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Old 24th July 2006, 04:25 AM   #11
Trout is offline Trout  United States
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Quote:
Originally posted by poobah
Gene,

I didn't see the PSU on your schem. But I think a bad/incorrect (shorted?) inductor could send your B+ through the roof.

Just a thought...

Did the amp sound right?



EDIT: Yeah George.... hold up on the "O" word -

The Amp sounds great, Totally normal in fact. Very loud, I did not use a choke in the PS, The schematic posted was just more or less the basic output section and bias supply.

Oh please do not use the "O" word lol.
I will know more tomorrow after I get a couple new meters, Right now its more or less guessing.

1 question though, How can I get the plate voltage and not blow another meter?
My notes show my other amp has a bias of -40.1V and plate of 397V. Good ballpark numbers.

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Old 24th July 2006, 05:29 AM   #12
Duo is offline Duo  Canada
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Well, if you have a good meter, 400V should not fry it unless something is terribly wrong...

I know asking you to use a fluke 179 or something is a bit much; but most entry level digital meters will handle something near 1000V unless they're really low end...
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Old 24th July 2006, 01:46 PM   #13
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The cheapie Harbor Freight meters are good to 600 volts. If you want to test a higher voltage use a voltage divider. I use two 100K resistors in series (yeah there is a small error) with the meter across the bottom one. Double the meter readings. This will protect the meter in case there is "something" causing a high "RF" voltage on the plate. If the readings are grossly wrong, suspect "something". The plate voltage should be slightly less than the supply voltage with no signal input.

It is possible for "something" to occur and the amp to function normally. There are other possibilities, but if the meter was on the voltage function, "something" caused it to draw enough current to fry. The impedance of the meter is high at DC but not at "RF".

On the other hand, maybe the meter's time was up, and it just blew up.
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Old 24th July 2006, 04:17 PM   #14
Trout is offline Trout  United States
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OK,

I got a few of those harbor freight multimeters. They seem to work well in fact!..

I ran around the not mounted extra turret board and checked all the resistors with all three meters, Them do indeed seems to match each other very well.

Now,

I'll start at the PS, No output tubes or preamp tubes, Just the rectifier(5U4).

Next I will put in the rest of the tubes, and re-check the voltages at the PS again.

It is possible that older meter was ready to go, But I find it odd that it did it when it did.

Here are a couple pictures,



The Older Working Unit

The New Not Working Unit

Basically, there are only a few layout differences.
#1- I moved the ground buss up onto the board requiring 2 less chassis holes.

#2- I moved the bias pot up onto the board requiring 1 less hole and making it less accessible to non-qualified fingers.

#3 Used a much sturdier board material, The original was 1/16th perfboard, The new unit has 1/8th inch garolite.

#4 Used all 20awg wire for hook-ups rather than the older 22awg I had originally used.

Other than those items, Everything is basically the same.
I cant help but to think I botched the under board wiring.
I ran around it with a meter and of course some readings interact but I do show connections where they are supposed to be
( I Think)

I'll post up the full schematic later, Right now, I want to see if I can actually get so voltage measurements.

Trout
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Old 25th July 2006, 12:33 AM   #15
Trout is offline Trout  United States
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Here is the working units Schematic

After a couple of hours of self debate, I decided clearly something's not right on the board, SO,
I am in the process of a transplant using a spare new board.
Interestingly, I have the board removed and all the wires are indeed correct on the old boards underside.

The only thing I am not certain about are the actual components. Possibly a bad cap, Eh, who knows.
Maybe the schematic is fundamentally wrong.

Should be complete in a few hours.
Results soon.
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Old 25th July 2006, 01:30 AM   #16
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Nothing fundamentally wrong with the schematic. I would add a small resistor in series with the screen grid of each output tube mounted right at the tube socket. I use 100 ohm 1 watt. It helps prevent "something" from happening.
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Old 25th July 2006, 03:16 AM   #17
Trout is offline Trout  United States
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Quote:
Originally posted by tubelab.com
Nothing fundamentally wrong with the schematic. I would add a small resistor in series with the screen grid of each output tube mounted right at the tube socket. I use 100 ohm 1 watt. It helps prevent "something" from happening.
OK, I think I understand this, I have seen this resistor setup commonly on Marshall clone amps, But there was just 1 shared 100/3W like in this draft.

Giving a peek at my layout, A single resistor would drop right in between the board & the socket very easily.

How about moving the CT of the OPT to 1st cap in the PS? I have seen both my original and the red edit in this draft.

Rough Draft

I am on break LOL, I have the new board 1/2 installed and momma made me take her to Dinner.

Trout
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Old 25th July 2006, 11:06 PM   #18
Trout is offline Trout  United States
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OK, Transplant complete!!

After a complete inspection of the underside of the eyelet board I feel a little better, It was wired correctly

Now, The new board was made with turrets rather than eyelets and much easier to install and replace parts if needed.

The new EL Cheapo Harbor freight meters work great.

I pulled voltages off The PS & the KT77's also added them to
This Schematic

In summary, I have not yet added the screen resistors since I wanted to not make to many changes.

Error #1

I re-checked several items including the speaker, 15.2 ohms on the meter means it must be a 16ohm, not an 8ohm. The opt only has 2-4-8 ohm taps.

My new 100W Jensen Neo speakers will not be here until friday, My old celestions are history due to prolonged overexposure to excess wattage! LOL

I am still getting the glow at the screen grid but, volume has to be set fairly loud to occure, At idle with no instrument plugged in, there of course is no issue.

Voltages seems odd.

Pin #3 432V
Pin #4 434V
Pin #5 35V

Center point on bias pot reads 43V

V3 Voltages seem odd?

Pin #1 216V
Pin #2 0V
Pin #3 1.8V
Pin #6 276V
Pin #7 4.49V
Pin #8 64V

So, Thats where I am this minute, Mild pulsating glow at mid-upper volume settings with mis-match output impeadance.

Trout
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Old 25th July 2006, 11:29 PM   #19
poobah is offline poobah  United States
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Voltages on V3 look pretty good... you can't really measure the voltage at the grid of Stage 2. It is a grid leak circuit and the meter loads it down and gives a false reading...

Output stage all looks pretty normal too. Just to be picky... you should express the grid and bias voltages as negative.

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Old 26th July 2006, 12:42 AM   #20
Trout is offline Trout  United States
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Quote:
Originally posted by poobah
Voltages on V3 look pretty good... you can't really measure the voltage at the grid of Stage 2. It is a grid leak circuit and the meter loads it down and gives a false reading...

Output stage all looks pretty normal too. Just to be picky... you should express the grid and bias voltages as negative.


OPPPPPPs. I forgot the negative symbol lol

I am a bit puzzled about what would be considered the correct voltage at those bias points, I do not know the correct " Ideal " Setting.

Here is the exact spec sheet for the tubes I am using
KT77 Specs PDF

I know there has to be a formula to determine this at least to a ball park setting.

Now, The pulsating glow from the tube, Could that be as a result of the impedance mis-match? That under normally matched speakers I am already on the edge and this is just pushing it over the top?

Just for peace of mind, perhaps I should add the 100/3w resistor to the screen just as a precaution?

This is basically my first ventures into adjustable fixed bias so please forgive my lack of experience.
I only went this direction to grab the bit of extra power Click the image to open in full size.

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