Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 6v6Pin1 on November 01, 2015, 10:54:32 am
-
I am looking at this image:
(http://el34world.com/Hoffman/images/Dscn0294.jpg)
And this layout:
http://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf (http://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf)
In the layout all filter caps are grounded to a single power transformer lug. In the image, filter caps 2-5 are grounded to the preamp ground bus. In the layout, it looks to me like:
Filter cap A -> power tube plates
Filter cap B -> power tube screens and vibrato plate
Filter cap C -> phase inverter plates
Filter cap D -> preamp and reverb plates
Does it make any difference where the filter caps are grounded? Is there an advantage to grounding the preamp and power amp filter caps at different points? How about the phase inverter filter cap ground?
Also, from the image, three power transformer bolts are used as ground. Do you think of all four power transformer bolts as the same grounding point, or are there reasons why you'd want to use separate bolts?
Thank you!
-
I just a newbie around here but I'm going through similar things with my 1st amp, and I have the strong feeling that separated ground from lugs to the chassis are the recommended thing, other guys that are way more skilled around here will probably say the same and more.
just my 2 cents :smiley:
-
Check out this post first
http://el34world.com/Forum/index.php?topic=19361.0 (http://el34world.com/Forum/index.php?topic=19361.0)
Look at all the documents for the AB763 boards on the library page
The install document has a link to my grounding scheme
http://el34world.com/charts/grounds.htm (http://el34world.com/charts/grounds.htm)
Use the document above as a guide on how to run ground and filter caps
My AB763 build may confuse you and it is not how many other people would run the caps
The bolts on the tranny don't matter, but not many people do it that way
I probably would not do it that way the next time I build
You are missing a bunch of important info by not looking at all the info under each board on the library page
Look at my Princeton reverb build pics here to see how I do most builds these days
http://el34world.com/Hoffman/Princeton_Reverb_Board_Build.htm (http://el34world.com/Hoffman/Princeton_Reverb_Board_Build.htm)
-
I read through the documents. Please confirm my analysis.
The grounding document suggests grounding the preamp filter caps and B+ filter caps at different points.
The installation instructions document says "The filter cap grounds should technically be connected as close as possible to the cathode of the tube that they go to"
Applying that to the AB763:
A is connected to V6, V7. These cathodes are connected to the PT. A should be grounded to the PT.
B is connected to V2, V4, V6, V7. V2, V4 cathodes are connected to the preamp ground. V6, V7 cathodes are connected to the PT. I'm not sure how to choose here.
C is connected to V5. This cathode is connected to the PT. C should be grounded to the PT.
D is connected to V1, V2, V3. These cathodes are connected to the preamp ground. D should be grounded to the preamp ground.
Thank you for your help. I promise I'm not trying to be difficult :)
-
You can get a headache trying to analyze grounding schemes. There are many variations that work very well. No one scheme is the only right one. My grounding schemes are slightly different from Hoffman's. And there will be slight variations in my scheme for different builds. These variations are usually due to differences in overall layout of different amps. But I do try to follow logical (at least to me) guidelines in all my projects.
A is connected to V6, V7. These cathodes are connected to the PT. A should be grounded to the PT.
Correct.
B is connected to V2, V4, V6, V7. V2, V4 cathodes are connected to the preamp ground. V6, V7 cathodes are connected to the PT. I'm not sure how to choose here.
This one is confusing. There are inconsistencies between what you have read and the layout pdf. Cap B should be connected to power ground. Technically speaking, V2 grounds should be split away from the preamp board buss ground and connected to the power ground at the PT. V4 grounds are already separated from the preamp ground buss on the board but are connected to about mid point to the pot buss. Again, technically speaking, this should be connected to the power ground. But what to do with that 47K? It's at the output of V3 and should be connected to the preamp ground.
C is connected to V5. This cathode is connected to the PT. C should be grounded to the PT.
Correct.
D is connected to V1, V2, V3. These cathodes are connected to the preamp ground. D should be grounded to the preamp ground.
Almost correct. Cap D does not connect to V2. Cap D should be grounded to the preamp ground. Ideally, it should also be physically mounted as close to the circuit it feeds, kinda like the attached pic.
I totally understand the confusion and I'm just trying to help you sort it out. But, if you intend to build this amp, I highly recommend you follow Hoffman's board layout EXACTLY. Cap D gets grounded to the preamp ground. Caps A, B, and C get grounded at the PT bolt. It's a proven design that works well.
-
I read through the documents. Please confirm my analysis.
The grounding document suggests grounding the preamp filter caps and B+ filter caps at different points.
The installation instructions document says "The filter cap grounds should technically be connected as close as possible to the cathode of the tube that they go to"
Applying that to the AB763:
A is connected to V6, V7. These cathodes are connected to the PT. A should be grounded to the PT.
B is connected to V2, V4, V6, V7. V2, V4 cathodes are connected to the preamp ground. V6, V7 cathodes are connected to the PT. I'm not sure how to choose here.
C is connected to V5. This cathode is connected to the PT. C should be grounded to the PT.
D is connected to V1, V2, V3. These cathodes are connected to the preamp ground. D should be grounded to the preamp ground.
Thank you for your help. I promise I'm not trying to be difficult :)
It's hard for me to follow all that text
I think you are over thinking the whole thing
This picture on the grounding page link I gave above tells the whole story
It's easy to follow for any amp
(http://el34world.com/charts/images/ChassisGrounds.png)
-
I try to think of my grounding like I were building 2 separate things. A Preamp and a Amp. I have actually put a shield between the sections in the chassis with one hole and grommet. Usually I will ground preamp to a dedicated location, usually a buss terminating at an input jack. On the PA end I use a single transformer bolt.
I have used Doug's scheme on a 5F6a out of curiosity and the amp is nice and quiet. No it is not hard to change a pot either if you have wire cutters.
High gain amps are a different story, but similar idea. I just use more shielded cable for grids and loop insertion like a reverb or even a Tube Loop.