Mismatch power tube current balance

[TIMBO]

I don't know what made me think of this but could it be done   This schem is only a guide. Thanks

http://diyaudioprojects.com/Tubes/6L6-5881-SE-Tube-Amp/SE-6L6-5881-Tube-Amp-Schematic.png

[kagliostro]

Someone has used final tubes in preamps and in the forum there was a tread about

but I'm not able to find the link at the moment

and ........... yes, if you want .............. it can be done (with a lot of current consumption)

Ciao

Franco

[jjasilli]

???? in the posted schematic the "preamp" tube is a 6SJ7; the power tube is a 6L6. 

[kagliostro]

Ciao Jjasilli

TIMBO specified

This schem is only a guide

K

[John]

In one of Merlin's books he mentions that it has been done (he attributes it to marketing more than anything). FWIW.

[jazbo8]

Using power tubes as preamp tubes - lots of ideas here.

[HotBluePlates]

6L6 as a preamp tube?????

Triode mu of a 6L6 is less than half the mu of a 12AU7 (6L6 amplification factor is around 8).

Running as a pentode with a 10k load resistor, 300v B+ and 250v screen: the plate voltage swing from cutoff to saturation is ~288v and the absolute value of the grid voltage has to swing from -19v to about -42v to get it done. That's 23v of input signal for 288v/23v = a gain of ~12.5

Seems like a waste of a big-cathode tube. Miniature pentodes take less heater power, make less current available, have higher triode mu and seem to give better gain at low current. But then, of course they do, because big tubes are optimized to deliver power.

Using power tubes as preamp tubes - lots of ideas here.

102 pages!!! I confess to reading only the first 3 to get an idea of what was going on.

Line "preamp" the way they used a 6V6/6AQ5 is really a misnomer. They used a power tube strapped as a triode as an output device where gain was needed, but either power output capability (to drive line capacitance) or low source impedance (triode mode cathode followers, with or without transformer coupling) are important.

Voltage gain (which is what your guitar amp preamp provides) was present/important in none of what I saw in the brief skim of the early part of that thread.

So "Can it be done?" Yes. But smaller, cheaper tubes will do the same things or more for less dollars and heater power.

[TIMBO]

Thanks HBP, Got a pair of mismatched RCA 6l6s and when flicking through some stuff on the net I briefly saw something on the subject but couldn't find it when the idea hit me. Thanks anyway, I hoping to do a dual adjustable cathode or fixed bias so I can use them.

[HotBluePlates]

... hoping to do a dual adjustable cathode or fixed bias so I can use them.

If you add a rheostat between the 68Ω and the tied ends of the 100Ω resistors, then it can adjust overall tube current in the schematic below. The existing 100Ω pot adjusts the balance of idle current between the tubes.

Pin 3 of the 807 is the screen; of course you don't have to use 100Ω screen resistors.

The "100Ω" resistors which connect directly to the coupling caps should be 100-120kΩ.

68Ω may be swapped for whatever value you think is a minimum resistance below which you won't go (because the tubes will idle too hot). Imagine the pot shown set in the middle; the total resistance is then 68 + 50 + 25 = 143Ω, because of the parallel paths. If you added a 100Ω pot as I suggested above, the total resistance is just under 250Ω which often works for reasonable B+ and cathode-biased 6L6's. You could bump the 68Ω up to a fixed 100Ω and have a range of adjustment from 175-275Ω (or go a little higher on the fixed resistor if you think you should).

I used a version of exactly this in my 25L15 copy amp.

[stratele52]

6L6 as a preamp tube?????

Triode mu of a 6L6 is less than half the mu of a 12AU7 (6L6 amplification factor is around 8).

Preamp is 6SJ7 not 6V6

[kagliostro]

Stratele52

read post #2 and #3

K

[darryl]

I can verify that using a 6L6 as a cathodyne phase inverter is a simple task. This amplifier was the result of a challenge issued on the AGGH forum.



It has a 6L6 phase inverter driving a pair of 6V6's. 

The challenge was met, the amplifier worked fine, but it was a waste of resources.   

[kagliostro]

Hi Darryl

the 6L6 PI was connected as triode or not ?

K

[TIMBO]

Thanks HBP, I've drawn a schem just for reference and put in a PPIMV as well just to make sure I've got it right.

The 360r in brackets is the resistor I used for my TOS so some adjustments could be made.Thanks

[kagliostro]

May be I'm wrong but I think the PPIMV pot must go direct to ground

K

[shooter]

I'm still learnin here but I found this datasheet on another thread here, pg 12 has a suggested circuit that looks like the one you're discussing.  Anyway, hope it helps....somewhere, before we all become C batteries.

[shooter]

Sorry wrong schematic!  this should be the one I was referring to

[sluckey]

May be I'm wrong but I think the PPIMV pot must go direct to ground

I think he had it right. The 100Ω balance pot operates by varying a positive voltage sent to the grids. If you ground the MV pots you lose the balance feature.

[tubeswell]

Using power tubes as preamp tubes - lots of ideas here.

Wow! - a 248 page-long thread! (and counting - no-doubt)

[HotBluePlates]

Thanks HBP, I've drawn a schem just for reference and put in a PPIMV as well just to make sure I've got it right.

For cathode bias only, you don't need the 2.2MΩ resistors.

I'm not sure this is a good system if you're trying to cram both fixed-bias and cathode-bias in one amp.

[TIMBO]

HBP, The schem below maybe a bit easier to do and won't effect the PPIMV

[kagliostro]

To do that you require high W pots

you can achieve a similar result with standard pots in parallel with the cathode resistors

The value of the pots must be enough large as to avoid big current across it

Franco

[TIMBO]

Hi guys, I thought I was just about sold on using a dual fixed bias to accommodate these mismatched buggers. Well I found these (pics below) 2000 ohm @50w adjustable and all.

They aren't real heavy but I was thinking that I might be able to cut them through the middle just enough the cut the winding to make two 1000 ohm ends.

[sluckey]

I might be able to cut them through the middle just enough the cut the winding to make two 1000 ohm ends.

Why risk destruction? Just use them as they are.

[PRR]

> might be able to cut them through the middle

You won't be able to solder to the special wire.

If you want two resistors going to the same place (like two cathode resistors to ground), just set the slider dead-center, ground that, and take the two ends to cathodes. Or since you are fighting unbalanced tubes, slack the slider a bit and offset it until "balanced".

[HotBluePlates]

... Well I found these (pics below) 2000 ohm @50w adjustable and all. ...

Is that really necessary? There's got to be an easier way...

[tubeswell]

To do that you require high W pots

you can achieve a similar result with standard pots in parallel with the cathode resistors

The value of the pots must be enough large as to avoid big current across it

Franco

That's the gem of a good suggestion. I would add a (large enough) resistor in series with each pot to ensure a minimum parallel resistance (parallel to each Rk that is)  that didn't put too much current through each pot (as you dime-down the pot resistance)

[HotBluePlates]

To do that you require high W pots...

I don't consider 2w to be "high w" pots. But yeah, not the common 1/2w pots; however, not exotic, either.

Also, I know first-hand from use in one of my amps that the plan I presented works as-advertised.

[TIMBO]

HBP, As always I value your input and limitless knowledge. My problem, living here in OZ is that anything that is valve related is hard to acquire. The local electronics store only caters for low voltage stuff and any thing over 1/2w is not available and if by chance they have anything hi watt/ hi voltage its very limited.

Its a bit like these tubes I am trying to use, I could use them in two single ended builds but then i would have to find two SE OTs and I already have two SE amps.

I might find that having majorly mismatch tubes a no goer and as a back I have a couple of matched pairs of 6L6s, so if I have a bias that will cater for both matched/mismatch tubes it will make it easier to swap over

I have no doubt that your schem works, and to adapt this circuit and to be able to adjust it enough to get the two tubes drawing similar current.
I can get 100 ohm/3w pots @ $8 or 1k @ the same price from the local store but there is no guarantee that they have them in stock.

Because of this I then look to another option of the Fixed Bias as they are low voltage/wattage parts I can get locally.

The two adjustable resistors ARE NOT something that I can buy here at all but these were in a old electrical substation that I happened to be working in and they were in a pile of parts that were removed from an old switch board, lots of high voltage and high wattage.

And of cause tweaked the idea of how I could use them.

I guess that I probably would not have worried able all this but after all they ARE RCAs