Express Harmonics and Feedback

[Richie]

If the amp is stacked sometimes moving closer to the speaker to encourage feedback is negated by hum pickup from the head which chokes the notes.

Glen, i'm sure you are aware of this, but others may not. The hum when you get closer is from the transformers. If you move any guitar [pickups] closer to any amps transformers ,you'll hear the hum pick up quite a bit.

[Richie]

I think it's interesting that KF would up with exactly the PI component values used in late tweed Fenders (and thus in early Marshalls). Very much an "if it ain't broke, don't fix it" kind of thing. But this might be a fun (and pretty simple) area to experiment with. In some blonde amps, Fender got the tail resistor down as low as 6K8 (with a 12AX7), before turing tail (heh) and going up to 22K and even 27K (with 12AT7s).
Randall Aiken suggests replacing the tail resistor and the cathode resistor with trim pots and playing around with them to get the desired effect. Lower tail resistor values should throw off the (already tenuous) balance of the two sides a little. Large values will make them more balanced, but eventually reduce headroom. One of the things I'll probably end up fooling round with.

I posted about the different values.. over on Metro amps a guy posted pics of his amp. And it is exactly the same.. Notice the tail resistor and tone caps
http://forum.metroamp.com/viewtopic.php?t=17277&start=0

[BluzMike]

I think it's interesting that KF would up with exactly the PI component values used in late tweed Fenders (and thus in early Marshalls). Very much an "if it ain't broke, don't fix it" kind of thing. But this might be a fun (and pretty simple) area to experiment with. In some blonde amps, Fender got the tail resistor down as low as 6K8 (with a 12AX7), before turing tail (heh) and going up to 22K and even 27K (with 12AT7s).
Randall Aiken suggests replacing the tail resistor and the cathode resistor with trim pots and playing around with them to get the desired effect. Lower tail resistor values should throw off the (already tenuous) balance of the two sides a little. Large values will make them more balanced, but eventually reduce headroom. One of the things I'll probably end up fooling round with.

I had a very early fender [untouched]5f6 bassman here,and it had the 6.8k tail.
And it also used different tone caps values, it didn't use the .022..it was more similar to later fenders

One of my three 5F6As also .047uF/.1uF tone caps, and I have seen two others like that before. Most had .022uF/.022uF though.

[gearhead]

Have tried a couple of things. One was to change the 150k V2 grid (leak?) resistor to higher values. Used a 250k pot in series with the 150k. Interesting, did thicken it up a bit (part of a high pass filter), and a bit wilder, but not quite the territory I was seeking.

On a whim, detached the short, 1", V2 grid to 150k/.0022uF junction wire at the later junction and soldered on an 8 inch piece of wire. Wrapped it over the top of the board so it runs parallel to the V1a grid wire (tonestack) under the board before going to back to the junction. Pretty much a complete loop.

Much better, hehehe. Quite a bit wilder, . Not sure if it's solely due to the 8" length, the loop, or proximity to the V1a grid wire. Takes a bit off the top end, but that's next.

How much funner can it get? LOL.

[Buschman]

Very interesting gearhead. Show a pic if you can. Sounds like an internal feedback antenna.

[gearhead]

Recognize that this was a "hmm, let's see what this will do" hack, so it's not the neatest. Used unshrunk shrinkwrap as a sleeve to cover the shorter/longer wire joint.

[gearhead]

Can someone do me a favor next time you fire up your express and just feel like measuring things

What is your V3 (PI) common cathode voltage ( pins 3/8 ) ?


Also V2 and V3a/b plate voltages if you have time.

Thanks,

Dave

[gearhead]

Alrighty folks, would appreciate your help. I am about ready to just rip it apart and start over.

I HAD it in the zone for about 5 minutes. Redid a LOT of solder joints, flicked it on, and Kabam, noisy sucker-city. Wow, didn't have that before. Tapping anywhere on the chassis led to a loud and ringing, pinging, thunk. Very, very sensitive. Mein Gott, I had it! Noticeably louder than anytime before. And had a certain vibrancy to it that wasn't there before either.

However, within a couple of minutes, turning the volume knob past 11 0'clock made the noise go way down and/or led to a very loud screech. Similar to the swapped OT lead screech, but not nearly as loud. Within another minute more or so, the volume knob had absolutely no effect.

Huh? Completely disconnected the pot, and it checked out. Have redone a few other component, but not all, checks/

Put everything back together, and lo and behold, it is now back to it's old, sedate, self. Volume works, etc, but with lower voltages and olume/vibrancy.

One thing I did manage to do before it puzted on me was to take some measurements. Across the board, no matter which one, they were all higher voltage readings. The PI cathode was at 36-37V vs a rock-steady 33V before.

Any ideas?

[Allynmey]

For 1, V2 pin 2. The wire should be 1 inch long coming from the .002 cap. Why did you wrap it around the board? Take some more high res pics and I'll try to figure out what else.

[gearhead]

That pic is of a hack I did trying to get some more harmonics; read the text 5 posts back. Buschman asked for a pic. It's now back to the normal 1 inch length.

Before I did the resoldering (which temporarilly opened things up) I played around a lot with lengthening various grid input wires to run parallel to other wires for V1 and V2. Was trying to purposefully induce crosstalk/feeback.

This included running an un-shielded line from the input jack to the socket -and- bending that V2 wire in the pic over to run parrallel to the input line for about 6 inches, since V1b and V2 are in-phase. If this was 95% of the other builds posted here, it should have been screaming it's head off at that point, but nope. Did gain some feedback/harmonics, but seemed to cut into the headroom and mush out the top end a bit.

Tried a number of similar permutions, with similar results.

Will try and get pics up, but I'm pretty sure that everything, from a visual inspection standpoint, is connected up correctly. Have spent probably 40 hours in the last three weeks rechecking with schematics, layouts, going back to Francesca pics, as well as the mods to kick-start it's heart. About enough time to build another. LOL.

I was mostly wondering from a symptom standpoint, did anything ring any bells? Meaning any tell-tale clues as to bad components or shorting/grounding issues?

BTW, did notice something that has me puzzled; plate resistor voltages don't add up. Measured the B+ at the top of all of the pre-amp plate resistors, and then at the corresponding plates. The difference should match voltage across the plate resistor, or so I thought. It's not. In each case the voltage across the plate resistor is -Less- than the difference between the B+ and Plate voltage measurements by 10-20 volts. Huh?

Thanks all,

Dave

[skyboltone]

Gearhead:
I hate to tell you this, but the super sensitive, pinging and plunking, ringing and all that is what's it's supposed to sound like. More or less. This sedate little romper room number with the 6 inch loop antenna has got to be a miss wire or a wrong component. I know you've looked 100 times but something is wired wrong.

I don't think you have a scope yet. So here's what's next. Make up the stethascope that has been talked about here. A probe with a .01 cap on the end, connected to a headphone amplifier. Disconnect the grid to V1B and check for signal there. If everything is OK, hook up V1Bs grid and disconnect the input to the clipper etc. etc. If you don't have a signal generator I think there are CDs around with tones on them that you can run into the input of the amp and pick it up as it goes along.

I lament the demise of the VTVM. It's a perfect tool for looking at tube voltage multiplication to see if you're getting the gain you should working through the amp. If you've got a .5 volt input into stage one you'll see 25 volts into stage two. No you don't get 1250 volts out of stage two because there isn't 1250 volts available, so put the same .5 into stage two and see if the 25 volts come out the other side. Work through making sure that each stage does it's job. Unfortunately your fluke won't cut it here. It loads the circuit too much. Scopes and VTVMs don't load the circuit, thus giving you a true sense of what the stage is doing.

Remember that this amp is basically just short of being out of control. It's a gain monster. You had it working right for just a minute there.

[gearhead]

SB,

Yeah, when it was there, it sounded like what I knew it should sound like. That puppy was wild and crazy; the much-higher noise level was music to my ears, LOL. That's whats so frustrating; something is draining the life out of it, and am beginning to suspect a bad or intermittent component, or a short/ground where I can't see it. Measured all the components before soldered them in, but things happen.

No, I don't have an OScope yet. Been trying. Just lost one (Tek) on ebay two days ago. I will get one!

Will look at doing the stethoscope.

Or, I can just make this my designated Express LV (low volume). LOL.

[skyboltone]

Lift the NFB wire temporarily. That takes one variable out of the mix. Pull the clipper and clip a jumper direct from V1a to the PI. See what you get. Add one stage at a time.

Jump off a bridge

Yer pal
Dan

[skyboltone]

BTW, did notice something that has me puzzled; plate resistor voltages don't add up. Measured the B+ at the top of all of the pre-amp plate resistors, and then at the corresponding plates. The difference should match voltage across the plate resistor, or so I thought. It's not. In each case the voltage across the plate resistor is -Less- than the difference between the B+ and Plate voltage measurements by 10-20 volts. Huh?

Thanks all,

Dave

You need to rephrase this. The way I see it, at the top of Rp is B+ at the bottom of Rp is the plate. Are ignoring the dropping string? What? I'm stuck.

[geetarpicker]

I suspect a good old fashioned loose connection...

I had an issue with one of my two clones on start up. Turning the treble control kicked the treble control in and out of the circuit. Maybe it was the bass or mid knob, can't quite remember. Anyway I played a gig with the amp and all was well. However in time it came back. At first I thought it was a bad pot. Turned out there was a bad solder joint on one of the tone pots that looked perfectly flowed out, but the wire wiggled when I pulled on it and looked very closely under a bright light.

Solid core wire doesn't give much, and if it moves a tiny bit while the solder is cooling you can have a crack in the joint.

So, that said I'd reflow EVERY solder connection in your amp especially those on the pots.

You could even buy some freon cold spray which can sometimes help find intermitent issues in connections and components.

On a different and doubtful note, you mentioned it wasn't very loud but is it clean and clear? If it has low volume but it's kind of distorted you might have a ultra high frequency oscillation going on. That would zap your volume because the circuit would be clipping already albiet way out of the range of hearing. Just a thought. However usually when amps do this you can hear the degrading effect the oscillation has on the guitar range, even if the oscillation is way above the hearing range. These amps have a very large bandwith. Heck in my old house my original Trainwreck Express would send out enough RF (when it was cranked) to start the satelite TV dish spinning as it was on an RF remote albiet on the other side of the house from the studio! The solution was to wrap the output tubes in a small tight layer of foil and ground them...