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Hoffman Amps Forum image Author Topic: Stereo SE KT88 power amps for live rig  (Read 64299 times)

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Offline jjasilli

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Re: Stereo SE EL34 power amp for live rig
« Reply #150 on: February 19, 2016, 02:40:31 pm »
Wait, I think I over-reacted to the 25K output impedance of the Triaxis.  For voltage matching (bridging, actually) the input impedance should be 10X the source impedance.  So if the 1st stage grid leak resistor is at least 250K this should be fine.
That 25K rating for the Tri. output is the printed spec for a MAX output setting...does that number increase as I lower the output from MAX to about 50% ?


You're fine, according to this:  http://forum.tcelectronic.com/topic/4907/triaxis-tone-vs-output-connection/
Triaxis output impedance is from 0 - 25K.  Lower output setting yields lower (not higher) output impedance.  At high Triaxis vol, turning down the 250K input pot on the power amp technically will create an impedance mismatch (less than 10 : 1) & drain away hi's and signal strength.  But this may be a desirable feature, rather than a bug to be fixed.  Since you like the way it sounds, looks like no problem here. 

Offline SILVERGUN

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Re: Stereo SE EL34 power amp for live rig
« Reply #151 on: February 19, 2016, 03:37:16 pm »
Funny....that's where I've been reading up.
Cause I'll be using the Triaxis with a TC G-system so there may be another impedance hurdle to get over to get it right.
There's a guy over there that has taken a lot of the guesswork out of it for me:
http://www.guitaraffair.com/pdfs/g-system-configuration-and-troubleshooting-guide.pdf
 
The G-system utilizes a preamp by inserting it via an 'insert send/return' loop
 
Should go like this for me:
G-system insert send (low-Z) > into Triaxis input (1M) GOOD
Triaxis output (0-25K) > G-system insert return (24K or 34K) NOT SO GOOD
---That is where I might have to use a DI box or buffer to get the Triaxis low-Z enough to play nicely with the insert return on the G-sys
 
G-system outputs (low-Z) into power amp inputs (250K or higher if needed...maybe just a fixed 270K resistor)

Offline jojokeo

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Re: Stereo SE EL34 power amp for live rig
« Reply #152 on: February 19, 2016, 03:46:42 pm »
SG I am reminded of your experiment with ppicf you did about a year ago when you explain the power tube grid driving stage except for now being single ended it may be slightly different? But in this way close to the same idea?

I dont see a huge problem to overcome in your idea and an au7 is a good tube to use to try. After the pi outputs I'm thinking there may be the need of a voltage divider after the coupling caps? You'll know by the range of the controls once you get things wired up. Then utilize a dual ganged pot to control both sides at the same time just like ppimv going into the au7's grids.
The other trial would be to have a voltage divider prior to the au7's grids and then coupling caps after the au7's plates with the dual ganged pot after the au7?

There's only these two ways to see what's best but I like the first option going on instinct.
« Last Edit: February 19, 2016, 03:49:13 pm by jojokeo »
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Offline SILVERGUN

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Re: Stereo SE EL34 power amp for live rig
« Reply #153 on: February 19, 2016, 04:07:56 pm »
SG I am reminded of your experiment with ppicf you did about a year ago when you explain the power tube grid driving stage except for now being single ended it may be slightly different? But in this way close to the same idea?

I dont see a huge problem to overcome in your idea and an au7 is a good tube to use to try. After the pi outputs I'm thinking there may be the need of a voltage divider after the coupling caps? You'll know by the range of the controls once you get things wired up. Then utilize a dual ganged pot to control both sides at the same time just like ppimv going into the au7's grids.
The other trial would be to have a voltage divider prior to the au7's grids and then coupling caps after the au7's plates with the dual ganged pot after the au7?

There's only these two ways to see what's best but I like the first option going on instinct.
Yeah jojo...I think there is something there...and I promised Ed I 'd always give up the good stuff.
 
I'd also like to see what tubenit would think about this....
It requires way less preamp gain to be able to push the output tube grids effectively...and that seems to be where the "TONE" is.
 
I'm out for the weekend but I'll try to check back when I get a chance....to the mountains,,time for some R+R!
Maybe we'll start another thread with that idea when I get back and if anyone else wants to give it a go, I think it's worth the time...
..and when I'm done here we can move forward with something like that on the breadboard

Offline jojokeo

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Re: Stereo SE EL34 power amp for live rig
« Reply #154 on: February 19, 2016, 05:00:25 pm »
Yeah jojo...I think there is something there...and I promised Ed I 'd always give up the good stuff.
 
I'd also like to see what tubenit would think about this....
It requires way less preamp gain to be able to push the output tube grids effectively...and that seems to be where the "TONE" is.
 
I'm out for the weekend but I'll try to check back when I get a chance....to the mountains,,time for some R+R!
Maybe we'll start another thread with that idea when I get back and if anyone else wants to give it a go, I think it's worth the time...
..and when I'm done here we can move forward with something like that on the breadboard
Speaking of tubenit (king of the cathode follower for a while) I was thinking about the possibly of taking the signal out from the cathodes of the au7 as third/fourth options since not a lot gain is required??
« Last Edit: February 19, 2016, 11:16:13 pm by jojokeo »
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Offline PRR

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Re: Stereo SE EL34 power amp for live rig
« Reply #155 on: February 19, 2016, 07:16:16 pm »
> according to this:  ...output impedance is from 0 - 25K

It computes as 21.4K down to zero. Close enough to rock.

Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #156 on: March 01, 2016, 08:57:47 am »
OK, one down, one to go...this one is quiet as a church mouse, and I wound up adding a R/C node before the plate node (as per reading all of your posts on other threads)...excellent punch, presence, clarity, and depth....really happy with it!
I feel like I have the perfect amount of drive but I haven't checked the scope yet to see what that looks like.
 
377V a-k
356V g2-k
33V across 300ohm rk
105mA @ idle
 
Changed the title of the thread to be a little more accurate...
Waiting on an octal hole cover after deciding that another cap can was a bad idea because it was too close to the 88.

Offline Willabe

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Re: Stereo SE KT88 power amps for live rig
« Reply #157 on: March 01, 2016, 09:21:38 am »
Very nice!  :icon_biggrin:

Even has a MOTS face plate!    :icon_biggrin:

Offline sluckey

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Re: Stereo SE KT88 power amps for live rig
« Reply #158 on: March 01, 2016, 10:26:28 am »
Very neat work. Do you have any insulating sleeve on the filament wires?
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline shooter

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Re: Stereo SE KT88 power amps for live rig
« Reply #159 on: March 01, 2016, 10:40:55 am »
Very Nice!  I'm gonna be *tuning* my-(Son's) SE 88 in a week.  He wants more sparkle, He says it has plenty of spank :icon_biggrin:
Went Class C for efficiency

Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #160 on: March 01, 2016, 10:51:08 am »
Very neat work. Do you have any insulating sleeve on the filament wires?
Thanks!
No insulation.
Bad idea?

Offline sluckey

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Re: Stereo SE KT88 power amps for live rig
« Reply #161 on: March 01, 2016, 11:02:34 am »
Only if one of the naked wires touches the chassis. I'd be nervous about that possibility.
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #162 on: March 01, 2016, 11:23:38 am »
Only if one of the naked wires touches the chassis. I'd be nervous about that possibility.
Ok,,,I think I'm good.
I went out and tried to press them against the chassis and because they are 18gauge they don't really move.
I could get the one that's floating over the power tube cathode to touch but it would take a good deal of force, and I would be bending it more than taking up slack.
There's about a 1/4" clearance along the run  as-is.

Doesn't make me nervous, and I'm easily nervous'd 'cause I watch the carnage of failed electrical components all the time.

« Last Edit: March 01, 2016, 11:26:18 am by SILVERGUN »

Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #163 on: March 01, 2016, 11:39:46 am »
Very Nice!  I'm gonna be *tuning* my-(Son's) SE 88 in a week.  He wants more sparkle, He says it has plenty of spank :icon_biggrin:
Cool...did you ever post a schematic?...I'd be interested in seeing it, and seeing what you wind up doing.

Spank is a good adjective. That it has for sure.

Offline sluckey

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Re: Stereo SE KT88 power amps for live rig
« Reply #164 on: March 01, 2016, 11:40:47 am »
Probably a non-issue. Good luck. It's just a PT. I can afford that.   :icon_biggrin:
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #165 on: March 01, 2016, 11:54:30 am »
Probably a non-issue. Good luck. It's just a PT. I can afford that.   :icon_biggrin:
That earns my first emoji since coming back off injured reserve.
 :undecided:
I'll keep all touchy-feely gremlins from entering the chassis.

One thing I didn't address which will make an easy addition if deemed valuable:
Would you guys put a B+ fuse on the OT like I had on the breadboard, and what approx. value? (I used an 1/8 amp SB)
DL said OT is rated @ 120mA
Does placement matter? between B+ rail and OT or between plate and OT?
« Last Edit: March 01, 2016, 11:56:42 am by SILVERGUN »

Offline Ritchie200

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Re: Stereo SE KT88 power amps for live rig
« Reply #166 on: March 01, 2016, 12:05:45 pm »
Wow, that is too cool! :worthy1:  I can't wait to hear the soundclips.  I see the Mesa KT88 that you bought, what, 5 years ago!  :laugh:  Your a good man to finally cross over!  If I could make a suggestion for the eventual cabinet?  With the name and the KT88 front and center....there HAS to be some sort of cutout showing the master tube in all it's glory!

Oh wait.....those fender knobs gotta go.....  I think this deserves some custom aluminum lathe spun knobs that would do this build justice!

Now about the royalties.... :evil5:

Jim

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Can we have everything louder than everything else?

Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #167 on: March 01, 2016, 12:41:11 pm »
Now about the royalties.... :evil5:
Oh, I changed the name again....sorry
It's now the Smoking 88
...totally original and necessary so no one will associate me with "the dark side"

If I could make a suggestion for the eventual cabinet?  With the name and the KT88 front and center....there HAS to be some sort of cutout showing the master tube in all it's glory!
That's the plan...I was thinking about glass, and I wanted to look into some heat resistant plexi

I like how Doug's chassis put the tubes across the front like that, so yeah, that's gonna happen

How's these knobs? I bought about 200 of them when Radio Shack was closing...they were in a big bag and all plastic bag components were 95% off....I left with a bunch of different plastic bags

Offline shooter

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Re: Stereo SE KT88 power amps for live rig
« Reply #168 on: March 01, 2016, 12:51:35 pm »
Quote
did you ever post a schematic
here's the old link with schematic, but it's been modified WITHOUT re-documenting, still a short-coming  :think1:
I like your new knobs, I got the same deal :icon_biggrin:

http://el34world.com/Forum/index.php?topic=18394.msg187753#msg187753
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Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #169 on: March 01, 2016, 01:25:46 pm »
....but it's been modified WITHOUT re-documenting, still a short-coming
I do the same thing...I skipped about 4-5 amps because I just couldn't keep up with the documentation.
 
Sorry I missed that thread...bad timing
 
It's never too late to reopen it (as others have recently proven)...I think there's interest in how it gets sorted out.
I also enjoyed the nonchalant mention in the opening post...nice to know that I have a similar effect on others that they have had on me.
 
Your comment about drive signal (56Vp-p) really got me thinking early in this thread and at some point during this process I realized how important it is to REALLY drive the grid of this monster...and then I realized that I was gonna have to give myself a little more headroom with higher B+
 
This thing pulls B+ down in a big way....unloaded I'm around 460VDC(ish)

Offline jojokeo

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Re: Stereo SE KT88 power amps for live rig
« Reply #170 on: March 01, 2016, 01:33:22 pm »
This thing pulls B+ down in a big way....unloaded I'm around 460VDC(ish)
This is what I figured and why the earlier suggestions of getting that B+ up the higher 400s.
 
What did you end up doing about the driving tube situation?
 
I think you should name the thread "Stereo SE KT88 amps for live rig" as these aren't just simply "power amps"???
 
For the amp name:   SmokinG  88  because then your name SG stands out (pretty obvious yeah?)
« Last Edit: March 01, 2016, 01:36:57 pm by jojokeo »
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Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #171 on: March 01, 2016, 01:53:48 pm »
This thing pulls B+ down in a big way....unloaded I'm around 460VDC(ish)
This is what I figured and why the earlier suggestions of getting that B+ up the higher 400s.
 
What did you end up doing about the driving tube situation?
 
I think you should name the thread "Stereo SE KT88 amps for live rig" as these aren't just simply "power amps"???
 
For the amp name:   SmokinG  88  because then your name SG stands out (pretty obvious yeah?)
Oh yes Sir...I was listening about the B+ conundrum....had to see/hear it for myself and once I did I knew you were right.
I was thinking more about limiting output power with B+ and didn't realize the effect on headroom until I saw it on the scope.
Very glad everyone is here and hung in there with me....well, almost everyone.
 
I told you he was gonna start....I still swear he's like Beetlejuice, where if he comes up 3 times in a thread you can be sure he's gonna show up spouting off about something. It doesn't even have to be his name....just strat, KT88, tele, major, Ritchie, Blackmore, DP, the color purple,,,anything deep....etc., etc., etc., etc.,
At one point he pm'd me and said some crap like "THE GODS OF BLACKMORE HELLFIRE WILL RAIN DOWN UPON THEE"
What does that even mean?
 
 I like the name suggestion.
The SG88...has a nice ring to it
 
Driving tube(s) is as it was in the schematic, single AU7 into parallel AU7 into KT...nothin fancy
 

Offline jojokeo

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Re: Stereo SE KT88 power amps for live rig
« Reply #172 on: March 01, 2016, 02:25:16 pm »
SG I'm reminded of a long ago discussion regarding Plate Resistance rather than Tube Gain as being a prominent factor for consideration of design and how your adjectives are seemingly spot on to this. In using the au7s not just in a single stage or two but in all of the preamp stages - you are substantially increasing the bandwidth of the entire amp and remember, the greater the BW, the more “sparkle” and “chime” we’re likely to perceive.
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Offline DummyLoad

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Re: Stereo SE KT88 power amps for live rig
« Reply #173 on: March 01, 2016, 02:37:35 pm »
mojo758. drops right in stout chassis. B+ should be about 440V

doug gets them for 112 bux.

make that 88 sweat. squeeeze it!

--pete

Offline Willabe

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Re: Stereo SE KT88 power amps for live rig
« Reply #174 on: March 01, 2016, 02:38:29 pm »
Why is rp (ohms) listed twice? And why the rp and rp'?

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Re: Stereo SE KT88 power amps for live rig
« Reply #175 on: March 01, 2016, 02:51:00 pm »
Why is rp (ohms) listed twice? And why the rp and rp'?
One is it's listed "static" value and one is the actual "dynamic" value in operation. When you see the apostrophe you would say "prime" as in "rp prime"?
« Last Edit: March 01, 2016, 02:53:28 pm by jojokeo »
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Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #176 on: March 01, 2016, 02:59:16 pm »
... your adjectives are seemingly spot on to this. In using the au7s not just in a single stage or two but in all of the preamp
Ah-Ha!....I knew there had to be a reason for this awfully large head.

I think the AU7 gets a bad rap....in the guitar amp world.

I recently started with a 5F6-A 'ish design and sat with the guy for 15 different tweaking sessions and wound up with 1- parallel AY7,  2- AU7 gain stages and an AU7 FX loop in the preamp....got lots of drive without a lot of clipping

The guy is thrilled and thinks there's magic dust in there....in reality it's just the effect of cascaded 'low gain' stages.

So that carried over to this design and I wouldn't do anything differently....the Triaxis into this amp is literally the best amp tone I have had at my bench since I started. Especially the pushed clean sound which retains note separation and clarity, but feels rubbery and spanky.
The distortion is just how Dr. Destructo described it and offers up an edge I've never heard before...had to drive the grid to get there, but man am I happy I did.

Offline Willabe

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Re: Stereo SE KT88 power amps for live rig
« Reply #177 on: March 01, 2016, 02:59:36 pm »
Thanks Jojo.   :icon_biggrin:

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Re: Stereo SE KT88 power amps for live rig
« Reply #178 on: March 01, 2016, 03:18:35 pm »
mojo758. drops right in stout chassis. B+ should be about 440V

doug gets them for 112 bux.

make that 88 sweat. squeeeze it!

--pete
there must be a name for the illness we suffer from


One thing I didn't address which will make an easy addition if deemed valuable:
Would you guys put a B+ fuse on the OT like I had on the breadboard, and what approx. value? (I used an 1/8 amp SB)
DL said OT is rated @ 120mA
Does placement matter? between B+ rail and OT or between plate and OT?
?
« Last Edit: March 01, 2016, 03:23:51 pm by SILVERGUN »

Offline DummyLoad

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Re: Stereo SE KT88 power amps for live rig
« Reply #179 on: March 01, 2016, 05:56:34 pm »
use a 250mA fuse replace with 500mA if it pops.


--pete

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Re: Stereo SE KT88 power amps for live rig
« Reply #180 on: March 01, 2016, 06:29:24 pm »
there must be a name for the illness we suffer from

there is no cure. we can only treat the symptoms with doses of inductors.

--pete

Offline pompeiisneaks

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Re: Stereo SE KT88 power amps for live rig
« Reply #181 on: March 01, 2016, 07:09:34 pm »
there must be a name for the illness we suffer from

there is no cure. we can only treat the symptoms with doses of inductors.

--pete

Oh I thought we could transform ourselves into some output that was better?

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tUber Nerd =|D

Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #182 on: March 01, 2016, 07:45:40 pm »
After extensive play testing I have determined that I was wrong about a few things...
-I thought it was gonna be perfectly mated up with a Greenback G12M-25...nope, can't handle it
--Plugged it into my Eminence EM-12 and it took over...much wider response, and no speaker mud at all
 
-I thought I could handle 2 of the same amp for a stereo pair, but I'm gonna still need more clean headroom, for clean tones and effects.
--I'll make the next one push-pull, and I'm thinkin' 6L6 so hopefully when I blend them together they will fill each other out. (?)
 
I'll get it laid out on the other board and see what happens.....Board Wars...it's the only way.
 
there must be a name for the illness we suffer from

there is no cure. we can only treat the symptoms with doses of inductors.

--pete
...or bridge rectifiers (seeing how I now have a large stock of those )
 
And so we go on......................
 
 
 

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Re: Stereo SE KT88 power amps for live rig
« Reply #183 on: March 01, 2016, 08:46:01 pm »
Quote
the AU7 gets a bad rap
+1
my next - current build is a 2 tube AU pre, thanks to PRR for helping dink out the kinks, driving a PSE 6V6

Quote
there is no cure
   :nice1:
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Re: Stereo SE KT88 power amps for live rig
« Reply #184 on: March 01, 2016, 11:09:33 pm »
After extensive play testing I have determined that I was wrong about a few things...
-I thought it was gonna be perfectly mated up with a Greenback G12M-25...nope, can't handle it
--Plugged it into my Eminence EM-12 and it took over...much wider response, and no speaker mud at all
 
-I thought I could handle 2 of the same amp for a stereo pair, but I'm gonna still need more clean headroom, for clean tones and effects.
--I'll make the next one push-pull, and I'm thinkin' 6L6 so hopefully when I blend them together they will fill each other out. (?)
 
I'll get it laid out on the other board and see what happens.....Board Wars...it's the only way.
 
there must be a name for the illness we suffer from

there is no cure. we can only treat the symptoms with doses of inductors.

--pete
...or bridge rectifiers (seeing how I now have a large stock of those )
 
And so we go on......................


i know nothing of what you speak.  :angel


--pete

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Re: Stereo SE KT88 power amps for live rig
« Reply #185 on: March 02, 2016, 06:41:09 am »
Pete, how many of those things do you have?   :l2:
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline DummyLoad

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Re: Stereo SE KT88 power amps for live rig
« Reply #186 on: March 02, 2016, 09:15:34 am »
Pete, how many of those things do you have?   :l2:


i had 2k. it's a smaller pile now...  :icon_biggrin:


--pete

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Re: Stereo SE KT88 power amps for live rig
« Reply #187 on: March 02, 2016, 09:24:42 am »
How did you come to have so many?
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline jjasilli

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Re: Stereo SE KT88 power amps for live rig
« Reply #188 on: March 02, 2016, 09:48:52 am »
-I thought I could handle 2 of the same amp for a stereo pair, but I'm gonna still need more clean headroom, for clean tones and effects.


The reason I suggested KT-88's (on page 1) is that I thought they were the best SE approach to sufficient clean headroom.  Which raises the musical question: where is the dirt in the power amp coming from -- the small bottles, the big bottles or both?  If it's the small bottles, you might get more clean headroom from a higher mu tube, despite your affection for the 12au7.


Also, if you add a second identical amp & speaker, you will get a 6dB boost, which may be good enough: 3dB for double amp power + 3dB for double the speaker cone area. 


You can measure this stuff.  To judge this you need an SPL meter.  These can be downloaded for free on your smartphone, but these programs don't work well below 100Hz.  Or, for about $35 Radio Shack SPL meters are good enough & are used by pro's and can measure down to 50Hz.  So play a guitar passage (above 100 Hz for a smartphone SPL).  Note the SPL level.  Record your passage; play it back through the power amp or your hi-fi.  Turn up the vol of the playback until it matches the SPL of the original passage.  Then turn it up more until the SPL meter reads 6dB more.  Now you will hear the SPL level of 2 identical amp-speakers, before building anything.

Offline DummyLoad

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Re: Stereo SE KT88 power amps for live rig
« Reply #189 on: March 02, 2016, 10:39:37 am »
How did you come to have so many?


m.c. howard surplus store. closed sometime around 2008. richard and i bought 4 packs of 500 for a $20 bill.


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Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #190 on: March 02, 2016, 11:05:30 am »
-I thought I could handle 2 of the same amp for a stereo pair, but I'm gonna still need more clean headroom, for clean tones and effects.


The reason I suggested KT-88's (on page 1) is that I thought they were the best SE approach to sufficient clean headroom.  Which raises the musical question: where is the dirt in the power amp coming from -- the small bottles, the big bottles or both?  If it's the small bottles, you might get more clean headroom from a higher mu tube, despite your affection for the 12au7.


Also, if you add a second identical amp & speaker, you will get a 6dB boost, which may be good enough: 3dB for double amp power + 3dB for double the speaker cone area. 
Some of my thought comes from the idea that I won't need this much KT character and drive X 2
I got this amp to a point where I love it and the drive is incredible and I now I cant turn around,,,so that's enough of that for the DRY side.
The character of the KT88 is there and it stings like a bee.
So now I'm thinking I want the other side to be a cleaner reproduction (or my vision of "more headroom") so that the cleans don't drive so hard and there will be enough "headroom" to reproduce the effects well....this side should float like a butterfly.
 
And I'm thinking of a p-p setup for that so it will be a lot like running 2 completely different amps (through different speakers) at the same time where they fill each other out. I just don't think it would be the best idea to duplicate the 88 amp and wind up with too much of this good thing.
 
As far as the 12AU7s go....it's time for me to learn something because I thought I was using this tube to provide the biggest swing without clipping.
The KT88 output appears to square before the input signal does AFAICT,,,but that character is worth keeping, and it cleans up just enough as I turn the volume down on the geetar so that it is useable (and desireable).
Going forward I will need to figure out how to drive the p-p side cleaner, so if a higher mu tube will do it 'better', I'm all ears.
It is counter-intuitive to my thoughts about 'gain' to use a higher mu tube for 'less gain'(?)
 
One last thing, while I'm typing...
Even though I like the EM-12 speaker better than the Greenback, it just opened my eyes to the possibility of the speaker's effect.
I'm not sold on the EM-12 as THE speaker for the KT88 (dry) side, but at least now I won't just settle for the Greenback because I thought it was the 'right' choice.
I will probably wind up using the EM-12 as more of a full-range speaker for the p-p (wet) side, and keep experimenting with different single 12"s until I find one that matches the character of the 88 output. The Greenback sounded congested. The speaker in the pic below is the G12H-30 70th anniv. and it has way too much 'sizzle' to pair well with the 88.
I'm gonna look at some frequency curve charts and see if something stands out that will counteract the extended response of the EM-12
 
Here's the other cab I'm building for the 88 to push...it will be fully open back, and angled up like this to act as a monitor.
 
« Last Edit: March 02, 2016, 11:17:19 am by SILVERGUN »

Offline jjasilli

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Re: Stereo SE KT88 power amps for live rig
« Reply #191 on: March 02, 2016, 12:56:01 pm »
Sounds like a good way to go. 


Meanwhile, I've searched the web for a useful image of a 12au7 loadline, w/o success.  I think that a typical 12au7 loadline runs pretty close to the X-axis; closer than typical for a 12ax7.  The result is that the 12au7 has less voltage swing, before the bottom of the curve cuts-off, or the top saturates.  That's OK because the 12au7 has little mu anyway.  You can get more clean voltage swing from a 12ax7.  If the output voltage is too high for the next stage, it can be cut back with a voltage divider.


OTOH, sometimes people think of headroom in terms of how far they can rotate the vol knob before overdrive kicks in: more rotation = more headroom.  Of course, with a lower mu tube, the vol knob can be further turned up and maybe never overdrive: "infinite headroom"  :icon_biggrin: .  But the overall vol of the amp may be lower. 

Offline pompeiisneaks

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Re: Stereo SE KT88 power amps for live rig
« Reply #192 on: March 02, 2016, 01:24:28 pm »
Quote
It is counter-intuitive to my thoughts about 'gain' to use a higher mu tube for 'less gain'(?)

From what I understand, the tube can get a lot louder (due to higher u) without distorting, therefore has more headroom.  you can turn it up to 70% of its capacity and it is at 70 u and still clean, whereas for lesser amplification of say a 12AU7, it causes the breakup at its '70%' point which is like u of maybe 25 or something (I think its about 30 on that one?).

I could be completely out in the dark, considering how new I am to this though :P  (I'm an older nerd though so I read like a madman, and have less practical experience.  So the concepts are sinking in faster than the actual use type stuff).

As far as a load line, you're right, I did one based on the design of my recent Vox AC100 with the preamp input 12AU7, and it was really low down on the curves, unlike the 12AX7 with identical 100k Anode and 1.5k Cathode resistors.  It was odd to me to see, but it makes sense now that I think about it.  I'll have to get that image I created and post it here to show it compared to load lines we've probably all seen for the 12AX7.  (its at home and I'm at work)

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Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #193 on: March 02, 2016, 01:46:00 pm »
Meanwhile, I've searched the web for a useful image of a 12au7 loadline, w/o success.  I think that a typical 12au7 loadline runs pretty close to the X-axis; closer than typical for a 12ax7.  The result is that the 12au7 has less voltage swing, before the bottom of the curve cuts-off, or the top saturates.  That's OK because the 12au7 has little mu anyway.  You can get more clean voltage swing from a 12ax7.  If the output voltage is too high for the next stage, it can be cut back with a voltage divider.
This is a great time to make sure I am getting this right....
I choose the AU7 at these operating conditions because I wanted low gain that could handle a big swing cleanly.
Looking at these load lines, I would think that the AU7 does a much better job of not clipping than an AX7 that is biased relatively similar.
(I use PRR's rule of thumb equation that I took as meaning---center bias can be approximated this way:
relation of plate resistor value to cathode resistor value should be approx. rk x mu = ra )
So I have the AU7 @ 47K ra with 2.2K rk
...and the AX7 @ 100K ra with 1K rk
 
If I'm seeing this wrong I would love to get it right.

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Re: Stereo SE KT88 power amps for live rig
« Reply #194 on: March 02, 2016, 01:58:48 pm »
(I'm an older nerd though so I read like a madman, and have less practical experience.  So the concepts are sinking in faster than the actual use type stuff).
p-sneaks, and anyone else who may be lurking and learning....
I found this video to be helpful in seeing this concept.
jj, and others will straighten me out and we all can learn something here.
 
https://vimeo.com/33244955

Offline pompeiisneaks

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Re: Stereo SE KT88 power amps for live rig
« Reply #195 on: March 02, 2016, 02:33:14 pm »
I will check the video, I thought that the load line should include the cathode, so you should do 49.2k not 47,k on that 12au7? no? 

I have been doing a lot of reading, bought the valve wizards preamp design book and am almost done, the first chapter covers a lot of this, but again, it is all book learning so I could be way off.  He mentions going too low down on the scale becomes difficult because it causes more current to flow, and more current clipping. 

I also think maybe I'm confusing headroom and volume.  The lower the slope is, as above, the higher the current, the lower distortion, and the higher the voltage swing.  This means more headroom.  This doesn't specifically mean more volume.  I.e. you get a cleaner tone right up until the tube hits maximum output.  The volume is what I meant 'goes up and stays clean' with a higher u tube.  i.e. if they were both biased with identical headroom, the 12AX7 would be louder and still clean because it has more magnification no?  On the other hand, if you were to bias a 12AU7 so that it has a low slope and high headroom, and then a 12AX7 with more chance of OD and the slope is up higher (negative slope for both of course) the more crunch you get at output from the 12AX7 at higher input, but you also get more overall volume due to the tube having higher u.  In theory, if balanced right, the 12AX7 could have more volume at lower input, without distortion, than a more headroom biased 12AU7 just due to its u factor. 

I was calling it headroom and meant volume.  I hope that clarifies it?

If someone can help me understand what I'm missing please do :) 

I'm guessing one of the PRR/Sluckey/Willabe guru gents is going to show where my thinking is wrong :D

Edit:  worded it as 'I'm confusing' not 'we're' because it may more be me than anything lol

~Phil
« Last Edit: March 02, 2016, 02:39:26 pm by pompeiisneaks »
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Offline pompeiisneaks

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Re: Stereo SE KT88 power amps for live rig
« Reply #196 on: March 02, 2016, 02:47:53 pm »
The question on including the cathode resistor in the line, comes from this image example from his book.

I.e. you do the load line without it initially in trying to get a general idea, but once you add the chosen cathode resistor in, you use it for both the load line and the bias line. 

See attached image.

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Offline SILVERGUN

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Re: Stereo SE KT88 power amps for live rig
« Reply #197 on: March 02, 2016, 02:52:53 pm »
If someone can help me understand what I'm missing please do :) 

I'm guessing one of the PRR/Sluckey/Willabe guru gents is going to show where my thinking is wrong :D
You can always start a fresh thread with those questions/comments and some of the guys will be happy to teach you.
This stuff can get hidden and overlooked in these marathon threads and I imagine some guys give up on these threads once we start drifting off topic.

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Re: Stereo SE KT88 power amps for live rig
« Reply #198 on: March 02, 2016, 03:00:59 pm »
Oh no worries, I was just trying to talk about what you were about the reason that you were told a 12AX7 would give you more... I'm just enjoying this thread a ton overall seeing the cool stuff you do to create circuits from scratch etc.  I'm still so far away from that it's not funny :P

If what I'm saying is right, and helps, I'll be stoked because it means I'm starting to make positive progress lol.  If I'm not, I won't be shocked. 

~Phil
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Offline jojokeo

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Re: Stereo SE KT88 power amps for live rig
« Reply #199 on: March 02, 2016, 03:13:40 pm »
p-sneaks, and anyone else who may be lurking and learning....

This stuff can get hidden and overlooked in these marathon threads and I imagine some guys give up on these threads once we start drifting off topic.
:sleepy2:   :hello:   :l2:
 
To steal ideas from one person is plagiarism. To steal from many is research.

 


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