One Bottle preamps. Strange...

Edit 12 aug 2011: posted actual schematic.

Hi everyone!
I'm finishing a build of 4x6SN7 preamps (one bottle, no feedback) in an old Geloso chassis, it will look pretty cool!

ACTUAL SCHEMATIC:


-FACTS:
-I plan on running the outputs unbalaced at first, no output transformer, just the output cap.
-I have no resistor loading the output yet, the output cap is hanging until I connect it to some input.

Now I think I'm not getting this:
-The B+ gets to one side of the output coupling cap (330V approx)
-And the other side measures 170V even though it's not physically connected anywhere.

-QUESTIONS:
-Shouldn't it measure 0V? I mean, it's the point of that blocking cap, right?
-Could it be that as soon a I connect it to ground with my meter (or my finger as it has happened!!!) some current starts coming back from the PSU or from the other preamps?
-Will loading the output with a resistor solve the problem?

I would like to be sure that there is not something really wrong that I will hide in some way (like by dumping current through that loading resistor for instance).

This is the last step in this build, if you guys want to help me not blow it all up, I would really appreciate it!
Best
jules

Hi Jules,

Looks like a fun project.

There is nothing to ground the other end of the cap at the moment, so even a tiny bit of leakage will bring the voltage up. You could try connecting say a 1 meg resistor across and measure again. If you still have a significant voltage then maybe the cap is bad.

Cheers!

Stewart

Add the tiedown resistor and see if you get a correct reading.  Anything from 100K-1M should be sufficient.

I would like to be sure that there is not something really wrong that I will hide in some way (like by dumping current through that loading resistor for instance).

Click to expand...

You can verify that the output tube is wired and working correctly by checking the cathode bias voltage.  Something around 3-5 VDC would seem reasonable for that circuit.  May as well check the plate voltage while you're at it and make sure the numbers add up.

Hi guys!
Ok so I added a resistor and the DC is gone. Lassohap I will  check those voltages to see if they're fine.
In the meantime the resistor I put is way too small: 10k. (I got confused, mixing things up as there is no output transformer...)
This could explain strange audio I get out of it.

Output is low, distorts quickly and badly and mostly half the waveform is clipped:

Kind of suck right? Is this because of that stupidely low resistor or do I have another problem coming up?

Thanks for your help guys! How are things going stewart?
I took pics of the whole thing but I will only post them when the thing works!

best
jules

j.frad said:

-QUESTIONS:
-Shouldn't it measure 0V? I mean, it's the point of that blocking cap, right?

Click to expand...

If you leave the resistor floating, it always be whatever voltage is at the other end of the cap, which in this case is the plate voltage.  170V sounds about right.  And when you connect a voltmeter, the high resistance of the voltmeter will provide a DC reference point in any case.  I wouldn't worry about it.

So you put a 10K resistor on the output?  That is way to much of a load.  The output impedance of that stage must be close to 7-8K which means nearly half of your signal will be eaten up right away.  A 10K:600 output transformer will reflect a 10K load impedance to 160K as seen by the plate which seems like a much better value (and follows the 1:10 rule for bridging).

What kind of input signal are you supplying the preamp?

Output is low, distorts quickly and badly and mostly half the waveform is clipped

Click to expand...

That sounds in the ball park for the conditions.  2  factors I see:  1) No OT for loading/impedance transformation and better drive capability.  2) The low 10K // w 12K plate resistor gives a little less than matching load condition.  Not the worst thing as triodes are tolerant of loading below plate resistance but it probably is adding more distortion than if you were loading the plates in say 15K.

As you may already know with no OT (even with large tiedown resistor) you will get better results going into a higher Z load (10K or more) and get lousy results for 600ish ohm loads.

If I recall NYDave speced his output transformerless version to do 'ok' (but not great) with 10K loads and short cable runs.  This may not be good enough for what you want, but get a larger tiedown resistor in and make sure everything else in the circuit is working correctly first.  There may be problems elsewhere contributing to the poor results.

I tried the preamps with sm57s and speech or snare, and an re50.

Since I have a DAOC with no output transformers and I remember kevin doing some preamps with transformerless outputs (but different tubes), I thought it would be easy.
I still plan on going transformerless output to keep things cheap, I got most of the parts for free and this is meant to be a "specialty" pre, nor for everyday use, but if I really need to , I guess I'll add output TXs.

I'll do a few tweaks and see if it gets better. I will try a direct input, to remove the input transformers from the list of suspects. I have 2 philips and 2 altecs and they all do the same thing so I'm pretty confident it's not the problem.

Also the 6sn7s I got are all really old RCAs and NUs, do you think more recent productions would handle the situation better?

I have 2 philips and 2 altecs and they all do the same thing so I'm pretty confident it's not the problem.

Click to expand...

Which model Altecs? 

Also the 6sn7s I got are all really old RCAs and NUs, do you think more recent productions would handle the situation better?

Click to expand...

I doubt it unless you just happen to have tubes that are right at the end of their life.  The lesser performing output you're seeing is likely due to lack of output transformer.

> do you think more recent productions would handle the situation better?

No. New-production is, at best, as good as average healthy vintage.

> Output is low, distorts quickly and badly and mostly half the waveform is clipped

Something wrong. 10K is a lowish load, but it should drive OK.

> B+ gets to one side of the output coupling cap (330V approx)

B+ is noted as 250V.

Coupling cap must be tied to the _bottom_ of R8. When V1B is alive, you should have roughly half of supply voltage across R8, half across V1B. So the left end of C3 should be near 100V-200V, NOT 300V.

Basic tube triage: what are the Plate voltages? If not about 1/3rd to 2/3rd of supply voltage, WHY NOT? Look at grid and cathode voltages. Grids should usually read essentially zero (<0.1V here). Cathodes should be "a few volts", 3V-6V here.

Popular goofs are unconnected grid or cathode resistors, or connected to a "ground" that doesn't actually go anywhere. Lower in popularity, but never off the chart: confusing brown red and orange stripes to put a 4K3 or 43K where should be a 430.

The tubes do light-up, right? (I can't know anymore who has basic tube savvy.)

> I took pics of the whole thing but I will only post them when the thing works!

It will look 99.9% the same. Meanwhile some sharp-eye here might spot your wrong-color or missing-wire.

Hi!
Ok so I posted the actual schematic, the other was an example as I couldn't find the real one anymore.
Here B+ is stated 300V, which I have roughly, this doesn't explain the reading I have.
I will proceed to complete readings and post back.
Thanks!

Here it is:

R7 are really 10k, (I remeasured them).
The coupling cap is tied to the bottom of the resistor.
The tubes light.
Grid and cathode resistors are connected to the same ground as the rest (i rechecked).

B+ reads +258V (I have a dropping resistor there, removing it makes B+=+320V)

V1A seems fine:
I have 220V at the junction of R9 and R4.
Plate = 80V
Grid = 0V
Cathode = +3.1V

V1B is wrong:
I have 250V at the junction of R7 and R8.
Plate = +249.9V !!!
Grid = 0V
Cathode = +3.12V

If I remove the output cap and loading resistor:
There is still less than 1V drop across R7
V1B Plate = +244V
V1B Cathode = +19V
V1B Grid = 0V

I don't get why this would happen.



EDIT: Oh and the altecs are 15095, I know they're meant for line use, but I don't think it is the source of the problem as the philips transformers do it too (they are el6805 and I've used one successfuly in a fetboy preamp).
Now I could have gotten the pinouts wrong but I think I checked that, I may need to try a direct IN just to be sure.

I'm not sure if it will help, but I just did something very similar to what you're doing.  I put 3 channels of NYD's one bottle 6SN7 into an old piece of audio test equipment that already had the all the tubes and power supply.   

http://www.groupdiy.com/index.php?topic=42864.0

Unfortunately I won't be much help otherwise since I'm new to tubes, but you're in very capable hands here.

Hi Jules,

can you please post some pics of the wiring including the tube sockets ? It will definitely help to get what's wrong.

Thanks

Thanks Millzners I'll go through it!

Here are pics: Hope they fit the page.

B+ comes through the big blue resistors, the big green 10k resistor where the problem seems to be.
Grey wires are the heater voltage.
The black electrolytics are going to another ground bar and are the filtering caps.



On the first pic, the output caps and loading resistors are removed.
You can see 4 copper bars that serve as ground, I know it's a bit unorthodox but the  resistance to my "star ground" by the power supply measures 0.

If there is anything you can't see please tell me.

have you tried swapping tubes yet?

It seems strange to have no drop accross r7, but still have a drop across your cathode resistor. Try poking around a bit with a chopstick to find any cold/bad solder joints.

Yep, I did and the same thing happens on all channels...

V1B is wrong:
I have 250V at the junction of R7 and R8.
Plate = +249.9V !!!
Grid = 0V
Cathode = +3.12V

Click to expand...

Your cathode voltage suggests the  plate voltage should be around 124V .  Did you measure the plate voltage directly on the tube socket pin?  Looks like pin5 right where the big green 10K is tied to.
I'll study the pics in more detail but that's the simplest thing I can think of ATM.

Jules,

One other thing I noticed on the bottom pic - It appears from the numbers written in blue pen that the octal sockets are mislabeled.  I'm squinting hard but it appears that the numbering is backwards.  From the bottom of an octal socket the numbering is clockwise from the keyhole.  In the pic it looks like pin 1 is labeled 8, 2-7, 3-6 etc . .

I assume these are for the plug in OTs ?    The red ground wire looks like it's tied to pin 8 which is part of the pri on the altecs.

Hi!
Yes, I measured on both sides of that 10k resistor, including right on the socket, but I only get less than a volt difference.
I even took out that resistor to verify it was really a 10k and it is!
If I disconnect the resistor from the tube socket (just keep it hanging there) and  measure voltage on either side of it, I get the same value...
I don't get how such a simple circuit could exhibit this...

The blue labeling has no numbers,  I just circled (plus) and (minus) numbers to identify my choice of terminal on the transformers.
Right now all transformers are used as input transformers, even the altecs. Those are wired 150:10k. So the red wire is on what side of what I chose to be the secondary.  I could lose 2 channels of preamps and use them as OT.

I can post bigger pics if you want!
thanks a lot

it's the connections around that 10k r7 that are suspect in my mind. Check that something is not shorting across it.