12AX7 - 8 pin sub

[TIMBO]

 Hi guys, got a little amp that I want to add a plexi type preamp to.
It's a 10w PA and the chassis is in good nic.
Four sockets, 3x9pin and 1x8pin.
I was going to use the 8pin for the cathode follower prior to the tone stack.
I'm thinking 6SL7 or 6SN7.
Would either do the job in that position, or is there another 8pin that would do a better job. Thanks

[John]

With my limited experience, I really like the 6SL7 myself.

[TIMBO]

Thanks John, When searching 12AX7 8pin the 6SL7 and 6SN7 are what comes up.

[sluckey]

The gain of the 6SL7 has a gain of 70 (12AX7 is 100) and works well with slightly different plate and cathode resistors. It will also work as a CF to drive a tonestack.

The gain of the 6SN7 is only 20. It works well as a cathode follower or PI.

[TIMBO]

Thanks sluckey, Updated schem to be similar to your 6V6 plexi.
I've got a couple of options for the BIAS supply..
One of the old Diason amp I have uses the heater supply but its grounded on one leg, so I could use a small transformer.
OR, adapt a marshall style bias supply, but need to get down to around -10v.

[TIMBO]

Hi guys, Bias supply sorted.

[TIMBO]

Hi guys, Power valve grid leak resistors.
I'm sure this has been discussed. 
These range from what I've seen, 100k - 470k
The 6gw8 usually have 470k.
How do these resistors effect the power valves. Thanks

[shooter]

I took this quote from the link, maybe it helps

"I think the thing to bear in mind is that there is not a 'correct' grid leak value, just a range of acceptable values. If it is too small then the bias will be stable, but it will load the previous stage - RDH says at least 4 times the anode resistor of the previous stage. If it is too big then the bias shifts. The data sheet gives the maximum, usually for cathode bias. For fixed bias RDH suggests a ratio of 1.2-2 (e.g. 1.5 close enough?) smaller - my figure of 5 was a wild guess, so please ignore it."

http://www.diyaudio.com/forums/tubes-valves/169041-choosing-grid-leak-resistor-value-partly-subjective.html

[HotBluePlates]

"... grid leak value ... If it is too small then ... it will load the previous stage ... "

Read this as "possibly reduce the gain and/or maximum output voltage swing before distortion of the phase inverter."

If you go too big, leakage current at the grid tends to charge the coupling cap at the output tube grid.  This can shift the bias of the output tube (in either direction, but "more-off" seems to be problematic) such that a big transient afterwards causes blocking distortion.  The usual cure for blocking distortion is to reduce the R-C time constant for that grid leak/cap, by making the R or C smaller (or maybe both) and/or raising the grid stopper R.

If you were designing the amp at the drawing board, you'd select the grid leak value based on data sheet recommendations and its loading effect on the previous stage (for a.c., the plate load of the previous stage is its actual plate R in parallel with this grid leak R).  Then you'd size the coupling C for the bass response you want (and perceived risk of blocking distortion).  Test on the breadboard and jigger R & C as needed to prevent blocking and decrease mud when overdriving the output tubes.

[TIMBO]

Thanks guys, I kinda understand 
My aim for this circuit is to get the 6GW8 to show their "ANGRY' side.
The 6GW8 is used in many of our vintage low watt amps up to 15w and most are cathode bias and are fairly subdued.
I'm am using a fixed bias and the screen voltage close to the plate voltage.
Happy to take some guidance.Thanks

[TIMBO]

hi guys, wiring finished and fired up well with only one problem.
The "TONE" isn't really working.
With the knob at "0" all is good with some good bass, but when turning up it gets a kinda blocking distortion.

Also sluckey mention that the plate and cathode resistors of the 6SL7 may need some adjustment.


Missed voltage Pin 9 V3 - 248
                      Pin 9 V4 - 255

[John]

I think you need a grid stopper on your PI ... V3-1.

[TIMBO]

Thanks John, Found this in schematics.
http://el34world.com/Forum/index.php?action=dlattach;topic=567.0;attach=43374;image

[TIMBO]

Hi guys, a bit more tweaking.
I found a schematic at diy audio, http://www.diyaudio.com/forums/instruments-amps/255890-could-use-second-opinion.html
Seems all he did was change the cathode resistor from 820 to 1.5k.
All seems to sound great, except when turning the tone pot to half way it get ugly but gets better when turned up full.

[John]

Hmm. On DaGeezer's SoHo, coming off the cathode follower he had a 470K || with a .01 (? I think) I believe that was for frequency/tone shaping, but blocking distortion is just the audio signal getting chopped off, or something like that.    Merlin does an excellent job of explaining it. I'm guessing the tone pot is reacting with capacitance in some way, but I'm not nearly smart enough to explain why (or probably understand it when someone else does)


Something to try is increasing that 47k grid stop to 220k, just to see if it helps. Also, I usually put larger grid stoppers on my power tubes, anywhere from 47k to 82k. FWIW.

[shooter]

A WAG, halve, or double the 15K to ground in the TS and see if that "moves" the problem up or down the dial

[TIMBO]

Thanks guys, I'm having no joy with solving this problem.
I changed components, pots, put in grid resistors even a master volume and the problem doesn't change.
I'm using a 100k plate resistor and 1.5k cathode, I'll go back to a 820 cathode resistor.
Sluckey mention that these values may need to be tweaked.

[kagliostro]

Give a try to the VOX AC30 values



Ciao

Franco

[John]

just for something to try, if it's not too hard, is move that .022 before the tone pot. IOW, coming straight off the cathode, like you normally see it drawn coming off the plate. No idea why it would make a difference though.


Have you tried bypassing the tone pot altogether? Like, using clip leads, hook in that .022 straight from V2-6 to V3-1.

[sluckey]

That cap is necessary to prevent the tonestack from interfering with the bias on the LTP PI. There's a healthyb voltage on the grids of V3 and V4.

I wonder if you are just overdriving the 6GW8s?

[TIMBO]

Hey sluckey, I think you have hit the nail on the head.
I changed the cathode resistor on the 6SL7 back to 820r and this seemed to fix the tone pot problem.
Now that I have done that I'm getting the blocking distortion when I increase the volume on the amp, but I am able to increase the amp volume if I decrease the volume on the guitar.
So I have added a few bleed resistors to reduce the signal strength.
The dark channel is worse as you would expect.
I have the 6GW8s bias to 14w.
I'm thinking of increasing the 6gw8 screen resistors to 2.2k as the plates and screens are close in voltages.
I may drop the Node "D" voltage to see if that helps or even add another RC filter.

[John]

That cap is necessary to prevent the tonestack from interfering with the bias on the LTP PI. There's a healthyb voltage on the grids of V3 and V4.

Ah, so that's why it's after instead of before. Thanks!

[sluckey]

It's all a matter of your point of reference. I like to think of that cap as being BEFORE the LTP PI grid. 

[sluckey]

Timbo, give this a try... Put a 100K resistor between V3 pin 1 and the junction of that .01µF and 1M resistor.

[TIMBO]

A few pics of this monster.

[sluckey]

Timbo, give this a try... Put a 100K resistor between V3 pin 1 and the junction of that .01µF and 1M resistor.

Did that resistor help?

[TIMBO]

Hi sluckey, Pretty much had all the wiring tidy up but was a bit late to give it a bash.
I'll post a final schem a voltages today.
From the little bit of quiet playing it seems to have helped.

[TIMBO]

Hey sluckey, No real progress.
I'm only able to get to 3 on the dial before it gets ugly.
I'm thinking of rewiring the 6sl7 as the input and the 12ax7 as the TS driver.

[John]

Hey, I just thought of something. Try elevating your heaters by 60 or 70 volts DC. That cathode follower might be having issues with too high a voltage between cathode and heaters.

[kagliostro]

I was wondering if 100K will be too much for the CF cathode ???

Franco

[TIMBO]

This is where I'm at...........
I measured the output with the scope and found the point that with wave starts to clip at the OT output.
This is about 2 on the volume for each channel.
Tone pot set at mid point of the pot.


TP1

TP2

TP3

TP4

TP5

TP6

TP7

TP8

Dark volume off

Bright Volume OFF

[shooter]

the point that with wave starts to clip at the OT output.

looks like all the scope shots were from the OT?  If so walk it back, I'd start at V2 pin 2.  You might need to use/change voltage divider to "tame" the pre-amp drive.  I try on my builds to get clean pre-out til I hit around 6-7 on gain pot.

EDIT:, your PA bias tap shows -6 to -17, anything bigger than the actual bias will start bending the PA tubes,  I believe.

[TIMBO]

Hey shooter, The boys over here are certain that the PI is overdriving the 6GW8s.
I have now rewired the PI and output similar to the Marshall 1930.
Some new pics.
Both channels set at 1.5 on the dial, this is the point of onset of clipping at the OT output.


TP1

TP2

TP3

TP4

TP5

TP6

TP7

TP8

Dark OFF

Bright OFF
The amp still has an ugly distortion after 3 on the dial.

[kagliostro]

Just my 2c

Give a try to a partition divider between TS and PI and/or Split Load resistors on PI

Franco

[VMS]

my 2c:


Bigger grid stoppers on output tubes.


Split the 100k resistor on the cathode follower and take the tone stack from the junction.

[kagliostro]

Split the 100k resistor on the cathode follower and take the tone stack from the junction.

Yes, I would do a test also in that way

Franco

[TIMBO]

Hi guys, If you wish to see what is unfolding. http://www.guitargear.net.au/discussion/index.php?topic=47673.0

[John]

Interesting stuff!

[TIMBO]

Hi guys, A new name.
Thanks the one of the local blokes, the amp is now sorted.
I can't really explain what he has done, but now I have more useable volume to 8 and good drive after that.







I'm not much of a git man, it sounds like a small marshall.
I'm hoping that i'll get someone to do a demo.

[tubenit]

Timbo,   

I just read this thread & some of what you described were similar problems I found developing the SoLow Watt amp.  It had too much overdrive too quickly and some of the mids sounded harsh.

Ultimately, the SoLow Watt turned out to be a great amp with remarkable bass tone using those small tubes. The fixes were:

-  smoothing caps across plates resistors
-  .0047 & .02  LTPI cap values
-   enhance cap across LTPI plate resistor
-  .01 post LTPI coupling caps.
-  sort of unusual tone stack cap values
-  PPIMV

The amp is now owned by a guy (lead guitarist) of a band that opened up for Dave Matthews and also Derek Trucks. He is a great guitarist, IMO.  Last year when I spoke with him,  he said it was his #1 amp for playing at home for years.

With respect, Tubenit

[TIMBO]

Thanks T, I'm trying to find someone to do a clip, but no joy so far.

[shooter]

Nice rebuild!

I'm trying to find someone to do a clip, but no joy so far

I have 2 amps I'd like sound clips for, I'm having the same problem here in farm country, my next best shot is FEB

[tubenit]

I will look forward to the sound clips when you (both) are able to make those happen!

With respect, Tubenit