Nice RCA console

[electra225]

Even RCA used the dreaded 6EU7. I thought they used a 6CG7/6FQ7 for the "little" amplifier. The cabinet color is different, but it's really nice.

https://www.ebay.com/itm/373764882871?h ... SwW-thcotG

[William]

I have seen that chassis in other RCA consoles and I'm guessing it would perform quite well. I think the cabinet finish is a later version of limed oak and on this unit it really looks nice. A little pricy, but it would make someone a nice stereo.

Bill

[electra225]

This thing has your name on it, Bill. The price is pretty steep.......

[William]

Way to pricy for cheap me, besides I have it's big brother sitting in the basement waiting restoration.

Bill

[Firedome]

Very nice little RCA, like that cabinet finish.
Curious about the 6EU7 angst though, Heath used them a lot and I've never had any issue with any of the amps they were in or the tube itself for that matter. In fact their little AA-32 stereo amp with 6EU7s in the preamp stages and 4 x 6GW8 outputs is just about my favorite lower power stereo integrated amp of all time. So sweet! And the bigger bro' AA-151 is no slouch either. I'd imagine it must be how it's used in the circuit. Have a stash of them just in case any future Heaths appear.

[electra225]

I have experienced problems with the 6EU7 in Magnavox. The dirty tube sockets are a Magnavox tradition, but the shorts and "one half dead" situation is more serious. I know they are basically a low-noise 12AX7, but I'd feel better if Magnavox had used the 12AX7 and passed on the 6EU7. Anytime I have a performance problem with a Magnavox that has a 6EU7, it is always at the top of my list of suspects. But that's just me.

[Firedome]

IIRC although both are lo-noise hi-mu dual triodes you have to re-wire to sub a 12ax7 for a 6eu7, or make an adapter.

[back-2-mono]

FYI.
Current prices for NOS 6EU7's $20.00 each. 6CG7's $12.50 each.

[Motorola minion]

I just got finished with a few repairs on equipment using the 6EU7 instead of 12AX7 and 6CG7.

The '62 portable Magnavox phono used one, it tested good but the gain and use in a series-heater application with two 50EH5's made it very susceptible to hum. Subbing in another 6EU7 made no difference.

A V-M amplifier from a model 853 console had one, it tested good and was a GE replacement made in Great Britain. I will be testing this soon.

A small blonde 61 Magnavox used one in the 86-00 stereo amplifier. My simple EICO 625 tester showed a heater-cathode short in one section. This one clue made it necessary to replace it and I confirmed issues driving the one channel's 6BQ5.

The 6EU7 has the filament pins as 1-2 only, assumed to improve on the noise over a 12AX7, both tubes which GE tube data sheet lists amplification factor of 100!

In comparison to a 6EU7, and having worked on equipment from RCA recently that uses many more tubes (and lousy OEM parts ), both the "newer" 6CG7 and older 12AU7 have an amplification factor of only 20. This is why at least two, maybe three triode sections of this type is needed in a 6BQ5-tubed amp, making RCA's a bit unique. Even more unique, are Orthophonic flagships 6HF1 and SHF1.

The RCA Orthophonic SHF-1 uses two monoblock amplifier chassis RS147, also witnessed to by Firedome as the famous "SP-20", sold as a component. This amp uses two 6AU6 pentodes to drive the four 6V6 outputs. This is an unusual choice of driver tube for a reason, while the tube itself is common as bugs, the mandatory "better than typical RCA parts" to use it add up. Resistance values drifting in the 6AU6 stages will cause problems with phasing and crossover that is critical due to all the 5% resistors I had to change. There are three more 6CG7's on the preamp-tuner chassis. Is all these parts replacements worth it? Always worth it, even good resistors are cheap!

Here's a toast to 12AX7 , versatile enough to run on 6.3 or 12.6 volt heater supply, yet proving durable in most of our equipment that uses it. I recommend and upgrade from 6EU7 if you don't mind rewiring sockets. Upgrading an older set that uses a 12AU7 would require changes to resistance and capacitance values as well as a possible modification to the output stages' gain, anti-oscillation and feedback loops.