Ecological recycling amplifier (6CU6)

[Gold_xyz]

Hi guys

I have recovered these beautiful valves from an old television destined to scrap. From this I have saved the 6CU6 (h.deflection) and a double triode without name. And in an old radio I have pulled out also a 6K7G 😀

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

My intention is to build a single ended guitar-amplifier with these tubes
6CU6 plate max = 600V & 11W

The schematics that I would like to use is very similar to Fender Princeton but "tuned" for my 6CU6

An externally hosted image should be here but it was not working when we last tested it.

Every suggestion is welcome ! 😀

 

[SY]

The Achilles Heel of high perveance sweep tubes is a very low maximum screen voltage rating. You could potentially use a separate screen supply at 150-200V.

 

[Tubelab_com]

The 6K7 is a remote cutoff pentode intended for gain controlled amplifier use. These tend to distort a lot when used for audio. It still may be useful as the input stage in a guitar amp. Look for the old Fender Champ schematic that used a 6SJ7 for the input tube.

Your dual triode is probably a 6BL7 or 6BX7. These have a relatively low gain for guitar amp use. It may be a 6SN7 which has similar gain.

The 6CU6 is a sweep tube that has a 200 volt maximum screen voltage rating. I have built a few guitar amps with sweep tubes. They usually sound awesome. Some sweep tubes don't seem to mind having their screen grid voltage violated, while others glow red in anger. Only testing will tell how the 6CU6 responds. The technically correct thing to do would be to regulate the screen voltage to about 200 volts. What I would try would be a resistor in series with the screen. Adjust the value until there is no glow on the screen grid wires, and then increase it slightly. This is not optimum from a distortion stand point, but it may work for a guitar amp.

When I was young (and tube TV sets were in trash dumps for free) I made guitar amps entirely from old TV parts. If the TV is still around, grab the power transformer, and the vertical output transformer. Also get any other tubes that are available. The power transformer can power up a 50 watt guitar amp without breaking a sweat. The vertical output transformer can be used as an OPT in your SE guitar amp. I recycled the caps and resistors, but this was in the 1960's. The parts were not that old then. Don't bother with them today, they are at least 50 years old.

 

[Gold_xyz]

The 6K7 is a remote cutoff pentode intended for gain controlled amplifier use. These tend to distort a lot when used for audio. It still may be useful as the input stage in a guitar amp. Look for the old Fender Champ schematic that used a 6SJ7 for the input tube.

Your dual triode is probably a 6BL7 or 6BX7. These have a relatively low gain for guitar amp use. It may be a 6SN7 which has similar gain.

The 6CU6 is a sweep tube that has a 200 volt maximum screen voltage rating. I have built a few guitar amps with sweep tubes. They usually sound awesome. Some sweep tubes don't seem to mind having their screen grid voltage violated, while others glow red in anger. Only testing will tell how the 6CU6 responds. The technically correct thing to do would be to regulate the screen voltage to about 200 volts. What I would try would be a resistor in series with the screen. Adjust the value until there is no glow on the screen grid wires, and then increase it slightly. This is not optimum from a distortion stand point, but it may work for a guitar amp.

When I was young (and tube TV sets were in trash dumps for free) I made guitar amps entirely from old TV parts. If the TV is still around, grab the power transformer, and the vertical output transformer. Also get any other tubes that are available. The power transformer can power up a 50 watt guitar amp without breaking a sweat. The vertical output transformer can be used as an OPT in your SE guitar amp. I recycled the caps and resistors, but this was in the 1960's. The parts were not that old then. Don't bother with them today, they are at least 50 years old.

unfortunately I have saved only the tubes 🙁
I would like above all to make to relive even these old valves using some conservative tensions 🙂
I think to supply anodic with 310V and limit the screen around 150V


I now look me the schematics of Fender Champ
http://www.drtube.com/schematics/fender/champ-5c1-schem.gif

Tnx ! 😀

 

[Gold_xyz]

Perhaps to use one 6K7G as audio driver is a bad idea.
even however for an guitar-amp it can plays well 😀
Then I can use as audio driver a 6BH6 sharp-cutoff pentode.

mumble mumble... the schematics of fender champ is without
global NFB. How can I add this ?
I need to place a cathod resistor on audio driver...

 

[Gold_xyz]

Off Topic.

I have found this online Tube-museum http://www.r-type.org/static/museum.htm
http://www.r-type.org/inx/ages.htm

I am constantly to the search of information on the tubes 😱

 

[Gold_xyz]

Some question around the Fender Champ schematics...
does it seem me that the first valve works in A2 calss ?
I have added the nfb and polarization to the first pentode.

now "my" schematics is like this :

An externally hosted image should be here but it was not working when we last tested it.

and i do adjust the value of circuit 🙄

 

[Tube_Dude]

Some question around the Fender Champ schematics...
does it seem me that the first valve works in A2 calss ?
I have added the nfb and polarization to the first pentode.

Hi Gold

The first valve works in grid bias mode.

If you use feedback ( a good idea ) change the 1 MOhm pot for a 1 M resistor and put a 100k pot at the input of the first valve.

A feedback amplifier will try to maintain constant the voltage gain and the inclusion of a pot inside the loop will completely defeat the situation.

Ci vediamo 🙂

 

[Gold_xyz]

Hi Gold

The first valve works in grid bias mode.

If you use feedback ( a good idea ) change the 1 MOhm pot for a 1 M resistor and put a 100k pot at the input of the first valve.

A feedback amplifier will try to maintain constant the voltage gain and the inclusion of a pot inside the loop will completely defeat the situation.

Ci vediamo 🙂

Hi Tube_Dude 😀
Thanks for the answer !

Now i have modified the circuit with Your suggestions

An externally hosted image should be here but it was not working when we last tested it.

Ciao

 

[Tube_Dude]

For the lower arm feedback resistor , from the input tube decoupling capacitor to ground , i will use 100 Ohms.

The other feedback resistor, to the output transformer secondary depends of the overall voltage gain you want.

V (gain) =( R (fed) / 100 ) +1

The cathode decoupled resistor depends of the operating point you choose .

Discard the 5 MOhms grid resistor, because you are note using grid bias anymore and use 470 Kohms or 220 k instead...

The 0,02 uF input capacitor is not needed anymore...

Arrivederce 😎

 

[Tubelab_com]

I offer the following suggestions:

The capacitor at the grid of the 6BH6 is not needed since both sides of it are at ground (DC) potential.

The 2 meg screen resistor on the 6BH6 will need to be changed. The 6BH6 will draw a different screen current than the 6SJ7. Experiment with the value to get a reasonable tube current. I am at work now and I don't have access to a data sheet. The 250 K plate resistor may need reducing also. I just tweak these for the best sound.

I would also reduce the 5 meg grid resistor to 1 meg. Very high value resistors tend to be noisy and drift prone. Any reasonable value resistor will work here.

Again, be prepared to adjust the values of the screen and cathode resistors in the output stage to get a good tube current and a good sound.

Your 6CU6 tube looks like it has seen a bit of use (worn getter). I would keep the plate dissipation down to 8 watts or so since it is the only tube that you have. Sweep tubes tend to be underrated since they run at full power every minute that the TV is on. I have been known to push them beyond the published ratings, but I do this with tubes that are plentiful and cheap.

does it seem me that the first valve works in A2 calss ?

It was common in the old days to develop "grid leak bias" by using a large (5 meg) value resistor in the grid circuit. There is a small (1 or 2 volt) negative voltage developed on the grid because some of the electrons emitted from the cathode land on the grid. This works but the bias voltage is highly dependent on the quality of vacuum in the tube. Resistors and capacitors were not cheap then, so this was done to save a few parts. This design is no longer common since the results are unpredictable. This is why I recommend reducing the grid resistor. Keep the grid at zero volts, and use cathode bias.

 

[Gold_xyz]

Thanks Tube_Dude & Thanks Tubelab.com
for your suggestions 😀

I have modified some value...
Schematics V1.3

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

Now i need draw the load line static and dinamic of endstage tube...
for to determine the correct value of the components.

- Loadline static :
For Anode Current = 0 --> Vak=Vcc (on X axis)
For Voltage Anode = 0 --> Vcc/(Rk+Rdc) (on Y axis)
where Rk = cathode resistor and Rdc = dc resisrance of output trasformer.

An externally hosted image should be here but it was not working when we last tested it.