Cathode bias 6L6s, Voltage rise(50v) on one power tube when treble is turned up????

[Enzo]

Oh, you are still doing internet? I signed up for internet, but I finished it.

[km6xz]

Hi Steve
Yes, certainly there are people who know what they are doing but it seems that a lot of the posts are by an intermediate group...those who know just enough internet buzz words and axioms to post things that are counter productive. This forum is better than most but overall there is too much conclusion jumping that serves the person asking for assistance poorly.
I've posted a number of times (after several posts appear telling a person to change many of the parts, or blaming bad sound on not having a $350 transformer) about fixing the problem before attempting wholesale parts replacement and been told to butt out because internet wisdom says that all problems are not spending enough on transformers or not replacing all tube or not modifying the amp.
Often the best advice is to have a pro look at it because some measurements need to be done that the musician does not have access to. From what I have seen, very few home repairs are cheaper then a better and more complete repair in a good shop. A symptom might go away or lessen but what caused the problem that damaged the part that resulted in the symptom? One of the reasons SMD units are more reliable is that fewer people screw with them.

[Steve Conner]

Oh, yeah, I forgot that classic piece of Internet wisdom. If something is wrong with a tube amp, it's always the output transformer. I blame the free troubleshooting booklets handed out by Mercury Magnetics.

I agree with your analysis, but you'll probably never change things. (You're welcome to try.) The people who're any good at troubleshooting are too busy doing it for a living to argue every little point that comes up on a forum.

And the trouble with these new Iphones is, they have Internet on them. That means if you drop one, you can break the Internet.

[Cosworth]

Perhaps, So only put factory tires on your car. Never improve the exhaust or intake system since thousands work just fine. Only allow union workers with approved licenses do your work. That your cheep overseas equipment had every bell and whistle installed that possibly could have been conceived at design time and throughout the assembly process. That the manufacturer did not cut any corners by eliminating a part here or there to save money or assembly cost. That just because it works means that no matter what it could not be improved. And don't you dare place a non-original factory part on your 57 chevy! Your amp broke due to divine intervention since it was designed perfectly and was immaculately conceived. No, never think that anything could possibly be done better than it already is..... Geeezzzze!

[MWJB]

"seems good & strong", or is good & strong? I know it's a judgement call but we need a decision? Are you getting 25-30W clean?

It's very possible that connecting the meter is causing instability, as Steve suggested. As km6XZ suggested, if this instability was present without the meter, then you should hear it.

425vdc with 6L6s and a 200ohm cathode resistor is making alarm bells ring for me, 70mA per plate? I wouldn't be surprised if you had some shorted OT primary windings (check dc resistance from each power tube plate to main B+, or pull power tubes & inject a small AC voltage (0.5VAC or 1VAC - measured with a meter) on to the OT secondary to determine the turns ratio, by measuring the resulting primary voltage from 6L6 pin 3 to other 6L6 pin 3. Also check that the OT primary AC voltage from main B+/OT centre tap to each power tube plate is identical....but as with the oscillations, you should be hearing a bad OT?

[pdf64]

Whilst the maxim 'there's nothing so good that it can't be improved' will always hold true, amp mods are one of those many topics where some people just have to take things a step too far, see
Images may be disturbing to some viewers - The Gear Page
and
YouTube - Bad Amp Mods... video by O. www.whatosaw.com crazy

[jrfrond]

What Stan (km6xz) is trying to say here (and I can attest to his knowledge and wisdom firsthand) is NOT that you SHOULDN'T mod an amp, but it needs to be 100% functional first. In addition, if an amp has worked well for 10, 20, 30, 40, or 50 years or more and suddenly develops an issue, it's not in the layout or design UNLESS someone has screwed around with those aspects. If someone HAS, then you need to put on your "Technical Toilet Paper" hat, and proceed to wipe the ass of the person or people before you who screwed it up in the first place by cleaning up old goofs, and all of us here who've been at this for awhile know exactly what I am saying.

Once you have it all sussed-out, then you can proceed to mod and experiment, but if you just jump in and mod first, it's tough to qualify your progress. Using the car analogy: if you are attempting to boost performance of an older car, you'd want to due compression and vacuum tests to determine the condition of your piston rings and valves first. If you have valve leakage or piston blow-by, you are losing horsepower. Those need to be addressed BEFORE you can maximize performance.

But then again, there are A LOT of hack mechanics for both electronics and cars. The "enough knowledge to be dangerous" category of technician is a huge grey area.

[km6xz]

Perhaps, So only put factory tires on your car. Never improve the exhaust or intake system since thousands work just fine. Only allow union workers with approved licenses do your work. ..... Geeezzzze!

The mods that are so fashionable are assumed to be improvements, yet are they? There is no free lunch in engineering, it is a creative process of compromises. When people come asking for a mod they heard will make them play and sound like Hendrix, I ask what they want specifically, and what are they willing to give up.
There are no mods that have only positive changes.

The point of making a more methodical and scientific approach than advocated on the internet and apparently by you is that few mods or "upgrades" can be demonstrated as improvements, they might be, they might not be but the owner has no point of reference, now system analysis and baseline measurements. Some will counter, "I will know if it is better when I hear it" but time and time again, in double blind experiments, sounds are darned hard to evaluate and an impression one has quite often is based more on expectation, wishful thinking or having a short memory. Actually, we all have short memory of sounds, we have long memories of our impressions of sounds however. Many tests on hi-fi golden ears have shown that what they hear has little to do with reality. For example, since few of the claimed improvements in sound based on high end interconnects or line cords can be measured in any way with any known criteria in mind, tests have been set up where the listeners were asked to pick out the "improved sound". When their score cards were collected, the claims of one being like "having a veil lifted" in improved sound stage with cable "B" vs the terrible "constricted, limited breath image" of cable "A" seemed hollow when the reality was that no cables were changed just a short .5 second muting of the signal to sound like a switching over. The number of variables in guitar amps and guitars make sound testers in hi-fi audio look like paragons of well grounded engineering. I've done listening tests that suggest that it is even more disconnected from reality and more subjective when a guitarist plays and listens while evaluating. There, they have a great deal of influence on the sound and full in the feedback loop determining the sound. When removing that variable, using re-amp techniques the differences between their judgement of garbage and their supreme amp becomes very blurred and inconclusive.
So what did it sound like before the mods? What did the mods do that might lower the performance or reliability?
Years ago, a well known producer came to do a project in my studio. After listening to the monitors he said he wanted them EQ's to remove the dominate mid and make them brighter. The prior occupant claimed the exact opposite..."OK, we do it". They were large soft mounted systems that measured perfectly as we intended them to be and I had no desire or willingness to change them on subjective notions of a producer. I had one of the seconds change the lighting angles so they were illuminated the monitors a little more, they are normal unseen in the dark region above the canted window into the main room of the studio. The next day, the producer listened, and announced that he was happy but thought be brightened them a little too much. It was expected that a change in appearance changed his perception of sound. We were used to tricking singers into pitch and mood changes by tweaking lighting and eq in the cans. This producer, I have no doubt, could "hear" circles around anyone who claims to be a golden ear modder, yet he could not separate his subjective notions from the reality.

You bring up the 57Chevy as an example of mods improving it. Does it, maybe in selected narrow criteria but overall it will be very hard to support that claim. I've built enough sports cars and even campaigned a TransAm 5 liter sedan(Z-28) for a few years during the heyday of that series. Some very specific improvements were gained while accepting tremendous downgrades in other areas of performance and longevity. And that was with some pretty knowledgeable engineers and testing programs.
I am still waiting for someone to show that slapping a $350 transformer in a amp makes an objective improvement in sound. Is a MM transformer better than a $100 Hammond. In some respects, possibly...not proven, but in other criteria it is definitely a downgrade. Pick your benefit and accept the compromises. Basically, good engineering is not behind the mod craze, neither is good music. I've yet to hear of an amp that made ANY difference in the appeal of a song or the sales of a recording. Have you? So, if it doesn't influence listeners judgement, what is it all about?
I am all for hobbyests creating new systems and designs, but that is not what the mod hobby really is involved with. There is little understanding of what they have and what they want to achieve. I don't even thing the goal is to understand anything or apply reason and planning to the modding. Instead of buying the unneeded extra set of fake NOS tubes, if they were really serious they would buy a few theory and math books, and some test instruments. Anyone who can afford several hundred in NOS on a whim just to see if it makes a difference, but not afford a scope, variac, good generator and AC voltmeter is not serious about getting any improvement, any more than the kid down the street through on headers on the 57 Chevy without measuring instruments and a strong understanding of the physics and engineering involved. It is for show, not result.

[billyboy]

"seems good & strong", or is good & strong? I know it's a judgement call but we need a decision? Are you getting 25-30W clean?

It's very possible that connecting the meter is causing instability, as Steve suggested. As km6XZ suggested, if this instability was present without the meter, then you should hear it.

425vdc with 6L6s and a 200ohm cathode resistor is making alarm bells ring for me, 70mA per plate? I wouldn't be surprised if you had some shorted OT primary windings (check dc resistance from each power tube plate to main B+, or pull power tubes & inject a small AC voltage (0.5VAC or 1VAC - measured with a meter) on to the OT secondary to determine the turns ratio, by measuring the resulting primary voltage from 6L6 pin 3 to other 6L6 pin 3. Also check that the OT primary AC voltage from main B+/OT centre tap to each power tube plate is identical....but as with the oscillations, you should be hearing a bad OT?

MWJB,,, OT Centertap to plate is 211 and 197 ohms left & right 6l6. Putting .926v ac on ctr tap gives me .926 v on both plates and there are o volts plate to plate, although one of my meters said 0-.001 or .002 I think its the meter doing its thing. I have 465 v on the plates, 410 on sqreens, with 30-31 v bias and about 65 ma current draw. Is that out of wack? They seem spot on with a B15 schematic voltages in a Gerald Weber book, Kendrick amps, which seems mostly identical to the M16 schematic I found on google. I don't know how much power it has. it's louder than my 15 watter, when you get above 1/3 volume and you play it hard it distorts a bit, at full throttle with the treble and bass down at 3/4 to half it just sings if you like that overdriven brown sound.

YO Steve Conner, I appreciate your response and I respect you because I can tell you know your poop. I responded after you and another gentleman pointed me to the meter lead trouble and I said that yes indeed the meter was affecting the readings.

After doing a basic cap job the amp did sound good until I dimed the volume and the treble with a guitar plugged in, there was some scratchiness in pots and more high end hiss of course with the treble up and then when playing power chords and leads I started hearing ghostnotes , treble distortion. So I started looking closer at voltage and thats where it all began.Thats when i went in and really cleaned the jacks and pots by pulling them and cleaning the chassis mounts, washers, nuts,,, Then I poked around at the various componets on the board and there was lots of crackle, pop which was cured mostly from reflowing the grounds on the circuit board. Perhaps I haven't explained myself very well ie. symptoms wise. I have read that some amps are prone to parasitic oscillation. I'm getting to the point that this amp is what it is and that it's probably working as it should. I just might take it to my somewhat local tech when I get a minute or two and have him look.

Thanks for the replies, Gentlemen

[Cosworth]

Studied electronics back when the only transistor to be found was in a hand held Japanese radio. Studied tube technology back in 73, Semiconductor theory for 2 years in college. Electronics for 4. Had my own engineering business for 5 years. Been running cad systems since they were invented. Schematics, Engineering Drawings, Printed circuit boards, Solid Models, Circuitry Design, Troubleshooting, Drop me a line. Just some of my recent work. Hey, but what do I know... Peace Ya'll!

[MWJB]

The difference in DCR isn't immediately shouting there's a problem, there is usually an imbalance, so long as it's not much over 10%.

With the voltage test, you are supposed to be applying the .926VAC at the secondary - the speaker wires, then measuring from plate to plate of the 6L6s & from centre tap to each plate. But from what you say, there's not obviously a problem...won't hurt to check/eliminate though.

[kg]

After doing a basic cap job the amp did sound good until I dimed the volume and the treble with a guitar plugged in, there was some scratchiness in pots and more high end hiss of course with the treble up and then when playing power chords and leads I started hearing ghostnotes , treble distortion.

these are some of the effects of parasitic oscillations.

[52 Bill]

Studied electronics back when the only transistor to be found was in a hand held Japanese radio. Studied tube technology back in 73, Semiconductor theory for 2 years in college. Electronics for 4. Had my own engineering business for 5 years. Been running cad systems since they were invented. Schematics, Engineering Drawings, Printed circuit boards, Solid Models, Circuitry Design, Troubleshooting, Drop me a line. Just some of my recent work. Hey, but what do I know... Peace Ya'll!

That's great, I wish that I had your education and experience level, but how does this help fix the guy's amp?

[billyboy]

The difference in DCR isn't immediately shouting there's a problem, there is usually an imbalance, so long as it's not much over 10%.

With the voltage test, you are supposed to be applying the .926VAC at the secondary - the speaker wires, then measuring from plate to plate of the 6L6s & from centre tap to each plate. But from what you say, there's not obviously a problem...won't hurt to check/eliminate though.

MWJB, Thankyou for the heads up! So I put 1.12v ac into secondary and I get 51.2 plate to plate. Ctr tap to plate is 25.7v, on both. Thats good to go isn't it? I can see why you raised an eyebrow looking at the M16 schematic voltages while my voltages reflect the M15. Maybe I have the revised version sans schematic or something,

More to the story... I was talking with a friend who plays and has his own repair shop for some 30 odd years, He, like Enzo said he'd really have to see the layout and scope it to really see what's going on. The next day I sent him the schematic and then I tried "KG's" idea of checking the stages by shorting grid to cathode with capacitor and it pointed to V3a with the .005 cap on the cathode which isn't on the schematic and I had a feeling, a wonder. I called my friend that evening and the first thing he said was "pull it". And for you less informed " like me" he said the input jacks were not a good load for guitar pickups, that the Fender setup would be much better, but did I want to mod a vintage amp? Not very badly, I replied. He then said that it was a split load PI and it would never clean up like a" long tailed pair". He said if pulling the cap didn't help as a last resort try a very small cap between the .05 PI caps,cap to cap, and the 6l6 grids. One of the Webber books does a good job of explaining this stuff,FWIW. Anyway, today I pulled that little V3a cathode cap, the one that isn't on the schematic, and the amp sounds much better. I believe that cap was put in the day this amp was born, maybe it was Friday, or it was in the wrong parts bin, or ,,,,
I need to play the amp some more but I think I got what I want.

Which brings me to WTF is it with you dudes who have your flame throwers on the hip. No one said I had to have a PHD in guitar, amps, computer, or music to join. I thought it was a place to learn , discuss, help, blah,blah,,, A while back there was a thread on wether or not you should help us idiots for fear we might kill ourselves. Hell, There's too many of us now and it would clean up the gene pool, just kidding. Sheezse, Anyone with half a brain would know to fix before you mod, Is A tweak a mod?Is a tweak a repair? As others have said, who wants to clean up a mod mess . The junkyards are full of great engineering. An older fella once said to me that all these circuits are from western electric to sell tubes and there were plenty of mistakes and so/so engineering in them. As Mr. Weber said, "they didn't think we'd turn up past 3". How about those fine little monkey wards amps where the tube values add to 115 or something and they were capable of killing you? For Criss sakes uknot your shorts. This is supposed to be "Hey I got an idea, try,,, blah blah blah,,,,,

Thanks to the fella who loaded the pics of how to really mod your amp. I'm still laughing.

Thanks to all who chimed in and helped, I truly appreciate it, even if you dissed me. I can take it. THANKYOU!!!!!!!!!!!!

[Cosworth]

Sorry to have "dissed" you all. I figured I could open up some creative discussion with what I though were probably like minded souls. Obviously I was wrong and am not welcome here. My 35 years of practical experience in electronics is no comparison to the "Wad you hand in a fist and give a hard Whack". Mentality, as recently posted here and met with total acceptance. Even Rudolf got better appreciation. Take Care Folks!