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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Fri Jun 08, 2012 1:58 pm
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tyronne wrote:
That's why I suggested what I said above..."I'd take the back panel off first and inspect if anything loose is visible.
If nothing apparent, I'd turn it on with the guitar plugged in and strum a bit while looking and gently probing to see if it could be isolated."

And again, that's with access to the inside of the cab. So you can isolate from the inside. Not the outside.


While I appreciate your suggestion, I can assure you that there is nothing loose in the chassis that is vibrating. The top panel is clearly vibrating and is most likely due to stress caused by the way the chassis is mounted in the cab or one of the two transformers or the cap doghouse touching the underside of the cab. That is a different situation and fix than if something were loose.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Fri Jun 08, 2012 3:38 pm
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Bill I think your best bet is to try adding that little bracket/strap, as Arjay mentioned it's probably the fix Leo came up with to solve the same issue you've come across. Instead of drilling the mounting hole you could initially try fixing a small flat square of rubber, something like a cut piece of tyre tube comes to mind, to the face of the bracket that meets the underside of top panel.

If you slotted the other mount hole on the bracket face that attaches it to the tranny, it would give you a little up/down adjustability and you might be able to set it up with just enough pressure to simulate pressing on the panel with your hand to eliminate the vibration, without having to drill a hole in the top panel.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Fri Jun 08, 2012 3:58 pm
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No457 Snowy wrote:
If you slotted the other mount hole on the bracket face that attaches it to the tranny, it would give you a little up/down adjustability and you might be able to set it up with just enough pressure to simulate pressing on the panel with your hand to eliminate the vibration, without having to drill a hole in the top panel.


+1!

Arjay

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Fri Jun 08, 2012 4:53 pm
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No457 Snowy wrote:
Bill I think your best bet is to try adding that little bracket/strap, as Arjay mentioned it's probably the fix Leo came up with to solve the same issue you've come across. Instead of drilling the mounting hole you could initially try fixing a small flat square of rubber, something like a cut piece of tyre tube comes to mind, to the face of the bracket that meets the underside of top panel.

If you slotted the other mount hole on the bracket face that attaches it to the tranny, it would give you a little up/down adjustability and you might be able to set it up with just enough pressure to simulate pressing on the panel with your hand to eliminate the vibration, without having to drill a hole in the top panel.

Snowy


I am going to look at doing a bracket, but the reason is slightly different than for what you describe. Please excuse me while I slip on my engineer's lab coat and whip out my pencils and sliderule. :lol:

If you look at the vintage amp photos I posted, you can see that the chassis control panel, power transformer, and bracket are all in practically the same plane. It appears that when the chassis is installed in the cab, the power transformer and bracket may actually be touching the underside of the cab or very near to it. The chassis mounts to the cabinet using two bolts through the control panel and top of the cab. This mounting results in the heavy power and output transformers, as well as the choke and cap doghouse, hanging out behind the amp chassis. The net result of this is a moment arm that is trying to twist the chassis and tilt the transformer down and toward the back of the cab. The bracket when installed does not press on the underside of the cab. Instead it pulls down on the cab from underneath, preventing the amp chassis from twisting away from the top of the cab. In other words, if you loosen the two bolts that attach through the control panel, the transformer side of the chassis will rotate downwards. The bracket prevents that from happening. I suspect that given the weight of the vintage transformers compared to modern ones, that Leo added the bracket to keep things from twisting, not to prevent panel resonances, although in my case that might be a desired side effect.

When I examine the cab and chassis (Might not be tonight as I have a pretty bad headache following a bad week at work. Due to budget constraints, I was cut from the program I was supporting. I am now in an overhead position and have about 30 days to find a new job or face potential layoff. Not a good situation for someone 58 years old with a mortgage and a wife with major health issues. :( ), I'll be looking at all the things discussed earlier as well as checking for any interferences between the transformers and cabinet or speaker magnets. The solution may be as simple as just slightly shifting the mounting of the chassis to change the tension on underside of the cabinet top or applying some damping material (like Dynamat) to the underside of the cab top.

Ok. I'm out of engineer mode now. I greatly appreciate everyone's comments and suggestions. I'll post what I find when I get a chance to look at things. :D

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Fri Jun 08, 2012 4:57 pm
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Bill,

Let me be clear, I'm not suggesting anything IS loose in the chassis.

I'm suggesting that by looking inside the cabinet and strumming with the guitar at the level you first heard the vibration, that you then isolate where it is. That's all! Not trying to say you left a nut loose here or a screw or whatever.

Was never trying to imply that you had done anything wrong in the build, which I enjoyed watching. Was just making suggestions on how to possibly find the cause.

I'll go away now. Best of luck.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Fri Jun 08, 2012 5:08 pm
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tyronne wrote:
Bill,

Let me be clear, I'm not suggesting anything IS loose in the chassis.

I'm suggesting that by looking inside the cabinet and strumming with the guitar at the level you first heard the vibration, that you then isolate where it is. That's all! Not trying to say you left a nut loose here or a screw or whatever.

Was never trying to imply that you had done anything wrong in the build, which I enjoyed watching. Was just making suggestions on how to possibly find the cause.

I'll go away now. Best of luck.


Please read my last post just before yours. It may help explain my thinking and approach to fixing this issue. Stick around for when I post what I find. :D

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Jun 09, 2012 4:25 pm
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Ok. I finally got around to taking a close look at what might be causing the panel resonance in my 5F6A clone.

First, a rehash of what is happening:

With my Fulltone Full Drive 2M set with the volume, overdrive, and boost controls between 9:00 and 12:00 and using the Vintage/Standard overdrive settings, and both volume controls on the amp set to "6", there is a very strong resonance in the top panel of the amp when plucking the "D" string at the 10th fret. Eventually the amp will go into feedback. The same thing will occur with the overdrive turned off and the amp volume controls set to "10". Pressing on the amp handle, top panel, or chassis faceplate will stop the resonance. Granted that none of this is a normal playing situation for me. I just happened to stumble on it while noodling around the other day.

So, to begin troubleshooting, I pulled the back panel off and with a bright flashlight took a look around inside the amp looking for anything loose or touching, or any cracks in the wood or joints, etc. A thorough exam revealed ....... absolutely nothing. :?

There is plenty of clearance between the top panel and the PT and OT as well as between the cap doghouse and the back of the speaker magnet. There is nothing loose inside. No cracks in the wood or joints. Nothing looks amiss in any way.

I did notice one thing that got me to thinking. When the top panel was vibrating, pressing anywhere on the top panel stopped the vibration dead. However, pressing on the chassis faceplate first changed the tone of the vibration and pressing harder stopped it. When I initially assembled the amp, I installed the chassis so that the open back of the chassis pressed on the upper back panel of the amp to which I had affixed some aluminum tape, hoping it would create a better shield for the back of the chassis. This meant that the edge of the chassis that slides under the top panel was barely touching the top panel. That would seem to account for the fact that it took greater pressure to stop the vibration by pressing on the chassis faceplate then on the top panel itself.

So, on a hunch, I loosened the two chassis mounting bolts and slid the chassis as far into the cab as it would go (really only about another 1/4" or so), tightened them back up and reinstalled the upper back panel. There is now about a 1/4" gap between the back panel and the chassis (might actually aid in venting hot air from inside the cab) and there is that much more of the chassis pressed tight up against the underside of the top panel (fullfilling Snowy's suggestion of applying pressure to the underside of the top panel).

The result? The resonance has been greatly reduced. What little bit seems to be left can be dialed out by changing the Full Drive settings slightly (Switching to Flat Mids instead of Vintage and turning down the volume and boost a notch. The OD and Tone controls stay the same.) to reduce the mids slightly. There is still a very, very slight resonance, but the amp no longer breaks into feedback like it was also doing.

Is there anything else that I can do? Well, I ordered a sheet of Dynamat Extreme that I might try installing to the underside of the top panel. I wouldn't want to use too much as that would then deaden the pine cab too much and that is part of the sound of the 5F6A. I will also look at adding the strap between the PT and top panel, although I don't think that it is as necessary now. In any case, I need to find somthing to use as a strap in my junk drawer or buy something to make one. In the meantime, I'm going to continue using the amp as I have things set now and see how it goes. It is interesting to note that even playing the amp clean but loud, it is clearer sounding due to the reduced resonance.

Thanks for everyone's suggestions. :D

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Jun 09, 2012 6:24 pm
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Check your local Ace Hardware store, Bill. They usually stock small 5" x 8" sheets of metal in the hobby section for the RC modeler crowd. Ours has various thicknesses -- from .003 shimstock up to .020" for strapping fuel tanks, building wing-spar gussets and the like. I've seen copper, brass, aluminum, and mild steel at our store.

HTH

Arjay

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Jun 09, 2012 9:09 pm
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Retroverbial wrote:
Check your local Ace Hardware store, Bill. They usually stock small 5" x 8" sheets of metal in the hobby section for the RC modeler crowd. Ours has various thicknesses -- from .003 shimstock up to .020" for strapping fuel tanks, building wing-spar gussets and the like. I've seen copper, brass, aluminum, and mild steel at our store.

HTH

Arjay


I used to have a supply of metal straps used to hold up the rear of car radios back in my car stereo installation days. They were made out of some sort of fairly soft and bendable metal. Not sure I have any left, but I probably have something lying around the house.

Its clear that the C note at the 10th fret on the D string is the resonant frequency of the amp/chassis combination. Like I said, I only discovered it while just noodling around. I normally don't run into this problem during normal playing. However, it is interesting to note that when this sort of problem is found and fixed there is a noticable improvement in clarity of the amp sound. I think a small piece of Dynamat (What I ordered is actually a 4"x10" sheet of Dynamat Extreme used to dampen license plates. $15 plus free shipping from Crutchfield in Charlottesville, VA) applied to the underside of the cap top panel plus the chassis strap should completely cure the problem. Or cause another one. :lol: :lol: :lol:

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sun Jun 17, 2012 4:42 pm
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Success! :mrgreen:

I applied the 4"x10" sheet of Dynamat to the underside of the top panel and the resonance is gone. The amp and cab sound much cleaner. It begs to be played LOUD. Of course, cranking the amp up loud brings out all the other resonances in the room that I always seem to hear no matter what I do. :lol:

I ended up not putting a strap to attach the PT to the top panel as in the original and Reissue 59 Bassman. Upon closer examination I found I would have had to remove the transformer from the chassis, disconnect all the wiring, and then reassemble the whole mess. It is just not something I care to do. The chassis is quite firmly attached to the cab and does not appear to be going anywhere anytime soon. :D

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Jul 21, 2012 9:57 pm
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I had an idea for a cool mod to my 5F6A clone, but it didn't work out they way I had hoped. Maybe someone can come up with a suggestion on how to make it work.

I have been experimenting with bypass caps for the bright channel. The stock cap is a 100 pF silver mica cap. For my guitars and ears, that results in too bright a tone with the settings I like. After experimenting with different values, I settled on a 68 pF cap. Works great. But I still wanted the option of the 100 pF, and I thought an option of no cap on the bright channel might come in handy.

The mod is very simple. The Weber kit has a ground switch installed for cosmetic purposes only. For the mod, I took a DPDT (center off) switch and soldered a 68 pF silver mica cap across one pair of outer terminals and soldered a 100 pF silver mica cap across the other pair. I then used a shielded cable to connect the center pair of switch terminals to the bright channel volume control.

The result? The switch works to switch values and change the brightness, but at the expense of a significant increase in hum in the bright channel. Unfortunately, the switch and caps are right next to the power transformer wires, and in particular, the heater wires. Here is a photo (with the ground switch installed) to show what I mean (This is an old photo. The unused ground switch is the two lug switch on the far left. This was replaced with the DPDT switch and caps. I did not photograph that configuration):

Image

I even tried to redress the wires coming out of the transformer with no luck. Any suggestions on how I can do this mod and eliminate the hum without taking everthing apart? Should I just abandon the idea (I've already put it back to the stock configuration for now).

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Last edited by bluesky636 on Sat Nov 03, 2012 11:51 am, edited 1 time in total.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sun Jul 22, 2012 7:37 am
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It's a "real estate" problem, Bill......one of those location location location issues.

The only two possible remedies I can think of are:

1. Use shielded wire to and from the switch, and shield the hell out of the switch itself.

2. Convert your heater and pilot lamp tap to 6.3 VDC.

The bright channel has two inputs, no? It might be easier to solve your problem there by configuring one of the inputs with the 100pf cap and the other with the 68pf cap. It won't offer the convenience of the toggle switch but it may get the job done.

Arjay

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sun Jul 22, 2012 9:05 am
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Retroverbial wrote:
It's a "real estate" problem, Bill......one of those location location location issues.

The only two possible remedies I can think of are:

1. Use shielded wire to and from the switch, and shield the hell out of the switch itself.

2. Convert your heater and pilot lamp tap to 6.3 VDC.

The bright channel has two inputs, no? It might be easier to solve your problem there by configuring one of the inputs with the 100pf cap and the other with the 68pf cap. It won't offer the convenience of the toggle switch but it may get the job done.

Arjay


Hey, thanks for the input Arjay.

As noted, I did use a shielded cable from the switch to the volume pot. Unfortunately, there really is no way to easily shield the switch as it is fairly large and as can be seen, there is not much "real estate" there. :( I got the idea from a 5F6A clone I saw at a local dealer that specializes in vintage amps. There was a clone there (I forget which one, but it was a well known name.) that used the ground switch location as a bypass for the 5F6A tone stack. I doubt the shop would open up the back of the amp to let me take a look. :lol: They also had a REAL 59 Bassman for sale. Looked really nice. I wanted to try it out but there was some guy there that spent the entire time I was there (about an hour) trying out different wah pedals. He was loud and a much better player than me. :lol: :lol:

Converting to a DC heater is not an option. Besides, all the other AC wires are right next to the switch anyway. There really is no other place to mount a switch away from the AC wiring unless I mounted it to the wood back panel.

Converting the inputs is an idea. That would be similar to what I did with my Champ 600, using a cap to bypass the 68 kOhm input resistor. The values would have to be different due to the different location in the circuit, but I do have a calculator for that. It sounds a bit different than a bright cap on the volume control as the cap is always in the circuit and the bright boost doesn't vary with volume.

Well, I think I will just put this idea on the back burner for a while and think a little more about how to get what I want. I think the idea has merit, it just has turned out more difficult to do than I had hoped. :|

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Nov 03, 2012 11:57 am
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Just a little note, no changes to the amp, it still sounds great. :D

All of my photos had been hosted by Webshots. Webshots is going away ( :cry: ) as of 1 December and being replaced by a crap hosting site called "Smile". The new site sucks. So I have been rehosting many of my photos on Flickr. I tried Photobucket, but the native format of the photos from my camera is .jpe and Photobucket only supports .jpg. Flickr supports both. So I have replaced all the Webshots hosted photos in this build thread with Flickr hosted photos. I'm pretty sure I got all of them but will check again. I'm also going to do the same thing with my Champion 600 Mod thread and one or two others.

In any case, if I make any other changes to my 5F6A clone, ya'll be the first to know. :D

PS: In case I didn't mention it in a earlier post, I finally went back to and have stuck with, the original 100pF bright cap in the bright channel. Leo DID have it right to begin with. :lol: :lol: :lol:

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Aug 17, 2013 7:19 pm
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Some time ago and I discovered that the normal and bright channel volume controls of the original Fender 5F6A Bassman were 1 Mohm linear pots, vice the audio taper pots that came with the Weber kit. This was confirmed through conversations with the owner of an original 59 5F6A on The Gear Page. He also confirmed the change in the Presence control to the "modern" design in late 59/early 60. The Fender 59 Bassman Reissues (both the 90s version and the LTD version use the "modern" presence control and linear volume pots). I installed the "modern" Presence control in my original build. However, I finally got around to swapping out the audio taper pots with linear pots. I wish I had done this earlier!

The linear taper pots completely change the response of the 5F6A circuit. Before, I had to turn the volume controls up to almost "8" to start getting some overdrive sound. Now the amp starts to overdrive around "4" and has a really good overdrive sound at "6". But because the gain of the 5F6A circuit is fairly low (especially with a 12AY7 in V1), the volume is not an on/off switch. Even with linear pots it is nice and smooth. Also, when set to a good drive around "6", the amp cleans up better with the guitar volume controls than with the original audio taper pots I had in the amp. I am really happy. :mrgreen:

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