Low B+ voltage on EF86 preamp section.

[HumBuzzThing]

Finally finished assembling the Roberts reel to reel mod.
I didn't know if I should post this in that thread...that was over a month ago.
Work got in the way and delayed this project. Figured I should post to a new thread and include a link to the older one.

Here is that link.
http://el34world.com/Forum/index.php?topic=22857.0

I borrowed jojokeo's circuit for most of this. Here is the updated schematic I used for this project.
See pdf at the bottom.


The components around the EF86 – R6, C2, R7, R9, C3 are the same as the original components.
For the EL84(6BQ5)  - R17 and C15 (Cathode resistor and Bypass cap). All of these value were very close to jojokeo's, if not the same.

I was dealing with unknown B+ voltages, so I kept  the B+ dropping resistors the same as the original.
I measure 287V and 278V(B+1) at the 32uF/32uF filter caps.  B+2 = 242V and B+3 = 128V.
These voltages are with all tubes in.

Sounds good. Plenty loud for my needs.
The only thing I am wondering about is the seemingly low B+3 voltage for the preamp section. Does this seem a bit low for running a EF86 and the 12AX7?

The 500K at the gain stage starts to “brown” out too quickly.
It would be nice to keep it a little cleaner. From what I understand this could be from a lack of headroom caused by the low voltage at B+3.
So my question is:

If I lower the value of the dropping resistor ( R13) from 47K to a value that would give me in the ball park of 180V-200V for B+3  would that upset the apple cart so to speak?
Would this mean (probably) adjusting the other values around the EF86, such as the Cathode and Plate resistors. I would appreciate some guidance.

I found on the internet what looks like a generic “Average Plate Characteristics” graph would I be able to use this with the EF86, or is there a specific one out there for this tube?
If there is I have not found it yet.

Hope someone can help.
Cheers.
--... ...--

[sluckey]

I would definitely increase the voltage to the EF86. Here's a schematic with voltages for an EF84 based amp (Vox AC-15 lite) I did a few years ago. Sounds good. Just try increasing the voltage. Heck, I'd probably start by taking that 47K down to 1K. You're not gonna hurt anything.

http://sluckeyamps.com/dual_lite/dual_lite.pdf

[terminalgs]

it looks like SW3 is way to bypass the top-boost-esque gain stage/CF.. I don't think the ef86's high output impedance (250K) will drive it. You might try moving SW3B's "out" pin.5 all the way to R15.  I guess if you flip both SW3 and SW2 the right way it'd be ok.. I'd still try bypassing all the tonestack and tone caps to see how the ef86 sounds.


I'm not sure what you mean by "Brown out" sound.  with a 250K plate resistor on the EF86 I'd expect loads of gain coming out of it, which would overdrive your 12AX7 pretty quickly.  If you want more clean range on the 500K gain pot while going through the 12AX7,  I'd consider a voltage divider between SW3A pin.3 and the grid of the 12AX7. stick a 500K trim pot in there to adjust.  (also you do want ground reference (resistor) on the 12AX7 grid for when you are bypassing the 12AX7).


no problem lowering  R13 to 1K to 10K.  You might even separate that power supply filter cap C7 into one for the EF86 and another for the 12ax7/gain/cf (with a 10K resistor in between the two).


What is the voltage on the plate of the EF86?

[HumBuzzThing]

Hi ... thanks for helping.

I should mention that there is a Right Mono Block and a Left Mono Block.
The two are/will be in separate head boxes. So everything I measure/ change is on both.

Changed R13 from 47K to 8.2K.
Voltage at B+3 is now 202V. B+2 = 234V and B+1 = 275.

Voltage on the Plate of the EF86 is 155V for the Left and 148V for the Right.
Before I swapped the Dropping Resistor I was getting 94V at the Plate.

The both amps have cleaner gain and louder volume now.

Other readings I am getting for the EF86 are:
At pin3 I am getting 1V on both. On pin1 68V on both.
All other measurements regarding Left and Right have a 5V difference with the Right
being 5V hight than the Left.

Before I dropped the 47K to 8.2K I should mention that they were fairly close to matched
in sound level with maybe the Right being ever so slightly brighter in sound than the Left.
Now all settings except the Cathode Follower section are somewhat matched.

With the Cathode Follower switch In the Right amp is WAY louder than the Left.
If I can get the Left side to match the Right I will be pretty happy.

I am just about to check through the wiring to make sure I didn't disturb anything on the Left
Hope that's all it is. Here are some voltage reading from the Gain/CF stage.

Just thought I would post this now so people don't think I am ignoring...had to wait for the neighbours to wake up before I started making noise.

12AX7                 Right:            Left:

b-3 to pin8      -     0V                 0V
b-3 to pin3      -     120V             131V
pin8 to pin6    -      119V             131V
pin6 to B+3    -      81V                70V
pin3 to pin1    -      82V                71V
pin8 to pin7    -      1V                   1V
pin3 to pin2    -      1V                    1V
b-3 to pin7      -      0V                    0V
b-3 to pin2      -      120V              132V
b-3 to pin1      -      203V               203V
b-3 to pin6      -      120V               132V

Going to poke around the Left to see what I may have disturbed. Will post again later.


--... ...--

[HumBuzzThing]

OK...the reason for the huge volume difference on the CF stage was a faulty switch. I just bypassed it and everything seems OK.
Left mono block is plenty Loud. 

 (also you do want ground reference (resistor) on the 12AX7 grid for when you are bypassing the 12AX7).

At this point I am thinking on disconnecting switch for CF Bypass on both units. Would not need a ground reference resistor then.
I am wondering if input to gain stage should have a Grid resistor though. My last build had a 270K on the input of the Gain Stage to the CF.
Not quite sure what value would be appropriate here.

Cheers.

[terminalgs]

At this point I am thinking on disconnecting switch for CF Bypass on both units. Would not need a ground reference resistor then.
I am wondering if input to gain stage should have a Grid resistor though. My last build had a 270K on the input of the Gain Stage to the

If the grid of the 12AX7 pin 7 is directly connected to the RV10 gain pot, then there is no need for a resistor to ground (the pot provides the necessary ground reference). 


putting a resistor there  (besides changing to the taper of the pot) will change the overall value of the pot between the outside terminals. This will change the impedance of the pot which factors into the the load that the EF86 sees as well as the low freq. roll off of the C1/R3/RV10.   When the pot would be at "10",   impedance of the pot would be 500K||R.   If R is 10M, it doesn't matter.  If R is 270K,  it matters quite a bit.