[78-L] RCA dealers' phono? (OT?)

[Malcolm Rockwell]

If the 5Y3 is the rectifier and the 6K6s are the push-pull power amp 
(they are),
then it only stands to reason that the other two (6SQ7s?) are very 
probably the preamp stage.
Mal

*******

On 3/11/2011 10:09 AM, J. E. Knox wrote:
> Greetings from FixitLand!
>
> Robert M. Bratcher Jr. wrote:
>
>>> Well the 5Y3-GT was the rectifier. I don't know what they used the
>>> 6J5 for (anybody know?)&  either the 6K6-GT's or the 6SQ7's were
>>> the final amplifier tubes in push pull I'm fairly sure. What the
>>> other pair of tubes (either 6K6-GT or 6SQ7) were for I have no
>>> idea without looking at the schematic for this amp. If anybody
>>> knows more about this amp&  how it works please let us know......
> Steven C. Barr wrote:
>
>> At a guess...and based on decades of collecting, repairing and
>> restoring
>> old radios...the 6K6's were the output "power tubes" and the 6SQ7's
>> were the stage preceding them. The 6J5 (a common triode) was
>> probably a "phase inverter" feeding one of the two 6SQ7's.
> Mr. Bratcher wrote:
>
>> Now that I think about it I'm pretty sure that your right about the
>> circuit.
>> What do you think those 6K6's put out to the speaker? About 10
>> watts perhaps? Or
>> less like maybe 5 or 6 watts?
> I'm looking at the schematic now. The 6J5 is a phono preamp ("1st
> AF") for the crystal cartridge. One 6SQ7 is an audio amp ("2nd AF")
> which follows the volume control, and the other 6SQ7 is a phase
> inverter. The tone controls are situated between the 6SQ7s. The diode
> sections of the 6SQ7s are grounded and unused. Output tubes are 6K6-
> GT power pentodes in push-pull with common cathode bias via a 390-ohm
> 2-watt resistor bypassed by a 20-uF capacitor. I would estimate the
> output rating at around ten watts (tubes are spec'd at 9.8 W into 12K
> plate-to-plate load impedance with a 285-volt plate and screen
> supply, but this circuit shows 335 V at the 6K6 plates).
>
> For those who are interested and can read DJVU documents, the schemo
> is at
>
> http://techpreservation.dyndns.org/schematics/47/47_RCA_66E_122.djvu
>
> Here I am talking like the technician I am...Maybe this thread should
> be taken to the Fun With Tubes Yahoo! Group...?
>
> Take care,
>
>
> Joe the solder jockey
> --
> It's my cats' world. I'm just here to fill their food dish.
>
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