HRM needs some Lovin'

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martin manning
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Re: HRM needs some Lovin'

Post by martin manning »

2.8V seems low given the heater winding current capacity.
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didit
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Re: HRM needs some Lovin'

Post by didit »

martin manning wrote: Tue Jan 02, 2018 8:54 pm 2.8V seems low given the heater winding current capacity.
Unloaded, it should be at least 3.3-3.5V on that Hammond. What's the voltage across the "heater" tabs on the first power tube socket in the string?

Best .. Ian
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

2.8v unloaded and 2.7v when loaded.

I have hooked the OT 2ndaries to the power tube though. but I did not connect the OT primary. Should I remove these in order to take my measurements correctly?


Humble and Dumble,

Fred.
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pompeiisneaks
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Re: HRM needs some Lovin'

Post by pompeiisneaks »

Make sure you're measuring AC across the two sides of the tube, not with reference to ground or you'll get half the voltage (Ex on a 12AX7, measure on pin 9 and over to pin 4/5 not from say 9 to ground or 4/5 to ground). This is also AC right, not DC?

~Phil
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HRM needs some Lovin'

Post by fred.violleau »

Another lesson down today [emoji6]

I measured with reference to ground, in AC (got that right!). I have a steady 6.1v on all the heaters without tubes in the sockets.

Thanks Phil, Martin and Ian !


Humble and Dumble,

Fred.
Last edited by fred.violleau on Wed Jan 03, 2018 4:28 am, edited 1 time in total.
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martin manning
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Re: HRM needs some Lovin'

Post by martin manning »

You can measure either way. In fact most people will want to check both sides to ground, as you did, to confirm that the voltage is centered. Your voltage still seems to be too low, though. Here's the spec sheet for 274BX: http://www.hammondmfg.com/pdf/EDB274BX.pdf You can see the no-load voltage in the lower-right corner: 3.42V to the CT on the 6.3V winding (Green to Green-Yellow), 6.84V Green-to-Green. Is you meter working correctly? Replaced the battery lately?
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

Thanks Martin for the infos on the PT. Mine was generic, and not as detailed. I have not changed the battery on my multimeter since I put one in, so I might do that right before taking new measurements.
I am still puzzle on the fact that my yellow wires do not return any value... Gets me worried about the possibility I damaged some filament when I removed the PT from the donor amp, akthough I do not recall any brutal force used nor anything going wrong...

Is there on the forum a good schematic of the HRM diagram with voltages?

I am trying to give a name to the connectors that expose through the chassis the bias pot values. Don't know how to call these, and I want to put some so I can adjust my bias without having to remove the amp from the headcab. I am sure there are some on mouser.com, but need a name ;)

Thanks again for your time and expertise, it is much appreciated.

Humble and Dumble;

Fred.
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Re: HRM needs some Lovin'

Post by sluckey »

I am still puzzle on the fact that my yellow wires do not return any value
That's probably because the yellow wires are not referenced to chassis. Set meter to AV volts. Connect one meter lead to one of the yellow wires and connect the other meter lead to the other yellow wire. Should read correctly now.
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martin manning
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Re: HRM needs some Lovin'

Post by martin manning »

For the bias test points, look for a 4mm banana jack. (edit: not 3mm)
Last edited by martin manning on Sun Jan 07, 2018 1:22 pm, edited 1 time in total.
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

Thanks for jumpin in Sluckey!

Battery changed on my multimeter, here are the readings:
- Red on red : 758v, 378v with reference to ground
- Yellow on yellow : 4.7v, 0v with reference and to ground
- Green on green : 6v, 2.8v with reference to ground

On my main AC switch : 120v

All readings are lower values than what they should be... is this a potential problem along the way?

Thanks Martin for the reference!
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pompeiisneaks
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Re: HRM needs some Lovin'

Post by pompeiisneaks »

another thing to check would be your incoming wall voltage, if it's a bit low, say 110 or 108 it may be causing the slighly low voltages. Basically the transformer is probably set to expect 120V incoming?

~Phil
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fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

pompeiisneaks wrote:another thing to check would be your incoming wall voltage, if it's a bit low, say 110 or 108 it may be causing the slighly low voltages. Basically the transformer is probably set to expect 120V incoming?

~Phil
Phil, I measured 120v on the main ON/OFF switch right after the fuse, so it should give me the wall reading right?

The transformer is set to expect 120V (I used the black wire for 125V instead of the grey wire that accepts 110V).


Humble and Dumble,

Fred.
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didit
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Re: HRM needs some Lovin'

Post by didit »

fred.violleau wrote: Thu Jan 04, 2018 7:53 pm Thanks for jumpin in Sluckey!

Battery changed on my multimeter, here are the readings:
- Red on red : 758v, 378v with reference to ground
- Yellow on yellow : 4.7v, 0v with reference and to ground
- Green on green : 6v, 2.8v with reference to ground

On my main AC switch : 120v

All readings are lower values than what they should be... is this a potential problem along the way?

Thanks Martin for the reference!
Transformer has two "hot" primary feeds, one spec for black=125VAC & another grey=115VAC. And 758V HT secondary is within 0.1% of expected for black with 120V in. But 2 x 378V is 2V below, which suggests something amiss. A few millivolts of difference might be expected. Something with your grounding or with how the probes are applied?

Reason I asked a few days ago for direct measurement of filament primary (green-green) was to eliminate poor grounding as an issue, as 2.8 is again not half of 6V; which by-the-by is ball-park correct for 120V feed on a 125V primary, except unloaded it should be about 8% higher. All to say the transformer outputs seems close to, if not 100%, fine.

Suggest going carefully through the grounding fasteners & wiring, and see what could be limiting good conductivity. Also, still wonder about calibration/accuracy of the multimeter. Is it digital or old-fashioned gauge?

Best .. Ian
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norburybrook
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Re: HRM needs some Lovin'

Post by norburybrook »

Is your Chassis anodised?

You really need to dremell away the anodising for your ground points on the chassis and I would add a separate earth connection from the output jacks etc if you're expecting them to ground through the chassis.



Marcus
fred.violleau
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Re: HRM needs some Lovin'

Post by fred.violleau »

didit wrote: Transformer has two "hot" primary feeds, one spec for black=125VAC & another grey=115VAC. And 758V HT secondary is within 0.1% of expected for black with 120V in. But 2 x 378V is 2V below, which suggests something amiss. A few millivolts of difference might be expected. Something with your grounding or with how the probes are applied?
I rechecked my wall AC is now 119.3v so it seems to fluctuate quite a bit.

Also on the HT readings, one filament is 378v and the other was 379v.

As for my probes, the red one melted a little ( i don’t remember when but long ago) so it’s definitely not as sharp as it used to be.
didit wrote: Reason I asked a few days ago for direct measurement of filament primary (green-green) was to eliminate poor grounding as an issue, as 2.8 is again not half of 6V; which by-the-by is ball-park correct for 120V feed on a 125V primary, except unloaded it should be about 8% higher. All to say the transformer outputs seems close to, if not 100%, fine.

Suggest going carefully through the grounding fasteners & wiring, and see what could be limiting good conductivity. Also, still wonder about calibration/accuracy of the multimeter. Is it digital or old-fashioned gauge?

Best .. Ian
My multimeter is digital, but probably low quality (less than 30$ probably).

I will try to redo the PT grounding and see if it does any good.


Humble and Dumble,

Fred.
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