B15N to ~100W output?

[afinitemind]

I recently gutted a non-functioning RCA PA amp and rebuilt it with a B15N circuit, octal pre and all. Love the tone, a perfect bass amp for small-to-medium rooms. Of course now I want more! I'm wondering about a new build, taking the B15N's simple 6SL7 pre/PI and pushing it into a more powerful power amp.


Any suggestions or potential pitfalls to avoid? Bigger iron and 2xKT88?

[shooter]

if you wanna keep it incestual;
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_v9powera.gif

[Tony Bones]

That 6SL7 PI should be able to drive a quad of KT88. It might struggle with a six pack.

[jjasilli]

You can make 100W clean (5% THD) with a pair of KT-88's: 600V plate; 300V screen.  Edcor can wind a PT.  Checkout my Stromberg Carlson APH-1100 schematics which Doug has posted.  (These PA's are Very Heavy!)


Or go UL and get 75 - 80W, which is pretty close.   UL with KT-120's should get you 100W.


You may need more signal voltage into or out of the PI to fully drive even a pair of KT-88's compared to the Ampeg's 6L6's.  Shouldn't be hard to do that.

[kagliostro]

6146 tube as in the SVT ?

https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_svtpoweramp6146b.pdf

https://frank.pocnet.net/sheets/049/6/6146.pdf

a pair B+ 750V / G2 200V / 8K Load / 120W / 5.7% distortion (double to 4 tubes and you have 240W on 4K Load)

https://www.thetubestore.com/6146a

https://www.thetubestore.com/shuguang-6146b

Franco

[VMS]

One option would be to use 4 output tubes like ampeg did with the b15r version.


That would keep the voltages similar and would be easier to find transfromers.


Schematic can be found here:


https://www.talkbass.com/wiki/schematics/#19-b-15r

[afinitemind]

if you wanna keep it incestual;
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_v9powera.gif

Woah. That'd be lots of fun! Don't really want to carry that much weight if I can get away without it.


But still, tempting...

[afinitemind]

You can make 100W clean (5% THD) with a pair of KT-88's: 600V plate; 300V screen.  Edcor can wind a PT.  Checkout my Stromberg Carlson APH-1100 schematics which Doug has posted.  (These PA's are Very Heavy!)


You may need more signal voltage into or out of the PI to fully drive even a pair of KT-88's compared to the Ampeg's 6L6's.  Shouldn't be hard to do that.

Looks like this might be the ticket. I can start drawing up a diagram...how would you suggest modifying the 6SL7 PI to push a little more voltage out? Or are you suggesting I might need to modify the APH-1100's 7199 PI?

[afinitemind]

One option would be to use 4 output tubes like ampeg did with the b15r version.


That would keep the voltages similar and would be easier to find transfromers.

I might be interested in this route, too. Does seem fairly simple to beef up the power supply/OPT and add a couple more 6L6s.


Any reason not to use one of those Antek toroidals to power something like this? Hard to argue with the price, but then again I've never used a toroidal in a tube amp.

[Tony Bones]

I figure that KT88's @ 600V will bias at around -80V compared to -50V for 6L6 in the B50N. I think that the preamp and PI could provide the extra signal swing to if their supply voltage is increased by 100V or so. The KT88's could still be overdriven, but it would start a little higher up on the volume knob.

I think an important part of the sound of the B50N comes from the paraphase PI (even if they are sneered at by the illuminati.) If I were looking to recreate the sound of your Ampeg, I'd avoid changing the PI to something else. But that's just me and my opinion.

[jjasilli]

-40V bias @ 600V plate, 300 screen per Stromberg Carlson schematic.

[Tony Bones]

Ah, 300V screen. Could use a bridge rectifier for 600V and get ~300 from the center tap.

[afinitemind]

Ah, 300V screen. Could use a bridge rectifier for 600V and get ~300 from the center tap.

Interesting. I get the bridge rectifier for 600V, but how does 300 from the CT work? Is there a schematic I can look at that would enlighten me?

[kagliostro]

When I saw it for the first time I was astonished



explanation here
http://el34world.com/Forum/index.php?topic=11999.msg111066#msg111066

Franco

[Tony Bones]

Interesting. I get the bridge rectifier for 600V, but how does 300 from the CT work? Is there a schematic I can look at that would enlighten me?

There are four circuits in the attached diagram.

A. An ordinary full-wave rectifier using a transformer with CT.

B. The same as A, but with the diodes turned around to make a negative supply.

C. The circuits from both A and B combined. This is commonly seen in SS equipment requiring a dual rail supply.

D. The circuit in C, but with the ground reference moved.

[afinitemind]

Crazy! Thanks for the insight. Love learning something new!


That idea makes the S-C power amp seem even more approachable.

[afinitemind]

So could I use this antek transformer in such a configuration to generate both 600 and 300?

[kagliostro]

No, seems you can't

the transformer has 2 x 0-6.3v windings

and seems to have 2 x 0-430V windings

assuming you connect the two HV windings to have an 860V AC with CT

with that transformer (rectified with Solid State bridge) you will have 1200VDC and 600VDC at CT

--

The transformer to have 600VDC and 300VDC at CT must be 440VAC with CT (220VAC - 0 - 220VAC)

Franco

[Tony Bones]

So could I use this antek transformer in such a configuration to generate both 600 and 300?

You need a transformer with either a CT or two identical secondary windings (connect in series, use the junction as CT.)

A 220-0-220 should work. You might find a toroid with two 220V secondaries at 150VA or more.

[afinitemind]

with that transformer (rectified with Solid State bridge) you will have 1200VDC and 600VDC at CT

Wait, so 1200VDC on the plates is a bad thing? (I'm kidding)


Obviously still learning the math behind all of this. Thanks for keeping me from doing something dumb!

[VMS]

There is a way to make half voltage without CT, but looks pretty complicated to me:


http://ampwares.com/schematics/300ps.pdf

[VMS]

...but voltage doubler circuit on the picture 4B looks pretty simple:


http://sound.whsites.net/valves/design2.html#s5

[afinitemind]

Looks like this guy from Edcor would fit the bill?


Although maybe I should take jjasilli's initial recommendation and just have Edcor wind one that matches up to the S-C schematic proper.


How about an OPT? I think I'm understanding 4500Ω for P-P KT88s? Does this seem like a good fit?

[shooter]

Don't really want to carry that much weight

that's what Roadies are for 

the Iron you picked looks good, except I'm not totally clear of where you "expect" to land B+ wise, guessing from skimming you're going FWB and north of 400vdc ?

when I worked for GE there was a saying "you can buy better, but you can't pay more" 
With Edcor, you pay a premium, but you get the best!  been using their Iron since I started this odd little journey

[DummyLoad]

https://www.mouser.com/datasheet/2/410/media-1067468.pdf

use a supplemental 6.3V 4-6A filament transformer.



--pete

[afinitemind]

that's what Roadies are for 

wish i had that luxury!

the Iron you picked looks good, except I'm not totally clear of where you "expect" to land B+ wise, guessing from skimming you're going FWB and north of 400vdc ?

Thats the plan...600VDC plate, 300 screen..pulling from the Stromberg Carlson schematic that jjasilli suggested.

use a supplemental 6.3V 4-6A filament transformer.

good catch...12.6 won't work for KT88s or 6SL7s. 4A should be enough as I'm only planning on a single preamp.

[shooter]

Was that for the filaments Pete?

quick look through edcor I found this;
https://www.edcorusa.com/xpwr225

but its 300-0-300 so you'd have to do FW, NOT FWB, I think?

[DummyLoad]

Was that for the filaments Pete?

quick look through edcor I found this;
https://www.edcorusa.com/xpwr225

but its 300-0-300 so you'd have to do FW, NOT FWB, I think?

sorry that transformer i posted won't work - use the Edcor. that triad N-66A not 230V & 230V. it's a 230V split to 2 x 115V.

that link above is closer the 400V CT that you're looking for. the 400VCT edcor will take 6-8 weeks.

--pete

[kagliostro]

A bit expensive

https://www.tube-town.net/ttstore/Transformer/Power-Transformer/Audio/Hammond-369KX::5728.html?language=en

May be in the USA it is cheaper

Franco

[DummyLoad]

i was fiddling with multi-sim using the edcor XPWR182. 103USD + ship and 2 month wait. https://www.edcorusa.com/xpwr182

it's a large part.  https://www.edcorusa.com/Attachment/DownloadFile?downloadId=2437




with this part ground the 12.6 filament winding CT and run the tubes filaments in paralleled strings. need the marshall style CC bridge tap or a second transformer for the bias supply.

--pete

[DummyLoad]

FWIW - 2 circuit suggestions.


i like the B+ output of the edcor, but i don't like the size of the part or the filament string arrangement. OTOH, the hammond can be had in a reasonable time-frame. 


C6 may need to be a stacked pair.


--pete

[afinitemind]

FWIW - 2 circuit suggestions.

Thanks for the power supply schematics! Super helpful. The Hammond (188VA) is enough to get it done, right? Any reason (other than bomb proofing the amp and getting a better workout carrying it anywhere ) to get the Edcor instead? Dedicated bias tap seems like a benefit, too.

C6 may need to be a stacked pair.

Why is that? Voltage handling?

[DummyLoad]

yes - on a cold start-up.


--pete

[PRR]

> C6 may need to be a stacked pair.

Yeah, maybe.

After 22k, it won't get enough current to blow-up before the small tubes suck it down.

Ampeg did this in the 1970s and those "overvolt startup" caps were still good in 2004.

[DummyLoad]

figured that as well, i'm just uncomfortable applying overvolt to caps.


--pete

[shooter]

i'm just uncomfortable applying overvolt to caps.

Yup!  caps are mini ied's just bidding time 

[kagliostro]

The Ampeg Schematic show 470V on plates and 450V on G2



What about an Hammond 260K ?

https://www.tube-town.net/info/hammond/hammond_pt_260.pdf

secondary 325V - 0 - 325V @ 200mA

Franco

[Tony Bones]

The Ampeg Schematic show 470V on plates and 450V on G2

I think the goal is to power a pair of KT88's with something around 600V on plates and 300V on screens.

[kagliostro]

Oh, sorry, I was thinking to a quartet of 6L6GC with that transformer

Franco