So, I completed my build and I was really happy with the way it turned out...until I turned it on. I expected it to need some tweaking, but I figured it would come in closer than it did. There is WAAAAAAAY too much gain. I mean to the point of sounding like 2 Big Muffs chained together and maxed! It completely compresses the front end of the note and then sizzles with distortion. I fooled around with some things that I thought would help, but using 12AU7s seem to tame it the best. The problem with that is it seems to suck the liveliness out of the amp. I like how its on the edge and seems so "alive", but I really need to tame the gain so that I can get a usable sound so I can see what I got in this.
I did do a couple of things different from the standard. I have the tube sockets flipped 180 degrees from origionals, I did my power supply arrangement in a different comfiguration and relocated the bias adjustment (I know I only need two test poits - DOH!), and my board is wider than what is typical. I also have my controls switched around for my taste. I can see where there could be stability(parasitics) issues with these things, but that isnt the case. Well, any advice is appreciated. Im really proud of how hard I worked to get everything Hiwatt-like with the wiring. I did the same thing with my ac-30. Here are gut shots of them both.
[img752]http://img.villagephotos.com/p/2008-11/ ... G_1582.jpg[/img]
the vox is light because I use it as backround on myspace
[img:984:414]http://img.villagephotos.com/p/2008-11/1327865/vox.jpg[/img]
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I'm no Trainwreck expert (far from it...), but I wager you'll get a posting or two to the effect of "layout is everything" or "copy Ken exactly" or "layout, layout, layout", etc...
Okay, I'll say it. Your layout certainly isn't going to help you, particularly the long runs of wire between the gain pot and the bright switch, which pass right over the presence cap.
The presence circuit is a great place to get all sorts of unwanted interactions going on, so putting as sensitive and as comparatively high impedance a signal as the output from the gain pot over there is likely to cause issues to some degree.
You could demonstrate the effect of this by temporarily disconnecting the bright switch wires at the gain pot to see if that's a major area of concern at this stage.
Also, you've got your NFB wire running directly alongside the power tube grids, which looks very neat but won't help things at all. You either want to move it away from them laterally across the chassis or do it Komet-style and elevate it so it effectively sits on the bottom of the chassis rather than the top.
A shot of the back of the pots, showing the wiring more clearly would help.
But before doing all that, have you verified all of your component values and checked voltages? I always find that printing a layout and methodically comparing values and actually ticking them off can help to spot things that you have have overlooked a dozen times before because you're subconsciously assuming it's correct.
What trannies are you using, and what primary impedance on the OT?
Component locations aside, how do you run the amp? At what volume setting does it reach the highly compressed stage?
Operation of a typical Express is to run the Volume at 12 or 1 O'clock. You control the amount of distortion; clean, hairy, crunchy, to singing lead solely with your guitars volume knob.
If you have a scope and signal generator you can check on which gain stage is clipping 1st on the volume knob. If it's one of the preamp stages, that might indicate a bad or wrong value component in the preamp. You could also verify that both sides of the PI and output plates are functioning. The most appropriate solution to the problem depends on which stage is not in sync with the Express mojo directive.
Thanks to all of you that responded. I can see the layout seems to be pretty important from what you guys are saying. I did notice the prescence seems to be acting like an antenna - especially as I turn it up.
I did verify my voltages at the filter caps and the most any one of them was off was 3 volts. That seemed like normal variance. I didnt verify each component by measurement, but that would be another interesting thing to have, wouldnt it? The exact value. I understand Ken Fisher did just that - measured each component looking for the "right" value before installing it.
I am using the toneslut transformers since they seem to be universally accepted as correct. I am also using all mallory caps for filter and coupling, CDE for smaller values.
As for operation, I have just been trying to adjust out the sizzle distortion. As soon as any overdrive starts to come in, it lops off the front of the note and gives me 60s style fuzz - even at low volumes if the eq is adjusted up. I can get a semi-clean sound, but as soon as I lay into the string, the compression hits the ceiling.
I do have a scope and signal generator at work. I will probably give that a try first just to take a look at it, but I think Im going to end up "standarizing" my layout. It seems with an amp this unstable, going with a tried and true layout seems the direction I should go. Thanks for all the great advice. I will probably tear into it next week. Im still trying to get the Megasquirt tuning dialed in on my truck this weekend. Too many projects!
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If you can take the time, try and do a voltage chart like the one below, and post here. A good number of folks here could spot something pretty quick if it's out of whack.
V2 cathode looks low. Double check the cathode (and plate) resistors on it. It may be clipping from idling too cold or hot. The scope would show that plate clipping early on the volume control if that's the problem. That plate should clip late on the volume control.
That's a typo on the screen pin 4 on v5 right? If not, that screen resistor is an issue.
You could switch tubes around to verify the voltage "problems" don't follow the tube(s.)
Gary's voltage chart is listed under the TW files -> compiled voltage readings.
I stuck it on a scope and it looks like it is only overdriving half the waveform. Im going to go through the wiring to make sure everything is correct and check all my solder joints. I did find out that the output transformer was out of phase. My experience has been that if you put an OT in an amp and its out of phase, the whole amp howls and you swap the leads. I guess this condition only showed up when the presence was cranked. Read the Phase Matters More Than I Thought thread and spotted it. The amp still sounds like a Big Muff with two batteries. Its a process, I guess. Appreciate all the input.
JBD
eBay: The place where a fool and their money part.
752]http://img.villagephotos.com/p/2008-11/ ... G_1582.jpg[/img]
