Sunn pre-amp to much gain, hard to read schematics

[nateflanigan]

A long time ago I built an amp based on a sunn 100s.  It came out sou ding really good, but one thing that's always bothered be is that it get's loud way to fast and I'd like it to stay clean a little longer.  Like the usable range on the volume pot is 0.5 to 2. 


Last night I fed a sine wave in to the the amp and scoped it at different stages.  It comes out of the first triode, clean and with much much more amplitude.  Then I put the scope on the plate of the second triode with the volume all the way down and slowly started to bring up the volume.  it's pretty much impossible to get a clean signal at this stage.  If I bring up the volume control to the point where the amplitude of the signal at the second triode is about equal with the amplitude of the original it is heavily clipped.


My conclusion is that the first stage has too much gain.



In re-studying the schematics I noticed that the values of the cathode resistors are really hard to read.  On some schematics it looks like 15k on other 1.5k
http://el34world.com/charts/Schematics/files/sunn/sunn_100s.pdf
http://el34world.com/charts/Schematics/files/sunn/sunn_200s.pdf


I've got 15k resistors in the amp.  I'll start with trying 1.5k but I don't mind going off script to get the amp performing the way I'd like.  Any advice besides random tinkering for tweaking the pre-amp tube?

[sluckey]

I clearly see 1.5K on the 100s schematic you linked to. There are plenty of other schematis in the Sunn folder that clearly show 1.5K also.

I have an original Sceptre amp. The value of those cathode resistors is absolutely 1.5KΩ, not 15KΩ. You can see one of those resistors in this pic (lower right hand corner)...

     http://sluckeyamps.com/sunn/sceptre1.jpg

My amp stays clean all the way up to bleeding ears level. It does not overdrive. You definitely need to change those 15K cathode resistors for V1 in your amp. Another thing that may be causing your preamp to overdrive is the level of signal you injected. What is that level?

[drgonzonm]

Steve posted before me, I would have given the same answer.

I see your point regarding the sunn 200, and the schematic you posted. The poor quality of the schematic can result in points, periods, dots missing. 

I would also check the voltages on your V1, and compare to the sunn 100 schematic (20% variance would be acceptable). 

I did not look to see if Sunn posted a frequency for testing.  I have seen amps at 1K Hz, and others at 400Hz. 

It probably wouldn't hurt to plot your information on a 12ax7 chart based on your voltage readings, current resistors, and any changes you plan to make.  The forum hasn't had this type of discussion in a while.  The procedure can be found in RDH4.  Spec information on the tube can be found on various sites. 

The gurus will correct me if I am wrong, but a quick way to reduce the gain on the V1a, (6,7,8 pins) is to open the circuit on the bypass cap on the cathode.  It should cut the gain about half.

[eleventeen]

Nate, please report back on how you come out shifting to 1.5K. Your quandary suggests a way to increase stage gain pretty painlessly and geez, do I have a load of 5K pots sitting around? (ans: yes)

[nateflanigan]

Great, thanks for the confirmation.  I just got into it a little more, and the plot thickens...


1) I WAS feeding it too hot a signal 3v (1k).  I turned that down to 100 mv now I can get a clean sine wave anywhere in the amp. 
2) I was able to find where the abrupt volume jump occurs.  It is after the plate of the first half of the 6AN8 (pin 6).  I've got the totally wrong cathode resistors on there.  68k and 47k.  I really don't know how I screwed all this stuff up like this.  


So, it seems obvious that step one is still to just get the right parts in there and see what happens.  Hopefully I've got the correct values in a box somewhere.


I'll report back.

[nateflanigan]

Alright, so proper cathode resistors have been installed.  The best way I can describe the effect is that the amp has chilled out a little bit, all in all it's very surprising that I could have had resistors THAT far off and it didn't make THAT big of a difference.


There is still a problem that I'd like to get sorted out.  If monitor my test tone on my scope off the wiper of the treble pot (end of the pre-amp stage) I get a nice smooth increase in the signal level as I turn up the volume.  However, if I monitor after the first half of the 6AN8 the signal still jumps up in volume very abruptly and dramatically. 


I'm wondering what I can do to get that gain stage to chill out a little bit. 


I should also mention that after the PI, it stops being a SUNN.  I've installed a Lar Mar MV and the output stage is a standard fender style push pull amp, I'm not using an ultra-linear OT.  This probably accounts for some of the drive I'm getting around 3 on the input gain (which sounds great).

[nateflanigan]

Some more deets...


The voltages on the 6AN8 are a little weird. 


Pin 6) 125v
Pin 7) 25v


Node C) 313v


Since the power rail feeding that stage is pretty spot on is it possible I just have a really hot 6AN8?

[sluckey]

Pin 7 is OK. Pin 6 is high, indicating the tube is cold.

What voltage do you have on pin 9? What resistance do you measure from pin 9 to chassis ground? What value resistor is connected to pin 6?

[nateflanigan]

Hey Sluckey,

Pin 9) 0.85v and 10m resistance to ground (amp on).
Pin 6) Resistor = 270k (I even pulled a leg out to measure it to be sure)


What's that get us?
 

[sluckey]

You don't measure resistance with the amp on!

Turn it off and try again.

[nateflanigan]

Ok, amp off 711r.


Thanks!

[sluckey]

Voltage and resistance looks good. I'd like to see a schematic for your actual amp.

[nateflanigan]

Me too!  Can you recommend a good tube eagle library?

[sluckey]

Me too!  Can you recommend a good tube eagle library?

I don't know what that is?

[kagliostro]

I hink he is looking for something like this


https://github.com/chiengineer/Eagle-Libraries/tree/master/Vacuum%20Tubes


Franco

[nateflanigan]

Yup.  Eagle is a free schematic/PCB design program.