biasing PI tube with a split load and a pair of EL84's

[phsyconoodler]

a friend and I built an amp with basically a Princeton 6G2 circuit but used a pair of EL84's instead of 6V6's.
  It has a distortion element way too early and was wondering if it's because the PI bias drives the tubes too hard?
 El84's being their own animal after all.
    I used to have an old PA amp with a pair of EL84's that had a slit load PI but I can't remember what it was.It sounded divine.
The goal is low distortion.
   Any ideas would be welcomed!

[catnine]

 What if you lifted the cathode bypass cap off V1 or reduced it's value . Is there a NFB on this build.

[zendragon63]

Any ideas would be welcomed!  

Yeah those EL84 are plenty sensitive. If you are working off the 6G2 schematic, you probably already have the NFB loop and share the tremolo and PI 12AX7. If low distortion is the goal, you might even rebias for a 12AU7 on the front end. You certainly have more experience on the bench than I but the 12AU7 really have a full sound but not lots of swang. Regards

dennis

[DummyLoad]

split the load. use 2 27K resistors for Rk and 2 27K resistors for Ra. leave the 1K bias R alone. if my thinking is coherent, Vout will be about 1/2 what you're getting now without upsetting bias.

please see attached.

--DL

[phsyconoodler]

We tried lower gain in the preamp but the early distortion is still there.I thought it was an oscillation too and upped the grid stoppers but it's still there.
   I'm thinking dummyload may be on the right track.We will give it a try.
Thanks guys.

[jjasilli]

basically a Princeton 6G2 ??? Schematic with voltages would help. The devil must be in the details.  Is there a tremolo?  If yes, disconnect it for test purposes.

[phsyconoodler]

The voltages are pretty much the same as the Princeton and yes it has tremolo.
  The only difference is it uses EL84's and cathode bias.
 I think it's still an oscillation as he is an older gentleman and is very set in his ways.He wired it on terminal strips and ran a pile of wires parallel from the power section to the preamp all bundled together and nary a crossed wire near the phase inverter.
  But still,the phase inverter is for 6V6's and not EL84's so it might be a factor here.
It's very difficult to convey this to him as he originally wanted to do a Gibson style amp and insisted on using a 6AU6 preamp tube as well.
  When he could get all the values for the Gibson build he caved and let me convince him to heading toward the princeton style amp with a Traynor tremolo similar to the YGM-1.
  He made several mistakes in his wiring but it was difficult to repair because he triple twisted all his wires on the terminals and tube sockets.
So then he goes to the Gibson schematic and assumes the problem is my idea for the princeton circuit,so he changes a bunch of stuff to be like the Gibson.It obviously didn't help.
   I'm hoping it's in the phase inverter or this one may be a problem in more ways than one.
  I found a Gibson amp schematic with a cathodyne PI that we can swap the PI values to so that may help.Or not.They are quite different than the princeton.

[jojokeo]

Lol, it's all your fault for helping. Geez
If I remember, you also will get less output going to 2 100ks right?
I think DL is right for lowering signal into el84s. But you also may be correct
so he may well have several issues.

[HotBluePlates]

A scope would tell the tale on oscillation.

A meter would tell you if it's a matter of too much signal to the output tubes.

[jojokeo]

A meter would tell you if it's a meter of too much signal to the output tubes.

How would you hook it up and test?

[topbrent]

Just a thought, but perhaps you might look at some of the Hammond organ power amp designs for inspiration.  More experienced eyes than mine would be able to tell you if this is smart or foolish.  Those organs were designed to stay clean, and many models ran EL-84 power amps.

[HotBluePlates]

A meter would tell you if it's a matter of too much signal to the output tubes.

How would you hook it up and test?

The first fact to grasp: If the grid is driven momentarily more positive than 0v, the grid draws current and can cause distortion of both the output tube, and the phase inverter. The phase inverter gets distorted because the grid no longer looks like a ~infinite impedance, and becomes a heavy load. That drags down the gain of the PI, while at the same time causing it to distort.

Additionally, the grid current can cause blocking distortion by placing a charge on the coupling cap between the PI and output tube, which then causes the grid to latch at a certain voltage despite a changing input signal. Cause all kinds of weirdness.

So we might play most guitar amps in a manner that drives the output tubes to distortion, but we simplify design by assuming the grid will never be driven positive. "Driven positive" means a positive incoming signal is more than the value of the bias (ex: -34v bias, 38v peak input signal).

So measure the bias voltage. Then, measure the signal at the output tube grid while injecting a test signal which approximates a guitar signal's strength. The test signal will usually be a sine wave, and a guitar doesn't produce sine waves. So, you would likely want to take a guitar and measure it's output with you banging on it, while the meter is set for a "Max Hold" function. You will want to know if your meter performs this reading based on maximum RMS signal or maximum peak voltage.

Depending on which it is, you apply a test signal with a signal generator that matches this peak at the input jack. You may have to convert a measured RMS voltage to equivalent peak voltage or vice versa. Checking the peak guitar signal using a scope may be a good idea if your meter's peak hold function measures RMS or average voltage (because the guitar's non-sine nature will make the meter lie to you).

Once you've established an appropriate test signal based on the guitar/pickup typically used, you can measure the value of the voltage at the output tube grid (converting RMS or average to peak, if necessary) to see if your typical settings of volume, tone, etc. drive the output tube excessively. Obviously, you can fine tube with a single-coil Strat and have an amp that doesn't sound right with a humbucker guitar, depending on how far on the edge you get (or how unwilling the player is to turn down until the amp sounds right).

Ken Fischer suggested that many great amps (likely assuming you don't need a master-volume gain machine) are designed such that the output tubes start distorting first, the phase inverter shortly after, then the other preamp tubes. It's part of what makes an amp touch-sensitive, where soft playing is largely cleanish, and a heavy attack is distorted.