Fender Bassman 5F6A Build help needed

[jimmy74]

Just finished building a Fender Bassman 5F6A clone with some major differences. Firstly the seller of the kit had bent the chassis in such a way (or maybe the turret board just isn't the right sort for this chassis) that I'm forced to place the input jacks on an external plate because they won't fit inside the chassis, also I'm forced to use only one output jack instead of 2 (following Ceriatones layout).Next big mess was how the seller of the kit indicated which PT wires have which function, there's a sticker on the PT with colour coding which is completely wrong and non compatible with the actual wire colouring. Sorted that mess out too and continued wiring the amp up. There was a fried 0.1uf cap in the bias voltage splitter which was bleeding +250v into the bias to pin 5 on the power tubes, so one of the power tubes may have been badly damaged (or maybe the seller of the kit just sent me 2 tubes he had laying around +10mA difference between them). Anyways I'm waiting for a new pair of matched tubes to arrive but in the meantime I do have some questions:
- seeing that the seller of the kit has made a mess on the PT, I'm wondering if he has done the same thing with the sticker on the OT. The primaries are ok, it's secondary windings I'm worried about. It has 0 4 8 and 16 taps, how do I actually verify that they are what the sticker says they are?
- symptoms the amp was suffering from loud hum and very sensitive presence and treble which would become a high pitch whistle when turned up. I rewired the input jacks with shielded wires, that seemed to take a bit of the hum away. I also tried as best I could to keep the NFB wires as short as possible and as far away as possible from any HV wires. That did not fix the problem.
- other symptoms made me believe that the power tubes (maybe damaged) are not doing what they're supposed to do and are setting out a very confused sound which after a while would become flabby and farty (though this may also be because the speakers don't have the right power rating - they were already in the cab that I'm using as a combo, though there is no indication of how many watts they are, no brandname or model, just a sticker indicating the impedence). Is there any way of testing the power handling of a speaker?
- lastly... the seller of the kit did not include the mains cap board so I used a hobby board I had laying around, and wired it according to ceriatones layout. I did place it next to the main PTP just behind the power tube sockets. Could this eventually be a cause of interference and hum or the whistling I'm getting through presence and treble?

If any photos are needed just say the word and I'll pop some up.

thanks for any help at all
J.

[jimmy74]

here are the voltage readings on the preamp tubes that I'm getting. No power tubes installed.
V1 12AY7
Pin 1= 178vDC
Pin 2 = 0
Pin 3 = 2.92vDC
Pin 6 = 182vDC
Pin 7 = 0
Pin 8 = 2.92vDC

V2 12AX7
Pin 1= 204vDC
Pin 2 = 0
Pin 3 = 1.29vDC
Pin 6 = 364vDC
Pin 7 = 199vDC
Pin 8 = 202vDC

V3 12AX7
Pin 1= 235vDC
Pin 2 = 18vDC
Pin 3 = 35vDC
Pin 6 = 430vDC
Pin 7 = 31vDC
Pin 8 = 35vDC

Anything wrong here?

[jimmy74]

here are some photos:

http://www.flickr.com/photos/73835524@N ... hotostream

http://www.flickr.com/photos/73835524@N ... otostream/

http://www.flickr.com/photos/73835524@N ... otostream/

this last photo was done with the old input jack wiring so don't take notice of that. It's just to show the cab and speakers I'm using:

http://www.flickr.com/photos/73835524@N ... otostream/

[jimmy74]

I changed the NFB resistor to 56K and that got rid of the whistle but the hum is still there. Following the Ceriatone layout the only differences are basically the grounding points I made: one ground point for mains, one for the PT + filter cap board (which I also modded to fit the bias trimmer on) and one ground point for the main PTP + output jack. I did not do a main ground point for the pots and left them grounded to the chassis as single points. So I took the input jacks ground wire from the volume pot and soldered it to the 100k/820r resistor junction, but that did not change the quantity of hum I'm getting. Here is a clip of the hum I'm getting. The guitar is attached, both channels are connected together and I turn up both volumes to max, then presence to max, other controls are half way up. When you hear the hum going back and forth that's because I'm moving the input jack plate aroundAfter a bit, I turn both volumes to halfway and strum on the guitar:

http://www.flickr.com/photos/73835524@N ... eame7b4365

[jimmy74]

I decided to do some tapping around with a screwdriver, very carefull not to hit the resistor and cap leads. I started around the 10uf 450v cap, as soon as I tapped it the hum got worse. I measured its capacitance with the amp off and drained and it measured around 10uf. I changed it out with a radial 10uf 450v cap I had but the hum is still there, just like it was after I tapped the previous cap. I've checked for continuity all through the board and everything is right. The only thing I noticed is that I had inverted the power switch wires.

Help!!

[jimmy74]

Just got the new power tubes and that didn't change anything except now I've got around 2mA difference between each tube instead of over 10mA. I also completely recabled grounding of the PTP + pots, used a solid metal bar as a ground buss for both pots and PTP going to 1 single ground point on the chassis. No change at all. Does anyone have any ideas? here's a clip of the hum I'm getting:

http://www.flickr.com/photos/73835524@N ... hotostream

thanks

[stef]

preamp tubes maybe?

[Froumy]

I've only read a little of your post, but you've been kicking around forever. Sorry, brother. I was hoping some one else would answer.

This should be a relatively simple build. My noise/hum issues have always been an issue of lead dress, or placing components too close to one another, but always in higher gain builds. Running signal wires too close to B+, and issues with my grounding. I haven't looked at your pics, but pay attention that your signal isn't running parrallel to your B+. Frankly, i wonder if that 90 degree confuses half of us on the led dress....

Are you bonding your input plate? Or are you grounding with no reference? In other words, if your trying to pull the ground off your input chassis, make sure you bond the plate to the chassis. If your input is isolated, ground it where you please. You did that, yes? Many amps of this time period, referenced half the filaments to the chassis,but I don't think it's your prob. This amp shouldn't require shielded input cables, unless you screwed something up.. Shielded cable could mask it, til you figured out what you did wrong.

You could float your filaments, or even reference them to Bias voltage to eliminate some hum, but once again, this amp worked great, never taking this in to consideration. This should be a moderately forgiving build.

There are some simple test methods you can use. Wire a spare cap(in series) to a test lead. Ground the other lead, and run it into a POS boom box, or a throwaway PC sound system, you have lying around. It's a poor man's oscilloscope. You can isolate stages, by attaching to the plate, or after the coupling cap. Emphasis on isolate.


I'll check your pics tomorrow, and see what I can see.

[jimmy74]

I took some better photos this morning, and recorded what I'm getting, the newest photos are dated 22052013 so they're the ones you should look at.

http://www.flickr.com/photos/73835524@N ... 569509141/

I have also posted a photo of what's under the board. The lower loop wires were the previous ground setup and are no longer present. For the heater wiring I followed exactly the same layout as what's present on Ceriatone, so there is a grounded CT for the 6.3v windings. Signal and HV wires being too close.... hmmm I can see one that may be a culprit.... looking at the photo of what's under the board... you can see the white wire going from the 10uf 450v filter cap to the 100k resistor junction for the V1 plates, this wire passes under and very close by to the signal wire from V2 pin 2 to the 270k resistor junction. Also I twisted together the OT primaries and passed them through the same hole as the choke wires... could this be another cause of hum?
BTW I do have a scope, and a good one too passes on to me by my late uncle, it's a tek 465B + DM44. I also have the 2 scopes one is a P6105A and the other is a P6063B.... this may seem silly but I just don't have the faintest idea of how to use it... so if the use of a scope can help out here... I would gladly like to understand atleast the basics of how this can be done.

thanks

[julkke]

Your solder joints look pretty messy, many of them might be cold

[jimmy74]

B+ = 431vDC right now
DC resistance on the 2 halves of the OT primaries gives me 49.8 ohms and 49.9 ohms, is this normal?

V1 12AY7
Pin 1= 159vDC
Pin 2 = 0
Pin 3 = 2.22vDC
Pin 6 = 160vDC
Pin 7 = 0
Pin 8 = 2.22vDC

V2 12AX7
Pin 1= 173vDC
Pin 2 = 0
Pin 3 = 0.9vDC
Pin 6 = 293vDC
Pin 7 = 172vDC
Pin 8 = 174vDC

V3 12AX7
Pin 1= 229vDC
Pin 2 = 24vDC
Pin 3 = 40vDC
Pin 6 = 226vDC
Pin 7 = 26vDC
Pin 8 = 40vDC

V4
Pin 3 = 400v
Pin 4 = -46
Pin 5 = 401v
Pin 6 = 404v

V5
Pin 3 = 402v
Pin 4 = -46
Pin 5 = 401v
Pin 6 = 401.v

One other thing I should add, that clip was done with the 2 channels patched together, and the guitar attached. If I do the same test without anything connected to the input jacks, and placing the input jack plate towards the front of the chassis (as if it were mounted inside the cab) I get a loud hiss (not a squeal or whistle) just hiss.

[Froumy]

Sry. Forgot about this. Use to be 10 posts a day on something like this.

Can't tell if your inputs are shielded. Looks like they may be. If so, is that breakout plate grounded(or shield), so it doesn't act like a receiver? How are you connecting it(to the chassis) and are your input jacks grounding to it, or is there no ground connection to that panel? Personally, Id have no ground connection to the breakout, isolate the jacks, and grab my ground from signal chassis ground, to cross off any difference in ground potential.

Your build looks respectable.

I keep my filament wiring much farther from the tube than you did. They are a huge source of noise potential. I noticed some were wrapped closer than my comfort level around the tube. I've virtual center tapped, and floated some of mine, to squash the AC noise. Have a bunch of 6.3 volt transformers on hand, as well, as this is a pet peeve of mine. Not saying that's your prob, but if yer fishin'....Throw a line.

Concerned with your NFB resistor swap curing your whistle. Just masking the prob. I think your prob is earlier. I've heard that sound when.....

I placed a filter cap too close to signal.
When my input lead dress was suspect.
When my grounding scheme wasn't right. (usually higher gain, though. I have some disgusting spaghetti tweed builds from way back, that still didn't do this)
When my filament wiring needed improvement.
When I needed to reflow my solders.

Not knowing what you did, I'd first verify that input plate, and corresponding grounding. Don't leave that plate flopping around when testing. If you ground to it, with no bond to chassis, you're screwed. But bonding it can screw you, too. You can also develop a ground loop if that plates potential is slightly different than chassis, when you bond it. Skip Leo's grounding. Isolate the input, ground to signal ground, skip the bonding. Normally, this should be a fairly forgiving build. My old tweed builds are total massacres, because these forums didn't exist. They sound fine.

Your prob is a simple one.. It's ONE thing. Don't give up.

[Froumy]

Shit balls.

Hope you haven't given up. That's a great amp. Years back, it was a frenzy on here. Hope that comes back.

My kids have had more fun with my o-scope than me(they like watching the lines move when they talk into the mic). That clip->cap-> cheap ass radio is great for isolating stages, when you clip onto the signal path. Good for determining interstage or global issues. I've only used my scope for ripple, and output signal...

G'luck!