hybrid B.B. amp

[shooter]

I’m starting a hybrid build, old parts from old attempts but using terminal strips for easy moding.  This attempt I’ll use a pre section i scratched out over the winter, basically a Reverend Knight plus Bogner ecstasy mated to a single ’88, into a 1628seaOT.  If it works n sounds good, I’ll probably convert the PA section to tranny PI with ‘34s self biased.  I scratched up a 2nd pre section but for now we’ll see if I can make this work

PS was tested using a 5K 50w D.L., came in at my target 400vdc, (394v), on the 1st tap, 365vdc at the 2nd.

Hopefully I’ll have V3 and 4 up and running by the weekend, then I have 3 free days!!!!

Random ideas,
A/d to stamp to pc that monitors cathode V & calc I on all tubes with plot-able data.

Mini incandescent lamp power soak for output power “adjusting”.

Play with “standard” cc CF verses dc CF

[PRR]

V3b grid is floating.

[Tony Bones]

Very cool. SE KT88 is guaranteed to please.

> V3b grid is floating.

If that's the only thing wrong he's doing OK!

[shooter]

V3b grid is floating.

glad you got your new glasses dialed in, thank you.  I even wired it as drawn, adding and R now.

EDIT:  I think i'll just remove a C and re-read direct coupled CF

[shooter]

updated the original schematic.  V3 and 4 are wired, ready for DC testing.  I added another PS tap, installed a 10ohm 50W DL at the speaker jack as a safety/convenience feature. 

[shooter]

It was 88/88 at 9pm last night! so all I accomplished was putting wires on pots and getting them mounted and jumping in my frog pond with my alt-beer was NOT an option for cooling down


hopefully i'll get the TS/vol wired in, DC checks so I can do some AC testing this weekend

[SILVERGUN]

Are you intentionally omitting the 3rd gain stage on the Ecstacy side?
Just wunderin. I'm a schematic checker at heart.

[shooter]

Are you intentionally omitting

for now, ya.  I want to get a "sense" of just the 2 gain stages of each.  I have 2 more channels I call NOR for a total of 4 pre's mixed down so the tube count is getting outta hand

If these 2 combine well, I'll note mods/tweaks, then put the NOR together and do the same.  I have a working turret layout for all 4 that include verb and trem

we'll see how far it goes

[shooter]

The amp be workin!

Initial fails;
V3 had 350ish on the plate n CF
V4 cal’d north of 50W Pdis!

I also had V3b’s grid DC coupled to the wrong side of V3a’s plate R 
Fixed the issues
Biased cold, PS taps changed, the schematic on top is updated.

Plate at 300ish, 29Vdc at Rk = 470ohms =62mA ~~ 16.8W Pdis

V3a plate 151vdc, V3b cathode 222vdc, (thinking that’s close enough for max fil?. )

Re-wired grid, plugged in a speaker and played!!   
The TS range seemed fine, vol pots worked, just a whisper of hiss, might be a barn thing, a no bottom thing, or an issue.

Next, get V4 sweating like the rest of us
Scope out test signal and see how much gain I need from V1 & 2
Fix any noise issues

Start wiring the pre’s

[shooter]

got the 88 breakin a little sweat;
Plate 348vdc
Vk 34
Ip 79mA
24W idle
good enough, scope time, after dark, it's 88/88 again

[shooter]

scope shot, my sig gen only puts out 1vrms;

[shooter]

Got the “top” channel wired n workin!!

1st fail, missing ground wire found during ohms checks
2nd fail, powered up without V1!

Sounds n plays well, a little weak but I used an AU for V1.
Got Friday off so hopefully I can scope out the signal path and tweak where needed.

[John]

2nd fail, powered up without V1!

I once spent 1/2 hour trying to figure out why I had no signal coming through on a new build. Then I realized the vol knob on the guitar was all the way down.

[SILVERGUN]

2nd fail, powered up without V1!

I once spent 1/2 hour trying to figure out why I had no signal coming through on a new build. Then I realized the vol knob on the guitar was all the way down.

Some things are better kept unshared...
"I" would've said "I had a friend who once...."
 
Shooter...
Maybe AY7 in V1?
I think V3 is gonna be a good spot for an AT7...with those values left in place

What are the blue dashed lines and red X's on your schematic.....is that an "end around"?

[shooter]

"end around"?

Ya, building it from back to front, tested good to there, next update i'll have them removed.

I think V3 is gonna be a good spot for an AT7

I hope to get a scope on the Sig path Friday and get enough signal to push the 88.  I'm biased about 34vdc so I need at least that much.  I might have to get V3's B+ up some, I'm setting about 150 at plate.

I can't play, but I can make enough sounds that players make to say she's sounding like a good straight up Rock amp, hoping to get it louder n dirtier over the next day or so.

[shooter]

  YippySkippy I bout pee'd myself!
I wasn't in the mood for signal n volts, so I just put the AX(wb) in, dialed for annoying at 10pm

now I'm like some 5yd old waiting for santa
I'll put into the 2X12 at 10am but I think this one beats expectations, and I won $20 on a $2 scratch off!

[shooter]

ok, she has some technical issues, scratchy noisey pots, possible dc crossing cap boundries, normal expectations for a BB with abused parts.

Cleans are good, treble might be a little to glassy, Bass sounds right, still needs more umph, you have to dig in hard to get any breakup.  So I guess I have to do some techy work, later, its 75 n sunny

[SILVERGUN]

Likin' the updated schemo
So now you have an AX7 in V1 and V2?
I was gonna suggest adding another B+ node, but I was picturing it the other way around with the 1K where you have the 22k
 

YippySkippy I bout pee'd myself!
I wasn't in the mood for signal n volts, so I just put the AX(wb) in, dialed for annoying at 10pm

now I'm like some 5yd old waiting for santa
I'll put into the 2X12 at 10am but I think this one beats expectations, and I won $20 on a $2 scratch off!

These are some of the best times we get...puttin down the rule book and just make it great
..besides hittin it big on the lottery
 
 
 

[SILVERGUN]

...basically a Reverend Knight plus
 
What is a Reverend Knight?...Google failed miserably

[shooter]

What is a Reverend Knight

my bad, it's a Reverend kingsnake, don't ask how I got from there to here

my 1st task is get rid of the build induced bad things.  Once that's done I can start tuning both PS rail and any gain tweaks.  Then if needed TS.  I've proto'd enough to know this one just wants to roar

[SILVERGUN]

Since I feel some kinship just by being in the SE KT88 club (not to mention the same first name thing), I felt myself trying to project my project ideas onto your project, so I'll give you the Cliff's Notes.

I had a revelation when I put 2 completely different speakers into a 2x12 cab and felt like I had discovered a cure for one dimensional tone.
Seeing your 2 parallel paths makes me want to accentuate the differences in those 2 paths so as to come up with a more complex sum. I assume that is one of your design objectives and I applaud it.
Dudes have been jumpering channels since there have been multiple channels to jump and I am sure it has more to do with a complexity of tone than just an increase in gain, so I am digging this concept and following along with interest.

Once you get the drive signal up, don't forget to overdrive the crap out of it and completely disregard max.
True max happens when something melts, until then your just following the rules. 

My 15W 88 sounds louder to me than a cranked Deluxe Reverb and that is partially because I have no regard for the health and safety of that tube...I drive it hard and I carry an extra 

Just spoutin' off

[shooter]

Just spoutin' off

spout at will Sir, I value your insight.

I had a revelation when

that's too funny, I did the same thing in my 2X and it was, goose-bumpy n smiles
that's when I did my "Pleximon" build and got all goosebumppy when 2 channels that sucked, sang out sooo pretty through the 2X, and then....

this concept started according to the date on the schematic, which I always leave alone, last Aug.  I'll post the concept schematic later, I call it the Daddy 5.6.  Great classic climb in the Linville gorge NC
Matt, my Son and I talk about gear ideas, etc, so I said, i'll take the V1ish from 4 "cool" amps, he rattled off a dozen, I schematic porn'd for a week and picked  4, then mix it down to a "tried n tested PA" and see.

so here's the signal path pics
(The signal leaving V1apin6 splits into it's and ch2's .02uF thru both gains n TS, they have some interesting interaction as 1 channel feeding 2 gain knobs n 2 TS knobs)

Notes;
Here’s the quickie; my brains still not interested enough to troubleshoot, but data collection, I can do that
All readings are Rms
Vin = 85mV 1k
V1a-1 1.72v
V1a-6 4.47
Pretty sine waves, no distortion
The pics finish out the signal path, the file names are hopefully decipherable.
(10_10 = gain 10, tone 10)
Full out square wave @ speaker  14.4W
10_5 @ speaker ~~ 10W

[shooter]

got all the loose screws tight, (on the amp ), no drop-outs or fades, played for 20minutes.  So i'll nail down the voltages then start hunting up parts for ch2

SG, can I ask what values you landed on for your '88?

mine breaks up sooner than I want, good breakup, just looking to move it from 4/5 on guitar to 7/8.  Loudness is good, more louder is better

[SILVERGUN]

These were the last notes from my thread:
373VDC a-k
350V g2-k
33V across 330ohm rk
95mA @ 35w pdiss

Into 4.8K -- One Electron UBT-2 OT (Thanks DL!  )

mine breaks up sooner than I want, good breakup, just looking to move it from 4/5 on guitar to 7/8.  Loudness is good, more louder is better

Did you try an AT7 in V3 yet?....it'll help

[shooter]

Your initials?  DL?  or mine
Thanks, I think I'm gonna sweat the '88 a little more
after I get the 2nd ch going to see how well they play together
i'll get the AT in also, thanks

updated the schematic with most the sigpath DC readings

[DummyLoad]

curious about the 68k grid stoppers. just may be shaving off more HF than you probably want to. with 68k your f-3dB knee is 31KHz. quite noticeable but maybe what you want. fyi, typical 33k grid stopper has f-3dB knee at ~63KHz.


your KT88 is loafing. very much loafing @ 22.7W Pdiss. 



--pete

[shooter]

68k grid stoppers

I saw a couple other builds using that value, n my brain said, hey 2 68k in || = 34K

22.7W Pdiss. 

Ya, I want to sneak up on it, like a frog in warm water
I'm doing the measure into a 10ohm DL, forgot to note that, 
not that it makes any sonic difference.

Thanks for the ideas n tips, maybe i'll tweak 1st, then build ch2 since the next 4 days will be hell at work.

[shooter]

got the 88 to 32.2W
343vdc   31vdc  300ohm
I tweaked the 1st tap to 390ohm from 470, still lost 7 volts at plate, but gained watts, i'll listen in the AM.

sorted out all the parts for ch2

happy fail, since I have only 1 channel, the 2 68k's are in || , just like I planned it

[PRR]

> your KT88 is loafing. very much loafing @ 22.7W Pdiss.

With 5K OT and ~~350V(loaded) supply, that may be as good as it gets.

He wants to get to around say 440V 88mA (39W Pdiss) to "make the most of" that big bottle and take the chill off this weather. I don't think the PT he has has it in it.

Alternatively, since he has 340V and 103mA, he could load in 340V/0.103A= 3.3K more or less. However it seems he already blew $140 for the 5K OT and may not be in a mood to go back again.

[DummyLoad]

> With 5K OT and ~~350V(loaded) supply, that may be as good as it gets.

agree.

mr. shooter needs a new power transformer (650VCT) to wake up KT88. Kinkless Tetrode sounds much better when she's pushed hard-er.

--pete

[shooter]

agree.

440V 88mA

Thanks gentlemen,  I can get there with current hardware, my PT is rated 550vac 250mA
but when I hooked it up as such it was far away from my 400 target so I "cheated" n used CT to 1 leg.  I can still move one tap back on the PS rail, probably 10 volt gain, I'll keep tweaking.

[SILVERGUN]

Wanted to get you some real-time numbers, so I went upstairs and flipped it over and saw:
PT: Hammond 290CX  = rated 650VCT @ 207mA
Actually putting out 345-0-345 into SS rectifier @ 119VAC input - showing 338VDC on standby
Off standby at idle:
370V a-k
349V g2-k
32.3V across 300 ohm rk (so I actually had a 300 in there instead of 330)
102mA @ 38W pdiss (assuming 5% g2 current)
 
Confirmed approx. 5% g2 idle current by measuring a 5V drop across 970 ohm g2 resistor (5.15 mA)
 
Pa+ g2 = 107.67 mA @ 39.83W total
 
 
 

[shooter]

Thanks for taking the time SG, I just "got there"
I'm using the 270HX, but couldn't get close with SS using red/red, so with CT/red
and moving to the B+ tap from A tap, new numbers;

376Vp
34.3Vk
300rK
114.3mA Ip (didn't minus out g2, or measure yet)
39ish Watts

played about 10minutes and she's close enough to Beast to move on, for now.

I don't have a 330, but I think that's gonna be the sweet R value.

[PRR]

> putting out 345-0-345 into SS rectifier ...- showing 338VDC on standby

345V AC times 1.4 is 483V DC. Why are you SO much less?

Bad first cap off the rectifier??

[SILVERGUN]

> putting out 345-0-345 into SS rectifier ...- showing 338VDC on standby

345V AC times 1.4 is 483V DC. Why are you SO much less?

Bad first cap off the rectifier??

Standby switch is directly off of the rectifier, 'before' 1st cap

But that doesn't explain the big sag once I throw it on

Sorry for the hijack shooter; thanks for the help PRR

[shooter]

NP SG, fwiw, my inrush vac is about 440vac which I'm guessing is like having standby off.  I also can't figure how you can use a similar PT end-to end, SS,  and wind up where I am CT to end SS

[SILVERGUN]

Ok, I went out and checked...
I think I initially threw off PRR with standby switch semantics
I said "standby on" which I meant as "unit is on standby; in standby; or standby switch is open"
 
I also pulled the power tube and I'm seeing 495VDC unloaded B+
Put the 88 back in and it pulls it down to 403VDC at 1st node
Is that a reasonable power supply sag?
 
370V reading I was giving was accurate plate to cathode reading

[shooter]

Is that a reasonable power supply sag?

I'll wait for PRR to answer yours n my newquestion

Is there any NONO in re-setting my PS rail like the snippet below?

[SILVERGUN]

That drawing was slightly confusing...this was my interpretation..is this what you meant?

[DummyLoad]

Is that a reasonable power supply sag?

I'll wait for PRR to answer yours n my newquestion

Is there any NONO in re-setting my PS rail like the snippet below?

not really but you are putting more stress on the 1/2 secondary winding. current will be near doubled through the active 1/2 of the winding. FW is more efficient use of the HV secondary for that transformer. 


--pete

[SILVERGUN]

Is that a reasonable power supply sag?

I'll wait for PRR to answer yours n my newquestion

I'm OK
I forgot about the drop across the OT...I'm losing 46V there @ idle
So, 403V B+ plus 46V is 449V at the actual 1st node (I had mis-worded that before)
That's close enough to Hammond's rating for me.
 
For the record, I've got mine wired as a Pentode.
Back to your regularly scheduled thread

[PRR]

As Pete says, you want the 2-diode affair to work both sides of the winding. This may get you toward 430V under load.

An SE output usually wants more filtering than just a first cap. I suggest moving the OT feed over one stage and changing some values.

[SILVERGUN]

This may get you toward 430V under load.

His 270HX PT is only 275 - 0 - 275

shooter, just curious, why were you trying to use half the winding into a bridge?

[DummyLoad]

best you're going to do with that PT is just under 400V loaded. with a 100mA load SIM indicates very close to what you have - 370V after 1st RC filter with about 385V at first filter.


272HX will yield about 420V at first filter - 405V after 1st RC filter with 100mA load. closer. better. 



--pete

[shooter]

shooter, just curious

when I 1st powered up using a 5K 50w DL using whole secondary she was >500vdc
moved to ct 5K DL just over 400vdc @1st tap, my target V
stayed pretty close loaded and since it's spec'd @ 230mA and I'm about 1/2 that, I'm happy.  since down-sizing, whatever is already paid for is good
also the bridge n caps n bleeders were rescued from a previous build.

SE output usually wants more filtering than just a first cap

ya, I had that config, but moved to the 1st tap to get B+ up some, hence the "extra" 47uF for the more filtering.  Last time I checked ripple at 1st tap was <2vac (without added 47), i'll re-check now that I'm cooking the 88

If someone says "Dude build me that", I will do a proper PT

this whole build is basically to see what blending channels (hopefully 4) sounds like. Along with having a goodish BB platform for quick change tweaks

[shooter]

couple wires left then V2 will be ready, here's my layout, not2scale!

[shooter]

Got V2(ch2) up and running, played about 15min, there's not as much channel difference as I thought the schematic suggests.  #2 was pretty treble hissy, like to much gain, swapped in an AU, now both channels have the same "loudness" with ~~same knob setting.  I'll scope things later, "the beers to cold, the skies to blue........."

[shooter]

Ok, so I just played more, quietly, 3 on gain, 4/6 TS, 3-5guitar
Played , (my version ), orchestral, serious Church, lounge, n kinda sorta country, she exceeded my expectations for clean & chimmey !  Hopefully in the AM I find "that spot".


hum is noticeable, but it's a breadboard n looks like my old stuff, which hummed

To Sluckey, you don't need to hear the sweet spot, you can feel it
but, ya, all the cats did run!

[shooter]

I updated schematic on original post.  Added an AC chart; voltage vs freq.

with 150mVrms in the signal is clean out of V1&2's plates, knobs = V10, TS5 yields top clipping similar to spk10-5.jpg (except inverted) coming out of V3-8

[shooter]

Final tweaks, putting this one in the worth building/tested box

Updated the schematic, here’s the changes;
V1a bypass cap to 2uF from 1uF
V2 “mid” point voltage divider, raised R from 220k to 330k
V2’s TS I ||’d a 120pF with the existing 500 and .0022uF with the .0047

All the above tweaks added to the amp, the V2 side became more of an equal to V1 side.  I can get more possibilities with the TS, gain is still over a smallish range (4-7) for what I consider gigging range, noodlin, 2 to 8.

It has that plexi twist, where either side on it’s own just doesn’t seem “good enough” but both, together, = 4 instead of 2   To be fair, for a 1 less tube version ,V1 could easily carry the load with some minor tweaks.

To anyone considering, use the Fullwave diode config, not my CT bridge config, or spec a PT closer to 400vdc loaded.

Thanks for the inputs n insights.