Realistic Carnival 6V6GT Tube Amp Help

[misterc57]

I ran my iPod into the phono-tuner input, set the volume on the iPod so that peak levels were about 100mV, the amp worked well and the sound distortion free. I would say the amp is working as it should under these conditions. Not exactly high fidelity (IMO) going on here.

I measured the heaters, all had 6.3 VAC. Pins 4 & 5 on the 12ax7 are soldered together. I get 6.3 VAC between those pins and pin 9.

I removed the 12ax7 and recorded the following at pins
1: 125 VDC
2: .05 mVDC
3: .2 mV
6: 125 VDC
7: .05 mVDC
8: .02 mV

I grabbed a good JJ 12ax7 and then recorded the following
1: 95.6 VDC
2: .3 mVDC
3: 0 (readings move around)
6: 82.5 VDC
7: .3 mVDC
8: .02 mV

My intentions were to convert this to a guitar amp, so I would like to follow DRH1958 suggestions to modify the mic input to cathode bias (could you please detail the steps to convert to cathode bias?), and add the 3 prong power cord and add a fuse.

The amp is drawing .25 amps and there is never a spike when powering on, what size fuse would be recommended here? Slow of fast fuse?

Thank you!

[DRH1958]

First of all it's probably best to just send a schematic of a simple cathode biased amp, the Champ 5F1 which is about as simple as it gets. This shows how the input changes and if you look where the power comes in, it shows how the fuse is wired as well. It's so much easier to look at the schemo because this eliminates the "take the wire off the input jack and connect it to..." type instructions. This can cause confusion. But in a nutshell, C4 is replaced with a grid stopper resistor and R4 is changed to 1 meg. The cathode is then disconnected from ground and a 1.5k resistor is wired in series. This sets the bias voltage for this half of the tube. If you want only one input input jack, just use a 32k-68k grid stopper resistor and connect it right to the socket, pin 7. It does a better job this way. Really, just look at how input #1 is wired if you want just the one jack. The 1 meg resistor is the grid leak which connects to a switching jack to ground to provide a reference for the grid to ground. This is the grid leak resistor.

As far as replacing the cord goes, you can just use one of the tranny's mounting bolts and use a crimp-on connector for the end of the green ground wire. It makes a better connection than just twisting the end of the wire. Or for a better job, drill a separate hole and use a nut and bolt. This ground should be separate from other grounds. The two other wires are then connected to where the two browns are at right now with the fuse in series with one of them. Find a fuseholder and figure out what size hole to drill for that. I would mount it close to where the power cord comes in and use a .5A slo blo or 1A fast since your current draw is .25A. Don't forget to leave clearance for how long the fuseholder is so it doesn't hit the tranny.

Maybe replace the cord first as the bias voltage on pin 3 should be 1 volt according to the schematic. This could be a problem to solve first. You measure 0V moving around right now and this voltage should be stable at around 1 volt. Check the connection to ground to make sure it's solid and also you could take a resistance reading as well from pin 3 to ground. Are you sure the B+ to the preramp section isn't moving too?

Fender_champ_5f1_schem.pdf

EDIT: Not sure why this won't open. Here's the link: http://el34world.com/charts/Schemati..._5f1_schem.pdf

[misterc57]

I will focus on the cord first. I will check B+ to the preamp to see if it is stable.

I am looking forward to all the mods.

Thank you!

[The Dude]

And, as DRH1958 mentioned in an earlier post, clip out that death cap.

[misterc57]

3 prong cord is installed (no fuse installed yet) with green wire to transformer bolt. Removed C9 death cap. B+ to preamp is stable at 117.8 VDC.

Readings at JJ 12AX7 pins;

1: 95.8 V
2: -7 mV
3: .99 V

6: 82.5 V
7: .08 mV (kept dropping very slowly)
8: 0 mV

I will do the 5F1 changes next chance I get.

[DRH1958]

Did you notice how the bias voltage changed on pin 3 after you changed the power cord? It went from a 0v reading moving around that you listed in post #16 to a stable .99v? Dead nuts on the 1 volt shown on the schematic. It appears that having a stable ground does affect readings in addition of providing the safety of a grounded chassis. Really, without having the chassis grounded, it could be floating at any voltage with reference to your house ground.

[olddawg]

Doesn't the phono input have RIAA equalization incorporated and is probably optimized for an old style crystal cartridge?

[DRH1958]

Doesn't the phono input have RIAA equalization incorporated and is probably optimized for an old style crystal cartridge?

Yes, it is for the old crystal phono cartridge. That's why it skips the first stage as its output was quite high. I've noticed this in other old PA amps with this circuit architecture. This input always bypasses the first stage that the mic input goes to.

As far as RIAA EQ being incorporated in this input, all I see in the schematic is a direct connection to the 500k volume pot and then on to what I would assume R6 is, a 500k channel mixing resistor as there is one on the end of the mic input as well(R7) just before each channel inputs to the grid of V1B, pin 2. So I don't think there appears to be any extra circuitry for this phono EQ. I'm aware this exists but only have come across it in designed home audio amps of that era. There is usually a selection for this on the compensator selector knob that says RIAA as in this listing. Vintage Tube Amplifier Electrohome PA 330 6V6 P P Design | eBay

[Jazz P Bass]

The old crystal cartidges had a 'built in' RIAA curve.

There output was perfect.

It's when the magnetics came out that the RIAA was incorporated.

[misterc57]

Did you notice how the bias voltage changed on pin 3 after you changed the power cord? It went from a 0v reading moving around that you listed in post #16 to a stable .99v? Dead nuts on the 1 volt shown on the schematic. It appears that having a stable ground does affect readings in addition of providing the safety of a grounded chassis. Really, without having the chassis grounded, it could be floating at any voltage with reference to your house ground.

Yes!

[misterc57]

new input jack

I am getting a little confused on wiring the 1/4" input grounding jack, has three tabs on it (see image). This is replacing the rca connector for the "MIC" input.



The lug in the background is the positive tip, this is where the 68K grid stopper connects and goes to pin 7 of the 12ax7.
The lug in the foreground is the negative tip. Do I need to run a wire to ground?
The lug in the center is the auto grounding tip. Anything connect here?

Where does the 1 meg resistor connect and is there anything else to connect on the other lugs?

Thank you!

[DRH1958]

I am getting a little confused on wiring the 1/4" input grounding jack, has three tabs on it (see image). This is replacing the rca connector for the "MIC" input.

[ATTACH=CONFIG]37895[/ATTACH]

The lug in the background is the positive tip, this is where the 68K grid stopper connects and goes to pin 7 of the 12ax7.
The lug in the foreground is the negative tip. Do I need to run a wire to ground?
The lug in the center is the auto grounding tip. Anything connect here?

Where does the 1 meg resistor connect and is there anything else to connect on the other lugs?

Thank you!

Again, a pic is better. Do it like the upper right jack. Connect the green that's already there on the RCA connector to where the green is in the pic. Or the other way is to just establish the ground with the locknut. This green connection is redundant. Sometimes these jacks can establish a ground loop, so try whichever way is less noisy. Of course the yellow goes to the 68k resistor soldered to the socket. I suppose I should ask if the existing green goes to the chassis ground. I looked at your pic again and can't really tell for sure now but the green in my pic goes to ground.

If you want more gain, reduce the 68k to around 30k or parallel it with another 68k and if the signal is still too hot, reduce the 1 meg to 470k or parallel on another 1 meg. You can also change the 1.5k cathode resistor to change the bias voltage too. Take a reading at the cathode to see what bias voltage you have. The 1.5k is sort of a standard value but can be changed to give the input a different sound.

EDIT: Now that I have another look, I bet that green is the signal from the phono-tuner input going to the vol control, so disregard hooking that to the jack if this is what it does. Seeing green made me think "ground" but it might not be.

[misterc57]

The green wire in my pic on the rca connector is going to the tuner-phono volume control, it is the signal wire. The black is the ground wire.

[DRH1958]

Glad I had a second look and you confirmed it. The black goes where the green in my pic goes then.

[misterc57]

5F1 mods installed for gain stage #1. Readings at JJ 12AX7 pins;

1: 96.4 V
2: -5 mV
3: .66 mV

6: 79.8 V
7: -22 mV
8: .55 V

Have not tried a guitar yet...

Just tried a guitar. Sounds pretty good. I am wondering if it is all too loud and breaking up to quickly.

Need to keep the amp volume at about 3, and it really responds to the guitar volume pot; above 4 is starts to break up a lot and get pretty loud.

I am using the original 8" speaker that came with this amp. Maybe I should try a more modern 12" cab and see how it plays in that space.

I used a 56k resistor for the grid stopper as I did not have a 64k.