[Amps] Time to ask for help.

[TexasRF at aol.com]

Hi Roger, a couple of thoughts come to mind. The 400 mA idle current is  
problematic. This would not cause an oscillation but it allows the tubes to  
operate pretty much at maximum gain and that can make the taming process more 
 difficult. In addition, the plate dissipation rating is being exceeded 
before  there is even any rf drive applied. There may be thermal run away if 
the screen  voltage is not stiff enough. The negative screen current will try 
to charge the  screen voltage upward and depending on the screen voltage 
source impedance can  rise as high as the plate voltage.
 
Under these conditions the tube gain is way greater than that  expected.
 
The purpose of neutralization is to purposely create a feedback path from  
plate to grid that is of opposite phase in respect to the normal internal 
grid  to plate feedback path. In addition, the amplitude is supposed to be 
such that  the internal feedback path is effectively canceled.
 
You have the perfect test equipment to make the test and adjustment. This  
is not related to any parasitic suppression as that is a separate issue. 
While I  don't have direct experience with this aspect of using a 4CX250, I 
have tested  other larger tetrode tube amplifiers. They showed about 40 dB 
input to output  isolation when an rf signal was connected to the input and 
measured at the  output. Also, as a check. the signal can also be applied to the 
output and  measured at the input. Of course this is done with the 
amplifier completely cold  and at the expected operating frequency.
 
I would expect the neutralization null to be fairly narrow band but that  
will depend on the degree of neutralization needed to make the null  happen.
 
Once you get to this point, you can then start looking at possible  
parasitic oscillation issues. The 4CX250 family of tubes are quite small  
physically so it would seem that they would be a lot less prone to parasitics  than a 
larger glass tube that inherently has more inductance in the  internals.
 
Actually, I am thinking your root problem is more likely due to a run away  
issue as discussed above.
 
The idea of swamping the grid is a valid method of reducing the gain but if 
 that does not help, that again points to the run away scenario.
 
Please let us know how you make out.
 
73,
Gerald K5GW
 
 
 
In a message dated 7/25/2015 3:20:45 P.M. Central Daylight Time,  
ai7rogerroger at gmail.com writes:

I've  been building a 6m amp from scratch based on quad 4x150a's for more
than 3  years. I've never built any type of rf producing device before  and
unbeknownst to me at the time, chose a project that was way over my  head.
I've read and studied volumes of material written on 6m amps and  tetrodes.
The power supply was also built from scratch and is sound. It has  been
successfully tested on a 2m rf deck running a pair of 4cx250b's in  push
pull. HV: 1680vdc, Screen: 360vdc, Bias: ~ -50vdc, when not jumping  off the
bench idle current 400ma. Though not currrently designed for  individual
adjustment and not perfect, the tubes have been selected and are  very close
to matching.  I'm at the point of irradicating runaway,  oscillations and/or
parasitics. I have access to quality test equipment and  am fairly
knowledgeable with its use. I'm conducting a cold test using a  tracking
generator with spectrum analyzer to observe the output while  injecting a
signal into the grids. This test is suggested in Care and  Feeding Chap
5.5.2 (Feeding the Grid Circuit). This states: "The  neutralization can then
be adjusted for minimum feed through." One question  of many: 1) Is the
author suggesting that the tester knows the frequency of  the parasitic and
that is the frequency being injected and neutralized?  Continuing. I have
indications of runaway, in that, when the amp is keyed  without excitation,
excessive grid, plate and negative screen current is  drawn. Some of the
fixes I've tried include raising the cathodes off ground  by as little as 10
and as much as 33 ohms; various configurations of  parasitic suppressors,
grid swamping with various values of resistors (all  non inductive) and
different methods of neutralization.

I have no  intention of dropping this project, but would really appreciate
any advice  or suggestions beyond that to rein this thing in.

Thanks so much for  your time.

73,  Roger
AI7RR
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