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  #1  
Old 02-25-2011, 08:54 PM
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Preamp for using low output ceramic cart in place of crystal cart?

What I am trying to come up with is a way to boost the typical .5-1V modern ceramic cartridges to the typical 3 volt output level that the older crystal cartridges had.

I'm working on a '48 Decca 78 rpm phonograph that used such a 3 volt cartridge to drive the output tube directly. Not wanting to spend the money for a cartridge rebuild, I installed an Astatic 89T; which, has an output of 1V. Although the volume level is OK, it could still be better.

On ARF, someone posted a diagram of a simple one transistor preamp that obtaines it's B+ from the cathode of the output tube. Either I'm missing something; or, the diagram is wrong because it does not work.

If anyone has any other ideas, I'd love to hear them. And, I'm sure others would, as well. Since these high output crystal cartridges are generally NLA, it would be nice to come up with a circuit that would allow use of a modern cartridge and the player still have the level of volume it did with the old cartridge.

Here's a link to the diagram, and the Decca record player that I'm repairing.

http://www.antiqueradios.com/forums/...c.php?t=122237
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Old 02-25-2011, 11:54 PM
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Measure the dc voltages on the preamplifier transistor and report back. You have to adjust the collector-to-base resistor so that the transistor is neither in saturation nor cutoff and that value depends on the specific transistor you are using.

It's not a good circuit. A FET would work better. To keep the aura of its age perhaps you should use a tube as recommended by one of the posters.
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Old 02-26-2011, 12:59 AM
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I tried several different value resistors for R1 and still got the same .5 volts on the collector.

I did find a schematic for a JFET based preamp for a guitar pick-up. I may experiment around with that and see what happens. I would consider using another tube; but, there is really not enough room on the chassis.
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Old 02-26-2011, 12:31 PM
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That is too low; the transistor is saturated. Raise the value of the resistor some more.
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Old 03-14-2011, 12:48 AM
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David Roper David Roper is offline
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I built one for a TV/RP combo which had the same ceramic-for-crystal retrofit and it works very well, but I think I ended up with 1 megohm for R1. The value range given in the schematic is way too low.
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Old 03-14-2011, 02:01 AM
bob91343 bob91343 is offline
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If you have room, punch a hole and mount another tube in there. You will have to adjust the filament dropping resistor but otherwise it's a fairly simple thing.

Just clone one of the other circuits of the time, maybe with a high mu triode for an input stage.
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Old 03-16-2011, 10:27 PM
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It looks to me like the 220k resistor and 10k-47k are swapped. This is a touchy circuit configuration because the bias depends on the current gain ("beta") of the transistor. Since the base current is the collector current divided by beta, you want the resistor from base to collector to equal (very approximately) beta times the resistor in the collector. You have about 4.3 volts to work with once you discount the base-emitter voltage, and want half of this to appear across the collector resistor and half on the collector. So, I would try R2 = 10k and then play with R1, using maybe 100k for a starting value. The low value of collector voltage is to be expected with the values posted. You should be shooting for about (0.7 + (4.3)/2) = 2.85 volts on the collector to center the range.
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Old 03-17-2011, 03:00 PM
bob91343 bob91343 is offline
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Trouble is, you can too easily overdrive the transistor, given its low voltages. Better to use an entirely different concept.

With one Volt of signal, if the gain is more than two or three it will overdrive it so the circuit won't sound good. Plus, it won't supply enough drive to the output tube.
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Old 03-17-2011, 05:43 PM
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Powering something from the cathode bias of a tube is not a bad
idea in itself, but the circuit you are referring to uses base bias,
and is a textbook example of how NOT to bias a bipolar transistor
properly. Furthermore, as drawn the amplifier won't work because
the collector resistor is way too big: if the supply voltage is say 11 volts,
you can at most get .05 ma collector current which is maybe 50 times too
small. Reducing the collector resistor, and adding a proper emitter resistor
to control the gain should help getting it to work. Good luck!
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Old 03-23-2011, 11:02 AM
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If you used a 6SQ7 or 6AT6 or similar tube, as long as it had the same heater ma. requirements of the existing tube, you wouldn't have to do anything about the dropping resistor. An extra six volts wouldn't matter in the heater circuit and the tubes would like it. The circuit around the new tube could be copied from the schematic of any AA5 radio that had a phono input. At any rate there would probably be only four or five components anyway.
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Old 03-23-2011, 12:56 PM
bob91343 bob91343 is offline
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Agree.
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Old 11-14-2011, 12:58 AM
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Several months ago, I picked up a beat up early '50's Webcor "one tube wonder" record player at the flea market. This model used a selenium rectifier with the 25L6 tube filament wired in series with the 90V turntable motor. This seems to be a very common design among these types of players. The cartridge was a flat metal high output type with a 2 mil. "all speed" needle.

Of course, the old cartridge was dead. I had a Pfanstiehl P188 ceramic cartridge that I salvaged from a junk '90's stereo. From my research, this cartridge puts out .3 volts output and I installed it in the "one tube wonder" just to see what would happen. It actually gave moderate volume on some 45's. The results were not as good with an LP.

So, I stripped the chassis, drilled a hole for a 7 pin miniature tube socket, rounded up the needed parts, and added a 6AV6 preamp stage. Now, this record player has plenty of volume. There was no tone capacitor across the output transformer in the original amp; but, I added a .047 cap to the modified amp. The high frequency response was way too pronounced for my taste. With the cap, the tonal quality is much better. I will likely be doing this preamp modification to the other "one tube wonder" phonographs that I restore. This modification will allow me to use a modern ceramic cartridge that will be less likely to destroy a record and I won't have to worry about finding a NOS 3 volt cartridge that may not be much better than the dead one that was in the player.
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Old 11-14-2011, 01:51 PM
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maxhifi maxhifi is offline
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i modified a two tube amp using a 35z5 and a 35l6, with filaments in parallel.

Replaced with 25l6, 12sq7, and silicon rectifier - filaments in series.

Gets the job done.
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