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 HMV F4 - A2 - a factory experiment?
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 Return to top of page · Post #: 1 · Written at 3:06:11 PM on 16 August 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1269

This is the same chassis I mentioned before about the original 6R3. It actually still has most of its original valves despite the chassis showing evidence of having done a lot of work. All Aus Philips Miniwatts, Lane Cove production. Not bad for 1957.

Original 6BM8s are long gone of course, killed off by leaky paper caps.

Couple of things about this resto, after the usual re-cap:

1. This chassis reinforced my view that you can usually re-form Ducon electros but never UCCs! All Ducons were good, all UCCs were bad. The JJ 500 volt can electros from the Czech Republic I have mentioned before - they showed their worth again in this job. Extremely low ripple on the rails after they were fitted in place of the UCCs.

2. This set has a 6AW8 (branded Admiral) as the sync separator. Since I couldn't find an F series circuit that used a 6AW8 I assumed it was a ring-in and replaced it with a 6BL8. When I got no sync I put the 6AW8 back to be rewarded with perfect sync. 6AW8 pinouts are different to 6BL8 but socket wiring appears to be original factory. What gives? A short-run experiment or a valve shortage? There is not much difference in the characteristics of both valves (except for plate dissipation which is hardly relevant in this circuit) so it's hard to understand why such a substitution would have been made.

3. Excessive height, bad vertical linearity. After replacing out-of-tolerance resistors without benefit, found O/C primary on the current to voltage transformer in the feedback path. Wire rotted away inside the sleeve. Looked like this was caused by the use of acid flux, a long time ago. Cleaned it up, resoldered, OK.

These F series HMVs are sweet, well-engineered performers in all respects when they are working as intended. High end audio with push-pull 6BM8s and excellent weak signal and noise (fringe area) performance. Full black level retention. Video resolution out to 5MHz. Rock-solid raster geometry. A pleasure to watch.


 
 Return to top of page · Post #: 2 · Written at 4:43:50 PM on 16 August 2018.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 317

In those early television years, all manufacturers were using enormous numbers of 6BL8’s.
So I guess there was a shortage at some point and the 6AW8 was used to keep the production line going.
Of course the opposite occurred in the ending days of valve radios when Kriesler used 6GV8’s for audio outputs.
Suppose they had to get rid of them someway.
JJ


 
 Return to top of page · Post #: 3 · Written at 7:00:59 PM on 16 August 2018.
BringBackTheValve's Gravatar
 Location: Linton, VIC
 Member since 30 December 2016
 Member #: 2028
 Postcount: 210

I came into TV servicing just prior to the B&W to CTV transition. I vaguely recall the 6GV8 as a Vertical Output valve.
I suppose if they worked well at 50Hz they would well be suitable with audio frequencies, particularly at bass reproduction??

Another thought, many years ago I owned a Dyson guitar amp which used a pair of 6BM8's in push-pull. I got rid of it because it it could
not keep up with the 6L6 based amps. Big mistake---when I think back, I doubt if I ever come across a cleaner distortion free output as
the Dyson(20W RMS) In those days distortion was king.

So, I,m thinking one day to build an amp based on that design, but using 6GV8's instead. I have bulk of these valves.

Any thoughts?


 
 Return to top of page · Post #: 4 · Written at 7:24:42 PM on 16 August 2018.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 317

I don’t think a 6GV8 has the power capability of a 6GW8.
Recently built a guitar amplifier using 6BQ5’s, around 20 watts with 450 volts on the plates.
Very good result.
There is a lot of info on the net using 6GV8’s.
Good idea as they are very plentiful
JJ


 
 Return to top of page · Post #: 5 · Written at 9:16:32 PM on 16 August 2018.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1038

I just checked the HMV F4 service manual.

In regards to V11, 6AW8A & 6BL8, page 13: "Substitution of the 6AW8A in the Sync. Separator V11 makes possible better hold in fringe areas. The values of the associated components have been changed also, as shown in the amended circuit".

The amended circuit is included in the manual.


 
 Return to top of page · Post #: 6 · Written at 11:37:56 AM on 17 August 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1269

Thanks Monochrome, I only have the JR circuits. I don't recall ever seeing an F series chassis with a 6AW8 in the V11 position before and I have seen lots of them back in the day. Still learning!!

Hard to see why the 6AW8 per se would have made that much difference, maybe it was the component changes that did that.

Re 6GV8s, Kriesler used them in the 11-99 mantel radio to use up stock of 6GV8s that failed testing in TVs due to slow lockup (drift) of the vertical oscillator from cold. They are spec'ed differently to 6GW8s because of their intended application but an examination of both types reveals they are very similar internally, save a missing shield between the triode and the pentode in the 6GV8 and different pinouts.


 
 Return to top of page · Post #: 7 · Written at 11:41:48 AM on 17 August 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1269

20 watts from a pair of 6BM8s is wishful thinking. Perhaps at 50% distortion!


 
 Return to top of page · Post #: 8 · Written at 9:19:48 PM on 22 August 2018.
Pitchersj's avatar
 Location: Albury, NSW
 Member since 2 July 2017
 Member #: 2134
 Postcount: 113

QUOTE: These F series HMVs are sweet, well-engineered performers in all respects when they are working as intended. High end audio with push-pull 6BM8s and excellent weak signal and noise (fringe area) performance. Full black level retention. Video resolution out to 5MHz. Rock-solid raster geometry. A pleasure to watch.


Agree with Ian wholeheartedly here, My F3 (that Ian brought back from the dead) has a glorious picture, better than I could ever have hoped for


 
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