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SEplex style build questions (fizz, fuzz, fart?)
SEplex style build questions (fizz, fuzz, fart?)
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Old 21st December 2017, 10:00 AM   #21
GeorgK is offline GeorgK  Austria
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Agreed it's strange behavior.
But measuremants are always a good starting point ;-)
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Old 21st December 2017, 06:09 PM   #22
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by jarrodthebobo View Post
I'm so frustrated right now.
You have my sympathy!

When I first got into electronics, this type of frustration was a frequent companion with every new project. I was a kid at the time, with no diagnostic tools at all, so it was hard to dig myself out of the holes I got into due to my novice skills.

One of the things that can happen is that each attempt at a repair creates a new fault (or faults). This happened to me a lot at first, mostly because of my poor soldering technique as a novice.

Anyway, my first suggestion is to take a nice long break. Do whatever relaxes you and makes you happy - take a long hot shower, play with the dog / cat / kid, go for a walk, enjoy that piece of chocolate that's been sitting in the pantry, whatever.

When you're refreshed and no longer frustrated, and ready to re-visit your amp, make a plan as to how you're going to approach the fault-finding.

I understand that you don't yet have a 'scope. That leaves you a little bit blind, but there are still at least two different fault-finding options, both involving tracing the progress of an audio signal through the chain (as you have already started to do).

Firstly, you need some sort of source of a steady audio signal. Playing your guitar as a test signal doesn't work well, because a guitar is large, cumbersome, and occupies at least one of your hands. So use your resources here. Ideally you would have an audio signal generator or function generator. If not, you can solder together a couple of transistors, resistors, and caps to make an astable multivibrator (Google if you don't know how). As a last resort, do you have an old MP3 player or phone you could play audio files through?

(Please don't use your shiny new $$$ iphone or Android phone, there is a possibility of frying it, and that will not make you happy!)

With the signal generator ready to go, first unplug the speaker from the amp, and connect the signal directly to the speaker. Do you hear a faint sound? Good, the speaker is working. No sound? You have either a dead speaker, or bad wiring to the speaker. Find and fix the problem.

Now plug the speaker back into your guitar amp, and power it up. Using extreme caution to avoid electrocuting yourself, apply your test audio signal through a small capacitor (say 0.01 uF / 500V rated) to the control grid of either output tube. You should hear a much louder tone from the speaker.

It works? Okay, try the other output tube (assuming push-pull). If applying the test audio signal to either output tube's control grid produces audio from the speaker, your output stage is working.

If not, break it down. Did one tube work? Then focus on the other one. Make DC voltage measurements. Power down, discharge filter caps, make resistance measurements. Inspect and measure continuity through solder joints. If you are methodical and test every single point where the circuit might have failed, you will find your problem, and be able to fix it.

Okay, let's say you have achieved success, and now you know the output stage of your amp, including OT and speakers, is working.

So you move your signal source back one stage, to the control grid of your phase inverter (if cathodyne), or to both control grids of the phase inverter (if a long-tailed pair). Best to disconnect any global negative feedback while doing this - it can confuse the issue a lot. As before, fault find if necessary, one step at a time, until you are 100% sure your phase inverter is working.

By now the method should be clear - you start at the speaker, and work back, one amplification stage at a time, until the signal disappears. When it does, you find and fix the fault (it will be in the stage you are currently testing), then resume the process.

At some point, you will find yourself applying a signal to the input guitar jack, and hearing a loud sound through your speaker. Now your amp is working.

There is an alternate approach to signal-tracing, which is closer to what you did: apply a small test audio signal to the control grid of the input stage, then use a small value cap (say 0.010 uF 500V) to pick off the audio at each successive stage of the amp, and run that audio into a small, low-power test amplifier so you can hear it.

The disadvantage of this method is that you need both a source of audio signal, and the little amp that you use to detect if the signal is present at each stage.

-Gnobuddy
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Old 21st December 2017, 08:02 PM   #23
GeorgK is offline GeorgK  Austria
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At least you can rule out some faults by simply taking DC mesurements on the tubes. This will give you a hint whether you have trouble with a tube, its connections or supply.

A decent used scope won't cost you an arm and a leg either. I personally would even prefer some old-style analog CRT model for this work. They are bulky, but I definitely prefer the response of a CRT trace over a LCD screen. Plus a - for me analog - function generator to make it complete.
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Old 21st December 2017, 11:40 PM   #24
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by GeorgK View Post
I personally would even prefer some old-style analog CRT model for this work. They are bulky, but I definitely prefer the response of a CRT trace over a LCD screen.
+1 At almost* any price point a used, good analog CRT will be better value for money. Bulkier, hotter, less automated but better at seeing what's going on. And tougher & maintainable.

The new "stop here" point is about $500US where you can get into HP/Aglient/Keysight "InfiniiVision" product (used and the bottom end of new). These have "persistence" done in hardware (ASIC) rather than software so you've a much better chance of catching "the glitch".

Plus the automation and integration now available on modern digital gear...
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Old 22nd December 2017, 06:12 PM   #25
AquaTarkus is offline AquaTarkus  Canada
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Quote:
Originally Posted by jarrodthebobo View Post

The amp in question started its life as a bit of a project one of my buds made up for me; it was an Epiphone Valve special that he gutted (aside from the power and output trannies) and built a brand new all tube amp circuit inside.
Take a look at this thread on sewatt.com for how to mod the Valve Special successfully, www. sewatt.com/node/11921 It may be some help. You will have to login to acceds the thread.

Last edited by AquaTarkus; 22nd December 2017 at 06:25 PM. Reason: correction
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Old 22nd December 2017, 06:14 PM   #26
AquaTarkus is offline AquaTarkus  Canada
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Last edited by AquaTarkus; 22nd December 2017 at 06:23 PM. Reason: user error -please delete
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Old 22nd December 2017, 06:17 PM   #27
AquaTarkus is offline AquaTarkus  Canada
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Last edited by AquaTarkus; 22nd December 2017 at 06:24 PM. Reason: Finger trouble - please delete
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Old 22nd December 2017, 11:33 PM   #28
jarrodthebobo is offline jarrodthebobo
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I have some updates:

So I made an audio injector probe and started poking around in the circuit; and there is life! A lot of it actually... most of the circuit path will allow signal to pass... all except for the pin 1 and pin 6 of both the 1st 12ax7 of the marshall channel, and the only 12ax7 in the clean channel. V2a+b all function as they should; putting the audio into the grids or plates yields pretty load audio so amplification IS occuring.

But my main issue here is what the hell is going on with pin 1 and 6 on the 12ax7 stages? SHouldn't injecting audio push signal through the tube for amplification? I've tried different tubes in these spots too but to no avail; it isn't being fixed by that. I even bridged the gain pot on the marshall channel just to see if signal will flow past it and be amplified; but all that I get is very miniscule audio if you inject audio before the 470k resistor paralleled with the small cap, and miniscule audio if you inject it after words. Injecteding audio into pin 7 yields loud output of the music being played. The pins of the socket all have continuity, so what gives? What could be causing this??

I'm so confused, hahah

On a further note, it turns out if you play the guitar very loud, quiet output can barely be heard from the speaker.
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Old 23rd December 2017, 04:26 AM   #29
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by jarrodthebobo View Post
So I made an audio injector probe and started poking around in the circuit
Excellent, you are now on your way to fixing the problem. All that's left is to be 100% systematic about your fault-finding process.

Quote:
Originally Posted by jarrodthebobo View Post
...all except for the pin 1 and pin 6 of both the 1st 12ax7 of the marshall channel, and the only 12ax7 in the clean channel.
Okay, this opens up specific possibilities. Perhaps you have mis-wired all your 12AX7s. Some possibilities:

1) Is the oddball centre-tapped heater wired properly? You have to parallel the two parts of it to run it off a 6.3V heater voltage (4 & 5 shorted, 6.3 V AC between pin 9 and the shorted combination pin 4&5).

Once you are sure the heater is wired properly and getting hot as it should, do the following DC voltage measurements on V1:

2) What are the DC voltages on pins 1, 2, 3, and the B+ end of the 100k anode resistor connected to pin 1?

3) What are the DC voltages on pins 6,7,8, and the B+ end of the 100k resistor connected to pin 6

Quote:
Originally Posted by jarrodthebobo View Post
But my main issue here is what the hell is going on with pin 1 and 6 on the 12ax7 stages?
There are a lot of ways to get it wrong, and only one way to get it correct (and working)! That 12AX7 might not work for a number of reasons. For example, you may have got the pin connections wrong, mis-wired the heaters, maybe there is a cold solder joint so no B+ on the anode resistors, et cetera, et cetera.

Do the faultfinding steps I outlined above, keep thinking through every step, you will find the problem. The DC voltage checks I recommended will either point you straight at the problem, or narrow down the search so you can do your next diagnostic test.

With fault-finding a system like this, you just have to be 100% logical and test every possibility, until you find and fix every problem (worst case, there may be multiple problems).

Quote:
Originally Posted by jarrodthebobo View Post
I've tried different tubes in these spots too but to no avail; it isn't being fixed by that.
And this is telling you it's not the tube, it's the circuitry around it. Most likely something is wired up wrong. Again, do the diagnostic tests I listed in this post...that's the way to find out what's wrong!

Quote:
Originally Posted by jarrodthebobo View Post
all that I get is very miniscule audio if you inject audio before the 470k resistor...
In my experience, getting miniscule amounts of signal usually tells you nothing. A high-gain, high impedance circuit like a guitar amp will respond to all sorts of tiny unwanted signals, in all sorts of ways that nobody anticipates, and that doesn't help you narrow down the source of the much bigger problem you are looking for.

Quote:
Originally Posted by jarrodthebobo View Post
What could be causing this??
Most likely a miswired socket, or bad solder joints. But guessing is rarely helpful when troubleshooting - so (if you ever watched anything Star Trek related) be as Vulcan as you can be, set aside your emotions for now, be calm, methodical, systematic, do all the diagnostic tests I just listed in this post.

You will get it working if you are methodical and systematic, and when you do, that's the time to become human again, and enjoy the surge of satisfaction and happiness that comes with success!

-Gnobuddy
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Old 23rd December 2017, 07:39 AM   #30
jarrodthebobo is offline jarrodthebobo
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V1 pins
1:229v
2. .3mw
3. 1.956v
4. 4.16v ac (6.8ac between 4 + 9) 20.1v dc... seems a little high yet tubes are dim
5. Bridged with 4
6. 261v
7. .5mvish
8. 4.4v

V2.
1. 304v
2. 262v
3. 263v
4+5+9 all the same
6. 200v
7. 1.2mv
8. 1.537v

V3 (seperate channel)
1. 135.5v
2. .8mv
3. .333v
6. 175.6v
7. 135.4

B+4 304v
B+3 308v
B+2 342v
B+1 355v

6v6 pins
1. .1mv
3. 333v
4. 353v
5. .2mv
6. .1mv

Output transformer
357v primary common
333v 5k primary
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