1965 Realistic Carnival 6AQ5 Conversion Ideas

[misterc57]

I could do a 5K and 10uF cap on the first stage also.

I do not understand why and how to choose this.

[Helmholtz]

Do you think it still needs a input cap or can that be left out if you have a 1meg?

A tube using grid leak bias always needs to be AC coupled.
Otherwise the DCR of the PU would kill the negative grid bias caused by the tiny grid leak current.

[Helmholtz]

misterc57:
Please post an updated schematic showing actual wiring, component values and voltages (including plate and cathode voltage if applicable).

Do you still have a hum problem?

[misterc57]

misterc57:
Please post an updated schematic showing actual wiring, component values and voltages (including plate and cathode voltage if applicable).

Do you still have a hum problem?

Will do. No hum

[misterc57]

Updated schematic. New voltages are in red. Original values in black

[Helmholtz]

What are the voltages at the far ends of R5 and R8?
Acc. to original schematic they would have been connected to the same 100V power supply node.

[mozz]

No input cap. Either you need 1 or convert totally to cathode bias, no in between.

[misterc57]

What are the voltages at the far ends of R5 and R8?
Acc. to original schematic they would have been connected to the same 100V power supply node.

212V. Yes they are connected together at node 3 of the filter caps.

Updated schematic attached.

[misterc57]

No input cap. Either you need 1 or convert totally to cathode bias, no in between.

Sure I can convert to cathode bias like the other triode. 10uF and 5K

[Helmholtz]

Sure I can convert to cathode bias like the other triode. 10uF and 5K

I recommend to use a 1.8k cathode resistor and a 47µ bypass at the input triode for best center bias and low heater buzz.

[misterc57]

I recommend to use a 1.8k cathode resistor and a 47µ bypass at the input triode for best center bias and low heater buzz.

Installed. Only had a 1.9k. All V are up. Should the plate V of 6AQ5 be lower than the pin 6 grid?

Updated schematic attached.

Thank you!

[Helmholtz]

Should the plate V of 6AQ5 be lower than the pin 6 grid?

Plate voltage being a few volts lower than screen voltage is no problem.
That's not uncommon.

Also with signal plate voltage is generally much lower than screen voltage most of the time.

[misterc57]

On the output transformer the schem shows blue to 4 ohms and yellow to 8 ohms. However the factory wiring is opposite that acc. to the speaker terminal designations on the chassis.

I see the following resistance

black to blue 0.55 ohms
black to yellow 0.43 ohms

Can you tell which is which based on the readings?

[g1]

I see the following resistance

black to blue 0.55 ohms
black to yellow 0.43 ohms

Can you tell which is which based on the readings?

Usually the lower DC resistance is the lower impedance tap. So the factory wiring rather than the schematic should be correct.

[misterc57]

I will post final pictures and final schematic when all is done.

I am very happy with the sound of this amp! Very pick responsive and guitar volume responsive with the perfect amount of saturation.

I had the 6V6 version of this amp a few years ago and felt the same way about that one.

The bass control is subtle. Treble control is very noticeable. I am using the original 500K pots.

I have installed an internal 1A fuse. I still have to install a 3 prong AC cord and a back plate for the output jacks.

The indicator bulb does not work. It is an odd one. Does not look like the front part lens can be unscrewed. There is a nut inside the chassis that holds the assemble in place. Anyone familiar with this type of assembly and bulb? Pics attached.