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81

Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by sluckey on April 14, 2026, 12:42:18 pm »

Hmm. What do you think?      

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 12:34:08 pm »

I’ll give it a try.  Is E the figure I measure for the voltage drop?

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by sluckey on April 14, 2026, 11:48:09 am »

The original looks like a 3 watt that Hoffman sells. Put the 10K in then measure the voltage ***ACROSS*** it. Use the power formula P=E²/R to calculate actual wattage dissipated. Double the calculated wattage for safety.

EDIT... Many times when a resistor gets hot, the color bands will change a bit. The colors don't have to change much to make orange look like brown.

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 11:28:33 am »

If I replace that 100 ohm resistor with a 10K, what wattage should I use, and what might the resulting voltages look like?

That resistor links to the C+ 100uf cap that I had to replace because it was shorted to ground.

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 11:24:00 am »

Sure. In my Reply 78 above, the grids which are pin 5 of the 6L6’s read -50VDC.

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by astronomicum on April 14, 2026, 11:18:28 am »

No, it only lights up if the power tubes are installed.  If I remove the 2 of them, and even if I leave all the other tubes in, then no light up on the current limiter.

…verify voltages are good on pins 3, 4, and 5. Check voltages with tubes pulled…

I apologize if I missed it but what were your voltages on pin 5?

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 11:07:57 am »

If you look at the other large resistor above those 2 caps, it appears to read 470 ohms, and that’s what it also measures.  But it may be serving a different purpose.  It’s connecting to the B+ node. 

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by shooter on April 14, 2026, 11:02:36 am »

Brown, Black, Brown

when you HEAT up RED or ORANGE, they tend to turn brown 1st, then they smell, then they......

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Tube Amp Building - Tweaks - Repairs / Re: minor Major 7 combo

« Last post by GroundhogKen on April 14, 2026, 10:33:32 am »

I think your driver voltages look reasonable based on the 12au7 datasheet.   And you did say it sounds good.
The B+ voltage on a real Major is close to 600.   The 12au7 plates on this schematic (see page 2) are 250v.   https://el34world.com/charts/Schematics/files/Marshall/Marshall_major_1966_200w_pa.pdf
I say if it ain't broke, don't fix it.
Nice job.

Ken

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 10:28:09 am »

If you look at Reply #14 of mine above, in the first image posted there, the resistor in question is the big one directly below the 2, Black, B+ node filter caps.