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Old 22nd November 2005, 12:29 AM   #1
Trout is offline Trout  United States
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Default New Nightmare (Guitar Amp)

Oh Boy, Here We Go again.

After setting aside the champ I had problems with, I decided to finish another project I had partially done.

I had a bunch of PT's off some old consoles that seemed about right for a deluxe 5E3 fender kind of project.

Well, Its done, but something's not exactly correct. This time, I pulled off lots of voltage refs to possibly aid in the mystery. The real problem is I am not exactly sure what voltages are correct to the original as the original schematic had no voltage references.
Here's what I have

The PS in the green outline is what I have built, The original is still shown.

I am suffering from very low output . What I get is about 1W at full volume setting and It is rather distorted which running at full volume would be considered about normal on this amp.
The 6V6GT's seem to get rather hot, Nothing is glowing but the glass seems a bit warmer than I normally see on my other amp. Might Be nothing, I dropped in 6L6GC's but it really did nothing.

Gene
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Old 22nd November 2005, 12:42 AM   #2
anatech is offline anatech  Canada
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Hi Gene,
The plate supply seems higher than "book" normal, but you have more negative bias to compensate (VTL ish). I thnk the 6V6 wants a higher plate load impedance (around 8K in the book). Still, you should get over a watt easily.

Are you getting full drive to the output plates? 'Scope it if you can.

-Chris
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Old 22nd November 2005, 01:02 AM   #3
Trout is offline Trout  United States
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Quote:
Originally posted by anatech
Hi Gene,
The plate supply seems higher than "book" normal, but you have more negative bias to compensate (VTL ish). I thnk the 6V6 wants a higher plate load impedance (around 8K in the book). Still, you should get over a watt easily.

Are you getting full drive to the output plates? 'Scope it if you can.

-Chris
Ok, I am not much of a wiz, So I take it plate load is the OPT right?

This OPT is Triodes drop in replacement @ 6600ohm. Slamming 6V6's with 350V is fairly common in guitar amps. Is the 25V reading possibly High?I am not finding many schematics similar to this with voltage refs.
One thing Nice,, No Oscillations!! This things dead silent idle. but then again, Its almost silent running too
Quote:
Are you getting full drive to the output plates? 'Scope it if you can.
I am not exactly sure what your asking,

I am going to recheck (again) all connections and make sure I havent missed anything.
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Old 22nd November 2005, 01:07 AM   #4
anatech is offline anatech  Canada
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Hi Gene,
Run in a sine wave and look at the drive to the 6V6 grids with an oscilloscope. Good drive and bad output means the output circuit, bad drive means something else is wrong.

Your 6K6 transformer is lower than the "book" 8K transformer (too low impedance), but it might be okay. Tubelab may know since he builds guitar amps often enough.

-Chris
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Old 22nd November 2005, 01:14 AM   #5
Trout is offline Trout  United States
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I Have a second working unit.

HAHA, here we go again. The other unit is deadly loud with the same OPT. So, I kind of ruled that out.

Im going to beat this one I SWEAR I Will !! lol

Ya know I sat here and copied other unit piece by piece , wire by wire, pin by pin, and got 2 entirely different results. Other than the brand name on a few caps, the only real difference is the PT. Based on my voltage readings, I think the PT is putting out plenty
Even the chassis layout is Identical.
Gene ( dazed and confused )
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Old 22nd November 2005, 01:56 AM   #6
PB2 is offline PB2  United States
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I suggest a table of cathode, grid, and plate voltage for the working and non working amps, for each tube, tubes in circuit.

Pete B.
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Old 22nd November 2005, 02:27 AM   #7
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You seem to have the worst luck with guitar amps. The reason that the 6V6's are getting hot is the fact that they are running at (or slightly above) their maximum ratings. Your voltage readings point this out. If you have 25 volts at the 6V6 cathode, and a 250 ohm resistor to ground, mister ohm tells us that 100 milliamps are flowing. The tubes have 355 - 25 = 330 volts across them. Uncle ohm tells us that 33 watts are dissipated through the tubes, or 16.5 watts per tube. The book says 14 watts plate + 2.2 watts screen or 16.2 watts max, per tube. This assumes that both tubes conducting equally. This will make the tubes wear out quickly, but does not explian the low power.

The DC voltage readings that you give do not indicate an obvious problem. I would suggest a comparison with the good unit to make sure. Check the grids (pin 5) of the output tubes. They should be zero to slightly negative, any positive voltage here would indicate a leaky cap, which would cause a weak distorted output. Check the wiring of the output transformer, especially if you used the extra tap to feed an extention cabinet jack like was done on some old Fenders. A shorting jack here will cause a weak distorted output.

The 6600 ohm transformer will give you more power output than 8K, with slightly more distortion, especially when cranked hard. I have used 6600 ohm transformers with 6V6's primarilly because I have a bunch of them, but they work OK.
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Old 22nd November 2005, 03:33 AM   #8
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"One thing Nice,, No Oscillations!! This things dead silent idle. but then again, Its almost silent running too"

Just because you can't hear anything does not mean that there are no oscillations. Given the problem of low, distorted sound, hotter than normal running of the finals, and the "dead silent idle", I strongly suspect RF oscillation. Either 'scope the output, or if you can't do that, try running it near an SW receiver or TV set. Either way will turn up RF.
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Old 22nd November 2005, 03:34 AM   #9
Trout is offline Trout  United States
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Oh Darn,

Looks like all my transformers are always to big,,

I Updated schematic info , Items in pink are volts from working unit.
Items in red are the non_working unit.

I forgot how loud the good one was untill I just twanged out a few.

It appears I have to much transformer AGAIN~!!
I am surprised nothings melting now looking at the differences.
Quote:
Check the grids (pin 5) of the output tubes. They should be zero to slightly negative, any positive voltage here would indicate a leaky cap, which would cause a weak distorted output
Pin 5 Reads Dead "0" Volts

Quote:
If you have 25 volts at the 6V6 cathode, and a 250 ohm resistor to ground, mister ohm tells us that 100 milliamps are flowing. The tubes have 355 - 25 = 330 volts across them.
OOOPS, thats a 220 Ohm resistor I didnt have a 250.

Quote:
Check the wiring of the output transformer, especially if you used the extra tap to feed an extention cabinet jack like was done on some old Fenders. A shorting jack here will cause a weak distorted output.
I have a selector switch for 4 or 8 ohm. I never put in the shorting jacks, I just be sure to never turn on until everythings plugged in correctly.

Now,, How can I tame this beast?
Gene
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Old 22nd November 2005, 04:00 AM   #10
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A cheap choke (high resistance) will drop you some volts, probably not enough.

To test your theory you could add a 250 ohm 10 W from rect pin 8 to first cap, then 10K 10W from first cap to ground. Full power will be limited by this so you'll probably want to look for a better solution.

Rebias and run 6L6GC or 5881s is a better solution.

Adding capacitance across the first cap will up your peak power before the supply sags.

Pete B.
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