New Express with VVR....and issues!

[Richie]

maybe look back to what i suggested..

Also try a shielded cable coming back into the second part of V1.
Try these simple things first, it may help narrow it down.

If you look at some express amp pics, Ken used a grid stopper on the wire coming back from the volume pot to the other section of V1..820ohm or 1k see if it helps.

[paulster]

+1 on Richie's suggestion. Just try the shielded cable first and then you can easily add the grid stopper if you need it.

[ampgeek]

Thanks all!

Yep...should have remembered that the grid stopper values were previoiusly identified.

Anyway...some updated pics in a couple separate posts to follow. Although I have tried to maintain lead dress as close as possible to the reference documents, there are a couple of spots where I left nubs of wire/leads a tad long knowing that I would have to come back and spiff them up later. I don't believe that they should make a huge difference since I am still looking for some gross change in condition.

I clipped in the new OT with no observed change (e.g., "singing" from the OT body when playing guitar notes, highly mismatched PI current draw with a signal and an ugly overall distortion over the signal).

Shielded cable to the second stage and various values of grid stoppers tested (up to a max of 1.5K) with no change.

I also un-installed the pre-amp board HOPING to see one of the underboard B+ wires thrashed for some reason. But...nada!

A number of different coupling caps in the PI have been tried with no change to the condition.

But...one new observation: with the temporary OT clipped in and the OT body isolated from the chassis and with my guitar plugged in but not playing, I can hear make the OT body singing character change to a high pitched whistle. The whistling character can be changed by waving my hand over the OT body, changing where my hands are on the guitar strings and moving my body when standing near the chassis. Kind of like a theramin!!

That whistling character alldisappears when the OT body is in contact with the chassis. I am following the standard plan and have gone back and inspected/reflowed all of the grounds (jacks and pots included) but this makes me wonder.

I am using PECs with buss rod through lugs from one of our members here. I am not using any type of pot isolation scheme (e.g., pot is grounded through the chassis through bushing/nut). I have heard that some high gain amp plans benefit from isolation and a single common ground at the input jack. Thoughts on that?

One of the only things left un-touched (other then reflowing connections) during this excercise is the power supply caps. I will go through the B+ string and sub in an off board 500 V cap (20uf) just to see if that changes anything. Hoping to eliminate those from the equation.

The search continues!!

Thanks much,
Dave O.

[ampgeek]

A couple more pics follow

[Structo]

In your first picture there appears to be a purple wire that is not attached to anything?

[ampgeek]

I am still using a newly received OT from rj that is connected via soldered roach clips at each point. The original OT is still mounted so the blue, purple and green primaries and the yellow and black secondaries are just hanging around.

There are also a couple turrets that need to get touched up with a bit-o-solder too. I was getting late. Finished those up this morning.

Thanks for looking Tom!

I also jumpered in a 20mf@500 V Atom, one by one, at each filter cap position. No changes observed.

Cheers,
Dave O.

[marcoloco961]

The bias pot is in the usual place on the PS board. I procured the part suggested in the BOM. It is a little bitty thing but works just fine. I didn't like the idea of soldering it's little leads into a turret so I liberated some pins from a relay socket, soldered them into the turrets and now have a "push fit" bias pot!

Are you saying there is no solder on the actual legs of the bias pot?

I don't see a jumper on the wiper of the bias pot to the grounded leg. Is it under the board??


See attached pic, Is the white bias wire hooked to the same pin I have highlighted?

There is a jumper between the white bias wire where it hooks to the main component board and the (2) 220K bias resistors, correct?

[marcoloco961]

Also, Have you ever tried the amp hooked to a speaker cab instead of a dummy load? Maybe the dummy load is malfunctioning??

The blue wire coming from the board to the V5 socket grid looks pretty buggered up where it bends on top of the board. From my angle it looks really thin in one spot. That is a quick change. Might consider replacing the grid resistor in the process since this is the problem socket. (?)

I'm hoping you get it soon. Your patience is a virtue. It almost always ends up being something simple. A forgotten jumper, bad solder joint, bared wire, bad cap, etc. You will get it.

[ampgeek]

Thanks Marco!

Correct. The bias pot is pushed into a socket which is soldered to the turret. They are nice and tight. But...I might just re-think that arrangement if I don't find something obvious soon.

The bias pot includes jumpers to ground from the wiper and another terminal as indicated in the reference layout drawing.

There is an underboard bias wire as you sketched out. It terminates at the VVR board "Bias In" point right near the chassis surface. It then emerges from the board on a white wire at the opposite elevation and runs over to the normal bias in lug on the pre-amp board.

Yep! Lots of mangled wires about as I have done a LOT of component replacement/re-flow solder joints. I will have quite a bit of clean-up work to do when all is said and done. No matter....it is a labor of love!

I end nearly every trouble shooting session by placing the metal cover over the amp, tightening it down with a couple of screws and playing into either a 1X12 partially closed back Avatar cab with a G1275 or an early 70's Ampeg V4 cab with original speakers. My very first, "childhood" amp/speaker set up. I have not observed anything much different in that configuration than what I hear using the stethoscope connected to the speaker pot. Slight "coloration" differences but the distorted character is the same.

I KNOW that I will get it sooner or later. No amp has totally beat me yet!! I can really hear the Trainwreck character right up to the first PI grid and I want it BAD. Sounds VERY much like Strato17's(?) clips which is what really turned me on to building this plan in the first place. Glen K's clips didn't hurt much either!

I can't help but believe that there is something awry in the PI but I need to understand the technical background better to figure it out pragmatically. I think that it is being overdriven too much and/or not biased correctly.

Also hoping that others with similar builds could check some of their readings for reality sake. For example: Maybe it is normal for one power tube to draw nearly double the other. I can't help but think that the "character" of the wave form from each PI half is quite different (other than phase) given how different each plan is.

Thanks again,
Dave O.

[Richie]

are the protection diodes correct? this pic looks like one is not.
https://tubeamparchive.com/download/file.php?id=11977

also, for the heck of it.. remove the diode from the pin one and eight connection,,and move it to ground,.

or try removing the diodes connected to pin one and eight..

or remove the 1 ohm resistor and just tie the pin one and eight to ground.

the diode string from pin 3, should be tied to ground , and not on pin one and eight if your using the 1 ohm resistor..

[rooster]

Richie - Wow, you're good! As you say this is a good possible.

One other thing to consider is the speaker jacks ground plane? I realize the chassis is aluminum, but I would still connect the ground lugs on the speaker jacks together for possibly better chassis contact.

[Firestorm]

I just re-read this thread from the beginning (cheaper than a book and sort of like a murder mystery, except we haven't got to the point where someone shouts out "Colonel Mustard in the PI with a leaky cap!")

The one thing I see in all of it is the PI voltages: all of your measured voltages are lower than the "expected," except the PI plates. It's only a few volts, but that's quite a bit when you compare it to the 5-9% below "expected" you get everywhere else. This implies that less current than expected is flowing through the tube, which might suggest a less than perfect ground in the PI.

You have good signal up to the V3A grid; it sounds "ugly" from there on. The V3A cathode ought to have a signal nearly identical to the grid, but even this sounds ugly. Since the signal to V3B is fed from the cathode, all of V3B will sound ugly too.

So what I would check is the grounds. Clip one lead of your DMM to the main ground or to the input ground and make sure everything that is supposed to be a ground really is. Especially check the junction of the 470R and the 10K in the PI: make sure you read about 15K to ground from that point (the Presence pot is the lower third of your PI "tail" resistor). Also make sure that the grounded end of C10 is 5K to ground. If it's not grounded or the resistance is too high, the PI won't work. Make sure the feedback resistor is actually 100K and connected properly. If there is a bad ground in the PI, the PI will ground itself through the feedback resistor and the speaker, so it will still work (sort of).

Since you got some interesting changes in voltage and current when you removed V1 (suggesting something parasitic going on), I suppose you ought to try putting 1,000-2,000pf caps to ground from the output tube grids to see if that changes anything.

I don't really know anything about Dana's VVR (except the basic operation), but I can't help feeling that something could be amiss there, but I wouldn't know where to look.

Good luck!

[passfan]

are the protection diodes correct? this pic looks like one is not.
https://tubeamparchive.com/download/file.php?id=11977

also, for the heck of it.. remove the diode from the pin one and eight connection,,and move it to ground,.

or try removing the diodes connected to pin one and eight..

or remove the 1 ohm resistor and just tie the pin one and eight to ground.

the diode string from pin 3, should be tied to ground , and not on pin one and eight if your using the 1 ohm resistor..

I thought about those too but discounted them due to him swapping the grid wires and the 180ma condition following the swap. Firestorm is chasing something perhaps. There have been a lot of ground problems ,especially with pots. Even though he used PEC he could have a bad one.

[gahult]

Have you tried switching the primary leads?

Purple to V4 and Green to V5.

That may not be the issue in this case....just a thought.

Gary

[ampgeek]

LOL! Clue!! I took a couple of days off from the frustration and perused the whole thread to better understand the sequence of events and couldn't agree with you more.

And....came back with "Professor Dumbell in the bias circuit with(out) a soldering iron"!! Morcoloco gets the detective award on that one.

Soldered up the bias pot (had previously used push sockets), biased it up (required a MAJOR adjustment of the bias pot which was a clue that something had changed!), plugged it into a 1X12" loaded with a G1265 and let it rip with the VVR dimed and everything else at high noon and... !!!! It sounds REAL close to Strato17's "Shaky Ground" clip which was my motivation to give this a go in the first place.

I wouldn't say it is perfect yet and there is lots of knob tweaking and much housekeeping to be done as identified in all of the gracious posts...but...it is the good kind of tweaking left to be done (I think!).

I still have a factor of 2X power tube current draw mismatch over all volume settings when playing (perfect match at idle). I would like to sort that one out first.

Has anyone ever done similar measurements on their builds? Could that be a normal condition?

I have a shielded V2 grid line and a whopping ~22K grid stopper currently and the amp is way stable. Lying on it's back with no cover plate and nothing plugged into the input jack, I can dime everysetting and get no feedback. A bit of hiss is present but that seems normal.

I also appear to be experiencing excessive voltage drop across the diode string. I have 601 VAC going in and ~389 VDC coming out. That appears to be way off from expected. I will change those out and report back.

I will mention that I have the power tube plate to ground diode bridge disconnected as Richie suggests so that doesn't appear to be playing a role in the low B+ observation.

Regardless...I am a happy camper! I am resisting plugging in the NOS Siemens until the mismatch power draw observation gets resolved. But can't wait to give them a whirl!

TIA,
Dave O.