AB763 KT88 Question

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RB
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AB763 KT88 Question

Post by RB »

I have a head that is basically a AB763 reverb only - 12AT7 PI - 2X6L6
OT is Classic Tone BFBassman 50watt 4K - PT is 200ma 320-0-320 with solid state rectifier.
Heater spec is 5A so no problem with 2x12AXY 2x21AT7 and 2xKT66/6550

I am looking for the shortest path to a little more power/SPL - clean headroom (more speakers not an option).
Is there any thing to be gained using KT88s with current PT and OT? What would I do to run
KT88s in this amp without replacing OT/PT?
Would 6550s be a possible option?

If the above is a ridiculous approach, What PT and OT values would result in a realistic and economical solution?

Thanks in advance for any and all thoughts and recommendations.

Regards
Randy
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martin manning
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Re: AB763 KT88 Question

Post by martin manning »

6550 and KT88 are basically the same thing. Just swapping tubes won't get you very far, though. To get any power increase you'd have to run a lower primary impedance, and increase the PT current and OT capability accordingly.
Stevem
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Post by Stevem »

All of the above like Martin posted, with the PT current being a big part!
In light of the cost of all that you might just concider doubling up on the speakers which will do more then even jumping up to 80 watts of good clean power!
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pdf64
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Re: AB763 KT88 Question

Post by pdf64 »

Yes, by far the shortest path to more SPL is a more efficient speaker.
What are you currently using?
Unless it's an EVM12L there should be a few options.
RB
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AB763 KT88 Question

Post by RB »

Thanks for all the replies.

Well the speaker is an EV. Also have a couple other 101+db speakers by eminence.

So looking around at possible xfmr choices I favor the a431s OT and the p782s PT Dynaco iron offered by Triode Electronics

After you figure in the cost of tubes and the xfmrs and power supply parts, screen supply issues and new bias circuit the cost and complexity exceeds that of installing 4X6L6 and twin iron (OT and PT). Which was what I intended to do in the first place. But I convinced my self that 2 tube should be simpler and cheaper.

The current chassis (which was to be small special with reverb)has enough room for the twin power section. I thought I would save weight etc with the KT88s but at this point I don't think so.

If anyone has another option for xfmrs I would be happy to know about it.

Regards
Randy
pdf64
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Re: AB763 KT88 Question

Post by pdf64 »

I just had a play with KT88 using IVDS and it looks like >70 watts may be achievable providing that the HT is reasonably stiff and maintain ~420V and the LTP can provide sufficient signal swing.
http://www.bmamps.com/Tech_tds.html
The PT must be able to provide another 1.4A heater current!
That's compared to 57 watts with 6L6GC, so 1.8dB more power with KT88.
Last edited by pdf64 on Tue Feb 14, 2017 11:18 pm, edited 1 time in total.
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M Fowler
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Re: AB763 KT88 Question

Post by M Fowler »

You can do whatever you want but I would not be placing any order with Triodestore right now.
RB
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Re: AB763 KT88 Question

Post by RB »

Thanks Mark

I had just finished reading the remarks about Triode in another area of the forum when I found your post.
There was a real great guy that worked for them a few years ago. He really gave me great service but
when he left they jacked me around big time on a large order and I have never been back. Dog only bites me one time.

I was going to contact Mag Componnts and see if could get the iron directly from them or maybe amp parts direct.
If not I will see what Heyboer could do for me.

I think on this chassis I am going with the twin power section - I have all the parts and I can knock it out in an afternoon.

I think I will do one from scratch with a Jon Mayer/TwoRock style front end and something like a Sunn/Dynaco power section

Thanks for all the input I love this forum.

If any one knows of a another source for Dynaco style iron please let my know.

Regards
Randy
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Mr. dB
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Re: AB763 KT88 Question

Post by Mr. dB »

You will need more B- voltage for the bias supply of a 6550/KT88 amp than is usually supplied by the bias tap in a power transformer made for 6L6 or EL34 amps.
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Mr. dB
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Re: AB763 KT88 Question

Post by Mr. dB »

RB wrote:
If any one knows of a another source for Dynaco style iron please let my know.
http://www.dynakitparts.com/dynakit-pro ... ansformers
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Leo_Gnardo
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Post by Leo_Gnardo »

RB wrote: Wed Feb 15, 2017 8:08 pmIf any one knows of a another source for Dynaco style iron please let my know.
Bob Latino, I think he's in Sturbridge Mass. Lots 'o Dynaco and Dyna inspired iron, kits, modern drive boards etc.

Gets good reviews, also gives good advice on Audio Asylum & other spots.

http://bob01605.50webs.com/

Have a look at Sunn's early tube amps, they were built with Dyna iron, output amps were copied directly from Dynaco.
down technical blind alleys . . .
pdf64
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Re: AB763 KT88 Question

Post by pdf64 »

With another diode and cap, a voltage doubler can easily provide heaps of bias voltage.
For reasons unknown, some 60/70s UK amps had barely adequate bias supplies, and so voltage doublers could be a handy mod.
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