• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

ccs, direct coupling and stuff

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beamnet

beamnet

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Joined 2004
geocities.com/beamnet
#1
Here's my problem

I have an amp using a long tail phase splitter. I am terribly unhaapy with the performance.

I want to use a ccs.

The circuit is half a 6sn7, directly coupled to an ecc99 ltp with a 10k tail


I am not very good with ss design, so i decided to copy the a css

How do i manage to get the gorrect voltage at the kathodes to keep the direct coupling?

Or should i decouple with a cap?

I am not unwilling to do some calcs, i just need a few pointers
 

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#4
Balance the anode loads. Match them if you like.

You will have to make a CCS from scratch because it must withstand 100V or so. Unless AD have changed their policy, they don't make valve support circuitry...
 
#6
IXYS makes a CCS that is rated at 450V. IXYS 10M45. Sold at Digikey etc.

Texas Instruments makes a linear regulator that can be used as a CCS and will tolerate up to 125V between in and out if you need alot of current. TL783.

Theres some fairly simple CCS designs of good performance using just a couple cascoded mosfets. This picture from Gary Pimm's site illustrates the idea:

An externally hosted image should be here but it was not working when we last tested it.
 
#7
The simplest CCS by far is the IXYS 10M45 chip. It works good too. Get the amp working with it, then you can upgrade if you want.

You need two resistors and the chip. The resistor from the "K" pin to ground sets the current, and you need a 1K resistor in series with the "G" lead to supress oscillations. The "stopper" resistor is not shown on the data sheet, but bad things happen if you don't use it.
 
#9
beamnet said:
Here's my problem
I have an amp using a long tail phase splitter. I am terribly unhaapy with the performance.
Click to expand...

Although that is not going to be a perfect splitter, your problem may be elsewhere: your 330nF coupling cap and 47k grid leak frot he GU50 make a 64Hz highpass filter. This REALLY needs to be MUCH lower. Although seeing that you use pentode mode with no NFB or even local FB, are you maybe attempting to equalize the resonant peak of your speaker?

How do i manage to get the gorrect voltage at the kathodes to keep the direct coupling?
Click to expand...

Quite easy actually, just set your CCS to the same current that passes through the 10k tail resistor now. That way the DC conditions remain unchanged.
 
#10
is it! damn!

That are the things i can't calculate. Increasing the coupling caps to 1uF won't take that away would it? This is a design by a highly respected designer here in holland (in the real design he used kt88's)

I thought he'd done his calcs right...Ii'll drop him a mail...

any suggestions to lower the filter? 100k leak/2uF coupling?

It does have solid bass, but might be increased if the transfer function is optimized below 64hZ.....

I want to experiment with feedback in the future, for now it sound very acceptable to me. The opt's don't require nfb to be flattened out. I want to decide on the final seakers and then go the fb way (or not)

There are recent rumours that UL could be achieved (there's a thrread here that i started or contributed to, don't remember)

Really appreciate the help here, finally my bodged together circuit is analyzed in an uplifing way!

Bas
 
#11
ARGH my bad - it's not 64Hz but 10.2Hz, sorry.

Still, maybe there is a mispring with the values - 47k as grid leak does sound a bit low, even for large tubes. Was it maybe 470k?
The frequency is easy to calculate - 1/(2*PI*Rleak*Ccouple). Still, even 10Hz sounds a bit on the low side if you intend to use NFB, unless it is taken as plate-to-plate from power tubes to splitter. You are adding phase shift where you already start getting some from the OPT, so you could get in trouble - but this depends greatly on OPT.
 
#12
I knoe few things obout the opt, they are excepionally BIG. C-core. They came from a beard kt88ppp 100watt amp.

I'm not sure what that means in terms of phase shift, but i know that it doesn't require the feedback because i starts to drop too early in the bass. If feedback is implemented it must be to lower Zo because my speakers might like that.

The original circuit has 39K resistors going to bias supply.

I am totally in the dark here, but would 470k be better?

For the req calcs: i knew that formula. I thought you had to account for Zo of the driver and Zi for the tube

The Zi of the tube dictates the bleed resistor too i suppose.

Looking at mu curves and bias power supply, it is obvious that my power supply can't deliver much current, and as far as i ca see, the grid current is not very high...

Compared to other amps,my ampsi a little low on the bass, but i suppose many commercial speakers raise the bass to sound good.
(especially ol marantz amps..)

Measuring with my tone generator and a scope, i don't see any early dropoff. I even remember it dropping below -6db at 8Hz

Bas
 
#19
This thread shows a pentode CCS in the cathodes of a 6SL7 long-tail pair splitter in my amp. The key is to use the pentode at an operating point where the plate curve is essentially flat, i.e. the plate current doesn't vary singnificantly over a wide range of plate voltage. In this case, plate current in the 6AU6 is a bit less than 2 mA, screen-cathode voltage is a little under 100v and plate-cathode a little over 110v.
 
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