stereo SE kt88 build ... abdellah diyaudioprojects design

[jmillerdoc]

Hi,
I am too in the process of collecting parts for this amp. I am still finishing a set of Oddblocks from the same site. The first one sounds spectacular, second one is half way done. I mention this because I was thinking of doing this SE KT88 design as a monoblock too but, as I am nearing completion of the Oddblocks I am finding I wished I would have done them on a single chassis to cut down on time and parts. Anyway, I am thinking of using the high end Edcor 5K OPT for this design. I had such good luck with Edcor on the Oddblocks I thought I would try them here. Plus I would have no idea of where to get a James trafo. My question is has anyone used Edcor for this build. Also, how much and where can one find James OPTs if I changed my mind?
Jeff

[porkchop61]

That's why I did mine as dual monoblocks on one chassis, saved me the duplication of the power supply.
I could also use the whole single 6N1P

[hoerath7]

hey i have scrounged together all my parts now ... i was going to go with the jensen signal caps but the kimbers are cheaper so i just ordered those from parts express.. I am starting to make some layout considerations and would really like to have the tubes forward. Normally I know that the mains tranny sits comfortably between the output trannys' but with the choke and huge capacitors I am not quite sure what to do. I will post some pictures of layouts that I am sketching up as soon as I get the time. If anyone has photo's of their finished se kt88 amp that they can post links to that would be greatly appreciated. I know a lot of people have this amp built. Maybe I can just copy someone else's layout.

Last big question. Is there a way to make it so I don't have a standby switch? What needs to happen so I can get by without one. I assume it is mostly related to capacitor voltage ratings but what should they be? I would like to have a simple lone power switch if possible.

thanks guys!

[jmillerdoc]

In reference to the schematic above I have 2 Questions....If you throw the switch to drop the resistance on the cathode of the power tube is that to allow the EL34 tube (i.e. leave the switch open for the KT88)?

How are the 3 star grounds connected? Why not use a single bus ground?
Jeff

[tim614]

Last big question. Is there a way to make it so I don't have a standby switch? What needs to happen so I can get by without one. I assume it is mostly related to capacitor voltage ratings but what should they be? I would like to have a simple lone power switch if possible.

thanks guys![/QUOTE]

as i understand STANDBY is so you can let the filament heat up before you apply power to the plate. you dont need it than just take it out.

[jmillerdoc]

Tim,
Take a look at Bruce Heran's Oddblock design at diyaudioprojects.com

He uses a very unique Octal plug relay that gives 30 sec before connecting the HT. The contacts are rated for 125VAC nominal with a "max when new" (whatever this really means) of around 500V. Because of this he uses the 60N30 relay to set off another higher rated relay. I have used the 60N30 with 450V running through it without any problems so far, ommitting the second relay. It does give quite a spark display within the 60N030 however. I figured I would try to omit the second relay knowing if the 60N030 failed with the higher voltage I was using it to switch, I only risked a $4.00 60N030 tube relay device. I have switched my amp on probably 100 or so times without failure using the relay at higher than rated voltages without any problems.

The device I am talking about Bruce uses is found on the KT88 Oddblocks project.

It looks really nice too and can add some asthetic symetry to the tube layout in a project as well. I have plans to use this same device in my build of this project too.

Sincerely,
Jeff

[Tube-Head]

Why use a standby modus if you got a tube rectifier ? tube rectifier turns up the voltage very slowly

[tim614]

Originally Posted by jmillerdoc

Tim,
Take a look at Bruce Heran's Oddblock design at diyaudioprojects.com

He uses a very unique Octal plug relay that gives 30 sec before connecting the HT. The contacts are rated for 125VAC nominal with a "max when new" (whatever this really means) of around 500V. Because of this he uses the 60N30 relay to set off another higher rated relay. I have used the 60N30 with 450V running through it without any problems so far, ommitting the second relay. It does give quite a spark display within the 60N030 however. I figured I would try to omit the second relay knowing if the 60N030 failed with the higher voltage I was using it to switch, I only risked a $4.00 60N030 tube relay device. I have switched my amp on probably 100 or so times without failure using the relay at higher than rated voltages without any problems.

The device I am talking about Bruce uses is found on the KT88 Oddblocks project.

It looks really nice too and can add some asthetic symetry to the tube layout in a project as well. I have plans to use this same device in my build of this project too.

Sincerely,
Jeff

i see!
i have to research this relay tube.

[tubelab.com]

I added the 100R resistor per Tubelab.com's suggestion. I believe he commented that it didn't matter if this was left in place for all three modes of operation (correct me if I'm wrong George).

I prefer to have the resistor in the circuit for all 3 modes of operation. It will help prevent oscillation and may save the tube from a blown screen grid if the amp is heavilly overdriven. This is especially important with EL34's.

EL34's may work better with a slightly larger resistor value, but 100 to 150 ohm seems to be the best compromize if you intend to try EL34's and KT88's (and 6L6GC's).

[88man]

I've been away for a while... Thanks Glenn (Porkchop61) for responding to the question of the 100R. I am curious if there is any affect on the microdynamics, frequency bandwidth, or the output impedance?... I'll try it.

I don't know why an uncommon Russian preamp 61NP tube was used in the design? I read that some have tried using a 6DJ8 and 6922 tube, but seems to suffer from limited bandwidth. I am wondering what brand of NOS 61NP tubes will work well with an EH KT88?...

jmillerdoc, I am found the James 6123HS transformers on ebay. They're quite expensive. You can Google it, that's what I did.

hoerath7, I am deciding against a standby switch since it's going to be overkill in that the rectifier tube will have an inherent "soft start" as the filaments heat up in the start up cycle before it goes to the amplifier tubes.