Vibrolux Reverb AA964 Build

theprofessor

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I've found this series of videos by Psionic Audio helpful so far, since I've never built or worked on any BF Fender, other than a Princeton Reverb. One thing I wondered about (I think it's in the third or fourth video) is why he decided to tuck away the heater CT in the PT and to create an artificial center tap off the pilot lamp. I suppose this is to reduce heater hum? Which reminds me of how Blencowe says he always designs amps with a Humdinger balance pot for heater hum -- something I've not done or seen done before.









 
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Lowerleftcoast

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One thing I wondered about (I think it's in the third or fourth video) is why he decided to tuck away the heater CT in the PT and to create an artificial center tap off the pilot lamp. I suppose this is to reduce heater hum?
I figured he would install a humdinger pot or... perhaps he wanted to return the amp to original configuration. By his description the PT is original. IMO replacing an original CT with an artificial CT is odd. I would trust an actual CT rather than two low resistance resistors to provide less hum. Changing an original amp when nothing is wrong is not how I would do business. OMMV.
 

theprofessor

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I figured he would install a humdinger pot or... perhaps he wanted to return the amp to original configuration. By his description the PT is original. IMO replacing an original CT with an artificial CT is odd. I would trust an actual CT rather than two low resistance resistors to provide less hum. Changing an original amp when nothing is wrong is not how I would do business. OMMV.
I thought that strange, too, though I’m clearly no expert.
 

theprofessor

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I have a question about wire: I will use mostly solid-core cloth hookup wire on this build, Fender style. Though Blencowe recommends stranded-core for any place where wires will be twisted (like heaters) and also for connections to tube sockets, I find solid core easier to work with. But of course the cloth-covered pushback wire isn't rated for the very high-voltage sections of the amp. This didn't keep Fender from using it there, but I'd like to have a higher-rated type in the high-voltage section.

I have used the thick cloth-insulated wire from Tube Depot that's rated at 600V in the past, but the problem with that is shoving more than one or two strands through the grommet to the doghouse. I have a bunch of Alpha wire that I like using as well, but that's only rated at 300V. Then I have a small amount of 22 ga Tefzel wire, also from Tube Depot, that's rated at 600V. It would seem that's the stuff to use for high voltage. It is surprisingly thin!

And of course, one can use cut-offs from the power transformer. I keep those and have done that in the past. But it seems the 22 ga Tefzel stuff would save the most space going through the grommets and is rated for the job.

Any other thoughts or advice?

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Lowerleftcoast

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Though Blencowe recommends stranded-core for any place where wires will be twisted (like heaters) and also for connections to tube sockets, I find solid core easier to work with. But of course the cloth-covered pushback wire isn't rated for the very high-voltage sections of the amp.
The Gavitt vintage cloth pushback wire I have used is stranded and pre-tinned so it acts much like solid core in holding shape. In a recent search I did not find a voltage rating. I am pretty darn sure it was rated 22AWG 300v. Not anymore I guess.

For our amps I think it is important for the wire to hold it's shape. The ability of the wire to hug the chassis so the wire does not act as an antenna is desired. A lot of stranded wire will not do that.

I like vintage cars, vintage radios, and vintage amps so I very much like to see cloth covered wire in vintage inspired amps. If the wire offered is not rated for the voltages in our amps, I think there is a liability issue that must be reckoned with. Maybe we are all headed for that teflon stuff.
 

theprofessor

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The Gavitt vintage cloth pushback wire I have used is stranded and pre-tinned so it acts much like solid core in holding shape. In a recent search I did not find a voltage rating. I am pretty darn sure it was rated 22AWG 300v. Not anymore I guess.

For our amps I think it is important for the wire to hold it's shape. The ability of the wire to hug the chassis so the wire does not act as an antenna is desired. A lot of stranded wire will not do that.

I like vintage cars, vintage radios, and vintage amps so I very much like to see cloth covered wire in vintage inspired amps. If the wire offered is not rated for the voltages in our amps, I think there is a liability issue that must be reckoned with. Maybe we are all headed for that teflon stuff.
I agree on the choice of cloth pushback. I have used Alpha wire when building an amp on one of Doug Hoffman's layouts, but I don't love it even then. And with a traditional layout, cloth pushback is more than appropriate and looks "right." I too find the solid-core the easiest to work with and mould, and so I use that exclusively.

I _think_ I'm correct about most of the 22AWG stuff being rated at 300V. Thus my query about higher-rated wire. I've found PT cutoffs that I think are long enough for the stuff over 300V in the amp, so I'll use those.
 

theprofessor

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Some "sad" pictures: A bit of prep, but waiting on some crucial parts. I should get the boards in the next day or so and can start populating those. I'm waiting to install the transformers once I have the cabinet and can drill for the chassis straps without those huge things hanging off the chassis. And I like to do heaters last, so....

You'll see that I decided to build a bus that kind of floats, but I used the ground points on the relevant pots to hold it up. Otherwise, I'll try to ground stuff from the board in their respective "vectors" of circuitry, as @Lowerleftcoast suggested in a post above.

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slider313

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The Professor: I don't see any lock washers on the input jacks. Did you use them on the pots?

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tele_savales

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why he decided to tuck away the heater CT in the PT and to create an artificial center tap off the pilot lamp
I took a quick peek at the chassis of a SF Bassman I bought last week (very quick), and plugged it in and check the controls and so forth. Sounded great but had MASSIVE ac hum...
Took it home, pulled the chassis and there was the 6.3v CT, shrink tubed and tied off.
Cut out the virtual ct, bolted down the actual center tap, hum gone.

I found the whole thing rather surprising, tbh. A) I mean, there has to have been more than one guy inside the thing since 1968, right? B) I can't really be that smart, can I?
 

theprofessor

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I took a quick peek at the chassis of a SF Bassman I bought last week (very quick), and plugged it in and check the controls and so forth. Sounded great but had MASSIVE ac hum...
Took it home, pulled the chassis and there was the 6.3v CT, shrink tubed and tied off.
Cut out the virtual ct, bolted down the actual center tap, hum gone.

I found the whole thing rather surprising, tbh. A) I mean, there has to have been more than one guy inside the thing since 1968, right? B) I can't really be that smart, can I?
Wow. That's odd. Glad you got it sorted!
 

theprofessor

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I've gotten some work done, but slowly. I've been populating the board. And then I realized that I had a .033μF coupling cap instead of a .0033μF cap, and that I had a 2.2k resistor instead of a 2.2M. I went out to a local electronics store and picked up a 2.2M metal film resistor and a .0033μF mylar film capacitor, but I may still wait on my Tube Depot order to come before finishing out the board. I'd rather have the board be more uniform. I think I'll start in on the filter cap board.

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theprofessor

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Speaking of two-conductor signal wire, I got some Belden 8451 and plan to use that for the inputs. Can't I simply put a 33k resistor and a 68k resistor at pin 2 and then hook the two conductor leads up to those and to the tip of the hi and lo inputs respectively? As you can see in the pics, I have the 1M grid leak wired as usual at the inputs.

While I await some more small parts, I got the bus finished.

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Lowerleftcoast

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Can't I simply put a 33k resistor and a 68k resistor at pin 2 and then hook the two conductor leads up to those and to the tip of the hi and lo inputs respectively? As you can see in the pics, I have the 1M grid leak wired as usual at the inputs.
It is a little more complex than 68k vs 34k with 1M to ground. When the low jack is chosen it creates a voltage divider 68k/68k. The 1M is bypassed with a shunt to ground.

Here is an illustration:

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Lowerleftcoast

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If only one jack is used as the ground point for the circuit, it would avoid the other three paths to ground.
 

theprofessor

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If only one jack is used as the ground point for the circuit, it would avoid the other three paths to ground.
By the other three, I assume you mean the lo input jack on the VIBRATO channel, plus the grounds from the two volume pots, the reverb pot, and the intensity pot (that's five, though)? I decided to ground those pots there, because I needed something to hold the bus bar up.

I'd be very happy to hear suggestions on how to do it otherwise, as I'm not totally committed to this layout yet. As I've said before, I've got a lot to learn, and there's a whole lot I don't totally understand.
 
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Lowerleftcoast

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By the other three, I assume you mean the lo input jack on the VIBRATO channel, plus the grounds from the two volume pots, the reverb pot, and the intensity pot (that's five, though)?
I meant the other input jacks.
From what I see in the pictures the pot cases are not connected to the ground bus bar. The pot cases are only acting as shielding for the pot internals. The pot cases are touching the chassis not the ground bar.

Your choice of ground scheme mainly has two ground points, one near the PT and one near the input jacks. There is some variation in your design because the output jacks, the power tubes pin8, and the input jacks maintain their respective local ground points. I would like to point out there are many amps with very similar designs that work well.

When the vibrato channel input jacks were included as ground points for the input side ground bus bar there is now more than one point to ground across several inches of the chassis. The ground bus bar and the chassis span the same several inches which forms a loop. This can lead to hum. It is not a good idea.
 
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