Would You Guys Replace This OT?

[chinrest]

I was hoping to get a few informed opinions before I drop some coin on a replacement OT.

The symptoms:
A slightly harsh high-end when overdriven.

The Goods:
- Hammond 125E
- parallel single ended 6v6
- I've done some tube and speaker swaps.

The History:
This OT has served well in a few different amps since the late 1990s, including a parallel 6L6 SE, and a few tweed deluxe circuits. The OT's previous amp was a somewhat high-voltage deluxe. This amp suffered a gnarly flyback voltage spike from a defective, 150ohm speaker cable. It arced across the 6v6 plate and heater lugs in a dramatic way. The amp wasn't a very successful project to begin with, so I scrapped it for parts.

Basic testing showed that the OT wasn't shorted, and it works in its latest rig, but I'm wondering if the unsavory tone is caused by a haggard OT, and maybe this poor old thing is going belly-up. I hate to throw money at a problem, especially knowing that every amateur thinks the OT needs replacing, and knowing that the harshness could be somewhere else in the circuit.

Seeking sage advice before spending. Thanks.

[J M Fahey]

A slightly harsh high-end when overdriven.

In no way points to a "damaged transformer".

the harshness could be somewhere else in the circuit.

You bet.

[pdf64]

Yes, the only failure modes I can think of in this scenario are continuous or intermittent shorted turns on the primary.
Try RGs test to check for continuous shorts, though I think that if that was the case, you would notice the output and tone to be really bad as the primary inductance would collapse.
http://www.geofex.com/FX_images/xform_test.gif

Intermittent shorted turns, where the insulation fails when the signal level reaches a sufficient peak, results in big static crackles coming though with the signal, which you would surely have noticed and mentioned.
For a harsh top end, I would look at the VB+ level being too high and checking the stability, may be parastic oscillation or just a resonant peak due to positive feedback from poor lead dress, layout etc.
Pete

[km6xz]

Do you have test instruments? If not, get it to someone who does to see what the distortion looks like on a scope. I would be very surprised if the transformer is causing it. If it is that audible it will be seen with low distortion signals so figure out it is there with any level or frequency or if it is present only on high level transits or overload.
Shorted turns are somewhat frequency dependent in symptoms, impacting lower frequencies first. SE means a strong continuous magnetic field in the core and if too much current, saturate the core dropping the impedance very low.
Too many variables here to suggest a cure from the description given. Measure it and come back with the information.
Measure the level and distortion of the drive signal, the plate current at idle, presence of any oscillation or amp instability as seen on the scope. Look for bursts of high frequency signal riding on the peaks of signals as you vary the drive signal level.
This sort of problem is much easier to find and correct with a minute of visual inspection with a scope than writing about it or guessing. What the distortion looks like will point to the source.

[chinrest]

bursts of high frequency signal

Now that you mention it... Are we talking ultra-high frequency, or might it be audible? I say this because I just noticed bursts of high-freq tones, ~10kHz, coming through at idle. I usually ignore these kind of noises because digital pedals often create such noises, but I have it now plugged straight in with my 3 foot troubleshooting cable.

I don't want to derail this thread into group troubleshooting, so I'll try the few things I can: shielding, lead dress, playing with grid stoppers. if that doesn't smooth things out, I'll bring it to a tech and report back. Sound good?



Thanks for your responses.

[soundguruman]

"A slightly harsh high-end when overdriven."

The bias is wrong.
Tubes have undergone improvements in efficiency, and they are not the same as when this OT was made.
That is correct. The design was changed.
It is not surprising that the same circuit made the old style work correctly...
but must be updated to make the new style work correctly.

The same for EL34 and many others.
Example: EL34 never needed a 1K screen resistor in the old style.
But now the new EL 34 MUST have a screen resistor. Or the old style circuit will draw too much current.
As you can see, original Marshall EL34 had no screen resistor.
But now, to run new EL34, you must add one, or blow fuses. Use the OLD style EL34, and no problem.

Many tube designs have been updated and changed.
They took the Mercury out of 6V6...that was just one of the recent changes made.

You will probably need to select a better cathode resistor and bypass capacitor, to reduce distortion, with the NEW style 6V6.
or more...
the new style is becoming more like 5881 and 6L6. And much less like the original little dinky 6V6.


To conclude that the OT needs to be changed, because the amp sounds harsh, is a really silly misguided assumption.

Yes, for sure, incorrect impedance load on the amp WILL cause arcing and burning of sockets, etc...that applies to ALL tube amps.

Burst of high frequency tones: probably in the electricity.
Utility company is now using remote control system, through the electrical lines.
Causes all kinds of problems and noises.
Stuff like this also caused by welding equipment, electric saws, etc, etc...

[km6xz]

Do the bursts of HF become audible when some signal comes through or does the HF get swamped by any signal or so away. It is periodic or continuous? Do tone and volume controls change the HF?
Is it possible that there is a steady state HF sound at a low level and when playing, the signal gets modulated by the HF noise?
If the tone control, particularly the treble control impacts the level or presence of the HF, the preamp is either detecting an external signal or a stage in the pre-amp is oscillation. If the volume control and tone controls do not impact the HF, the power amp probably has a parasitic oscillation. If that is the case, just moving the wired to and from the OPT should change the sound or make it disappear.
Do you have a scope to look at the waveform? If you look on the scope, and the HF is present for most or all the a cycle, look in the preamp, if it just rides on top of the wave form for a few degree of the cycle, look to the power amp.

[chinrest]

To conclude that the OT needs to be changed, because the amp sounds harsh, is a really silly misguided assumption.

Yep, I'm a silly misguided ass. That's why I'm here.

The bias is wrong.

It's certainly worth tinkering with the bias. Thanks for that suggestion.

Burst of high frequency tones: probably in the electricity.

I guess I can rule this out (or in) with some simple tests.

Do the bursts of HF become audible when some signal comes through or does the HF get swamped by any signal or so away. It is periodic or continuous? Do tone and volume controls change the HF?

They are period bursts, are present with or without signal applied, and are not affected by the tone/volume controls.

Do you have a scope to look at the waveform?

Why exactly did I give that o-scope to the Salvation Army? I think I had determined that my manic obsession with amplifiers had turned against me, and my health and family life were degrading and falling apart. I'm feeling much better now and I wish I had my scope back

I think you've confirmed my decision to fiddle with the lead dress a little before going to a real tech to receive a serious well-informed brow-beating



-Thanks

[Gtr_tech]

Early Mar$halls (and several other 60s amps) that didn't use screen resistors ate EL34s for lunch....often. The quality of the old Mullards was much better than we have now, but the screens still failed when abused by hard clipping. The addition of the 1K screen R prolonged tube life. It is mandatory to use them with current mfgr tubes.

Old Mullards didn't care so much about control grid stoppers, new mfgr tubes *need* them in almost all cases. Every old Traynor I've worked on that didn't have stoppers motorboated wildly. Add the stoppers...all is well.

[soundguruman]

Early Mar$halls (and several other 60s amps) that didn't use screen resistors ate EL34s for lunch....often. The quality of the old Mullards was much better than we have now, but the screens still failed when abused by hard clipping. The addition of the 1K screen R prolonged tube life. It is mandatory to use them with current mfgr tubes.

Old Mullards didn't care so much about control grid stoppers, new mfgr tubes *need* them in almost all cases. Every old Traynor I've worked on that didn't have stoppers motorboated wildly. Add the stoppers...all is well.

The design of old EL34 WAS DIFFERENT!
When the tube was re-designed, screen resistor became NECESSITY. That's WHY old Marshall DOES NOT have screen resistor!

Where did I LEARN THIS?
From the Engineer TECHs at the MARSHALL factory in Milton Keynes.

[g1]

The design of old EL34 WAS DIFFERENT!
When the tube was re-designed, screen resistor became NECESSITY.

EL34's were never re-designed. Doesn't matter if Jim Marshall himself told you so, it's not true.
Post some data sheets that show a difference between "old design" and "re-disigned" EL34.