Layout thoughts, KT88 SE build

[Diverted]

I just tried to post a followup to the KT88 single ended build, with a few photos. Something went wrong with the website and now I can't see the thread. Anyway, I've spent a few days trying to work out a layout for Dummyload's KT88 reverb schematic that isn't too convoluted. Does anyone see anything alarming with respect to my layout and grounding scheme?
Before I lock myself in to a particular layout I would love to have a few extra sets of eyes scope it out.  Only major difference from Dummyload's original schematic is the addition of a tone pot. Thank you!

[DummyLoad]

1) the 100R under the V3 recover/driver 3.3KΩ/4.7uF is missing - should connect to the presence pot.
2 i assume for item 1) that the 100Ω located next to the 390Ω/47uF for the KT88, which is incorrect. there IS a 1Ω/1% R under the 390Ω/47uF that's correct.
3) the 390Ω/47uF KT88 bias R and bypass cap are connected to the presence pot - this is incorrect.
4) you are missing 2 diodes - you really need those two diodes unless you are using 1.5K PIV 1A diodes.
5) the presence controls is wired incorrectly.


there may be more errata,  i stopped there. will check again after you update. 


respectfully, 


--pete

[DummyLoad]

looks like the mods deleted your original thread.


--pete

[Diverted]

Thanks. I’m using 1N5408 diodes which are beefy ones @100v/3A.

Thank you for the notes. Im going to get in there today and tweak it.

[Diverted]

Here's a revised version. Thanks, I see what I did.
What's the purpose of the 1 ohm resistor off the KT88 cathode? Easy bias checking like what you would see with a fixed bias amp?

[DummyLoad]

Here's a revised version. Thanks, I see what I did.
What's the purpose of the 1 ohm resistor off the KT88 cathode? Easy bias checking like what you would see with a fixed bias amp?

yes. bias checking - with SE usually running so close to maximum plate dissipation, it's nice to know what it actually is, since a variance in tube characteristics between tube mfg. could push one brand type over the recommended limits.


--pete

[Diverted]

Thanks.

Overall do you see any issues with the way I laid it out so far? Getting all the preamp/reverb grounds to one point was one of my questions. Would you do your ground scheme differently? I ran most of the cathodes to the ground buss along the sockets to try to keep the board uncluttered and wiring around it to a minimum, with as much clearance as I could make work.

I am looking at the chassis and trying to lay out the components in a logical way. I'm thinking about swapping out the can caps for axials inside. That'll save room up top to give the transformers some clearance.

[DummyLoad]

the presence pot is wired reverse. pins 1 & 2 are jumpered and connect to the 100Ω - pin 3 connects to the 1.2KΩ resistor. 

2 of the 4 power supply diodes are still missing - 1N5408 are overkill - you really want the 2K PIV with 450V B+.

the colored dots on the layout don't correlate colored dots on your marked up schema. on the layout, the red dot goes where the blue dot is, the blue dot goes where the green dot is, the green dot goes where the yellow dot is.  the yellow dot goes to pin 1 of the volume pot.

some of the signal leads are long with the turret layout, but that's life. otherwise it appears to be wired correctly. 

--pete

[Diverted]

Thanks. I’m disregarding the color codes..that version of the schematic has the tone control on it, I just used that one as it shows it but I knew those codes were off. Sorry, will replace it.
I’ll double up those diodes and reverse the pot.


As for the signal leads I’ll move the pots sockets and board around to keep lead lengths at a minimum. I’m using shielded wire on V1A and B grids. Recommend it anywhere else?
Thanks!

[Diverted]

How does transformer orientation look? I'm using a Fender Princeton Reverb-sized cabinet which is 18.25x7.25x2.
Tried two different ways. If I go with the filter caps to the left of the board version I will probably put the reverb transformer inside the chassis, between V2 and V3. There would be plenty of room between them for it.

Any one favor one over the other?

[sluckey]

Any one favor one over the other?

Same pics. Put some tube sockets on the chassis too.

[Diverted]

How about this?

[DummyLoad]

keep the reverb driver OT as far away as possible from any other inductor. laminations should be perpendicular to the closest inductor's laminations. if the winding window will point into a tube, the rotate 45 deg. off axis - hint look at the placement on a super/twin reverb, however IMO that's still too close to a tube.

keep the laminations of the output transformer and power transformer perpendicular. the choke laminations should be perpendicular to the power transformer laminations.

keep the choke as close as possible to the main filters. at least close enough so that you can install it without extending the leads.

--pete

[Diverted]

Thanks Pete. How about this? Spaced out V2 and V3 to fit reverb ot to fit between them. There is also enough room for this ot under the chassis, between the sockets. Would that effectively shield it from closest transformer (output)?

Thanks!

[DummyLoad]

reverb transformer under the chassis can still interact with circuitry. suggest rotation of the reverb transformer 45deg on it's Z axis.


--pete

[Diverted]

Got it. How's this?
The output transformer is a little farther away from the power section to give some breathing room to the reverb ot. Will I have any issues running the primary wires all the way across the chassis? Should I stick them in hollow woven shield and ground it along the chassis somewhere?

[sluckey]

I've had good results with layouts based on this pic. I like to keep the OT close to the power tubes and far from the preamp tubes. Most of my builds have been based on this layout.

[Diverted]

Thanks Sluckey.
How's this? I had to bump the filter caps in a bit to leave room for the power cord. Other than that, looks pretty much like yours.

[Diverted]

Here's an updated layout based on Sluckey's input. Don't pay any attention to the fuse over at far left. I forgot to put it in place when I exported the layout. It's going to go on the front panel now instead of bottom due to moving the cap cans. Thanks!

[sluckey]

I could work with that. Put all the tubes and caps in a straight line. I would not drill any holes until I had a working board layout to put in the mix. The board, little tubes, and front panel all need to work together. Treat them as one unit. And don't forget, you may have to shuffle stuff slightly to accommodate mounting screw holes.

I like to put wide painters tape on the chassis. Easy to mark up and keeps chassis scratches to a minimum.

[Diverted]

Thanks. I’m going to build a board, figure out mounting and go from there on the sockets etc.
thanks!

[EL34]

I just tried to post a followup to the KT88 single ended build, with a few photos. Something went wrong with the website and now I can't see the thread. Anyway, I've spent a few days trying to work out a layout for Dummyload's KT88 reverb schematic that isn't too convoluted. Does anyone see anything alarming with respect to my layout and grounding scheme?
Before I lock myself in to a particular layout I would love to have a few extra sets of eyes scope it out.  Only major difference from Dummyload's original schematic is the addition of a tone pot. Thank you!

The original post got corrupted and could not be displayed
I was able to pull the post from the database and save them in a csv format

All the replies are in the .zip file I attached to this post

[Diverted]

Thank you for doing that!

[jojokeo]

*2 corrections noted on your layout by arrows. Also, I highly suggest you don't just lay your iron anywhere without testing/using a listening circuit prior. Insulate ALL of your output wires then temporarily plug in your power tranny where you want it. Then take any headphones you have and connect to your output leads on your OT. Make the two hot leads into a mono plug to connect your headphones or you'll only hear in one of your ears. Then move the OT around on the chassis, turn it 90 degrees, etc... you'll actually hear noise get louder and quieter as you move it closer and farther from the PT. Install it where it's quietest and in the position that it's quietest. The PT puts out the most interference the direction of the laminations and gets worse the more power it provides therefore I'd turn it 90 degrees to face/run parallel to the front and back of the chassis - going by your layout - picture, not the one printed and corrected. Then move the output tube a little to the left away from this area. Typically I put the OT all the way across the other side of the chassis and install the preamp tubes more inwardly without any issues. Mounting them all of the way to the backside of the chassis prevents this. To each his own...(but many modern amps are made this way).

I'll defer to Pete for anything else.


jojo

[Diverted]

Thanks JoJo.
Your message is appreciated, but it kind of intimidates and very much confuses me. I would have no idea how to set up the amp wiring, with everything in place on the chassis 100 percent completed, while having the ability to rotate transformers to see how things sound.
I'm also a little confused too after reading your suggestion that I rotate my power transformer 90 degrees from what was shown in the photo I took and the drawing Sluckey posted. It seems to me that if I rotated that, I would also need to rotate every other one after it but I could be wrong.
After reading your comment I'm kind of at a standstill and don't really know which way to go, or which configuration is better.

[Diverted]

I came up with a variation on the layout because it was too tight along the line the tubes are on with both large filter caps there. So I deleted the preamp filter cap can and put individual 50/500 caps in what I think are appropriate places along the circuit board.
With this there's more wiggle room along the first cap can/kt88socket/power cord area.
How's it look?

[DummyLoad]

jojo, he had those markups of yours correct as according to my plans. please see attached,


respectfully,


--pete

[Diverted]

Hi,

Quick question about the layout: There is a 220K resistor shown in schematic from B+ to ground. I'm assuming this is to quickly bleed off B+ when the amp is powered down?

Thanks. Waiting on parts now, fine tuning layout and am excited to get started over the next week or so.

[sluckey]

Quick question about the layout: There is a 220K resistor shown in schematic from B+ to ground. I'm assuming this is to quickly bleed off B+ when the amp is powered down?

yes

[Diverted]

Final version of layout. Will build board, then home in on exact placement of all elements. Thanks to everyone for the help so far.

[Diverted]

Finally got pretty much all the parts assembled and started drilling the chassis, installing components and building the board this morning. I'm pretty happy with how much I was able to get done working off a template.
Will install and wire up the board and rest of the circuit tomorrow, then trace it out a few more times to make sure all is OK.

[Diverted]

Finished up the wiring today apart from a few odds and ends. I'm going to trace it out another two times or so, go over it all and bring it up some time tomorrow. Thanks for the help so far.

[Diverted]

Gingerly powered it up for the first time about an hour ago. Massive squeal, reversed OT primary wires, all good. First impressions? Holy cow the amp has presence. It's mean, overdriven and beautiful. Only issue I'm having so far is NO REVERB. I'm using an 8AB3C1B tank which I think(?) should be a usable swap for the 9ab2c1B dummyload suggested.
Here are my voltages. Power transformer (rated at 320-0-320) is putting out voltages maybe 10 percent lower than what I was expected. But it sounds great so right now I don't care:

B+ 400
B+1 389
B+2 366
B+3 330

V1A
P 225
K 1.5

V1B
P 222
K 1.5

V2
P 316
K 14
G 316

V3A
P 224
K 1.5

V3B
P 225
K 1.9

V4
P 384
SG 362
K 32.2v (390 ohm resistor)
MA (reading taken at junction of V4 RC network and 1 ohm to ground: 81.4ma



I traced out the reverb circuit a few times and all looks good. When I tap the tank I can hear the reverb, and I can also hear it when I turn it way way up ... but very, very slightly.


All in all, I'm happy so far. If I could root the issue out I'd be very happy!

[shooter]

It's mean, overdriven and beautiful.

welcome to the SE'88 side 
take your time, you'll find the gremlin

[Diverted]

Well that was easy. Found it. Tank wires were reversed. I tried swapping them earlier and it didn't work in either position. But there must have been a bad connection or something. In any case all is well and the reverb is fantastic with dwell control. Getting a few microphonics in the amp ... sensitive to touch but I haven't tried swapping out different preamp tubes yet to see if one of them is ringing. I'm pretty sure all my solder joints and connections are solid. Hopefully it isn't the brand new Genalex KT88 in there !

Many thanks to all who chimed in and helped. What a beast this thing is.

[DummyLoad]

it is a fairly high gain ckt. you are going to have some microphonics with any tube in this ckt. back off a bit on the throttle.    use the presence control to get the most gain possible without inducing oscillation.

i use JJ 12AX7 in mine, i have also used NOS RCA short plate 12AX7 for even better tone. i tried some older sovtek 12AX7LPS and they sounded good, but to me the J&J sound better in this amp.

my tube line-up is: J&J for V1 and V3, sovtek for V2 and V4. unfortunately, sovtek ceased production of the KT88 some time ago. mine are dated 1999. i have not tried with 6550, i have a couple of sovtek laying around here somewhere, and plan to test with one of those later.


--pete

[Diverted]

I’m using NOS Amperex 12ax7as, a Telefunken 6bq5 that I found somewhere and is strong, and the Russian KT.

The presence control is fabulous! Great great feature on this amp.

Many thanks for your guidance. Will post a vid this weekend some time.

[shooter]

brand new Genalex KT88 in there !

once you let 'em outta the box and feed them, they get cranky for the 1st 10 hours or so, keep feeding, they find a happy place