Custom Amp Build - 5F8-A

[jly56]

Hi Everyone...


I am working on a 4 tube 5F8-A type build and I have a few questions regarding the chassis layout.  The chassis is a Hammond 10x17x3 and attached is the layout that I am considering.


1.  Will the 6.3v tube heater wires cause any problems/interfrence with the audio controls/pots?  In past 5F6-A builds, I put the heater wires into the corner of the chassis to cut down on interference.  The other alternative would to put the wires above the sockets, like I've seen in some JTM45 builds.


The plan is to have the tubes as far as possible from the PT/Choke/OT, which is why the the tubes are at the front and the iron is at the back... with my 'wall-o-cap cans' in the middle.  I can easily switch the tubes and cap cans, but then the tubes would be too close to the iron!?!


2.  Since I have room between the choke, rectifier, and PT... I have seen some custom builds where the choke was 45 degreesfrom the PT instead of the 'normal' 90 degrees that use.  Any advantage to doing this.


Any other suggestions or comments?


Thanks - Jeff

[thermion]

In such proximity to the kt66s, those caps are gonna get pretty hot.
Controls and pots will pick up stray hum from adjacent heater wires if they are run with AC.

[DummyLoad]

you'll need greater spacing between the can capacitors for the mounting clamps to clear one another. you may wish to add a heat shield between the output tubes and the capacitor row.


why so many filter caps? you have tube rectifier.. ;-) fender plan uses 40uF (20+20) for B+, choke & 20uF to screens & PI and 8uF to preamp. you have an awful lot of 100u and 47u @ 385V. if you're going to use 400V B+ as fender plan did, then 500V F&T or JJ cans will work.


suggestion for tube recto and keep the cans:
 
                                             OT                                     Screen + PI           Preamp                 FX loop
                                               |                                             |                         |                          |
PT --->5U4GB --->  50uF/500V | ---> choke + 50uF/500V ---> | ---> 16uF/500V | --->  16uF/500V | 




--pete

[jly56]

In such proximity to the kt66s, those caps are gonna get pretty hot.
Controls and pots will pick up stray hum from adjacent heater wires if they are run with AC.

Hi Thermion. 


The caps are 1" (25mm) from the KT66s... I can move them back farther to the iron, but that still may not be enough.


Yes, the plan was AC for the heaters... and the stray hum is what Im worried about.


thanks

[Cliff Schecht]

The bodies of the pot will act like a shield if hooked up to a solid ground. Running a buss bar along the pots or preamp board tied to the pots automagically takes care of this. If you twist up the heaters decently tight and tuck them into the back of the chassis or fly them well over everything, they usually don't cause too bad of hum problems, even in tight spaces. I prefer tucking them into the back corner of the amp (where the backside meets the top where the tubes are mounted).

[jly56]

you'll need greater spacing between the can capacitors for the mounting clamps to clear one another. you may wish to add a heat shield between the output tubes and the capacitor row.


why so many filter caps? you have tube rectifier.. ;-) fender plan uses 40uF (20+20) for B+, choke & 20uF to screens & PI and 8uF to preamp. you have an awful lot of 100u and 47u @ 385V. if you're going to use 400V B+ as fender plan did, then 500V F&T or JJ cans will work.


suggestion for tube recto and keep the cans:
 
                                             OT                                     Screen + PI           Preamp                 FX loop
                                               |                                             |                         |                          |
PT --->5U4GB --->  50uF/500V | ---> choke + 50uF/500V ---> | ---> 16uF/500V | --->  16uF/500V | 




--pete

Hi Pete..


The cap cans are JJ snap-in caps on a 2"x3" pcb, so there aren't any clamps needed.  As far as the quantity, the cans are single caps and they hooked in series.  I had a bunch of JJ snap-in caps that I wanted to use up. So the power stages are 50uF/770V, 23.5Uf/770V , 23.5Uf/770V, 23.5/770V.... yes, a little overkill on the voltage, but the uF is right where I want it.


Attached is a pic of a single cap series mounted on the PMillett pcb compete with 220k bleeders.


Thanks for your comments

[jly56]

The bodies of the pot will act like a shield if hooked up to a solid ground. Running a buss bar along the pots or preamp board tied to the pots automagically takes care of this. If you twist up the heaters decently tight and tuck them into the back of the chassis or fly them well over everything, they usually don't cause too bad of hum problems, even in tight spaces. I prefer tucking them into the back corner of the amp (where the backside meets the top where the tubes are mounted).

Hi CLiff


While I normally don't do a pot buss bar, this build is using a Hoffman board layout and he recommends using a pot buss bar to ground... Might be a good time to try it.  On my 5F6-As in the standard (little) chassis, I always tuck the wires into the corner, but my concern was with the pots also being on that same side (as opposed to just some output jacks and an output selector switch, like a 5F6-A)..... but I've seen other builders who swear by the fly over method.  If I keep this design, I might have to decide on the method, when I get there.


Thanks
Jeff

[EL34]

IMO, bad grounding schemes may have more of an effect on hum than the heater wires


Twist them up nicely and don't run them close and parallel to signal wires

[2deaf]

Ground the center tap on the 6.3V winding.  This will make your heater wires 3.15V with respect to ground.

[jly56]

IMO, bad grounding schemes may have more of an effect on hum than the heater wires


Twist them up nicely and don't run them close and parallel to signal wires

Thanks Doug...  The plan is to use your grounding methodology (along with your 5f6-a board layout (with 4 power valves)) for this build.  I'm just trying to find the best layout for the stuff on top of the chassis.  Easier to do it on paper first, but I have to keep all of the inside wiring in mind too!


Jeff

[jly56]

Ground the center tap on the 6.3V winding.  This will make your heater wires 3.15V with respect to ground.

Hi 2deaf...


While the PT has a center tap for the 6.3V, I was going to use Doug's method of grounding it to a pair of 100 ohm resistors on the board instead.  If there is a short or (god forbid) mis-wire, it will just melt the resistors and not damage the PT.


Jeff

[jly56]

Here's another question/comment...


DrGonzoM and DummyLoad showed using a 5U4G as the rectifier.  The goal of this amp (we'll see if I achieve it ) is to be a (moderately powered) bass amp.  I was looking at using either a 5AR4 or a SS rectifier because I really don't want sag!   I'm looking for headroom!  That's why I was contemplating using KT66's instead of 6L6's.  The PT has the needed 6.3V for the higher heater draw of the KT66s.


What does this group think of this?  Am I on the right track?  From my research, it seems like the KT had 'better' bass response than the 6l6 or EL's.... I think I could even go to KT88 if I plan for it now.... Maybe?


Thanks to everybody who has replied to this thread!  Your knowledge is appreciated!

[DummyLoad]

then go SSR and size filter around 1uF/mA at idle state.


another option is to use 2 x 5AR4 in parallel, however, the PT recto winding would need to be rated @ 4A.


--pete

[terminalgs]

the four novals in the corner will be difficult to wire,  and your placement of a circuit board will be less than optimal.  you'll need to locate the circuit board  either above all the electrolytics  (which make wiring them difficult), or all the way at the back under the transformers.

putting the board under the transformers is handy,  but for you,, that would mean lots of long runs back and forth to the tubes. 

for those reasons, I'd suggest a redesign.

first: the cooling fan:

The cooling fan is taking up valuable space.  Not only that, cooling is about airflow through the cabinet,  so it should be mounted to the side of the cabinet (inside is fine),  and blow air out, preferably from the hottest area of the cabinet, near the KT66's.   If you want to cool a room in a house, you don't open a window and put the fan in the middle of the room on the floor (basically what you have in this layout),  you put the fan in the window, and open a window on the other side of the house to draw air through.


one idea:

move the choke to the fan's previous location, then move  Stage-1 and Stage-2 electrolytics to where the choke used to be,,  and arrange the KT66 in a 2x2  instead of a 1x4 arrangement.  now straighten out the string of novals along the front.  from left to right:  PI, FX, Pre2, Pre1.   now,, your board  can run down the middle from the right side to about where the KT66's start.

here's another:


the big box with a "x" in the middle is where you can arrange the lytics  and the choke.   Do you need two rectifier's?  to drive the amp draw of 4 KT66's? if you do, there is two here.  If not, put a lytic or the choke there.    solid-state is probably the way to go with a 4xKT66 amp.

[jly56]

then go SSR and size filter around 1uF/mA at idle state.


another option is to use 2 x 5AR4 in parallel, however, the PT recto winding would need to be rated @ 4A.


--pete

The Hammond 373ex has 6A on the 5v and 9A on the 6.3v windings, should be good for two 5AR4s, 4 KT66s, and 4 12A?7s.  I should be able to heat a room in my house from all of the tubes!
 


Jeff

[jly56]

the four novals in the corner will be difficult to wire,  and your placement of a circuit board will be less than optimal.  you'll need to locate the circuit board  either above all the electrolytics  (which make wiring them difficult), or all the way at the back under the transformers.


The 'plan' is to put the board under the caps.  The caps mount on top of the case and only need access holes for the + and the -.  The different stages should line up pretty close to the power entry points on the Hoffman layout.

putting the board under the transformers is handy,  but for you,, that would mean lots of long runs back and forth to the tubes. 


Exactly why I wasn't planning on mounting it there

for those reasons, I'd suggest a redesign.

first: the cooling fan:

The cooling fan is taking up valuable space.  Not only that, cooling is about airflow through the cabinet,  so it should be mounted to the side of the cabinet (inside is fine),  and blow air out, preferably from the hottest area of the cabinet, near the KT66's.   If you want to cool a room in a house, you don't open a window and put the fan in the middle of the room on the floor (basically what you have in this layout),  you put the fan in the window, and open a window on the other side of the house to draw air through.

There will not be a cabinet.  The plan is to mount a 'cage' over the top of the case to protect the components mounted on top of the case.  The fan was added in just an attempt to keep the temps down and can easily be moved or removed from the final design.
one idea:

move the choke to the fan's previous location, then move  Stage-1 and Stage-2 electrolytics to where the choke used to be,,  and arrange the KT66 in a 2x2  instead of a 1x4 arrangement.  now straighten out the string of novals along the front.  from left to right:  PI, FX, Pre2, Pre1.   now,, your board  can run down the middle from the right side to about where the KT66's start.


Interesting... where would stage -3 and stage-4 caps go?..... let me fire up DIY LC and see what it looks like.

here's another:


the big box with a "x" in the middle is where you can arrange the lytics  and the choke.   Do you need two rectifier's?  to drive the amp draw of 4 KT66's? if you do, there is two here.  If not, put a lytic or the choke there.    solid-state is probably the way to go with a 4xKT66 amp.

The reason I didn't consider a layout like this was  due to my desire to keep the preamp and power tubes away from the PT!  Would this not cause interference with the KT66s being between the PT and the OT.. also consider the amount of heat that will be generated between the PT and OT..

Update:  There won't be enough room between the PT and the OT to put the KT66s in this configuration.  The spec sheets for the tubes recommend 3.5" between the center of the tube to another... about 1.5" between the 2" tubes.  This would leave very little room (<1") from the outside edge of the tubes to the PT and the OT..... too close for my comfort.

Thanks for your comments and suggestions!!!

[DummyLoad]

recto holes become can cap holes with SSR.    


--pete


EDIT: all parts are to scale: 1" = 1" D size sheet. 

[terminalgs]

The reason I didn't consider a layout like this was  due to my desire to keep the preamp and power tubes away from the PT!  Would this not cause interference with the KT66s being between the PT and the OT.. also consider the amount of heat that will be generated between the PT and OT..

Update:  There won't be enough room between the PT and the OT to put the KT66s in this configuration.  The spec sheets for the tubes recommend 3.5" between the center of the tube to another... about 1.5" between the 2" tubes.  This would leave very little room (<1") from the outside edge of the tubes to the PT and the OT..... too close for my comfort.

what if you rotate both your PT and OT 90deg.?  in your original diagram, it looks they they take up more east-west space than north-south.  if you rotate them 90deg, it seems like there might  be enough room for the 4 kt66's in between.



regarding the fan:  if your cabinet is completely open, then a fan isn't going to do much (or anything).


whatever you do, try to lay out all the novals in a straight line.  you'll have metal cans on top of them for shielding, so what you might give up with one of them being sorta close to the PT, you'll gain in ease of board-to-tube layout, shorter wire runs, and ease of heater wire routing.  I've tried similar tube layouts with twin triodes jammed in a corner in an "L" fashion.  my experience: it sucks.

BTW: I don't think there is anything to worry about with Preamp tube proximity to OT.  At least not if its at least 3", and you have shielding cans on top of them...

regarding rectifier: its not like you are hoping for an amp with sag with a 2x5U4 configuration.  go the SS route, save $50-70,, and have the option of putting as much reservoir cap after the rectifier as you can stand.  plus,, buy yourself a lot of chassis real estate.

[DummyLoad]

terminalgs plan with some personal spins. 


--pete


EDIT: fixed board right-side holes.

[eleventeen]

Thing of beauty, DL. That layout gets my vote.

[DummyLoad]

thank you, eleventeen.


--pete

[jazbo8]

Hard to beat - HiWatt stylee...

[jly56]

terminalgs plan with some personal spins. 


--pete


EDIT: fixed board right-side holes.

Wow DummyLoad... Looks great.... 

I made some modifications....  the Effects Loop (London Powers') is on a small PCB, so I moved it to the back, where it would have better clearance.  I also added an octal socket for the SS rectifier... that way I can play with using a 5AR4 or copper cap style SSR.  I also added my cap cans, but since they are top mounted, they won't interfere with the turret board... which is 12x3 in size.

One thing I like about your design is the fact that the tubes and pots are on the correct sides of the turret board.  My designs required that I run the pot wires under the turret board.

The first attachment are the mods made to your and TerminalGS' design. 

The second attachment is my latest attempt.  I moved the LTPI to the middle, where it would easier to wire... and also moved the preamps to the correct side of the board.  I will still need to run the  pot wires under the turret board and the tube wires on the top.

[jazbo8]

I still prefer Dummyload's version (which is also closer to the HiWatt)... in your revised layout, there are many "crossed paths", e.g., the ones between the power tubes and the OPT, and the wires from the front panel to the turret board. Just seems like that there are perhaps quite a few high current wires runing next to the low current ones - why take that chance?

[terminalgs]

idea: inside the chassis, underneath the OT, if you have unused space, put the FX-PCB and tube there, on its side.   You'll need to pull the chassis to service the tube, but how often will that tube get replaced?


There is no need to put the PI noval between  the pairs of kt66's.   it will ease your heater wiring tasks.  probably ease your kt66 and PI tube wiring layout as well.


having the OT nearer to the kt66s will make your life a lot easier.


everything about the hiwatt layout is excellent.  including the aluminum heat shield (lowes/home depot sheet metal department, $5-10).

[sluckey]

I think you're begging for trouble if you mount those power tubes near the same side of the chassis that you will be mounting pots and input jacks. There will be a lot of sensitive, low level signals wires from the board and/or preamp tubes crossing over high level signals of the output tubes. At least study the Hiwatt layout and try to understand why it is a superior layout to your proposal.

And if you are not locked into that particular chassis, consider abandoning this layout altogether and go for a more conventional Fender layout.

[jly56]

idea: inside the chassis, underneath the OT, if you have unused space, put the FX-PCB and tube there, on its side.   You'll need to pull the chassis to service the tube, but how often will that tube get replaced?

Yes...  I'm coming to the conclusion that the FX-pcb is being a real PITA trying to locate it on top of the chassis!  There is lots of room underneath and this, along with your design(and DummyLoad) is probably the better way to go.  And remember, there isn't a cabinet, so servicing the FX is just a matter of flipping the chassis over.


Thanks for all of your assistance with this!

[jly56]

I think you're begging for trouble if you mount those power tubes near the same side of the chassis that you will be mounting pots and input jacks. There will be a lot of sensitive, low level signals wires from the board and/or preamp tubes crossing over high level signals of the output tubes. At least study the Hiwatt layout and try to understand why it is a superior layout to your proposal.

And if you are not locked into that particular chassis, consider abandoning this layout altogether and go for a more conventional Fender layout.

Yes... I agree with your (and others) concern with the tubes and pots on the same side.  I've been trying to combine aesthetics with function... But function must be the driving factor.


I'm not 'locked' into a particular chassis, I just wanted to design a chassis where (most) everything is on top and visually pleasing.  I will be using the Hammond chassis cage over the top and maybe some nice wood sides attached to the chassis.  Not mounted inside a head cabinet.....  This is why I was going this route and not the standard 5F8-A chassis and layout.


Thanks for your input!  I have learned that it is relatively easy to follow a 'tried and true' chassis/board layout to build an amp, but doing a custom design has LOTS of other concepts that need to be considered.....  Then you have to worry about how it's going to sound!


Again, thanks to you and all of the other posters!   I have a bunch of other choirs to do today, but want to come up with a 'final' chassis this evening.   I will certainly post it here for all to comment on.

[jly56]

terminalgs plan with some personal spins. 


--pete


EDIT: fixed board right-side holes.

DummyLoad ... What software are you using to create these chassis layouts?  It looks a lot more powerful than DIY LC, which is what I am using.


Thx

[DummyLoad]

visio 2010 professional.

--pete

[eleventeen]

My opinion is that you will find that the more "square" a chassis is, the more design problems you are likely to run into. Is it because guitar amp chassis, whether for head or combo, are traditionally so elongated? I don't think so; it's not just "tradition".


Yes, certainly you can go look at dozens of 50's hi-fi amps and they seem square (by "square", I obviously don't mean square with sides:length = 1:1, but let's say the long way is no MORE than 130% of the short way, whereas your typical guitar amp might be 3:1 or 2.5:1) And a Dyna Mk 3 is an obvious and very noteworthy counterexample to what I am saying. I have an older Pilot SA-232 stereo quad EL84 amp that is long and thin. It wasn't necessarily a design objective to make electronic stuff small back then! A bigger amp must have weighed more and was worth more and so you could charge more for it, right? 


Why is this? Because most of those amps took in a 1 volt post-preamp signal and had no tone controls that required back-and-forth wire runs to tubes. With the exception of only a few, like LEAK and McIntosh, they generally used point-to-point wiring (but again, those amps had no tone controls, implying they saw a 1 volt post-preamp input) which maybe isn't as impressive-looking and perhaps not as rugged as turret-board type construction but the leads were shorter, and that matters. Obviously, such amps weren't made to go "on the road"; they were supposed to live on a bookshelf or in a cabinet their whole lives. A guitar amp typically has to start with 1/4 volt and has to run that sized signal back and forth to tone controls. Often, there is a[n extra] gain stage to make up for the losses of RC tone controls. This is where you stand to pick up hum, and frankly, that's the nuts and bolts of it. It is not so much the post-preamp portions of the circuit: it is the tone controls and how you have to wire them and what you have to run their wires next to.


The big smelly problem with experimental, non-proven layouts, is that you may well find out you have a bad hum monster, but you only find this out after you have hacked away all that metal and mounted all your parts. It's "impossible" to go back and change it and you definitely will not want to. So this means that you're stuck investing all this metalwork into the chassis, only to find out you *might* have a problem that resists a cure with all its irritating might. 


I believe anyone making suggestions here is only trying to save you that particular form of grief. That's really all there is to it, as far as I'm concerned.

[jly56]

.
.
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I believe anyone making suggestions here is only trying to save you that particular form of grief. That's really all there is to it, as far as I'm concerned.

I'm sorry... If anybody has taken offense to my posts, this was certainly not my intentions.   I am a new member here, and I appreciate ALL suggestions and comments regarding this personal undertaking.


Thanks..

[jly56]

Good evening all...

Well, I think that I have come up with the design for my amp chassis..... actually many of YOU have helped me come up with the design and I thank you all for that!

The first attachment is more of a drilling template, showing where everything will go.  I have put in two rectifier sockets, in case I decide to play with 1 or 2 tubes or a SSR... but the option is there.

The second attachment is the same thing, but with the turret board and BFX pcb locations, which will both be mounted inside the chassis and not the top. 


Any comments or questions are always welcome.


Thanks

[eleventeen]

You took my post the wrong way---your questions are perfectly welcome, and we all learn things even from people asking questions we think we know the answers to.


I only meant to say that if you decide upon a layout which causes you problems we, meaning some of us with more experience, have gone through, it's kind of a lousy situation because you have so much work invested in the thing at that point: Metalwork, parts, costs of chassis, troubleshooting, etc; etc; For most us who do not have magnificent top-grade metalworking tools, the metalwork is potentially a lot of kind of tedious work.


So if you get to the end and WOW! It works! But it has troublesome hum problems (due to the layout) then you won't want to use the amp...and even if you rip it all apart and still have your trannies and tubes and parts board and pots, etc; there is still all the work you did and the chassis is presumably toast by that point.


That's all I meant to say. It's a common result of having a strange layout and we're trying to help you avoid that condition.

[jly56]

You took my post the wrong way--

No, I understood your post perfectly... No need to restate it.  You didn't state anything I do not already know.


Thanks

[terminalgs]

Any comments or questions are always welcome.

I like it!


the FX board is sort of an awkward thing to locate as-is.  you might play with it, but since it is a self-contained board,, it can probably go anywhere. 


another note,, with your KT66's, you've got two pair of parallel kt66's.  I would orient them as the top two are one pair, and the bottom two are the second pair.


I look forward to seeing how it progresses!

[jly56]

Any comments or questions are always welcome.

I like it!


the FX board is sort of an awkward thing to locate as-is.  you might play with it, but since it is a self-contained board,, it can probably go anywhere. 


another note,, with your KT66's, you've got two pair of parallel kt66's.  I would orient them as the top two are one pair, and the bottom two are the second pair.


I look forward to seeing how it progresses!

Thanks!   Now it's time to do some drilling!