MOD3 build

[shooter]

I pulled the trigger on my next build, parts on order.
I want the ability to add a 2nd KT88, but will probably dial in as a single.

I chose this PT;

XPWR105  360vac (180-0-180) @ 250mA (would use without CT)

Doing the 1.414 I get 509vac, I’m expecting ~~20 - 30v drop loaded, Good Guess???
Looking to land around 460vdc at plate:  Is there a better way than “kinda what I got last build” to determine loaded unloaded voltage?

With 5k OT, 460 plate, expects, wants ~ 92mA
From that I get;

400ohm Rk * 92mA  = 37vdc Vk
460Vp - 37Vk =  423vdc *.092A = 38.9W

It’s about 100 volts higher than BB testing, but math still shows ~ same Pdis

Design questions;
I mis-wired the pots on the BB, reflected in the schematic below, is there reasoning/logic that “wants” the Vol/gain pot AFTER the 1knob TS?
I kinda want the 100k/330k “fixed” for internal gain adjusting, but all ideas welcome


thanks

[kagliostro]

Ciao Shooter

The way you arranged the TS + Vol pot seems odd to me (may be I'm wrong, but ....)

Doing the 1.414 I get 509vac, I’m expecting ~~20 - 30v drop loaded, Good Guess???

Try x 1.35

Franco

[shooter]

Thanks K, I borrowed Sluckeys "re-draw" of the '74 Marshall 18W TS/Vol.  Added to schematic below.

Try x 1.35

I can get there, 1.28 works great
I'm trying to remember how to "solve the math", I know what I want from the tube, ~39w, I get it from 460plate, 92mA, 400ohm series R.  Now I'm trying to figure out how to subtract that from the unloaded PT logically

I forgot to order an AU7 , the only one I have is way old n well used, so i'll probably start with an AX and see if I can keep it tame

[DummyLoad]

 > start with an AX and see if I can keep it tame...

lose the bypass cap of V3a.

--pete

[shooter]

lose the bypass cap

I'll be gator clipin them to taste
since I'm going with about 80 - 100 more vdc on the 88, i'll have to re-tweak the drive signal from my BB tests.  I'm expecting to start bending the 88 hard around 7/8 on the volume pot, so voltage dividers and caps are good starts.
thx

EDIT:
I keep staring at the 2 R's n 2 pots (TS/Vol) and I flashed to this;
https://en.wikipedia.org/wiki/Wheatstone_bridge
and my brain seized, it's quitting time

[shooter]

some progress;
I'll be doing V3 turret soon   I wasn't sure which caps, got both axial and radial, the radials needs testing, got 'em $1 ea., but I got Dougs FT, just in case

[shooter]

Tested caps to 400vdc, no DOA's!, I did pull 2 for less than scientific reasons.
I went back and edited the original layout, got the board from 11.5" to 9", turret count from 73 to 60.  Sorry Doug
still a month till Edcor gets all the wires right in my PT

[sluckey]

I like the dog legs in the diagonal jumpers. Makes it look fast or sumpthin'. 

[shooter]

I like the dog legs

it's my reminder, those are the wires with "lightning"
I did mess up and do 2 that are ground

[bnwitt]

From the title of this thread I thought you were building and old Kalamazoo solid state amp model 3

[shooter]

old Kalamazoo

won't waste my time on SS, but since I'm about 30miles from Kalamazoo, I have fixed 3-4 model 1's
Mod 3 is just the iteration of what I call the Daddy5.7 (I name all my amps after classic rock-climbs I've done )

[ALBATROS1234]

newbie question here . why the double coupling caps between v1a and v1b do you need both the .0047 and the .001? also why the resistor to ground in between the 2? i know you need to set a path to ground between stages but there are 2 paths to ground between v1a and v1b in this schematic. granted i have only poured over a limited amount of tube amp schematics but i dont recall seeing this before on anything i have looked at or thryed to build.

[shooter]

why the double coupling caps

I um, borrowed that section from a OD channel on a commercial amp.  It's a "T" (?) filter, notch filter.

It "adds" tonal happiness to the circuit

[2deaf]

It looks more like an "n" filter with a diacritic mark to me.  Looks like a second order high pass filter with a mutation added in.  This filter will severely cut the lows.  There is no notch or bandpass involved.



 

[shooter]

looks more like an "n" filter with a diacritic mark to me.

I will except all answers , I had to "learn" all kinda stuff like that 35yrs ago, it just never came back up fixing broke things the next 35yrs

(Don't tell, but it's a cut n paste from Bogners OD channel)

got a little more done;
Started the underside

[shooter]

snowblowed twice and still found time to solder

I forgot a wire on the layout, fixed

I can't find in my notes what goes on the 3rd pin of illuminated power switch, the sellers data sheet is just a mechanical, surfing I get everything from ground to line I'm leaning towards ground

[sluckey]

I can't find in my notes what goes on the 3rd pin of illuminated power switch, the sellers data sheet is just a mechanical, surfing I get everything from ground to line I'm leaning towards ground

See if this helps...

     http://sluckeyamps.com/supro/supro.pdf

[shooter]

Thanks sir, I bought 2 just in case

[sluckey]

I've become fond of those neon illuminated switches. I've used them on all my later projects. Simplifies wiring quite a bit.

[shooter]

those neon

Yup, this the 3rd one, but the 1st that didn't say neon, led, anything about the 3rd tab
from surfin they could be either
I do remember experimenting with getting the last one to light, of course I made no notes

[shooter]

wired as much as I can til EDCOR finishes my PT build, ~3weeks more!
I do like the thought, it's being made, with care, just for me

[shooter]

fired it up, makes guitar sound, and others
started with DC checks, voltages on schematic. Scope work next time.
the KT is only loafing.. for now

[shooter]

scoping I found V1 had HF oscillation, swapped, fixed
gain with an AX was huge, V1 now AU7, gains still very hot, in progress
Wired the TS backward   in progress!

getting low level fil hum, but she plays well, sounds pretty good, so boost the 88's watts, tweak on the drive signal some n see.

One ? if anyone is playing along;  there is a 100k(slope?) R between top of vol and bottom of TS pots, does this act as a AC voltage divider in conjunction with the volume pot?

initial ballpark #'s
outta V1b I'm at 16.7vac rms with 130mV rms in.  Not much loss with TS at 5, 40+Vac rms at pin 8 V2b
(values are all measured just at the edge of clean)

plan to lose the 2.2uF at v1b, drop the 22uF at V2a to 10uF
maybe tweak v3A's plate R down some, maybe tweak 470/560k to 560/560 between v1a&b

thanks as always

[shooter]

I updated the schematic in post 21

fixed the TS
took out 4 $1 caps at B+,(47uf total), and swapped in 2 FT's, (94uF total), dropped my ripple volts from 4 to 2, bumped B+ up 2 vdc.
boosted the 88's bypass cap to 220uF
cut the 560k in half at V1bs grid
All those were positive changes
tried 250k for volume and .007uF at TS, neither made better changes, maybe slightly less than good.

did some quick scope work, seems fine, but I can't "find" an annoying buzzy HF thing when ODing the 88, but I don't plan on it getting square-wave OD when I'm done.

still working on getting "full range" volume, and more balanced TS.  Volume is 0-5, TS about the same.
probably study Pete's monster for some NFB ideas.

All in all, it's a big "champ" sounding amp that seems to sustain a note long time Joe
also feeds back very well, a feature some music types seem to like
she doesn't have the cleans I typically strive for, but with 380vdc plate I think it's what I get.

[shooter]

quick scope shots taken at speaker;
(V@3 = volume at 3)

The harmonic distortion sounds bad
I don't have spare tubes on hand (AU's), probably picking up a couple Monday
not sure that's my problem, just easy to eliminate

[VMS]

Few things i would try:


Add grid stopper to V1b, remove the cathode cap on V1b.


Split the 84k resistor in two on V3b and use those as a voltage divider to get more range on the volume control.

[shooter]

thanks for the ideas,

Add grid stopper to V1b

I was thinking about that, I'm not sure the interaction with the 470k filter circuit.  tonally I'm 90% there, just trying to tame gain, and "re-shape" the square wave to something closer to a VERY asymmetrical sine.  (image below is close)

I'm gonna try an AU in V3's spot to help knock gain down some, I'm afraid if I split the 84k the drive signal will square out even sooner

[VMS]

I was thinking about that, I'm not sure the interaction with the 470k filter circuit. 

Grid stopper shouldn't have affect on that.


Here are two more ideas: making the 280k resistor smaller should tame some gain, easy way to try is to tag another resistor parallel with it.


Adding resistor before the volume pot should give it more range, start with 470k resistor.

[shooter]

making the 280k

the original was a 560k, I halved it, and got the drive into the grid where I wanted it, went from 7ishV to 1.4v
I also halved my Vpot, no real effect.  I was gonna tweak on the 100k in the TS, since there seems to be a pseudo Vdivider 150/500kTS  - Vol.

[VMS]

There's no voltage divider for everything that passes thru .0047 cap, that is why i suggest voltage divider with resistor and vol pot. Changing values on tone circuit might affect how it sounds

[shooter]

I went tube hunting in my boxes, settled on a 12BH7A, now I get full volume control.  I have 5 so i'll roll them in and see.  Think I'm on to NFB and some minor tweaks to the TS.  I just got parts in for a bench repair so I'm probably done for weeks  Re-caping the QX747

[shooter]

  My original PT showed up!!, ordered in Jan.
Installed on the BB ,bridge rec, series caps, 486vdc into a 5k DL

been playing about a month, like the tone, but little to crunchy over to big a range, so hopefully I get more swing with this PT.

amp has to much 60 cycle for studio, marginally acceptable for gigging, so I wanna verify the kt88 isn't the source before I swap in the new PT, which I can tweak if there's unhappy OT/PT fluxing

[shooter]

need to re-work the 1st tap filter to handle bigger B+

will I get enough extra filtering to justify the extra soldering 

I don't need to drop volts, but I have about 20vdc that I can play with.  IIRC, there is a possible "issue" with a too small dropping R, possible motorboating source 

thx
dave

[shooter]

took the extra solder route;

B+ tap now at 440 vs 400, haven't checked at the plate yet, so #'s are alittle skewed
ripple at B+ tap went from 2.2vac to .44vac with the dropping R and 6 caps!

Vk 40vdc / 350Rk = 114.3mA  440-40* 114.3 = 45.7W/42W (max plate) = 109%

tonally nothing really changed, probably tweak some when I get back from the Desert.
The biggest change, NO 60cycle hum!!! 

updated schematic;

[shooter]

Dropped off the amp with guys with guitars and went to the prairie 

got 3 thumbs up, some tonal suggestions, I put in the 2 switch circuits, (without switches :).  combined I get a gain boost enough to turn guitar from 6 clean to 3ish for the same clean-volume.  no #'s yet, too much fun playing, besides a head cab, this one is done. (I might wire in the TS shown on schematic, the one in has 2 "good" places, the rest is just to subtle to bother with)
consensus it's a really good blues amp, that can be dialed back to reach into jazz.
After the mods, I think I got Jimi sound