12ax7 with 6cg7 driver

[joshuahhhhhhh]

Hello all I have a Thomas 6973 stereo amp That I'm redoing and have some questions on how the front end works.

The amp uses:
1 12ax7 voltage amp
1 6cg7 driver
4 6973 beam-power tetrode

The cathodes of the output tubes are grounded and has a negative bias supply.

In the front end the 12ax7 and 6cg7 share the same decoupling resistors 680k so the plates on the 12ax7 and the grids on the 6cg7 see about 100v.

My question is how come the B+ isn't fed to the plates of the 6cg7 and a coupling capacitor used between the 12ax7 and 6cg7 ?

From what I have read In my tube manual for the 12ax7 It can take plate voltages up to 300v. Why such a low operating point ?

Also The 12ax7 has a 82pf cap on each plate that goes to ground is this for plate load ?

Sorry for all the questions just trying to learn some stuff here so I can bring the gain up a bit on this amp.

 

[GordonW]

This is what's called a "direct coupled concertina" setup. The concertina (or split load) phase splitter has to be biased at about half-way to B+, so it can swing up and down, to drive the output tubes... one of which is coupled to the anode of the 6CG7, the other to the cathode.

So, some bias voltage is required on the grid... and running the 12AX7 to a plate voltage that meets this requirement, makes for a really simple, effective circuit. Also, with no coupling cap, there's one less source of distortion and phase shift in the circuit! This can improve stability, making feedback tuning quite a bit easier.

BTW: They're not the only ones that do this... some Scott, Harman-Kardon and other amps also use this arrangement. This type of setup is popular with input pentodes (such as seen in the 6U8, 6GH8 and such in Scott amps), since a pentode is oftentimes very happy with lower voltages (sometimes as low as 65V) on the plates. IMHO, that's probably a theoretically better operating point, than with triodes such as 12AX7s, which, as you mention, can use higher voltages...

Regards,
Gordon.

 

[joshuahhhhhhh]

Thanks Gordon direct coupled sounds good to me.
I have always been curious about the 6973 12watts out of such a small package and being designed especially for audio use.
I have a few more resistors to replace before I start tinkering with the feedback.
I am also thinking of using this front end on a rca rs-177 waiting to be rebuilt.
Would I haft to use neg bias supply on the output tubes as well?

 

[kvflyer]

Here is a partial schematic from an HH Scott LK72B. Note the plate of the pentode section (V103-A) of the 6U8, voltage amplification, is directly connected to the grid of the triode section (V103-B), the phase inverter. Is this what you are asking about?

 

[joshuahhhhhhh]

Yes that kinda looks like what I'm tinkering with here. I can see where the plate of the pentode is directly coupled to the grid of the phase inverter, and the feedback comes back to the cathode on the pentode.
Is this dc balance like what Gordon was talking about in the
"Yet another magnavox thread"
Split load ?

 

[Thespeakerdude8]

FWIW, I believe the 6973 was the ancestor to the 6BQ5. I think they are electrically equivalent but different pinout. 12 watts would be plate dissipation, you would need two in push pull to produce 12 audio watts.

 

[kvflyer]

FWIW, I believe the 6973 was the ancestor to the 6BQ5. I think they are electrically equivalent but different pinout. 12 watts would be plate dissipation, you would need two in push pull to produce 12 audio watts.

I think that they went down separate and distinctly different design paths. The 6BQ5 was more of a radio output tube or used in TV receivers. The 6973 was an RCA design specifically for HiFi applications. I think that Sylvania made the 6973 as well. 7199 falls into that same category, an RCA design for HiFi use. Some people will rewire the 7199 socket for use with a 6U8 because the 7199 is very expensive. 6U8 is a readily available tube and the price is right. If I remember correctly, you only have two pins to sway, maybe three.

The improvement of the 6BQ5 was the 7189 and then the 7189A. I think that the 7189A is a little different in that some pins were tied together inside of the envelope. Pin 6 (G2) is tied to pin 9. You may not always be able to substitute a 7189A in the socket for a 6BQ5 especially if the manufacturer used a spare tube socket terminal for a tie point.

 

[joshuahhhhhhh]

I can see how they are similar the old 6973's really look like 6bq5's.

 

[Thespeakerdude8]

I think that they went down separate and distinctly different design paths. The 6BQ5 was more of a radio output tube or used in TV receivers. The 6973 was an RCA design specifically for HiFi applications. I think that Sylvania made the 6973 as well. 7199 falls into that same category, an RCA design for HiFi use. Some people will rewire the 7199 socket for use with a 6U8 because the 7199 is very expensive. 6U8 is a readily available tube and the price is right. If I remember correctly, you only have two pins to sway, maybe three.

The improvement of the 6BQ5 was the 7189 and then the 7189A. I think that the 7189A is a little different in that some pins were tied together inside of the envelope. Pin 6 (G2) is tied to pin 9. You may not always be able to substitute a 7189A in the socket for a 6BQ5 especially if the manufacturer used a spare tube socket terminal for a tie point.

I stand corrected. That is some really useful information there! :thmbsp: I also forgot about the 7199, which has a higher Vp and Pdiss rating than the 6BQ5.

 

[kvflyer]

I can see how they are similar the old 6973's really look like 6bq5's.

6973 is really a sweet tube. It has a much higher plate resistance and lower transconductance than the 6BQ5. Similar plate dissipation and much lower filament current. They look a lot alike but are much different. Very expensive to find NOS!

 

[Tom Bavis]

6973 is close to 6V6 in characteristics. I think the 6CM6 and 6CZ5 came first - they have lower voltage ratings, but will sub for the 6973.

 

[joshuahhhhhhh]

yes nos quads are big$$$ And I do haft to say this amp never seems to run
out of gas.

I still have lots of work to do but its starting to come around. The power transformer is huge theres another 6v heater secondary not in use and a secondary for what I think was for the B+ in the preamp I haven't measured this tap unloaded yet but it was on a rather large rotted out selenium stack that was in there.
Hoping this secondary has enough current to allow me to dc heat all the tubes on the amp.

 

[Jim McShane]

6973 is close to 6V6 in characteristics. I think the 6CM6 and 6CZ5 came first - they have lower voltage ratings, but will sub for the 6973.

As usual, Tom nailed it...

But the 6973 is in production, the EH 6973 has been out for a while. The 6973 was used in a number of guitar amps, and I suspect that's why it made it into production again. I've never carried the EH (just due to low demand), so don't bother asking me what I think - I can't give you a good answer!

I do like the 6CZ5, but they are getting scarce too. The 6CM6 is a nice tube, but it (and the 6CZ5) may be a bit overstressed in some 6973 apps. The Design Center plate and screen voltage ratings of the 6CZ5 and 6CM6 are 100-ish volts lower.