FET preamp stage question...

[MakerDP]

Hey solid-state experts...

I have an Egnater Tweaker 15 head on my bench that is extremely noisy - white noise type of noise. ALL of the controls on the amp affect the noise in some way and a complete tube swap with known good tubes doesn't fix it. So based on the schematic you can find in the library here, I am suspecting the problem is somewhere in the FET preamp stage of this amp (which is why this is in the SS section even though the rest of the amp is all-tube.)

Looking at the attached article from Blue Guitar the second page called "High VOltage Solid State Preamp Stage" the Tweaker's FET input is pretty much identical to this article.

OK here is where it gets weird... THE ZENER DIODE IS MISSING from the PCB. Would this be the cause of the extreme white noise??? Unfortunately I don't have any 24V z.d.'s or obviously I would just throw one in there and see what happens.

Second question... in general, since that zener diode is a 24V z.d. does that mean the voltage is set at 24V or does that mean the supply voltage is reduced by 24V? My SS knowledge of these types of circuits is a little weak.  FWIW the voltage at the zener tp ground reads about 31V.  Theoretically should this circuit even work without that z.d. in place? I'm not sure what it's exact function is here.

[Ed_Chambley]

A zener diode, when reversed biased will regulate voltage.  If it is a 24 volt, it should only get to 25.2 volts or  close.   Yes, missing the diode will allow for the first Q1 Gate to receive too much voltage which can cause noise.  Just like an effects pedal turned up.  Keep in mind zener diodes have resistance and wattage.  I don't see the part number on the schematic.

[Ed_Chambley]

Also, on some of the early Tweakers they has a Cathode Resistor of 220 ohms.  This caused the stock Ruby Tubes to red plate.  It was changed to 510 ohms to provide tube swapping abilities, but 6V6 tubes run too cold.  I ended up with 250 with just over 90% dissipation and of course more headroom and wattage, but the amp sounds like it should with 6V6 tubes.


My first tubes overheated and died very fast.



The first stage actually is modded often by installing a tube for those who do not want MOSFET and JFET as their first preamp.  There are other changes, like removing snubbers to "open" the sound and make the Hot Channel have more highs and a raunchy tone.  Considered Desirable. There is one that uses the loop tube as preamp V1 and bypasses the SS components, but it is easy to see if it has been done.


I just biased the output tubes correctly, reduced the 10K resistor (R14) to a 5.1K and increased the coupling caps after to .1 and changed the bass cap to a .47.  This seemed to be plenty as the bass on these limited.  The was just for me and my tastes, the amps sound pretty good out of the box, but are biased too cold.  If it does have the 510 ohm, most simply series another to achieve the 250 to 330 ohms for the 6V6.

[shooter]

this is the schematic I'm looking at;
http://el34world.com/charts/Schematics/files/egnater/Egnater_Tweaker_15.pdf

assuming D1?  does it look like it was EVER soldered in?
did it make Noise new?
does the white noise go away when the gain knob is on 0?

[MakerDP]

this is the schematic I'm looking at;
http://el34world.com/charts/Schematics/files/egnater/Egnater_Tweaker_15.pdf

assuming D1?  does it look like it was EVER soldered in?
did it make Noise new?
does the white noise go away when the gain knob is on 0?

Yeah D1 is the diode in question. It does look like something was there in the past because there is that characteristic "leftover solder" in the lead's holes.

I bought this as a "non functioning" amp off Reverb. Previous owner said it's been to a tech twice already. I think the issue he was having must have been blowing the output tubes because of the 220R cathode resistor like Ed mentioned and the tech didn't really know what he was doing??? I have no other explanation as to why that diode would be missing. I suppose it could have been a manufacturing defect and that diode was missed to be fair. I'll reach out to the previous owner and ask about the noise.

The reason I think the problem is in the FET circuit is that the TIGHT and BRIGHT switches color the tone of the white noise in a way that you would expect them to and they immediately follow the FET circuit. And yes the gain knob does affect the level of the white noise.

[MakerDP]

Also, on some of the early Tweakers they has a Cathode Resistor of 220 ohms.  This caused the stock Ruby Tubes to red plate.  It was changed to 510 ohms to provide tube swapping abilities, but 6V6 tubes run too cold.  I ended up with 250 with just over 90% dissipation and of course more headroom and wattage, but the amp sounds like it should with 6V6 tubes.


My first tubes overheated and died very fast.



The first stage actually is modded often by installing a tube for those who do not want MOSFET and JFET as their first preamp.  There are other changes, like removing snubbers to "open" the sound and make the Hot Channel have more highs and a raunchy tone.  Considered Desirable. There is one that uses the loop tube as preamp V1 and bypasses the SS components, but it is easy to see if it has been done.


I just biased the output tubes correctly, reduced the 10K resistor (R14) to a 5.1K and increased the coupling caps after to .1 and changed the bass cap to a .47.  This seemed to be plenty as the bass on these limited.  The was just for me and my tastes, the amps sound pretty good out of the box, but are biased too cold.  If it does have the 510 ohm, most simply series another to achieve the 250 to 330 ohms for the 6V6.

Wow thanks for all that Ed. Yeah this amp has the 220R cathode resistor. I was scratching my head about that one because it didn't match the schematic and is obviously the wrong value for 6V6 cathode bias. Of course the schematic's 550R didn't sit well with me either. Talk about from one extreme to the other! So I've already been thinking I need to change that. Looks like I need to take a drive and pick up some zeners and a couple bypass resistors to experiment with but I may have a 250R around here somewhere already.

I just want to get rid of that noise and fix the cathode bias value then I'll either keep it for a while and to evaluate whether it's a keeper or just flip it for an easy profit. I kind of like it's light compact format.

[PRR]

You seriously need the 24V Zener or a resistor to set that node to 20V-30V.

If it has been run without, the JFET is probably blown.

Get a full set of voltage readings, we may know more.

[MakerDP]

You seriously need the 24V Zener or a resistor to set that node to 20V-30V.

If it has been run without, the JFET is probably blown.

Get a full set of voltage readings, we may know more.

Thanks PRR.

I don't think the FET is blown because signal is getting through and the amp sounds quite nice actually, except for the extreme white noise of course.

But yeah I'm going to try and take a drive to get a zener and an assortment of replacement cathode resistors tomorrow. The resistor that feeds it off the main B+ drops it to about 32-ish volts.

[MakerDP]

OK finally had some time to dig into this some more...

I added the zener back into the circuit and no perceptible change at all... still extreme white noise.

Now here is where I am seeing some funny stuff:

I audio probed the circuit with a 1k Hz sine wave at appx 70mV.
Nice strong signal at the input jack... no noise... just as I would expect.
No noise on either side of R15... so far so good.
Nice strong signal at gate and source of Q2 (2N5485) - same as input signal - nice and strong, noise free.
At the drain (output) of Q2, the sine wave is very weak and there is a LOT of white noise. Noise is present from here on out in the circuit with all switches and controls having their expected affect on the noise.

So, would you guys agree that probably Q2 is the culprit and should be replaced?

This amp also cuts out from time to time when it is mounted in the head cabinet...  Something is getting pushed loose when it is slid into the box! Now that might be a tough nut to crack.

I also have a new cathode resistor for the 6V6's to get those to play a little nicer.

[shooter]

Q2 is the culprit and should be replaced?

It's possible, the "missing D1" at the same circuit section seems odd
for less than a buck might as well replace C1 n C13 also.

[MakerDP]

Yeah this amp has been to the tech twice before I got a hold of it. I think he just took it to Guitar Center in Fresno so no surprise it never got fixed right.

I actually have several IRF820'a on-hand so that's probably a good idea to replace it as well. I'll have to search around for the other one because Mouser only carries an SMD version now.

Thanks.