ECH83 tube preamp

[rankot]

I built the pedal using this schematic:



But I have nothing at the output. I tried to follow signal using oscilloscope, it is present only at the pin 9, and then nothing. Voltages (DC) on pins:
1: 7.54
2: 0.01
3: 0.00
4: 0.00
5: 6.05
6: 6.05
7: 7.54
8: 8.83
9: 0.00

This is my first tube build, so please help! Really have no idea what's wrong. Tube is ECH83, and it is completely cold. Power supply is capable of 1.5A, so I presume there is enough current for heater. Maybe BC549C I used is not capable to deliver enough current?

Tube symbol in my schematic is not correct, my PCB software doesn't have it, but pins are connected by numbers as in original schematic.

[rankot]

Looks like I have reversed tube pins

[rankot]

New PCB finally arrived and now I can tell that this baby works fine! I tried with 9/18V switch, but found out that 9V sucks, so I put a jumper to 18V instead of switch. Also, I get approx. 16V instead of 18V for some reason. LT1044 used as a charge pump.

One thing I noticed is that it is always working as some kind of distortion, e.g. no clean sound, so I Googled a little bit and tried with potentiometers instead of 47k anode resistors. Now it is possible to get almost clean and really distorted sound, so I like this much more than original. I used stereo A100k pot to control drive simultaneously.

Someone somewhere noticed that it is better to put 1M pull down resistor instead of 47k at the input. Didn't try that (this one is not easily accessible on my PCB), so I would appreciate if anyone could comment this. Why it should be better?

Also, the sound is OK, but there is some kind of hiss. Part of it definitely comes from pedal still not being boxed, so I will report about this when I put it into the enclosure and properly shield.

[tca]

Looks good!

Care to share the PCB drawings?

Cheers.

[rankot]

Here they are:



Please have in mind that DRIVE pot is stereo 100k, mid lugs are connected together and to mid hole on PCB, and CCW lugs go to PCB separated (one to CCW hole of drive pot, another to CCW1 hole on PCB). CW lugs are not connected.

[rankot]

And this is the final schematic:

[printer2]

Looked at your schematic and thought it won't work then realized you have not swapped tub parts to the correct pin. Looking up the original, pin 9 is the grid which has the 47k input resistor. The lower resistor loads down your pickups. As far as the hiss goes, that is what happens when you keep adding more elements between the grid and plate. When used in the application it was designed for, radio receiver, the hiss is not an issue.

[rankot]

Well, I have updated schematic and drew triode/pentode symbols, so the last one is just like original. I only modded it to allow variable grid resistance. But I don't understand what do you mean with "adding more elements between grid and plate". Actually, I can see only one capacitor between B grid and A plate?

[rankot]

I put this into the box and shielded, but noise is still there. Some kind of continuous high frequency hiss. I will investigate further. Maybe this LT1044 generates that?

[rankot]

This is what can be heard when I turn this pedal on. Sound is cricket-like and it is continuous, doesn't stop or change and when I play it is heard together with guitar. Does anybody have an idea what it could be?

[tonyharker]

Sounds like the LT1044 switching frequency bleeding through.  You may have to force it to oscillate at a higher frequency beyond audibility.

[rankot]

I thought so, but I have already connected it to run at a higher frequency (pin 1 connected to +9V). Maybe I bought a remarked 7660? Who knows? I will try to replace it with LT1054, which I am sure is original one and can be boosted to higher frequency. Unfortunately, I didn't socket that IC, so it would be a little complicated to replace.

[rankot]

Now I replaced LTC1044 with LT1054 and the hiss is gone. Seems it was really remarked 7660A. Really nice sound!!!

[rankot]

This is my first attempt with tubes. It is a modified version of the Engel preamp with ECH83, btw which cost me 2.5Eur at a local store. Here is the schematic



There is a similar version here. My mods are in red.

Is there any difference in sound regarding change of pull down resistor from 47k to 10M? Why did you do that?

[PRR]

47K off a naked guitar is maybe a little more loading than we like. It would tend to reduce the treble brightness. It may be about-the-same on some guitars, and if you are coming from another pedal rather than direct from guitar.

10Meg is plenty high for any axe. It may be a bit high for the tube, though at this low voltage this is not a concern.

tca knows what he is doing. You could just take his values.

Or for another few cents, you could try 47K, 1Meg, 10Meg, see if there is any difference in your setup. Put the 10Meg in permanently, then tack 1Meg or 47K across it for lower values. It won't be "wow!" (unless the tube is off-bias with large resistor). It may be "bit more sizzle" or something.

[rankot]

OK, I'll try that. What do you think about putting double potentiometer in series with anode resistors, just like I did here (see my previous schematic)? I like the sound I get, but I'm in doubt if that will do any harm to the tube?

[graemestrat]

I am a bit confused by these drawings but  the triode and pentode don't appear to have cathode resistors.

If this is the case what you have are what's known as Mu amplifiers which have unrestricted gains, which would be sure to cause all sorts of horrible noises.

The gain of each stage is approximately the anode resistance divided by the cathode resistance so with no cathode resistors the gain is flat out or as much gain as the tube is capable of - hence the name, Mu.

Typical values are about 1k to 1.5k (1.5k are what are used in Fender amplifiers)

[rankot]

OK, but I don't have strange noises at all after replacing LTC1044 with LT1054. This sounds from pretty clean to overdriven, and that's all. I was also a little bit puzzled with absence of cathode resistor, but I tried it and it works.

In Philips data handbook it is also shown without cathode resistor, see here:
http://www.radiomuseum.org/forumdata/users/1511/Ph-DataECH83.JPG

[graemestrat]

Ok, that's fair enough. I am not familiar with this type of bottle so I don't know what it's mu is.
I used to have an old MusicMan hybrid amplifier and they put one 12ax7 in it for the distortion sound. Half of it was wired up as a mu amp and it sounded like crap. They ended up changing that part of the circuit in a new revision.

[amptramp]

If you added a cathode resistor, you would get a cathode-coupled multivibrator that would oscillate.  I don't know what value of cathode resistor would do it but I would avoid any cathode resistor.  The standard use for this tube is an oscillator-mixer in a radio where the triode oscillator generates its own negative grid bias and the pentagrid side is biased by the negative automatic volume control bias derived from the detector stage.  But at 9 volts on the plate, you don't have to worry about excessive dissipation.