AX84 SEL 6BM8

[Auke Jolman]

Hello all,

I drew up a schematic with a AX84 SEL preamp combined with a 6BM8 PP powerstage. The schematic is attached.

I wonder if I did it all correctly. Specific the HT section. Further more I'm planning on using an pri 8K OT. Will this be suitable or is there a better option?

[sluckey]

V4-A plate resistor should connect to node C rather than ground.

8K OT is probably fine.

Power supply looks functional. But I don't think the 1M and extra diode are needed. What is your intended purpose?

You are using node A for your 'elevated heater' voltage reference. That node has a lot of 120Hz ripple. I would use node B or C or even node D or E for a much cleaner B+ reference.

[kagliostro]

Before I was on my cellular phone and there I can't open .sch files

maybe it will be easier for someone to have one image of the schematic



Franco

[Auke Jolman]

Thanks for the replies sofar.

@ Kagliostro:

Thanks for converting the schematic. Did you convert it into a bitmat from within ExpressSCH or did you used an external program. I can always print it as a PDF if that's ok.

@  Sluckey:

Indeed the plate should connect to node C.

The diode and 1M res come from ValveWizzard (http://www.valvewizard.co.uk/standby.html). The diode should prevent possible arcing due to the DC-power and the resistor is there for ground reference. I'm not sure if it's really necessary or wether the 220K res is still necessary in case I leave the diode and 1M in.

Your advise on connecting the elevated heater connection to the HT makes a lot of sense. I will change that.

[Auke Jolman]

I've added a new schematic. I've deleted the diode and 1M res after the standby switch. Furthermore I corrected the plate-connection of V4-B and changed the elevated heater connection to node E.

I'm going to try and make the layout. I will post it as soon as I've got a working version.

[kagliostro]

Ciao Auke Jolman

Did you convert it into a bitmat from within ExpressSCH or did you used an external program.

When you have the .sch file open with Jschem program, on the File options you can select between Export Sheet and Export Schematics

if you select Export Sheet then you can select also an image format for the exported file

--

Sometime I use Adobe Acrobat to save .pdf files generated with Jschem as an image file

this is from your .pdf file using Acrobat



Franco

[Auke Jolman]

Hi All,

Hope you've had a wonderful X-mas and I wish everybody a successful but most of al healthy 2018.

I've added the layout based on the added schematic which has changed a little with regards to the previous version.

I'm looking forward for your support and comments. 

[Auke Jolman]

I attached the wrong schematic pdf  . The right one is added in this post

[Auke Jolman]

Hi All,

Finally I finished the amp today.

All the voltages seemed to check out alright without the tubes in it so I decided to put the tubes in. Fired it up again. This time sound came through with a lot of hum and oscillation. After switching the OT-leads that problem was solved easily. And that was it! I never ever build an amp with so little changes to get it running.

May be I got lucky, but in all honesty I must say that I never planned and drew up the plans so exhaustively as with this amp. Also the help I got with checking the schematics here on the forum was a big plus. Thanks for that.

The voltages are a bit lower than what I expected them to be, especially on the preamp tubes and the sound was a bit on the dark side. So I decided to change the dropping resistors to 500R and see how that effects the tone. The preamp seemed to open up a bit more and not so dark, at least to my ears.

Is it Ok to use such low dropping resistors?

I've attached the last schematic an added some pictures of the build also.

[92Volts]

Looks good, very neatly built!

Small dropping resistors provide less power supply filtering. If you notice noise (120hz "buzz") it might be improved with larger resistors. Stages closest to the input are most sensitive, while requiring less voltage to handle the small signals present there. So the last dropping resistor is a good candidate to be a higher value, while keeping the others as you have now.

[PRR]

> Is it Ok to use such low dropping resistors?

I don't know why they are so low.

Small drop resistors invite trouble, both motoboating and buzz.

Those final tubes are very sensitive; you don't have to have huge supply voltage on the stages before them.

You have three gain knobs; you will diddle to shape where the distortion happens.

You only have three volts on those drop resistors. Insignificant. Thirty volts drop won't hurt, and gives 10X the reduction of sneakage through the B+ line. Get some 2K 5K and 10K resistors.

[Auke Jolman]

Thanx for the replies 92Volts and PRR.

The PT I used has a 240V and 220V tap. Initially I used the 240V tap. After checking the heater voltages I found that they where 5.8V. This made me thinking I was using the wrong tap, so I changed it to 220V. The heater voltages are now 6.3V.

After reading the comments I changed the dropping resistors to 4K7, 5K6 and 10K for the first preamp tube. I've added the new schematic reflecting these changes. Also I wrote down the voltages of each tube.

The amp now sounds overly bright to my ears with piercing highs. So I clipped the 0.001 cap on the first voltage divider after gainpot 1. The amp sounds less bright, but not as articulate as with the cap in, which made me decide to put the cap back in.

My question now is what is the best way to tame the brightness a bit

[DummyLoad]

swap the .68uF bypass with 2.2uF-4.7uF. use test leads to strap in a 1uF to 2.2uF before soldering. try bumping up values of the remaining bypass caps as well - use test leads.

--pete 

[Auke Jolman]

Some progress I would like to share.

Tried different things with clipping them into the circuit as Dummyload suggested. This is a neat trick to get where you want to go without the need of soldering everything in and out of the amp.

I got some oscillation when I turned the gain pots up. This made me want to try a JJ ECC81 in V1. This did it for me. If somebody has some other suggestions that I can try, please let me know.

Thanks again for all the help and support. I get back when I've done some woodworking. All my builds are in need of a nice cabinet. I don't seem to get around to do that. This time I want to change that.

[terminalgs]

The amp now sounds overly bright to my ears with piercing highs. So I clipped the 0.001 cap on the first voltage divider after gainpot 1. The amp sounds less bright, but not as articulate as with the cap in, which made me decide to put the cap back in.

My question now is what is the best way to tame the brightness a bit

I would remove all the cathode bypass caps on the first 3 gain stages and just focus on one (maybe the first, because that's traditionally the one that produces the max gain and overdrives V2),  I'd start with a big cap, like a 25uf to make it simple,  then see if you can fiddle with your tone-stack to get whatever range of highs, lows, and mids you want the amp to be capable of.  If you take low freq. out of the signal before the tone-stack, you can put them back in at that point.


On your tone-stack, I would change the way you've wired the 500K pot to be a variable resistor between those two junctions (like you commonly see with vox,marshall,fender,etc).


Also, isn't the cathode bypass cap 10uf on the output stage quite a low value?


As for the ECC83 vs ECC81:   survey all the great tube amp schematics,  you'll never find a ECC81 at V1.  the ECC81 is a good work horse tube (LTPI,reverb driver,etc) but isn't anything special when it comes to preamp gain duty.  All you've done is lower the overall gain exiting each stage as it hits the grid of the next.  I would stick your best ECC83 back in  V1 and change your voltage dividers to render the same +dB at each stage that you have with the ECC81.


It might be easier to tune if you change your first couple of preamp voltage dividers to what is pictured below right.  If you make R1 & P1 = 500K  you'll get the same end result as what you have at left.  When pot is at "10" A->B is 50%,  When pot is at "5" A->B is 25%.  There is a change to low freq. roll off from the CR filter, but you move the cut off from something like 30Hz to 10Hz... so no big deal. It might mess with your .001||220K, but something like a 330pf||500K might give you same thing.





the advantage if this is you can stick some 1M or 2M pots in there as "R1" trim pots,  and then tuning the amp can be done while the amp is powered on,  and you don't have solder/desolder, etc.. 


Stick a ECC83 back in there, fiddle with the trimmer "R1"s (in that R1/P1 arrangement) and don't be worried if the second stage has a 1M on top of a 500K pot.  If that's what sounds best!

[Auke Jolman]

Hi All,

Back again with some changes.

I've put an AX12A7 in the V1 position again and changed both voltage dividers to a set of 470K to grid and 220K to ground. That gave me kind a what I wanted. Decided to put in a switch to alter the cathode resistor and bypass cap of V2 (ala Dirty Shirley). This gives me some more gain options. Also I replaced the 100Ohm dropping resistor for a choke I had lying around. This was a huge improvement.

I struggled with the cathode resistor of the poweramp. After searching the internet I filled in the calculator on Rob Robinette's website (https://robrobinette.com/Tube_Bias_Calculator.htm). After filling in the forms i got 470 as the cathode resistor. I calculated the bypass cap based on 10kHz cut off (33 uF). I made these changes to the amp.

The schematic is attached as are some pictures of the amp in it's cabinet.

When I play the amp loud with a lot of distortion there's something going on that I don't understand. The sound is coming through alright, but when I stop playing the amp is dead silent for a fraction of a second just as if there's no signal coming through. When I play another note the amp responds. When I dial the gain down enough I don't hear this happening. Can somebody explain to me what is happening?

[Auke Jolman]

Hi All,

Turns out I was hitting the PI to hard. That caused the problem I described in my previous post. To resolve this I wired in a pre PI MV as a trimpot. Turning the post PI MV all the way up and both gain knobs dimed, I turned the pre PI MV so far down so that the notes did not degenerate.

I changed most everything back to the original schematic (see attachement).

The amp sounds super now and very versatile gain wise. I'm very pleased with how it turned out.

Thanks for all the help and support.

[Auke Jolman]

Hi All,

I stated in my last post that the problems where caused due to hitting the PI to hard. I learned now that that  was not the case. I discovered that when I had nothing in the passive loop everything was Ok. As soon as I put something in the loop, like a Boss DD7 delay, the sound changed as described earlier.

This made me decide to put an active fx loop in the amp, like the one described by Tubenit in his post about his HoSo56 minimalist & 6BM8 tubes.

I played the amp for like 1,5 hours and all looked ok, then suddenly the bypass cap of the cathode resistor blew up. I'm at a loss what to do.

Can somebody give me some pointers where to look please.

A photo of the blown resistor is added.

[sluckey]

Replace the cap. Be sure to connect the positive end of the cap to the cathode of the tube.

[Auke Jolman]

Well I be damned... Somebody wired the cap in wrong. Don't know who that could be

@Sluckey: thanks for putting me on track. Much appreciated.

The amp sounds super now.

[PRR]

> Well I be damned... Somebody wired the cap in wrong. Don't know who that could be

In this house, I just blame the dog.

[sluckey]

Well I be damned... Somebody wired the cap in wrong. Don't know who that could be

Don't feel special. You ain't the first! 

[Willabe]

> Well I be damned... Somebody wired the cap in wrong. Don't know who that could be

In this house, I just blame the dog.

 

[Auke Jolman]

Hi All,

For some time I've been enjoying this amp very much. Until today.

Suddenly the amp got way louder with hum that increases when I turn the PPMV up. Also it distorts almost right of the bat even with the gain turned way down. I've pulled V2 and the hum did not go away. Turning the PPMV up makes the amp go into oscillation (squeals like a pig).

Please some advice as to what to do

[Auke Jolman]

Hi All,

It's been a struggle to find what was wrong with the amp. It turned out to be a little piece of wire making a short to the filament of one of the powertubes 

Everything is ok now.