One more 4P1L SE

[Pano]

I'm really happy with my light bulb RCRC power supply trick. I'm using a very small 15W light bulb as the 2nd R in the filter. It just glows, but keeps the voltage to about 250V with the output stage drawing 40mA.

Isn't 40mA too much for this tube? The cathode/filament really lights up at that current. I think that tube is dissipating about 10W.

[Wavebourn]

Originally Posted by Pano

I'm really happy with my light bulb RCRC power supply trick. I'm using a very small 15W light bulb as the 2nd R in the filter. It just glows, but keeps the voltage to about 250V with the output stage drawing 40mA.

Isn't 40mA too much for this tube? The cathode/filament really lights up at that current. I think that tube is dissipating about 10W.

From my experiments, if not to exceed 11W on screen+cathode it is fine, anode even does not start glowing. :-)

[RajkoM]

Originally Posted by Wavebourn

You forgot about R21. You will get R28 + R21 + Rds(1 + gm*R21)

With gyrator as drawn you will get the same resistance that you get from CCS as drawn, that will depend on value R21 and parameters of your particular MOSFET. Of course, add roll-off on lower frequencies.

If I personally go with pentode load on CCS or gyrator, I would first of all increase many times value of R21, and short R28, or at least decrease R28 to the minimal value desired for HF response. Then define amplification factor by a plain linear resistor effectively in parallel on AC. Otherwise you add to your R28 resistive load some non-linear portion of MOSFET resistance.

Accurately compute CCS Zout, again:

Zout= Rp x (Mu +1) // Rload // R7 = (300000x10) // Rload // 24000

Rp x (Mu +1) = 300000 x (10) = 3 million

Rlad = 1/Gfs + 6500 = 6507 (data from DN2430; Gfs at 20 mA is 0.14 mA/V)

Now we have:
Zout = 3 million // 6507 // 240000 ~ 6596.9 ohm.

We can conclude; Zout with CCS have same value as CCS output resistor.


But it is not so important.
I had the opinion that DIYers better known fundamentals tube amplifiers.

[regal]

Originally Posted by merlin el mago

Can I use only one SSHV2 to power all four tubes? if yes have I to adjust SSHV2 to 140mA?

No, you only need the SSH2's for the output tube one for each channel.

You should prefilter the SSH2 with CLC, then pull from before the SSHV for the driver ccs/gyrator.

There is little gain and too much to ask the SSHV2 to supply both the driver and output and it is not needed with a ccs/gyrator load.

I run a huge heatsink on my headphone WE417 spud amp (20mA per channel) and the SSHV's still get the heatsink very hot. Trying to run the SSHV2's much over 100mA total is asking for one ugly tube amplifier

[merlin el mago]

Thanks for support regal, so I have to adjust SSHV2 for 80mA (60mA for 4P1L & 20mA for the reg) for the two output tubes, for the CCS I have some DN2540 laying around so I will do the same CCS like SSHV2 but p2p, how have I to calculate the input voltage to have an output voltage of 250V or how many volts draws the CCS? I need to make two CCS: one per each tube driver, right?

[Pano]

Thanks Wavebourn. True, I didn't see any glow on the plates - but I think I'll back them down to 8W and see how that goes. Might crank it up later.

It's fun to see the changes on a DH tube. Running the filament alone at 4.2V the tube just gets warm. Somewhere around house cat temperature. Running current thru the tube really lights things up. I never had a separate filament supply on direct heated tubes, so never seen the changes before.

[RajkoM]

Just answer on some questions derived from pentode driver lovers.

Quick simulation says yes, 4P1L in pentode mode can drive 4P1L in triode mode. It is very easy.
Stay with CCS, reduce Rsurce value, reduce the number of red Leds in string and reduce Ug2.

4P1L in triode mode on output have Ugk around 15-16V.
We need 15Vrms (42Vpp) from driver stage. If Vin=2Vrms, total MU of driver stage must be 7,5.

If use 5K:8 OPT with about 15 henrys in primary, You can get about 1.8 Wrms output power with 2.3% THD and Fb 12Hz-84KHz @ - 3dB.

My recommendation is run away from any feedback in two stage SE amplifier. It's very bad. And of course, use well regulated PSU for Ug2 voltage.

Cheers,
Rajko

[merlin el mago]

Originally Posted by regal

No, you only need the SSH2's for the output tube one for each channel.

You should prefilter the SSH2 with CLC, then pull from before the SSHV for the driver ccs/gyrator.

There is little gain and too much to ask the SSHV2 to supply both the driver and output and it is not needed with a ccs/gyrator load.

I run a huge heatsink on my headphone WE417 spud amp (20mA per channel) and the SSHV's still get the heatsink very hot. Trying to run the SSHV2's much over 100mA total is asking for one ugly tube amplifier

Tango NP-126 can be OK for headphones OPT transformer?

[Pano]

Originally Posted by RajkoM

4P1L in triode mode on output have Ugk around 15-16V.
We need 15Vrms (42Vpp) from driver stage. If Vin=2Vrms, total MU of driver stage must be 7,5.

Yes, that's what I'm finding on mine. I get a gain of about 10 on the triode output stage using a 5K load (5K:8R). I figured a similar voltage for the grid. I'm using battery bias.

I will, however, use a bit of GNFB. I am not afraid! Having seen and heard what a mild amount (~6dB) of FB can do for most SETs, I'll be giving it a try.

[regal]

Originally Posted by RajkoM

Just answer on some questions derived from pentode driver lovers.

Quick simulation says yes, 4P1L in pentode mode can drive 4P1L in triode mode. It is very easy.
Stay with CCS, reduce Rsurce value, reduce the number of red Leds in string and reduce Ug2.

4P1L in triode mode on output have Ugk around 15-16V.
We need 15Vrms (42Vpp) from driver stage. If Vin=2Vrms, total MU of driver stage must be 7,5.

If use 5K:8 OPT with about 15 henrys in primary, You can get about 1.8 Wrms output power with 2.3% THD and Fb 12Hz-84KHz @ - 3dB.

My recommendation is run away from any feedback in two stage SE amplifier. It's very bad. And of course, use well regulated PSU for Ug2 voltage.

Cheers,
Rajko

This is why the 4p1l driving another 4p1l is such an attractive proposition. And if you have a balanced source an input transformer can even drive a 2A3 with a 4P1l. Want more? balanced source -input xfomer- 4p1l -1:2 interstage -300B. All DHT two stage tube 300B SET! Its such a fabulous tube.