67 Super Reverb ugly distortion

[J M Fahey]

[ATTACH=CONFIG]55354[/ATTACH]
15 VRMS
Overload
(2 Ohm/2 x 6L6GC)

Load Ohm	Power W
2	112.5
4	56.25
8	28.12

For FSR/ 50W/ 2 Ohm
Speaker voltage 10 V RMS

For FSR/ 70W/ 2 Ohm
Speaker voltage 11.8 V RMS

Read my answer in context.

Are there places to avoid with the scope so I don’t damage it?

In principle, measure at speaker out only, easy 15 V RMS or so.

OP is NOT asking me what´s the amplifier power output, what is the overload point, NONE of that.

He´s worried about damaging his scope, which is reasonable because there are hundreds of volts inside.
So I tell him not to worry, it´s an easy job, I am close enough by saying "15V or so".

Obviously I am wasting my time here.

[earache]

JM and all, you’ve made loads of effort to explain and help me figure this out - thank you! Now I’ve got to get the load resistor sorted out and I will proceed from there, most likely after my short vacation, returning Tuesday

[earache]

So I’ve got part of the gear sorted out I think. While waiting for Amazon to send the resistors that you suggested (thanks Chuck H), I found these resistors at my work



They measure exactly at 2ohms in parallel. They are rated at 50 W each, what is their power rating in parallel?

I dont have a signal generator, so here’s my signal source - my old iPod...



So now to make a jack that I can solder to the resistor leads...

[The Dude]

3) 8 ohm 50W resistors in parallel would be a 2.666 ohm load with 150W capability. You'll need one more if you want exacty a 2 ohm load. They are 1% resistors, so I'd believe the stamp over the meter. If you're not concerned about accuracy, they'd probably be close enough. Also, for them to actually be able to safely dissipate full power, you'll need to mount them on some sort of heat sink. Mine are on a big aluminum plate.

[earache]

JM Fahey

After rereading your excellent post more thoroughly, I will try to measure and post the waveform(s) using the original speakers as you suggested.

[earache]

Thanks Dude!

[Enzo]

It takes four 8 ohm resistors in parallel to make 2 ohms.

[earache]

So, here’s some scope readings taken across the speaker jack, using the iPod Touch as a sound source.

If there are obvious errors in procedure, please let me know, just learning how this stuff works...

First I measured the output of the iPod and tried to set it to 500mA



The scope is a digital Tektronix set to auto ranging. The scope probe was clipped across the speaker output



Here’s a picture of the scope reading with the signal into the amp and the volume barely cracked.
The iPod is playing a 440hz tone.



Next is the Amp turned up to 4 and that was as loud as I could stand...it actually looks better! It didn’t sound distorted that way either...



This made me think it has to be in the attack of the note, so I decided to play the guitar into the Amp and take a video of what was going on

[earache]

Hmmm can’t seem to upload the video from the IPad...

[earache]

Hmmm can’t seem to upload the video from the IPad...

I will get this video up on Dropbox later today

[SoulFetish]

Save a lot of time and SCOPE IT.

Friggin’ THIS ^^^^!!

Run a 1kHz sine wave in the input, and check the output on a scope. This will tell you alot. For instance, whether or not you’re only getting one half of the pushpull signal output. If both sides are conducting, then you should see a fairly clean/symmetrical sine wave to the point of clipping. If you’re getting a clean output like I just described, then you know that you’re PI tube, 6L6s, PI plate resistors, and screen resistors are all good enough to not likely be the cause.

[SoulFetish]

It takes four 8 ohm resistors in parallel to make 2 ohms.

OR you can run all three 4, 8, and 16 ohm loads together in parallel to get 2 ohms (~about)

[Chuck H]

Friggin’ THIS ^^^^!!

Run a 1kHz sine wave in the input, and check the output on a scope. This will tell you alot. For instance, whether or not you’re only getting one half of the pushpull signal output. If both sides are conducting, then you should see a fairly clean/symmetrical sine wave to the point of clipping. If you’re getting a clean output like I just described, then you know that you’re PI tube, 6L6s, PI plate resistors, and screen resistors are all good enough to not likely be the cause.

???

Isn't that just what was done in post #38? Well, except that he used a 440Hz signal instead of a 1000Hz signal. Anyway... Symmetry achieved as far as I can tell.

What does look odd is the peaks of the wave form. Thick and hashy. Not sure if this is a pixel artifact and is just ripple or if there's some ringing already present as the wave form barely begins to clip. That low level signal looks strange and seems to indicate a silly high HF peak. So there may be something to this. Some kind of positive feedback that's making the amp unstable. But I'm just thinking out loud now based on limited information.

[SoulFetish]

???

Isn't that just what was done in post #38? Well, except that he used a 440Hz signal instead of a 1000Hz signal. Anyway... Symmetry achieved as far as I can tell.

What does look odd is the peaks of the wave form. Thick and hashy. Not sure if this is a pixel artifact and is just ripple or if there's some ringing already present as the wave form barely begins to clip. That low level signal looks strange and seems to indicate a silly high HF peak. So there may be something to this. Some kind of positive feedback that's making the amp unstable. But I'm just thinking out loud now based on limited information.

No doubt. I saw Juan’s post it got a little excited. I made the classic mistake of not seeing page 2

[Jazz P Bass]

"First I measured the output of the iPod and tried to set it to 500mA" (mv?)

That is a pretty 'hot' signal level.
I would go with 100mv.