4 x KT88 Guitar Amp using M-125 Iron Set, now with pic of driver circuit

I would like to thank Bob for allowing me to buy a set of transformers that are used in the M-125 Monoblock





I am planning on using them to build a high power (up to 200W) guitar/bass amp with 4 x KT88 or possibly 4 x KT120 amp. The complete driver circuit consists of a EF86 DC coupled into a 12au7 long tail pair with an ECC99 being used as DC coupled cathode follower off the 12au7 plates to drive the output tubes. The grid leaks for this amp are about 50k so in order to provide the largest possible signal into that kind of load a cathode follower is used due to a very low output impedance. They also can provide some nice compression effects when driven very hard, one of the signature aspects of the early Marshall 60s era tone. The EF86 is a small signal pentode which provides the bulk of the drive into the phase inverter, and it has an adjustable gain level via adjustable bypassing on the screens which acts as a infinitely variable pentode/triode control. The chassis above is a really great powder coated chassis I just got from a new supplier.

Thanks

Nick

I build a fair amount of KT88 based amps, heres the latest example using some 2 x KT88 dynaco repo iron



Hi res gut shot found here, click the link as it is way to big to directly link here

https://2img.net/r/ihimizer/img13/8645/img0163wh.jpg

NICE!

Keep the posts coming on this cool project and photos are appreciated!

mantha3 wrote:NICE!

Keep the posts coming on this cool project and photos are appreciated!

thanks its going to be a little bit. I have 2 more amps before it to build, 1 prototype amp, 11 Fuzz pedals, 2 delay/verb units.... Lists just keep getting longer.

Heres the one thats currently on my bench, this is kind of an artsy farsty shot

Very nice indeed. Do post pictures as you have time. Where do you source the componet holder/carrier (long black strip with posts)looks nice and clean.

Mark

metlmrk wrote:Very nice indeed. Do post pictures as you have time. Where do you source the componet holder/carrier (long black strip with posts)looks nice and clean.

Mark

Those are from Watts Tube Audio, they are really nice double sized turrets on a small piece of turret board, a touch pricey but well done. They come in sizes from 2 turrets up to probably 20+.

I use the same board and turrets for the main circuit boards I custom make for the amp.

If people are further interested I can be found on facebook (search dunwich amps)or you can check out my webpage

dunwichamps.com

Very cool...you've got another "like" on FB!

Blitzen wrote:Very cool...you've got another "like" on FB!

nice, more fodder for spam. ha just kidding.

Heres an older build I did, 2 x KT66, 40ish Watts.



1W stereo headphone amp with 2 band EQ on both sides, for my grandpa

I'm not sure I should ask this and probably normally wouldn't but since you posted a schematic of the KT88 amp could you possibly post a schematic of the headphone amp?

I am 100% NOT familiar with guitars or guitar amps. What do the Mode and Presence pots do?

ALL of your work is fabulous, just stellar!

tomlang wrote:I'm not sure I should ask this and probably normally wouldn't but since you posted a schematic of the KT88 amp could you possibly post a schematic of the headphone amp?

I am 100% NOT familiar with guitars or guitar amps. What do the Mode and Presence pots do?

ALL of your work is fabulous, just stellar!

sure sure no problem heres the schematic for one channel of that headphone amp, one change you should make is change volume pot to 100k, too much miller effect with a 500k pot there, zaps some presence on your music when not cranked

Heres a link, its a huge image

https://2img.net/r/ihimizer/img685/1227/tubeheadphoneamp.png

As far as the Mode and Presence are concerned. The Mode is an infinitely adjustable variation between Pentode and Triode operation on the EF86 driver tube. The EF86 is a small pentode tube often used in guitar amps dating back to some of the old Vox amps. Its also sometimes found in phono preamps or areas where extremely high gain is needed. Small signal pentodes can have gains of 200-300 compared to say a 12ax7 which has a theoretical max gain of 100 unloaded. So if you look at the mode when turned all the ways to one extreme, the Screen of the EF86 will be completely bypassed by the 470nF cap which is grounded while the 470nF cap on the plate is more or less floating with a 1M load to ground. This is pentode operation

When the Mode control is at the other extreme, the screen and plate caps are connected in series, and effectively from an AC standpoint, the screen and plate are shorted, which is Triode mode. However since they are not DC connected this produces an interesting triode characteristic where the tube curves are shifted by the screen voltage.

Now anywhere in between is some variation between those 2 modes, and even somewhere in the middle is a psuedo ultra linear operation. When operating in pentode, the stage has maximum gain, while in triode is has ~70% less gain, while in the psuedo ultralinear it is the minimum gain. This control allows you to adjust the gain into the phase inverter, effectivly controlling headroom. The clipping characteristics also change accordingly, and this makes the control very useful for tailoring the sound

Presence controls how much higher freq content in the feedback loop is cancelled in the long tail phase inverter. So what this does is boost the upper freq ranges in the power amp by not cancelling them in the feedback loop. When you shunt the highs to ground (max pres) all freqs below the cutoff are feedback while anything above is not. This is a very traditional guitar control dating back to early Fenders and Marshalls

Very interesting. Is this what it's "all about" with respect to tube guitar amp sound? I guess what I'm asking is a 25 word or less explanation why the best guitar amps are tube based. I suspect there are even more compelling reasons for a tube guitar amp than a tube hi fidelity amp.

Finally is it possible to replicate (gasp!) these effects with solid state devices?

tomlang wrote:Very interesting. Is this what it's "all about" with respect to tube guitar amp sound? I guess what I'm asking is a 25 word or less explanation why the best guitar amps are tube based. I suspect there are even more compelling reasons for a tube guitar amp than a tube hi fidelity amp.

Finally is it possible to replicate (gasp!) these effects with solid state devices?

When I think hifi versus guitar/bass, I think hifi is about reproduction, and guitar/bass is about production. In guitar amp designs we are purposely coloring the sound to our liking.

Why do tubes make better guitar amps than solid state, it has to do with clipping mechanisms, the onset of clipping in a tube is much more subtle and most people would describe it as more musical in the end of the day while solid state tends to clip harsh and quickly. Given the whole idea behind a guitar amp is to introduce distortion into the signal, the way a tube clips makes it more liked than solid state. NOW, to understand, this is just some of my opinion in there 2. Take this with a grain of salt.

As far as replication, yes there are a lot of amp companies who make DSP based emulations of tube amps in the hopes of emulating them. Theres lots of debate over whether or not this sounds good or bad. IMO, they can try to imitate but they never truly emulate the feel and sound of a tube amp

Interesting thread... I'm following it!

Nice work Mr. Dunwich!
The project I'm saving for is a ST-120
with a custom layout and I'm inspired
by your phone amp.
Thread well done!

Also a question:

Does the phono/line switch on your headphone amp
powerdown the phono tubes (are there any?)
to preserve their hours when not in use?

Tom

Tom wrote:Nice work Mr. Dunwich!
The project I'm saving for is a ST-120
with a custom layout and I'm inspired
by your phone amp.
Thread well done!

Also a question:

Does the phono/line switch on your headphone amp
powerdown the phono tubes (are there any?)
to preserve their hours when not in use?

Tom

Thanks

No it does not power down the tubes, just changes the signal input. No problems yet. It would have become complicated to say remove all the cathode grounds for 4 total stages and also switch the inputs

Heres some more spamola

The bias section in my amps looks like this from the inside,



And from the back panel



this way users can bias the amp without removing the chassis. you can measure the B+ on the output tubes which is conveniently converted down into mV to avoid shock, and you can also measure the cathode current on each tube via 1 ohm 1% sense resistors on the cathodes of the output tubes. There are also 2 locking screwdriver 1W pots to adjust the negative bias on the grids of the output tubes

Thanks, I've heard the "too complicated" comment before.
I built the SP-8 preamp with phono and line stages on the same board,
and it burns me that the phono tubes are burning
while I'm listening to the line stage.

Haha!

Tom wrote:Thanks, I've heard the "too complicated" comment before.
I built the SP-8 preamp with phono and line stages on the same board,
and it burns me that the phono tubes are burning
while I'm listening to the line stage.

Haha!

As far as I know, no commercially produced (single chassis) tube preamplifiers have ever offered a provision for separately powering up the line and phono stages. Yes, it increases the complexity which also increases the cost. The difference probably being enough to purchase a spare set (or two) of replacement tubes.

Best Regards,
Charlie

Ah! That rings of challenge.

If cost, complexity or even rationality
had anything to do with,
we'd still be listing to our first Close-n-Play.
(albeit with a cartridge upgrade)

It could be done but requires some thought, You could have a 3PDT switch which 2P switch the signals while a 3P actuates a bunch of relays which remove the cathode grounds on the phono pre stages. that would be costly in terms of space and complexity but doable if you really wanted it

dunwichamps wrote:It could be done but requires some thought, You could have a 3PDT switch which 2P switch the signals while a 3P actuates a bunch of relays which remove the cathode grounds on the phono pre stages. that would be costly in terms of space and complexity but doable if you really wanted it

And do you really WANT relay or switch contacts on the cathode grounds? Everything that flows in the plate circuit...and that is
what constitutes the sound that the tube stage "makes"...flows through the cathode, too. Whatever you do at the cathode, affects the sound.
Last time I looked, a straight soldered connection is still better than the BEST contact.

I say, "not worth the effort."

GP49 wrote:

dunwichamps wrote:It could be done but requires some thought, You could have a 3PDT switch which 2P switch the signals while a 3P actuates a bunch of relays which remove the cathode grounds on the phono pre stages. that would be costly in terms of space and complexity but doable if you really wanted it

And do you really WANT relay or switch contacts on the cathode grounds? Everything that flows in the plate circuit...and that is
what constitutes the sound that the tube stage "makes"...flows through the cathode, too. Whatever you do at the cathode, affects the sound.
Last time I looked, a straight soldered connection is still better than the BEST contact.

I say, "not worth the effort."

well i merely suggested how it could be done not that I would do it. I dont see much harm just letting the tubes idle all day which is why I just had switched inputs.

dunwichamps wrote:
well i merely suggested how it could be done not that I would do it. I don't see much harm just letting the tubes idle all day which is why I just had switched inputs.

Agree with you, there! Typically tubes in low level stages have a very long life anyway, so the gain isn't worth the effort.

GP49 wrote:

dunwichamps wrote:
well i merely suggested how it could be done not that I would do it. I don't see much harm just letting the tubes idle all day which is why I just had switched inputs.

Agree with you, there! Typically tubes in low level stages have a very long life anyway, so the gain isn't worth the effort.

i dont do much hifi stuff so I am typically ignoring hifi concerns in the design, that headphone amp was a special gift for my grandfather's 50 wedding anniversary and my 1st foray into building a stereo amp. Most of the time I am thinking about how I can absolutely make people deaf with huge guitar rigs.

build is just about to get rolling

here are the turret boards I fabricated for the build



the choke, power cord, and that small enclosure is for a footswitch



some other random items, tubes, a small hammond PT for the bias supply

main circuit board and main power board are coming along