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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 19th Mar 2017, 4:23 pm   #1
Andrew B
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Default Effect of cathode strapped suppresor grid?

Hi all.

A quick question about strapping the suppressor grid to the cathode instead of directly to ground. If I wanted to change the 6KD6 in the diagram for an EL84 (much easier to get), apart from the wiring on the valve base, would the tied-to-cathode suppressor grid have any effect on the operation of the circuit as shown.?

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Old 19th Mar 2017, 4:41 pm   #2
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Default Re: Effect of cathode strapped suppresor grid?

Given that the cathode is essentially at RF-ground by virtue of the decoupling capacitor, connecting the SG to the cathode shouldn't be a problem.

Unless you already have an EL84, why not try using a 5763/QV03-12? It's got a separately-connected SG and was designed-from-the-outset as a RF amplifier.

http://www.r-type.org/exhib/aam0048.htm
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Old 19th Mar 2017, 5:01 pm   #3
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Default Re: Effect of cathode strapped suppresor grid?

Well, I have a couple of EL84's that I could try, whereas I would have to "buy in" the 5763/QV03, which are more specialized and seem to command a higher price (on a certain well known site). I was also thinking about ditching the 6KD6's (if they were duff and rebasing the finals for EL509/519's as they are easier to get hold of.
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Old 19th Mar 2017, 5:09 pm   #4
G6Tanuki
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Default Re: Effect of cathode strapped suppresor grid?

As an alternative to the 5763 - in the past I've strapped both halves of a QQV03/10 together! Works quite well as a 'single ended' RF output-bottle, and they're quite easy to find, having been used in loads of 1950s/1960s two-way VHF radio stuff (Pye Cambridges and Vanguards...)
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Old 19th Mar 2017, 5:18 pm   #5
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Default Re: Effect of cathode strapped suppresor grid?

That sounds to be an idea, I intend to get around a few ham rallies this year. . The main bone of investigation is why I have a low (25w) RF output, is it the driver or the finals and how to go about testing then "in situ"? I have advertised on the board for anyone that can test the 6KD6's, but probably unlikely with the compactron tube base.
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Old 19th Mar 2017, 6:25 pm   #6
G6Tanuki
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Default Re: Effect of cathode strapped suppresor grid?

Does the Tx not have the facility to measure grid-current in the output-bottles (sometimes labelled "drive" on the meter-function switch)?

If you can't get the level of grid-current you need then the driver's at fault.

if you can get the specified grid-current but there's not enough RF coming out then the output bottles are 'tired'.

Must admit, a lot of rigs using TV 'sweep-tubes' as the output-bottles do tend to work them extremely hard so they have a short life.

If you're interested, a lot of good stuff on using PL519-style TV line-output valves has been written up by the Dutch ham PA0FRI: his "FRInear" amplifiers are things of intriguing beauty.
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Old 19th Mar 2017, 10:09 pm   #7
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Default Re: Effect of cathode strapped suppresor grid?

Understood G6Tanuki, the nearest it has to monitoring grid current is the "ALC", which monitors the finals grid voltage, turning down the drive if they go positive. I have a pair of PL519's in good condition and a pair of ceramic bases, so just need to sort out a filament transformer 40v @ 0.6A. The finals heater come from a separate winding on the main transformer, so no heater chain problems.

The only other thing that I have noticed is that when loading into the dummy load, the "loading" capacitor has little effect on I.C., and little on power out, although it seems to need to be set on full capacitance to achieve any power out. Maybe one of the switched loading caps has gone O/C? Any opinions are valued.

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Old 19th Mar 2017, 10:56 pm   #8
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Default Re: Effect of cathode strapped suppresor grid?

This excerpt provides one explanation as to why RF pentodes usually had a separate suppressor grid pinout:

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RCA stressed this feature separate suppressor grid pinout for its RF pentodes at least from the 6CB6 onwards. It was said to allow an unbypassed cathode resistor without risk of regeneration, and in turn a partially unbypassed cathode resistor minimized input capacitance variations, and hence detuning, with changing agc bias.

So basis the RCA position, the need for direct earthing of the suppressor grid would appear to be circumstantial, and not necessarily general.

It is interesting that in moving from the EL42 to the essentially identical EL85, Philips also changed from an internally connected to a separately pinned-out suppressor grid. Whereas the EL42 was seen as an AF valve, the EL85 was positioned as an RF valve that could also be used at AF.


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Old 19th Mar 2017, 11:53 pm   #9
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Default Re: Effect of cathode strapped suppresor grid?

All taken on board Synchrodyne, I believe we have discussed suppressor grids in another topic, probably concerning suppressor grid modulation.. It appears that the driver has sufficient output to trigger the ALC limiter action, this being derived from a pair of diodes in the grid circuit to the PA's, it may suggest that the grids are beginning to draw current, although how to go about measuring the PA grid current eludes me, perhaps Synchrodyne has a suggest of how to do this?

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Old 20th Mar 2017, 6:09 pm   #10
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Default Re: Effect of cathode strapped suppresor grid?

I seem to have sorted some of the output problem !! someone had moved the final tap on the tank coil, effectively removing 3 turns on all bands. I only noticed it after I had taken the cooling fan assembly off to lube its bearings. I wonder what else I will find.........
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