[Amps] Non-linear linear

[Fuqua, Bill L]

   Turn on the amplifier and observe plate current. Increase drive slowly and watch the plate current.
If it jumps during this process, you may have a parasitic oscillation. Sometimes amplifiers can have
a parasitic oscillation only when driven and actually amplify and oscillate at the same time. It can
be in or around the band you are transmitting or VHF parasitic. This  can cause all sorts of non-linearity.
Mostly one that robs power at the frequency you are transmitting at while emitting power at an unwanted
frequency. 
   73
Bill wa4lav

________________________________________
From: Amps [amps-bounces at contesting.com] on behalf of Jim Thomson [jim.thom at telus.net]
Sent: Thursday, January 30, 2014 11:51 PM
To: Paul Baldock; amps at contesting.com
Subject: Re: [Amps] Non-linear linear

### have u tried another tube (s) ??
Can u hear any increased imd, off freq.... on another RX... or a spectrum
analyzer ?
Does the idle current remain constant on all bands ?
Something is amiss that is causing non linearity.
It appears that the gain stays constant across all bands with 75 drive + 1.5
kw out.    Its not a gain problem at max out, it appears to be a non linear
problem.   That imo, eliminates both the tuned input and the
PI  kw output.
Can u test on other freqs outside the ham bands.....into a dummy load.  Like
15-30 mhz ?
Sometime the bypass cap used at the cold end of a bifilar fi choke will
resonate....but that effect  would show up as  PO  dropping way off.   U get
1.5 kw out.
Can u vary the bias on 17 + 15M ??

If u have some spare good tubes.. u might want to swap tubes.     I cant
think of anything that would cause non linearity..except the tuned input,
and bias.

As noted above, most layout problems show up as a drop off in po..or low eff
on upper bands etc.

This one is baffling.... my bet is on a one off  non linear tube.... but
even that is a stretch.

Jim  VE7RF



-----Original Message-----
From: Paul Baldock
Sent: Thursday, January 30, 2014 4:42 PM
To: amps at contesting.com ; jim.thom at telus.net
Subject: Non-linear linear



I have a problem with my home brew HF linear being non linear on one band.

On 80, 40, 20, 12 and 10 with 75W drive my linear will give about
1500W output. If I reduce the input power by 9.5dB the output goes
down by 9.5dB. So all is good.

On 15 with 75W drive my linear will give about 1500W output. If I
reduce the input power by 9.5dB the output goes down by only 5.2dB. That's
bad.

On 17 with 75W drive my linear will give about 1500W output. If I
reduce the input power by 9.5dB the output goes down by only 7.6dB.
That's not too good but not as bad as 15.

Some info on the amp:

Single GS35B, about 3.7KV, Idle current 210mA. No parasitic choke.
Ameritron plate choke.
Shared PI output for 10 and 12. Shared PI output for15 and 17.
Shared PI input coil for 10/12 and 15/17 but with padder caps
switched in for the lower freq.

One anomaly I notice is that at the lower power input on 15 the plate
current is about 390mA, where as on 80/40/20/12/10 its about 300mA.

At the high power input Plate Current is about 650mA on all bands and
grid current is around 350mA. At the low power input grid current is
zero on all bands.

Any thoughts.

- Paul   KW7Y

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