1965 Realistic Carnival 6AQ5 Conversion Ideas

[misterc57]

What would you do with this? Looking for guitar amp conversion thoughts and/or pointers to a schematic to follow.

This is a 3 tube amp; 12AX7, 6AQ5, 6X4. Controls are MIC volume, PHONO volume, TONE ON/OFF pot/switch.

I have yet to find a schematic for this 6AQ5 version. I have the 6V6 version schematic which I converted 7 years ago.

There are two inputs, phono and mic. Mic input goes through both sides of the 12AX7, phono only hits one side. I believe there is no phono stage circuit in this amp.

The phono input will go away.

I was thinking I might use the Champ AA764 pre-amp circuit coupled to the 6AQ5.

I will be installing a 3 prong AC cord and a fuse. Probably get rid of the "Switched" tone pot, install a separate power switch. Have a VOL, Treble and Bass pot in the circuit.

Should there be a NFB?

Thank you all! MarkO

[Jon Snell]

You have the makings of a decent little amplifier here.
I would use an EL84, rewire the valve socket accordingly and put a safe mains lead on it.
Good to go as they say across the pond.

[mozz]

Wire it up as a champ, no need to change the output tube to something else.

[g1]

Agree with Mozz, the 6AQ5 is very similar to 6V6, plentiful, and cheap. As the amp is already using them, it will be running them at more appropriate voltages than you would find in a real Champ.

[misterc57]

Hoping to find the original schem to compare before and after changes and to merge into the new build schematic.

I want to go the Champ route with the existing tubes. Should I change the bias values on the 12AX7 to match the Champ or leave it as it came? I am probably going to see far less plate V on the 12AX7 then the Champ would have.

I am also changing out the filter caps (old one is humming). Old can had 40uF, 20uF, 20uF at 350V. I am planning on 47uF, 22uF, 22uF at 450-500V

I am attaching the 6V6 schematic version of the Carnival for comparison. Also the Champ AA764 schematic which I want to use the preamp section.

This will be a fun project. Thank you

Realistic Carnival.pdf

champ_aa764.pdf

[Helmholtz]

The 6V6 version would work with a 6AQ5 as well as voltages are low enough.

C'mon it's only 3 stages and few components.
Should be easy to draw the schematic, probably close or even identical to the 6V6 version.

[mozz]

I'm sure your ~250v on the plate will be higher with today's voltages. I wouldn't push a 6AQ5 over 300v if possible. Also, first stage is grid leak bias, you might not like that for guitar but i would try it anyway.

[Helmholtz]

I'm sure your ~250v on the plate will be higher with today's voltages. I wouldn't push a 6AQ5 over 300v if possible.

Where would such 20% increase in B+ come from?
With the 6V6 circuit plate voltage is 235V. The tube only sees plate voltage minus cathode voltage resulting in around 220V.
A 10% increase by higher mains voltage shouldn't be a problem with a 6AQ5.
The 250V plate and screen limits of a 6AQ5 are specified as Design Center Values.
Meaning that Design Max Limits would be 10% higher as shown in the Tung-Sol datasheet.
So 275V plate and screen limits acc. to the Design Maximum System.
I wouldn't expect any benefit from using a 6V6 or EL84 if B+ isn't considerably higher.

Of course it's always best to measure actual voltages.

[misterc57]

Tomorrow going to draw a schematic for review before doing anything.

Thank you

[g1]

Tomorrow going to draw a schematic for review before doing anything.

Thank you

I think you should be able to use the 6V6 version schematic and just pencil in any variations.

[mozz]

"Where would such 20% increase in B+ come from?"
Easily, schematics have tolerances too. I can bet it's very close to 300v on the plate. As i said, i don't care what the datasheets say you'd be really pushing a 6AQ5 at 300v, they don't take much abuse as a 6V6 would.

[Helmholtz]

Let's see what actual B+ is in the amp with tubes in. If not larger than 275V it should be fine.

[mozz]

Only other thing I can think of would be, if it has those cheap wafer type sockets, I would swap them out for a bakelite socket, maybe with a retainer. Then I might swap to 6V6. It's going to be a guitar amp so it might get bounced around more than it's former usage. You can also go to a solid state rectifier for more voltage and a little bit more punch. I see it has 4ohm and 8ohm outputs which is nice.

[misterc57]

Rough schematic attached. I prematurely removed the cap can so I am going by pics now to recreate the connections.

Voltages will be updated when available.

Let me know if you see any issues with following this new schematic.

Thank you

[misterc57]

New input jack and Champ value resistors.

Just finished the Champ tone stack install. I am using the existing pots which are all 500K. I did not have a 250 pF cap but used a 220 pF.

Controls in pic left to right are VOL, TREB, BASS

Next is the PS caps. I will breadboard those before installing to see if values need to change.