Combining Two Different Record Cutters?!

[HardlyHumanFX]

Hello Trolls

I have two machines. A Recordio 6B10 and a Recordio 9J9 (A70). I have only restored the 6B10. The 9J9 chassis needs a ton of work to get working right again. Anyway, I have noticed a key difference in the hardware:

--The 9J9 (large console unit) recording arm is heavier, has a non-flexible metal arm below riding the screw that guides the arm and it has speed adjustments "Slow" and "Fast."
--The 6B10 has a flexible copper leaf that rides a screw that guides the cutter arm. I've noticed it's lighter. No speed adjustment. Cutter just doesn't seem as robust as the 9J9.

I've included photos.

My question is, should I swap out the other record cutting platform for the heavier duty, speed option 9J9 cutter platform? I don't care about keeping things original, obviously, just functionally superior. Thoughts??? Yes, I know it's a home cutter and you guys generally thumb your noses at them but I'm a newbie and they're affordable and I intend to trick this old beast out.

Troll-y Yours,


Jason Anderson, 29
Orange, California

[markrob]

Hi,

If you don't care about being original, then why not junk all of the electronics and drive the head with a modern solid state power amp? Since the head is a high impedance crystal type, you will need to add a step up matching transformer from the amp to the cutter head. Or you can use a modern tube amp and drive directly from the the plate of the output tube(s) (see the schematic of you current unit for reference).

If you go this route, you will be able to use one or both mechanical systems or just choose the one that works best for you.

Mark

[HardlyHumanFX]

Hi,

If you don't care about being original, then why not junk all of the electronics and drive the head with a modern solid state power amp? Since the head is a high impedance crystal type, you will need to add a step up matching transformer from the amp to the cutter head. Or you can use a modern tube amp and drive directly from the the plate of the output tube(s) (see the schematic of you current unit for reference).

If you go this route, you will be able to use one or both mechanical systems or just choose the one that works best for you.

Mark

Hey thanks for being in my corner! That actually really de-mystifies this rig a lot for me. I will look into that next month and keep you apprised!

[markrob]

Hi,

If you go with the solid state power amplifier, then you can use an off the shelf 70.7 V PA style transformer in reverse to match the head to the power amp. There are several posts here about how that is done.

Because the crystal head is a constant amplitude device as opposed to a constant velocity type cutter (as are moving iron heads), the standard RIAA EQ must translated to work with this type of head. If you look at the typical published RIAA recording curve, you will see a +6db/oct boost 50 hz to 500 hz. then flat from 500 Hz to 2122 Hz followed by a +6d/ocb boost above 2122Hz. For a crystal head, curve would be flat from 50 hz to 500 hz, fall at -6db/oct from 500 hz to 2122 Hz and then flat from 2122Hz to 10 Khz and beyond. You will also need to EQ out any boost due to the head resonance (typically in the 10 Khz range).

Mark

[HardlyHumanFX]

Hi,

If you go with the solid state power amplifier, then you can use an off the shelf 70.7 V PA style transformer in reverse to match the head to the power amp. There are several posts here about how that is done.

Because the crystal head is a constant amplitude device as opposed to a constant velocity type cutter (as are moving iron heads), the standard RIAA EQ must translated to work with this type of head. If you look at the typical published RIAA recording curve, you will see a +6db/oct boost 50 hz to 500 hz. then flat from 500 Hz to 2122 Hz followed by a +6d/ocb boost above 2122Hz. For a crystal head, curve would be flat from 50 hz to 500 hz, fall at -6db/oct from 500 hz to 2122 Hz and then flat from 2122Hz to 10 Khz and beyond. You will also need to EQ out any boost due to the head resonance (typically in the 10 Khz range).

Mark

It sounds like the solid state amp with a 70v PA transformer is probably the way ill go for sake of the cost. How many watts generally does a solid state amp need to have to drive the record cutter head?

I wish I could get that hum to go away but i guess this thing is too old to troubleshoot it apart. All caps replaced. Its grounded now. New mic amp tube. Doesnt ring any more when you tap the chassis. But damned if i can make the hum go away.

Good idea about just getting new hardware. It is maddening lol

[HardlyHumanFX]

Hi,

If you go with the solid state power amplifier, then you can use an off the shelf 70.7 V PA style transformer in reverse to match the head to the power amp. There are several posts here about how that is done.

Because the crystal head is a constant amplitude device as opposed to a constant velocity type cutter (as are moving iron heads), the standard RIAA EQ must translated to work with this type of head. If you look at the typical published RIAA recording curve, you will see a +6db/oct boost 50 hz to 500 hz. then flat from 500 Hz to 2122 Hz followed by a +6d/ocb boost above 2122Hz. For a crystal head, curve would be flat from 50 hz to 500 hz, fall at -6db/oct from 500 hz to 2122 Hz and then flat from 2122Hz to 10 Khz and beyond. You will also need to EQ out any boost due to the head resonance (typically in the 10 Khz range).

Mark

https://www.ebay.com/itm/Paso-T3130BGM-Amp-4-Channel-30-Watt-PA-Series-3000/273140934967?hash=item3f9877e937:g:S88AAOSwNaRaw961

This 30 watt PA amp has a 500ohm speaker output. Would that save me the step of using a PA transformer and a solid state amplifier?

[markrob]

Hi,

Probably not. That would be great for a moving iron 500 ohm head. You need to go with a larger step up ratio to drive the crystal head. You need something on the order of 100-180 Vrms to drive the head. At 10 watts, that implies an impedance of about 3K ohms

R = V Squared / Watts

So

R = 180 Volts Squared / 10 Watts = 3240 ohms

10 Watts into 8 ohms is about 9 Vrms, so the step up ratio would be about 20:1 (180 / 9).

A standard 6.3V filament transformer might fill the bill. That has a ratio of 120/6.3 = 19. Get one that has a 10-20 VA rating and you should be good to go.

Mark

[HardlyHumanFX]

Hi,

Probably not. That would be great for a moving iron 500 ohm head. You need to go with a larger step up ratio to drive the crystal head. You need something on the order of 100-180 Vrms to drive the head. At 10 watts, that implies an impedance of about 3K ohms

R = V Squared / Watts

So

R = 180 Volts Squared / 10 Watts = 3240 ohms

10 Watts into 8 ohms is about 9 Vrms, so the step up ratio would be about 20:1 (180 / 9).

A standard 6.3V filament transformer might fill the bill. That has a ratio of 120/6.3 = 19. Get one that has a 10-20 VA rating and you should be good to go.

Mark

I will start doing some homework over here on some of your explanation. I'm out of my depth. In the meantime, I did find the specs on the cutter head transformer from my schematics. Loaded.

[HardlyHumanFX]

Hi,

Probably not. That would be great for a moving iron 500 ohm head. You need to go with a larger step up ratio to drive the crystal head. You need something on the order of 100-180 Vrms to drive the head. At 10 watts, that implies an impedance of about 3K ohms

R = V Squared / Watts

So

R = 180 Volts Squared / 10 Watts = 3240 ohms

10 Watts into 8 ohms is about 9 Vrms, so the step up ratio would be about 20:1 (180 / 9).

A standard 6.3V filament transformer might fill the bill. That has a ratio of 120/6.3 = 19. Get one that has a 10-20 VA rating and you should be good to go.

Mark

https://www.ebay.com/itm/Thordarson-21F08-Filament-Transformer-Primary-117V-Sec-6-3V-CT-at-1-2A/352309562293?epid=1622733960&hash=item5207494bb5:g:Qs0AAOSwcJNasAe9:sc:USPSFirstClass!92866!US!-1

So VA = Volt Amps which means apparent power. I'm not sure how that's different from wattage. Is it specific to transformers? If I understand it correctly, does the transformer with 117vac primary and 6.3v 1.2a output have a VA rating of 13.07?

[markrob]

Hi,

So it looks like I'm in the right ballpark for the cutter head impedance. That should help you get in the ballpark if you elect to use a power transformer. Transformers are usually rated in VA. In this case VA and watts are essentially the same, assuming the voltage and current are in phase (resistive load). So a a 6.3V/1.2A transformer would be about 7.5 Watts (or VA).

Here is a link to a 25 VA transformer available off the shelf from Digikey

https://www.digikey.com/product-detail/en/hammond-manufacturing/166N6/HM508-ND/270109


Mark

[HardlyHumanFX]

Hi,

So it looks like I'm in the right ballpark for the cutter head impedance. That should help you get in the ballpark if you elect to use a power transformer. Transformers are usually rated in VA. In this case VA and watts are essentially the same, assuming the voltage and current are in phase (resistive load). So a a 6.3V/1.2A transformer would be about 7.5 Watts (or VA).

Here is a link to a 25 VA transformer available off the shelf from Digikey

https://www.digikey.com/product-detail/en/hammond-manufacturing/166N6/HM508-ND/270109


Mark

Thanks! Okay, so I can use a solid state audio amp with an 8ohm speaker output, hook it up to this transformer, output of the transformer goes to the cutter head? How powerful of an amp should I need to drive the transformer? Thanks for your help!

Jason

[markrob]

Hi,

That should do it. I've never had a crystal head to try this approach, so proceed with some caution. You should wire in at least a 3K ohm 10 watt resistor in series with the head. The cutter head looks like a pure capacitive load and might cause the amp to oscillate if you try to drive it directly. The resistor will help prevent that by making the load more resistive. As far as power goes, 10-20 watts is probably fine to start. Remember to wire the low voltage side of the transformer to the power amp and the cutter head/series resistor to the 120V side. To start, play a 1Khz test tone and slowly bring the level up until you read 2.8Vac at the output of the power amp (represents about 1 watt of drive into 8 ohms). Then check to see if get about 50 Vac rms across the head. Might be a good idea to connect a speaker to the output of the amp to listen for any oscillation. Make sure the amp can deal with a 4 ohm load (8 ohms due to the head in parallel with the speaker 8 ohm load).

See the attached datasheet for a typical Astatic cutter head that was used back in the day. They show a 60K ohm resistor and a 500:60K matching transformer. This works out to about 11:1 step up ratio. 500 ohm was the typical plate load for a small single 6V6 output tube.

Astatic Cutter Head Catalog.jpg