Please help to choose schematics for a KT88 PP

[AnatolyAA]

Dear Colleagues,
My father and I made our first SE tube amplifier and now are preparing to create a more powerful KT88 PP.
In the internet, I have found several schematics (attached, including one from this site).
Could you please help and advice which one is the better to choose? Or may be something else...

Also I would be glad if you could advice where to get output transformers suitable for it.
Thank you in advance!

 

[bigwill]

Two of those schematics are basically Williamson designs. They will work well but will be difficult to make stable and will likely require a fair amount of experimentation to do so.

As is often recommended a Mullard style circuit will likely be a lot easier to implement, while yielding good results

 

[petertub]

All three are way to complicated versions that may have stability problems unless one has access to some instruments.

Build a dynaco MkIII, transformers are available at dynakitparts.com and others, so is schematics. Driver tube 6an8 is available as NOS at reasonable prices. That would be a first-class amp to get up and running.

Using that amp as testbench you can elaborate with the other designs and see for yourself what differences will show up.

 

[waltube]

Hi

try this

P-P with EL34/KT120


Walter

 

[AnatolyAA]

Thank you for the replies! My father has about 50 years of experience in working with electronic stuff (as being a head of a service on an atomic icebreaker), so I am rather confident in him. But he also asked me to find an oscilloscope for a reasonable price.
I will show him the MkIII scheme. Does it have any advantage over the Williamson design (besides being easier to build)?
How would a Mullard style PP look like?

 

[petertub]

Williamson has a longer signal path, which will make stability more of an issue.
The extra driverstage might be needed in some combinations, but other such as
MkIII has enough "driving juice" by a single 6an8.

 

[smbrown]

Try this one:
"Mullard 5-20 KT88 PP blocks!

I built the version using the 12AT7, also used 5k, 100W output transformers. I eliminated NFB except for UL operation and changed EF86 pentode to EF86 triode. Works great, love it and so does a buddy who also built it. It is now my main amp and has been for a few years.

 

[Eli Duttman]

I agree with the Mullard circuit suggestion, as the topology is "idiot resistant" and tends to work well with O/P "iron" whose "pedigree" may be slightly shaky.

As for an o'scope, I have no idea if a Soviet era item can be sourced, at modest cost. A USB model, with software for your computer, might be the best bet. Shipping charges can be very nasty, when a heavy item is transported over a long distance. USB o'scopes don't weigh much.

 

[AnatolyAA]

I was shipping electroguitars from the USA, so, yes, the transportation can add pretty much on top. I have been looking through the variety of USB o'scopes. Can you advice some model / producer?

 

[artosalo]

I can recommend Dynaco style circuit based on excellent soviet 6F12P triode-pentode.
I have used this circuit from 10 W 6V6 to 100 W JJ EL509 PP amplifiers.
Attached the schematic of pentode connected EL509. For KT88 I recommend to modify this to UL output stage and 100 k grid resistors, if fixed bias is used.

 

[Andrewbee]

JJ EL509 amp - Artosalo

Hello Artosalo,

Do you have a Power Supply to go along with your EL509 design to share?

Also, did you have any faulty JJ EL509's?

I have read on the Transcendent Sound forum where they are used in OTL's where they say to to buy extras as they often have failures.

I do not know how they are run in the OTL's though.

I am considering another project, high power Pentode to play Magnepan MMG speakers and your amplifier looks like it may be suitable.

Thanks


Andrew

 

[artosalo]

Yes I have. I have an universal type that has adjustable G2 and G1 voltages.
Schematic attached and a photo. The PCB at left is this version.
For JJ EL509 the screen grid voltage was 150 Vdc.

I have had no failures with EL509.

 

[Andrewbee]

Thanks Artosalo.


Andrew

 

[AnatolyAA]

Hi, Friends!
Thank you for all the posts. We had some issues and finally came to the step of searching for transformers and other parts.
The basic design would be the Dynaco Mk 3 as attached.
I was just wondering before we get started whether the KT88 SE 13Wt as attached is in any ways better that the Dynaco Mk3? Does anybody have thoughts, experience?