Pcl86 guitar amp HELP!

[slash1986]

Hi, I'm new to the forum, which was recommended to me by a user that is part of the Italian forum electroyou.

I request your help to understand what's the problem in my guitar tube amplifier that I homebuilt.
the amplifier is equipped with a 12AX7 and a PCL86 (the diagram is fitted with a ECL86 but I think they are the same valve with different filament voltage):
here is the circuit diagram: http://notinteractive.com/stuff/guitar/frankencombo/schematic.png

the problem is that when I raise the volume (even if there is no speaker connected) the output transformer makes strange noises. if I raise the volume all I can see electrical discharges between the anode of the pentode and the filaments.

I'm going crazy what could it be?

Thanks to all in advance for your help.

[tubenit]

My first guess is simply a bad tube. The ECL86 is a pentode/triode.

I am somewhat confused by what appears to be an artificial ground to the 100R resistors ........... in that there appears to be some type of bias (?) control with it going to your B+ rail. Not sure what that is about?  Are you trying to elevate the heater wiring?

Did you measure voltages carefully?  And do they line up with the data sheet?

http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/ECL86.pdf

With respect, Tubenit

[slash1986]

Hello, I tried to replace the pentode / triode with another pcl86 but electrical discharges are still there.
I measure the voltages are as follows:

Anode 12AX7 Pin 6 - 236v
Anode 12AX7 Pin 1 - 130v
Pcl86 Anode Pin 9 - 215v
Pcl86 Anode Pin 6 - 286V
12AX7 Cathode Pin 3 to 1.34 v
12AX7 Cathode Pin 7 - 122V
Pcl86 Cathode Pin 2 to 1.37 v
Pcl86 Cathode Pin 7 - 7v

the circuit with 2 resistors 100R has not been done. the filaments are connected normally.

[Heinz]

Please do not run your amp without a speaker or a dummy load! You are running the risk of damaging your output tube or your OT.

I guess your amp suffers from parasitic oscillation. This seems to happen pretty often with ECL86s. You may want to try the following:

• Add a grid stopper resistor to the ECL86, solder it directly to the tube socket. Start with a low value (e.g. 10k) and work your way up until either oscillation stops or the sound deteriorates
• Use a tube socket with a shielded base for the ECL86, ground the shield
• Move the ECL86 away from the other components

Good luck with your build!

[tubenit]

Based on your response, I would follow Heinz's advice. It's probably parasitic oscillations. I think the 10k (or higher) grid resistor is good advice.

It certainly could be a layout issue or lead dress issue?

IF none of those resolve it ..............

It may be worth changing the ECL86 triode cathode cap from 22uf to maybe 4.7uf or 2.2uf  .............. AND changing the coupling cap between the ECL86 triode and pentode from .02 to .01 or even .0047.

You might find that those mods will eliminate the oscillation.  You can also add a second "smoothing cap" from the ECL86 triode plate to triode cathode. Maybe a 250p-390p range.

Stick with it, you'll get it figure out.

With respect, Tubenit

[slash1986]

To help you better understand the problem, I made ​​a little video where I plugged the guitar into the amp.
With the cone connected, the electrical discharges are gone.
I also connected a resistance of 220k on the grid pentode.
up to a quarter of the volume the amplifier sounds, just get up more than a quarter, happens ones you see in the video.

Link: Tube Amp problem.

[tubenit]

The squeal at :48 sounds exactly like oscillation problems before I resolved them on past builds. I don't know what the odd sound was later on the film?

Sometimes it was a tube, sometimes lead dress, sometimes layout, sometime changing component values worked great.

I think for the ECL86 that I would lower the triode cathode cap value and the coupling cap value.  I looked at the schematic of the SoLow Watt amp that I built using ECL84's and I used a .0047 going into an LTPI (yes, I know yours is single ended) and .01 coupling cap into the ECL84 pentode.

Chopsticking the amp safely sometimes can reveal the problem area. IF you're not sure how to do that safely, PLEASE read up on it. Keep one hand in a pocket or behind you and away from the chassis.

With respect, Tubenit

[tubenit]

I am not sure about that Orange Drop cap with the long leads flying over the tube socket?  I am wondering if that could be an issue? 

Is that your .02 between the triode and pentode of the ECL86?  I might be tempted to try a .0047 there to see if the oscillation is gone. If so, then maybe slowly parallel caps with that and see if you can get up to .01 without oscillation?

[slash1986]

Hi, I would like to ask you a question: thinking about replacing the output transformer, I can use a transformer that I already have. HAMMOND 125E is a push-pull, can I use it anyway? If so, how do I connect the wires? what impedance should I use?

Thank you.

[tubenit]

Are you saying you want to convert the amp to push/pull?   OR that you're wanting to use a push/pull for a single ended amp?  If it's the latter,  I think that's a bad idea.

Have you got the original problem solved?   Not sure how the OT got brought into the equation?

With respect, Tubenit

[slash1986]

I still have not solved the problem, I'm waiting for two new valves, which hopefully those. if I wanted to convert the push-pull amplifier in what I could do?

[slash1986]

hi i was wondering, possessing a new EL84, I can modify the circuit? using the EL84?

[John]

I strongly advise that before you convert to anything you try to figure out your problem as it is now. If new tubes fix it, that's good! Then you can convert to an EL84. You'll need to re-wire the socket of course, but you should have plenty of gain from the 12ax7 to drive it. But if it were me, I'd find the problem first before doing anything else. I agree with Tubenit, I'd really advise against swapping the OT, especially since you'd have to jigger more stuff to make that PP work. Solve the problem you have before creating another one.

I am not sure about that Orange Drop cap with the long leads flying over the tube socket?  I am wondering if that could be an issue?

Yeah, that does look like an antenna, doesn't it?

[slash1986]

Hi, I noticed a few things that I did not say, and I do not know if they could be involved in the problem.

The resistance of the cathode of the triode of ecl86 is 2.7 k.
The 3 capacitors from 20Uf/400v, are from 47uF 400V.
The capacitors on the cathodes are respectively: 22uf/35V, 100Uf/63V, 100Uf/63V.
In addition to this, since the Pcl86 has the filaments to 13.5 V, also the filaments of the 12AX7 are fed with the same tensionem, although under load, drops a little. (12.4 V)

can you tell me also about the impedance of the output valve to see if the TU and correctly?

[John]

I am no expert, so someone may correct me on this, but

The resistance of the cathode of the triode of ecl86 is 2.7 k.
The 3 capacitors from 20Uf/400v, are from 47uF 400V.
The capacitors on the cathodes are respectively: 22uf/35V, 100Uf/63V, 100Uf/63V.
In addition to this, since the Pcl86 has the filaments to 13.5 V, also the filaments of the 12AX7 are fed with the same tensionem, although under load, drops a little. (12.4 V)

none of that should matter.

can you tell me also about the impedance of the output valve to see if the TU and correctly?

I am not sure what you mean by that? What does TU mean? I think we're running into the language barrier.

[slash1986]

I apologize for the mistakes that I make in writing. I'm Italian.

I want to know what impedance should have the output transformer to see if it is correct.

[slash1986]

So I double checked the tension after changing the 12AX7.

the only abnormal voltage in on the 12AX7 pin 1 is 100V.

pin 6 is 228v instead.

From what can be derived? 54v are Off!

[tubenit]

The capacitors on the cathodes are respectively: 22uf/35V, 100Uf/63V, 100Uf/63V.

Try something. Lift one end of the PCL86 triode cathode cap up disabling it.  Do you still have an oscillation problem?

IF not, then try a 2.2uf or  4.7uf there.  Please report back after trying this.

With respect, Tubenit

[sluckey]

In addition to this, since the Pcl86 has the filaments to 13.5 V, also the filaments of the 12AX7 are fed with the same tensionem, although under load, drops a little. (12.4 V)

So, you're running the 12AX7 filament at 12 volts. But, it appears the socket is wired for 6.3 volts, ie, one filament wire on pin 4 jumped to pin 5, and the other filament wire on pin 9. That's wrong for 12 volt operation.

Put one filament wire on pin 4 and the other filament wire on pin 5 and remove the jumper between pins 4 and 5. NOTHING CONNECTS TO PIN 9.

[sluckey]

So, you're running the 12AX7 filament at 12 volts. But, it appears the socket is wired for 6.3 volts, ie, one filament wire on pin 4 jumped to pin 5, and the other filament wire on pin 9. That's wrong for 12 volt operation.

Did you correct the 12AX7 filament wiring? Did that solve the squeal and other weird noise?

[tubenit]

Sluckey is making an important point. Part of the "culture" of this forum is if someone offers an idea (after someone has appealed for help) that the original poster offers some response of what they are trying and not trying.

It's OK to veto trying something but let's us know that ..... vs going a whole new direction that may not ever address the existing problem.

FWIW,  Sluckey holds the record easily of all the forum guys for personally bailing me out of trying to diagnose my amp problems. He has put me on to more solutions then the next 3 forum geniuses combined. Honestly. He is at the top of my list of "go to guys for help".

With respect, Tubenit

[slash1986]

Hi, sorry for the delay of my answer.
I tried to do as was suggested in the filaments.
I disconnected the capacitor on the cathode of the triode Pcl86.

Unfortunately, the problems on my amp have not been resolved.

Having a push-pull transformer Hammond 125E, I decided to remove everything and try to build this push-pull amp

http://paulamps.com/Schematic6gw8.gif

Thank you so much for the help you have given me, and for the professionalism and expertise that you have shown me.
I hope you can help me with this project.

[tubenit]

OK, thanks for letting us know!  Hope the new project goes well for you.

with respect, Tubenit

[slash1986]

Hi, finally some good news . I built the push-pull amplifier of pcl86 and I must say I am very satisfied. Everything works fine, belonging to a noise which I think comes from the filaments.

Do you know any trick or tip to reduce hum?
The filaments are connected in a simple manner, nothing was high or connected to ground.

Another thing: I'd like to have more gain. Can i insert a circuit (integrated in the amplifier), even in the solid state, that gives me more distortion?

Thank you very much for your help.

[tubenit]

Glad you have made progress!

How are these filaments grounded?  Are you using an artificial ground with 100 ohm resistors?   IF you carefully move the filament wire around with a wooden chopstick or non-conductive stick ............. does the hum change?

Are you following Hoffman's grounding scheme listed in the Hoffman Library of Information?

As far as increasing gain. Try a 4.7uf  to 10uf cathode cap on the first triode as a starting place. You can also lower the first triode cathode resitor from 2.7k to 2.2k or even 1.8k.  I'd try the caps first since you can use alligator clipped wires to parallel those very easily.

With respect, Tubenit

[slash1986]

The filaments are not connected to ground, only to the pins of the valves.
The capacitor on the cathode of the first triode is from 1UF, not by 10UF. I'd like to have a lot more gain. I can only try to vary the resistance on the cathode?

For the filaments how can I do?

[kagliostro]

Tubenit mean you can try a cap who has a value from 4.7uF till 10uF (so much more than the 1uF is now present)

About filament if the 6.3v hasn't a CT use two 100R (or 220R) resistors connected on each branch of the filament supply and connect it (on the other side) to ground

a humdinger will be a better choice, but not strictly necessary

http://www.tube-town.net/ttstore/product_info.php/info/p5687_Fender-Potentiometer-Hum-Controll-100R.html

Have you give a try to the 330k plate resistor indicated for V1 on the schematic ?

K

[slash1986]

I can also put the capacitor electrolytic?

I also tried laresistenza to 330k, and I like the sound much more.

[kagliostro]

Usually the capacitors used as bypass are electrolytic

The use of big poly capacitors in the bypass, is considered a refinement in guitar amps

---

So, you tried the 330k resistor with satisfaction

well, this is the first step on sharping your amp  

K