4X6V6 Cathode Bias

[stokes]

Building a 4X6V6 PP output,cathode bias.Would there be any benefit splitting the cathode resistor between the pairs as opposed to just one cathode resistor for all 4 tubes?I have a 25watt 500ohm rheostat I want to use for an adjustible bias.I would think it wouldnt make a difference but I thought I would throw it out here and see if anybody has any ideas.Thanks.

stokes

[markbul]

Splitting them would allow you to pull two tubes if you wanted, and would let you see unbalanced tubes better.

[Enzo]

DEpends on how you split them then.

[MWJB]

If you gave the inner pair of tubes their own cathode resistor & bypass cap, then gave the outer pair of tubes their own resistor & cap, then you could pull either pair without any further adjustments.

You could also sub 4x6V6 for 2x6L6 in either the inner 2, or outer 2 sockets (e.g. you still have one tube at either end of the OT primary and both share the same Rk).

Of course, you would need another rheostat to keep with your adjustable bias idea with the 2 cathode resistors. However, I'm not sure that the adjustable bias feature is really of that much value, unless you are going to be constantly changing rectifiers?

Even if you go with adjustable bias (500ohm rheostat) with 4x6V6 sharing one cathode resistor, you're still going to want to use a minimum value fixed resistor to ground, in series with the rheostat, to protect your tubes when making adjustments. With a rheostat that big you may want to ensure that your B+ doesn't rise to dangerous levels for tubes & caps, at the end of its sweep.

My preference would be to use a pair of cathode resistors & bypass caps, select fixed cathode resistors so that the 6V6s aren't dissipating more than their max PD with the tightest rectifier, then just simply enjoy the various tones available from differing rectifiers/4x6V6/2x6V6/2x6L6 etc.

[WMK]

Building a 4X6V6 PP output,cathode bias.Would there be any benefit splitting the cathode resistor between the pairs as opposed to just one cathode resistor for all 4 tubes?I have a 25watt 500ohm rheostat I want to use for an adjustible bias.I would think it wouldnt make a difference but I thought I would throw it out here and see if anybody has any ideas.Thanks.

stokes

There are builders out there that believe there's a
sonic benefit in doing this.

The use of two resistors will allow for self balancing
between the opposing sides of the output trans primary- something
a single resistor can't do. Not so important if using matched
tube compliments but should allow the two sides to self-adjust
dynamically.

It will also allow for reduced wattage capacity of the resistors used.

This can be found in the Mullard tube audio circuits book
as well in many models of the original Matchless designs.

[stokes]

Thanks for the replies.Most are points I have considered,and to me are not that important.I know cathode bias is "self biasing",but I still like to tweak the bias when I change tubes.I have three other cathode biased amps that I used Ohmite wirewounds that are adjustable.Those amps are either single ended,or 2x6-6 PP.I just wanted to make sure there were no serious design flaws in using a single cathode resistor in a 4 tube PP output.What I am building now is a 5E3.I have one with 2x6V6's which I love but sometimes it just aint got enough headroom to cut through with a full band.With 4 6V6's I figure it should cut it and hopefully retain the tone of its smaller brother.Thanks again for the input.

[Shea]

That 500 ohm rheostat is probably too big for four 6V6s. You'd be using just a small portion of its travel for adjustment. The 5E3 schematic shows a 200-ohm cathode resistor. You'd be using half as much for four 6V6s at the same plate voltage -- 100 ohms. Then if you put a fixed resistor in series with the rheostat, which I agree is a good idea, that will reduce the needed size of the rheostat even more. So even when we allow for the fact that you'll want some extra resistance for adjustability, and possibly for running higher plate voltages than the original 5E3, 500 ohms is still way too big.

So, I would say try a 100-ohm rheostat in series with a 50-ohm fixed resistor, both bypassed with a single 50 uf cap, if you connect all the cathodes together. If you use two separate cathode circuits, then you could use a 200-ohm rheostat and 100-ohm fixed resistor, both bypassed with a 25 uf cap, for each circuit. Or, you could use two fixed 100-ohm resistors, each of which is connected to the cathodes of one pair of tubes, and bring those together to a point that is in series with a 100-ohm rheostat. And run a 25-uf cap from each pair of cathodes to ground.

Shea

[Bruce / Mission Amps]

OK time to jump in....
My original 5E3X2 amp used four 6V6s, a homemade, solid state version of a 5U4g rectifer with 1n4007s, 2 power resistors and a thermistor, a 2k7 2W screen node dropping resistor, a single 150 ohm 15 watt cathode biasing resistor on all four cathodes, a 100uF@100v bypass cap, a 4K@30 watt OT, 320vac@200ma PT. All the 6V6 tubes I tried idled hot around 10-13 watts each... all of them.
I would have needed two 47uF@100v cathode bypass caps and two 300 ohm 5 watt resistors to do the same thing with pairs of self biased power tubes.

If you want to use one or two 500 ohm rheostats, you can figure that out from here but I think it is over complicating a very simple circuit and really don't see any need for it, .... based on my findings that all the tubes tried idled around 10-13 watts.
I don't think 500 ohms is a good choice to use for this anyhow as it seems too much resistance to be that useful.
You could, use a smaller value 100 ohm 5 watt resistance resistor under each pair of poaer tubes and stick the grounded rheostat wired as a variable resistor under their common point. NO balance but you can alter the actual idle current a bit.
Or you could use the rheostat with it's wiper grounded, then each pair of power tubes sitting on relative ends of the rheostat and use the rheostat to balance the pairs. Watch that one as you could turn two pairs on full blast and blow the tubes, OT or at least the fuse.
You could parallel resistor across each leg of the rheostat to it's wiper for less total resistance and the put the wiper above ground with a third resistor.

[stokes]

I have a set of Thoradson tranny's that I figure will give me about 350v on the plates.The OT has taps for 4,8,16,32ohm taps for a pair of 6L6's.With a .5vac on the secondary I figure a 3000ohm primary should work for my quad of nos Marconi 5871's.Thats using the 32ohm tap.I figure it is best to use this tap because it uses the entire sec.winding and the #'s work out.I have to check but in my 2x6V6 5E3 I am pretty sure I have somewhere around 300ohms on my cathode using an adjustable 500ohm wirewound Ohmite.I just happen to have 2 25watt 500ohm rheos I got when I was ordering other parts from Weber.I know I only need around 150ohms for the cathodes on a quad,but the 500 is what I have.As far as a balance pot on the output I never saw a need for these as I dont see a need to have the tubes matched any closer than +/- 5ma.The Marconi's I have are closer than that anyway.I also have a quad of Sylvania 7591's I've been saving since I bought them new back in the early '70s I want to try in this circuit.I got that quad for $25 from a local electronics store for an Ampeg that I ended up retubing with 6L6's.If I only knew then what I know now.That guy was practically giving tubes away,said tubes were on the way out,SS is the way to go.Anyway,back to the subject.I was wondering about the bypass cap.Bruce,you think 100mf is the way to go?I was thinking about using a 50mf.I have a 35mf on my other 5E3 (pair).You think it'll make a big difference?I have both so I'll try them.Thanks again for your thoughts.
stokes

[LarryLarry]

Howdy - Just joined here. Been over at 18watt.com for awhile. I'm doing a 5E3x2 and am having problems getting my 6V6 plate voltages down. They are at 380v right now, idling at 40mA. I'm using a 150 ohm 10w cathode resistor bypassed with a 50uF 150v cap.

Stokes - did you get yours sorted out ok? Is there a layout or schematic for this amp somewhere besides Weber?

[MWJB]

1) Is that 380v before or after you have subracted the cathode voltage?

2) How did you measure the 40mA?

If you used ohms law to determine bias current, and have not yet subtracted screen dissipation and you still need to subtract your cathode voltage from the plate voltage...then you are OK.

Even as it stands at the moment, if you have not subtracted your cathode voltage, your 6V6s are dissipating 15.2W. So back to Q2 - if you determined bias by ohms law & still need to allow for screen current you are probably still OK.

A larger cathode resistor will push up your B+ further, beyond classic 5E3 voltages, so you might want to stick where you are and see how you go? If on the other hand you can't drop any B+ and feel, you need to get below 14W plate dissipation, then the easiest thing would be to go up on the cathode resistor and live with it.

[LarryLarry]

I did not subtract the cathode voltage (24v in my case) from the 380v. I used a bias probe on 1 6V6 at a time. Is this not applicable for cathode biased amps?

Thanks!

[MWJB]

Cool, that works fine. So you have 40mA x 356v = 14.24 plate dissipation. You might be OK, give it whirl and look out for redplating or any other unusual symptoms.

[LarryLarry]

I am using the Weber 5E3x2 layout. It shows a 25w 100 ohm resistor between the standby switch and the OT CT. That was giving me a lot of home so I pulled it out. It also has a 1/2 w 100 ohm resistor on the CT of the 6.3 taps going to the cathode of one of the 6V6s. This was the last thing I pulled. I may put it back as it raised my plate voltage and introduced some hum...

[LarryLarry]

So I put back the 1/2 watt 100 ohm resistor on the 6.3v CT. It got rid of a bit of hum and brought my voltage down some. I also switched from the 120v tap to the 125v tap which also lowered my plate voltage. Finally, I went with a 5U4GB which of course also lowered my plate voltage. Here's where I'm at now:

With GZ34
B+ 347v
362v 6V6 plates
349v 6V6 grids
22v 6V6 cathode

With 5U4GB
B+ 332v
342v 6V6 plates
328v 6V6 grids
21v 6V6 cathode

I think I'm ok with either rectifier. I kind of like the headroom/response with the GZ34 better...