EL84 / Champ Design and build

[Dave H]

I've seen 6V6's with the 220K. On the 6V6 datasheet it has a max value of 0.5M. So 220K is around half that.
The EL84's max value is 1M on the datasheet. So I thought, around half the max value is in the ballpark. 680K would make it a little more sensitive, but the 10K stopper will help calm that down.
If my logic is flawed let me know.

The gain difference between 680k and 220k will be less than 1dB. I'm not sure I'd be able to hear that. The 10k grid stopper won't calm audio frequencies. It's for HF.

[eschertron]

So, I designed the plate voltage at 270 with a cathode bias of 8 volts. You are saying I should add these values together for the B+, 278v?

Yes. The anode only 'sees' its potential relative to the cathode's voltage, which is pulled up towards B+. And I'd suggest starting with a round-ish 10v as a first approximation of the cathode voltage needed. The cathode resistor you've chosen will produce a bias voltage around what you've calculated, but it may be a tad hot dissipation-wise. I'm seeing over 13W here: Interactive Valve Data Sheets Swapping cathode resistors is part of the design process as I've come to know it.

[Tony Bones]

Hello. This is a design I came up with combining a Champ preamp/tone stack and an EL84 output. Not sure on the grid resistors for the EL84, so I just made some educated guesses. No feedback circuit yet, I was thinking I would add that later if need be. The transformers were rescued from old equipment, so that's the deal with the over powered PT and dropping resistors. I got it wired up but wanted to get some thoughts and opinions before I started soldering. Thanks.

My advice, which is standard for people in your position, is to build the simplest thing possible first and plan on adding and changing later. Seriously, if this is your first build then leave the tone stack out completely. Try adding different types later. Building amps is a journey, not a destination. It's certainly not sitting at the kitchen table looking at maps. Just get up and go. Not only CAN you back track and try different roads, but you SHOULD.

[rickyl]

Test #1!

Hello everybody,
First of all, I'd like to thank everyone who has provided advice so far. It has really helped fill in the gaps. Soooo I got the amp soldered up and turned it on, and it worked! Well, kinda. It did amplify sound from the input!
Upon taking some real world measurements, as you guys said, the voltages were low. I updated my schematic below with the voltages I got in bold. The one I really don't understand is the 96v at the plate of the first stage, when the second has 174v? I plan on lowering my dropping resistor setup to around 1500R in order to get 300+ at the first takeoff point.
The tone stack did not work as expected. The volume and treble seem to do ok, but the bass doesn't seem to do much at all. I may follow Tony's advice and omit the tone controls for now. But at the same time I would like to understand why it did not work. Maybe it is because I'm not using the special pots they used in the Champ tone circuit, maybe I just don't have it wired correctly(probably).
Also, there is a prevalent 60hz hum that gets louder and quieter with volume adjustment. I think referencing the filament circuit to ground will get this under control, I hope.
I look forward to get everybody's thoughts on the results of my first test run!

[Jazz P Bass]

That power supply is giving me a headache.

Why not make it like a Champ?

The output tube should come directly off of the 371v tap.
The screen has two resistances. Drop the 470.
I don't get the parallel 5K's. This should be the screen node. 1K.
After that, a 10K for the PI & first stage.

[Chuck H]

Since your first and second triodes are biased the same and both are supposed to have negligible loads they should have similar plate and cathode voltages. Something is likely wired wrong and causing the first stage circuitry to draw excessive current on the downstream side of the 100k plate load. This is possibly also the cause of your low amplification, non functional bass pot and excessive hum. You should correct this before continuing any other circuit modifications. Start by unplugging all the tubes and checking voltages at each plate pin. I'll guess that your first triode plate pin will show a voltage drop even with no tube in place.

[Chuck H]

That power supply is giving me a headache.

Why not make it like a Champ?

The output tube should come directly off of the 371v tap.
The screen has two resistances. Drop the 470.
I don't get the parallel 5K's. This should be the screen node. 1K.
After that, a 10K for the PI & first stage.

Jazz, rickyl is using a Pi filter to reduce voltage to optimum and increase ripple filtering. And having a dropping resistor and then an individual screen resistor looks like a hundred other amps to me.?. 2.5k seems like too much in the Pi filter but it's not the bigger problem right now.

[Jazz P Bass]

Optimum what?

Looks great if you want a low powered, never gonna breakup, no negative feedback amp.

[Dave H]

The triodes have different plate voltages because they have different cathode resistors. The first is 1k5 and the second 2k2.

Is the bass pot wired correctly? You didn't by any chance wire to the cw and ccw terminals but forget to connect the wiper to the cw terminal? That would set it to full bass with no adjustment and amplify any hum picked up by the first stage.

[eschertron]

Since your first and second triodes are biased the same and both are supposed to have negligible loads they should have similar plate and cathode voltages. Something is likely wired wrong and causing the first stage circuitry to draw excessive current on the downstream side of the 100k plate load. This is possibly also the cause of your low amplification, non functional bass pot and excessive hum. You should correct this before continuing any other circuit modifications. Start by unplugging all the tubes and checking voltages at each plate pin. I'll guess that your first triode plate pin will show a voltage drop even with no tube in place.

+1 to this hypothesis. the first stage is drawing ~1.5ma across the anode resistor, while the 2nd stage is drawing half that. Something in the tone stack is not according to schem and is drawing the plate voltage down.

Rickyl,
You have good data here with the voltage readings! A voltage drop of 116vdc across the 2.5k pi filter resistor comes out to about 46ma. Ballpark, that What I'd expect. If you want 270 (make it 280-ish to add the 6V6 cathode voltage to the plate-cathode potential) on the power node, figure r = v/i or r = (371-280)/.046 which comes out to 1978 ohms (even figuring on 8v on the cathode you'd not get closer). So make the pi filter resistor 2k and you'll be on target plate voltage

[Dave H]

Optimum what?(

Optimum plate voltage to centre bias it with a 6.3k OT without exceeding the 12W PD of the EL84. I would think that's a B+ of about 330V. I like the Pi filter. It gets rid of the SE hum.

[eschertron]

...and a couple other thoughts.

When you say 'referencing the filament to ground', do you mean it is floating? if so, then any reference will make great improvement in the hum problem. Fender's solution was to tie one leg to ground. You can do that with a test lead to see if there's a difference. Another solution that is more effective is using two 100R resistors (1/2W is OK) to reference each leg of the 6.3vac to ground. This is the simplest 'virtual ground' circuit. Very common and works very well.

You have drawn the tone stack with the traditional rheostat symbol for the bass control. a 3-tab pot will need the wiper (center tab) and one of the outer legs wired up. commonly the tab is jumpered to the 'other' outer leg to minimize scratching sound that can occur if the wiper intermittently loses contact with the track for any reason. One thing to look at when troubleshooting the tone stack will be how the bass pot is wired.

[Jazz P Bass]

330V plate voltage I can see. Now we have a guitar amp.

That 241V simply struck me as extremely odd.

[g1]

1st stage data is incorrect. Either there is an incorrect voltage reading or resistor value or wiring to supply node.
Solving for current, cathode and plate do not match.

Edit: Or possibly what was mentioned above about a parallel current path through the tone stack.
But double check measurements & parts first.
I suppose a leaky cathode bypass cap is also a possibility.

[Dave H]

Yeah, 241V is far too low. The equation says 275V plate voltage for a 6.3k OT and 12W PD but then you have to add voltage to make up for the losses. 25V for the EL84, 20V for the OT and 10V for the cathode resistor say gets it up to 330V B+