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1965 GA-30RVT Invader Bias Issue

#1 User is offline   Bobh1959 

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Posted 19 August 2017 - 02:44 PM

I recently picked up a decent condition project Invader amp minus speakers and tubes for $80. I recapped and retubed it, put in a speaker, and fired it up. The tubes were red plating so I shut it down. I looked and saw that someone had changed the screen grid resistors to 150 ohm, instead of 1k ohm. I put them back to 1k, and replaced the old control grid resistors which should be 1.5k ohms. I measured voltages and had 440v plate and -10v control grid, they should be 385v and -18.4v. Does changing the control grid resistor affect the amount of negative voltage to the control grid, and does that affect the plate voltage? I know this may be the whole idea behind biasing but as you can see I have gaps in my knowledge. If it does, any suggestion as to what value might get me to 385v plate and -18.4v control grid? BTW- I have an original manual if anyone wants copies.
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#2 User is offline   badbluesplayer 

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Posted 20 August 2017 - 06:20 AM

The 440v and the 10v that you're reading- is that with the tubes installed?
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#3 User is offline   Bobh1959 

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Posted 21 August 2017 - 03:55 AM

No, that's power tubes pulled. Are the voltages printed on my schematic with tubes installed? If you could just clarify that I'll be good, I won't suck you into another bias thread. If you or anyone knows of any good, condensed, biasing instructions for fixed bias amps I can work off that, I have a Hoffman Amps biasing kit. Thanks.
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#4 User is offline   badbluesplayer 

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Posted 21 August 2017 - 04:59 AM

View PostBobh1959, on 21 August 2017 - 03:55 AM, said:

No, that's power tubes pulled. Are the voltages printed on my schematic with tubes installed? If you could just clarify that I'll be good, I won't suck you into another bias thread. If you or anyone knows of any good, condensed, biasing instructions for fixed bias amps I can work off that, I have a Hoffman Amps biasing kit. Thanks.

Yeah - the voltages on the schematic should be with the tubes installed. The plate voltage will go down once you install the tubes. The control grid voltages shown on the schematic seem funny. The same 18.4 volts is shown at two different points along the voltage feed and that shouldn't be right. Anyway, I'd install the tubes and see if it biases up alright. I believe that the bias probe should work fine with the 7591's. 7591's are rated at 19 watts plate dissipation so 19 watts divided by roughly 400 volts (use the actual plate voltage that you read instead of the 400) is 0.0475 amps or 47.5 milliamps. That's 100% plate dissipation for each tube. Hopefully the amp will be running at say 50 to 80% dissipation somewhere - it's not adjustable, right?

So each tube should read at about 50 to 80% of 47.5 ma, or say about 25 to 40 milliamps. If it's in there somewhere, you're good to go. If not, then you can change a resistor and fix it. The last 2 pages of this instruction sheet explain how to use the probe. Your amp is "fixed bias" but does not have a bias adjustment pot as far as I know.

http://www.mojotone....ction=overwrite

Let us know how you make out and I'll be happy to help more if I can.
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#5 User is offline   Bobh1959 

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Posted 21 August 2017 - 01:26 PM

Wow, you helped a bunch, thanks. My reverb and trem don't work, I've tried several different tubes to no avail. I'm going to just replace every old film cap, including the .022's coming off pin 6. Assuming that may have voltage implications I'll bias after that and let you know.
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