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 How many watts from a 300b cathode-follower?
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 Return to top of page · Post #: 1 · Written at 2:42:20 AM on 9 April 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 745

Just got an idea for using a pwr triode in cathode follower to drive my JBL 2440 compression drivers (16ohm), thus a 16 ohm cathode resistor with coupling cap for direct drive to speaker (300hz high pass) (would only require a watt or two.)


 
 Return to top of page · Post #: 2 · Written at 5:13:31 PM on 9 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

Instead of the 300B which would need an isolated 5 volts DC . 1.2 amps filament supply, how about a 6S4?

https://tubedepot.com/products/6s4a

and

https://frank.pocnet.net/sheets/127/6/6S4.pdf

Unlike the audiophool hyped 300B they are very cheap, are more suited to this kind of service and you could afford to use more than one.
The cathode current rating is quoted at 30mA but that is for high voltage operation, I'd say you'd need 3 of them. That would give you a very low output impedance.

Your drive signal needs to be low impedance too, so that you can drive the tube(s) into positive grid current.

This might be an interesting thing to model in, say, LTSpice.

Edit - the 6EM7 is worth looking at, but it has been "discovered" by the tube audio brigade and is thus expensive.

Further edit - Look up "Futterman Amplifier".


 
 Return to top of page · Post #: 3 · Written at 10:12:20 PM on 9 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

Oh, I just looked up how much you could pay for a couple of 300Bs.

Prepare for a shock!!

https://tubedepot.com/products/nos-western-electric-engraved-base-300b-black-plate-late-1930-s-matched-pair

Quite a lot to pay for a mono amplifier with a best-case THD of 2%!

Of course, 2% THD was pretty good in the 1930s, but nothing like the .002% that's easy to achieve today.

Face it, guys, that's not an amplifier, it's a musical instrument!


 
 Return to top of page · Post #: 4 · Written at 1:04:37 AM on 10 April 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 745

Yes, I had searched for "cathode follower headphone amplifier" and see most were a "Futterman" type topology with the Cascode dual output (for improved current delivery?) meaning I would need dual W.E. 300b's per channel Sad

Actually I was envisioning using Soviet clone 300b's

Thanks for alerting me to 6EM7's, had just been sorting my (1,000's of) tubes and will move the 6EM7's from "TV" to "Hi-Fi" category.


 
 Return to top of page · Post #: 5 · Written at 3:16:25 PM on 11 April 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 745

The 6BM8 has a big audio following, very common in Aust TV's (save them from junked TVs) but you'd more likely find 6EM7's in US TV's, strange...


 
 Return to top of page · Post #: 6 · Written at 4:26:20 PM on 11 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

The HMV F series chassis (except for the F2) actually has 3 x 6BM8s, 2 of them as push-pull audio. This in the 50s.

Pentodes are voltage sources, triodes are current sources. A deflection yoke is best driven by a current source. Hence a triode is an obvious choice for a vertical output stage. With a pentode you need to add feedback. HMVs used a current to voltage transformer in series with the yoke to get an automatically correct vertical linearity. There are no vertical linearity controls in these sets.

Only one chassis in Australia ever used the 6EM7, and that was the short-lived TX-31 series AWA. AWA tended to follow RCA practice.

Disadvantage of a triode vertical output is, due to plate - grid capacitance, hor scan energy cross-talking through the yoke gets fed back into the vertical oscillator and can more readily degrade the interlace performance.

And of course the linearity depends on the tube's characteristics which change with aging and need regular adjustment. By comparison an HMV will run its 6BM8 into the ground before there is any change in the picture.


 
 Return to top of page · Post #: 7 · Written at 10:16:02 PM on 11 April 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 745

What did the Aust Admirals use for vert? I once sold an 'Olson' stereo amp that had PP 6BM8's, made in Japan for Olson-Electronics chain (similar to Tandy but went out of business in '80s) This amp had potted O/P tranny's, could have been very good?, also sold a Sansui 1000 - noted for their O/P tranny's, also sold a W.E. '1086' amp - PP 300b, no neg f'back - in early 80's.


 
 Return to top of page · Post #: 8 · Written at 11:54:16 PM on 11 April 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4346

I have a real Philips sheet noting that there was a compromise with 6BM8 as is was needed for TV deflection & audio. While I did hear comment on 6GW8 for reliability and had to repair the aftermath of one shorting, Philips claim 6GW8 was made exclusively for audio.


 
 Return to top of page · Post #: 9 · Written at 1:03:00 AM on 12 April 2020.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1380

My recollections was 6BM8 for vertical osc & output in older TVs, and 6GV8 for the newer ones. I only ever saw 6GW8 used for audio.

Of course, 6GV8 was used in the Kriesler 11-99 radio, but that has been exhaustively discussed elsewhere.

Before the 6GW8, the 6BM8 was used in TVs and radiograms for audio, or alternatively the 6AQ5.

6EM7 very uncommon here. I might have one in my collection, not sure.


 
 Return to top of page · Post #: 10 · Written at 8:19:44 AM on 12 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

"What did the Aust Admirals use for vert?"

90 degree - 6S4 single triode
110 degree - 6DB5 beam tetrode


 
 Return to top of page · Post #: 11 · Written at 11:40:38 PM on 12 April 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4346

Using 6GW8 for audio only, instead of 6BM8 was the intent & retain 6BM8 for TV.

There is a stereo amp circuit in this data.


 
 Return to top of page · Post #: 12 · Written at 6:34:15 AM on 13 April 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
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 Postcount: 745

stereo amp circuit in this [Philips] data.

Is it PP or SE type amp?
There was a stereo SE 6GW8 kit in Aus in the sixties [RCS]


 
 Return to top of page · Post #: 13 · Written at 8:42:39 AM on 13 April 2020.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

6GW8 was used in one of the Playmaster guitar amplifier projects.


 
 Return to top of page · Post #: 14 · Written at 2:56:43 PM on 13 April 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 745

The HMV F series chassis ... 3 x 6BM8s, 2 of them as push-pull audio

I saw one or two of these on service bench, incredible: 4 x I.F. stages for fringe performance, ext. crt magnets for razor sharp raster, lots of transformers under chassis dipped in tar!


 
 Return to top of page · Post #: 15 · Written at 3:45:10 PM on 13 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

Engineering wise, they were straight out of the textbook. And yes, many were expected to work in deep fringe areas before the regional transmitters opened. The picture may have been deep in the snow but at least it was stable thanks to the belt-and-braces synchrolock circuit that HMV continued to use in various guises in all tube chassis, except for the 1960 M and P series chassis where they went to a simpler circuit.

That was not tar, it was Berry-Wiggins Compound!

When did YOU see F series on the service bench?


 
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