Quad 6V6 Plexi

[Ed_Chambley]

She's alive, and she's a real killer.   

I had a few issues after completing the wiring.
That is one of the challenges with re-using salvaged parts.
Rectifier diodes were the major problem.
Exploded an e-cap to find that out.
1 signal cap was leaking, so I needed to replace 4 parts to start.

Then she squealed something awful.
Swapped the OT leads, no more squealing.   

I really don't understand how that OT lead swap works.   
But it definitely does. 

Some fine tuning and the completion of her paint job are all that's left.

The "Black Widow" is a killer amp!

Thank you for your guidance HBP!   

Nice work using salvaged parts. 


I am interested in why you want your Bias individually adjustable other than thinking you can use tubes that are not matched?


Also, since you are not looking to get the Maximum output from the 6V6 tubes, why not cathode bias?  I understand the learning process and you have dome a couple of neat things, but I am rather afraid of using chinese boards for Bias supply.


Do you have a fail safe if you lose negative bias voltage?


I am asking because I have a Quad 6V6 amp, but my idea was different as I used 2 OT's and a half power switch.

[Paul1453]

Nice work using salvaged parts. 


I am interested in why you want your Bias individually adjustable other than thinking you can use tubes that are not matched?


Also, since you are not looking to get the Maximum output from the 6V6 tubes, why not cathode bias?  I understand the learning process and you have dome a couple of neat things, but I am rather afraid of using chinese boards for Bias supply.


Do you have a fail safe if you lose negative bias voltage?


I am asking because I have a Quad 6V6 amp, but my idea was different as I used 2 OT's and a half power switch.

Thank you for the compliment Ed!

Learning process was one factor, another was being able to closely match unequal performing tubes.
A third reason, and I'm a little leery to try this just yet, was to match say a 6L6 with a 6V6 or EL34.
This approach seems to offer maximum flexibility and possibly unique sonic flavorings.

Fail safe???
I too was a little concerned with the reliability of Chinese parts, but consider these points.
Most fixed bias circuits use 1 diode to generate the - voltage.
If that single diode fails, open or short, we lose bias without warning too.

This board has a full wave rectifier with 4 diodes on the board.
So maybe if 1 of the - supply diodes opens, we still have 1/2 wave - bias operating.
If any of the diodes short we still lose our bias, but will likely explode an e-cap from applying AC to it.
That should give us at least a little indication of a problem from the bang and smoke coming out of the amp.   

Cathode bias is inherently safer, but does not allow for simple manipulation of our biasing.

I'm running her very conservatively right now.
I've got a +313 B+ and all 4 6V6s are biased at 30mA of cathode measured current.
I know from my other 6V6 Plexi I can safely push that up to around 44mA without red plating my tubes.

My C,D, and E node voltages are somewhere between + 20 to + 40 V over what Doug's plans show.
My PSU simulation told me to use 1.5K for my 1st PS resistor, and those elevated voltages are what I got.
One of my remaining tweaks is to up that resistor to 2K or whatever turns out to be the value I need to hit Doug's target voltages.   

[pompeiisneaks]

yeah basically it creates oscillations because half of the output is being increased instead of decreased causing things to grow out of proportion with each cycle of the sound through it.

~Phil

[Paul1453]

Do higher preamp voltages increase the noise floor level?

I don't have any hum or other noise issues with this Quad build,
but it does seem that the noise floor is higher on this than on the 1st one I built.
It is not bad at all, most people might not even notice.
Is it just that since this one has more output, the noise floor is more noticeable?
The NFB presence pot setting also seems to affect the noise floor.

I've upped the 1st resistor in my PS to 2.7K and my C,D, and E nodes only came down 5-10 volts.
It did boost my B+ a few volts.  I decided to up my bias current to 40mA.
I didn't recheck my voltages after that to see if it dropped the C,D, and E nodes any yet.

[HotBluePlates]

... the noise floor is higher on this than on the 1st one I built.
...
Is it just that since this one has more output, the noise floor is more noticeable?
The NFB presence pot setting also seems to affect the noise floor. ...

Noise could just be a noisy tube or resistor. In rare cases, some folks notice less noise when replacing filter caps, but I sometimes wonder if the noise reduction is simply reflowed solder in those cases.

The higher output could be a contributor to higher noise. And of course, when you adjust the Presence control for more highs, you're effectively increasing the gain for those highs, where hiss lives.