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andronizer 30th September 2012 10:29 PM

DIY 50W Plexi Preamp Problem
Hello guys.

I`ve built a 50W Plexi, but it`s not working :D

When I turn it on it only makes some random noise (hiss, hum and random crackles) and doesn`t react at the input.

So I`ve pulled out the power tubes (6L6) and started testing the preamp using a primitive probe, which is a radio into the input and an amp, with negative grounded to the amp and positive being in my hand as a probe.

First of all, the heaters have 6.3V and Voltage at the 1st & 6th pins of V1 is around 260V.

Now about the probe results. The R3 and R4 are OK, the sound is passing through, but as soon as it enters the tube, I can`t hear it anywhere (I mean the 1 and 6 pins).

So I`ve changed about 4 tubes, resoldered the tube nesting but nothing solves it.

Here is the diagram, don`t know where to go next...

pinkmouse 1st October 2012 11:44 AM


Thread moved to appropriate area.

grommeteer 1st October 2012 06:45 PM

I do not like the idea of poking the circuit with what you call a simple probe.
There are (should be) high DC voltages at the plates of the ECC83s. The coupling capacitor in your test-amp is probably not up to these voltages. And the AC signals are amplified to tens or hundreds of volts in tube circuits. So, please don´t you use your test amp! You might fry it!

Most tube circuits will show you what is wrong if you measure the DC voltages of the tube´s electrodes, the operating point, without input signal.
Give us the voltages of the first two triodes and folks are most likely to help you.

andronizer 1st October 2012 07:09 PM


Thanks for the reply! I found this idea (probe) from a youtube video, and as you can see in the videos it works flawlessly. You`re absolutely right, the couplng cap should be of at least 600V, I have the 630V cap.

Audio Troubleshooting.wmv - YouTube
Make an Audio Test Probe.wmv - YouTube

I have disassembled the amp (goin` to do everything from the beginning), so I can`t measure the voltage right now, but it was around 260V on the V1 and 265V on the V2.

Loudthud 1st October 2012 10:16 PM

Measure the voltage from pins 3 and 8 to ground. It should be about 1.5VDC.

There should be a 100K resisitor to pins 1 and 6 of the first 12AX7. Measure across these with the power off and the power supply caps discharged. Your meter might give a strange reading at first but eventually read 100K +/- 5%. Now measure from pins 3 and 8 to ground. What reading do you get. Can you post any clear well lit close up pictures of the insides of the amp?

andronizer 2nd October 2012 11:27 PM

Hi, Loudthud.
The load resistors are OK. The problem is, welll....

I started from the end of amp.

Today I reassembled the amp and here`s what I have:

1. Random crackles, the more the voltage - the louder. When the power tubes are pulled off the crackles are STILL present, but a lot quieter.
The bias V is not stable, it's riding from 0 to -70, the voltage on D1 is 170, the Bias Adjust VR1 isn't adequate, it's hardly making any changes.
When the bias is somewhere around -35 or lower the crackles are hardly audible, so I assume that all this is the bias problem. The tubes are OK, the signal pop is OK, Voltages:

pin3 - 443
pin4 - not stable, from 0 to -70
pin5 - 437

Replaced the bias adjust VR1, by resistor - no changes.

Instead of TX3 choke I have a 10KOhm 10W resistor + 115Ohm choke

2. Plate voltages on the V1 and V2 are around 370 - 400V (without tubes), I don't understand, I made all according the scheme, so why is it so high.

P.S. I live in Georgia, so it`s midnight here and I can't take any DECENT photos right now, sorry.

ImageShack® - Online Photo and Video Hosting

So my question is: "How to stabilize the Bias Voltage?"

Enzo 3rd October 2012 01:41 AM

2. It is high because there are no tubes. When B+ is 300v and the plate is 200v, where did the 100v difference come from? It came from Ohm's Law, telling us 1ma was flowing through the 100k plae resistor. And where does that current come from? Through the tube. With no tube in teh socket, zero current flows. Ohm's Law tells us how many voltas are dropped across a resistor at zero current? Zero volts. That means if ther is 300v at the B+ end of that resistor, there will also be 300v at the plate end.

Your voltages can;t be right. Pin 3 is the plate on the power tube, and should have the 400+ volts on it, pin 4 is the screen, which should also have 400+ volts. Pin 5 is the control grid and it is where the bias voltage should appear, not pin 4.

Use clip wires to connect your meter probes to the socket pins, possibly part of your reaqdings being unstable is your meter connections.

1. Um... if you still get noises with the power tubes removed, you have a problem. Pretty much what makes sound come out the speaker is current through the output transformer primary, and with the tubes removed, about the only source for that would be arcs and sparks on the power tube sockets or within the transformer itself.

andronizer 3rd October 2012 10:11 PM

You're absolutely right abot the Ohm's law and the wrong voltages (I was tired so i messed up pin4 and pin5).

At last!!! My amp is working! So the problems were:

1. The noise was because of some strange dirt in the tube sockets, I`ve cleaned them (should've done it in the beginning) and the noise was gone.

2. Instead of 12AX7 I have some USSR military tubes, called 6N2P-EV, so they have different pins for heaters, and they were off all the time... LOL

So the amp is working, but here come new problems.

1. On the normal channel, when turning down Presence a very intensive high frequency sound appears and is gone only when Presence is full on.

2. Bright channel has the same problem + it`s very distorted and lacks power, I mean when you play hard you hear the sound but when you play very softly it`s muted, sounds like some cap problem.

3. Lot`s of 50Hz hum.

4. The voltage on the 1st tube is around 100V, 2nd - 120V, 3rd - 230V.

So I'm off for searching answers to my questions....

Thanks guys.

Struth 22nd October 2012 05:51 PM

Hi Guys

It may be that the OT is wired backwards - brown and blue transposed. Open the feedback loop - lift the 100k from the speaker jack - to see if the high-level oscillation disappears. If it does, then reverse either the brown and blue leads to the OT, OR reverse the drive wires between the splitter and output tubes.

Wiring layout and grounding are critical to low noise operation and stability. TUT3 illustrates the correct ways to achieve both along with the best note articulation.

Have fun
Kevin O'Connor

andronizer 22nd October 2012 06:50 PM

Thanks for the reply, Struth, I've already solved all my problems and wanted to post when the combo is 100% complete.
I'm in the process of making FX-Loop and other mods, so when it's done I'll share my experience.

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