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Author Topic: Substituting 811 tubes for 811A's in hf amplifiers? Problems or not?  (Read 3178 times)

Posts: 392

« on: September 06, 2017, 02:51:44 PM »

Wondering about using "straight" 811 RCA tubes in place of the 4) 811A tubes in my hf amplifier (Collins 30L-1).  Have 6 of them, so a replacement or two if needed.

I know about the lesser dissipation and thinner plates and the resulting need to limit tuning time and driving power.  Will be running around 400-450 watts output SSB only on 40 meters for short QSO's.

Concerned about possible problems with difference in bias requirements and any instability, oscillations or runaway conditions that might affect the amplifier or power supply.  

Any experiences or input will be greatly appreciated.  I hate to not use available "free" tubes that might work.....just do not want to pay dearly for some unforeseen eventuality that I am unaware of.  

I did try substituting one of them in the amp and running it with 3) 811A's....did not notice any problem.  Just wanted to check before changing all 4 and getting in any unnecessary trouble.  Thanx

« Last Edit: September 06, 2017, 03:11:14 PM by N6QWP » Logged

Posts: 572

« Reply #1 on: September 06, 2017, 05:26:08 PM »

no problem for me,as ive done the same over the years. had me 28 tubes at one time,and just kept the gooders,and sold the weaker ones,but I myself have never had a problem with 811 vs 811a,and I have 2 amps. 811 and 811h

regards  ve7ren

Posts: 392

« Reply #2 on: September 06, 2017, 06:27:24 PM »

Thanx, nice to know that someone has already tried with success.  Any pointers as to power level or drive level?  No other differences noticed?  Appreciate any further input.

Posts: 392

« Reply #3 on: September 06, 2017, 08:27:55 PM »

Just installed the 4) 811's and made contact with another station on 40.  Good audio report and S9, but after speaking for a while noticed (in the dark) that the plates were turning orange.

Didn't wait for them to turn red and ceased operation at once.  Guess I was overdriving them?  Was told that orange was not necessarily damaging, but where is the warning cutoff point?  Didn't want to burn up the 811's.

Is having orange plates acceptable practice while talking?  Never noticed that happening before (might be because most of the time operating in daylight).....and was using RCA 811A's with thicker finned plates.

I also, since it was dark, noticed that one of the tubes had a purplish glow.  I assume that this indicates a gassy tube?

Did not seem to cause audio problems.  Wondering about what the effect of this situation will be....and what to expect?

« Last Edit: September 06, 2017, 08:39:40 PM by N6QWP » Logged

Posts: 392

« Reply #4 on: September 07, 2017, 12:48:03 PM »

Well, I can answer the last question I had about the "gassy" 811.  Amp was working fine until I turned it on again last evening to check on the purple glow.....then a loud bang and flash......blew the tube and both fuses.

Just replaced the fuses and left the socket empty and all appears fine with the rig.  Tube shows no visible signs of the problem, so it must have shorted internally.  Will replace that 811 and see what happens.
« Last Edit: September 07, 2017, 12:52:29 PM by N6QWP » Logged

Posts: 392

« Reply #5 on: September 07, 2017, 04:06:48 PM »

Well.....the saga continues.  Amp has good high voltage, but after replacing the tube, tried to transmit and output dropped down until it was only the driver putting out.

Replaced the 811's with my good 811A's and have the same the tubes are not the problem any longer.  Can not get any response to the amps controls and no current being drawn-regardless of driving power.  High voltage is still present.

Obviously I blew up something in the amp....would appreciate any pointers on where to start looking.  Other than flashing inside the one shorted 811, I noticed nothing and smelled nothing and since the high voltage is still good, the power supply would seem to be ok.
It took some time for the output of the amp to gradually fall away to nothing (other than the driver's output).  

Any obvious conclusions from the above described scenario (other than "not to experiment with unknown tubes and don't fix it if it ain't broke")?  I would certainly appreciate any help on where to begin-as this is my first amplifier.  

I just checked and do not hear the t-r relay inside the amp when I key the driving xmtr.  Is it possible that the keying buffer (Ameritron) was damaged during the mishap---or the relay itself???  It is acting like I am not able to activate the amp and driver is just working straight through.  Just tried grounding the lead that shorts out the t-r relay directly and nothing.  Suspect relay.

Thoughts?  Suggestions?  Test procedure?  I sure am learning the hard way....but any path to knowledge is a good one.
« Last Edit: September 07, 2017, 04:28:54 PM by N6QWP » Logged

Posts: 392

« Reply #6 on: September 07, 2017, 05:25:30 PM »

Since the original question has been answered somewhat with limited response, and the thread has now shifted into repair, I want to move and start a new thread to deal with that problem.

I hope my flailing around in unknown waters regarding changing tube types and testing out unknown tubes and the results thereof, will caution other new users of amplifiers about the pitfalls that can result.

73 to all.

Posts: 472

« Reply #7 on: September 13, 2017, 11:30:48 AM »

I would start with the schematic and try to identify the major circuits.

Don't overlook the power supply, there are more that High voltages there.  You could have a burned resistor or bad cap on one of the low voltage outputs that just won't let the amp work.

good luck,

...bc nr4c

Posts: 6655

« Reply #8 on: September 13, 2017, 10:04:22 PM »

An orange glow of the tube plates is normal.  What is abnormal is when the plates get so orange that it gives you an uneasy feeling.  The average person will sense what is too orange.  After the orange color comes the yellow and that's absolutely unacceptable!

I've never seen a red plate. 

A Pessimist is Never Disappointed!
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