Ameritron AL-811H Parasitic Choke Problem

[N5IIA]

Hello,

I recently purchased an 811H amplifier new. The amp works fine on all bands except 10 meters. During a relatively short QSO on 10 meter FM, I noticed smoke coming from the vent holes. I was driving the amplifier with about 35-40 watts with an output of about 400 watts. Considering the rated output and drive maximums for the amp, I thought I was certainly within a cautious operating envelope. I opened the amp and found the parasitic suppressor board was charred, and two of the 100 ohm 7 watt resistors were very crispy! Ameritron sent me a replacement board under warranty and I installed it. I fired back up on 10 meters, now driving the amplifier to 250-300 watts output. During a lengthy ssb contact, once again I could smell the odor of hot components. This is only an issue on 10 meters. On any other band I can operate the amp at 600 watts all day long without a whiff of burning resistors. Any advise is welcomed other than telling me I should have bought a more expensive amp, or that Ameritron is crap, etc. I am very happy with the amp other than this one issue.

[WB2WIK]

400W on FM for transmissions longer than a minute or so seems excessive to me for that amp. I have an AL-80B, which is a bit more "robust" (bigger tube, bigger power supply, etc) and I wouldn't do that.

However, operating SSB I would think you should be able to transmit as long as you want to; SSB duty cycle is pretty low, unless you're using a lot of compression. My AL-80B will take SSB transmissions that last forever without getting hot or doing any damage. (Big diff between SSB and FM!)

The 811H uses tubes that are a 70+ year old design (811As) which may have stability issues nowadays. Back when RCA was making them they were really good tubes, but that was a very long time ago and now they're all Chinese. The 811H has a neutralization circuit and I'm not sure if that needs adjustment for the specific tubes used or not.

Maybe Tom W8JI can chime in on this one. I know he designed that amp but isn't very happy with 811A tubes nowadays.

[AG6K]

Hello,

I recently purchased an 811H amplifier new. The amp works fine on all bands except 10 meters. During a relatively short QSO on 10 meter FM, I noticed smoke coming from the vent holes. I was driving the amplifier with about 35-40 watts with an output of about 400 watts. Considering the rated output and drive maximums for the amp, I thought I was certainly within a cautious operating envelope. I opened the amp and found the parasitic suppressor board was charred, and two of the 100 ohm 7 watt resistors were very crispy! Ameritron sent me a replacement board under warranty and I installed it. I fired back up on 10 meters, now driving the amplifier to 250-300 watts output. During a lengthy ssb contact, once again I could smell the odor of hot components. This is only an issue on 10 meters. On any other band I can operate the amp at 600 watts all day long without a whiff of burning resistors. Any advise is welcomed other than telling me I should have bought a more expensive amp, or that Ameritron is crap, etc. I am very happy with the amp other than this one issue.

 Operation on the 28MHz band is the Achilles' heel of VHF parasite suppressors. ref: [March 1989, QST Magazine "Calculating Power Dissipation in Parasitic-Suppressor Resistors"]
If a VHF suppressor didn't get hot at 28MHz it would not be doing its assigned job of reducing VHF amplification - which would be an open invitation to VHF oscillation. Thus one trades heat for stability. In your present suppressors, R dissipates most of the heat while the Cu L dissipates virtually none. A workaround to this problem is to distribute the heat burden between the suppressor R and the suppressor L by using resistance-wire for L.

RE: resistance-wire in VHF suppressors:
“The combination of both resistance and inductance is very effective in limiting parasitic oscillations to a negligible value of current.”

- - F. E. Handy, W1BDI 1926 Ed. The Radio Amateur's Handbook, p. 72.

If you would like to try this type of VHF parasitic suppressor in your amplifier, send your usps adr to r@somis.org and we will send you a freebee suppressor retrofit kit - which we will write off to advertising.
cheers
• Rich, ag6k

[W8JI]

You are far over the ratings for FM.

How are you measuring SSB power? What power meter? Is the measurement on voice?

[AG6K]

400W on FM for transmissions longer than a minute or so seems excessive to me for that amp. I have an AL-80B, which is a bit more "robust" (bigger tube, bigger power supply, etc) and I wouldn't do that.

However, operating SSB I would think you should be able to transmit as long as you want to; SSB duty cycle is pretty low, unless you're using a lot of compression. My AL-80B will take SSB transmissions that last forever without getting hot or doing any damage. (Big diff between SSB and FM!)

 INDEED. FM broadcasting has about 12x the duty-cycle of not 11m-processed 2-way SSB.

The 811H uses tubes that are a 70+ year old design (811As) which may have stability issues nowadays. Back when RCA was making them they were really good tubes,

 Really good? • According to my 1955 Radio Amateur's Handbook, the 811A has 0.6pF of feedback C. That means that an amplifier with 4, 811As has 2.4pF of feedback C. - so at 100MHz, they have a –j650-ohm feedback path between their output and their input -- which is pretty obviously a tacit invitation to VHF oscillation.

but that was a very long time ago and now they're all Chinese. The 811H has a neutralization circuit and I'm not sure if that needs adjustment for the specific tubes used or not

 The RCA Transmitting Tube Manual's "811 neutralization circuit" - which is used in the AL-811H is the same one that Heath tried in their HA-10 "Warrior" 4, 811A amplifier. This circuit neutralizes at the operating freq., but it does not neutralize at the VHF parasitic resonance in the anode circuit - and even worse, due to phase-shift, RCA's HF neutralization circuit increases VHF feedback. . . . RCA's circuit was so squirrel-ridden that Heath abandoned it in future amplifiers.

.

Maybe Tom W8JI can chime in on this one. I know he designed that amp but isn't very happy with 811A tubes nowadays.

 There's nothing wrong with using a tube with that much feedback-C, but it's like F. Henry Royce (1863-1933) said: good engineering practices overcome all hurdles. . . . Since we can do nothing about the –j650-ohm feedback path @100MHz for 4, 811As, the solution to improving their VHF stability is to artificially reduce their VHF amplification. This is the job assignment of the 4 VHF suppressors. They do this job by decreasing the RL presented to the anodes at frequency of the anodes' parasitic resonance -- which the curious can find by coupling a dip-meter to either side of the anode's DC blocker C.
Game Plan: The lower one makes the VHF-Q of a VHF suppressor, the lower the VHF-RL presented to the anodes, the lower the VHF amplification, and the less chance of VHF oscillation. However, the lower the VHF-Q, the more heat will be produced in the suppressor at 29MHz. Thus a compromise must be made in order to keep the suppressor from burning out on 10m - and yet avoiding big-bang parasitic oscillations.
- note - this subject is discussed in the March 1989 QST article "Calculating Power Dissipation in Parasitic-Suppressor Resistors"
• Rich, ag6k

"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."
— Mark Twain