Hello All,
I plan on making an amp that originally used 8 6LQ6 tubes. I really
don't feel like spending the $50 per tube that these cost. I see that
there is an equivalent but with a 24 volt filament; the 24LQ6.
My question is this: If I use these tubes with a 24 vct transformer and
ground the center tap of the transformer, will there be more "hum"
present at the speaker output? The output is "Unity Coupled" so there
is no voltage gain in this stage. I don't know if this makes any
difference or not.
The 24LQ6 looks very attractive because of it's price.
Thanks, Daniel

Surely you are talking about the Mac MC3500/MI350. If you can wind the
OPT or get someone to wind it for you you are way ahead of this game.

I would series two of the 24* heaters on each side with a ballast
resistor and build a DC supply running 55 or 60 volts, or rather two of
them, one for the pushes and one for the pulls. There is a lot of DC
offset and AC differential heater-cathode voltage in the Mac circuit.
See how they solved the problem in the MI200.

A pair of Vicor brick switchers might be the ticket here if you are
not anti-silicon.

Yes, Thanks for your reply.
I was lucky enough to acquire some outputs from a pair of MI350's. I
purchased them from a HAM who bought the amps as scrap. All he wanted
was the B+ transformers to build a transmitter with. I consider myself
extremely lucky.
As for using 24vac for the output tube's heaters, I want to keep it as
simple as possible. I know that with Directly Heated Triodes, the
higher the AC heater voltage, the more "hum" present at the output. I
am hoping that tubes with a unipotental cathode (like the 24LQ6) don't
suffer this same drawback.
As for heater - cathode voltages, I don't see them getting much higher
than 150v in this circuit, maybe 200 volts max. Each winding in the
output transformer is 150 ohms. These amps get there swinging lots of
current!
If anybody with experience using high heater voltage tubes can share
with me their knowledge... Thanks!
Daniel



wrote:
Surely you are talking about the Mac MC3500/MI350. If you can wind
the
OPT or get someone to wind it for you you are way ahead of this game.

I would series two of the 24* heaters on each side with a ballast
resistor and build a DC supply running 55 or 60 volts, or rather two
of
them, one for the pushes and one for the pulls. There is a lot of DC
offset and AC differential heater-cathode voltage in the Mac circuit.
See how they solved the problem in the MI200.

A pair of Vicor brick switchers might be the ticket here if you are
not anti-silicon.

They are worth a _lot_ of money, so he would have been far better off
to sell them and buy new transformers.

wrote:

I would series two of the 24* heaters on each side with a ballast
resistor and build a DC supply running 55 or 60 volts, or rather two of
them, one for the pushes and one for the pulls. There is a lot of DC
offset and AC differential heater-cathode voltage in the Mac circuit.
See how they solved the problem in the MI200.

I leave the heater supplies floating unless theres a problem with hum.
When that happens a .1uF cap from one side of the heater to ground
clears it up. Although I'm sure this is a big no no for some reason or
another, seems too simple.


Adam

DanF wrote:

Hello All,
I plan on making an amp that originally used 8 6LQ6 tubes. I really
don't feel like spending the $50 per tube that these cost. I see that
there is an equivalent but with a 24 volt filament; the 24LQ6.
My question is this: If I use these tubes with a 24 vct transformer and
ground the center tap of the transformer, will there be more "hum"
present at the speaker output? The output is "Unity Coupled" so there
is no voltage gain in this stage. I don't know if this makes any
difference or not.

Biasing the heater around 50VDC above the cathode will
reduce any hum you might have had if the centertap were
tied to ground. You don't need much current capability
in this bias voltage, a voltage divider hung off the B+
using resistors around 100K and 27K would do.

On 16 Apr 2005 14:26:01 -0700, "DanF" wrote:

Hello All,
I plan on making an amp that originally used 8 6LQ6 tubes. I really
don't feel like spending the $50 per tube that these cost. I see that
there is an equivalent but with a 24 volt filament; the 24LQ6.
My question is this: If I use these tubes with a 24 vct transformer and
ground the center tap of the transformer, will there be more "hum"
present at the speaker output? The output is "Unity Coupled" so there
is no voltage gain in this stage. I don't know if this makes any
difference or not.

"Unity coupled" means a gain approaching two, but yeah, noise
sensitivity is reduced.

If you want to null any residual 60 Hz hum, check Steve Bench's
pages for an elegant universal solution (injection at grids).

Chris Hornbeck
6x9=42 April 29