have noticed that the 2
> transformers also emit a hum...or vibrates very fast...wonder why...??
> Reversing the primaries also do not help in this case...
> Regards,
> li_gangyi
A certain amount of vibration is normal, it will depend on the
transformers also. Better ones tend to vibrate less.
They will also vibrate more if excessive current is drawn. They'll also
overheat if this is the case.
> "li_gangyi" <li_g...@yahoo.co.uk> wrote in message
> news:bag3l3$8nd$1...@mawar.singnet.com.sg...
>
>>Hiyee,
>> I have just tested out Fred's amp...I am getting high-pitch
>>whining..maybe I should reverse some of my transformer's primary
>
> windings??
>
>>Thanks in advance.
>>Regards,
>>li_gangyi
Whining? Are you hearing it from the speaker? Does it change with volume
control setting?
Cheers,
Fred
Tom Schlangen
> I have just tested out Fred's amp...
You mean Fred Nachbaur's MiniBlok or MiniBlok II?
> I am getting high-pitch whining..maybe I should
> reverse some of my transformer's primary windings??
In the case of aforementioned amps that would most
likely not solve the problem, since they have no
global negative feedback path.
Reversing of primary _or_ secondary transformer windings
probably could cure the probem if there would be a global
feedback path from OPT secondary to the input tube section,
since this whining is a common effect if the feedback signal
is 180 degrees out of phase, which means it is no
negative, but positive, feedback. Reversing the connections
on one side of the OPT would cure that.
So, if you are building a MiniBlok, the problem must
be somewhere else.
Maybe there is some capacitive coupling between stages due to
how you layed out the wiring - tube circuits are often more
prone to this than silicone circuits because tube circuits
often have much higher impedances. Redesigning the wiring
and parts arangement so that coupling between wires and parts
would be reduced could help in this case.
Tom
--
All the true gurus I've met never claimed
they were one, and always pointed to someone
better. -- Bill Vermillion
Tom Schlangen
> In addition to the last post...I have noticed
> that the 2 transformers also emit a hum...or vibrates
> very fast...wonder why...??
This seems to be a mechanical hum. Oh well, power
transformers tend to hum mechanically, when they are cheap, not
impregnated, driven beyond their specs and whatelse and
any combination thereof.
Given that the PTs are used within their specs (you can
find out by measuring the drawn currents) there shouldn't
be any excessive mechnical hum.
If there is, you could try to decouple the mechanical
hum of the PT from the chassis, e.g by bearing the
screws which hold the PT to the chassis in rubber.
When you switch your amp on, is the (mechanical) hum
especially loud for the first seconds or two?
> Reversing the primaries also do not help in this case...
As I suspected in my former article.
Tom
--
Live is too short to be taken seriously.
- Oscar Wilde
li_gangyi
Ok I found the problem...a power supply smoothing cap...(350V 100uf)
had some kind of internal short or something...I used a photoflash
cap...that cap started to smoke when I turned it on the second time...so I
took it out and series connected 2 PSU 200V 470uF caps together...seems to
work fine now...I am getting sound...but no bass...due to OPT?? Also I
cannot turn the volume to full...I will get a high-pitched whine...and there
is a noticeable amount oh mains hum coming through...maybe my connection to
pin 4 of the tube is a bit too long...I will try shortening that...any other
ideas?? Thanks in advance.
Regards,
li_gangyi
"Tom Schlangen" <t.sch...@ndh.net> wrote in message
news:jmnsp-...@skye.snakes.ndh.net...
Tom Schlangen
> Ok I found the problem...a power supply
> smoothing cap...(350V 100uf) had some kind of internal
> short or something...
Okay, that shorted cap was probably the reason why the
PT was overloaded and hummed mechanically.
> I am getting sound...but no bass...due to OPT??
Well, you can't expect much bass from a very low
powered triode amp, at least not with todays usual
low efficiency speakers.
And of course it could be the OPT, too, if it runs into
saturation at low frequencies. I suppose you built the
Miniblok SE version, not the Miniblok II PP variant?
> Also I cannot turn the volume to full...I will get a
> high-pitched whine...
This sounds like a instability depending on gain ...
maybe Fred himself has an idea how to solve that.
> and there is a noticeable amount oh mains hum
> coming through...
The hum can have quite many reasons. But most often it
is an internal ground loop problem, e.g, having grounded
the circuit at the inputs _and_ at the outputs, forming
a so called hum loop.
There have been quite some extensive discussions on RAT
about solving hum problems and I suggest to read some
of them, e.g by using Google's newsgroup archive.
But, as said above, there could be quite some other
reasons, too. The best thing would be to work and measure
consequently thru the circuit and find out, where the
hum leaks into it.
For example, you could test the driver stage alone by
feeding its output signal via a voltage divider to
another amp, and so on.
> maybe my connection to pin 4 of the tube is a bit too
> long...I will try shortening that...any other ideas??
Okay, here are some hints what could lead to excessive hum
and their antidotes. This list surely isn't complete, but
could give you some basic hints:
- ground loop problem
Could be solved by _consequently_ using _one_ of two
commonly used grounding strategies: _either_ use
a star ground scheme (bus ground or multiple star
ground being variants of that), _or_ tie every ground
connection as short as possible to the (grounded) chassis,
- magnetic field induction problem
PT may couple into OPTs if magnetic fields overlap. Could
be solved by more space between trannies and/or changing
core orientation between them. It is good practice to
arrange transformers so that their bobbins don't have
the same orientation in 3D space,
- PSU problem
shortened caps, dead diodes, etc., you name it :-)
- heater supply wires
The wires from the PT to the socket pins for AC driven
heaters of the tube can produce quite some field injecting
hum in other wires nearby. Therefore itt is good practice
to twist AC heaters wires tightly to prevent this. Also it
is good practice to lead the twisted heater supply wires
near to the (grounded) chassis and along the corners of
the chassis.
Tom
--
The large print giveth, the small print taketh away.
- Tom Waits