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D150 monster amp and funny tooobz
D150 monster amp and funny tooobz
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Old 17th September 2019, 05:05 PM   #71
koja is offline koja  Canada
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last update: reassembled the amp with no OP tubes. applied power. the mains measured 126V on the front dial. measured voltage at pin 5 of V21/V22 grid Vregulator sockets and got 345V (350V called out on the schematic). So the 7 zeners checked out. then plugged in two newly purchased Sovteks into V21/V22 and measured voltage at pin 4 of all OP tube sockets and got 374.5V (nominal 360V on the schematic).

p.s. just by looking over the smaller tubes on the back plate, all had some glow in them except for the four 12AX7s in the middle. not sure of their intended function but I guess that was normal without the OP tubes biased for work?

so I guess the amp is ready for plugging in the original 8 GE6550As. I will remove the plate and screen fuses and run only the heater on them for some hours. And then we see about the next step of getting the anode voltage up there.

p.s. another thing which i noticed last time and it bothered me: when the clamps are applied to the tube bases they tend to pull on the tube and pull the pins out on the opposite side. Not sure how to get around that unless I just plug them in and let the clamps hang loose?
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Old 17th September 2019, 05:09 PM   #72
rayma is offline rayma  United States
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The 12AX7s are likely heating normally, feel the envelope for warmth.
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Old 30th September 2019, 01:32 PM   #73
koja is offline koja  Canada
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finally ran the tubes at the 120V PS setting but with all bias trimmers turned all the way down. Nothing sparked. However when I switched the knob to look at individual tube bias current the front indicator showed nothing. (in the manual it says the trimmer are capable of settings between 25mA and 85mA which puts the recommended 55mA in the middle. So I expected the hand on the dial to start showing something immediately. I turned the trimmers up (clockwise) about a 1/4 turn and still no indicator movement.

@Rayma: do you think the dial circuit might be faulty? or it takes turning trimmers more before they come alive? I am afraid of turning them up and overbiasing if something is wrong by any chance.

p.s. another thing that bugs me is that at approximately the same trimmer setting some tubes feel like they are running hotter than others (just by feeling the envelope with hand). Thx.
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Old 30th September 2019, 02:57 PM   #74
rayma is offline rayma  United States
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Quote:
Originally Posted by koja View Post
do you think the dial circuit might be faulty? or it takes turning trimmers more before they come alive? I am afraid of turning them up and overbiasing if something is wrong by any chance.
Check the voltage directly across the 1R 3W current monitor resistors.
This should vary with the pot setting, nominal 50 or 60mVDC each.
Unless the pot is turned up enough, there will be little or no reading, but be careful.
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Old 10th October 2019, 06:43 PM   #75
koja is offline koja  Canada
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forgot to post the last update: it turned out that the front knobs (5 pole switches) for pointing dials to each tube for bias adjust were (still are) an issue.

After rotating them back and forth like crazy I got the left one to work reasonably well i.e. measure repeatably and I set the bias on those tubes. the right dial only works in one position though. in the two middle positions when I jiggle the shaft (there is some plunge free play) I can get the hand on the dial to jump up and measure something, but in the end left position (no free play) the dial just sits at the bottom. I ordered DeoxIT on amazon and will try that next but I am not so sure that it will fix it. AR should have really used gold plated contacts for this price range amp .
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Old 10th October 2019, 06:47 PM   #76
Zen Mod is offline Zen Mod  Serbia
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D150 monster amp and funny tooobz
in most cases contact sprays/solution can bring more trouble than benefit

sole proper way of solving that problem more permanently is to disassemble switch and clean it (medical benzine or similar solvent) , then grease again with Vaseline , either technical or medical
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Old 10th October 2019, 08:46 PM   #77
rayma is offline rayma  United States
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Quote:
Originally Posted by koja View Post
I ordered DeoxIT on amazon and will try that next but I am not so sure that it will fix it.
AR should have really used gold plated contacts for this price range amp .
Gold contacts would not be suitable for a power switch like these. This amplifier is 43 years old, and would
be expected to require service for proper function. Do not operate the amplifier further until the contacts
are properly cleaned. Do not rely on only moving the switch to clean the contacts. Follow Caig's instructions carefully, and do not spray any other part of the amplifier than the switch contacts. Do not spray into the tube sockets, or near the pc boards.

Last edited by rayma; 10th October 2019 at 08:49 PM.
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Old 11th October 2019, 12:35 PM   #78
koja is offline koja  Canada
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Ok. I will take a closer look. I believe the switches are mounted on pcbs behind the front panel which I was hoping not to have to take apart. so I was thinking of just sticking the straw in (I could see little openings on the switch body) and just spraying in. It is also pretty dusty there and hard to reach. maybe I use some compressed air first.
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Old 11th October 2019, 12:56 PM   #79
Zen Mod is offline Zen Mod  Serbia
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I told ya

better attack them properly right now , then later ....... cleaning what you need now , plus contact spray residue and mess
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Old 11th October 2019, 02:41 PM   #80
rayma is offline rayma  United States
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Originally Posted by koja View Post
It is also pretty dusty there and hard to reach. maybe I use some compressed air first.
No compressed air, it would make things worse, only vacuum if available.
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