Following 2 Power Tube Amp Design

[Andy]

Hold a second here. I think I’m confused now. Here is my understanding of how the input is wired in respect to the 68K resistor.

The TIP of the input jack goes through the 68K resistor into pin 2 on the 12AX7. The 68K resistor is a grid stopper resistor. In general, this serves to eliminate radio frequencies by working with the tube capacitance to create a filter. So the signal flows from Guitar Plug tip to Input tip through 68K resistor to pin 2.

The shielded wire is not entirely necessary. Grounding the shield on one end serves to better keep outside hum from entering the amp at this wire. The more noise that gets introduced early means more noise gets amplified along the way. A quiet input is important.

I am pretty sure this is the right way to wire this up. I think Deep that you have it right there.

[DeepBlue]

Thanks Aj I think you are correct too. I am pretty sure that Robin mistyped and meant to put be sure to only ground the shielded wire not the inner. I was pretty sure I had that correct also, thats why my surprise in my reply!

Robin clearly knows his stuff, so I am sure when he logs on will verify it was a mistype. At least I hope!

[Robin]

You are correct, my instructions are clearly wrong. Andy’s post straightens it out. Sorry.

[DeepBlue]

Hello everyone. Well tonight Ill post a photo of my amp. I have not been able to work on it much as I have been busy. However I checked the continuity and it seems to die off at right before the 68K resistor coming off the input jack to the V1 pin 2. I have no more loud hum just complete silence now? I am going to check all my wiring and hopefully that will correct my problem. I have a question regarding ground…so the input jack where is the easiest place to ground this? It is OK to ground of a bolt on the V1 or something similar? I will add some more info when I get more information. I will also post a photo. Maybe someone will see an error in my wiring. I have checked numerous times and I feel fairly confident its correct. I guess I was happy that my hum is gone, however the continuity test is leading me to believe that my problem is the input jack or a bad solder there perhaps?

[DeepBlue]

So I checked DC voltage at Pin 8 on the Rectifier. I am expecting about 379V DC as the rectifier has a 1.13 conversion and I am using 330V or 660V center tapped. However I seem to be getting more like 319V DC? Any suggestions as to perhaps why? In addition, now when i discharge my cap’s there is no voltage present? I notice the mounting bolt for the board is very very near the negative terminal for the capacitor right above it. However this line is grounded as well, so could the contact between the bolt be causing a short? I was thinking no since it was grounded already but is this sound?

[DeepBlue]

Alright so I check voltages. First I seem a bit low on DC. Showing 312V DC at the rectifier still and weirdly enough my 12ax7 is running very hot, like 260 volts or which I believe it should be down around 120-140 am I correct?

The other issue, is I am sorry if this is a nub question, but my continuity at my input jack like I mentioned seems to die at the 68K resistor. However I noticed I get continuity when my cable is unplugged and the tip and switch are connected, but as soon as I plug a cable in the tip has no continuity?

[beelzebum]

what is the voltage above and below the plate resistor on your 12ax7. also are you wiring you input jack like the pic that was posted. if you are using the picture as a guide then just remove the jack labled 2, it is wired strangely. use the jack labled 1 as a guide.

[Andy]

I’m not concerned on the rectifier voltage as much. You might check your AC power. That tends to vary from place to place so if your AC Voltage is like 110V vs. 120V, the amp voltages would be lower.

You are correct on the tube voltages seeming high. Check pin 1, and pin 6, right at the tube socket. Those are where it should be around 140-160 range. Though, if you test on the power supply side of the 100K resistor, it will be higher, likely in the range you measured.

As for the input, that sounds like a problem If you measure with an ohm meter, at pin 2 of the preamp tube, and then at the tip part of the input jack, you should have 68K. No continuity tho. If you dont have continuity from the Tip to to the 68K resistor, then there is definitely a problem and it wont pass signal. Likewise, if there is continuity from the tip of the 68K and ground, there is a problem.

[DeepBlue]

Hi guys thanks, when I get home tonight I will check the voltage at these points. I am curious how do I check the voltage above and below the plates? Also, as far as grounding goes is there a specific place I should ground to when checking these values?

Also another nub question, so if I check one of the terminals on the 6.3V supply to the pilot light, and the other end grounded say at the star ground, I get about 3.1 V. So is this correct should I be getting only have the volts from each Green wire from the PT? The tubes seem to be fine as far as heater element lighting up and the bulb is glowing, so just making sure. I thought I was right getting only 3.1V at each wire, but just to verify. I will post my new voltages tonight when I get home. I have some photos of my board, but its hard to see the wires so was not sure it was worth posting. I will though just in case. I also will check the Resistance across the tip to pin #2 on V1 see if I get 68K.

One other concern, is when testing the resistance of the 68K resistor that joins the 1.5K resistor in the center of the board, it reads more lik 20K? It reads 68K before I wire it in the board but as soon as it is wired it drops? Did I do something wrong or is this correct? I was concerned that an improper resistance might lead to excessive voltage…

[beelzebum]

above an below the “plate resistor”. its the 100k resistor above the plate. reading above it will show the B+ and reading below it will show whats actually on the plate. its usually easier to take a reading from there than to read your plate voltage from the socket.
with the 68k resistor reading different when its in circuit, that is because the 1.5k is grounded on one side, also the 68k goes to your output transformer that is also grounded on the other side through the speaker jack. so in circuit using an ohm meter it will show a different resistance because you are also reading other resistances in parallel.
as for the heater voltage if it is center tapped or referenced to ground with a hum balance pot, then you will see about 3.15v from either side to ground. if you read from one side of the pilot light to the other you should see 6.3v.
and for your input problem, like i said before if you are using that picture of the input jack that was posted as a guide. it is wrong. you will have problems.

[djroge1]

Wow this has been a busy discussion. I’m about to head out to band practice but I’ll try and catch up later this evening.

as for the input jack ~ I don’t know why but this is always the part that confuses me. I’m sure it’s fairly simple but sometimes the simple things cause confusion I guess. I even get mixed up when building pedals. The ring, the tip the N.C…. :huh:

[DeepBlue]

So in the middle of some testing…the Plate voltage is like 280 on BOTH sides of the resistor! Could my resistors be bad? They both seem to read fine when i check resistance. I am going to verify that they are still showing that value. I also have 68K resistance from tip to far side of the 68K resistor on the way to pin 2. I also have no continuity when the jack is plugged, between tip and ground, which I believe is good! On to collect more numbers.

So would a high voltage on the 12ax7 cause no sound? Interesting also, when i connect my VOM to the preamp side of the 100K, my speaker has a low hum, but when i disconnect its completely silent again. I dont see a bad solder, but could a cold solder do this? And perhaps my VOM is fixing it temporarily when attached? Ahhh the joys and nightmare of trouble shooting!

[DeepBlue]

Food for thought…I found some forums saying that using EH 12ax7 in the V1 position sometimes leads to failure. Is it possible my 12ax7 is bad? Sorry I keep posting, I am just home working on things now so while i have the time to post I will! I happen to be using the Electro Harmonix tube as V1.

So yes the voltage across the 100k doesnt drop very much. 300 to say 280-290 seems to be what I am seeing as measure to ground at the star ground.

[Andy]

I dont think that the fact that a V1 tube is EH should make a difference, that sounds odd to me BUT, it may very well be a tube issue here. Hard to say but it almost sounds like the tube is not drawing current (I could be way off on this) and not working. So one test would be to replace that tube. But then the question always is, what caused the failure, if in fact that is the issue.

[beelzebum]

if you are getting the same voltage on both sides of your 100k resistor, and the resistor is good, then your tube is not conducting. it could be a bad tube, or the cathode resistor not being grounded. whats the voltage on pin 3.
also you should read 1M from tip to ground with a plug in the jack, but not plugged into an instrument. when you unplug from the jack it should read 0 ohms from tip to ground.

[Author]

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