Vox AC100/2 build

[sluckey]

Just unsolder the end of the cap that connects to the pi plate and stand the cap up so that lead does not touch anything. Do this for both caps. Don't read anything else into what I'm saying.

[pompeiisneaks]

Okay, sorry It didn't make sense, I did it.  And is still doesn't make sense.  Now that my coupling caps are gone, there is no route to the power stage, and therefore I get no readings in the power stage, coming in... Or do you want me to measure the plate current on the power tubes now that the PI is disconnected?

~Phil

[Willabe]

Or do you want me to measure the plate current on the power tubes now that the PI is disconnected?

Yes.

If you are concerned that the pi is somehow causing the problems with your output tubes, just disconnect the coupling caps from the pi plates. Then you will be dealing only with the output tubes, OT, B+ supply, and bias circuit.

[pompeiisneaks]

Doh, it dawned on me as I hit send.  I retested, and with that disconnected, I get the same imbalance

[Willabe]

Then it's not coming from the PI.

[pompeiisneaks]

So what could it be?

[Willabe]

If it's not the PI, not the OT and it's not the power tubes, what's left?

[pompeiisneaks]

I don't know?  Sockets?  The 1k resistors all tested as good, the 1 ohm resistors in the power section tested good. I doubt two of the four sockets I just bought form Doug a few months ago are bad.  If it is one, what do I do to find out? 

~Phil

[Willabe]

You have 1 socket that is drawing low current, why?

What you have left is;

1. socket.
2. the wires that go to that socket.
3. the solder joints on that socket and on the other end of the wires/parts that go to that socket.
4. cathode ground connection.
5. -bias and the wires that deliver it to the power tube grid return R's.

[Willabe]

Which socket is the 1 drawing low current?

[pompeiisneaks]

I just put all 4 back in to see what was what, and I think I may have the one that is the bad socket or at least bad place. 

The second from your left pic, what I'd call V6 (V5-8 are power tubes) Here's what I just read with all 4 back in:

V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV

That may indicate either something isn't soldered well on V6, or that V6's socket has something wrong. 

What else could make that one pull so much current?

~Phil

[pompeiisneaks]

Oh sorry I actually didn't answer your question right, the right two sockets are low, the left two, higher, but it seems that, as you can see, one is a lot higher than the other.

~Phil

[sluckey]

What are the voltage readings on pin 5 of each tube?

[pompeiisneaks]

From the same below:

V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V

~Phil

[pompeiisneaks]

Oh and side note, not sure if it matters, but touching those makes a lot of static noise and scares the crap outta me each time :) bad hum type thing that's pretty loud in my speakers that have volume set to 0.

~Phil

[Willabe]

You have 1 socket that is drawing low current, why?

What you have left is;

1. socket.
2. the wires that go to that socket.
3. the solder joints on that socket and on the other end of the wires/parts that go to that socket.
4. cathode ground connection.
5. -bias and the wires that deliver it to the power tube grid return R's.

I forgot;

6. power tube grid return R's/value/wires to & from/wired incorrectly.
7. power tube grid stopper R's/value/wires to & from/wired incorrectly.

[pompeiisneaks]

So to recap all 7... 1. I am suspicious that one socket may be bad, but lets go over the rest
2. the wires that go to the socket.  I've been testing them with the continuity tester to ensure things are going where they should, by touching the base of the pin, not the wires themselves, and then connecting to the previous connection or even just past a turret and always have gotten good continuity.
3. I've resoldered them all over again ensuring I got good continuity from 2 above.
4. I can test the ohms resistance for all cathodes to ground with no issues (as well as continuity because low ohms under like 20 count as 'continuity' on my meter)
5. I also can touch the bottoms of the pins and get the -grid values I reported already.  Each pin is getting that voltage
6. Sorry, I'm not familiar with the 'return' name?  Is that the 1k, from the B+ resistor or the 1 ohm to ground?  the only other resistor is the grid stopper which is the 1 47k resistors.  I'll report on those. 

7.  Same as 6 above, the report on all resistors will be below.  I have tested that the wires give me the connections I expect, as for 'incorrectly' I can describe them to you:.

For the 1 ohm, I go out of the pin 8 which is jumpered to pin 1, and go directly to a grounding bus on each one.  I can measure their resistance by connecting my ground to the far end of the ground rail away from all three and get the same reading as if I put the ground just under it.  I test the ohms by touching the base of the pin itself, not the wires
for the 47k I have tested that I get the voltage of each on the pin, and I've tested each of them before and after the resistors and the voltages seem to make sense and indicate they're all flowing as expected.
for the 1k resistors, I've tested voltages on them and they all are showing correct.  I've tested them by touching the pin 4 base and then read resistance on the other end of the line and I get the readings I'll report.  As for their wiring, I've got one end of the 1k resistor in pin4, the other in pin 6 and then I've got jumper wires between all 4 pin 6's that then leaves the V8 tube off to the B point.

I hope that describes them, I can hunt down my pictures too if that will help, they're on this thread but I don't mind avoiding you going hunting for them.

Here are the measurements:

V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V
0.6ohm, 1.4ohm, 0.9ohm, 1.3ohm
0.976k, 0.978k, 1.001k, 0.981k

Does that help?

~Phil

[pompeiisneaks]

Here's a pic of the first three output tubes, missing the 4th in the picture.  https://goo.gl/photos/Z2av6nsFeQF2cxv26

~Phil

[Willabe]

In the Vox schemo, R21 & R22, 330K/350K?, I can't read it, are the grid return R's.   

[pompeiisneaks]

Oh gotcha, since they're in parallel, I can't measure them independently, but either side measures 172.7 Ohms (which would indicate about 345.3 each).  They're supposed to be 330k so that would be  = 165 is expected.  its a bit over.  If they're actually out of balance in some way, would that cause the sides to diverge?  Should I break them out of circuit and measure?

~Phil

[Willabe]

No, I don't think that's needed. Wait for Sluckey or 1 of the other guy's to confirm.

One other thing, most meters can't measure a 1 ohm R accurately. And there's been a few threads where it's come up that because of this and that modern 1 ohm/1% R's are made well enough to not even worry about them being off enough to throw off the K current measurement.   

(I've been sick for a week and I'm going to bed.  )

[pompeiisneaks]

Oh I may have 1 ohm 5% resistors!  That definitely would be part of why they seem so off.  I definitely don't think my Fluke 17 can read them super well, it seems to have only .1 accuracy for ohms.

I just got over being sick, thanks for your help, get some rest!

Should I go buy some 1% ones and replace them?  (I replaced a few I popped being stupid, the ones I got from Dough are 1% I believe.)

~Phil

[Willabe]

Here's his last power tube readings;

Here are the measurements:

V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V
0.6ohm, 1.4ohm, 0.9ohm, 1.3ohm
0.976k, 0.978k, 1.001k, 0.981k

[Willabe]

Oh I may have 1 ohm 5% resistors! 

Should I go buy some 1% ones and replace them?  (I replaced a few I popped being stupid, the ones I got from Dough are 1% I believe.)

I would.

Yes Doug's 1 ohm's are 1%.

Heck, for the price ($0.15 from Doug) buy at least a dozen or 2 dozen and be done with it for a while.      

[SoundmasterG]

Which socket is the 1 drawing low current?

Willabe,


This pic you linked here is of my amp that I showed for example, not of pompeiisneaks's amp.


Greg

[pompeiisneaks]

Heh I thought he'd just pulled in some random one to show 4 tubes for me to specify :)  Yeah I posted a pic of three of them (The 4th is the one that mirrors the other lower output tube, so since it seems identical to 3 I think its good, or I hope so).

~Phil 

[Willabe]

This pic you linked here is of my amp that I showed for example, not of pompeiisneaks's amp.

Doh, my mistake.   

[SoundmasterG]

Here's a pic of the first three output tubes, missing the 4th in the picture.  https://goo.gl/photos/Z2av6nsFeQF2cxv26

~Phil

Phil,


In this picture your 1 ohm resistors coming off the sockets and wrapping around a buss with solder to tack the connection into place isn't the best approach. That buss can easily contact the chassis at any point along its length which could compromise grounding. A better approach is to either use terminal strips near each socket that you connect the resistor to from the socket and then connect your buss or wires to those. Another approach is to use tip pin jacks along the back of the amp as in the Sunn 200S example that I attached to this post. Years ago I modified this Sunn and added the tip pin jacks. The wire goes from the socket to the tip pin jack, then the resistors go between the red tips to the black tip pin jack which is then connected to ground. With either of these approaches that I mentioned, there is no way that the wires or resistors can move and then have your connection points change as with the approach that you used. You can see the tip pin jacks at the back of the chassis in the Sunn. I believe Hoffman sells those tip pin jacks too.

This pic you linked here is of my amp that I showed for example, not of pompeiisneaks's amp.

Doh, my mistake.   

Its cool Willabe!    Just wanted to head off any more confusion...haha


Greg

[pompeiisneaks]

Not sure I follow, if the tubes ground sooner that wouldn't be a problem would it?  The problem you often have is that the preamp side can ground before the power stage leading to hum, I recall being taught and reading that it is often best to get the power section and output sections to chassis ground before the preamp side. So if somehow the power section grounds to chassis earlier than they already are, isn't that a 'win win'?   There is also a really thick paint coating over the chassis anyway which reduces the chance of a ground that I shouldn't need to worry about anyway.  (a lot of older amps used to ground pins 8 directly to the chassis right next to them, instead of running a bus to a ground somewhere else).  This is why I wasn't concerned about the grounding for the output tubes.  Is that a misunderstanding?

~Phil

[SoundmasterG]

Not sure I follow, if the tubes ground sooner that wouldn't be a problem would it?  The problem you often have is that the preamp side can ground before the power stage leading to hum, I recall being taught and reading that it is often best to get the power section and output sections to chassis ground before the preamp side. So if somehow the power section grounds to chassis earlier than they already are, isn't that a 'win win'?   There is also a really thick paint coating over the chassis anyway which reduces the chance of a ground that I shouldn't need to worry about anyway.  (a lot of older amps used to ground pins 8 directly to the chassis right next to them, instead of running a bus to a ground somewhere else).  This is why I wasn't concerned about the grounding for the output tubes.  Is that a misunderstanding?

~Phil

Phil,


It isn't about grounding sooner or later....it is that you want to control your chassis grounding because poor grounding can and does lead to hum and noise. What you have there is a random ground occurring randomly.    It is best to control all of your grounds in either a galactic grounding scheme like O' Conner and Merlin recommend, or use specific ground points like Hoffman recommends. Powder coated chassis' will not stop parts from grounding btw. I found out that the hard way on a previous project some years ago. In my example amp I posted above, everything grounds to a buss that is on the boards. They all ground in order from preamp, phase inverter, and power amp, and finally that buss and any other grounds anywhere else in the amp all ground to a terminal attached to a bolt that is secured to the chassis by itself. it is best practice to put that bolt near the input of the amp, as any noise on the input would then ground out there before it gets into the amp, but in the case of my example amp it was quieter bolted near the power supply end of the chassis so that is where it was connected. With that setup there is only one point on the chassis that is connected to ground and it works very well if done correctly. With Hoffman's approach, the grounds are controlled so that the noisy power amp stuff grounds to one point and the quieter preamp stuff grounds to a different point. With either system though the ground connections to the chassis are controlled and any ground issues then are easy to solve if they occur as there are only a couple places to look. With the setup you used, it is possible to randomly have that buss grounding, or not grounding at various points along its length. This can create issues depending on where that buss runs in relation to other noisy areas of the amp. It is also not best practice to connect wires and resistors that way and there is a reason that no commercial amp ever made used that approach. It would never pass any regulatory bodies anywhere in the world either. It is your business if you want to build something with questionable practices. I'm just offering a suggestion....you can use the advice or not. For myself I always try to build the best and safest amp that I can.


Greg

[pompeiisneaks]

First off, I take all input and suggestions very seriously, I'm just trying to understand the 'whys' of things.  I'm still unsure I understand, because it 'seems' to me that you've just re-confirmed what I'm saying, but also stated against it as well, so I'm trying to understand.

There isn't an 'optimal' way to ground per se, right, many that work. 

1 is that you ground the noisiest stuff on a bus to the near chassis ground, and then ground the preamp connections to a bus that is slightly farther away,  less potential for noise.  I am using this right now with my bus bar, I ahve two of them, one that connects the power stage and output tubes directly to a bus that is grounded near to a PT bolt.  The second bus grounds a few inches away from the PT and is on the other side of the narrow side of the chassis from the PT. 

2. Ground the noisiest stuff directly to chassis.  I.e. power and output tubes go to ground asap so they get out of the grounding plane the fastest, and defer the grounding of the preamp side still similarly to 1 above.

so in my current grounding scheme, what you're saying if you ground via 1 or 2 randomly it can add noise?  Why would the two optimal grounding options, at random, cause a problem? 

I.e. right now it is grounding well to the PT... oops it just grounded directly to chassis... but that's the same high quality connection as the previous? 

What am I misunderstanding?  What does a good ground that accidentally gets a different good ground do to contribute noise? 

If I were to put some kind of grounding anchor on the left hand side, which means that I have a dual ground for that bus, is that good, or bad? 

Is your suggestion to simply put a 3 hole Terminal strip and connect the first tube on the left to it so the rest of that ground is lifted off the chassis, or do I put several of them along the length? 

Ultimately with this build, I'm trying to finish it in a sane way, since I've put in about 1000 into it and its driving me nuts not getting it working and I'm trying really hard not to sink even another penny in unless it will definitely help. 

The hum is not gone, but it is significantly cleaner since I restored the 2 100meg resistors to ground for the heaters.  I don't necessarily like my layout or how the heaters are wired, but I don't have a lot of room to improve that due to the layout that I'm using that mirrored exactly how Vox did their previous one.  The only major difference right now is that they also had about 2 more grounding connections (They were grounding each board to chassis, the main power one twice). 

Also a lot of my hum, as stated, was due to me standing between my speakers and the chassis with single coil pickups.  When I wandered a few feet back it was almost gone.  (I still want to fix that last annoying little bit, though, I think its my heater dress... I may rip it all out again, and give it one more shot, but maybe after I put coax cable from each connection of board to grid of the preamp tubes, to reduce input noise. )

Thanks for your help. I'll implement whatever is the right way to do this, just trying to understand it.

Edit:  Clarity, one statement didn't make sense.
~Phil

[Willabe]

..........I think its my heater dress... I may rip it all out again, and give it one more shot, but maybe after I put coax cable from each connection of board to grid of the preamp tubes, to reduce input noise. )

I would not do that. Do the shielded cable on the grid wires 1st.

You *might* have a ground loop or 2 in that amp, but you do have some ~long grid wire runs that are running parallel to other wires that could be a problem. The longer they run together, the more 'hash' they can/will pick up. Given your layout, the overall size of the chassis and # of tubes/grids, ie, several ~long grid wire runs, each possibly adding/picking up hum/noise, I would do this 1st before messing with the grounds.

You kinda have use chasing our tail here. And you still have more than 1 thing going on that your trying to fix.

Sluckey told you on the 1st page of this thread; Divide and conquer.

The hum issue started on page 1 of your thread, which is fine. You found out that 1 or both 100 ohm heater faux CT R's blew and Sluckey told you to leave them out until you get the B+ on the heaters fixed. 

Then you say you didn't put the 2 x 100 ohm heater R's back in after you straitened out the shorted tube(s) that blew them.       So that helped a lot, good. Then you mention that the hum was again reduced by moving the guitar (single coil PUP's/P90's) away from the amp. Single coil PUP's are noisy that was the reason humbucker PUP's were invented, to buck the hum that can be picked up.

With your 100w amp, you have a bigger PT with more current than a smaller amp, so, your amp PT will put out/radiate more 'noise' to be picked up. And because your amp is a 100w it will be amplified a lot more than with a 20w amp. Yes there still is noise to signal ratio no matter what the amps power but still a 100w amp's hum will be noticed a lot when turned up. 100w & bigger PT + single coil PUP's = hum/noise.

....I'm just trying to understand the 'whys' of things.

I think I posted this link for you before;

http://www.valvewizard.co.uk/Grounding.html

[pompeiisneaks]

Willabe,

Promise, I didn't bring up the hum issue, that was soundmasterg.  I'm fine with discussing it, but my primary problem is that of the imbalance in the power section, it has to be why I'm getting such horrid sound out of the amp and the runaway pops that start happening as well.  As I see it, if I really am understanding what's happening, the two right power tubes are set at a point that they're probably fine, but the left two have one that is always biased near runaway if not already in runaway, it then climbs any time too much input signal comes in, and pops as it overloads and is arcing in the tube itself?  Or from one pin to another in the chassis becase it was never meant to do so. 

I think if I can find out why socket 2/V6 is so high compared to the other three, I think that will resolve the majority of my problem.  I just can't seem to track that down. 

To reiterate, last measurements for anyone that could maybe help (staying on the primary issue):

V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V
0.6ohm, 1.4ohm, 0.9ohm, 1.3ohm
0.976k, 0.978k, 1.001k, 0.981k

This is with no input into the amp, volume at 0. 

why oh why would the V6 be at 45.0mV??  If I use the associated ohm readings thats the following in mA
44mA, 32mA, 13.9mA, 9.6mA 

(Of course, as was stated earlier, the measurements I'm getting on those ohm's could be off due to measureing that small being hard.  If not, then we just estimate they're at 1ohm and get the same mA as mV)

I also seem to remember the V5 resistor being at .8 ohms the other day, maybe those have been worked too hard with something else I did wrong in the past and are slowly dying? 

Edit:  SoundmasterG, that in no way is a negative comment to you, just that I've definitely led this thread all over the map, and I'm trying really hard to stay focused on what I need to do to resolve the major issue facing me.  I may even start a separate thread on hum removal once I clear up the major problems here.

~Phil

[shooter]

Try this, leave the PI caps un-soldered, completely unwire everything from the 4 PA tube sockets, re-wire with new wires, new R's, THEN recheck bias volts at each tube socket(at the tube side NOT under side).
leave tubes OUT till all 4 have *close* bias

[pompeiisneaks]

I was going to try something similar tonight, I don't have time to do it all tonight, but if what I was going to do doesn't work, I'll try that.  I was going to disconnect the socket that is significantly higher, pull it out completely and try to remove any solder on the socket.  Get it really clean, and test every pin with my DMM.  Ensuring that every pin has no continuity to the other pins by plugging one side in with my DMM probe like a tube pin and touching everything else.  Then once I"m sure that's good, I'll resolder it back in, and give it a try.  I'm suspicious of that one tube that is so significantly off, and want to really give it a once over. 

~Phil

[SoundmasterG]

Phil,


No worries. it is good that you are trying to learn and don't take what I said too harshly. There are many problems with this amp including layout, poor connection methods at various points, etc., but as you said, you need to get the amp stable before you can do anything else. Willabe's suggestions about using shielded cable on the grid connections is very good....I made that point earlier also. Having those grid wires running next to plate and cathode wires and in parallel with other grids from other tubes is a recipe for oscillations and crosstalk. The grounding article Willabe posted is very good reading and you should really read up on that whole site as there is much useful info there. The 1 ohm resistor connections to the bus is another area that you should address throughout the amp. Any connections like that are not a good way to connect things, as Sluckey mentioned earlier about your screen grid connections. You want things connected up in a such a way that if the amp gets tossed around or dropped or thrown into a van or something that no connections or parts will move so that they could short out etc. There are books and websites out there that talk about the proper way to connect things and it may also be useful to research some of the UL requirements so you can see what regulatory agencies around the world consider appropriate construction and change your methods in the future so you can make stuff that flows with those recommendations. The hum sounds like it is mostly your guitar so I wouldn't worry about that per se. First rule of troubleshooting hum is to eliminate the guitar from the equation by unplugging it and seeing if the hum goes away. But, address the bias issues before you do anything else. You need the amp to be stable before you can continue troubleshooting.


On the power tube bias, when you are checking it, do you have a signal running through the amp? Do you have it connected to a resistor load or a speaker? All controls at zero? There should be no signal coming into the amp, all controls should be at zero, and a resistor load attached to the amp instead of a speaker. (In the absence of a resistor load you can use a speaker but the reading will change with frequency.) Then check your bias and set it to say 65% of max dissipation so the tubes will be happy. No need to run them hotter than that until you are doing the final dial in for tone after everything else is fixed on the amp. So 65% of the 25 watt max is 16.25 watts. At 470 V B+, that would be between 34mA and 35mA. The tubes should be stable and happy at that point. you should be able to fire up the amp going into a speaker with no signal going into it and have it sit there and work without the tubes running away or anything else happening. If the tubes start to get unstable on their bias, and start to runaway, then either there is something wrong with that tube, the socket, the connections inside the socket where it contacts the tube, the bias supply, etc. It may be worthwhile to take the power tubes out and adjust every socket so they all the pins tightly grip every pin on the tube when you insert the tube, just to be sure.

Greg

[pompeiisneaks]

Greg,

Thanks, I've already taken the sockets and tightened them up, they all feel nice and tight when inserting.   (some didn't at first).  When I'm checking it, I have no input into the amp and volume at 0.  I've had a speaker connected for some of the biasing, but I can go back to the dummy load after I clean up the one socket that seems suspicious.   

~Phil

[Willabe]

........ suggestions about using shielded cable on the grid connections is very good....I made that point earlier also.

Yes you did. 

[Willabe]

I also seem to remember the V5 resistor being at .8 ohms the other day, maybe those have been worked too hard with something else I did wrong in the past and are slowly dying?

I'd get some 1 ohm/1% R's from Doug instead of those 5%/1 ohms you have.

[pompeiisneaks]

They're on order now, got 20, and a tag board for the other end of that power bus, as well as a chassis for my gibson eh-185 build that's coming up soon :)  (gotta keep it under the monthly budget allocated by swmbo)

~Phil

[pompeiisneaks]

Okay I ended up doing V6, cleaning everything up really well, removing excess solder, testing continuity on all pins and found nothing shorting and good connectivity on the pin itself, resoldering them all very carefully, and restarting it, and still the exact same imbalance issue 25mV, 50mV, 12mV, 12mV.

I then went and repeated the process for V5, removing excess solder, cleaning it up testing continuity etc, and this time it started going into runaway badly again, I couldn't get consistent measurements so I dialed down the bias more until it stopped.  I then measured and it is now balanced per side, but not for both sides.  The reading now is: 24mV, 24mV, 5mV, 5mV.  I now think if I can take the time to clean the other two out as well, and get them setup, things may be fine.  I'll report back on it when I can get it done.

I think ultimately this may yet be another side effect of me using too thin of solder at the onset.  I did 'resolder' the connections with new solder, but I didn't clean up big blobs at the bottom.  They may be causing some kind of bad arcing or bad connectivity, who knows.  Ultimately we'll see if the same cleanup will now balance out both sides of the power tubes side. 

Hope to get to that tomorrow or thurs.

Thanks to all for the help, crossing fingers this will do it.

~Phil

[pompeiisneaks]

Well, good news, and bad news. 

I've completely redone all 4 power tubes and now I have very consistent readings across the board on the power section.  I just read the mV on them and got

30.0mV, 29.8mV, 30.1mV, 29.9mV

Voltages on the other pins all seem in line.

I then put in the speaker and I've got horrid noise and distortion.  Lots of clicks and pops.  I'm going to have to look over the areas I desoldered for the testing, I may not have gotten those resoldered in very well, but I'm at a loss as to how I had clean tone before at low volumes, now it sounds horrible at low volume.  I'm already done for now, nursing a sore back, I'll take a look at it tomorrow again, and do some checking.  I tried the tube removal from V1 - V4 and the horrible static that is back disappears until the PI is back, but again, that could be between the PI and the Output side.  At least I now have a stable power side. 

When I play it has a lot of static and pops as well.  hopefully tomorrow I can get a look at it with fresh eyes.

~Phil

[SoundmasterG]

Well, good news, and bad news. 

I've completely redone all 4 power tubes and now I have very consistent readings across the board on the power section.  I just read the mV on them and got

30.0mV, 29.8mV, 30.1mV, 29.9mV

Voltages on the other pins all seem in line.

I then put in the speaker and I've got horrid noise and distortion.  Lots of clicks and pops.  I'm going to have to look over the areas I desoldered for the testing, I may not have gotten those resoldered in very well, but I'm at a loss as to how I had clean tone before at low volumes, now it sounds horrible at low volume.  I'm already done for now, nursing a sore back, I'll take a look at it tomorrow again, and do some checking.  I tried the tube removal from V1 - V4 and the horrible static that is back disappears until the PI is back, but again, that could be between the PI and the Output side.  At least I now have a stable power side. 

When I play it has a lot of static and pops as well.  hopefully tomorrow I can get a look at it with fresh eyes.

~Phil

Thats good news Phil. Now that the power section is behaving, you can try more troubleshooting to see if you can pinpoint where the problems are. If you pull the PI tube and the noises disappear then the problem is before the PI circuit somewhere, not after. It could be caused by oscillations due to poor layout, or poor soldering connections. If you have any shielded cable, then now might be a good time to connect that on the grids in the preamp. If you do this, remember to only connect the shield on one side to ground....the other end of the wire should have the shield floating. You may also see if you can route the wiring going from the board to each preamp tube differently so stuff doesn't run all in parallel. Perhaps use zip ties to secure stuff in places so it will have some space between each stage and cross at 90 degree angles if you have to cross?

Greg

[pompeiisneaks]

Sounds like a plan, I'll give that a go.

~Phil

[pompeiisneaks]

Okay so tonight I just had time to resolder the areas I'd removed previously and go over a few other parts on the main board that seemed suspicious, I then turned it back on and got things going.  I had to dial the bias down to about 25mV just to keep it stable, or it kept going into the clicking state (is that runaway?).  Once I did that, though, it seemed to really settle down. 

I could then play pretty damn well, it was clean a bit muddy/dark sounding, but all in all it actually sounded like an amp.  It did do one thing that was odd.  When I connected the scope probes to one half of the 47k resistors into the grids it gave me perfect output of the input, but the other side, as soon as I'd connect the scope would start screeching like just introducing the scope caused some massive feedback. 

What would cause that?  Is it yet another soldered connection somewhere that's not working right?

I think the bulk of the problems on this build may be chalked up to just using too thin of solder honestly!  /sigh.  If anyone has any ideas I'd be happy to hear them.  If not, I'll poke at it more tomorrow or Sat, got caught up in other things after only an hour or so of work on it tonight.

~Phil

[SoundmasterG]

These amps can be a bit muddy sounding sometimes...they don't have the crisp upper frequency clean like an AC30 or a blackface Fender...that is just the nature of the beast. They do give a very touch responsive and harmonically rich sound though which many other amps do not. I added a cut control to mine to be able to crisp up the top end and I like the addition.


It is strange to me that you have to keep dialing back on the bias to get the amp to behave....I am still suspicious of those power tubes myself due to what was happening to them previously when they would run away.


I think the scope probe thing could be the fact that they can act as an antenna of sorts when you connect them up to a grid. I didn't have that problem when I was messing around with mine specifically but every amp and layout is unique.

Greg

[pompeiisneaks]

Yeah I realized its basically the normal channel on the vox, which is also not so clean.  I may just add a 'top boost' switch in it so that it alters that cap from the .047uF to the 500pf one in the top boost channel on the AC30.  (edit: Seems like I missed the additional 100pF cap on the volume pot itself... does that being on the volume pot make the extra 100pF dynamic where whatever portion of the sound goes through that works like a variable tone control?, to make that switchable, do I need to also use a dpdt switch and add a second line for that cap in the path with the volume?)

That's pretty much the majority of the difference that I can see, and then its switchable. 

Sound like a logical option?

As for the tubes, are you saying you think they've been damaged due to previous runaway conditions and therefore go into runaway sooner? 

~Phil

[SoundmasterG]

Yeah I realized its basically the normal channel on the vox, which is also not so clean.  I may just add a 'top boost' switch in it so that it alters that cap from the .047uF to the 500pf one in the top boost channel on the AC30.  (edit: Seems like I missed the additional 100pF cap on the volume pot itself... does that being on the volume pot make the extra 100pF dynamic where whatever portion of the sound goes through that works like a variable tone control?, to make that switchable, do I need to also use a dpdt switch and add a second line for that cap in the path with the volume?)

That's pretty much the majority of the difference that I can see, and then its switchable. 

Sound like a logical option?

As for the tubes, are you saying you think they've been damaged due to previous runaway conditions and therefore go into runaway sooner? 

~Phil

If you forgot the cap across the volume control then that would do a lot to affect the tone at any volumes lower than max and adding it in there will make the amp have better top end for sure. I would just add it rather than making it switchable as it sounds a lot better with it in place. The stock AC100 only has one channel and it isn't really the same as the normal channel from an AC30 as it has more gain stages and more tone controls and uses different tubes.


Yes on the tubes. Since you were running them in a condition where they kept going into runaway, it couldn't have been good for them. I had that happen on my AC100 project with JJ KT77's because the screen grid resistors I was using were undersized and they were cooking themselves and then the tubes would start to act funny. I also had trouble with the fixed bias and my switching and getting the fixed bias mode to work correctly. So basically the tubes got really cooked once and after that the whole quad didn't want to behave. I could try one of the pairs and it would be ok but as soon as I tried the quad it would want to go out of control eventually. My tube tester showed that they were still good, but their performance parameters had changed from when they were new by quite a bit. I had to run the negative bias voltage a lot higher (current a lot lower) to get them to behave and they weren't happy at all in the cathode bias mode even with larger cathode bias resistors that made the tubes cooler running. I replaced them with a new quad and fixed the issues that were causing the tubes to have problems and it worked fine after that. So if it was me I would be suspicious of those tubes myself.


Greg

[pompeiisneaks]

Ok I had a buddy over today to fix his amp and in going to buy a few caps I got myself the 500 and 100pf and replaced the one, and put the 100 on the volume pot.  Major difference in tone, and I really like it. 

So I didn't have much time to do more than that, being that I was helping my friend get his amps tuned up (peavey and an electar, both resounding successes, filter caps going out on the peavey and I just changed a 3.3k cathode resistor on an 12AX7 on the electar to a 1.5 k to bias it for more tone overdrive, it sounded great)

I'll work on it some more tomorrow.  I did get one more bit of testing done, though. The still annoying clicks and pops happen every time with 4 tubes.  As soon as I get down to two it sounds amazing.  This has happened though, with EVERY set of tubes I've ever put in it, even these brand new ones do it.  Why?  If I bias them really hot, like 25mV or lower, they do okay but can still climb out of control if I'm not careful.  If I bias them too cold, nearing the full 70% it can barely go over 3 or 4 out of 10 without going nuts.  Why does the amp sound so great with only 2 tubes?   It can literally be any two tubes, inside two, outside two, and any of two of the four.   I just can't get the full 4 to settle down. 

Any pointers are greatly appreciated.  I can't see myself buying another 4 power tubes after now going through a total of 10.  (first four were doing the same, so I pulled two and it seemed good, we figure two tubes were dead so I bought two more, the other two, one had a short on pins 2/3 and blew the heaters on all the other tubes.)

~Phil

[shooter]

can still climb out of control

I'm not well versed in P-P but my understanding is the bias supply, if correct should be quite stable.  Maybe do the same thing there you did with the sockets, rebuild it?