New Nightmare (Guitar Amp)

[Trout]

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

 

[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

 

[Trout]

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

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.

 

[anatech]

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

 

[Trout]

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 )

 

[PB2]

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.

 

[Tubelab_com]

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.

 

[Miles Prower]

"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.

 

[Trout]

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.

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

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.

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

 

[PB2]

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.

 

[Trout]

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.

6L6's arent out of the question, Triodes data on the opt states:

Response flat within 1 dB 50 Hz-20KHz at 20 watts, you can actually put more than 20W through it at a more limited bandwidth, eg: at lowest note on a lead guitar (approx 80 Hz), this transformer can pass 50 watts or more.

But, I still have way to much verb hitting the 2-12ax7's?

I am guessing Even the working unit is hitting the 6V6's pretty hard. But its within the grey area at least.
Gene

 

[PB2]

Change the 22K in the PSU to 33 or 39 K if you want closer to original voltage on the 12AX7s. This would be without the 250/10K in the PSU of course.

The Delux Reverb AA763 ran 6V6GTs with 415V on the plates, probably not a good idea, but the bias was lower.

Increase the cathode resistor until you get about 37V across it, this will lower the bias. 430 - 470 ohms might be about right.

Pete B.

 

[Trout]

OK, Got Ya,

With 6L6GC's in place,

Change the 22K in the PSU to 33 or 39 K if you want closer to original voltage on the 12AX7s. This would be without the 250/10K in the PSU of course.

39K resistor in place.
1st 12AX7 Pin1 194V - Pin6 188V
2nd 12AX7 Pin1 209V - Pin6 258V
Still high but should be a safer range for the tubes?

The Delux Reverb AA763 ran 6V6GTs with 415V on the plates, probably not a good idea, but the bias was lower.
Increase the cathode resistor until you get about 37V across it, this will lower the bias. 430 - 470 ohms might be about right.

Raised the the 220 ohm to 470 ohm, Voltage changed from 25V to now at 33V.

Voltage at plates(pin3) of 6L6's now at 417V
Pin4 386V

Looking at the specs sheet and trying to learn where and why these modifications influence each other, It appears that at these voltages I should be running conciderably higher watts output than I first thought.

Specs Sheet

I know that I really dont understand most of it, But reading the data sheet I see my OPT is ball park.
I am still suffering at about 1W output power. But I checked everything yet again in comparison to the original I built and can find no differences. well other than the PT.
gene

 

[anatech]

Hey Gene,
What does the drive to the outputs look like?

-Chris

 

[anatech]

Gene, it looks like the first section of the preamp is starved for current. You could increase teh 22K resistor to point "C" to bring the plate voltage more in line with the original. The extra voltage will only run the plate hotter.

-Chris

 

[Trout]

Gene, it looks like the first section of the preamp is starved for current. You could increase teh 22K resistor to point "C" to bring the plate voltage more in line with the original. The extra voltage will only run the plate hotter.

-Chris

I raised the 22K to 39K to get it lower already, Compared to the original voltages which werevery high.

39K resistor in place.
1st 12AX7 Pin1 194V - Pin6 188V
2nd 12AX7 Pin1 209V - Pin6 258V

It Was originally
1st 12AX7 Pin1 - 211V Pin6 - 223V
2nd 12AX7 Pin 1 - 233V pin6 - 273V

Didnt seem any hotter than normal on the 12AX7's at either setting, The 6L6's seem a bit hot yet but not near as bad as before.

Is 210ma rectified just plain to much?
Gene



So the 1st 12AX7 is just a few volts over the working unit.
the second 12Ax7 is still a bit high compared to the working unit though.

 

[anatech]

Hi Gene,
What are your pin 3 and 8 readings on the 12AX7's? Also point "C" reading?

The available current isn't a problem that I know of, it's the high point "A" voltage that was bothering the 6V6's before. That and the low grid bias voltage as PB2 pointed out.

-Chris

 

[PB2]

Gene,

Good to see Chris helping you out, I'm a bit busy today.

I think you've got the DC bias OK on all the tubes, and it seems that an AC problem remains.

It could be a wrong resistor value including the pots.
A bad cap.
A miswire, post pictures of both if you want us to take a look.
Bad (defective) OT.
Oscillation, but I hope not.

I don't think the voltages on the 12AX7s were too high, but at least now their nearly the same to rule out that issue. Debug the AC problem, then we can talk about getting the most power out of the amp.

Pete B.

 

[anatech]

Hi Pete,
Hey, feel free to add when ever. Your input is always welcome.

I am curious to see if any stages are starved for current. That will distort and kill gain. A photo comparison of bth amps is a good idea too. When ever I get stuck, it's great to have a second pair of eyes to catch what I missed. Usually staring me right in the face too!

-Chris

 

[Trout]

Gene,

Good to see Chris helping you out, I'm a bit busy today.

I think you've got the DC bias OK on all the tubes, and it seems that an AC problem remains.

It could be a wrong resistor value including the pots.
A bad cap.
A miswire, post pictures of both if you want us to take a look.
Bad (defective) OT.
Oscillation, but I hope not.

I don't think the voltages on the 12AX7s were too high, but at least now their nearly the same to rule out that issue. Debug the AC problem, then we can talk about getting the most power out of the amp.

Pete B.

Ya Know, The Pots never occured to me, Hmm let me re-phrase.
The pots are new as is everything except the PT.
BUT, The pots are from the same order from parts express as the pots that are in my Non-Working mystery Champ !
All three pots in this amp are the same value, 1 Meg.

Just for the sake of making every componet identical to my working unit, I am about 15 minutes from done replacing the only 2 caps that were a different type & brand.

I'll post pics hopefully later or early morning.

Interestingly, this is the third amp I have built using the same output stage and layout, Odd that both the others which are assembled the same work perfectly.

As far as a possible mis-wire, Its a very simple circuit, I have gone over it very carefully. I am 110% sure its wired correctly.

Tomorrows is going to be a non-productive day also. I have to re-arrange the west end of our basement to clear the way for the 10,000 tubes we just bought. I need to get racks/shelves set up for those and the 3000 in our garage. I dont think the un-heated garage is a very good idea this time of the year.

When ever I get into a hobby, I do it large, When I was flying RC areobatics nationally, I had a 4000ft shop/warehouse for fiberglass kit production. 15 years in that hobby was enough, Basically ruined my ears and eyes. Its amazing how much hearing loss you get hovering over a 115db model aircraft engine for a few years.
Couple that to 20years cabinet making, 35 years of guitar playing, Its a wonder I can still hear a siren.

Gene