Blackface Fender Twin Build

[Charlie Wilson]

Hello, I few months ago I posted photos of a Fender Deluxe Reverb Reissue conversion I did and the same customer wanted a Twin also. This one is not a conversion but Mojo chassis and cabinet. The power, choke, and reverb driver are Mercury and the OT is a custom Heyboer single 4 ohm tap twin. I guess it is kind of obvious what the components are. I used some very nice PVC 1000v solid 20 gauge wire for the yellow, red, and orange and style 1007 for everything else. Amp sounds great. I think it helped to throw in a few vintage ceramic discs. Also, the lead dress around the oscillator tube is on purpose. I had to keep pushing the wires closer and closer together to keep it from coupling to the reverb.
CW

[ToneMerc]

Sweet!

TM

[Colossal]

That is freakin sweet

[xtian]

That is very groovy, waavy lead dress. Like being at the beach.

[Charlie Wilson]

I built it for a Surf music guy and I live at he beach so I guess I can't help the wavy vibe. Now for the bummer part, after being the guy that sticks up for fiberboard I find that this board is a bit conductive. I get a couple volts on the board around the b+ eyelets on the hot side of the board and less than half a volt between the plate resistors. I usually see at least this with vintage amps that are working just fine but the problem with this one is my vibrato channel volume control is a bit scratchy. I have never liked the plate supply eyelet being so close to the low side of the treble cap and I think that is the problem. I get about 1 volt between the plate supply eyelet and the treble cap eyelet. I think I may take a deep breath and proceed to route the plate supply wire all the way to the top of the plate resistors. This is a pay the rent build so I hope I don't wipeout on this one.
CW

[xtian]

Fiber board!

[Charlie Wilson]

Yeah maybe the stuff they make surfboards out of is better for amps. Anyway, I moved the wire and now the scratchy is gone and it seems like the treble is a little clearer.
CW

[Stevem]

Nice nice work ! but you should have used shielded wire from the grids to the tone stack , or better yet mount all the tone stack parts on the pots themselves as that gets rid of atleast 1.5 feet of grid wire that acts like a high impeadance antenna!
Yeah, it does no looke pretty when built like that ,but the amp sounds better and all the oscillations and instability goes away!

Also the the wire's from the input jacks should be shielded .

Also the way Fender grounds the preamp amp section filters makes for hum, a seperate ground wire should be run back to the PT grounding point

[martin manning]

I like it CW, very tidy. I'm not a fan of fiberboard, too many reports of stray voltage. Did that come from Mojo too?

[Charlie Wilson]

Thanks guys. Yeah Stevem I guess there are some improvements that could be done to Fender's original layout but I find there are trade offs. Sheilded wire makes things quieter but dulls the high end a little. I am not so sure about running the preamp ground to the same spot as the power amp. I have had a few repairs of vintage amps were a previous tech did that and it actually caused hum. Same thing with the fiberboard, obviously it has its drawbacks but a vintage style Fender build does not sound the same with other materials. Yes Martin, it is from Mojo. The weird thing is when the amp is first turned on, the voltage around the B+ eyelets is around 3.5v. After about 15 min of being on the voltage drops to .3v or less. I also find that my VTVM measures these board voltages much less than a good Fluke digital meter and I know the VTVM is calibrated. Anyway the amp really does sound good and has that vintage blackface vibe. I think the real winner is the Heyboer single 4 ohm tap output transformer. It is the closest to the sound of a original Schumacher I have heard yet.
CW

[dorrisant]

Very nice Charlie!!

[Charlie Wilson]

Interesting note. I was playing around with the amp and it seemed like the treble was a bit hard sounding and not blending as well as I wanted with the mid and bass(vibrato channel) so I put a vintage 2-35 250k pot in for the treble control. Wow, instant vintage amp sound. It seems that particular taper was chosen for a reason.
CW

[martin manning]

Not looking to argue, but this is where I go off the rails... Taper is just the angular rotation needed to get a given resistance ratio from input to wiper, and wiper to ground. It's still just a resistive attenuator, so what could that have to do with "vintage" sound? Could it be something about the material used in the resistive element?

[Charlie Wilson]

Not looking to argue, but this is where I go off the rails... Taper is just the angular rotation needed to get a given resistance ratio from input to wiper, and wiper to ground. It's still just a resistive attenuator, so what could that have to do with "vintage" sound? Could it be something about the material used in the resistive element?

Yes I agree with all of that but we are talking about the treble control. Correct me if I am wrong but the resistance between mid/bass and wiper and treble and wiper are different depending on the taper which means the perceived mix would sound different. I suppose it is possible the trace is a different material but I have never heard anyone verify this so I guess it is possible it is not. Maybe age/wear smears the sound giving the effect of a sweeter high end. The thing is, I have not even read a definitive explanation of what a 2-35 taper is. I just know that when that pot is used in the treble position it gives what I perceive as a very Fender like sound.
CW

[martin manning]

Taper is just the percent of total resistance on one side of the wiper at 50% rotation. Regardless of the taper, there is an angular position that will produce any given ratio of resistance on one side of the wiper with respect to the other. In the tone stack pots, it's still just a resistive divider, regardless of what the rotation angle is to achieve a given ratio. If there is a difference in sound between pots of the same total resistance, it has to be something else that causes it.