Stereo build- Mikael Abdellah's KT88 SE amp

[porkchop61]

Originally posted by kmtang
I believe the Red wire goes to B+ and Blue wire goes to the plate. To make sure you get the best performance from the OPT, try connect it both ways and measure it's frequency response.

Johnny

Originally posted by kmtang
I just noticed that Hammond's OPT has new configuration. The new version having all 4, 8, and 16 ohm tappings available. The oldere version either you can configured it to 4 and 8, or 16-0hm only.


Johnny

Thanks Johnny, I'll try it both ways as there seems to be some conflicting thought as to how to hook this OT up.

Yes, the OT's now have 3 taps on the output.
I'm actually adding a DP 3 position rotary switch to my build so I can select between all 3 if needed.

Glenn

[Sherman]

Originally posted by porkchop61
...I have a question about the 6N1P.
I see it's anode current is rated anywhere between 5.6-10.5mA. ...

I should double that for a stereo version?...

I forgot to ask, what are you using for a schematic for the guitar amps? I think you said it was part of 2 different designs?
Please post pix when you get the chassis done. I'd be intersted in seeing them.
Glenn

Yep, I have only 1/2 the tube connected in my monoblocks, each triode would be at about 5mA so 10mA for both running.

My guitar project is sort of a combination of the AX84 P1 and P1 Extreme. I'm planning on using the EL84 output stage of the P1 and the preamp stages from the P1 Extreme.

The circuits are very similar but the P1 Extreme adds drive and master volume controls (the P1 has only volume). Both also have bass, mid and treble tone controls. The P1 Extreme's output stage is an EL34.

With the EL84 I'm hoping for about 5 watts in pentode and UL modes and maybe 2 in triode.

I'll post pics if it doesn't explode!

[porkchop61]

Yes, I love the master volume!
Of course it only preamp 12AX7 tube distortion. The output tubes add an extra level of distortion that sounds different.
I used the "High-Octane" preamp design on my McMartin MA-20 rebuild. A LOT of gain in the preamp section! This amp uses a PP 6L6 setup for the power amp. It only has a Baxandall type tone stack, but it's pretty good sounding after a few component changes.
Good luck with the build.

Back on the subject of the KT88 SET amp-
Does anyone here use soft start circuits in their power amps? I've seen transmitters that use a resistor in the circuit for startup, and then switch it out after with a relay. I think this is more than I would like to tackle. I was thinking of using a thermistor for a soft start. I know they get quite hot when running, but they should work well. Has anyone tried these in a tube amp?

Also, I think I would like to add a "standby" switch (thats' what guitar amp builders call it) for the B+ in my build. Do builders here worry about hitting the tubes with the high voltage before they are hot? Is it really an issue?
Thanks in advance.
Glenn

[kmtang]

Hi Glenn,

I found the standby HT switch is very good and handy. I just love having it in every power amplifier I build.

Johnny

[porkchop61]

Originally posted by kmtang
Hi Glenn,

I found the standby HT switch is very good and handy. I just love having it in every power amplifier I build.

Johnny

Hi Johnny-
Yes, I always put them in all my guitar amps.
Nice to keep the tubes warm between sets so you can come back and have the amp ready to go.
Glenn

[Sherman]

[i]Originally posted by porkchop61...
Back on the subject of the KT88 SET amp-
Does anyone here use soft start circuits in their power amps?...

I was thinking of using a thermistor for a soft start. I know they get quite hot when running, but they should work well. Has anyone tried these in a tube amp?

Also, I think I would like to add a "standby" switch...

Do builders here worry about hitting the tubes with the high voltage before they are hot? Is it really an issue?
Thanks in advance.
Glenn [/B]

Glenn,
If you are using tube rectification you have both problems solved with no extra work. The rectifier won't start working until its filaments are warm enough and then the voltage slowly builds to its final value. (Actually in most cases it overshoots a bit and then settles back when the filter caps are fully charged and the rest of the circuit begins using current.) You get soft start and standby in one easy package!

I've got an amp with SS rectification and no standby switch that I've been using with the same tubes for about four years with no problems.

I've also got a preamp I built last winter in which I did install a standby switch. No way to know if it would be suffering any issues if I hadn't put in the switch.

<Guitar amp aside>BTW, UPS just delivered the package with the rest of the guitar amp parts. I'm looking forward to that build also. (It will use tube rectification. ) </Guitar amp aside>

[porkchop61]

How hot does the 25w 470 ohm cathode resistor get on this amp?
I'm just wondering about proximity to other components (coupling caps), and ventalation. Are they "back of the hand" hot, or "burn the hand" hot?
I know this is subjective, but are we in the 125 degree F range, or higher?
Glenn

[Sherman]

Originally posted by porkchop61
How hot does the 25w 470 ohm cathode resistor get on this amp?
I'm just wondering about proximity to other components (coupling caps), and ventalation. Are they "back of the hand" hot, or "burn the hand" hot?
I know this is subjective, but are we in the 125 degree F range, or higher?
Glenn

The 25W value is higher than it has to be. I think Mikael told me he thought that 10 or even 5W would be OK there but I never did the math since I had some closely match 25W resistor that measured about 485 ohms.

That being the case mine don't get so hot as to blister my skin, at least not right away. I mounted my coupling caps flush with the top and the cathode resistors so that they project down from the tube sockets. That gives at least an inch between coupling caps and cathode resistors and maybe 1/2 inch between cathode resistor and bypass cap.

[porkchop61]

Thanks, that's good to hear.
I think I'll mount mine on the side of my turret board opposite from all the other components anyway (the side facing up). This way all the heat will rise to the underside of the chassis plate.
I can then always add venting slot patterns over them if heat is an issue. I also thought about using the aluminum housed wirewounds and mounting them to the underside of the chassis, but it sounds like the heat is not that bad.

BTW: how are your guitar amp(s) coming?

Glenn

[Sherman]

I like your idea of using turret terminal board for the wiring. I soldered one end of the components directly to the tube sockets and the other end to a terminal strip. It worked just fine but a turret board would make a neater job.

Originally posted by porkchop61
...

BTW: how are your guitar amp(s) coming?

Glenn

I am trying to decide on how to hook things up. I was planning on using perf board as I don't have any turret terminal board. My original thought was using one long, narrow board but now I'm thinking of using three small boards, one for for the PSU, one for the preamp sections and one for the output stage. I think almost all the tone stack components can be soldered directly to the pots.

I just bought a couple hundred insulated press-fit turret terminals off that little auction site but I don't think I want to wait until they arrive to start.