Another 5B6

[Phil_S]

Phil, looking at your layout I see a couple of errors mainly centererd around the 6SL7; pin 6 R/C cathode network should not be connected to the ground rail, it should be elevated by the 25 ohm resistor above ground potential. Also, the wire pre the 6.8K NFB resistor is shunting all your FB to ground, that wire should connect between the 6.8K and 25R, with only the tail of the 25R connected to ground.

TM: How right you are! There is an under-board wire from the cathode to the NFB divider. I have fixed that, by removing the R/C ground on pin 6, but it doesn't fix the problem at the PI.

Maybe I didn't do this correctly? I have a wire from the tube side of the 6800 to the 4R tap on the OT. As I said, there is a wire from the divider to the ground side of the R/C for pin 6.

I did a preamp tube change. I still have 100V on the second triode of the 6SL7 and 160V on the first triode, and I'm getting 2V on the grid. I must have another wiring error somewhere. I will have to get the fine tooth comb out. I can't see why both sections are not at the same voltage. B+ feeds a pair of 250K attached to the same turret!

Oddly, I notice that at power up, the 2nd triode starts out north of 300V and slowly but surely declines to 100V. This seems very odd to me. It takes maybe a minute for voltage to drop and stabilize.

I changed V1 also and now have Va=121. I will probably lower that to 180K as you suggested. I was thinking about that as I was building it.

I'll ask again, what can cause DC voltage on the grid at idle? I'm just not seeing it.

BTW, the power section seems to be fine, though I may have to upgrade the cathode bypass cap as it is running very close to 25V.

Thanks.

Phil

[ToneMerc]

Phil, looking at your layout I see a couple of errors mainly centererd around the 6SL7; pin 6 R/C cathode network should not be connected to the ground rail, it should be elevated by the 25 ohm resistor above ground potential. Also, the wire pre the 6.8K NFB resistor is shunting all your FB to ground, that wire should connect between the 6.8K and 25R, with only the tail of the 25R connected to ground.

TM: How right you are! It is oh so clear now that someone else tells me. I can't thank you enough. I have a feeling this will make a serious difference. A few snips and a bit of wire and solder should make this right. I'll post results sooner or later. Standby!
Phil

10-4, not a problem, looking forward to the results.

TM

[Phil_S]

Having slept on this, I'm convinced that either a cap is leaking DC or I have miswired the inverter. The picture shows V1 (6SC7) at the top, and V2 (6SL7).

As ToneMerc pointed out the R/C for pin 6 (counting from the top, 6th pair of turrets) should not be grounded and I have corrected that. There is an under-board wire going to the NFB loop. You can't quite see the NFB loop -- there is a mid board turret at the bottom just above the sandblock R for the power tube cathode and that is where the under board wire goes, with the 25R to ground and the 6800 to the 4z tap. I am reasonably sre that is now done correctly. For good measure, I clipped the bridge on the ground side of the 25R and gave the other R/C for triode #2 its own ground wire to the buss (not that it should matter.)

At the risk of boring you, I'm going to review the wiring of V2. Counting from the top:
- Turrets 7 and 8 on the tube side hold the plate load resistors, fed underneath the board. I am seeing about 300V on the B+ side.
- Turrets 7, 9 (and a mid board), and 10 on the ground buss side hold the 270K - 6800 - 270K combination between the .1 caps.
- There is an under board green wire from the mid-board turret in position 9 that connects to the grid/pin 1 on V2.
- The buss side of the .1 caps on turrets 7 and 10 connect to the power tube grids.

No matter how many times I look at this, I can't figure how this is getting 2VDC on the second grid of the PI and why it follows through to the power tubes. I just don't see a source for it. The whole section between the two .1 caps is isolated from DC supply.

I'm convinced that 2VDC on the grid is causing the drop in plate voltage on the 2nd triode. Please tell me if that reasoning is not correct. I keep thinking both plates should be at around 160V. Triode 1 is at 160 while triode 2 is at 100. (Note there was a tube change and this raised plate #2 from 70 to 100V but had little effect on plate #1.)

Curiously, the cathode voltage on the 1st triode of V2 is at about 2V, but that gets bled off across the R/C at T6 on the board. I probed the pots. I don't find DCV there. So, where is 2VDC coming from?

I'm not quite clear on how to determine if a cap is leaking DC. This would be really disappointing in a new cap, but I've heard it happens. Please tell me how to establish this one way or the other.

For an amp with so few parts, how could it be this much trouble? Go figure!

Thanks for wading through this.

Phil

[rdjones]

Good grief man! That OT is STACKED!

Yikes !
I had to look at that thing 3x before I believed I wasn't "seeing things".

rd

[rdjones]

Jazz says, the OT is STACKED! I don't know what it was. The seller says it was PP 6L6 25W. There is a number on it: FT2386668. I am guessing 25W hi fi = 40W guitar. The turns ratio suggest it is about 4K to 4-8-16z. Solving for turns on a voltage test, I get 3658:4, 3457:8 and 4139:16. There is a 1 ohm tap as well that I capped and I thought it was odd to have a 1 ohm secondary -- maybe too low to test, or what? In any case, right or wrong, I judged it as OK for this circuit without further analysis.

Is the "1 Ohm" tap a separate winding ?
It may be a feedback winding of some sort.

There area couple of reasons I didn't want to cut the hole for a lay down unit. One was that I don't like to cut the hole, though I've got tools and skills to do an adequate job in aluminum. Another is that it helped me control placement. A third reason is that I don't like how much real estate gets consumed on the chassis when you do that. I wanted lots of room inside, which I got in spades.

Cutting the brackets for conversion was not that much work. I got a 3' bar of the aluminum at the home despot for about $3 or so. The one with the wire holes -- had to make it twice because I didn't quite understand about the screw holes for mounting to the chassis and didn't make the bracket wide enough. I actually had fun doing that, if you can believe it.

I've said it before but I do envy you guys with metal (and wood) working skills.

rd

[Phil_S]

Good grief man! That OT is STACKED!

Yikes !
I had to look at that thing 3x before I believed I wasn't "seeing things".
rd

I don't think I fully appreciate the comments. The chassis is 7" wide, so you can be sure of the scale here and the lam stack is 1.75"....What exactly are you saying? Is it that large? Remember, I'm an amateur. I don't always have a good sense of things.

And yes, there is a 4th separate tap which I thought was 1 ohm. You know, it's all a guess. Maybe this was meant to be a 6.6K primary?

[ToneMerc]

Having slept on this, I'm convinced that either a cap is leaking DC or I have miswired the inverter. The picture shows V1 (6SC7) at the top, and V2 (6SL7).

As ToneMerc pointed out the R/C for pin 6 (counting from the top, 6th pair of turrets) should not be grounded and I have corrected that. There is an under-board wire going to the NFB loop. You can't quite see the NFB loop -- there is a mid board turret at the bottom just above the sandblock R for the power tube cathode and that is where the under board wire goes, with the 25R to ground and the 6800 to the 4z tap. I am reasonably sre that is now done correctly. For good measure, I clipped the bridge on the ground side of the 25R and gave the other R/C for triode #2 its own ground wire to the buss (not that it should matter.)

At the risk of boring you, I'm going to review the wiring of V2. Counting from the top:
- Turrets 7 and 8 on the tube side hold the plate load resistors, fed underneath the board. I am seeing about 300V on the B+ side.
- Turrets 7, 9 (and a mid board), and 10 on the ground buss side hold the 270K - 6800 - 270K combination between the .1 caps.
- There is an under board green wire from the mid-board turret in position 9 that connects to the grid/pin 1 on V2.
- The buss side of the .1 caps on turrets 7 and 10 connect to the power tube grids.

No matter how many times I look at this, I can't figure how this is getting 2VDC on the second grid of the PI and why it follows through to the power tubes. I just don't see a source for it. The whole section between the two .1 caps is isolated from DC supply.

I'm convinced that 2VDC on the grid is causing the drop in plate voltage on the 2nd triode. Please tell me if that reasoning is not correct. I keep thinking both plates should be at around 160V. Triode 1 is at 160 while triode 2 is at 100. (Note there was a tube change and this raised plate #2 from 70 to 100V but had little effect on plate #1.)

Curiously, the cathode voltage on the 1st triode of V2 is at about 2V, but that gets bled off across the R/C at T6 on the board. I probed the pots. I don't find DCV there. So, where is 2VDC coming from?

I'm not quite clear on how to determine if a cap is leaking DC. This would be really disappointing in a new cap, but I've heard it happens. Please tell me how to establish this one way or the other.

For an amp with so few parts, how could it be this much trouble? Go figure!

Thanks for wading through this.

Phil

Phil, you need to ground pin 1 of the 6SC7.

TM

[rp]

you need to ground pin 1 of the 6SC7.

especially if it's metal, but it doesn't cause any voltage problems that I've seen. I think he's having trouble with the 6SL7 at the PI. BTW You can also play with the cathode resistor on V1 but as you're using cf you should be on spec.

[Colossal]

Good grief man! That OT is STACKED!

Yikes !
I had to look at that thing 3x before I believed I wasn't "seeing things".
rd

I don't think I fully appreciate the comments. The chassis is 7" wide, so you can be sure of the scale here and the lam stack is 1.75"....What exactly are you saying?

Phil,

I think the context of this comment came from your original chassis photo. The image compression (or however you formatted and posted) caused the image to be very squat, giving the appearance that your OT was about 4" thick.

[Phil_S]

I think the context of this comment came from your original chassis photo. The image compression (or however you formatted and posted) caused the image to be very squat, giving the appearance that your OT was about 4" thick.

ROTFL! I did not realize this happened. Most of the pictures display accurately. I must have slipped in something in the picture setting. In my spare time I'll see if I can fix that. It sure does look like 4"!

Meanwhile, what about the 2V problem on the PI? Is there any wisdom for that?

[rp]

Some of the heavyweights here are going to have to come to the fore, but I while back I got something like that in my 5E3, I was going to post and ask, figured leaky cap as I used black cats, or if it's the nature of the cathodyne, but amp sounds dynamite so I forgot about it. How's it work? How's it sound?

[ToneMerc]

I think the context of this comment came from your original chassis photo. The image compression (or however you formatted and posted) caused the image to be very squat, giving the appearance that your OT was about 4" thick.

ROTFL! I did not realize this happened. Most of the pictures display accurately. I must have slipped in something in the picture setting. In my spare time I'll see if I can fix that. It sure does look like 4"!

Meanwhile, what about the 2V problem on the PI? Is there any wisdom for that?

Phil, until I print out your layout and compare it segment to segment against the Fender layout, it appears to be an error in your layout off the top of my head.

Your PI has a extra 250K that should not be at that location. At your PI between pins 2 & 5 you have a 250K>6.8K>250K point and you do not have a 250K attached to the cathode ground and the power tube grid. Pin #2 of the PI should be decoupled from the 250K at the pwr tube cathode via a .1uf cap. I don't see that in your layout.

TM

[JazzGuitarGimp]

Good grief man! That OT is STACKED!

Yikes !
I had to look at that thing 3x before I believed I wasn't "seeing things".
rd

I don't think I fully appreciate the comments. The chassis is 7" wide, so you can be sure of the scale here and the lam stack is 1.75"....What exactly are you saying? Is it that large? Remember, I'm an amateur. I don't always have a good sense of things.

And yes, there is a 4th separate tap which I thought was 1 ohm. You know, it's all a guess. Maybe this was meant to be a 6.6K primary?

Hi Phil,

My original comment was based on the second-to-the-last picture in the link on your first post. Not sure what's going on in that pic, but the OT stack looks to be about twice as thick as it appears in other pictures in this thread. No offense intended.

http://home.comcast.net/~psymonds/5B6.html

Cheers,
Lou

Also, and this could just be another photo anomaly, but in the last chassis wiring photo, it appears as though one of the pins on V1 is not connected.

[JazzGuitarGimp]

Phil,

Have you tried pulling the PI 6SL7 and measured for DC on the grid of the upper 6L6? I am wondering if maybe the 6SL7 is bad.....

CHeers,
Lou

[Phil_S]

TM: It is a lot to ask you to print and compare and not necessary. I think maybe you are on to something and I can do the compare part. You have done plenty by pointing me in the right direction. I think you probably know how hard it is to check your own work because you see it the way you want to see it, not as it really is.

A quick look says I simply need a ground wire from the other end of the 6.8K to ground, but it is late and I'm tired.

As I said, either a leaky cap or a wiring error. Now I'm thinking the latter and I'll take a good look at it on Sunday. I'll let you know what I find and thanks very much for the pointer.

Lou: Yes, I looked at that picture earlier. It's only funny now that I understand what you were seeing. I wish I had an OT stacked like that! I have no idea why it displays that way. I thought I wrote the code for all the pics to display at 3:4 ratio...what the camera produced. I'll fix it sooner or later.

I did a tube swap on the 6SL7. I don't think that's it, but thanks for the suggestion.

Phil