HW A-7 Amplifier to Speach amp

[KA2DZT]

No lows through the 12SL7,  check the two .05 coupling caps, maybe one or both are partially open.  After that, try adding bypass caps to the two 1K cathode resistors.  Whatever you have, 20ufd 25V.

Also, I assuming that you're using a straight D-104, one without anything in the base.

Fred

[KX5JT]

......  Sounded OK, used to play Led Zepplin 1 as "test audio" into the dummy load with it, and the mod waveforms lookd pretty good on the old Hickock scope I had at the time, including the Zepplin bass lines from Heartbreaker.

Zep 1 .... do you really think "Communication Breakdown" was appropriate? *grin*

Maybe you meant Zep II (Heartbreaker was on Zep II)  

[AJ1G]

......  Sounded OK, used to play Led Zepplin 1 as "test audio" into the dummy load with it, and the mod waveforms lookd pretty good on the old Hickock scope I had at the time, including the Zepplin bass lines from Heartbreaker.

Zep 1 .... do you really think "Communication Breakdown" was appropriate? *grin*

Maybe you meant Zep II (Heartbreaker was on Zep II)  

You are correct...it was Zep II..corrected the post...

[ke7trp]

The 10 meg goes from input to ground.  Its not in series.  The input I used was right out of the handbook, the same one I use on my transmiters. c

The trouble was the bypass on the cathode.  Thanks Frank!   I clip leaded a 47 UF/450 cap and each time I hit the Cathode of the SL and SN tubes, the bass came way way up.   All I have is 450 volt caps and I hate to waste them. I will order some 20 UF and 47 UF 50 volt caps today and place them on each cathode of both the SL and SN tubes.  Then, This thing will really sound nice with a D104.

Now to make a simple voltage divider off the 180 volt supply to power some LED on/off indicator lamp.  Then I have to go back to work tomorrow and the fun stops

C 

Hey CT,

Just where is that 10 meg resistor?  In series with the mic line, or from grid to ground?

To do it properly, you should've removed the original grid resistor and subbed it.

Just a hint...

73DG

[KA2DZT]

No lows through the 12SL7,  check the two .05 coupling caps, maybe one or both are partially open.  After that, try adding bypass caps to the two 1K cathode resistors.  Whatever you have, 20ufd 25V.

Also, I assuming that you're using a straight D-104, one without anything in the base.

Fred

Glad the bypass caps improved the bass LIKE I MENTIONED ABOVE.  Who's Frank??

Too bad you're way out west,  I have some 47ufd 25V caps (NOS) not too old, not sure but, I think about 6000 of them.  I don't want to run my stock too low

DO NOT add bypass caps to the 6SN7.  The first 6SN7 stage has feedback applied to the cathode, that's why there is no bypass cap.  The second stage is the phase splitter.   Just bypass the two cathodes of the 6SL7

FRED

[ke7trp]

Sorry Fred.  I was thanking you. 

OK.  Will use caps only on the SL7 tube.  I will take some video of the little amp soon.  Need to mount the correct input connectors for XLR input and Two prong D104 to complete the conversion to speech amp.  Also need to make a screen cover.  Then to test it into the 304s!

C 

[ke7trp]

Here is a picture of the little amp.  Junk Drawer knobs, A neon lamp indicating its on. Dennis donated the 12a6 tubes.  Couple things left to do.  I need to hook up the 600 ohm transformer on input 2 and test it out.  I will clip lead it first. If it works and sounds good, i will mount it.  The D104 input is done. Lots of gain.  Sounds good with some bass boost and treble flat.

[KA2DZT]

Nice looking 12A6's,  I have a bunch of the metal 6V6's and 6L6's.  Don't think I have any 12A6's,  lucky Dennis had them.  Knobs look perfect for the amp.

[ke7trp]

Can I get some suggestions on negative feedback? I would like to try this as a learning experience. I have read up on this.  The gain of the first stage is very high and I would like to cut it back.  Where and how can I get started?

THanks!

C

[Opcom]

1.) The output stage cathode negative feedback mod as shown here will greatly improve its ability to regulate the output voltage over each cycle as your modulator's grids conduct and present a varying load.

It looks very 'busy' due to 4 tubes, but the idea is that the 4 Ohm tap is the CT and 0 and 16 are the ends. Unground "0" and make 4 Ohms the GND and then, 0 and 16 become a balanced winding to buck the cathode currents. The global negative feedback tap can stay where it is but the GFB resistor will probably need adjusted lower (4.7K or a little less) because the GFB voltage will be half with the new connection. The lower, the more GFB, but beyond a certain point it will become unstable.

You might need to use more grid to grid voltage drive from the phase inverter or more gain from its driver, but on this small amp, it should not be an issue because the fixed bias is only -12.5V with Eb=250V. A simple way there is to increase the B+ for the driver and phase inverter to prevent those stages from clipping, if it is an issue.

2.) Also apply fixed bias and get rid of the cathode bias. Bias can be either multiplied up from the 6.3V, or can be taken off one side of the plate voltage winding and rectified.

either or both mods can be applied separately or together.




--> What about the 12A6's plate voltage rating of 250V and this amplifier having 390V on the plate?

12V6's with a 350V rating seem more suited. Is it really running 390V?

[ke7trp]

I will study the schematic tomorrow.  Thanks for the information Patrick.  I thought about using Negative feedback to reduce the gain of the First stage.

The B+ is 400.  The second supply is around 200 volts.  This is with the factory rectifier. I thought about solid stating the supply, to reduce heat but then the voltages would be way up. 

C

[KA2DZT]

Just looked at the schematic again.  390 volts on the plate seems very high for a tube that's rated to run with 250 volts on the plate.  You can lower the plate voltage by adding a resistor, of some value, between the rectifier and the first filter cap.  Probably 1K-2K figuring about 60ma-80ma total current.  You're using a cap input filter in the PS which always puts a heavier load on the xfmr.  A resistor or choke input filter will drop the B+ and help cool everything down.  Probably better to keep the 5Y3 but, if you go to SS rectifiers then you should add the resistor or choke to the input to the filter.

As for adding FB to lower the gain of the 12SL7,  drop the grid resistor on the second stage to maybe 100K leaving the 470K in series with it.  This will act as a voltage divider and drop the gain of the first two stages.  Adding FB will be more problematic.  You could also replace the grid resistor and the 470K resistor with a volume control, 1/2-1meg audio pot.  You could mount the the pot somewhere on the back or top of the chassis.  Something you can adjust for your D-104 then leave the adjustment as set and use the main volume control to adjust audio level.

Fred

[ke7trp]

Fred. Thanks.  I will try lowering the voltage.  The transformer runs warm but not hot.  I have some DALE 5w and 10w resistors, I can test some out.

I am going to take your advice and just add a 1meg audio pot. I have a pile of old pots I ripped out of a heatkit O scope.  I will mount one with a knob and label it D104 Gain.  This is very usefull.  I can set the volumn to match the audio rack and this pot to match the D104.

Thanks!

Clark

[KA2DZT]

Fred. Thanks.  I will try lowering the voltage.  The transformer runs warm but not hot.  I have some DALE 5w and 10w resistors, I can test some out.

I am going to take your advice and just add a 1meg audio pot. I have a pile of old pots I ripped out of a heatkit O scope.  I will mount one with a knob and label it D104 Gain.  This is very usefulll.  I can set the volume to match the audio rack and this pot to match the D104.

Thanks!

Clark

Clark,

You'll need an audio taper pot.  Not too many of those were ever used in scope circuits.  Scope circuits would have been all linear taper pots.

Fred

[ke7trp]

Those pots turned out to be .1 meg.  Why would they even mark them like that???    Why not just say 100k?

I did find a 1.0 meg pot in the junk drawer. Its not marked if its audio.  However, It seems real slow in the first part from 0 to about 200K and then ramps up fast to 1.0 meg.  I will try it in a few minutes.

C

[KA2DZT]

Those pots turned out to be .1 meg.  Why would they even mark them like that???    Why not just say 100k?

I did find a 1.0 meg pot in the junk drawer. Its not marked if its audio.  However, It seems real slow in the first part from 0 to about 200K and then ramps up fast to 1.0 meg.  I will try it in a few minutes.

C

I'll bring that up (.1meg/100K) at the next pot meeting.

That 1meg seems to be an audio pot.

They're usually never marked as audio. Although, some will be marked as linear.

F

[W7TFO]

Some nomenclature is confusing.  They use both upper and lower case old Greek for lot of electronics notation.

M=million
m=thousand
w=Ohms (sort of a 'wiggly' looking w).

73DG

[KA2DZT]

Some nomenclature is confusing.  They use both upper and lower case old Greek for lot of electronics notation.
w=Ohms (sort of a 'wiggly' looking w).

73DG

The wiggly looking w usually refers to 2pi/t   radians/s (IIRC).  The upper case omega is ohms

F

[W7TFO]

Ah yes, Fred.  In algebraic formulae is does mean that.

Also, for vintage electronics I submit a snippet of an old WECo skizmatic:

73DG

[KA2DZT]

Ah! I see,  no wonder WE went out of business.

Fred

[ke7trp]

Fred, The 1 meg did not work.  To touchy.  I ended up wth a 20K resistor for the perfect level.  I removed the 1 meg and used a 100K I had.  Its still touchy but works. 

Is this ok??  It strikes me as odd, the resistor had to go from 470K to 20K.  Big change.  However, That input was made for a phono.
 

C

[KA2DZT]

Not sure exactly how you connected the resistors.  The pot should have been in place of BOTH the 470K resistor that is connected to the coupling cap and the grid resistor.

The high end of the pot should be connected to the coupling cap and the low end to ground.  The wiper of the pot should be connected to the grid, nothing else to the grid.

Is this what you did??

Old pots need to be clean with something like WD-40 before you try to use them.

Fred

[ke7trp]

No. I replaced the 470K with the pot.  Directly. 

From Pin 2 it goes to a .05 coupling cap which goes to Pin 4.  The 470K goes from Pin 4 to ground.  I removed the 470K and placed the pot there. 

The two caps and resistors that where tied to 4 are long gone.


I will redo it after lunch.   

C

[ke7trp]

Ok. That worked Fred.  Its to touchy still thought. The usefull range of the Pot is only 1 to 1/4.  I need a half value pot so I can get a wider range. Or just leave it.  This stage is super hot with sensitivity

Power supply tests:

At 126volts mains:

450 DC out of rectifiier.
First supply 450dc
Second supply is 420
Third supply is 171

A 2K only got this down to 400

I have a bunch of 10K power resistors.  These got the volts down to 190.  But the rest of the voltages where low and what concerned me was that the volts on the rectifier dropped all the way down to 350.  So I had a 100 volt Drop OUT of the Power supply. This is telling me that the reistor (which runs hot) is pulling down the supply.  Is this expected and how can this be easier on the transformer? The tubes ran cool at 190 volts. The amp seemed fine.  Maybe put two of these 10Ks together to get a 5K and leave it?  Or I can try on the other side of the Filter?

C

[KA2DZT]

No. I replaced the 470K with the pot.  Directly. 

From Pin 2 it goes to a .05 coupling cap which goes to Pin 4.  The 470K goes from Pin 4 to ground.  I removed the 470K and placed the pot there. 

The two caps and resistors that where tied to 4 are long gone.


I will redo it after lunch.   

C

I'm not completely following what you're doing with the pot. 

Pin 2 (plate of first stage) has the 100K plate load resistor as always.  Connected to pin 2 (also) is the .05 cap.  The other end of this cap goes to the high end of the pot (nothing else).  The low end of the pot goes to ground.  The wiper of the pot goes to the grid (pin 4) of the 2nd stage, that's it.  No other connections.

If you have the correct type of pot it will work.  You should be able to turn the gain down to zero or all the way up.  You will need an audio taper pot.  You can use whatever value you can dig up (100K-1meg).  If you use a lower value (like 100K pot) you can increase the value of the .05 coupling cap to maybe .1ufd.  This will allow more lower frequencies to be past.  The lower the grid resistor value is, the larger the coupling cap should be.

Fred