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 Return to top of page · Post #: 1 · Written at 4:52:13 PM on 22 November 2019.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 606

Why are they elevating filament to 142v.?
Are they applying feedback to driver stage (via 10k res.) as well as to pre-amp?


http://www.fisherconsoles.com/service%20manuals/fisher%20ambassador%20VI%20a69wa%20sm.pdf


 
 Return to top of page · Post #: 2 · Written at 8:25:39 PM on 22 November 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3711

I really would need to study this more carefully but:Two things often done'

Run the heaters on DC to eliminate hum modulation, from the heaters.

Run the heaters in series: Now that measurement would perhaps then be referenced to ground, or negative, and as one moves along the string the voltage would change by the voltage drop across each heater.

All tubes would need to draw the same current, & possibly saves using a winding carrying the current of a welder.


 
 Return to top of page · Post #: 3 · Written at 9:04:59 PM on 22 November 2019.
Gandhn's Gravatar
 Location: Windella, NSW
 Member since 5 November 2010
 Member #: 770
 Postcount: 320

In the amplifier circuit, the heaters are AC driven and this is where the 142V appears.

My guess is to reduce the difference between the heater and cathodes on the 6HU8s. The circuit shows 176V on the cathodes of these tubes.

Harold

EDIT. Have just looked at the specs of 6HU8 and the maximum heater to cathode voltage is 100V, so I think my guess might be right.


 
 Return to top of page · Post #: 4 · Written at 10:46:33 PM on 22 November 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3711

Actually looking at the circuit the heaters are above ground and the positive onto them would tend to reduce the DC between heater & cathode. 12AX7 is in on this to as its resistive coupled: H to K 180V DC.

Also note the Pentodes are wired triode and we have a cathode follower output, so it has to be that way, or one gets a flashover. Which would be the primary reason for the floating winding. Regulator in my PSU has the heaters floated.

So one could theorise that if the heaters have a positive charge, they are thinking along the line of like charges repel ?

Marc


 
 Return to top of page · Post #: 5 · Written at 2:01:12 AM on 23 November 2019.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 606

specs of 6HU8 heater to cathode voltage is 100V

That's interesting, did not know there was a heater-to-cathode spec.


 
 Return to top of page · Post #: 6 · Written at 7:11:09 AM on 23 November 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 574

Re the 6HU8 heater, yes, grounding the AC heater supply would risk a punch through as the cathode is at 170 odd volts.
They put the AC winding on a DC pedestal so the potential difference between the heater and cathode sleeve is then a mean of the AC signal and the pedestal.
Like Marcc says the rating is very important when you totem pole valves or use them as series regulators in power supplies.
In audio amps you have to worry about hum levels from AC injection specially in the preamp sections. Then you ground them through a pedestal.
Pass valves in power supplys may have 400 volt on the cathodes! I have left the heater supplies in that arrangement non grounded and floating.
Fred.


 
 Return to top of page · Post #: 7 · Written at 9:29:24 AM on 23 November 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3711

A trap for young players: One needs to have an understanding of what one is playing with. With a filament tube like 5Y3 & even some audio tubes eg #42 the filament is the cathode and they use isolated filament windings. In the later the winding is often centre tapped with the Cathode resistor to ground, on the CT.

Heater valves in having separate Heaters & Cathodes with often very close spacing; Introduce the problem of flash over between Heaters and Cathode elements. The nominal is 20V but as you see from above it will vary depending on what application the valve was designed for.

Eg Some transmitter tubes ran heaters at 12 phase LV at around 250A , whilst the plate was running 30KV (heater / filament floats. The Philips 124 here has a 6X5 and what a disaster they were. They had several structural mods. They were designed as a rectifier for car radios and robust they weren't.

This Philips, despite 6X5 being a heater tube, has it on a separate floating heater winding & I would bet that's got a lot to do with early 6X5's shorting Heater to Cathode & causing a very common (at the time) transformer meltdown.

This set looks and acts as a 124 but definitely has two 6.3V windings, whereas the 124 circuit in AORSM shows one. So there may have been one I missed, or they had a problem? No ID on chassis.

Works & well, so not particularly phased.

Marc


 
 Return to top of page · Post #: 8 · Written at 4:36:03 AM on 24 November 2019.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 606

This seems to be a daring early attempt at high power solid state output using stacked (not such a good idea) car radio transistors.
Notably, it also has no speaker protection mechanism for rogue offset voltage condition that is used in more modern Amplifiers Shock


 
 Return to top of page · Post #: 9 · Written at 5:20:28 AM on 24 November 2019.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 606

"specs of 6HU8 heater to cathode voltage is 100V"

Now while one could argue that a normal floating filament source (no centre tap ground on this filament winding) would not pose a problem for the 176v cathode....but what if (thinking backwards) one of the pre-amp tubes, for instance, developed a heater-cathode short which would then create an out-of-spec condition on the 6HU8 -- the designers then fearing possible resultant breakdown in the 6HU8 - sending a transient to the stacked Germanium output devices causing “secondary-breakdown”(Collector-Emitter short) which would in turn destroy the woofer? (apocalyptic scenario)


 
 Return to top of page · Post #: 10 · Written at 11:05:38 PM on 24 November 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3711

I think part of the thinking in placing a positive charge on the heater winding, would be to discourage Heater Cathode leakage.

However, should it short H to K that feedback to B+ may be enough to shut the tube down.

Would be interesting to run the scenario through a Spice program, or similar.


 
 Return to top of page · Post #: 11 · Written at 2:19:33 AM on 26 November 2019.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 606

Such are the perils of mixing transistors with tubes -- especially early power transistors that have to be in series due to low vCEo spec.


 
 Return to top of page · Post #: 12 · Written at 11:00:41 PM on 26 November 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3711

One would think that if you had to repair an arrangement like that, with blown transistors one would perhaps consider MOSFET.


 
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