• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Stereo build- Mikael Abdellah's KT88 SE amp

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I think I'd like to start a new thread on this topic as it might help others when doing this build.

For a little background,
I'm in the process of building a stereo version of this project.
I've built 4 guitar amps, but this is my first hi-fi project. This is also my first non push-pull build, so I have a few new questions.

Sherman has been helping me with a lot of my questions, so I'm very grateful to him for all the advice.

Stay tuned, and I'll post pix as my build goes along.

Has anyone else built this design (other than Sherman), and if so, what to you think of the sound?

Thanks.
Glenn
 
Here's the last few questions I had on this topic to move this thread:

porkchop61 said:
What voltage did you use for your .22uF Auricap coupling capacitors, 450V?
I just finished my "turret board", and all I need to order now are the 2 coupling capacitors. The Kimber Kaps look good from Parts Express.
Thanks
Glenn


Sherman said:
I did use the 450V Auricaps. I think they sound very, very good and the price is not too bad.

I considered all the usual suspects but the Auris were easily available and have a good reputation. At the time I didn't remember PE having the Kimbers available but if I were building today I might well use them.


porkchop61 said:


Thanks Sherman. I only have to buy 2, so price isn't that much of an issue :D.

On another topic, I'm looking at the wiring diagram for the Hammond 1628SE OT's. On the primary side they list the the blue lead as common, then there is a red lead and a blue/yellow lead for the 40% screen tap. My question is, does the B+ go to the black lead, and the plate of the KT88 go to the red lead, or the other way around? I assume the screen (pin #4) of the KT88 goes to the 40% screen tap.

This is my first SE amp, I'm used of PP designs so this is a little different.

Thanks for all your help!
Glenn

BTW: I'll be posting pix of my progress as I go along. I just got my 1/8" thick aluminum chassis plate from UPS today. I'm going to work on the red oak base this week.
 
Power supply decoupling?

Has anyone ever used a cap-resistor-cap for power supply decoupling in the power supply of this amp?
I typically do this for all my guitar amplifiers. I usually use (2) 50uF 450V electrolytics, and a 10k 1 watt in between the caps.
If anyone has done this, what did you use for the resistor and capacitor values?
Thanks in advance.
Glenn
 
Glen, here's the answer to your tranny question:

http://www.hammondmfg.com/1627.htm

As for the second one...i'll let it to those more enlightned.

BTW, i showned my amp to some friends and they were amazed by the sound quality of it (i was expecting that :D). They had never heard tube sound, and even though the amp was strugling at 88dB speakers at max volume, they couldn't hear distortion (that's the "difference" between sand and hollow state gear).

So, go on with the project Glen! :cool:
 
Re: Power supply decoupling?

porkchop61 said:
Has anyone ever used a cap-resistor-cap for power supply decoupling in the power supply of this amp?
...


When playing my amps at high volume on a passage with a lot of bass I got some LF oscillation. I put in a decoupling filter but used only 1 resistor followed by a 47uF cap then the plate resistor. I adjusted the values of the resistors (can't remember what I ended up with off the top of my head) to get about the same plate voltage on the 6N1P. It cured my oscillation.
 
An aside to Simpleton

Simpleton said:
Glen, here's the answer to your tranny question:

http://www.hammondmfg.com/1627.htm

...

So, go on with the project Glen! :cool:


Simpleton,

Today I finally plugged in the 6922 in place of the 6N1P. I haven't done any measurement and it is hard to do A/B tests as I don't have a balance control on my preamp, but it sounds at least as good as with the 6N1P.
 
Simpleton said:
Glen, here's the answer to your tranny question:

http://www.hammondmfg.com/1627.htm

As for the second one...i'll let it to those more enlightned.

BTW, i showned my amp to some friends and they were amazed by the sound quality of it (i was expecting that :D). They had never heard tube sound, and even though the amp was strugling at 88dB speakers at max volume, they couldn't hear distortion (that's the "difference" between sand and hollow state gear).

So, go on with the project Glen! :cool:

Thanks Simpleton, but that's where I started.
The Hammond schematic still does not tell me which lead goes to the B+.
I think what's confusing me is the label of "common" on the black lead.
Does this go to the plate on the KT88, or to the B+?:confused:

Again, this is my first SE amp,
I'm more familiar with PP designs as on all my guitar amps.
Glenn
 
Re: Re: Power supply decoupling?

Sherman said:



When playing my amps at high volume on a passage with a lot of bass I got some LF oscillation. I put in a decoupling filter but used only 1 resistor followed by a 47uF cap then the plate resistor. I adjusted the values of the resistors (can't remember what I ended up with off the top of my head) to get about the same plate voltage on the 6N1P. It cured my oscillation.

Thanks Sherman.
I thought I'd have to play around with the values anyway.
I always have to do this on my guitar amps.
I'm still learning all these "Hi-Fi" tricks :D
Guitar amps are pretty narrow frequency-wise as compared with this stuff.

Thanks for all the help.
Glenn
 
Re: Re: Re: Power supply decoupling?

porkchop61 said:


...
I always have to do this on my guitar amps.

...Guitar amps are pretty narrow frequency-wise as compared with this stuff.



Glenn,
I'm in the process of building my first guitar amp right now. A single-ended EL84 combining the P1 (output stage) and P1 Extreme (preamp stages, drive and tone stack) from the AX84 site. Since I've never built a guitar amp before I'm sort of applying what little hi-fi knowledge I have and keeping my fingers crossed!

In the schematic the B+ to both preamp stages is decoupled and so is the screen if operating in pentode mode. I'm planning on putting in a pentode/UL/triode switch so I'll put in the screen decoupling filter per the schematic.

Any suggestions on good 8" speakers? If this amp turns out OK I think I'll build one for each of my sons. One plays 99% acoustic and the other is 99% electric. I assume each would need a different driver but I don't know much about instrument speakers.
 
Re: Re: Re: Re: Power supply decoupling?

Sherman said:



Glenn,
I'm in the process of building my first guitar amp right now. A single-ended EL84 combining the P1 (output stage) and P1 Extreme (preamp stages, drive and tone stack) from the AX84 site. Since I've never built a guitar amp before I'm sort of applying what little hi-fi knowledge I have and keeping my fingers crossed!

In the schematic the B+ to both preamp stages is decoupled and so is the screen if operating in pentode mode. I'm planning on putting in a pentode/UL/triode switch so I'll put in the screen decoupling filter per the schematic.

Any suggestions on good 8" speakers? If this amp turns out OK I think I'll build one for each of my sons. One plays 99% acoustic and the other is 99% electric. I assume each would need a different driver but I don't know much about instrument speakers.

Hey Sherman-

Wow, something I finally know about!
Yes, definately decouple the preamp stages as shown. This is especially true for high gain amps.

I've never built the P1, I went straight for the November as my first amp. I've also reworked a McMartin 20w (6L6 PP) with the gain stages shown on the "High Octane" amp, really awesome gain. I actually don't use any overdrive/distortion peadals on this amp as the 12AX7's sound really good with the 2 gain knobs up, and the master volume down.

As for the pentode/UL/triode switch, my November build is a rev:8 version with the ultralinear/pentode switch. I can't tell the difference myself, but YMMV.

I built my own cabinet (4X12) copy of a Marshall slant-front design. I used 4 Jensen vintage alinco from Antique Elecrtronics Supply http://www.tubesandmore.com Jensen has some really nice vintage design speakers. I also looked at Parts Express as they have an excellent selection.

I'm very happy with the sound, and the price was resonable. Of course I was after a Marshall sound, but the Celestion
Greenbacks were out of my budget.

Hey, as a side note, what speakers do you use with your SET monoblocks? I have a pair of Altec A7's in the basement that I've been holding on to for years (much to the Wife's dismay).

Glenn
 
Re: Re: Re: Re: Re: Power supply decoupling?

porkchop61 said:


...
Wow, something I finally know about!
Yes, definately decouple the preamp stages as shown. This is especially true for high gain amps.

...

Hey, as a side note, what speakers do you use with your SET monoblocks? I have a pair of Altec A7's in the basement that I've been holding on to for years (much to the Wife's dismay).

Glenn

Thanks for the guitar amp info! I'll decouple just as shown. I do need the higher gain of the preamp stages. I plugged my guitar into my SE EL84 with a single 12AU7 input/pre stage and while it sounds pretty good the volume is too low and there is no chance of getting any overdrive distortion. I'm guessing 2 stages of 12AX7 which already has a mu much higher than a 12AU7 will fix that!

I like dealing with Angela and will take a look at the speakers there. My older son has a couple of 12" 2 way PA speakers and a 4x12 slantfront cabinet so I'll get to test it with those.

I'm currently using a pair of Advent/1 speakers with the KT88 monoblocks. They are 10" 2 ways. The sound is excellent, at least to my ears. I think they are about 89db/1W/1M and go loud enough for me without distortion. I've also used them with a pair of DIY line arrays built with 4" full ranges with a simple 1uF cap crossing to the tweeters. Great midrange detail and presence but no bass punch.
 
Power supply questions

Hi all-
I was trying to simulate the power supply for this amplifier using Duncan's PSU software. I'm using a Hammond 193J choke (10H, 200mA) because this is a stereo unit, and I need a higher current rating.



I had a few questions about what to use for values in the software.
1.) What do I use for resistance values for the 2 capacitors?
2.) What type of load do I use on the end of the power supply?
Resistive, inductive? What values would you suggest for current if an inductive load is used? What value resistance if it's a resistive load?

Thank you for any help you can give me.
I'm still learning :D
Glenn

An externally hosted image should be here but it was not working when we last tested it.
 
Glenn,
I just used the default of 2 ohms for the caps.

For the load I set it to be a constant current (the amp is running in Class A so current should be almost constant). Add the current of your two KT88s and of the 6N1P sections to get the load. I fudged and simulated it at 5mA and 10mA over my expected current draw just to make sure I didn't overload the rectifier.

I was also told that using a stepped load with the initial load about 30% to 50% higher than the expected draw gives a more accurate simulation but I'm not the one to ask about that! I did simulate it that way though and everything worked fine.

One thing I've run into with PSUDII is that I believe it assumes a value of 115VAC for mains and my mains are at 120V (most of the time!). With my simulations, for every PS I've used it for, my resulting B+ has been higher than the simulated value. My sims for my KT88 PS came out at 399 volts. My actual B+ is 425V. Still well within tube circuit allowances.

BTW, I'm hoping the final items (some caps and a couple resistor values I didn't have) for my guitar amp project should arrive today. I'm hoping to start inhaling solder fumes later in the week!
 
Thank you for that information Sherman.
I'm going to be running the KT88's at the specified 75mA (I'm using the shematic component values), but I have a question about the 6N1P.
I see it's anode current is rated anywhere between 5.6-10.5mA. I believe you said that you were running at around 5mA on your monoblock (I'm assuming you only have one plate connected). Does this mean I should double that for a stereo version? After all, I'm using both sides of the tube. Maybe it's safer to estimate the current at the max of 10.5mA?
I'm getting close to the max current rating of the Hammond 273BX (175mA). I'd better watch the final B+ outcome, and bias accordingly :)

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
 
Hammond 1627 OPT

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
 

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