Parallel Triodes for AC Coupled CF

[ricardo]

BTW, the inductance of an 'iron' cored transformer VARIES WITH LEVEL.

I posted frequency response measurements at various levels in another thread which I've forgotten.  The changing inductance means you get different bumps/peaks depending on your level.

I first saw this mentioned in the Williamson amplifier articles in Wireless World.

 

[emrr]

Good point.  Reminds me of watching a 10W tube amp response as measured (bridging) from it's 600 ohm output taps while an 8 ohm speaker was in play at that tap.  You can watch the cone excursions make the response curve dance around like a light show. 

 

[My3gger]

Thanks for your help Ian!

The follower is driving a transformer 4:1 (load on the tx about 1K) so I didn't think I needed the very low output impedance the white follower offers.

Why do you use output transformer when output can be unbalanced for short runs? Beside obvious balancing, galvanic isolation, maybe sound, i've seen one used on WE 141-A like CF and never really understood why it was used there. Iirc it was about avoiding sudden distortion CF is known about, don't really know if that was the reason and what is here.

 

[abbey road d enfer]

Why do you use output transformer when output can be unbalanced for short runs? Beside obvious balancing, galvanic isolation, maybe sound, i've seen one used on WE 141-A like CF and never really understood why it was used there. Iirc it was about avoiding sudden distortion CF is known about, don't really know if that was the reason and what is here.

I would think you can't ignore the benefits of low-impedance balanced outputs...?
What is "sudden distortion CF is known about"?

 

[My3gger]

Yes, having low impedance transformer balanced output is mostly advantage.
Instead of "sudden distortion" like i say, people here mostly use word "splat" to describe CF's sound when distorted.
I searched through the forum and can't find more notes about using output transformers for CF. Thought it was also about sound because step-down gives more current at the output, but i never tried if this is so.

 

[abbey road d enfer]

Yes, having low impedance transformer balanced output is mostly advantage.
Instead of "sudden distortion" like i say, people here mostly use word "splat" to describe CF's sound when distorted.
I searched through the forum and can't find more notes about using output transformers for CF. Thought it was also about sound because step-down gives more current at the output, but i never tried if this is so.

Applying NFB to a stage reduces its output impedance, but it doesn't change its capacity for delivering current. A "perfect" CF stage running at 4mA cannot deliver more than 2.8mAac into the load (in practice it is even less). That's about 1.6 Vrms into 600 ohms (about +6 dBu).
Adding a transformer, let's say a 5:1 will allow driving a 600 ohms load to 8Vrms (+20 dBu).

 

[emrr]

Adding a transformer, let's say a 5:1 will allow driving a 600 ohms load to 8Vrms (+20 dBu).

Scanning historical product data, you find examples of the same single ended preamp marketed as a standard plate driven transformer, and then also with an additional tube as CF driver.  The only difference on (sales) paper is higher max output.

 

[My3gger]

I have quite a few RCA, Gates, WE, along several other American manuals and papers about pro audio gear. Didn't notice possibility for adding CF tube in those notes, closest is RCA SK-1A/MI-12173-A originaly meant as a mixer, your notes tell it could be used as a line amp. Have to check some more to find examples like WE 141-A out, or what you mentioned if drive is good without using hefty tubes.
Already thought of using something like ECC99/6N6-P and similar for CF, since it needs higher current tubes for good max output.

 

[emrr]

Gates PRE4 versus PRE1 is an example, and one of the other Gates remote mixers has an oddball version with CF output tacked on.  There's a Gates SA-70 variant I've seen which does this too, and I know in that case they used an output transformer with significantly less inductance (790DCR / 19H versus 1K5DCR / 480H); someone that production cycle decided a cheaper transformer was worth the cost of an extra tube? 

 

[My3gger]

Thanks a lot for your invaluable input, really like historic part of it too. Information like this is rare, so hints are real nice along a lot of searching. Same to the others of course.

 

[PRR]

> decided a cheaper transformer was worth the cost of an extra tube?

The relative cost of iron and glass changed over the decades Gates was working. Early days you got as much gain in transformer as possible: any motor-winder can wind a transformer, but only Cunningham (and the folks he infringed) can make a tube. Later tube production spread, everybody was making 6V6 in their bathtub (wait, that's gin), and tube prices fell. Wide-range 480H was never cheap, and begs for more cathodes less iron.

 

[emrr]

Yup.

Gates also marketed what may be the first commercially produced stock item press mult by 1952 at the latest, a 1 in 10 out portable battery powered box, likely to have employed a CF or 10. 

 

[PRR]

> 10 out portable battery powered box, likely to have employed a CF or 10. 

If at MIKE level, I would suspect a battery-radio 100mW power amp, 15 Ohm output, and a lot of 150r resistors (or pairs of 68r). Plenty hot for 10 mike outputs.

At line level it does become a challenge. Tremaine shows a half-rack full of separate line-amps, but that is far from a practical battery proposition.

 

[emrr]

Advertised as 8dB gain.  Wish I knew more. 

 

[My3gger]

Gates PRE4 versus PRE1 is an example, and one of the other Gates remote mixers has an oddball version with CF output tacked on.  There's a Gates SA-70 variant I've seen which does this too, and I know in that case they used an output transformer with significantly less inductance (790DCR / 19H versus 1K5DCR / 480H); someone that production cycle decided a cheaper transformer was worth the cost of an extra tube?

If anyone is interested, i found catalog with Gates CC1 console which uses PRE4 and PRE1, later having CF output. At least circuit description says so, unfortunately there is only block schematic, description starts at page 120 here:
http://www.americanradiohistory.com/Archive-Catalogs/Gates-Harris/Gates-Catalog-Prices-1953.pdf
It looks similar to RCA BC-3C, although Gates insides are based on plug-in modules.
Searching Google and here doesn't show anything about CC1, only a pic or two.  It is interesting to notice PRE1 has much lower distortiom than PRE4, or i could say higher output at the same distortion, seems like OT makes this change.
I have schematic of all 4 SA70 versions which are in most catalogs and some more information. CF output with OT transformer must have been rare, it is hard to understand why would one design it so, maybe it was high cost of early iron (if used instead of later) you mentioned somewhere.