MAXIM Clone

[TIMBO]

Hi guys, This one has worked its way to the top of the "TODO" list.
http://www.ozvalveamps.org/maxim.htm
http://www.guitargear.net.au/discussion/index.php?topic=32531.0
The transformers came from a donor 100w PA kit amp and they are HUGE.

[jazbo8]

It's going to be a cool amp! Are you sure that V1a and V1b's cathodes are tied together?

[TIMBO]

Hey Jazbo, At this stage i'll have to accept it as correct.
The original amp circuit was traced and it had the two resistors in parallel and tied to both cathodes. The copy was then redrawn by a guy that knows his way around an amp, he redrew it as my schematic.
I thought I would try it this way first and see what it does.
There is now original circuit schematic to be found.

[jojokeo]

Hey Timbo,
That was a good read about that amp as I've never heard of one before. And with KT88s seemingly a hot topic lately it was fitting. Also a nice amp building pioneer story to boot. In looking at the design apparently the KT88 mounting and cathode bias recommendations were not followed by that gentleman for some reason? See attachments:

[TIMBO]

That's VERY interesting.

[TIMBO]

Hey Jojo, I have looked at a few other builds and I do see them at about 4" apart and further, but not aligning the pins.
Now that the heaters are sorted, I'm looking for good place to put the STANDBY and HT fuse.
 

[SILVERGUN]

Hey TIMBO,
I was just talking to jojo about having a couple driver tubes like this after the PI so it's a nice coincidence to see this here.
 
Is there another page to the schematic?
I see it now on the .sch file in the op
 
As an idea...this might make a good spot for a Master vol.

[jojokeo]

As an idea...this might make a good spot for a Master vol.

Agreed! This way the driver's inputs can be controlled in the same way as the grids of power tubes would be in a normal ppimv situation. With a Vol before and after the phase inverter you could/should be able to dial that sweet spot in at will? And if wanted - control not just preamp distortion but also phase inverter distortion too? But SG this is a cathodyne pi and not a LTP as you've been using correct?
 
Now I don't want to get Jimbo going but in the Major - it also used a cathodyne correct? (shush, we he doesn't know won't inflate him  )
Is that SK7 tube an octal version of a AU7??? (and it already has your 47k load resistors  )

[SILVERGUN]

But SG this is a cathodyne pi and not a LTP as you've been using correct?

Yeah...and while were at it I'd have to try a balance pot off of the cathode there
 

Is that SK7 tube an octal version of a AU7??? (and it already has your 47k load resistors  )

Well...it's listed as a variable mu pentode....so it also might be pretty cool to try VVR on just those plates as a Master?
..or even just VVR on g2?
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/6/6SK7.pdf
 
OK,,,I'll stop now...too much coffee

[jojokeo]

Have you seen the 6V6 tube being used as a regulated supply that feeds the KT88(s) from that site/schem? In contacting Mal he gave me the schems and info directly on this.

[SILVERGUN]

Have you seen the 6V6 tube being used as a regulated supply that feeds the KT88(s) from that site/schem? In contacting Mal he gave me the schems and info directly on this.

Yeah...Timbo shows it in his .sch file

[TIMBO]

Thanks guys, I'll bring this thread http://el34world.com/Forum/index.php?topic=19706.0 into here just to tie them together.
So with that out of the way I can push on with some fun stuff, OR can I.
The OT............
Nice big 100w chunk of metal, 4.5k PP with secondary taps of 500,279,124 and 31ohms.
So the 31ohms is a no brainer and as 30-32ohm speakers cabs are a bit rare and the head weighing a ton this will be a piggyback setup.
Now the secondary winding is made up of four separate windings of 31ohms each.
Is it better to use just one winding for my speakers or as it is at the moment the four windings are paralleled together for one output, or will two windings paralleled together for 2x31ohm speaker connections cause problems.Thanks

[TIMBO]

Some progress........

[PRR]

Parallel all four windings for 31 Ohms.

Unless you find 125 Ohm speakers, there is no other good connection.

One single winding loaded in 31 Ohms will have increased loss, you may be down 10% on maximum power output; worse at the top of the audio band. Each winding is not a "impedance", it is a voltage. 31||31 does not make 15. You can't put four 32r speakers on the four 31r windings, the amp will see 8r at sec and 1.1K at plates.

You can use a second transformer (more weight!!) to step-down 32 to 8 or whatever. In fact a 200-300VA *power* transformer with 120/240 windings may work well, cheaper than "audio" iron.

[TIMBO]

Thanks PRR, The four windings were paralleled when I got it. I'm hoping that who ever did it has grouped them with the right polarity, as I have read that this could be a problem.

[PRR]

> this could be a problem.

Yeah. Zero output would be a problem.

[TIMBO]

Thanks PRR, Next question.
I thought I better check where the best position for the speaker jack should be, where the wires won't upset anything.
Between the EF86 and the tremolo oscillator, OR
extend the wires and put them between the two 6SK7.
I would think as far away from the preamp tubes.


Also in the first pic, the shielded cable is just sitting on the chassis, but when the chassis is turned over there is nothing holding them in place. I have in the past put a few dobs of glue to hold in place, but can make it a bit difficult to remove wires if needed.

Any ideas???

[sluckey]

I would put the speaker jacks between the fuse holder and that pot (humdinger?) Keep all OT wires away from the preamp tubes and preamp circuit on the board. I would also swap places with the choke and OT to minimize OT wire lengths and to get the OT far away from the preamp area.

I think your shielded cables are fine. I would not tie them down. Your circuit board will hold them in place. I run my shielded cables under the board and don't tie them down.

[jojokeo]

Timbo going for the mustard mojo. Yeah baby yeah (channeling Mike Myers)

Looking good so far and agree with sluckey about OT wiring & output jacks' location. You can get away with this sometimes depending on wire placements but it can be real sensitive issue here. Placing them even just a little bit in different positions (when close to preamp stuff) can make a lot of difference depending on the build's gain and with high powered amps that generate a lot of current. with them being under your circuit board this can be problematic and you won't know for sure until you have everything installed and power up for first several times. If you leave things as they are I suggest you run them sideways to the right then turning upwards to the output jacks but you will likely have to lengthen some of the ones you'll use. But that's okay and better than the shorter direction near the preamp sockets currently.

[TIMBO]

Thanks guys, this is how the trannys sit.

[sluckey]

I could tell that from looking at the bottom. That's why I said I would swap places with the OT and choke. You may not have any issues with the OT where it is but I would not tempt fate. It's easy to do now, but may be a real chore to swap later on.

[PRR]

Hanging long shielded wire off a *resistor* seems risky to me. Not for a bedroom beast, no. But on your world tour the road-vibration will crack something somewhere between Tokyo and Lodi. Your choice.

The "right way" would use an honest terminal lug to hold the resistor-cable point, and perhaps some little wire-clips to take the quiver out of the longer cables.

[TIMBO]

Thanks guys, ON IT.

[TIMBO]

Yeah should look at the Marshall Major/JCM chassis before drilling.
Moved transformers, much better.
PRR, I agree with you on the resistor/cable joint.
This is the reason I put the heatsrink over the joint and end of the resistor.
The shielded cable has about six strands of hair thickness wires and even soldering it to a tag strip is flimsy.
I found some stickon cable fasteners.

Is there such thing as a 1Ma pot that has a 70%(350k)tap????? Thanks

[TIMBO]

Hi guys, My next problem.
There seems that a 1Ma tapped pot don't exist.
Doug has 350k and I have a 250k.
Should it make a huge difference. Thanks

[jojokeo]

Hi guys, My next problem.
There seems that a 1Ma tapped pot don't exist.
Doug has 350k and I have a 250k.
Should it make a huge difference. Thanks

I don't think so Timbo since it only feeds into the EF86 and is only part of the overall preamp but you may try either changing the lower 150K resistor to 470k and/or adding a resistor above it - say 100k - 220k and seeing how these affect things or not? It will only increase or decrease the bass response but as long as you're happy with the end product and have enough bass shouldn't be a big deal.

Another more drastic departure from being an exact clone would be to simply replace the tone stack with a standard Treb/Bass type but that would make J1 & J2 inputs a little closer to the same is all. J1 appears to be a bit brighter and J2 more full with a higher gain response.

[TIMBO]

Thanks Jojo, I'm not too out of shape with making an exact clone, close is good enough.

The interesting thing about this amp is that it has this odd tremolo channel, as the a lot of the amps that are out there are single/dual channel similar to the lower circuit and have the same output circuit.

[TIMBO]

Well guys I thought I'd search through my stash of recycled junk and there it was.
And as a bonus it has a pull sw. Might be good as some kind of boost sw.

[TIMBO]

Good news, This has fired up well.
With a few minor issues, I hope to be able to sort out.
It has a typical type hum that PA systems suffer.
The tremolo is the biggest problem.
The DEPTH pot is very scratchy and when increased the speaker cones nearly fly across the room, there is a faint effect but can't get the full effect as I do not want to damage the speakers.
The SPEED works but this also has flapping effect on the speakers with a lot of thumping.
The overall sound is great, but I don't have the speakers in a cab as yet.
Also I don't have the 6SK7, so I'm using 6SJ7.
The EF86 is having microphonic problems.

[jojokeo]

Good news, This has fired up well.
With a few minor issues, I hope to be able to sort out.
The EF86 is having microphonic problems.

Good news Timbo. I had a feeling the ef86 would have issues using the values I'm seeing. But first try to deal the tremolo since it's connected to it's screen. Both the load and screen resistors are very high values, even though I don't know what they B+ node voltage is? Using an ef86 in that position and set-up for high gain will scream with microphonics using even normally quiet/good pentodes. Normally lower load resistor & screen both together to keep the same ratio and allow a higher voltage to hit the tube thus lower gain and getting microphonic control.

[trobbins]

Speaker cone apoplexy - mounting the speakers in a cab will certainly help.  Perhaps use a safety resistor from depth pot wiper to top of pot, to alleviate DC voltage shift.  Also perhaps aim to attenuate LFO frequencies after the EF86 more - you have two 33Hz high pass filters before the PI, and PI output is just 15Hz.  Perhaps they all need to have their corner frequencies raised.  Another path of attack is to attenuate LFO harmonics prior to the EF86 screen, possibly by adding in an extra CR filter between depth wiper and coupling cap - sometimes a capacitor shunting V4a anode to ground can further attenuate harmonics by loading the internal plate resistance at higher frequencies.

I suggest the first remedial path for microphonics is to try all your EF86 collection to identify the best performer.  Even with suppressed gain circuit values, it is still a wise move to separate the valve socket from chassis by 1-2mm thickness neoprene rubber to allow the tube and socket to 'move'.  Best if wiring underneath does not restrict movement of socket.  Next step up is to add some mass to the valve, so that with the compliant mount, the valve doesn't experience vibrations that reach the chassis as much.  And then use rubber feet for the chassis.

[Paul1453]

Even with suppressed gain circuit values, it is still a wise move to separate the valve socket from chassis by 1-2mm thickness neoprene rubber to allow the tube and socket to 'move'.  Best if wiring underneath does not restrict movement of socket.  Next step up is to add some mass to the valve, so that with the compliant mount, the valve doesn't experience vibrations that reach the chassis as much.  And then use rubber feet for the chassis.

Excellent information shared.   
Not all of us are near your guys level of understanding.
But I'm learning more every day I read your posts. 

I had wondered why some tubes were mounted on those floating island type setups.
Microphonic shock absorption, OK now I get it.
I thought it was some kind of ground isolation thing.       
It also seems you have experience in improving the shock resistance of these setups. 

[trobbins]

http://dalmura.com.au/projects/Microphonics%20in%20valves.pdf

Link to one article on microphonics - if you're not sure whether those sounds are real or just in your head!

[TIMBO]

Thanks Tim, a few dobs of silicon and it works a treat.
I have built a cab for the speakers to rule out any problems that may arise from the speakers sitting on the bench.
There is still a problem with the tremolo and this is way over my head.
This is another circuit that is similar.

But depth pot wired different.
Is there away to cut and paste these circuit together???

I'm getting fizz and I would think that its coming from the rectifier, any thoughts on quieting it down. Thanks

[trobbins]

Nup - the 6BE6 is specially made for using control grid to modulate input grid signal with lower gain - there is still a decoupled screen - so lots of 'grids'.


Notice that the LFO has a lot of attention to filter out harmonics of the fundamental LFO frequency - both on the oscillator output, and after the 6BE6.


Why did you try and modulate an EF86?

[TIMBO]

Hi guys, There has been an interesting twist to this story. http://www.guitargear.net.au/discussion/index.php?topic=46089.0

[TIMBO]

Some great progress has been made....
First off, it turns out just because "J" comes before "K" that this is just the difference in output and the pin arrangement is the same, doesn't make it the same type of tube. Ones a sharp cutoff and the other is a remote cutoff.
So with a mistake of the tied cathodes of the 6SK7 resistor of 2.2m which should have been 22m. The purpose of the 6SK7 arrangement may not only be a driver for the output valves but also a type of compressor.
Two problems to tackle............
https://www.youtube.com/watch?v=8hxY6tHYkUY&feature=youtu.be
The sounds we have so far.......
https://www.youtube.com/watch?v=bJgygNskqj0&feature=youtu.be

[sluckey]

This filter circuit from a Vox AC-15 will probably eliminate that trem pumping. Here's the orig schematic...

     http://el34world.com/charts/Schematics/files/Vox/Vox_AC15.pdf

[SoundmasterG]

If you want to try some 6SK7's I think I have some good used ones around here somewhere. Though I think you're in Oz right, which would make shipping expensive.

Greg

[TIMBO]

Thanks Greg, got a supplier here in OZ.
Sluckey, the filter works well but I still have a bit of a swooshy heart beat kinda sound.

The tremolo was weak, so I had a bit of a play with the DEPTH pot as the tremolo would only kick in when the pot wiper got close to the plate connection of the pot. I reduced the pot to 100kb and this was much the same, added a 100k between the filter cap and the pot and got a stronger tremolo, up it the 220k and get a fuller range of tremolo.
Changing this has also given me a better range of speed as well.

[TIMBO]

Hi guys, I have been given an alternate circuit in the hope of escaping the thump.
It fitted well into the circuit board with little mods.

This works but is very weak and the speed is only kicks in at half rotation of the pot and way too fast at full.
Ideas. Thanks

The 1M beside the .0047 has been removed.

[Lauri]

Use a 3M rev log pot for speed like Fender did and you'll get much more usable range. If the tremolo is too weak lower the value of the 470k resistor and if that's not enough remove the 220k after the depth pot.

[sluckey]

You're gonna have a hard time driving the 6550 grids with that circuit. The 6G16 trem circuit has a cathode follower that will help get a stronger trem and works well with 6L6s, but I've never seen it driving 6550s, so I can't say if it will be as strong as you want. Maybe if you use a LED in the oscillator cathode.

[Lauri]

One way to get more tremolo signal would be to increase the values of the depth potentiometer and the 0.1µ capacitor but with a larger pot you would be way above the maximum grid 1 circuit resistance for 6550.

Another way to do the tremolo would be to take the LFO signal to the suppressor grid of the EF86. For this to work properly the screen voltage should be kept constant. But with this kind of circuit you might have the thumping and ticking problems you were trying to get rid of in the first place.

[PRR]

I think the trem oscillator could be a lot beefier. Don't starve the poor thing with big resistors. 100K plate resistor, 1K cathode resistor. I don't see why the 220K on the depth pot. The 470K feeding the pot reduces the max wobble-- you need it, but maybe not that much.

Whether beefing the LFO is enough, or you should buffer as Sluckey says, I do not know.

[SoundmasterG]

You might get a bit better trem depth with a mosfet source follower also. I tried that in an amp along with LED biasing of the oscillator and had way more depth than I needed.

Greg

[TIMBO]

Thanks Greg, I was going in that direction as I do not have a spare triode to use.
If you have a schem to guide me would be great.
I have a ready made one on a tag strip that's ready to go. thanks

[kagliostro]

Ciao TIMBO

Basically what you need to know is here




The Zener is a must you can see also in Tubenit's builds

Franco

[TIMBO]

Thanks K.

[kagliostro]

I think that all can be of your interest is in the schematic and files I previously posted

however I forgot to give you this link

http://milas.spb.ru/~kmg/irf_en.html

Ciao

Franco