A question about an Olson phono section

Elbowgeek

Active Member
I'm going through the phono preamp circuitry of an old Olson tube receiver. It seems that before hitting the grid on pin 7 of the 12AX7, the signal passes through a 47K resistor.

I had noticed that the gain was rather lower than I would have expected and was wondering if this resistor had anything to do with it. Other amps I've got schematics for don't generally have a resistor of any kind betwixt input plug and grid, so could this be attenuating the signal? And would removing it make a huge difference?

BTW, this is an interesting amp. It uses one half of each 12AX7 as phono preamp and the other half as line preamp, obviously to save a bit of money. And 6BM8's on the output. But a very clean unit overall. The sound through speakers is not great, but I believe the phono section has promise, and if I can use it as a phono preamp that would be great.

Thanks for any help :)

D
 
With the amp on measure the voltage on pin 7. I think it should be 0 volts. Someone chime in here if it can be higher but I was under the impression that it had to be less than 2 and close to 0.

It could be that the phono stage was made for the cheaper higher gain cartridges and the eQ is not what you need.
 
Grid voltage always has to be more negative than cathode voltage to operate the tube without distortion. There are two types of circuits usually used in the preamp for providing the proper negative grid voltage versus the cathode...grid leak bias with a high value resistor from grid to ground, or cathode bias with a resistor in the cathode circuit.

With grid leak bias you will usually read 0 volts or maybe -1 or so on the grid. The cathode is grounded. The current provided by this system is very limited, so the load of the voltmeter may prevent you from getting a negative voltage reading, even though a small amount of negative voltage is actually there.

With cathode bias a resistor is between the cathode and ground. You will have a small positive voltage on the cathode and 0 volts on the grid. The resistor allows some voltage to be developed at the cathode point..the cathode is more positive than the grid (hence the grid is "more negative") and the proper bias is established on the tube.

In both systems there is often a capacitor between the grid and the input terminals so that DC coming through the input lines somehow does not upset the bias. A resistor in series with the input should have a negligable voltage drop on the signal as the tube grid does not draw current.

How is the sound on the other input terminals of the amp...like from a CD player or tuner?
 
Thanks Chad - it's that sort of post that furthers my understanding of these lovely tube-ed beasts.

I've done quite a bit of experimentation and signal path mapping, as well as compiling a schematic using Visio as I can't find an official (or even unofficial) one anywhere. Too obscure a thang, I guess.

Anyway, it turns out that the output to the record connectors on the receiver from the phono inputs are taken just after the first phono amplification stage, before it hits the line preamp section. Therefore it is still at a relatively low level.

So... I soldered leads to the output of the line-level preamp tube half, just after the coupling cap, of course, and the sound now is at the correct line level. I even did this without burning down the house :banana:

However, as I should have expected, the sound is less than thrilling, probably due to the ceramic caps and no doubt the existance of resistors which may have drifted over time (ceramics are extremely poor performers in audio circuits). I now have Orange Drops on order and replacement high-quality electrolytics for the cathode circuit. We shall see if there's an improvement...

Cheers
 
If it is using ceramic caps (which usually have fairly low amounts of capacitance) in the coupling circuits...you may want to experiment with using larger values of caps (at proper voltage ratings) when you install orange drops. The higher the capacitance...the better the bass response.

New electrolytics are a good idea in the cathode circuit as leakage here can reduce the cathode voltage, causing distortion.
 
I just came back from lunch, over which I studied some tube amp circuit theory in depth. And yes, it had occurred to me that I should have ordered higher value caps, but they should be at least better in performance overall.

The only disadvantage to not having an official schematic is the lack of knowledge of the expected voltages at different points, but by studying other circuits for other amps intended to perform the same functions and with similar configurations one can make educated guesses as to what the circuit designers were after. And that can help ingrain the small amount of knowledge I've gained, which isn't a bad thing.

Cheers
 
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