Loading the General Electric RPX and VRII cartridges

[brian]

From the instruction sheet that accompanied new VRII cartridges:

Amplifier Input Connections

Practically all modern high fidelity amplifiers are equipped with a proper connector [...]for the General Electric Variable Reluctance Cartridge. No circuit change is required when substituting the new VRII for previous standard GE models.

When it is desired to use an amplifier having a "flat" high level input, or when replacing a crystal or ceramic type with the General Electric cartridge, an equalized pre-amplifier such as the General Electric Model A1-203 or UPX-003B is required.

Most amplifier input corrections for the General Electric cartridge have a high impedance (100,000 ohms recommended) and have interstage equalization. Separate pre-amplifiers such as the General Electric Model A1-203 or UPX-003B have a low impedance input of 6,200 ohms which provides RIAA high frequency equalization at the cartridge input.



The circuit for the UPX-003B pre-amplifier is given in the instruction sheet that accompanied all domestic use RPX cartridges. I believe it is not suitable for the lower impedance Professional models, which are identifiable by chromed case for the RPX and red body for the VRII.

Scans of these sheets are available over at the Vinyl Engine Library in the General Electric section.

Using the UPX-003B circuit for the replay of 78s allows "tuning" of HF response to suit varying equalizations by varying the input resistor in the range 3,900 ohms to 50,000 ohms. I recommend replacing the fixed resistor with a 47k, 50k or 100k linear potentiometer to allow subjective HF correction.

I am hoping to build the General Electric SPX-001 pre-amp (same as the UPX-003B circuit but without microphone facility), which is based on a single 6SC7 dual-triode valve, in the not-too-distant future for use with my RPX-040 cartridge.

[tipetu]

Hi, I`ve purchased the same preamp you are describing here and I also would like to have frequency potmeters!


As you can see I`ve highlighted some of the resistors.


I assume you are talking about installing potmeters through R1, R11 and R13.
But how should I do it?
I see most potmeters have 3 connectors and dont know which one to use?
You mention that you`d use either 47, 50 or 100k linear pots. Which one would you prefer?

[brian]

Hello tipetu and welcome.

My suggestion was to use a potentiometer simply to replace R1, allowing experimention with high frequency roll-off with a GE RPX cartridge on 78s.

This potentiometer would have its middle solder tag wired to the tag that is connected to signal earth (ground) to allow variation of the resistive loading of the cartridge. This then would function as a "treble control".

R11 and R13 form part of the "bass-boost" network and I did not propose to touch those.

I would use a 100k potentiometer since this was a common value in pre-amps of that era.

[tipetu]

Thanks! I`ll do that when I get it! Should I bypass the resistor or replace it with the potentiometer?

[tipetu]

You wouldn`t happend to know how old these amps are? I get the impression they are from the 50s

[brian]

Replace it. These amps are certainly from the Fifties tipetu. I think some companies like Lafayette and Calrad may have continued into the beginning of the Sixties with 12AX7 replacing the 6SC7.

[tipetu]

I appriciate you taking time to answer all of my questions!

the values on R11 is 39k and R13 are 910k
Do you think it would be possible to throw in a 40k pot at R11 and one 1m pot in R13?  Would this give me midrange and bass controls?? 

[Johannes LeBong]

Hi tipetu

nice little unit. It reminds me in some way of the SHURE M64 RIAA phono preamp, which came a little later to market.



The circuits of this kind of units are very basic, but just made right.

In the original state - as it is - the equalisation will not be very precise and you will not achieve a good S/N Ratio (high frequency hiss and hum).

But If you replace the old resistors and capacitors e.g. with new ones from today, the results can be be a surprise!

At least the power supply should be changed to a rectifier bridge with 4 diodes for full wave rectification, the value of the capacitors in the power supply should be made higher and you can add one more R/C stage for the voltage drop. The output voltage should be tuned to approx 100 VDC.

If you build an external power supply for this little unit and feed only DC in it - also for the tube heating - the S/N Ratio can be made very low.

[tipetu]

I`m going to get new carbon comp resistors and caps.. maby paper :

[Johannes LeBong]

I`m going to get new carbon comp resistors and caps.. maby paper :

Why not, if you want to preserve the 'retro sound', hiss and hum, as it was in the old days.

Otherwise I would not recommend that.

[tipetu]

We`ll see how it turns out when I get it. but doesn`t capasitors have a tense to "dry out" over the years? The seller says it works all fine, and if he`s correct I`ll leave it just the way it is 

[brian]

Have you a variable transformer ("variac") to bring the input voltage  s  l  o  w  l  y  up to 120VAC tipetu?

[Johannes LeBong]

Hi Brian,

good idea to be cautious with such an old device.

I think there are not to many variacs out there in private households.

Another trick is to connect a light bulb in series with the mains supply. It will work as a current limiter for test and nothing will be destroyed in case of current overload or short circuit. The electrolytic capacitors in the power supply may be reconstructing their internal structure by an electrochemical process then. The german term for this process is 'formieren'. I do not know th english term.     

[brian]

Reforming.

[Johannes LeBong]

Thank you Brian!

it makes sense to participate at LH to learn new english vocabulary