Linear Power Section KT88

[Ed_Chambley]

> is designed for 70 watts

At what B+ voltage?

Is that anywhere near the B+ voltage you intend to run?

Remember that if B+ is changed 1.4 times, the optimum impedance is changed *2 times*.

Cannot answer that.  It is a Saratoga SI-310-0, it was a pull from a phase linear amp from years ago.  I was wrong about the weight.  It may be too heavy for this project.  It weighs in at 14 lbs.  Pulled maybe 1981 from what I remember to be one of their models which was in a glass enclosure.  I am not able to find specs on it, but I do know it was a replacement.  May not work, but since I have had it for (wow) over 30 years I figure if I toast it I have not lost anything.  When it gets right down to it, I will just probably spring for the Hammond as it is $135 from Digikey.

[HotBluePlates]

HBP do you have any schematics we could look at that shows how they tapped 300v from lower caps and 600v from the upper filter caps. I have several sets of kt88 and want to use them in a guitar amp someday.
I dont really know anything about ulta linear outputs so regular push pull would be better for me, but I could read up a bit and see what would be good for a guitar amp .

Guys like us looking at guitar amps have a tendency to assume screen voltage has to be the same or close to plate voltage in an output stage. That's not the case, it's just easier and cheaper to do that.

The only difference between normal push-pull operation and ultralinear (as far as wiring) is that plate and screen voltage must be essentially the same, and you have screen taps on the OT that attach to the screens instead of a node of the power supply. It's generally easier to hook up the output stage UL than typical push-pull, and requires fewer components.

Depending on how clean you want your power supply to be, you might move the choke from between the plate and screen node (of a typical push-pull stage) to a point ahead of the plate node. You'd need to uprate the choke to handle all the amp's current, and you'll likely add a filter cap to ground ahead of the choke.

In other words, cap to ground -> big choke -> cap to ground, with the 2nd cap feeding the CT of the ultralinear OT. Screen taps of UL OT go to screens, which may have smallish 100Ω or so stoppers. The 2nd filter cap then would go to a probably largish resistor before a following filter cap feeding the phase inverter.

[Ed_Chambley]

I got to take apart the RedPlate.  There is really a lot going on inside in the preamp and a couple of tricks in the power section.  It does use cathode bias for 6L6 and has, you guessed it, a UL OT without a sticker.  There is a stamped number on the bell housing.  Looks like Heybour stuff.  Anywho, it looks fairly straightforward build if you remove all the bells and whistles.

Question
What is the (b) and (o) on the data sheet as in
Va,g2 (b)     460v
Va,g2 (o)     453v
I read where (b) is anode supply voltage and (o) is zero out.  Could someone elaborate.

Also I posted a gut shot of a Route 66 if anyone is interested.

[HotBluePlates]

Question
What is the (b) and (o) on the data sheet as in
Va,g2 (b)     460v
Va,g2 (o)     453v
I read where (b) is anode supply voltage and (o) is zero out.  Could someone elaborate.

"b" in all cases on the data sheet is the supply voltage. "o" is a condition at idle or "zero" signal output.

In your selected portion, 460v B+, and after OT d.c. drop, 453v measured at the plate and screen.

Of course, that sounds a little fishy to me, as the plate will have more winding resistance in the way, and in reality is probably a volt or 2 less than the screen.

[Willabe]

It does use cathode bias for 6L6 and has, you guessed it, a UL OT without a sticker.

So the UL taps are hooked up but no NFB loop?


               Brad    

[sluckey]

UL OT without a sticker

What does that mean?

[DummyLoad]

UL OT without a sticker

What does that mean?

i'd guess no part number, so no specs?

respectfully,

--pete

[HotBluePlates]

Which is fair... I took the stickers off the Hammond transformers I used in my 25L15. 

[Ed_Chambley]

It does use cathode bias for 6L6 and has, you guessed it, a UL OT without a sticker.

So the UL taps are hooked up but no NFB loop?


               Brad    

That is correct.

[Ed_Chambley]

UL OT without a sticker

What does that mean?

Pete nailed it.  No part number and no specs.  I am going to order the hammond HBP (Brian) suggested.  From the photo on hammonds Web site it looks the same and dimensions are the same.  Specifically 1650NA.  Should work well.

[Ed_Chambley]

Which is fair... I took the stickers off the Hammond transformers I used in my 25L15. 

I emailed Redplate and asked.  That is the difference.  If I asked you, you would tell me.  They wrote me back offering to sell me one of their "custom" OT's. $276.48 plus shipping.  It is not a Heybour.  I called Alden at Heybour and he told me they do not make the transformers for Redplate.  Strange, but the bell covers are not painted which is the only thing that makes me think it is not a hammond.

[Willabe]

They wrote me back offering to sell me one of their "custom" OT's. $276.48 plus shipping. 

At that price why not get a dozen?


             Brad     

[HotBluePlates]

....  Strange, but the bell covers are not painted which is the only thing that makes me think it is not a hammond.

Half the bell covers on the Hammonds I got had rust bubbling underneath the paint.

I had already planned to paint the endbells of my transformers, but the rust made it imperative (to me) to strip the paint, remove the rust and repaint.

Maybe Redplate had rust on their endbells and decided to use bare metal (or whatever they used).

I'll sell you my "BluePlate Special Transformers" for only $220/ea. 

[Ed_Chambley]

You guys are having waaaaay too much fun with the reply from Redplate.  Good information tho.  I know if I ever see a Redplate amp that needs help, I will have to get it cheap.  OTOH, they are really nice amps with some unique approaches.  I just think they smelled something fishy, like I was going to build one of their amps.  You all know I would never do anythihng like that.

Bet I can build it for less than $600 not counting that I have a cab and speaker and chassis already plus all the small parts.  The deal is I get the BF twin and $1500, but I have to give him a 75 silver twin I blackfaced.  Got that thing for $400 last week off craigslist.  Dude said he already had about 20 calls on it and the first one there got it.  Craigslist.  He lives about a mile from my office and had cash in my pocket.  He said he may have listed it too cheap.  Ya think?

[DummyLoad]

maybe redplate is using edcor iron? edcor offers unpainted end-bells and custom colours... the only other winder i can think of right now is magnetic components.

--pete

[Ed_Chambley]

maybe redplate is using edcor iron? edcor offers unpainted end-bells and custom colours... the only other winder i can think of right now is magnetic components.

--pete

Thanks for the info.  I forget about Edcor, but they do not have and specs for the UL Transformer, but will make it.  Magnetic Components only makes one currently and it is not sufficent.  I ordered the hammond yesterday, but now I have to put the build aside for a few weeks.  I have too many amps and I have to get busy getting them to a couple of stores on consignment so I can make more and have too many again.

[Tone Junkie]

Here's a simple way to do that...

...and here's another:

Daryl do you happen to have one of these with a center tap off the transformer or how i would draw it onto this one sorry. Im kind of visual and yes I am trying to take the easy way out by asking.
Thank you Bill
PS. I do have the Valve Wizards new book on power supplies I just havnt read much of it yet

[PRR]

> one of these with a center tap off the transformer

What Sluckey posted (Daryl just showed a different path).

[Tone Junkie]

> one of these with a center tap off the transformer

What Sluckey posted (Daryl just showed a different path).

The reason I asked is his showed 210-0-210 was getting 600v on the top node the the output transformer I want to use shows 410-0-410  i thought his full wave rectifier was a doubler to get that kind of voltage off of it. i need the regular rectifier set up . Actually I need to get farther in to valve wizards book sitting here in front of me then i will understand these things better.
Thanks Bill
PS off to read 

[Tone Junkie]

Ok finally  I understand the voltages showed are figurative not actual . That confused me that will work perfectly.
Thank you

[sluckey]

Ok finally  I understand the voltages showed are figurative not actual .

No, those are real numbers. A 212-0-212VAC PT is the same as a 424VAC with CT. And 424 x 1.414 = 600VDC unloaded.

Your 410-0-410  will yield 1160 and 580 volts!   

[HotBluePlates]

Yes, when I first brought up the idea of arranging the supply like this, I had the bridge rectifier in mind.

The transformer you want to use was wound for a full-wave rectifier (not bridge) configuration, and will output way too much voltage.

For what it's worth, Darryl's circuit is also a doubler, and kind of like a half-bridge.

So ultimately, you'll need a different power transformer to easily get full- and half-B+ voltage and at a reasonable voltage level.

[Tone Junkie]

I can get one and use darryls circuit with no center tap 0-385 400ma get just over 500 volts on the mains. but triode store has a bunch of dynaco replacement stuff good ul transformers could just go that route
4.3k rated at 60/120 watts i dont know how they figure the 120 watts but thats what it says. Like you said push it at 470v get 70 watts from a pair of KT88 tubes.
Thanks Bill

[darryl]

I can get one and use darryls circuit with no center tap 0-385 400ma get just over 500 volts on the mains.

This circuit is a voltage doubler. If your secondary AC voltage is 385, then the B+ is 385 x 1.4 x 2 ≈ 1078 VDC.   





You could use a bridge rectifier with your non-CT 385VAC secondary, to produce a B+ of about 539VDC, but this circuit will not provide a ½B+ voltage.

[Willabe]

Wouldn't these work with a PT with no CT and give B+ and half B+?

Or are they voltage doublers too?


              Brad     

[sluckey]

Wouldn't these work with a PT with no CT and give B+ and half B+?

I 'believe' both of those are disfunctional.

There are alternatives to getting 600/300 volts without using a hard to find transformer. You could just use a 425-0-425 (or close) to give you the plate voltage of approx 600. Then use a VVR to get the 300 for the screens. Replace the 1M pot on the VVR with two 470K resistors to get a fixed output from the VVR that's 1/2 the input.

[Willabe]

Wouldn't these work with a PT with no CT and give B+ and half B+?

I 'believe' both of those are disfunctional.

Thanks for correcting me on those Sluckey.

Sorry for the misdirection guys.


             Brad       

[PRR]

> Wouldn't these work

"B" certainly does nothing at all. Imagine ALTernating current through the transformer secondary. One way, one diode blocks current. The other way, the other diode blocks current. NOthing comes out.

"A" will charge-up the top cap, but there is no complete path through secondary and bottom cap. You will have useful DC on the top cap, no useful DC on the bottom cap.

[PRR]

The iron from a Dynaco Mk III, but with silicon rectification, will be close to your 80 Watt goal.

[Tone Junkie]

Thats what I was thinking also.