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Author Topic: BAD AUDIO WITH ALPHA 91B  (Read 3923 times)
LA4UOA
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Posts: 217




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« on: July 11, 2010, 12:59:59 PM »

Bouhgt a Alpha 91b second hand, amp has good output on all bands, but i get poor audio reports on 80m band, a little better on higher bands.
My radio is TenTec Orion 2, mic Heil Proset Plus/Ten Tec 707 Regal desk mic, ant at 80m , loop about 15 m over ground.
There is no swr, it s 1:1 at 80m,my station is very good grounded.
Ant on higher band 4el 5band cubex quad.
Now to my problems.
audio sounds not good , when i work without amp its ok.I bouhgt this amp because i was not happy with my old amp,so this was very boring,hope for some good advice,perhaps from other Alpha owners.
My other Amp never made any audio problems at same ant/power.
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K6AER
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Posts: 4682




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« Reply #1 on: July 11, 2010, 09:23:14 PM »

Several items could be the problem with the amplifier linearity.

I am making an assumption you power out is with in norm on the various bands. Your amplifier could for a number of reason be biased closer to class B.

Check the screen voltage. Also the Grid bias voltage might be off or the EBS system might be having problems. Bias should be around - 15 volts in the operate mode.

Is the input VSWR normal? Should be under 1.3:1. if not check the swamping resistor network on the input.

Alpha opens at 9 AM Denver time and the staff will help over the phone if you like.
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LA4UOA
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Posts: 217




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« Reply #2 on: July 12, 2010, 01:10:53 AM »

Thank you for answer,everything seems to be ok between rig and amp,i will do some more testing.
73 Torgeir.
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VR2AX
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Posts: 803




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« Reply #3 on: July 12, 2010, 02:28:22 AM »

I didn't encounter that problem when I had a 91b (ETO), however have you checked that you're not overdriving the 91b on 80m with the Ten Tec (which I presume does not have any ALC facility from the amp)?

The 4cx800a tubes in the 91b only require a relatively small amount of drive, compared to a grounded grid amp to produce a fairly high amount of power output. If your signal is clean on the other bands, there's probably nothing wrong with the amp. You could be getting RF into the radio, have you checked that the mic lead is shielded (and if it is supposed to be shielded, that the 'shield' is actually connected to the ground/chassis connection on the radio?
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LA4UOA
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Posts: 217




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« Reply #4 on: July 12, 2010, 05:34:11 AM »

Hi.
I am using low drive from Orion 2,if i use to much drive, it automatic going into protection also if i tune it wrong,i will check microphones to perhaps they pick up something.This never happen with my Drake L7 amp at same ant and same power.
But i have only used amp twice,will do some more testing.
Thanks
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KA7GKN
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Posts: 149




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« Reply #5 on: July 12, 2010, 07:43:54 AM »

"Audio sounds not good"Huh?

What does that mean?
1. distorted sounding audio
2. a roughness in the audio
3. a scratchy noise in between words

A very common challenge when using a linear amplifier [any brand]
is RFI in the shack. When you said the problem went away when you
shut the amp off is a major clue. And to make this even more frustrating it can
be intermittent.

RFI can be from a bad coax connector, bad coax jumper, antenna too
close to the shack, Equipment placement  etc.

what has changed in your shack?


Martin  KA7GKN

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G3RZP
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« Reply #6 on: July 12, 2010, 09:52:10 AM »

Is it clean on a dummy load? If so, it's an RFI case. If it's still bad, then it's an amp fault.
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LA4UOA
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Posts: 217




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« Reply #7 on: July 12, 2010, 10:34:47 AM »

The audio sounds a kind of rough in endings of words, it sounds like am very catch could hi hi,sorry my poor English,not easy to describe it ,nothing is changed but amplifier, ant is about 15 m away and 15 m over ground,never been a problem with my other amps at same output/ant/mic.
With a dummy i will not be able to get audio reports,and i dont have equipment to see what goes into the dummy either other than watts,think i would need more than just a dummy load to find out.I here nothing in my monitor.
But like i wrote earlier, i will just have to wait until i go up again and do some more tests,will also see if same thing happen on my KWD 870 rig.Perhaps the Orion 2 picks up more than other rigs do.
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G3RZP
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« Reply #8 on: July 12, 2010, 02:57:17 PM »

If you run into teh dummy load, you will get enough leakage that you will hear it on the Kenwood, quite likely without even a piece of wire in the antenna socket. Then you tell if it is RFI or an amp problem.
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LA4UOA
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Posts: 217




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« Reply #9 on: July 13, 2010, 01:17:43 AM »

Ok, i will try it.
Thank you.
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VK3IDX
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Posts: 1




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« Reply #10 on: August 15, 2010, 04:04:54 PM »

i also have a orion2 with regal707 mic. When using amp, if i place mic close to spe amp i receive rf. If i move the mic away from amp it is ok. Im not sure where to start with this one, so i just seperate the distance and it seems ok. Same situation with Heil headset and different radio, so i suspect amp is the cause. All equipment is grounded,so not sure what else to do?
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