From Scratch Amp Building Chronicle - 6/13/10 UPDATED!!

[sah5150]

Signs of life...

Last night, I made some manual switching fixes and then tried the guitar into loop return, which did not work...

I then followed my friend Olaf's suggestion of putting a jack in at the PI to ground and that worked. I got a clean guitar sound at the cab that I could adjust the volume of with the PPIMV. Even with the PPIMV up all the way it wasn't as loud as I'd expect a 100W amp to be though, but I got reasonable volume sound.

For the heck of it, I then decided to try just plugging the guitar into the regular inputs again. Now, I get somewhat what I'd expect - a reasonable volume distorted guitar sound at the cab that I can adjust the gain of with the plex gain control and adjust the overall volume with the PPIMV! Sounds like a guitar amp now! Both the high and low inputs work as I'd expect them to.

I can change the plex gain control and go from a completely clean sound to a distorted sound pretty much like I'd expect for the plex mode, however, once again, the amp isn't near as loud as I've experienced cranked plexis when the PPIMV is cranked. Also, none of the tone controls or masters (except the PPIMV, of course) seem to do anything. When I adjust the masters - the volume does not change. When I adjust the tone controls, the sound stays the same. This seems to be the case with the presence and density controls as well, which doesn't make sense to me since everything past the PI seemed to check out.

Troubleshooting continues and there is a long way and probably lots of problems still to tackle, , but I heard a distorted guitar tone out of my cab, last night!

Steve

[sah5150]

cool. you wouldnt beleive some of the bonehead mistakes ive made. In fact just recently Mark Cameron found a big mistake I had on my vactrols that were switching MV's .. once he pointed it out, I couldnt beleive I could get that twisted around. When you look at switches too long, you end up overthinking it.

Oh man - I am no where near getting my switching working... I'm sure I still have a lot of problems!

Steve

[jerrydyer]

got anymore closeups of it as it is now.

[sah5150]

got anymore closeups of it as it is now.

I'll take some tonight and post - what specifically would you like to see closeups of?

Steve

[erigm]

Could you just hardwire it in plex mode without all the switching and fx loop and all that other stuff and just getting working right that way, and then mod from there?

[sah5150]

Could you just hardwire it in plex mode without all the switching and fx loop and all that other stuff and just getting working right that way, and then mod from there?

Well, here's the thing. If the plex gain pot is getting signal through the plex relay, I should be getting signal through the default paths on all the other relays, which means the tone controls and the plex master should work by default. My next step is to check the continuity on all the default paths to make sure they are actually there. Then I'll just take it from that result. There must be some default paths that are not connected or not connected correctly...

Steve

[jerrydyer]

got anymore closeups of it as it is now.

I'll take some tonight and post - what specifically would you like to see closeups of?

Steve

I dont know Im just trying to keep your hopes up

[jerrydyer]

Could you just hardwire it in plex mode without all the switching and fx loop and all that other stuff and just getting working right that way, and then mod from there?

Well, here's the thing. If the plex gain pot is getting signal through the plex relay, I should be getting signal through the default paths on all the other relays, which means the tone controls and the plex master should work by default. My next step is to check the continuity on all the default paths to make sure they are actually there. Then I'll just take it from that result. There must be some default paths that are not connected or not connected correctly...

Steve

yep what you do is pick one channel and wire it manually and get the amp kickin ass first then go back and finish them relays.

[sah5150]

got anymore closeups of it as it is now.

I'll take some tonight and post - what specifically would you like to see closeups of?

Steve

I dont know Im just trying to keep your hopes up

Very funny, very funny!

Steve

[jasonp]

Hey Steve! Keep up the good work man don't worry I know it is really tough right now but as you start to figure stuff out you will learn and get better to understand how the switching works and how to problem solve. I had many many issuse with my slo clone which I implemented relay switching which imo was much easier then what you are doing.

A couple things I found with my build that might help you was the 470R resistor off the pi.. make sure that is not a K. Also what I found was if I had the 100k and 82k resistors swaped on the pi of the plates it would make the amp sound like a 25 watter, didn't have the punch and the gain or drive.

[jasonp]

I see a cap on your layout of .0022uf for the input of the pi. From most amps I've seen from schematic wise are .022uf. Not sure if that will do anything for you.

[sah5150]

Hey Steve! Keep up the good work man don't worry I know it is really tough right now but as you start to figure stuff out you will learn and get better to understand how the switching works and how to problem solve. I had many many issuse with my slo clone which I implemented relay switching which imo was much easier then what you are doing.

A couple things I found with my build that might help you was the 470R resistor off the pi.. make sure that is not a K. Also what I found was if I had the 100k and 82k resistors swaped on the pi of the plates it would make the amp sound like a 25 watter, didn't have the punch and the gain or drive.

Thanks Jason - I've checked those things and am good there - found that stuff in other threads here actually!

Thanks!

Steve

[sah5150]

I see a cap on your layout of .0022uf for the input of the pi. From most amps I've seen from schematic wise are .022uf. Not sure if that will do anything for you.

My latest layout and my amp do have a .022uF input to the PI. Thanks!

Steve

[Roe]

I see a cap on your layout of .0022uf for the input of the pi. From most amps I've seen from schematic wise are .022uf. Not sure if that will do anything for you.

this makes no difference. a 1500pf cap has a 53hz -3db point in this position.

[sah5150]

The latest...

I had to fix the mains switch as I had the indicator lamp wired to the wrong terminals. It was on all the time when the amp was plugged in. I just found this out because I did not have an indicator lamp until it showed up yesterday... (forgot to order one...)

I have now followed the default switching path from the input through each of the relays to the controls and I have "beep" continuity as expected (Loop off). Good...

I then tested the amp with my guitar again and was very careful this time. The "plex" gain control adjust gain from clean to distorted plexi type gain as I said. The "plex" master now does adjust the volume, but not how I'd expect. Even with the master turned all the way down, there is still a good amount of sound, but no where near as loud as when it is cranked. Not working right, but signal is getting there. Tone controls still don't seem to do anything, but I notice now that adjusting the density control reduces the volume of the amp. The amp is still not loud as a cranked plexi, but it is reasonably loud in the room. Lets put it this way, after a few minutes my wife asked me to turn it down (she thinks it sounds good though. HA!), but the one time she heard my PeaceMaker fully cranked because I had left my attenuator up full from when she wasn't in the house, she was completely freaked out! Thin, trebly sound with distortion - no bottom end or mids... does sound kinda cool though...

As I said before, I re-wired the manual switches because I misunderstood how they worked. Now, no matter how I switch them, I don't get the hum I was getting with them before in some settings. Good...

One strange thing is now I'm only measuring 5.9VDC on the preamp heaters... I was getting ~ 6.2VDC before... haven't changed anything in that circuit?? Hmmm.....

Measured all the voltages now again and here is where I'm at:

V1 (12AX7) - Pin 1 - 128.2, Pin 3 - .796, Pin 6 - 175.7, Pin 8 - 1.351
V2 (12AX7) - Pin 1 - 134.2, Pin 3 - .824, Pin 6 - 235, Pin 7 - 134.2, Pin 8 - 134.7
V3 (12AX7) - Pin 1 - 266.2, Pin 2 - 17.79, Pin 3 - 45.96, Pin 6 - 175.8, Pin 7 - 0, Pin 8 - 1.369 (Loop tube)
V4 (12AX7) - Pin 1 - 198.3, Pin 2 - 20.77, Pin 3 - 32.82, Pin 6 - 194.3, Pin 7 - 21.85, Pin 8 - 32.9 (Phase inverter)
V5 (EL34) - Pin 1 - 35.6 mV, Pin 2 - 3.261, Pin 3 - 459.7, Pin 4 - 458, Pin 5 - -40, Pin 6 - 459.7, Pin 7 - 3.445, Pin 8 - 35.6 mV
V6 (EL34) - Pin 1 - 33.7 mV, Pin 2 - 3.282, Pin 3 - 460, Pin 4 - 457.5, Pin 5 - -40, Pin 6 - 459., Pin 7 - 3.474, Pin 8 - 33.7 mV
V7 (EL34) - Pin 1 - 37.9 mV, Pin 2 - 3.307, Pin 3 - 460.2, Pin 4 - 457.5, Pin 5 - -40, Pin 6 - 458.8, Pin 7 - 3.521, Pin 8 - 37.9 mV
V8 (EL34) - Pin 1 - 38 mV, Pin 2 - 3.393, Pin 3 - 460.2, Pin 4 - 457.5, Pin 5 - -40, Pin 6 - 458.8, Pin 7 - 3.584, Pin 8 - 38 mV

Lotsa stuff look suspect in the preamp section...

Power section and PI look relatively correct, but the voltages are all down a bit across the board since my last measurement... I didn't change anything... wonder if this could be from fluctuation in the wall voltage I'm getting.... I dunno...

I'll try the VTL... Wonder why the "plex" master sems to adjust the volume, but not all the way off.... Since I'm getting continuity across the default path I can't figure out why the tone stack controls don't work... All the grounding looks good and I measure 0 volts where I should with the amp on...

Lots more to do...

Steve