EF86->CF->Tone Stack->EL84+6X4 Questions

[HumBuzzThing]

Hello.

I was going to rebuild some Roberts tube mono blocks from a reel-to-reel following the guidlines set out by Rod Childers and Casey Connor. Original Schematic that I started with.
http://lacinato.com/robertsmod/robertsakai_schematic.png

I soon started to get my own ideas about what I wanted to do with them and have  done some research on the internet to try and figure it all out.

I wanted to have a Gain + Treble, Bass + Master Volume. EF86->CF->Tone Stack->EL84.
I used the guidelines for the CF from Valve Wizard . The EF86 stage is unchanged from the original except for the 1/4” jack input and resistors to grid.

The James tone stack idea from:
https://www.ampbooks.com/mobile/amp-technology/james-tonestack-synthesis/    and borrowed the Raw Boost Switch from tubeswell's Blues Meister Mk II (Pro-Bolt).

I also looked at what J. Mesa did with his Tablebeast builds as he uses these Roberts Amps as well for some of his builds.  I borrowed his Line Out circuit.

The EL84 stage is basically the same except for the grid resistors which I followed from J.Mesa.
Now its time to ask some questions. I hope some here can help guide me through this.

Please see the circuit pdf link at the bottom. Thanks.


First question.
I decided to replace all the electrolytic capacitors in the circuit...just to be safe.

The power filter caps. They are currently 20uF/20uF @ 350V cans. One is an Elna. The other is a Nippon Chemi-Con which also has a 400V Surge rating.
I don't know what the Elna Surge rating is. Each mono block has a 6X4 tube rectifier.

I ordered 2 – F&T's that are 32uF/32uF @ 500V to replace them as I could not find exact values. I found out later that the recommended cap for the 6X4 is only 10uF.
I have seen examples where people have used 20uF  and have read that some have used 40uF.
The danger I hear is that on power up an in-rush of current can kill the 6X4.
So, I guess my question is, will the 32uF/32uF can be OK to use or am I just asking for trouble?
Are there some components I can add around the 6X4 to protect it from any tube killing in-rush? I should mention that the PT is 250V-0V-250V @ 60mA.


Second question.
There are some circuits(filters) around the the 6BQ5/EL-84 that I don't understand. They are AK5, AK3, and AK4.

Is AK5 and AK3 some kind of protection circuit? Should I leave them in, or would it be OK to remove them? I have seen that others have removed them...but I would like to know what they are before I do.
Maybe I would like to keep them...don't know.

I think AK4 is an NFB circuit, but I am not sure if I can still use it as I have re-purposed the 12AX7 as a DC Coupled Cathode Follower for driving the James Tone Stack. Is it an NFB?

AK4 use to go to the cathode of the last 12AX7 stage before the 6BQ5 where there was also a preamp out jack. Would I have to add another tube stage to use it.
Is there some place I could connect it to the circuit. I am not opposed to adding another stage if its the only way...there is room for another tube.

Third question.
Is where I have the Master Volume OK?..is there a better way of doing this? I am not sure about the capacitor or the grid resistors.
Would another tube stage help to implement a master volume as well as  an NFB stage?

Hope this has not been too many questions for a first post...I have been saving it up for awhile trying to answer as many as I can for myself. Guess I will have to find some good books on tubes, but for now
maybe someone will be kind enough to help out.

Cheers.

[kagliostro]

I would not use those three filters and I'll remove them (AK3 - AK4 - AK5)

about the first capacitor after 6x4, datasheet say max 40uF



and max 10uF

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6X4.pdf

if you want to be safe you can add one other PS node before the 32uF cap, a 10uF cap and .... a 150-220R resistor will prevent from problems

(on other tubes, like 5Y3, I'm sure you can use larger capacitors than those that are on the datesheet and you can find the same indications I linked for the 6x4 tube)

--

Remember that if you want to add one other tube it has a filament current consumption and we don't know if the PT can afford further load (OK, a bit of current is recovered because you planned a LED instead than a spy/pilot lamp)

--

Your configuration isn't EF86->CF->Tone Stack->EL84, it is eF86(V1)->Gain stage(V2a)->CF(V2b)->Tone Control->EF84(V3)

I'm not very skilled, but I suppose that V2a will increase highly the output signal that comes from V1 that is high because of the resistors arrangement, I think you can lower the values on V1 (so you will have also less microphonic effect) and then amplify with V2a

There are also other arrangements for your 3 tubes and I'll not add one other to have the NFB loop, one option, you can arrange

EF86>CF(V2a)>TS>Gain Stage with NFB(V2b)>Power Tube

Franco


p.s.: I noticed only now that V1 heaters are at 5.7v instead 6.3v, this will give a bit less humm and microphonic effect

[jjasilli]

You design goal is unstated.  Do you want a guitar amp or a Mic amp?  For harp or vocals?  You should make this clear.  IOW do you want overdrive? Presumably yes since you want MV. 


Only 2 placed for overdrive in this amp.  After the input tube, which conveniently has a goal pot. And power tube overdrive.


If the input tube can't overdrive the cathode driver,  then rework the input tube circuitry for more gain output.


Typically a tonestack, even a non-lossy James, is followed by a gain recovery stage to overdrive the power tube.  This might be needed if the power tube won't overdrive to your satisfaction
 


In any event, the MV is placed after the tonestack, either before or after the tone recovery stage, if any.

[HumBuzzThing]

Hi kagliostro,
...and thanks for the advice.

Remember that if you want to add one other tube it has a filament current consumption and we don't know if the PT can afford further load (OK, a bit of current is recovered because you planned a LED instead than a spy/pilot lamp)

I should have mentioned that these amps had a 6AR5 tube that was used for the oscillator circuit in the Left mono block. The Right mono block has a space for a tube socket...but no 6AR5.
So I think  the PT could handle another tube stage.
 

[HumBuzzThing]

Hello jjasilli,

You design goal is unstated.  Do you want a guitar amp or a Mic amp?  For harp or vocals?  You should make this clear.  IOW do you want overdrive? Presumably yes since you want MV. 

Sorry for not being clear. Yes, this will be used for Guitar. I don't want over the top overdrive...just the ability to dial in some dirt when I want it,  at a reasonable volume.
I am pretty sure that the EF86 has plenty of drive if set up properly.

[jjasilli]

I think the 1st step, if not done already, is to get the amp up & running stock. 


The purpose of an MV is to get preamp overdrive; then the power amp provides a clean boost to drive a speaker at whatever low vol you want.  You can't have power amp overdrive at low vol (though you use an attenuator later).

[jojokeo]

This was one of my first amps. Albiet originally it was an old Vox AC4 copy. Then I re-designed and modified it into what you are wanting: EF86 > CF > EL84 while still using the tube 6X4 recto. It's a great sounding amp! I put a switch on the CF stage to bypass it when I want and that still has all the volume and more clean & bell-like chiminess. You can bias the EF86 to stay cleaner longer or get really gritty. I also like to use the triode mode but with the decrease in gain/vol the CF is pretty much mandatory to drive the EL84. I also added another switch to bypass the tone stack for more tonal variety and here you can use the triode mode with the '86. I got a little switch happy and put the bypass cap on a switch to the EL84 - this is also a noticeable jump/decrease in gain and quality of gain. Play around like this and you'll learn a lot. I didn't like the baxandall tone stack at all on this one. I went with a Fender-ish Treb Bass. If you like, you can use another switch DPDT and make three different "mid shift" points but I'd only do something like this if you don't use a tone stack bypass or lift.

[HumBuzzThing]

Hi jojokeo,

Thanks for your input.

I got a little switch happy ...

Yes, all those unused holes on the front panel are tempting to fill...I've had some ideas too. 

...and put the bypass cap on a switch to the EL84 - this is also a noticeable jump/decrease in gain and quality of gain.

That sounds like great idea to explore. At present I have the stock 25uF cap and 150 Ohm resistor there...did you keep those values or change it up?

I would like to take a look at that Fender-ish Treb Bass tone circuit you mentioned. You wouldn't happen to have a link handy would you?

Did you ever use the NFB circuit that was in the original? Or did you remove it when you did your Vox AC4 build?
I have been wondering if I should try to keep it, or just remove it.

Again, thanks for sharing your knowledge on these.

[jojokeo]

No, no NFB was used nor wanted for this one. The pics should help answer things for you but if not then please ask away?

[HumBuzzThing]

Hi jojokeo,

Thanks for the pics...Wow. You were not kidding...lots of options.

I was checking out the EQ section. Nice! I like your EQ bypass set up too. I want to try this one out.
Looking at the schematic all the values are pretty obvious except the 250K Treble potentiometer. Is it a 250K-A, or is it a 250k-L? Hard to tell when you zoom in.

A friend of mine has some 6AB4's (7-pin) that I might try to use as a tone recovery stage after the EQ. There is a spare 7-pin socket on the chassis so maybe in the future. I have options. I am going to try it without first.
J. Mesa used a 6AB4 in one of his Tablebeast builds...probably because there was a spare 7-pin socket. Makes sense.

I am also going to remove the NFB circuit...from what I have been reading it's not going to contribute much to this build anyways.
On another note...I decided to order a WX4 Copper Cap. More for peace of mind since my filter caps are 32uF/32uF. This would also give me a spare 6.3V/0.6A to use if I need. Maybe I could use that if I decide to add a 6AB4.

Thanks for sharing.
Cheers.

[jojokeo]

Treb pot is linear. It'll work either way it's just that the taper will affect the control feel differently. So rather than what you'd get set at 50% with a linear you will have to have set maybe at 7 or more to get the same amount. Then the latter part of the control using an audio will be overly sensitive than it would.

I advise against using any extra tube stages. It is not needed and will change the character and sound for everything. You'll see how well it works even without the CF stage. There's not as much signal loss that most think there is. I even halved the signal if you noticed leaving the EF86? This is an important aspect also. Without it you get too much bad distortion at the upper parts of the preamp volume and makes the amp's setting work well all the way through the dial settings. You can't get a bad tone out of this amp the way it is and all settings work well throughout their settings.

I have it housed in a small combo donor cab fitted with a 10" Weber Blue Pup.

[HumBuzzThing]

Thanks for the advice and sharing.
 Have most of the parts I need to change on hand. Going to try to re-use as many of the wire wound resistors as I can too. All caps are going to be replaced.

I will post pics and schematic when complete. I will include voltage reading on the schematic too.
Hopefully before the end of the month. February is starting to get booked up in stagehand world.

Cheers.