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6C33C cathode follower, slimline power amp (design: Suncalc)
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Author:  laurie54 [ 10 Jan 2018, 03:24 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

Even at 12V for two filaments it's up to 7.2 Amps. Going to make a great room heater. If you
insist on DC heaters there are going to be a lot of TO-3's in the regs. How about a SMPS?

Author:  mwhouston [ 10 Jan 2018, 06:40 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

It is an issue with these tubes.

Author:  M. Gregg [ 28 Jan 2018, 07:05 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

mwhouston wrote:
It is an issue with these tubes.


mwhouston wrote:
It is an issue with these tubes.


Your just scratching the surface at the moment.
Heat from the tube heaters seems OK a bit hot but OK.
Then you run bias through them and oh crap. you need fail safe shut down on the bias I've seen one red plate :hot:

There are some strange anomalies with these tubes, you might be lucky and not have them,
But you can get cathode modulation and heater cathode interaction giving weird effects.
Low current series is the way to go, I run AC filaments with heaters reversed to help with hum cancelling but whatever floats your boat.
I also run mixed bias I think it sounds better. I also float the heaters.
Have fun.
There are quite a few OTL's commercially available (PIC), I switch heaters and run one side of the tubes as well.
So I have single left- right or inner left - right single or all heaters on to ensure equal wear.
I run 200V anode.

NB in the picture I like the granite tile because it looks good<<<wrong its a fire guard..LOL
However you are just running one tube so its probably all good.


Regards
M. Gregg

Author:  M. Gregg [ 28 Jan 2018, 07:16 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

NB I ran cathode bias on four of them,

LMAO the solder kept melting on the cathode resistors.
:D

So if your thinking DC heaters and cathode bias better mount the tubes in a flower box outside the window.

Regards
M. Gregg

Author:  mwhouston [ 28 Jan 2018, 17:13 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

You all have me worried. Still wading through other projects so sometime to go yet.

Author:  ILoveHiFi [ 28 Jan 2018, 19:29 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

I don't think this design is a good idea because as a cathode follower the voltage you need for 1amps into 8ohms is only a swing of 16v pk-pk.
It seems like datasheets tell you max current should be 0.63amps.

The voltage supply should be dropped, anywhere arround 50-200v should be good. Taking into account of internal resistance of valves one would find the best vcc for the follower.

Furthermore the output cap needs to be at least 2x larger. Cut off freq with 220uF = 90hz into 8ohms
https://www.electronicproducts.com/RC_F ... lator.aspx

Author:  mwhouston [ 28 Jan 2018, 21:37 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

Agree, cap should be larger.

Author:  Suncalc [ 29 Jan 2018, 19:54 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

I thought I might post the load line for this cathode follower so people could more easily understand the design. So often just having the schematic tends to obfuscate the design process and decisions. Here is the load line diagram:
Attachment:
6c33C Load Line.jpg

And here is the list of operating characteristics:
Attachment:
6c33c Load Line Characteristics.jpg

Hopefully having all this data will help folks understand the design.

The tube is operating at Vp=114v and Ip=185mA. This gives a plate dissipation of just over 21W and allows some margin for both tube and power supply variability. The very low µ of the tube limits the distortion reduction due to feed back but the final number is only ≈2.4% based on a ±20v peak swing. This is reasonable performance for this type of driver.

I would also like to directly address one comment.
KochiyaYamato wrote:
Furthermore the output cap needs to be at least 2x larger. Cut off freq with 220uF = 90hz into 8ohms
This would be correct if the driver had a very low output impedance. However, the effective cathode impedance is 47Ω. Using the 200µf capacitor shown on the schematic yields a -3dB low frequency rolloff point as follows:

f-lo = 1 / (2*pi*200e-6*(47+8)) = 14.46 Hz.

This rolloff point was chosen to control the size of the output capacitors. These parts must be carefully chosen so that they are fully capable of handling the ripple current. Although I usually don't recommend them in audio applications, a motor run capacitor might be a good choice for this particular part.

Hopefully this additional information makes the design a little more clear.

Author:  mwhouston [ 29 Jan 2018, 20:29 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

I can purchase from where I work 220uf NP caps which I have used (at least the 100uf version of) on other builds and thought they sounded excellent. I do bipass them with 0.1uf polies.

Thanks Matt for a further explanation. I'll stay with the original caps. Project is still a way off.

Author:  ILoveHiFi [ 30 Jan 2018, 04:52 ]
Post subject:  Re: 6C33C cathode follower, slimline power amp (design: Sunc

Quote:
Hopefully having all this data will help folks understand the design.

The tube is operating at Vp=114v and Ip=185mA. This gives a plate dissipation of just over 21W and allows some margin for both tube and power supply variability. The very low µ of the tube limits the distortion reduction due to feed back but the final number is only ≈2.4% based on a ±20v peak swing. This is reasonable performance for this type of driver.

I see zero reason to drop nearly 200v accross the emitter resistors, 130v is already sufficent to get same operating point without extra heat. A constant current load with high voltage transitor would be very suitable. Since v=IR and the valve will never see 1amps, max output swing is only 16v (8*1=8v for 1 amps into eight doubble because postive and negative swing)

This means a 10v drop on the consatnt current load is more than sufficent and it won't see much heat. using a 1.8v leds to bias a szlikai connected power tranistor yeild arrount 1.1-1.3v at the emitter, using 1.8v and another series diode and a darington pair will yeild about 1.1-1.3v at the emitter. 1.2/0.185a=6.85ohms emitter resistor equired to get 0.185amps.

Furthermore the resistors used are extremly underated, 0.185amps into 200ohms means 37v drop and 0.185amp load this means 6.845w into this 200r
820*0.185=151.7v*0.185=28W into 820r


The power rating should be at least double of the actual power

The emitter resistor of constant current load should never consume more power than the output valve or transistor.

Because the current capable of the valve i would try biasing it at lower currents to get higher output power

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