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[WJ3V]

Have a small issue with a newly purchased 91B amp. When tuning up I can only get about 1300 watts output. When I talk using SSB it will do the 1500+ on voice peaks.

 When I first purchased the amp I could tune up to 1500++ and IF I remember correctly the plate current would show a little over 1 amp and the max grid light would not come on. Now once tuned for peak power plate current is below 1 amp .
Not sure what to check. Here is what I have done so far.

HV is 2850 at idle and drops a couple hundred volts under load. I could get an exact measurement if needed. I have a HV probe.

Tuning up for peak output the plate current struggles to get to 1 amp. The red grid Max light will light up red with about 50 watts drive. Increasing the drive does not seem to raise the output much. At about 45 watts of drive power it seems to max out at 1300 watts out.

I have an LP-500 station monitor and talking on SSB once tuned to 1300 watts out I can see peaks over 1600 watts+ The LEDs on the amp output match the LP-500.

Output is the same with 2 sets of tubes. I just got a new NOS set and see no difference.

Its possible this amp may have set unused for a while. It came from an SK estate.
I'm thinking I might have some marginal HV caps but maybe I'm overlooking something?
Any ideas........

[K1VSK]

Ignoring that the difference between 1300 and 1500 watts is negligible, if you are seeing 1amp at 2500plate volts, what is the problem you seek to remedy?

[WJ3V]

I'm not really concerned about the difference between 1300 and 1500 watts, I just wanted some comments from someone more knowledgeable as to what might be the cause of not able to tune to legal limit. I usually run the amp at 1000 watts out when needed. Just would like to know what might be going on and if it needs something repaired, I can get it done before it turns into a worse problem.

Something is not quite right and want to find out what it is.

And to add to the above post. I checked the full load HV and at 1300 watts the voltage drops from 2860 to 2540.

[W1QJ]

You didn’t mention a word about adjusting the load after driving the amp with more drive.  Usually once the plate is adjusted it barely cjhangez much with that amp but the load control makes a huge difference in output when readjusted after more drive is put in.  This explains the grid warning.

[VE7RF]

What xcvr are you using ??  Switch amp to standby, then TX with a cxr, to what u used to drive the amp to 1300 watts. Measure the average PO.   Then switch xcvr to SSB..and measure the pep output.

My guess is, you have a different output on the xcvr in CW / CXR mode,  vs  SSB mode, with more PO on ssb, hence the 1600w pep out on ssb.

BTW, if the amp is tuned to 1300 watts, it's NOT tuned  for 1600 w pep.  The load control is not high enough, and once tweaked for max pep out on ssb, it will read even higher than 1600w pep.

[N2SR]

Ignoring that the difference between 1300 and 1500 watts is negligible, if you are seeing 1amp at 2500plate volts, what is the problem you seek to remedy?

Since the amplifier is capable of way more than 1300 watts out, other than being not tuned correctly or that the exciter is not driving the amp correctly, maybe the owner wishes to make sure that there isn't an underlying issue? 

[WJ3V]

I'm using a TS-890 to drive the amp. Before this issue started, I could tune the amp up with about 45 watts and it would produce over 1600 watts. Now, once I drive the amp with about 35 watts it will load up to 1200 to 1300 watts output. Increasing the drive and retuning the amp changes nothing.

This morning with the amp tuned up for max output with 45 watts drive, the most I can get is right at 1290 watts. The output meter on the amp and my LP-500 show the same amount. Now talking on SSB with the power out on the 890 set at 45 watts I can see peaks at near 2000 watts on the LP-500. The LEDs on the amp power out go into the red as expected.

Just not sure what to look for. My day job is an electronics tech but I don't have much experience with tube type amps. It seems to me this amp should load up to over 1700 watts out with 50 or so watts of drive.

Any help on this is much appreciated.

[WJ3V]

Hopefully final update. Don't know for sure what I did but I removed the HV capacitor board and checked the caps. All seemed fine. This amp has push on connectors for the HV and screen voltages. I cleaned those connectors. They were slightly black. Not corroded or burnt but kind of dirty. Put everything back together and now with 25 watts drive have over 1700 watts out.

How do you post a pic on this board? I don't see any options for attachments.

[N7ZM]

Todd, glad you solved your 91B problem. I'm sorry I sold mine in 2019. I bought it in 1998 when they first came out. Great amp.
Ron N7ZM

[AE0Q]

I cleaned those connectors. They were slightly black. Not corroded or burnt but kind of dirty. Put everything back together and now with 25 watts drive have over 1700 watts out.

A couple of possibilities...
1) You might have been running on just one tube.  Were they both getting hot, about the same temp, when it was struggling to get legal limit?  The Russian tube sockets can become intermittent.
2) Some of the 91B Control Boards use metal pots for all the analog adjustments, including the EBS and full-on tube bias settings.  They can corrode and become intermittent, changing the tube operating bias.
3) Were all tests on the same band?  If an Input RF Detect diode has failed, it can switch the tube bias (the EBS circuit) at varying drive levels on different bands.  If the bias isn't switching correctly, you can have low output.
4) Email me for a simple way to test the EBS switching to see if an input detect diode is bad.  Only takes a minute :-)
ae0q at arrl dot net

Glenn AE0Q