Weber Java build - some oscillation

[martin manning]

Perhaps you could post a really good picture of your build so people here can see what you have. Being that this is an Express-type amp, you have picked a challenging build as far as getting a stable result goes and you should follow the published layout exactly. I have to believe that it will work if you wire it as drawn. If you have made a number of alterations, your lead dress only needs to be "bad" in one unfortunate place to cause problems. I don't believe grounding is likely to cause oscillation, that would more likely be hum or other noise. If it is quiet when it's not squealing, then we can assume the ground scheme is okay.

[Colossal]

Lindz,

Although it may not be the root cause of (some) of your problem, I don't much care for R34 (1k) sitting on the preamp board. Grid stoppers are best put right on the grid pin of the tube socket where they are intended to stop blocking distortion and oscillation. That long run from R34 to the grid socket is IMO not a good idea. I would mount that resistor directly on the tube socket and use a piece of shielded wire to go from the volume pot out to the grid resistor. Ground the shielded wire only on one side. Even though that run is not shielded in a real Express, it might be required for your amp to get it under control.

Second, the way the grid wires run from the output section to their respective power tubes and the very long runs to the presence circuit I think is inviting trouble. To do again, I would use a vertical mount power transformer and move the whole power supply over to the left (looking down on the board) so that the output section can be appropriately positioned to provide the shortest wire runs to the power tubes. The NFB wire should be made short.

Sorry if this seems negative.

[Colossal]

Hmmm, using 250uF on V2a seem like a setup for muddy, undefined, and boomy bass response, especially with no grid resistor on V2b. It's a lead amp not a Plexi . I would be inclined to lower that cap to 2.2-22uF. I'm not sure the liberties taken with this design make for a 'better' Express.

[Zippy]

@Colossal - Care to comment on the addition/modification of V1B?

That seems to me to be a source of instability in an amp that is already on the edge.

[Colossal]

@Colossal - Care to comment on the addition/modification of V1B?

That seems to me to be a source of instability in an amp that is already on the edge.

In the Express, the first two stages are fully bypassed with 22uF caps which maximizes gain for all audio frequencies. However, to control blocking distortion, both stages are bypassed with very low values of coupling capacitors (500pF and 0.0022uF respectively). This limits the amount of low end that is passed from stage to stage. And, KF added a grid stopper of 820R-2k2 on some amps, presumably for those that required a bit of extra taming.

On a test-bed Liverpool which I recently completed with a paralleled V1a/b and an Express tone stack, I modified the treble cap from 500pF to 1000pF (to match the drop in output impedance) but immediately noticed that the low end increased but at the expense of definition. I changed it back again to the lower value. There is plenty of low end in the amp despite these small cap values and shows that KF really understood exactly what he was doing.

By using a huge cap value like what is used on the normal channel in a plexi (820R/330uF or 250uF), the 2k7 cathode on V1b (Java) is biased more stiffly but very low frequencies far below audible range are also bypassed for max gain. In a plexi this results in the big, rumbling low end as the normal channel volume is increased. The bass is big and boomy, but it is not defined or tight per se. In the Java design, I am guessing the addition of the 250uF cap might be an attempt to add bass which some complain about as lacking (or is not tight enough) in the Express. KF specifically limited the low end in the preamp and used a large PI entrance cap to let through those frequencies but again limited the output (0.022uF caps there) to keep the bias excursion time small and not invite blocking distortion in the output tubes which are being hit with a large signal. With V1 at 1k5/22uF this is the classic center-biased Fender stage but cascaded into 2k7/22uF which is slightly lower in gain. The total gain however is significant. The Express preamp is generating harmonic content in the 3rd stage asymmetric clipper (10k unbypassed) so any frequencies passed from the preceding two stages will have their fundamental raised to an octave. If subsonic frequencies are passed, then their octave would be a lot more audible low end. I would think this would cause the clipping stage to clip hard drawing grid current from the two 0.001uF caps before it. I'm guessing with the absence of a grid stopper at V2b plus the increase in gain coupled with the subpar layout is resulting in an exaggeration of the already-present instability. The gain in each stage in the Express is lower than the one before. That 250uF cap changes that relationship.

Of course, if that 250 is a typo and it is really 25uF, well then my above speculation is for naught I could be completely full of sh!t and welcome peer review and correction as needed.

[Lindz]

OK .. after trying the amp with the nfb disconnected and grounding the PI tail (thanks for that hint - should have caught that) it still squealed - played around with a chopstick a fair bit and I could actually change the pitch of the oscillation by moving the wires (on pin 6 in particular) on both V1 and V2. But they were cut too short to really move around much as are most of the tube leads I cut so I could not do much in that regard

Since these seem like 2 real trouble spots how would people address this? obviously lead dress and positioning, but would people use shielded wire or perhaps stranded vs solid core? snubbing?

Anyway I got frustrated thinking that I have too many issues with things as I have it wired up now, so I yanked the board a few minutes ago and am going to rewire it more carefully this time

to answer some of the questions

No, I did not use the alternate "express" wiring - I followed the supplied layout as I am pretty green sand it was easy to follow for wiring

This time I will try the alternate V1 wiring since that seems to be the consensus here

As for the cap values I might as well start stock and see if I can get it going then tame as necessary afterward since those changes should not be too difficult later assuming I can get it more stable after my rewire.

I will apply everyone's suggestions above as well as any others people want to add where I can and check back in a day or 2 when I fire it up next

Thanks!

[Lindz]

Better pic of my build below and another Java build I found that seemed like it had the most thought and careful lead dress of those I could find. I was using it as reference since I had read over at Weber that these amps can be a handful. Mine is the bottom one

Martin posted that I should not have mounted the 47 k resistor for the nfb right on the 16 ohm tap - my logic in the way I did it was after seeing this other build where the builder had used shielded wire for the presence pot to the board, with the resistor running across the board, then shielded again from the other end of the resistor to the 16 ohm tap - he has the shield jumped between the 2 connections so it is only grounded at one end

I thought that if he went to all the trouble to shield it, that it was necessary, so thought I could simplify this by using shielded all the way to the tap and put the resistor there, thus leaving even less exposed wire because there was no resistor or lead on the board.

Is there a reason what I did would be incorrect?

Any other comments on my build would be appreciated so I can incorporate improvements in the rewire (please realize I am a noob so if it looks like a rookie did it, its because a rookie did do it - ha ha)

thanks again!

[M Fowler]

Using shielded wire on the input with 33k to 68k grid to V1.

Also use a shielded wire to the volume pot.

Keep the wire for each triode on V1 away from the other triode if they have to cross do it at 90 degrees and keep your wire short.

[martin manning]

The location of the feedback resistor was not so much a concern as the path that the wire took. Locating the resistor on the board routes the wire as shown in the layout and anchors it. Note in the other build pic it runs along the back wall.

Chopsticking has located the sensitive area, and that is good progress. It tells you that you need to do a better job around V1 and V2. Looking at your photo I see that the 9-pin sockets are rotated 90 degrees relative to the layout. It's not impossible to deal with, but it is kind of lame on Weber's part, and forces beginners to improvise without the required knowledge.

That aside, wrapping the leads around the sockets as you did is not a good idea. The leads should generally be run along the chassis surface, cross at 90 degrees if they have to cross, and then approach the socket pins from a radial direction. The shortest path is desirable, but not always best if it compromises those principles. Grid wires (pins 2 and 7) are the most sensitive, and sometimes it is necessary to elevate them to shorten the run and distance them from other leads. Snubbers can work to tame oscillations, but should be a last resort unless you intentionally want to reduce treble frequencies. On the input leads I see you have the grid stoppers at the ends of the shielded cables, which is good.

[Zippy]

No, I did not use the alternate "express" wiring - I followed the supplied layout as I am pretty green sand it was easy to follow for wiring

This time I will try the alternate V1 wiring since that seems to be the consensus here

I don't know that I would call it "consensus" but I would encourage you to seek out the original Express schematic and layout as shared here at The Amp Garage. The Express is noted for being demanding of good work and attention to detail. I haven't a clue why Weber chose to take that path with the added V1b - there has been a lot of discussion of "What can I do with the extra (unused) triode and the answer is usually "Leave it alone, the Express has everything it needs already."

G'luck with your build. Hang in there.

[LOUDthud]

Most DVMs will show a reading on AC volts even if the AC is ultrasonic. Just connect the DVM across the speaker terminals. With no sound from the guitar, the reading should be something less than 100mV. An ultrasonic oscillation will be several Volts.

[Lindz]

Partial rebuild was a success! No oscillation even when dimed..

thanks for everyone's input!

For the rewire I used shielded cable for the master, presence, input jacks and the volume control, and I redid all the leads to the pots and tubes with a little more care. Picture below of revised build. Excuse the mess with all the shielding on the master ground lug - kind of a clusterfu%^%ck but I was more interested in getting it working vs making it a pretty build at this point. Not much hope of that with this layout anyway with all the odd and long lead runs

With the angle of the preamp tube sockets I still had to either wrap around the base of the tubes again or run some leads over the top of the socket so I opted to wrap again trying to keep the angles and spacing clean. Either that or I could have drilled holes and rotated the sockets to a more lead friendly angle - that was going to be next if this revision did not work out. What do you you guru types think of what I did there? - helpful criticism would be appreciated

Question.. when you guys run your leads to the preamp tubes and they have to cross the heater or other wires do you raise them up as well as try and pass over perpendicularly? Or do you try and keep them against the chassis?

I left in the cathode follower even though I thought it might make it a little harder to tame the oscillation. Actually, I initially tried without it but must have had wired something incorrect - the alternate wiring on the schem looked simple enough but though it made sound it was really faint and kind of faded out when I strummed - must have missed something ?... So I hooked it back us as per the layout and the amp worked - had some low frequency oscillation that went away when I reversed the OT leads

I'm stoked...

thanks again!

[Reeltarded]

In the Express, the first two stages are fully bypassed with 22uF caps which maximizes gain for all audio frequencies. However, to control blocking distortion, both stages are bypassed with very low values of coupling capacitors (500pF and 0.0022uF respectively). This limits the amount of low end that is passed from stage to stage. .

I have heard all of this 1000 times, and I think it's more you than me, but suddenly I understand how that all works together. A major breakthrough in understanding what/where/why. Some of my frustration is my lack of understanding the technical aspects, which other than being safe, I sometimes just defer to trial and error, the arrogance of the artful idiot. I am used to kind of pretending to know and winging it, like any Hollywood asshole. As Seen On TV.

I don't even know enough to ask the question.

Thank you, sincerely.

[rdjones]

Java build v2 sm.jpg

Don't ya just love it when a plan comes together ? Congrats.

You can reduce the risk of shock hazard when pulling the fuse by wiring the fuse holder with the "hot" (Line) to the tip of the holder.
Apparently Weber has the same wiring mistake as Ceria ...

[img:399:167]https://tubeamparchive.com/files/thumbs ... ul_290.gif[/img]

reddog

[martin manning]

With the angle of the preamp tube sockets I still had to either wrap around the base of the tubes again or run some leads over the top of the socket so I opted to wrap again trying to keep the angles and spacing clean. Either that or I could have drilled holes and rotated the sockets to a more lead friendly angle - that was going to be next if this revision did not work out. What do you you guru types think of what I did there? - helpful criticism would be appreciated

Question.. when you guys run your leads to the preamp tubes and they have to cross the heater or other wires do you raise them up as well as try and pass over perpendicularly? Or do you try and keep them against the chassis?

Great, glad you've got it working!

Re the socket wiring, what you've done there is much better. The undesirable wrapping I spoke of was where the wires came around very close to the socket pins. Truthfuly, you probably don't need all that shielded wire with careful routing. Maybe just the input and volume runs.

Generally keeping signal wires against the chassis is best. Heater wires can be on the chassis with the main run along the back wall (ala Marshall), and crossed with signal wires at a right angle, and/or elevating the signal wire a bit if needed. Fender elevated the heater wires above the sockets and avoided the crossing issue, but still kept the signal wires against the chassis. Either way works.