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

[J. E. Knox]

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.