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[VK3BL]

Whether or not 'Super Cathode Drive' is sound engineering practice I'm not qualified to say.

I will comment however that in my experience arcs / flashovers are something every 811A and 572B amplifier owner is likely to experience these days.

As Carl mentioned, NOS American Made 811As are often gassy; at least enough to arc over once.  In my experience, the Russian G-811 also suffers the same issue.  QC is so poor with Chinese 811As & 572Bs that even brand new stock is likely to arc over, sometimes repeatedly.  Just recently I had a Shu Guang 572B Arc over almost continuously from Plate to Filament, due to some idiot putting the anode bar too close to the filament tensioning springs. Unbelievable.

So, given the reality of the above, I completely support Tom's position that the grids are best off directly grounded, in the interests of preventing further damage.

Now, even if we accept that 'Super Cathode Drive' improves linearity, and removing the bypass resistors and capacitors will remove that benefit, should we really be worried if we can still achieve -38dB IMD3?  After all, the people who buy the AL-811 are not likely to have an exciter capable of putting out anything better than -30dB IMD3!

[K9MOV]

Now I know why the well respected antenna experts no longer post in that forum. KM1H, VK3BL, G3RZP, W1QJ very informative and accurate posts. I have played with the glass transmitting tubes from the late 50's( mainly 6146, 811,812,813) and have worked in the field for over 50 years( now retired.)
I have owned a AL-811 for over 15 years. It is the AMP and power level I want. When I bought it, I put JAN/RCA 811A tubes in it right away.I don't use it a lot, but when I do-- its usually close to max. ratings. My first set of 811 lasted 15 years, I just put in a new set of JAN/RCA tubes in last year. I guess I got lucky. I bought 6 NOS JAN/RCA tubes off of e-bay and all were perfect. I guess I broke them in ok, because none were gassy.
Now, the modern 811 and 572 are junk. Yes they make the plate structure better to meet the old specs., but the filament materials are not the same as the old tubes. They all suffer from short tube life due to filament emission. Even the " NEW AND IMPROVED " 572 sold by a US company is being reported to have short tube life due to this.
 I think W1QJ had the best remark, if I may repeat it-- If the AL-811, 811H were sold with the JAN/RCA tubes ( I know, not possible) we would not be having this discussion.

[K9MOV]

VK3BL-- boy , you bring up a good point. I'm ashamed to say after all these years of owning the AL-811, I never made that mod. Thanks for reminding me and will put it on my list.
Please don't bring up IMD. Even though I respect them both, I don't think I could take ZENKI and W8JX in the same subject.--- HI

Lane

[VK3BL]

Now I know why the well respected antenna experts no longer post in that forum. KM1H, VK3BL, G3RZP, W1QJ very informative and accurate posts. I have played with the glass transmitting tubes from the late 50's( mainly 6146, 811,812,813) and have worked in the field for over 50 years( now retired.)
I have owned a AL-811 for over 15 years. It is the AMP and power level I want. When I bought it, I put JAN/RCA 811A tubes in it right away.I don't use it a lot, but when I do-- its usually close to max. ratings. My first set of 811 lasted 15 years, I just put in a new set of JAN/RCA tubes in last year. I guess I got lucky. I bought 6 NOS JAN/RCA tubes off of e-bay and all were perfect. I guess I broke them in ok, because none were gassy.
Now, the modern 811 and 572 are junk. Yes they make the plate structure better to meet the old specs., but the filament materials are not the same as the old tubes. They all suffer from short tube life due to filament emission. Even the " NEW AND IMPROVED " 572 sold by a US company is being reported to have short tube life due to this.
 I think W1QJ had the best remark, if I may repeat it-- If the AL-811, 811H were sold with the JAN/RCA tubes ( I know, not possible) we would not be having this discussion.

Thanks for the kind words Lane, I am sure all of us appreciate it.

Satisfaction with the AL-811 also comes down to usage patterns, and given that I fully believe you will be satisfied with your AL-811 for a long time

RCA stated that ICAS ratings were published with the assumption that the average Ham used their station (with amplifier) around 300 hours a year; which equates to 3-5 years of service life for a set of RCA 811As.  Many Hams now have much more time than that to use their station due to retirement and the kids leaving, which can mean tubes don't seem to last as long.

You're definitely right regarding the quality of filaments in currently produced tubes.  I experienced 30-50% longer lifetime from a set of 1969 JAN 811As, and they still have around 70% output, which probably would not have warranted replacement in bygone eras.

The RCAs were much nicer tubes; one notable point was they developed less grid current than the communist variant.  They are also much less likely to cause catastrophic damage when abused due to anode re-enforcement; the Russian/Chinese 811 plate is not well supported and when abused it will sag inwards until a grid short occurs.

The above is probably the primary reason Tom W8JI recommends directly grounding the grids; completely destroyed anodes are not an uncommon site in amplifiers owned by inexperienced users.

[K9MOV]

VK3BL-- thank you for your info-- 73

Lane

[G3RZP]

My professional experience in building HF transmitters (which used about 9,000 6146Bs over the production runs) was that US made tubes had the edge over the best European ones, although not very much. We also found RCA were somewhat more repeatable than GE, although the difference was marginal.  The few times Chinese tubes were tried, we gave up on them, but that was in the mid 1970's when Chinese tubes weren't really getting into the West.

The time scales involved now I look back really make me feel old..................

[KM1H]

Out of interest do you get much plate glow at 1200 watts when running AM?

I missed this the other day. With the 125W PD Chinese tubes there is moderate color while tuning and just a little on AM voice peaks.
With Cetron and UE US tubes there is very litle during a fast tune and none on voice unless it is a long Hola or a Timtron belch

[KM1H]

This bizarre theory has been passed around for the last 40 years to discredit Orr and Senti both superb engineers

.

I dont believe Orr was an EE

Would I neutralize a GG amp --- of course.  Would I use the standard inductor wrapped around the input technique --- no because if it is neutralized at 29Mc it will become progressively under neutralized as you move to 3.5Mc because of the effect the inductance of the transformer has on capacitive reactance.  Ok, so what.  This is why the IMD and linearity of the lower bands progressively become worse using that method. 

   


What inductor? The neutralizing component is a capacitor and doing that even to a Clipperton L results in a very stable amp on any band which does directly ground the grids as did the Gonset and Heath 811A 80-10M amps and also the Gonset 4 x 572B version.
Collins 30L1 use of a single parasitic suppressor for two tubes was a poor design along with a clunky relay that often resulted in hot switching, especially on VOX. The Kenwood TL-922 has a similar problem that one meathead still claims are due to parasitics.

JI now holds court on the Yahoo Ameritron forum but has been rather tame sofar since he is not the site owner.

[VK3BL]

Out of interest do you get much plate glow at 1200 watts when running AM?

I missed this the other day. With the 125W PD Chinese tubes there is moderate color while tuning and just a little on AM voice peaks.
With Cetron and UE US tubes there is very litle during a fast tune and none on voice unless it is a long Hola or a Timtron belch

Thanks Carl.

Have you tried / seen the new Chinese tubes?  The plates are much thicker, so I suspect they should handle power a little better.

The trick of course is not getting one made by a retard.  One of my recent batch had the anode connection bar practically touching the filament springs!  It takes about 10-15 seconds as the filament heats up for the tensioning springs to get within arc distance of the anode, and then it arcs over like a welder.

Absolutely unbelievable.  Tested my ass.

[KM1H]

Jarrad, I fondled a new one at HRO a few months ago and at least that one looked good but I didnt have a Cetron with me to compare. The anode appears thicker than some other Chinese. I now have a tired Cetron and a gassy Chinese in the car in case someone there has another new one; I drive by HRO at least once a week.

Im still working down the 100 Shunguans I bought direct and they also have a thicker anode but I havent had even a report of a failure so I could break the glass and measure.

Ive mentioned it before that I suspect Shunguan does not like RFP due to the pressure put on them over the years to keep lowering the price and retaliated by shipping returns and rejects. The low emission is a huge concern and a favorite practice of the unsavory tube manufacturers over there from audio to industrial power tubes.

The RFP 3-500G's typically last 4-6 years in a contest or other heavy use environment yet my 1986 Eimacs in the LK-500ZC Ive had since new are still a solid 1200W even after 12 years of world class DX, and WPX contesting in CW and SSB and gobs of DXing and more recently as an AM linear also. OTOH I have a pair of no name Chinese (marked PRC and with a date code) in an old Hunter Bandit 2000C that are still full power after 12 years. That is a backup amp (never used for that yet) and also gets used on AM with a number of 50-60's low power rigs in the collection.

At one time there were ~6 manufacturers to choose from and 1-2 were supplying the military, I forget the details. Not all had the same products and some quit the ham stuff due to low volume and concentrated on the audiophools who have more money than brains.

[K9AXN]

BL this is your comment.

"And fyi, tube dissipation is often lower under full drive than standby"

Please explain how this can be in an amplifier that is properly biased and loaded to exploit the dissipation of the tubes --- AM or Linear (AB1 or AB2).

=

I'll take a guess at what BL is trying to say here.  Lets assume an Al-82 amplifier.  In SSB ZSAC of .225A, plate voltage 3500v=787 watts plate dissipation ZERO WATTS OUTPUT.  Load amp up to .5amps plate current, plate V=3400v Pin=1700 watts Ameritron says the amp has 65% efficiency .65X1700=1105 Pout=1105 watts.  Plate dissipation =Pin-Pout  1700-1105=595 watts. 
So in standby, I assume he means IDLE the plate dissipation is less when the amp is making 1105 watts output than when making zero watts output.  I am assuming that is what he means.

First thanks for the response.  A pleasure to see someone run calculations.  One thing about amplifiers is anyone who develops an amplifier designs it to fully utilize the dissipation design limits of the tubes.

In the AL-82's case it's 1000 watts.  Your calculations are correct for an AL-82 that's loaded so lightly that it will deliver a maximum of approximately 1/2 of the available output to the antenna.  If it was a one tube amp the idle current would be .110A and the idle dissipation 374 watts and max out dissipation 595 watts.  I don't believe any properly loaded GG amp would have less dissipation at max out than idle.

The efficiency of a properly loaded GG amp will naturally decrease as the drive is reduced.  If the amp that you described was loaded to .900A and driven to .500A the ep would have been 2000V ---  3400V-1400V and the efficiency at that drive level would have been about 45% not 65%.

Bl stated "And fyi, tube dissipation is often lower under full drive than standby" (Idle) and he is correct if the amp is under loaded; Properly loaded not so.  I believe he commented, don't load the amp to its max out when you plan to use significantly less power; just load it heavily enough to deliver what power you wish and use less drive, it will run more efficiently ---- this is also correct.

BL, I would suggest next time add the explanation.  Comments without explanation are usually suspect.

Thanks to BL and a special thanks to QJ for taking the time to run the numbers and bring attention to the concept --- interesting.
 
     
 

 

[K9AXN]

I dont believe Orr was an EE

What was he --- a cook in an oriental massage parlor that wrote several electronics texts at engineering levels? 

Your a senior RF engineer that worked for an aerospace company.  Why not share your papers and theoretical written material or maybe a thesis so we can learn what was going on in that great era --- everybody's story is interesting.   

[VK3BL]

Bl stated "And fyi, tube dissipation is often lower under full drive than standby" (Idle) and he is correct if the amp is under loaded; Properly loaded not so.  I believe he commented, don't load the amp to its max out when you plan to use significantly less power; just load it heavily enough to deliver what power you wish and use less drive, it will run more efficiently ---- this is also correct.
 

The problem is, most amps don't have enough loading capacitance to run less than 75-80% of their full power efficiently, this is especially a problem on bands like 40M.

The other thing that needs to be taken into account is load sharing in multi-tube amplifiers.  In a 4 holer like an AL-811H with no load sharing circuitry, it is very easy for one of the tubes to get right on or exceed its dissipation during zero signal.

Things are a little neater in theory than practice.  I didn't feel the need to provide data, as there is already a well written article about AL-811H efficiency, which can be found here:
http://www.eham.net/articles/18364

If you scroll down, you can see a definite kink in efficiency as power output is increased.  The data in that article mirrors my practical experience.

73,

Jarrad

[K9AXN]

The point that I tried to make is an amplifier that is loaded to use all of its design plate dissipation will not have higher plate dissipation at idle than at max output.

The logic is the amplifier is designed to reach full output at the same time all of the plate dissipation is consumed.  I don't believe the idle dissipation could be intentionally set to max dissipation when it can be assumed the dissipation will significantly increase with full output.

The AL-82 is a good example.  When it reaches .900A the plate dissipation will be right at 1000 watts.

Regards Jim     

[VK3BL]

The point that I tried to make is an amplifier that is loaded to use all of its design plate dissipation will not have higher plate dissipation at idle than at max output.

The logic is the amplifier is designed to reach full output at the same time all of the plate dissipation is consumed.  I don't believe the idle dissipation could be intentionally set to max dissipation when it can be assumed the dissipation will significantly increase with full output.

The AL-82 is a good example.  When it reaches .900A the plate dissipation will be right at 1000 watts.

Regards Jim     

You're right, amplifiers are designed to reach full output at max plate dissipation, but what I keep stressing is that such a design target means they are often significantly mistuned at low outputs; hence dissipation can also be high at low output.

I'm not sure if you own a linear, but if you do, it may be an interesting exercise for you to plot the dissipation / output as was done in the referenced article.  You may be surprised to learn that dissipation is not linear with power output.

If on the other hand you find it is, let me know the make and model of your amplifier, because I would like to buy one.

And just to be clear, no one is claiming that the AL-811 TOTAL dissipation is lower at full smoke than idle; only that it is not uncommon for 1 of the 3/4 tubes to show colour if the key is held down under zero signal.  This statement can be verified empirically by any AL-811 owner.  Its the very reason using more than 1 or 2 tubes at most is considered poor engineering practice, and why it is only done on cheaper amplifiers.

Anyway, thats the last I'll say on the matter.  Its clear to me that you have not owned an AL-811 series, or you would not feel the need to argue this point.