is there any sound in a 30A5 / HL94 ?

[Empee]

Dear all,

One guy at my work cleaned his house and found an old tube amp. And gave it to me

It's a really dented, all dirty and warped stereo thinghy, brandname "Palace" (?)

It worked, although with a terrible hum and one channel brighter than the other.

So I took it appart and found a pair of 30A5's driven by a 12AX7,
single ended (real tiny OPT's) as far as I can see. There's a power transformer putting out 72V on one tap, (used for the heaters),
and a 120V tap. There's no greatz bridge nor rectifier tube, just one diode and a capacitor.

Now I am not going to rebuild this little piece as I allready got enough amplifiers, but I was thinking to use all the components
to try to build an amplifier myself!

Now my question;
is there any schematic you guys can think of wich I can use ?

Something "hi-end", please

Even if it's a 0,2W amp, I'd like to build it !
Or maybe something to drive my headphones (sennheiser HD650) ?


Thanks in advance,

Empee

[Empee]

oh, and I got some spare EL84's and ECC83's, but no OPT's

power transformers and diodes, some elcos too, but no chokes



there must be a combination in all this

[gingertube]

30A5 data sheet here:

http://www.mif.pg.gda.pl/homepages/f...133/3/30A5.pdf

2.1 Watts at 11% distortion. Don't get your hopes up too high.

I'd suggest just change all the electrolytic caps and use it as is.

Cheers,
Ian

[Empee]

hmmm.....


Can I re-wire it to triode ?


my speakers are 96dB efficient


Too bad all the schematic & circuit-sites on the internet are all for the more popular tjooobz.
No schematic to be found using 30A5's !


Cheers,

Empee

[gingertube]

Trace the circuit yourself - that way you will learn a bit about it as you go.

YES you probably can put it into triode mode and then you would get less than 1 Watt per channel which is not really enough for 96dB/W/m speakers.

With any old amp it always best to get it going properly "as is" before trying any mods - so replace those electrolytic caps and see if that gets rid of the hum etc. first.

You may also have to replace the coupling caps between the 12AX7 and the 30A5 grids.

Cheers,
Ian

[Empee]

ok,
I will try to read the circuit


So, wired in triode will give less than 1 watt;
Isn't that enough for my 'cans ?

or is their 300 ohms input impendance a problem ?


Cheers!

Empee

[Empee]

bump!


Can nobody think of a schematic using 30A5's ?

please ?


eager to make something with it,
don't care if it's hifi or not,

just want to test if I can make something from scratch
(using the circuitry it's in now isn't such a challange)

[Eli Duttman]

Originally posted by Empee
bump!


Can nobody think of a schematic using 30A5's ?

please ?


eager to make something with it,
don't care if it's hifi or not,

just want to test if I can make something from scratch
(using the circuitry it's in now isn't such a challange)

Start by examining the data sheet. Notice that the conservative design center system is used. Dissipating 8 W. at the plates rates to be safe. 40% efficiency is a realistic target for Class "A" pentodes. Pentode operation requires regulated screen grid B+ and loop NFB for low distortion and decent damping factor. Regulating g2 B+ is easy. Use a VR90/0B3. Use a simple, straightforward, signal path. With a CDP as the source, a 12AY7 rates to provide the voltage gain needed.

Please clarify the power trafo situation. Is there a single, tapped, secondary or are separate secondaries for heater and B+ present?

No matter what, a negative bias supply is going to be needed. Cathode biasing the "finals" wastes Volts that can't be spared. Bridge rectify and CRC filter "25" VAC to get the bias voltage.

Other than the limited deep bass extension needed to protect the O/P trafos from saturation, a HIFI amp will be constructed.

[Empee]

the power trafo is tapped
72V & 120V

what's a VR90/0B3?
a voltage regulator ?
how does one apply it schematically?

not totally, but more or less still a newby on this....



notice this a first project using old parts

can it be done with a LM78MXX (got some lying around)

[Eli Duttman]

Originally posted by Empee
the power trafo is tapped
72V & 120V

what's a VR90/0B3?
a voltage regulator ?
how does one apply it schematically?

not totally, but more or less still a newby on this....



notice this a first project using old parts

can it be done with a LM78MXX (got some lying around)

The VR90 is a 90 V. nominal gas discharge regulator diode. You wire it up using a resistor, just as is done with a Zener diode. A Maida regulator is a reasonable SS alternative for regulating g2 B+. VR90 data sheet here.

How many secondary wires are there on the power trafo? 3 wires indicate a single winding that's tapped. 4 wires indicate separate windings for the 2 voltages. If 4 wires are present, verify (using an ohmmeter) that 2 separate secondaries exist.