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Simple Se build, Which tube?

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preludefan

Member
Joined 2011
Hi all,

I have a pair of 10 watt 5k outputs on my shelf and am looking to build an SE amp to utilise them.

Tubes I have in my stash,

Westinghouse 807
Pl500 sweep tube
El33 mini watts
Mini watt el84 ( parallel Se?)

I want to use the tube that will give the most power, I want the 807, but it's screen voltage is only 300 max vs its 750v plate voltage.
Would a triode connected 807 output more at 600V with a 5-6k load in triode mode than it would at 300v pentode mode?

Or am I better to go for the pl500?

Thanks in advance for your help

Mike
 
K

KenTajalli

Member
Joined 2014
preludefan said:
Artosalso, I like your circuit, thank you for sharing it with me, it says that you measured distortion at 3W. Is this the maximum output possible from a pl500?
Click to expand...
I am a "signal capacitor" hater!
There are so many capacitors in the signal path for my liking. I have counted four! not counting PSU caps (some consider those to be in signal path too).
The first 1uF can be replaced with a few kilo-ohm resistor.
Use the best 220nF you can afford, I like paper in oils.
For the larger electrolytic ones, use quality caps, at least nichicon fine gold or better.
Also for the pot, 100K 220K and 680R use low noise resistors, to get minimum hiss.
Good luck.
 
artosalo

artosalo

Member
Joined 2010
807 will produce some 8...10 W when pentode connected, but THD will be about 10 %.
As a triode or UL the output power will be essentially smaller, but THD still relatively high.

The input capacitor can be excluded if the signal source is producing no DC to output.

Power supply and filtering capacitors are naturally in the signal path.
How the signal (AC) current could flow otherwise ?
 
K

KenTajalli

Member
Joined 2014
artosalo said:
Power supply and filtering capacitors are naturally in the signal path.
How the signal (AC) current could flow otherwise ?
Click to expand...
Some debate if the PSU caps are signal caps, I am sort of undecided.
Signal naturally means "audio signal".
It is a question of whether a design relies on smoothing caps to short out audio signals or not.
It is a grey area for me.
Perhaps you could shine a bit more light on this for me.
 
artosalo

artosalo

Member
Joined 2010
KenTajalli said:
Some debate if the PSU caps are signal caps, I am sort of undecided....
Perhaps you could shine a bit more light on this for me.
Click to expand...

Attached is a schematic of a basic SE output stage.

Let's first examine it's currents without any audio signal fed to the control grid.
We can see that there is only the idle DC-current of 34 mA flowing thru V3, L3, L2, U2 and R3.
There is certainly no current flowing thru capacitors C1 and C2.

Next, 12 Vpp audio signal is fed to the G1 of the output tube U2.
Then the same 34 mA DC-current is flowing thru V3, L3 and R3.
In addition there is now 56 mAp-p audio current flowing thru C1 and C2.
And 34 mA DC current + 56 mAp-p AC-current is flowing thru L2 and U2.
So the signal path around the output tube is: C1, C2, L2 and U2.
The power supply V3 and cathode resistor R3 are not included into the signal path.

Actually, the current thru input capacitor is negligible, just some 4 uA.
Therefore I would say that the possible unlinearities of C1 and C2 could have some effect to output signal, but input capacitor C7 does not.
 

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K

KenTajalli

Member
Joined 2014
In your diagram, you have a voltage source V3 as PSU. Voltage sources have a zero internal impedance, accepting this, your first part of the argument becomes correct (i.e. no current flowing through C2). Otherwise a ripple current would flow through it (that's why it is there).
If our voltage source has low enough internal impedance then when the audio signal is connected, current would flow through C1 for sure, but not C2 - it'll go through V3!
If V3 has a large impedance, then C2 and V3 would share the current, however compared to the much larger ripple current, it would be almost negligible, hence my grey area!
C7's quality is of utmost as all of the audio current goes through it, any losses here would show up immediately. C1 also controls bandwidth and gain, again shortcomings here would be very audible (that's why I do not like autobias simple because of this cap).
Changes to quality of C2 can be audible, but in very transparent amps (gery area again?)
I do not claim to understand this completely, but that's my take on it, what do you think?
 
TheGimp

TheGimp

Member
Joined 2009
I built a 6P41S amp UL SE with a 250V Anode Supply. In order to maintain the 190V spec on the screen voltage I used a 47K resistor with a parallel 2.2uF Film cap from UL tap to screen 220R screen-stop resistor. I prefer the film cap to a electrolytic in this position.
 
TheGimp

TheGimp

Member
Joined 2009
Because the screens are only rated at 190V and the anode is rated above 250V. The 47K resistor drops the 60V needed to set the screen at 190V, the film cap bypasses the resistor so that from an AC standpoint the screen is connected directly to the UL Tap.

The 6P41S is a sweep tube. Many of the Sweep Tubes have very low G2 voltage limits that need to be respected. The 6P41S is similar to a PL508, but a bit beefier.
 
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artosalo

artosalo

Member
Joined 2010
KenTajalli said:
...If our voltage source has low enough internal impedance then when the audio signal is connected, current would flow through C1 for sure, but not C2 - it'll go through V3!
If V3 has a large impedance, then C2 and V3 would share the current, however compared to the much larger ripple current, it would be almost negligible, hence my grey area!
Click to expand...

Ripple current is outside of my study. There usually is an other capacitor before the choke L3, but not drawn. The signal current do not flow in any case thru V3.
Choke L3 prevents that.
 
artosalo

artosalo

Member
Joined 2010
TheGimp said:
Because the screens are only rated at 190V and the anode is rated above 250V....
The 6P41S is a sweep tube. Many of the Sweep Tubes have very low G2 voltage limits that need to be respected. The 6P41S is similar to a PL508, but a bit beefier.
Click to expand...

I have not found such specs. and if I had, I would not care. (I have this: http://www.rutubes.com/datasheets/6p41s.djvu)
For PL508 max. specified Ug2 is 275 V.
6P41S will also bear easily some 18 W anode dissipation in long term operation.
This is tested in practise by several 6P41S SE-amplifiers.
 
K

KenTajalli

Member
Joined 2014
Just wondering

artosalo said:
Ripple current is outside of my study. There usually is an other capacitor before the choke L3, but not drawn. The signal current do not flow in any case thru V3.
Choke L3 prevents that.
Click to expand...
OK I hear you. (BTW I'd missed L3 being in circuit)
Consider my model:
the circuit is class A push-pull, with no cathode resistor/capacitor (alternative bias arrangement) perfectly balanced.
With no audio, DC flows through the tubes, transformer and voltage source/choke at steady current.
If we apply audio (AC) but keep the operation to class A at all times, no AC flows through voltage source or the smoothing cap! correct?
is smoothing cap in signal path?
( I know this thread started as SE)
 
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