Lead Dress: OD V2a Grid wire

[ayan]

Wire type? I don't think so, but the contact/no contact is really odd.
I have to believe the problem is oscillation, and that means there is positive feedback coming from somewhere, and it's not likely to be the out of phase plate lead. A prime suspect would be the OD2 stage, and I can see where moving that wire might bring sonic differences due to its being closer to or farther from breaking into oscillation. The high plate configuration adds a little gain, but in either case the HF is rolled-off substantially by the snubber. With a 270p plate-to-cathode the corner is way down, around 6kHz.

Without experimenting with the placement of the V2A grid wire, I would agree with the statement above. From what I recall, I never ran into oscillation issues with the ODS layout -- but I had my share of trouble with oscillations building a pseudo ODS (+ reverb) in a Fender Deluxe Reverb chassis/board. But, I am curious and will mess with the V2A grid wire location in one of my amps and see what I find out.

I tried it on my 100K Skyliner amp, pictures attached. In one, the blue grid wire and the orange plate wire touch under the board, run parallel and then touch again as the blue wire loops back to go to the terminal strip. In the other, the grid wire is not even close to touching the plate wire and is always airborne. Differences? The biggest one might just be the way the wires look in the pictures. At low volumes, I couldn't hear anything. At higher volumes, the wires completely separated appear to give a pinch more sizzle to the sound. Not in a bad way and I actually prefer that, it's like turning the treble control from maybe 6.0 to 6.5 in a CTS 30% audio pot. So, these are my results. I'll clarify than in neither case does the amp sound "bad," or the controls not work, etc. I will try the other amp later and if I find something radical I'll report back.

What Scott describes about the amp in which the bass control acted weird reminds me of a friend's Marshall that had been modified from 6550s to EL34s, without changing the power tubes grid resistors. Operating the bass control would do all kinds of weird things and increasing the grid resistor values -- which should have been done during the conversion -- cured the problem. Since the effect, for those who have encountered it, is detected at higher volumes, I wonder if there is some oscillation going on inside the amp that stems from the power section. The signal level present at V2 is always the same irrespective of master volume settings, but the signal level at the power section obviously goes up with higher master settings. A simple question, in the cases reported, are the power grid resistors 1.5K? For what it's worth, all my amps use 5.1K instead.

[Audiodog]

To be clear.

The amp that had the problem, similar to Gil’s oscillation description, is a high end clone from a respected and major builder. Built to exacting specs. All parts measured, oriented, etc. I changed all the tubes after getting it here. Power and preamp. Still had the problem. Adjusted PI. Still did it. Checked bass pot. Swapped bass cap. Swapped mid cap. Changed treble cap. Still did it. Then I noticed the grid wire. If you have room, why anyone would intentionally put an emitter on a grid is beyond me. I moved the grid and problem solved, Did not touch the plate. Could there have been an oscillation? Sure. But only caused by that lead dress issue. All voltages checked out. I was very very thorough. The loudness creates vibrations. Not talking about signal levels.

I am still surprised that we have this many pages on the topic. Unless there is a space constraint, separating grids and plates is the recommended procedure for proper lead dress as per the RCA tube manual. Apparently, there may be other factors that influence the situation such as tube type, wire type, etc etc….. I am not crazy and all I did was move the wire. Like a new amp! Clearly some say it matters and others don’t notice. In that case, separated is still better as it is never worse.

Take it for what it’s worth.

[ayan]

I just finished having a look at the other amp, which is a 102 clone with a few liberties -- 1 uF presence cap, no extra power supply filtering for the preamp, and 1K screen grid resistors on the power tubes. While I was inside the amp, I took the opportunity to pluck out the master volume brightness cap. I'd started with a 68 pF, then went down to 47 pF, to 30 pF and finally to 20 pF. I didn't care for any of them.

Anyway, similar pictures and basically the same results. HOWEVER, in this amp, that V2A grid wire is a lot more sensitive than in the 100K Skyliner amp. With the grid wire elevated pretty high from the bottom of the chassis, getting my hand to the back of the amp to switch channels manually would change the "buzz" sound than one hears then the amp is pretty loud. Getting a finger pretty close to the grid wire would almost make the amp whistle. So I can see where placement of the wire in this case could change things more noticeably than in the previous one. When tucked down parallel with the plate wire, the amp sounded good, really, just not as much sizzle. For what it's worth, this amps grid wire is a little longer than the 100K amp's.

Checking two amps out, especially two different amps, is statistically meaningless. But, based on my observations I'd say that one could possibly mess with the wire placement to tweak the brightness and gain (which I think of as "sizzle") of the lead channel, to an extent. My wires, as well as many of the clone amps', are longer than most actual ODSs. Length of that wire would be something else to check out. Rule of thumb is that grid wires should be as short as possible. However, those of us who know they'll be back in the amp for "research" and tweaking purposes over the years are probably better off with some extra length just in case.

[martin manning]

Thanks for posting your findings. What value are the snubber caps in these two amps?

I agree completely that a sample of one or two doesn’t make anything like a definitive answer. Every amp is different, really.

[donvan]

I am still surprised that we have this many pages on the topic
...
separated is still better as it is never worse.

Audiodog,
I really appreciate your posts and have added what you have said to my notes. I believe your message that these two wires should not touch has been received and believed by just about everyone. Perhaps it has been discussed for this many pages because of the notion that Dumble *may* have moved these wires further apart or closer together (without touching) in order to shape the tone. Not sure if that is the case but looking forward to trying to figure it out.

[daniboy79]

Yes. Separate the snubbers. They are out of phase. There should be a gap between them.

Interesting topic for sure! I built my OTS as a kit from Ceriatone. It's the FM50 ME mod. And I basically followed the lead dress of the Flckr photos that are posted for helping those of us during the build.

This is my V2 lead dress, how does it look? Should I make any changes with the plate/grid lay? I do feel that my OD needs low end 'tightness' addressing.

This is what you guys had me doin today, lol



I separated the snubber caps and attempted to do a little more lead dress in the concerned areas highlighted in this thread.



I was more than happy to put it back together. I'll have to wait until tomorrow to hear if the OD sound any different

[ayan]

Thanks for posting your findings. What value are the snubber caps in these two amps?

I agree completely that a sample of one or two doesn’t make anything like a definitive answer. Every amp is different, really.

I believe 330 pF in the 102, maybe 350 pF. For sure 390 in the 100K.

[martin manning]

Thanks. FWIW, my low plate has 370p Treble cap and 330p snubbers.

[Audiodog]

I have a theory…. What if the issue is caused by the vibrations. Moving wires with chopsticks is not the same as 100-300hz cycles. Specific touching, combined with specific vibrations, causing some sort of oscillation or other issue with that lead dress. Maybe being in a combo adds to the issue.

I also think it may be more prevalent on 220k plates. I have tubes that work on 100k, but are too noisy on 220k. The higher gain could exacerbate the effect.

The amp in question is an exact 102. 250pf snubbers.

It is definitely real, question is how to conclusively replicate it. It is not power amp related.

[Audiodog]

A few things I see….

First, have you tried a 150k slope? Rare to see a 100k with a skyline. I use the 150k. Or maybe go with a 100k and .047 mid cap. Also, I would use a 500pf cap on the 220k resistor on V1b. It does effect the knee of what is bypassed in an audible range. I feel the 470pf is the lazy man route. Never understood why so many builders use 470pf when 500pf is easily found. Lastly, consider using a 270k into the OD trigger. Use a 50k pot with a 1.5k tail to ground (or similar, whatever is on hand). That way the noon point is the “Robben” setting and it is much easier to adjust. No brainer on that one….

How is the od after lead dressing?

Yes. Separate the snubbers. They are out of phase. There should be a gap between them.

Interesting topic for sure! I built my OTS as a kit from Ceriatone. It's the FM50 ME mod. And I basically followed the lead dress of the Flckr photos that are posted for helping those of us during the build.

This is my V2 lead dress, how does it look? Should I make any changes with the plate/grid lay? I do feel that my OD needs low end 'tightness' addressing.

This is what you guys had me doin today, lol

I separated the snubber caps and attempted to do a little more lead dress in the concerned areas highlighted in this thread.

I was more than happy to put it back together. I'll have to wait until tomorrow to hear if the OD sound any different

[ayan]

Thanks. FWIW, my low plate has 370p Treble cap and 330p snubbers.

Found an old picture, it is a 330 in the "102" -- and 390 in the low plate.

[Charlie Wilson]

I keep hearing about high end but the difference I hear is in the low end when moving the grid wire away from the plate wire. I am going to go with phase cancelation of low end from proximity of the wires as that grid wire is so sensitive. I don't feel very enlightened.
CW

[Audiodog]

Me too. Certainly seems that way.

I keep hearing about high end but the difference I hear is in the low end when moving the grid wire away from the plate wire. I am going to go with phase cancelation of low end from proximity of the wires as that grid wire is so sensitive. I don't feel very enlightened.
CW

[jazzbass]

Tony, I think you told me that you like your trimmer set fairly low. I set mine at about 40k to ground and the effect is very noticeable.
CW

.....Really I didn't get to spend a lot of time with it since my wife just got out of hospital. So I just wanted to confirm what you were hearing ..Perhaps when I get more time I can give you a more detailed response. So for now i am sticking with I noticed it. Either way I recommend keeping those wires apart.

Tony

Tony, I read your wife just got out of the hospital, I hope it's nothing serious and I wish you all my best wishes.

Franco

[bepone]

Excellent thread! I read everything and made me think about, thank to all for bringing this up revealing a lot of things and artifacts that were strange!
Sure that layout and lead dress are crucial for any amp! To me happened that changing one power tube socket in Fender 75 Rivera with rat's nest of wires below changed the bass, 35% to better. Why and how? Bass pot was uselles (to much bass so amp was played with bass on 0 or 1) and after the change, bass pot was useful up to 5! Means moving only few grid and anode wires on the socket.
Sometimes during repair you cant understand how something happen because effect is invisible.. would be easier if electric field around wire can have color, per example yellow. And magnetic field pink so we can clearly see the parasites and would be easy to avoid them, but things are invisible (at the moment ).

The theory behind this is constant changing of the electric field and grid wire which is inside, means electric coupling, between those
Similar is explained here https://learnemc.com/electric-field-coupling
In ODS is even more pronounced, because we have high resistances in the circuit and because source 1 (grid) is taking feedback coupling from source 2 (plate) through capacitive coupling but is even complex, source 1 is controlling directly source 2 through normal tube operation! Taken from this formula cures are to increase the capacitance between the A and g wires, in practice only to lift grid wire from the ground at first, and second to move it away from plate wire.

Why V2a wiring is so critical - when I see the standard D layout, maybe is not so ok to form hi-resistance “U” shaped turn (coming from V1b with internal node resistance od 40k, passing 47nF, going to sw. relay, coming back, increasing node resistance even more over 220k, arriving to trimmer, increase node resistance even more over the trimmer 70k+) with 300k od resistance on this “antenna” and sum this to V2b without shield? How I see this point with high resistance is very prone to all kind of problems.

Maybe will be better, this particular distance from sw. relay to the V2a to connect over coax without attenuation, with internal resistance of 40k (tube V1b), then on the socket tag terminal make attenuation with two resistors 300k-30k to the gnd, and center point connect to the tube grid. On this way 300k is grid stopper and attenuator in the same time and because of coax coupling problems can be avoided, signal is arriving on the socket with 5 times less resistance (40k, compared to the 300k before).
See here there is no gain trimmer anymore (no problem if people want to follow 27-30k to the gnd like D, put the resistor, more reliable then cheap trimmer) . Actually I like more sound od 60-100k’ish. I will test these ideas one day just to find free time...