OT questions

[marcoloco961]

I have read several times here that if the primary wires of the OT are reversed, the amp will be wired out of phase and squeal. On my first build, I used 6.6K Heyboer's, following the Express layout and schematic posted here and wired the green wire to V4 and the Violet wire to V5 and had no problems. V4 was hooked to the leg that connected to the 100K resistor and the 82Kleg connected to V5.

My second build uses 6.6K Pacific OT. Comparing the diagrams posted here, it looks as if the green and violet wires of the primary are reversed, although the Heyboer drawing does not have a dot to signify the start. Upon comparing the schematic to the layout, I noticed that on the Schematic the 82K leg hooks to V4, while on the Layout the same leg hooks to the V5. I believe I followed the layout in my first build.

I am now slightly confused as to what determines if the OT's primary is "out of phase" ? I realize that if it squeals when I plug it in then all I have to do is reverse the wires, but soldering around the power tubes is a PITA and I would like to only have to do it once...

My first instinct is to wire the grids according to the layout as before and to reverse the violet and green wires from what I did with the Heyboer. Anyone dealt with this situation before??? Is there a "rule of thumb" for determining the proper phasing? I know if I throw the dice and gamble on the 50 percent chance I get it right by accident, that I will be re-soldering...

[rawnster]

accidental post here...sorry.

[paulster]

You can actually work it out if you follow the various phase inversions that occur through the different gain stages, the PI and the output stage and, crucially, at what stage the negative feedback is reinjected into the circuit.

You also need to know the relative phase relationship between the primary and secondary of the transformer, which I always check on a scope before I install and cut the wires, just in case the manufacturer was having an off day.

In the case of the Pacifics and the Heyboers that I've had the start of the wind (for sake of argument) is the brown or green on the primary side and the black on the secondary side.

In the case of a Marshall-style PI (like the Express) the 100K side of the PI goes to the brown/green side of the OT primary and the 82K side to the blue/purple side. The Liverpool is the same, except that the default configuration uses 100K each side so it's not as easy to spot the correct way to wire it.

[marcoloco961]

You can actually work it out if you follow the various phase inversions that occur through the different gain stages, the PI and the output stage and, crucially, at what stage the negative feedback is reinjected into the circuit.

You also need to know the relative phase relationship between the primary and secondary of the transformer, which I always check on a scope before I install and cut the wires, just in case the manufacturer was having an off day.

In the case of the Pacifics and the Heyboers that I've had the start of the wind (for sake of argument) is the brown or green on the primary side and the black on the secondary side.

In the case of a Marshall-style PI (like the Express) the 100K side of the PI goes to the brown/green side of the OT primary and the 82K side to the blue/purple side. The Liverpool is the same, except that the default configuration uses 100K each side so it's not as easy to spot the correct way to wire it.

Thanks so much Paulster, you are a great help. I have to admit that you lost me on some of that. But I can deal with the green to the 100K side and violet to the 82K. Much appreciated.

[marcoloco961]

On the Wreckxpr.pdf file in the Trainwreck files section, the version I am following for a starting point is page 7 of 14. It shows the feedback wire off the presence pot hooking to the 16 ohm wire on the OT instead of the more commonly used 8 ohm tap. Typo?? Or was that done on purpose to tweak the sound a bit??

[paulster]

You don't want to use any of those schematics as they are the Blue Guitar ones and were unverified (and generally incorrect) attempts at cracking the Wreck code before Francesca and other Wrecks were properly documented.

The first post in this thread has the correct schematic:
https://tubeamparchive.com/viewtopic.php?t=5691

[marcoloco961]

You don't want to use any of those schematics as they are the Blue Guitar ones and were unverified (and generally incorrect) attempts at cracking the Wreck code before Francesca and other Wrecks were properly documented.

The first post in this thread has the correct schematic:
https://tubeamparchive.com/viewtopic.php?t=5691

I went ahead and moved the FB back to the 8 ohm tap. I used the "correct" schematic on the first build and was happy with the results, however I am looking for a bit more distortion. The only real differences in the two schematics are a 47k resistor on the input grid, a 820 ohm resistor on the entrance to the third stage grid, and a 150k resistor on the plate of one stage instead of a 100k.....ohh and a .001 coupling cap. I'm going to try it and see how it sounds.

[gearhead]

With the "correct" schematic, distortion is not insignificant. Sonically, what kind of increased distortion are you looking to get?

[paulster]

That's what I was thinking! Most people seem to be trying to tame the distortion on their builds!

[marcoloco961]

With the "correct" schematic, distortion is not insignificant. Sonically, what kind of increased distortion are you looking to get?

Hmm, I will try to elaborate on what I am looking for in words. I guess I would have to say more of an overdriven sound. The first build is great for blues, jazz, etc. The harmonic distortion and sustain are there, but it seems to lack the nice overdriven sound I have heard on so many clips of the different Wrecks. Everyone who plays through it seems to be looking for their overdrive pedal.

All moot point right now as I have some troubleshooting to do on the second build. Voltage checks went well with no tubes and current limiter and with tubes and current limiter, but upon final fire up I found a few issues.

First off, I ordered a 20K bias pot instead of a 25K so I had to adjust the 220K resistor to a 150k to get the bias between -30 and -40 with out tubes. After the tubes were installed the highest I could get the bias voltage was -35 and this only gave me 27mA on the power tubes. That is what I am going to try and fix first. Although it didn't look as if I was going to get to 45mA by the time I hit - 30, I will have to fiddle with it.

Also I have a extreme voltage drop between B+2 and B+3. Thgis is the biggest concern. It goes from about 395 on B+2 to 230 on B+3. And everything behind B+3 is accordingly low. I'm going to reflow solder joints and try again. I know I saw someone with this same problem in an earlier thread, just can't find it again .

Would the low mA on the power tubes cause this? I adjusted it and took measurements with the mA at a low and a high and it didn't seem to change the B+ voltages at all. I noticed that the voltage bleeds off of the filter caps at an extremely fast rate also. Turn the switch off and grab some test leads and it is already down to 5V.

Oh and I have some bad pots also. .....

[marcoloco961]

Bad 9.1K dropping resistor was the power killing culprit. Measured over 1M. Changed bad pot. Upped bias resistor to approx. 200K to get Tubes running where they should be @ 42 - 45 mA. Makes noise with a bass plugged in. Damn, I wish I played guitar!!! Anxious to hear what it sounds like.

[breakfastbuddy]

in general how will a transformer with lower impedanse sound compare to one with higer impedanse . i know a lot of you like the fender thing , sweet clean sound , but what if you like a bit of edge ? more like a marshall. is a lower impedanse the way to go ?

[marcoloco961]

in general how will a transformer with lower impedanse sound compare to one with higer impedanse . i know a lot of you like the fender thing , sweet clean sound , but what if you like a bit of edge ? more like a marshall. is a lower impedanse the way to go ?

I have read here that a 5.2K primary on the OT will give more of a hard rock guitar sound as opposed to the 6.6k primary. I have also read somewhere that changing the power tube grid resistors from the 1.5K to a 5.6K gives the amp a more "Marshall" sound. I was looking for a dual tap OT for the bench model I just built, it has a 5.2 and a 6.6 tap, but I ended up going with the 6.6K Pacifics. The bench model does seem to be a bit more agressive than my first build. It does have a few variances from the "proper" Express schematic. ( a 47K resistor on the input signal, a 820 ohm r on the grid of the second tube and a 150K on the plate of one of the 12AX7's and a .001 coupling cap). I am going to try the 5.6K grid resistors here real soon. I will post as to what I find.

[marcoloco961]

Quick Update:

Finally got a chance to have someone come and play some guitar while I did a little tweaking on the bench amp. As I said earlier, I started from the Blue Guitar file Wreckxpr.pdf file I found here, with the version on page 7 of 14. Not too different from the verified version. Power supply used is the one show on that page.

Was looking to get a little more of a "hard rock" / "thrash" sound out of the circuit without the extreme volume usually needed to get there. Quit laughing, I'm not a guitar player, I am just trying to give my Metal-head buddies what they want.

I did a little tweaking on the tone stack to get a little more bottom end and make the treble pot a little more useable, as it initially was way to bright and just added extreme hiss when turned up past 2 or so. I am going to post a schematic of the changes I made in case anyone wants to compare findings. Changes are in red.

We started out with a older Les Paul and used it to fine tune what we thought we liked. After that we plugged in a Kramer with some EMG actives...........Holy $hit, Those pickups sounded so good, I made him try them through the first build and it came to life. The circuit just loved those pick-ups. Excellent sustain and harmonic distortion.......EVH style.....and not quite as much gain as the first build which seems to make it easier to push it into distortion at a lower volume. Never even got to the grid resistor mod I intended to try. I was so happy with where it was at that point I was hesitant to change it anymore. I am thinking about using the opto switcher to toggle the resistance on the 100K R before the bass and mid caps between 47 and 100K, as the cleans are sacrificed some when it was dropped to a 47K.

[breakfastbuddy]

the experess is more of a lead amp then a rythm amp , if you want to make it more like a rythm amp put a 10h choke in the powersupply (like a comet ) i think i also would have that 56k dropping resistor as a pot . so you can adjust it to differtent pickups . it would be fun to hear some sound clips
Espen