First build 5F2-A: low output

[mac dillard]

You have something strange going on between point A where you are reading 368 VDC and pin 3 of the 6V6 where you are reading 294 VDC. There should be only about 5 volt drop across the OT primary windings. Make sure you have the OT wired correctly. You should be reading about 320 VDC at pin 4 of the 6V6. Do you have another 6V6 tube that you can try? If not pull the 6V6 and take voltage (DC) readings at pins 3,4, and 5.

[mikepukmel]

Don't be worried about posting photos. I posted some and my wiring is hideous! (Mrs Johnsons's second grade class gets a soldering lesson!)

[JDodd]

You have something strange going on between point A where you are reading 368 VDC and pin 3 of the 6V6 where you are reading 294 VDC. There should be only about 5 volt drop across the OT primary windings. Make sure you have the OT wired correctly. You should be reading about 320 VDC at pin 4 of the 6V6. Do you have another 6V6 tube that you can try? If not pull the 6V6 and take voltage (DC) readings at pins 3,4, and 5.

I checked the OT against the schematic and it looks good. I checked the website for the OT and saw this, so I reversed the leads, but it made no difference.


I also rechecked the resistor values (especially the one between point A and point B) and they all check out. The significant drop in voltage between A and B has me baffled. I don't have a spare 6V6 tube, nor do I have a tester at this point, though I am looking for someone local who does.

Here are shots of the board. It's a bit of a rat's nest at this point, and I've lifted the board so it looks strange.

[mac dillard]

Pull the 6V6. Power up and measure the voltages at pin 3,4 and 5.

[J M Fahey]

1) please, with amp OFF, disconnected from mains and after you checked +V discharged to zero, measure DC resistance across OT primary winding .

2) same conditions, measure resistance from 6V6 cathode to ground.

3) please post the actual schematic you used, so we all talk the same.
IF a .pdf, please convert schematic to a .gif, .png or .jpg so we can straight see it in the page itself, instead of jumping back and forth to another window, and we can also edit it, add measured voltages, etc.

4) for the next test, build an earphone to guitar plug cable and download http://www.mediacollege.com/audio/to...6bit_30sec.mp3 and http://www.mediacollege.com/audio/to...6bit_30sec.mp3

Playing those tones on a phone or MP3 player gives you constant 440Hz or 1kHz tones, about 100 to 200mV, perfect for amp testing.
Set player to "loop" or "repeat one" so tone plays continuously, original is just 30 seconds.

Then we´ll be able to follow signal from input jack to speaker terminals and measure actual power.

[Chuck H]

JDodd,

I'll open by saying that it's not my intention to offend but some of this may come across as overly critical.

I'm not even going to try and trace the complete circuit from the posted photos. There's simply too much wire going in too many directions. I don't understand why either.?. Did you intend to resolder the whole thing with shorter leads if it worked first with the longer leads? Did you intend to just stuff and mash all that lead wire down into the chassis before sending it home in the cabinet? While there may be a wiring error causing the low volume condition I think another possibility is a lead proximity induced positive phase error causing a parasitic oscillation in ultra sonic frequencies. This would make the amp work hard at amplifying something you can't even hear all the time and seriously limit it's ability amplify audible frequencies. It would explain your low volume problem and also the 1.25V on pin 5 of the power tube. That voltage shouldn't be there when the amp isn't conducting. That tells me your amp is trying to amplify something when it's just sitting there seemingly doing nothing. You really, really need to get all that lead wire under control and follow good lead path, layout and grounding in the build before you can go any further I think. Trying to trouble shoot it as it is could well be a waste of time anyhow since even if you did get it working that could all change when you stuff all that wire down on top of itself in the chassis.

JM12C

[JDodd]

LOL. In a word, yes, I was planning on cleaning up much of the wiring after assembley was complete, but I have been sidetracked on the volume problem. If this could be a cause then I could change gears and do that first.

No offense taken.
:-)

[JDodd]

1) please, with amp OFF, disconnected from mains and after you checked +V discharged to zero, measure DC resistance across OT primary winding .

2) same conditions, measure resistance from 6V6 cathode to ground.

3) please post the actual schematic you used, so we all talk the same.
IF a .pdf, please convert schematic to a .gif, .png or .jpg so we can straight see it in the page itself, instead of jumping back and forth to another window, and we can also edit it, add measured voltages, etc.

4) for the next test, build an earphone to guitar plug cable and download http://www.mediacollege.com/audio/to...6bit_30sec.mp3 and http://www.mediacollege.com/audio/to...6bit_30sec.mp3

Playing those tones on a phone or MP3 player gives you constant 440Hz or 1kHz tones, about 100 to 200mV, perfect for amp testing.
Set player to "loop" or "repeat one" so tone plays continuously, original is just 30 seconds.

Then we´ll be able to follow signal from input jack to speaker terminals and measure actual power.

Will do. thanks.

[Chuck H]

Here are a couple of things I see wrong in the photos. Notes in yellow.

[JDodd]

1) please, with amp OFF, disconnected from mains and after you checked +V discharged to zero, measure DC resistance across OT primary winding .

2) same conditions, measure resistance from 6V6 cathode to ground.

3) please post the actual schematic you used, so we all talk the same.
IF a .pdf, please convert schematic to a .gif, .png or .jpg so we can straight see it in the page itself, instead of jumping back and forth to another window, and we can also edit it, add measured voltages, etc.

4) for the next test, build an earphone to guitar plug cable and download http://www.mediacollege.com/audio/to...6bit_30sec.mp3 and http://www.mediacollege.com/audio/to...6bit_30sec.mp3

Playing those tones on a phone or MP3 player gives you constant 440Hz or 1kHz tones, about 100 to 200mV, perfect for amp testing.
Set player to "loop" or "repeat one" so tone plays continuously, original is just 30 seconds.

Then we´ll be able to follow signal from input jack to speaker terminals and measure actual power.

The easy ones while I have a moment, the other two later.
1) 284.8 ohms.
2) 461 ohms.

[JDodd]

Pull the 6V6. Power up and measure the voltages at pin 3,4 and 5.

3) 406v
4) 378.8v
5) 0.7 mv

[mac dillard]

Check your wiring from point A to the OT to pin 3 of the 6V6. No way you should have a 74 volt drop through the primary of the OT. Should be 5 to 10 volts.

[JDodd]

I pulled the 6v6 and measured the voltage at 3,4, and A,B, C Plus pin 1 on the 12AX7B, then reinserted it and rechecked as follows:

6v6 in. 6v6 out
3) 318v 410v
4). 255v. 383v

A). 375v. 410v
B). 257v. 385v
C). 219.5v. 327v

12ax7 1. 152v. 203v

[Jazz P Bass]

Schematic with voltage readings: http://music-electronics-forum.com/a...n-voltages.gif

Have you tried to measure the Vac signal that is at the 6V6 grid when driving the preamp hard?
What is it's amplitude?

If you have a low signal there, the output Will be low.

[mikepukmel]

Not being picky, just noticed a few things:


1) A few spots where your soldering iron melted off some insulation. REALLY inspect all of your wiring carefully for spots of bare wire. If you find any, unsolder one end, and put shrink tubing over the damaged area. I found out the hard way to get a thin piece of cardboard, cut it with scissors and with one hand hold it over adjacent wires to protect them when Im soldering in a crowded area. Important: Don't turn your amp on again until you make absolutely sure there are no melted areas on your wiring.


2) This looks like a "cold" solder joint, its not shiny and looks crystalline. Use a magnifying light and check all of your solder joints, they should all be shiny and "wet" to the wire or component lead:



3) Input jack tip shorting on case? While you're doing your testing on your new build, put a small piece of electrical tape over the END of the jack tip on your input jack. If that jack tip shorts on the case, you'll lose signal there!!!!!


4) Frayed wire. I have HUGE problems with this on my wiring: when you have a stripped end of wire, sometimes when you're pushing the wire through a jack lug or tube socket pin, one or two strands bend away and don't make it into the lug or pin hole. Unsolder those connections, re-strip the wire, and be really careful to make sure all of the strands go through the connection hole:


5) Your capacitor is too close to the power resistor. If it heats up, as they do when they're dissipating, it could kill your capacitor. Unsolder and move components away from power resistors:


6) When you stripped the wire for this connection, you cut several of the conductor strands. (same image as frayed wire above)

This could be really bad for higher current connections. You should unsolder, re-strip, and re-do any connections where strands were cut off when stripping.