Slow "Scope Creep" Amp Build

[jjmantele]

Not sure if it was mentioned but the presence pot is missing a wire in the layout. It needs a jumper from the center lug to the lug with the cap. Otherwise the wiper is connected to nothing and the pot is a mere 5k resistor.

[PeteG45]

JJman, I just went back and looked at a bunch of different layouts and I definitely have it wrong. Thanks for catching that. Another benefit of taking five months to build an amp. I'll fix it on both my layout and my amp.

If you see anything else let me know.

[PeteG45]

Here's the next most updated layout.

[PeteG45]

I got the transformers, the last resistor and the rest of the caps. All I'm waiting on now is the end bells to make it easier to mount the power transformer.

[Scott S]

If you want, you can just connect the cap to the wiper of the presence pot and leave the pin on the left unconnected. Same result in the end.

- Scott

[PeteG45]

Scott since I had already soldered and trimmed the leg of the capacitor it was easier to add a jumper between the left and middle lugs.

I got my heater wires in last night and finished up most of my board.



I was checking over the board this morning and I found that the first filter cap was backwards. Oops. I guess better to find that before first power up. You can see in the picture that the first filter cap is not lined up with the second that's because I soldered and trimmed the cap before I checked it. After I fixed it I went over the layout with a highlighter and rechecked everything.



Here is a shot with the shielded cable from pin 7 of V1 back to the volume pot and you can see the 2 Ohm and 4 Ohm wires from the OT that will be unused. To make soldering easier when I install the board I also soldered the wire to the tube socket that leads from pin 8 back to the NFB. I made sue that the shield of the shielded cable extended past where the cable crosses the heater wires.

[Phaze]

Nice job so far! Good catch on the cap.
The shielded input wire has solved some noise issues for me in the past. Always a safe bet.

It's gettin' fun now, ain't it? Good luck!

[PeteG45]

Here's another question regarding the ground bus for the preamp and input stages. I have a heavy copper wire soldered to the backs of all of the pots. Is this grounded to the chassis by the shells of the pots or should I connect a lug to the chassis via a nut and bolt and then solder a connection from the lug to the copper wire? If the ground is made via the input jacks and pot shells then I should remove both and sand away the paint on the inside of the chassis? I wish that I had thought of this before I soldered that shielded cable in place.

[PeteG45]

Another question for anyone that's used the Weber WSE15 OT. I think that I read that the brown and the blue are opposite on the WSE15. My layout has the brown going to the filter caps and the blue going to the tube socket, is this correct?

[tubescent]

go pete!

[PeteG45]

Hmmm... I guess that I have another question. Has anyone used the end bells from Weber to install there PT? If I do this it looks like the coil axis of the PT will be changed by 90 degrees putting both the OT and PT coils in the same plane. Is this a concern? Am I wrong about how the end bells work? I have my OT currently mounted like you see in this picture (not my amp). Should I change the orientation of my OT? Maybe I should wait until the end bells arrive and I see what they do.

[Ben Harmless]

The PT and OT of my Marshall build are right next to each other, and on the same plane, but rotated 90 degrees to each other, (laminations perpendicular) and I've had zero problems - zero.

...I'm sure I could just be lucky though.

On the other hand, I don't see any pressing reason to use the end bells when you've got a chassis cutout for that PT. Mounting the whole thing externally to the chassis will just make for more of a pull (not that it's that heavy) on the metal, and a slightly less balanced chassis overall.

[PeteG45]

I don't have a cut out on my chassis (it's home made) and I was hoping to avoid having to cut the hole for the PT. The only reason I wanted the bells was to make the project a little easier. Maybe I'm looking at this wrong. If the end bells make it so that the transformer flips up so that it stands on the left side face instead of the front face
(as shown in the photo) then the coils would still be oriented at 90 degrees.

I really need to chill out about this and wait until the stuff arrives. It wouldn't make sense for Weber to build the bells so that the coils would be oriented in the same plane. I am a 2D thinker in a 3D world.

By the way how's Albany? I grew up in Schenectady and lived there til I was 35.

[Scott S]

The pots should be pretty well grounded by virtue of their contact with the chassis, but you could check with an ohmmeter -- should be almost no resistance between the back of the pots and the chassis. As for the OT, I think sometimes they are manufactured with the wires backward to begin with, so it'll be good to double-check anyway. I would leave the NFB disconnected, then when you power up the amp, use a clip-lead to make momentary contact. If the amp gets quieter, it's hooked up right. Louder, and you need to reverse the leads.

- Scott

[big-daddy-59]

Pete as far as the ground bus goes I did mine in a fashion similar to yours. I formed a loop in the end and bolted it down to the chassis. Didn't rely on the pots to make the ground. And concerning the output transformer, just hook it up like you have it now, if it squeals(oscillation) then reverse the leads.When you get your endbells make sure you mount the PT's iron laminations are at a 90 degree angle to the core laminations of the OT. I'll see if I can't find a picture that shows this clearly.

[PeteG45]

I checked the resistance from the bus wire to the screw holding the rectifier tube socket and it was virtually non-existent. I guess that's a good sign. I thought that I had read somewhere that the blue and brown were reversed from the Hunter's original design but it may be my foggy memory playing tricks on me. In Hunter's build notes (which I re-read last night) he does discuss the blue and brown leads for the Weber PT and maybe I got that screwed around in my head. Even Weber's 5F2H, which uses the WSE15 OT shows the brown going to the board and the blue going to the tube. I will keep your test in mind at first power up.

[PeteG45]

When you say the laminations are you referring to the stacked metal plates that surround the windings? I would have thought that the coil orientation was the important thing.

[PeteG45]

I received a FedEx shipping notice from Weber and the rest of my stuff should arrive on Friday. I guess I'll go play my guitar.

[PeteG45]

Well after four months the last parts needed to assemble my amp are on the FedEx truck for delivery. It'll be clear and cold here in Austin so there shouldn't be any weather delays getting them to the house. If I thought that the parts would arrive before three, and I wasn't so busy at work, I might be tempted to call in sick today. :o)

[PeteG45]

I have the PT mounted and wired. I tested the heater filaments and got appropriate values. I read 722V across the pins 6 and 4 of the empty rectifier tube socket. It's supposed to be about 10% higher than the rated 660V which would be 726V right? I'm a little surprised to see my measured values so close to target.

Here are some photos:

[PeteG45]

I got it together last night but I have two problems that I believe are related. The top input works and the bottom input does not and there is some hum. At first I had it wired like you see in A and then I did some research and wired it into B (the difference is in the connection from the tip on the top jack to it's connection to the bottom jack via the black wire). The bottom input is dead either way. Can someone take a look and see if either of these are correct? I checked my resistance and continuity between the G and ground and between the tip and pin 2 on V1 and everything looks OK.

[PeteG45]

I just found that the connection to the tip of the failing jack is bad at the jack. I've got to run to Lowes, we're painting the dining room, yay :o( . I'll fix it when I get back and report back in.

I'd still like to know which input connection is correct; Fig A or Fig B.

[PeteG45]

OK I fixed the jack and 'viola' everything works. The hum is not too bad up to about 2/3 of full volume. The hum is definitely less when the voicing switch is in the middle which bypasses both voicing caps. Based on that I'm wondering if I should use a different switching mechanism for that switch. When it's in the middle position the amp is very quiet. Any suggestions?

The amp sounds great. Very dynamic and the tweed voice and the Marshall voice are beautiful. The middle position which is supposed to be black faced Fenderish is leaving me a little flat but it might grow on me. I'm still using the speaker from my ValveKing 112, I'll get the Weber Signature 12B installed in a little bit and give it a try. I really would like to get the tweed and Marshall voices as quiet as the black face voice. Maybe if I eliminate the resistors on the switch. Hmmm...

[big-daddy-59]

Congrats on finally getting it up and running! Might still want to poke around with the old chopstick and see if the hum isn't related to lead dress.

[PeteG45]

I'm going to have to figure out how to suspend the amp so I can poke around with a chopstick. I may have to cut a couple of boards to build a little frame that can hold the amp upside down so I can access the components while it's still running.

Is there a chance that the noise is coming from a bad voicing switch or one that was heated too much? It really is quite a bit quieter when it's in the middle, off, position. Would the voice with no bypass capacitor be expected to be quieter?

[celeste]

Pete. the easiest and most versitle amp ficture I know is just 2 wooden "V"'s that the amp sits down in. It self adjusts to what ever width chassis you have

[PeteG45]

Celeste that's a cool idea. Thanks.

[jeefberky]

Hi Pete,

I built nearly this same amp a couple months ago and have been tinkering with many of the mods that you have listed here. I'm really enjoy the sound this circuit puts out. Lots of fun.

To answer your question about the voicing switch: I tried the same mod with three voices and dropping resistors. The difference in hum that you're hearing is a result of the boost in frequencies that the .68 and 25 uf caps give the signal. Its unavoidable as far as I can tell with that arrangement, especially at higher volumes. If you remove the resistors, you just get a loud pop everytime you change the switch to the top or bottom position. With the resistors in place, you always have some contact between the capacitors and ground (albeit small) so that when you add a cap into the circuit, there isn't a giant instantaneous load dropped on it.

In the end, I settled on a SPDT switch that just adds the 25uf cap in parallel with the .68 uf cap. I agree that the middle position is kind of flat... it has it's own thing, kind of 50's low-fi, but it lacks the punch of the 25 uf and the chime of the .68 uf.


Thanks for sharing the cool build-along. I look forward to hearing your impressions about the resonance and presence controls.

[PeteG45]

Jeefberky (awesome name by the way) I'm really happy with the amp. The additional noise was mostly due to the crappy soldering job that I initially did on one of the input jacks. Once I straightened that out it came into it's own.

The presence control does add some presence to the circuit. It's loaded a little funny though. I turn the knob to between 3 and 4 o'clock and it sounds great. Any more is too much and any less than that just leaves me flat. The resonance can add some real boom to the bass and I usually keep it pretty low, down around 8 o'clock.

At first I didn't use the 0.47uF setting on the voice switch much but it's starting to appeal to me more as I try to play different things. I'm really amazed at how much tonal variation I can get out of so simple an amp.

The one thing that I would like to change is the lack of break up. I don't play loud and the amp never really gets close to breaking up. I may try a different preamp tube. It does take my dirt pedals well but I would love to hear some real tube distortion.

Here's a link to a bunch of photos that I put up on Picasa.

[Scott S]

Hey Pete, it's still too loud with only one 6V6 installed?

Some things I've tried with my 5F2A: disconnect one of the rectifier plates from the PT (pins 4 and 6) -- this turns it into a half-wave rectifier and drops the B+. Since the tubes draw less current with lower voltage, the B+ doesn't quite get cut in half like one would expect.

Another thing would be to raise the value of the 10K resistor in the power supply. With a lower screen voltage, the 6V6 will draw less current and have less voltage swing before it clips.

- Scott

[PeteG45]

Scott those are both interesting ideas. I think that I might try placing a SPST switch between the PT and pin 6 on the rectifier. It seems like that would be an easier fit given the amount of room. Then the switch would be accessible from the top of the chassis.

It's funny, the whole time that I was working on this amp I was afraid I wouldn't have enough clean headroom. I guess that I just need to play louder.

[Scott S]

Originally Posted by PeteG45

It's funny, the whole time that I was working on this amp I was afraid I wouldn't have enough clean headroom. I guess that I just need to play louder.

Yep!

When I have the negative feedback hooked up on mine, it surprises me with the legitimately clean sounds I can get out of it. All 3 watts or so of it!

- Scott