Drip Dual-STA 5167 Limiter

[Bowie]

Starting to realize some of the voltages are off spec on this build. For instance the original schematic says 282v on the 6v6 plates but I'm at 326v. This isn't something I'm hearing. It sounds great. Just something I noticed while rebiasing after a couple weeks of usage. Is this a problem? Some fender amps have their plate voltages as high as 400v.

Thats pretty much the only voltage that is off more than 8v that I've noticed.

Hard to say without comparing the build you have to the original schematic but if it were to follow the original then something may be off somewhere. Check your B+ and see where it deviates. The original schemo is very helpful here as it lists voltages at various points.

The concern is less that the 6V6s are in danger, but more that you may have built something incorrectly. If it were mine I'd want to find out where and why it's going off. It could be an error that affect the performance of the unit and the raised voltage is just a byproduct of it.

 

[FarisElek]

Hard to say without comparing the build you have to the original schematic but if it were to follow the original then something may be off somewhere. Check your B+ and see where it deviates. The original schemo is very helpful here as it lists voltages at various points.

The concern is less that the 6V6s are in danger, but more that you may have built something incorrectly. If it were mine I'd want to find out where and why it's going off. It could be an error that affect the performance of the unit and the raised voltage is just a byproduct of it.

Sorry I forgot to mention. My plates are the same as the B+. So what causes B+ to be too high?


Update:
The sta-level manual says that socket voltages are good within 20% of voltages listed. Does that mean I'm good seeing mine is closer to 16%.

 

[FarisElek]

Weird hack....

There is a 6386 balance. I've balanced it a million times but today I actually compressed with a 1kz tone as -10db and set the compression to around 15db of GR and THEN balanced them. Not sure if this is okay, or sound...but it sounds better to me. I didn't measure anything. This is completely based on aural perception. I can't find any information on 6386 balance and the proper way to do it. But I think this it being balanced while compressing makes more sense when idol, especially if it drifts when compression is happening. Not sure!

 

[FarisElek]

Some random things:

1. Greg said to connect B+ directly to choke because it workers better. I'm not exactly sure the idea behind this, I'm sure it's true, but when I connect it that way my B+ is at 325v and It means my 282v ideal plate voltage is impossible to achieve without a very big dropping resistor clumsily added between Center tap and choke. So I placed Center tap back on it's area on the main PCB and now my plate voltage is perfectly 282.

2. My 12k resistor off my grid is dropping my 287v to a 191ish. So my ideal 242 grid voltage is a bit off. It must be that for some reason my current is off what it's supposed to be. Reverse finding the voltage drop with a voltage drop calculator I see that I need 12k .75 watt and 8 milliamps to drop from 287 to 191, so I surmise that I'll need a .5 watt 5.7k resistor to get to 242v.

My only question is if that all sounds kosher to ya'll. All my other big voltages are looking good. I want this thing to be up to spec so I'm going to order the couple mouser parts and be done with it. I've read about the effect of different grid screen voltages and I certainly want my build to sound right.


Ryan

 

[scott2000]

2. My 12k resistor off my grid is dropping my 287v to a 191ish. So my ideal 242 grid voltage is a bit off

Looks like there are two resistors in play here..... A voltage divider with the 39k as well.....

Maybe since the power supply and regulation is different from an original, you'll see some variations from the original circuit voltages ???but idk...Maybe Gregory took this into consideration......  Are all the components (choke,transformer..etc ) used in your build exactly as they are in the bom???

 

[Bowie]

Some random things:

1. Greg said to connect B+ directly to choke because it workers better. I'm not exactly sure the idea behind this, I'm sure it's true, but when I connect it that way my B+ is at 325v and It means my 282v ideal plate voltage is impossible to achieve without a very big dropping resistor clumsily added between Center tap and choke. So I placed Center tap back on it's area on the main PCB and now my plate voltage is perfectly 282.

2. My 12k resistor off my grid is dropping my 287v to a 191ish. So my ideal 242 grid voltage is a bit off. It must be that for some reason my current is off what it's supposed to be. Reverse finding the voltage drop with a voltage drop calculator I see that I need 12k .75 watt and 8 milliamps to drop from 287 to 191, so I surmise that I'll need a .5 watt 5.7k resistor to get to 242v.

My only question is if that all sounds kosher to ya'll. All my other big voltages are looking good. I want this thing to be up to spec so I'm going to order the couple mouser parts and be done with it. I've read about the effect of different grid screen voltages and I certainly want my build to sound right.


Ryan

It's not totally clear to me, since your PSU is not going off the original but I recommend referring to the orig schemo and seeing where you vary. The power going into the choke is around +332. The B+ comes off the choke at +300, hits the 12k and reaches the plate at +282. you can just trace it back and see where your variance originates. That original schemo is really gold and not hard to understand, even for newer builders. I also found that screwing w/ the C12 caps as one of the Drip BOMs recommends, created problems w/ my B+.

 

[FarisElek]

Ah I see. Yeah, It appears my PSU is funky. It's got about 350 at the choke and off the choke it drops to around 340. That's why my B+ is so high when I connect it directly to the choke. But If I take the B+ from the mainboard, it gets regulated down to 300V. I don't really understand this PSU so much and not having a schematic for it is kind of stressful.


As for C12...I'm not sure if I have one. There is an area with four 10uf electrolytic that I think may be essentially the two c12's. Then again There is one big 10uf capacitor that also is labeled in smaller silk font 20uf. The original build details only say 10uf and since this is his universal PSU I didn't pay any attention to it.

A real schematic for this thing would be amazing.

 

[scott2000]

A real schematic for this thing would be amazing.

Was mikeij's schematic not verified as correct? Post 51.....

 

[FarisElek]

He did, but it's incomplete and the psu continues onto the main board. Also, there really any test points of listed voltages and the whole PSU differs from the original so I'm not exactly sure where things are getting dicey. It seems like the amperage may be high near the choke, which is why the choke isn't dropping the voltage enough, but then it seems low on the main board as resistors are dropping voltage too much.


Ryan

 

[scott2000]

He did, but it's incomplete and the psu continues onto the main board. Also, there really any test points of listed voltages and the whole PSU differs from the original so I'm not exactly sure where things are getting dicey. It seems like the amperage may be high near the choke, which is why the choke isn't dropping the voltage enough, but then it seems low on the main board as resistors are dropping voltage too much.


Ryan

It's tough because the drip power supply doesn't seem to resemble the original much although it should end up with similar results... The choke in the drip build is different for example? The original Sta choke was 400 ohms resistance....sounds like yours is only around 65ohms (10v drop @ 160ma) ...etc.....


Hard to tell from reading what power supply you ended up going with in regards to solid state vs tube diode, regulation vs no regulation.
Any chance you could recap your psu  decisions (tube diode vs solid state,transformer selection,choke selection,regulation...etc) again just to clarify? Maybe another pic of your populated psu could help as well.

Nothing wrong with clumsy dropping resistors if need be btw...

 

[FarisElek]

I. You're right on with the choke. Its 65 ohms 5H at 200ma. I didn't want to worry about overheating so I opted for the SS psu. Atleasy for the time being. I can always remove the diodes in the future when there is more information. The 6v6 already get really hot though and the case only has vents on one side of the top.

Everything I did was option one of the eratta. I jumped the regulated dropping resistor that was one of the newer additions.

 

[FarisElek]

The ultimate question after much deliberation. What would cause low current draw in the 6v6 screen grid?

 

[Heikki]

The ultimate question after much deliberation. What would cause low current draw in the 6v6 screen grid?

6V6 is a beam tetrode and the control grid and screen grid are aligned so the screen current is very small. The screen current can be very unpredictable because it depends on how well the grids are aligned. At some operating condition the screen current can be near zero.

 

[FarisElek]

6V6 is a beam tetrode and the control grid and screen grid are aligned so the screen current is very small. The screen current can be very unpredictable because it depends on how well the grids are aligned. At some operating condition the screen current can be near zero.

Thank you for the info, Heikki. Just to be thorough, we are speaking in regards to Pin 4, correct?

How can I expect the 12k resistor to drop voltage to 248 if the current can vary so drastically?

For me to hit that voltage I would need something closer to 7k.

 

[Heikki]

I have two different versions of Sta-Level schematics. One has just a 12k series resistor for the screen grids and the other one has 12k + 39k voltage divider. I don't know which one you have but it doesn't really matter. If your B+ is around 300V, as long as the screen voltage is somewhere between 200V and 300V I wouldn't worry about it. You are working with tubes, you don't need any precise voltages anywhere.

 

[FarisElek]

Good to know. I guess I got a little obsessed with it being closer to spec. Just worried the sound isn't within ballpark of where it should be. My screen grid one one unit is 192 and one is 195. Every single other voltages is within 5% of where it should be.

If you could help me with one more thing. When you balance the 6386 and the 6v6, Should you be sending a tone through? Or is with no signal fine?


Ryan

 

[Heikki]

Your screen voltages are what I would expect them to be with the 12k + 39k voltage divider, so nothing to worry about there.

Balancing the 6V6 tubes you just probably want set the idle current to be equal without any signal.

The Sta-Level schematics I have don't have a any balance control on the gain reduction stage. I can guess that the balance control is a pot on the 6386 cathodes just like the balance on the 6V6 tubes. There are a few methods how to balance the gain reduction stage. Some compressors have switch that changes the bias on the gain reduction stage, feeds common-mode signal to the grids and then you turn the balance control until you get minimum output. Anyway I wouldn't worry about balancing the gain reduction stage until the unit is otherwise working. I would start by just balancing the plate current at idle first. The original Sta-Level seems to have fairly slow attack time so good balance on the gain reduction stage might not even be that critical.

 

[FarisElek]

Your screen voltages are what I would expect them to be with the 12k + 39k voltage divider, so nothing to worry about there.

Balancing the 6V6 tubes you just probably want set the idle current to be equal without any signal.

The Sta-Level schematics I have don't have a any balance control on the gain reduction stage. I can guess that the balance control is a pot on the 6386 cathodes just like the balance on the 6V6 tubes. There are a few methods how to balance the gain reduction stage. Some compressors have switch that changes the bias on the gain reduction stage, feeds common-mode signal to the grids and then you turn the balance control until you get minimum output. Anyway I wouldn't worry about balancing the gain reduction stage until the unit is otherwise working. I would start by just balancing the plate current at idle first. The original Sta-Level seems to have fairly slow attack time so good balance on the gain reduction stage might not even be that critical.

Thank you for the info! My unit is working good. I just am being a little obsessive about the small details. This unit has many attack and release times so I find heavy distortion when used at a fast attack. I had it set to be balanced on both sides at idle. I turned down the pot with a tone coming through a used a multimeter to turn it down and watch the voltage drop on the output. It's at it's lowest when completely rolled off. Presumably this what you're supposed to do and you're only supposed to turn it when your 6386 isn't quite balanced?


Ryan

Update:
Used a 330ohm 25 watt resistor and mounted it to the inside of the base. Put output transformer CT's on one side and off the choke on the other. Dropped my B+ to 290 while allowing me to get a little more on the screen grid. Still pretty low. But it sounds like it's not terrible so I'm going to leave it there. I think I'm done. I haven't ran it for more then a couple hours straight because it gets really hot. Still want to install a fan in my rack.

Update 2: It appears the 6V6 are running noticeably cooler in this configuration. I can lightly touch the tops of the 6V6 after 30 minutes of operation which i previously burnt myself on accidentally while adjusting the balance.Could be the extra hear dissipation from the mounted resistor? Sound is very good.

Update 3: I'm currently trying to figure out if the eratta info I posted here below is relevant to me who is using a 270HX power transformer. He has the note about the 272 on it, but I figured that this would apply to me too and it was just a measure to keep heat down. However, I'm beginning to think it's the cause of the low current in the screen grid. Asked Gregory but haven't heard back yet. Also, still don't know for sure how to use the 6386 balance. I read the Collins 26-U manual to see how it's is used but it has different controls so I'm not really getting it. Sta Level sounds great though. This is more of the nitty gritty now.

Update 4: Still going strong. Upgraded pretty much all the tubes at this point.  Old 6AL5's caused weird distortion especially at higher speeds. Good 6V6 and 6AL5 are important it seems. Upgraded the 12AT7's and didn't really notice anything.

 

[JMan]

Any recommendations on the SMA cables/connectors?  This build is still wayyyy in the backlog for me, but I was just kind of looking at it for fun this evening, and I realized I’d never worked with SMA cables before.  I went online to check them out, but holy cow, I don’t recognize almost any of the manufacturers and the prices range from a couple bucks for several cables or connectors all the way up to like $30 for just one!  What should I look for when I eventually get around to this?  (Also partially asking because I’ve got a different Drip build up first and Gregory likes to use these connections in his recent projects).

 

[Sc]

Any recommendations on the SMA cables/connectors?  This build is still wayyyy in the backlog for me, but I was just kind of looking at it for fun this evening, and I realized I’d never worked with SMA cables before.  I went online to check them out, but holy cow, I don’t recognize almost any of the manufacturers and the prices range from a couple bucks for several cables or connectors all the way up to like $30 for just one!  What should I look for when I eventually get around to this?  (Also partially asking because I’ve got a different Drip build up first and Gregory likes to use these connections in his recent projects).

The reason why some SMA cables are very expensive is the RF use (transmitting) with rated cables. According to Drip you don't need these and you may use the cheep ones. Personally I think SMA is a good thing for must be shielded signal applications - but why use it for uncritical connections ? Like the IDC's better there. And maybe it's nonsense, but I don't use it for high power (heater) and high voltage connections unless I know the rating of the cables - that's the expensive ones.