5755 tube as audio pre-amp tube

[David Root]

OK, let's talk about tubes for a change. The 5755 is a noval based twin
triode designed as a DC amplifier. Raytheon made it as an alternate to the
Western Electric 420A. It has a weird pinout not the same as the 12A*7
series, so it's cheap in NOS. Mu is 70 and Rp is 140,000 approx.

Anyone used this as an input or pre-stage tube in MI or audio designs? I'm
interested as it has spring mounted cathodes to minimize microphonics and is
highly matched triode to triode.

I ran it thru Tubecad and it looks good in parallel grounded cathode
operation with 150K plate R and 1-2k cathode R, it gets reasonably balanced
voltage swing that way.

[Ruud Broens]

"David Root" > wrote in message
...
: OK, let's talk about tubes for a change. The 5755 is a noval based twin
: triode designed as a DC amplifier. Raytheon made it as an alternate to the
: Western Electric 420A. It has a weird pinout not the same as the 12A*7
: series, so it's cheap in NOS. Mu is 70 and Rp is 140,000 approx.
:
: Anyone used this as an input or pre-stage tube in MI or audio designs? I'm
: interested as it has spring mounted cathodes to minimize microphonics and is
: highly matched triode to triode.
:
: I ran it thru Tubecad and it looks good in parallel grounded cathode
: operation with 150K plate R and 1-2k cathode R, it gets reasonably balanced
: voltage swing that way.
:

I guess that if the model accurately follows those Raytheon datasheet curves,
that's amazing ,
rather wild bunch of curves there, apparently not very symmetrically build,
somewhat like half an 5751/ECC83/12AX7, minus the linearity, that is :-)

all is not lost, it actually could be quite useful in a current conversion
topology, 160V, 1.1 mA looks very promising as a 1.3 mA/V converter,
that you can then (see hybrid circuit) use with a 120K resistor for some
150 times voltage gain. (1 Vtt in max)

another interesting option would be, using several 5755's, plates parallel
with suitably adapted current source and lowered resistor, you'd have an
easy multiple input mixing amplifying stage, allbeit with fixed gain,
an opamp does it's input addition by current, also, but those are miniscule
so why not try the 'big bottle way ? :-)

R.

[David Root]

"Ruud Broens" > wrote in message
...
>
> "David Root" > wrote in message
> ...
> : OK, let's talk about tubes for a change. The 5755 is a noval based twin
> : triode designed as a DC amplifier. Raytheon made it as an alternate to
> the
> : Western Electric 420A. It has a weird pinout not the same as the 12A*7
> : series, so it's cheap in NOS. Mu is 70 and Rp is 140,000 approx.
> :
> : Anyone used this as an input or pre-stage tube in MI or audio designs?
> I'm
> : interested as it has spring mounted cathodes to minimize microphonics
> and is
> : highly matched triode to triode.
> :
> : I ran it thru Tubecad and it looks good in parallel grounded cathode
> : operation with 150K plate R and 1-2k cathode R, it gets reasonably
> balanced
> : voltage swing that way.
> :
>
> I guess that if the model accurately follows those Raytheon datasheet
> curves,
> that's amazing ,
> rather wild bunch of curves there, apparently not very symmetrically
> build,
> somewhat like half an 5751/ECC83/12AX7, minus the linearity, that is :-)
>
> all is not lost, it actually could be quite useful in a current conversion
> topology, 160V, 1.1 mA looks very promising as a 1.3 mA/V converter,
> that you can then (see hybrid circuit) use with a 120K resistor for some
> 150 times voltage gain. (1 Vtt in max)
>
> another interesting option would be, using several 5755's, plates parallel
> with suitably adapted current source and lowered resistor, you'd have an
> easy multiple input mixing amplifying stage, allbeit with fixed gain,
> an opamp does it's input addition by current, also, but those are
> miniscule
> so why not try the 'big bottle way ? :-)
>
> R.
>
I don't know how well the Tubecad model follows the Raytheon curves, not
great I think as I had to plug in the "actual" plate R at the point I wanted
to model i.e. by measuring off the curves and inputting that number into the
Tubecad model that I had already set up. Otherwise just using 140K didn't
cut it, the given bias point was way off the printed curve. I had to use
parallel operation to get any kind of halfway balanced Vo max out of the
model, which didn't surprise me. Varying the gm didn't seem to have any
similar effect, so I used 600 off the general area of the curve I was
looking at.

I also tried it in the Tubecad's longtail pair model, which is not quite a
standard MI amp stacked longtail, and could only get decent output voltage
balance at the cost of hopelessly unbalanced Vo max, and vice versa. This
disappointed me.

Did not try the I to V topology, but I will as it is in Tubecad. Thanx for
the tip.

Steve M. at Angela.com says he has heard the 5755 powering a DHT triode amp
with excellent results, but didn't disclose the operating point/design.

Still, where else can you get a NOS twin triode for $4.50? I bought four and
will play around with them a bit.

[Ruud Broens]

"David Root" > wrote in message
...
:
: "Ruud Broens" > wrote in message
: ...
: >
: > "David Root" > wrote in message
: > ...
: > : OK, let's talk about tubes for a change. The 5755 is a noval based twin
: > : triode designed as a DC amplifier. Raytheon made it as an alternate to
: > the
: > : Western Electric 420A. It has a weird pinout not the same as the 12A*7
: > : series, so it's cheap in NOS. Mu is 70 and Rp is 140,000 approx.
: > :
: > : Anyone used this as an input or pre-stage tube in MI or audio designs?
: > I'm
: > : interested as it has spring mounted cathodes to minimize microphonics
: > and is
: > : highly matched triode to triode.
: > :
: > : I ran it thru Tubecad and it looks good in parallel grounded cathode
: > : operation with 150K plate R and 1-2k cathode R, it gets reasonably
: > balanced
: > : voltage swing that way.
: > :
: >
: > I guess that if the model accurately follows those Raytheon datasheet
: > curves,
: > that's amazing ,
: > rather wild bunch of curves there, apparently not very symmetrically
: > build,
: > somewhat like half an 5751/ECC83/12AX7, minus the linearity, that is :-)
: >
: > all is not lost, it actually could be quite useful in a current conversion
: > topology, 160V, 1.1 mA looks very promising as a 1.3 mA/V converter,
: > that you can then (see hybrid circuit) use with a 120K resistor for some
: > 150 times voltage gain. (1 Vtt in max)
: >
: > another interesting option would be, using several 5755's, plates parallel
: > with suitably adapted current source and lowered resistor, you'd have an
: > easy multiple input mixing amplifying stage, allbeit with fixed gain,
: > an opamp does it's input addition by current, also, but those are
: > miniscule
: > so why not try the 'big bottle way ? :-)
: >
: > R.
: >
: I don't know how well the Tubecad model follows the Raytheon curves, not
: great I think as I had to plug in the "actual" plate R at the point I wanted
: to model i.e. by measuring off the curves and inputting that number into the
: Tubecad model that I had already set up. Otherwise just using 140K didn't
: cut it, the given bias point was way off the printed curve. I had to use
: parallel operation to get any kind of halfway balanced Vo max out of the
: model, which didn't surprise me. Varying the gm didn't seem to have any
: similar effect, so I used 600 off the general area of the curve I was
: looking at.
:
: I also tried it in the Tubecad's longtail pair model, which is not quite a
: standard MI amp stacked longtail, and could only get decent output voltage
: balance at the cost of hopelessly unbalanced Vo max, and vice versa. This
: disappointed me.
:
: Did not try the I to V topology, but I will as it is in Tubecad. Thanx for
: the tip.
:
: Steve M. at Angela.com says he has heard the 5755 powering a DHT triode amp
: with excellent results, but didn't disclose the operating point/design.
:
: Still, where else can you get a NOS twin triode for $4.50? I bought four and
: will play around with them a bit.

tube prices can still be amazing. eg., i have a box from 1970 with an EF860,
32.50 DM, probably a better part of a days pay in Germany, back then
today, gold pins 'n all, 2 euro's
crazy deflation
leads to vacuum
:-)
:
:
:

[robert casey]

David Root wrote:

> OK, let's talk about tubes for a change. The 5755 is a noval based twin
> triode designed as a DC amplifier. Raytheon made it as an alternate to the
> Western Electric 420A. It has a weird pinout not the same as the 12A*7
> series, so it's cheap in NOS. Mu is 70 and Rp is 140,000 approx.
>
> Anyone used this as an input or pre-stage tube in MI or audio designs? I'm
> interested as it has spring mounted cathodes to minimize microphonics and is
> highly matched triode to triode.
>
> I ran it thru Tubecad and it looks good in parallel grounded cathode
> operation with 150K plate R and 1-2k cathode R, it gets reasonably balanced
> voltage swing that way.
>
>
I just created a circuitmaker 3f4 spice model that seems pretty close to
the published curves. Simulated an SRPP, but it needed a B+ of 440V to
yield decent linearity (2nd H -40dB, 3rd -65dB, others lower still) and
with cathode resistors of 666 ohms a voltage gain of 20 (1Vp-p in,
20vp-p out). I wouldn't use this tube to build an SRPP, but this is to
give you an idea of what it would do. Besides the heater cathode rating
is only 75V, so the top triodes (in a stereo setup, a pair would be the
top left and top right) would need its own filament transformer biased
up to around 230VDC.

* 5755 CircuitMaker model
..SUBCKT X5755 P G K

*between this
Bp P K
I=((0.001135349709/1.0e3)+(7.678523205e-006/1.0e3)*V(G,K))*uramp((64.81029347)*V(G,K)+V(P,K)+(-31.32339601))^
1.5 * V(P,K)/(V(P,K)+(-4.234399626))
*and this should be one line

Cgk G K 1.55P
Cgp G P 1.4P
Cpk P K 0.7p


..ends X5755