bias advice

[dscottguitars]

Hello,

Can someone give me some advice on the bias circuit I am using for this amp.  It is about 100 amp, 4 6L6GC's; switchable dual rectifier or solid state; and switchable fixed or cathode bias.  It is one I built myself.

I wanted to have the bias pots, one for each side, so that one cannot go to low in voltage resulting in too much current that exceeds 70% of the plate dissipation.  And, cannot go to high to make dissipation really low.  I used a 10K pot and get a range of 8 volts to work with.

My main concern is that if someone buys this amp and puts other tubes in it can another quad/pair of tubes be such that the grid voltage would not be negative enough to keep it under the safe dissipation?  So far I only have two tubes, because I'm waiting for a couple more to come, but the ones I have are GE and test on my EMC Model 213 tester at: 48 out of 50 and 46 out of 50.

The voltage range I have is -27v to -35v. At -27v on the grid I get about 48mA at 420v on the plates for about 20 watts dissipation.  At -35v I get about 32mA, and 430v on the plates for about 13.7 watts dissipation.  On another similar amp but only 50 watts it has -29v and 435v on the plates for about 45mA, but that had a 25K pot for more range.

My reason for doing this is to allow a user to adjust the bias voltage without getting too far either way, red plates and cutoff, using a bias socket tool to measure the current.  Is this not a good idea?

The amp also has a low power switch which changes the load resistor on the bias circuit to match the plate voltage.

[dscottguitars]

Willabe,

It does work.  I've used it before.  When the cathode mode is selected, on the drawing, the right four pins on the switch have the point 'a' from the 6L6 cathodes going to the 220 ohm resistor and 68uF cap.  Point 'b' goes to ground from the grid's 200K resistors passing through the 10K pots, or through the tail resistor probably depending on where the pot wiper is at, but it only adds 22K which is still ok.  When in fixed bias mode, the left four pins have the point 'a' going to ground from the 6L6 cathodes and at point 'b', lets the negative voltage through to the pots to adjust for the grid voltage.

The tail resistors are there to keep the negative voltage where I choose.  There are two because that switch is shared with the high/low power DPDT and selects a different resistor for different bias voltage in high or low power.

It's complicated but works.

I would like to know what you think about my wanting to have that range of negative voltage and if other tubes will cause a problem.

thanks...

[Willabe]

To up the -bias you can up the value of the tail R from the bias pot(s) to ground, but you have something else/extra going on there.

What is the DPDT switch that's hooked up to the PSU and bias pots for?


                    Brad        

[dscottguitars]

That switch is for high/low power...

And yes, I can change that resistor but I am curious to know about the range I get with the pot. 

[Willabe]

Ok, I posted then took it down to look at your drawing a little longer. When I came back you answered my question.

If you say it works good and I think I understand what your going for/doing.

Try raising the value of the 2 bias pot tail R's.

I agree with you, 8dcv of bias swing is not enough. I think guys shoot for about 25dcv of bias swing from center. 12.5 up/more, 12.5 down/less?


             Brad    

[dscottguitars]

Hi,  The difference between ss and tube rectifier is opposite of what you thought.  Tube rectifier is a spongier response than ss.  SS is tight and punchy.  The spongier loose effect from the tube is kind of a fast wah sound, and I think it's due to the tube being a bit 'starved' in the current flow.  SS handles that better.  At least I think that's what I understand from what I read.

The phat/screamy boost switches are only due to the cap used.  The small cap gets a higher boost than the large one which is a bit more bass.  It's my own descriptions.

This amp is not going for clean too much.  The low voltage on the preamp tubes increases the gain vs headroom.  It can get pretty clean with the feedback control but not like your typical Fender.  I get anywhere from Nugent's Stranglehold sound to Clapton's Sunshine of your love. 

Switching rectifier and bias adds to the gain a bit and a spongier response. They also lower the output power a bit.

[Willabe]

I think it's due to the tube being a bit 'starved' in the current flow.  SS handles that better.  At least I think that's what I understand from what I read.

Yep. It's from the rectifier tubes plate resistance acting like a R in series with the B+ PSU. Pull enough current through the rec. tube and it can't pass it all, so the B+ drops voltage, sags per ohms law. This acts like compression to the guitar signal.

Bang a big chord hard, the amps power tubes draw a lot of current at first to reproduce it and the B+ drops, so less volume output. As you hold the chord the the strings start to die off and the amp needs less power from the B+ PSU. So the B+ voltage rises back to full B+ and this causes the volume output to rise with it. Built in compression. (This all happens very fast.) This can be part of the sweet spot of an amp, you need to get to a certain volume to get the B+ to sag enough. Helps the amp to sing/sustain.

The lower the note the more power it takes to reproduce it, so bass notes are spongier, sag more than higher notes.


                Brad      

  

[PRR]

When unlabeled switch in "Bias Circuit" is in the right-side position (see below), the 160V caps will charge to 500V.

It might be better to short-to-ground, such as the small-dot line I added.

You could reduce to a SPDT switch, tie the middle lug to ground, switch either grid-supply or cathodes to ground.

> one cannot go to low in voltage resulting in too much current that exceeds 70% of the plate dissipation. And, cannot go to high to make dissipation really low.

Don't make the garage door too narrow.

Trust furture techs (or let them learn).

Let your fattest tubes go 120% rated dissipation on the rich end. It is bad to go to dead-zero bias because that may be 500% dissipation and death-in-seconds. That would turn a momentary mistake into a costly mistake. But tubes will stand 120% for minutes, long enough for the tech to notice the warm glow and unusual heat.

There's no real limit how cold the bias may go; no harm happens. As you suggest, some tubes do need plenty of bias. The other issue is that with a huge range available, a fine adjustment gets very twitchy. However tube bias is not a fine adjustment. A dozen mA, several Volts, is all the same to the audience.

Based on your limited data, I would go 20V to 45V. That's 25V range, spread over 270 degrees rotation, 10 degrees per volt, easy to adjust yet able to cover a broad range of tubes and player/tech taste.

[dscottguitars]

Thanks for that suggestion.  If I understand you correctly, see drawing...

And I will change the pots to 25K for more range.