rh84 build/design questions

[kheper]

Been playing around with the simulation some more. I've found with an Edcor 285-0-285 and 5U4 rectifier I can produce almost exactly 300v B+ which I now believe should be OK given the voltage drops by the OPT's and resistors before the driver. The sim shows an RMS current draw of 188mA on the PT at max amp output, so I think a 200mA PT is perfect for this amp.

Look again. The 5U4GB is not a diode tube; It has 2 diodes. You have two tubes, labeled as 5U4-GB, in the simulation. You only need one 5U4-GB.

http://tdsl.duncanamps.com/show.php?des=5U4GB

[kheper]

Spec sheet for EL84 says Ik is 65mA. At 300v, that's 19.5W Pd, right? How do you limit the current draw? Max Pd is 12W right?

The Ik is 65mA for a much lower voltage. You can lower the current by changing the cathode bias resistor.

http://tubes.nekhbet.com/#cath

[jollyrogr]

Look again. The 5U4GB is not a diode tube; It has 2 diodes. You have two FW rectifiers, labeled as 5U4-GB, in the simulation. You only need one 5U4-GB.

http://tdsl.duncanamps.com/show.php?des=5U4GB

I don't know how to make it show only 1 tube unless I select a half-wave rectifier. Neat program otherwise...

[Alex Kitic]

A FWB "cap input" design cannot produce 100ma (or even 90ma) DC from a 140ma AC secondary, even if Zorsines, Đorđe Petrović or Gavrilo Princip built it themselves!

1) It does not need to produce 100mA (I really do not get it where did this figure come from?) since it needs to produce 90mA.
2) FULLWAVE BRIDGE Capacitor Input Load, I D.C. = 0.62 X Sec. I A.C. therefore I A.C. = I D.C. / 0.62, i.e. 0.145 A (which is a trifle higher, exactly 5mA.

As explained already, you will most probably not be able to obtain a 140 or 145mA rated secondary, but a 150mA rated... which some other manufacturer might consider as a 200mA secondary, depending on his seriousness.

Even if those 5mA were missing from the count, the result would be, as I explained already, a lower B+. Actually, with a lower B+ less current would be drawn by the tubes.. and at low current discrepancies the system would actually balance itself (since this amp has cathode resistors, not constant current source/sink devices).

As I said a few times already, it's really that simple (in the real world).

Now that I know who kheper is (or how does he see himself), I will refrain from commenting on Karađorđe or Gavrilo Princip... the second is Bosniac, BTW.

[kheper]

Look again. The 5U4GB is not a diode tube; It has 2 diodes. You have two FW rectifiers, labeled as 5U4-GB, in the simulation. You only need one 5U4-GB.

http://tdsl.duncanamps.com/show.php?des=5U4GB

I don't know how to make it show only 1 tube unless I select a half-wave rectifier. Neat program otherwise...

Just don't calculate a voltage drop for the second tube.

Correction: I meant "You have two HW rectifiers, labeled as 5U4-GB, in the simulation."

[kheper]

1) It does not need to produce 100mA (I really do not get it where did this figure come from?) since it needs to produce 90mA.

It only produces 86ma, which is not adequate current to run the amp. That is the point I have been making - over and over.

IDC = .62 x .140A = .086A = 86ma

[DeathRex]

Spec sheet for EL84 says Ik is 65mA. At 300v, that's 19.5W Pd, right? How do you limit the current draw? Max Pd is 12W right?

The Ik is 65mA for a much lower voltage. You can lower the current by changing the cathode bias resistor.

http://tubes.nekhbet.com/#cath

That's what Alex was trying to say. He was keeping it simple by putting 300 volts on the anode, and with a 270 ohm resistor on the cathode you would be near the maximum for the EL84, or in class A. If your anode voltage is lower you can decrease the cathode resistor, to increase the current, keeping the EL84 at about 11 to 12 watts.

300 volts (anode) - 10.8 on the 270 ohm resistor = 290 X 40ma = 11.6 watts.
250 volts (anode) - 9.6 on a 200 ohm resistor = 240 X 48ma = 11.52 watts. The current is appoximate.

[Alex Kitic]

Been playing around with the simulation some more. I've found with an Edcor 285-0-285 and 5U4 rectifier I can produce almost exactly 300v B+ which I now believe should be OK given the voltage drops by the OPT's and resistors before the driver. The sim shows an RMS current draw of 188mA on the PT at max amp output, so I think a 200mA PT is perfect for this amp.

Look again. The 5U4GB is not a diode tube; It has 2 diodes. You have two tubes, labeled as 5U4-GB, in the simulation. You only need one 5U4-GB.

http://tdsl.duncanamps.com/show.php?des=5U4GB

jollyrogr - pardon my French, I am under the impression that you do not even know what is it that you want...

kheper - I think this guy (jollyrogr) is pulling us by the nose If he really wanted to build the amp, he would have done it as per schematics and eventually asked about the size of the choke, which is erroneously indicated as 50mA...

When I think that I tought the amp was designed to be diyer-proof and anyone could build it... with 250V B+ it will have less power, but will operate normally... same is true of 300V B+... or even 330V B+. If you build it with an underpowered transformer, the transformer will get a little hotter, you will get a little less B+ and power... none would die.

In the meantime, others have built their RH84s, some of which can be seen in recent threads on this forum. I really do not know what to say.

[jollyrogr]

Spec sheet for EL84 says Ik is 65mA. At 300v, that's 19.5W Pd, right? How do you limit the current draw? Max Pd is 12W right?

The Ik is 65mA for a much lower voltage. You can lower the current by changing the cathode bias resistor.

http://tubes.nekhbet.com/#cath

Ok, so in this design, the cathode resistor is 270 ohm right? I don't know how to figure the cathode voltage. Guess that shows how much of a noob I am.

[kheper]

Spec sheet for EL84 says Ik is 65mA. At 300v, that's 19.5W Pd, right? How do you limit the current draw? Max Pd is 12W right?

The Ik is 65mA for a much lower voltage. You can lower the current by changing the cathode bias resistor.

http://tubes.nekhbet.com/#cath

Ok, so in this design, the cathode resistor is 270 ohm right? I don't know how to figure the cathode voltage. Guess that shows how much of a noob I am.

Yes. Look at the 270ohm resistor on the cathode of the el84.



http://tubes.nekhbet.com/#cath

[jollyrogr]

jollyrogr - pardon my French, I am under the impression that you do not even know what is it that you want...

kheper - I think this guy (jollyrogr) is pulling us by the nose If he really wanted to build the amp, he would have done it as per schematics and eventually asked about the size of the choke, which is erroneously indicated as 50mA...

When I think that I tought the amp was designed to be diyer-proof and anyone could build it... with 250V B+ it will have less power, but will operate normally... same is true of 300V B+... or even 330V B+. If you build it with an underpowered transformer, the transformer will get a little hotter, you will get a little less B+ and power... none would die.

In the meantime, others have built their RH84s, some of which can be seen in recent threads on this forum. I really do not know what to say.

So now you want to make friends with kheper by insulting me? WTF. I just want to learn about this stuff and use the correct parts to build a good sounding amp. I guess I should just blindly follow the schematic and use a 20H choke when it seems I could get better filtering with lower cost chokes. If I build an amp exactly to the schematic, and it doesn't work right, are you going to fix it? I don't think so.

[kheper]

Tried to delete this post - failed.

[Alex Kitic]

That's what Alex was trying to say. He was keeping it simple by putting 300 volts on the anode, and with a 270 ohm resistor on the cathode you would be near the maximum for the EL84, or in class A. If your anode voltage is lower you can decrease the cathode resistor, to increase the current, keeping the EL84 at about 11 to 12 watts.

300 volts (anode) - 10.8 on the 270 ohm resistor = 290 X 40ma = 11.6 watts.
250 volts (anode) - 9.6 on a 200 ohm resistor = 240 X 48ma = 11.52 watts. The current is appoximate.

Yes! I try to keep it simple, others like it complicated...

Regarding class A -- an SE amp can operate ONLY in class A since the device is on all the time (of the duration of the signal, if someone was wondering what time that was).
Most configurations (like grounded cathode, grounded grid, cathode follower, etc.) are actually SE and therefore only class A in operation -- but marketing guys have made class A into "first class"!

If someone is interested in the EXACT values of the current drawn at each given voltage, the answer to that is in the previously mentioned datasheets. The curves show you how much current will be drawn in which operation conditions. For those who are not used to that, the voltages shown as Ug1 can be deducted from the voltage drop across Rk.

Lowering the B+ lowers the voltage across the tube and the voltage drop across Rk... and the current draw. Higher B+, higher current draw, higher anode dissipation. Lower B+, lower current draw, lower anode dissipation.

Anyway, the Rk in the RH84 should stay at 270 ohm, as it should ideally operate with 300V across the tube and a given current draw, into a defined load, etc. in order to be able to produce the estimated power with the estimated distortion (for those who have not seen a sim of the RH84, let's just say it's approx. 4W with approx. 1% distortion).

[20to20]

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Ok, so in this design, the cathode resistor is 270 ohm right? I don't know how to figure the cathode voltage. Guess that shows how much of a noob I am.

You don't have to be a Noob to be thrown by the Vk problem. It is a more complex problem because the tube has its own internal impedance and with the cathode R creates a voltage divider that is different with every tube. Even with every different 6BQ5. So from tube to tube you can estimate it but it won't be EXACT. That's why it's important to build in a little slop to the conservative side for everything.

[Alex Kitic]

jollyrogr - pardon my French, I am under the impression that you do not even know what is it that you want...

kheper - I think this guy (jollyrogr) is pulling us by the nose If he really wanted to build the amp, he would have done it as per schematics and eventually asked about the size of the choke, which is erroneously indicated as 50mA...

So now you want to make friends with kheper by insulting me? WTF. I just want to learn about this stuff and use the correct parts to build a good sounding amp. I guess I should just blindly follow the schematic and use a 20H choke when it seems I could get better filtering with lower cost chokes.

No, I cannot be friends with kheper. He knows that words are important for Slavic people and they hurt more than blows. Besides, we are incompatible, totally.

WTF - is an F word, isn't it? Bad boy!

Based on the fact that you know very little, I would say you are safer with following the schematic BLINDLY, AS YOU PUT IT. You do not have the knowledge (at least for now) to be able to mod anything in the schematics with any chance for improvement.

When it comes to the power supply, it has never presented a problem for diyers. If you can find, or afford, or already have - a 20H choke, you just use it, just like Elvis, who has drawn the schematics, did. If you find the price of a 20H choke ridiculous, you find yourself one that suits your purpose. As simple as that.

AND, in case you do not know whether a 20H choke is important, you ask someone about it -- "do I really need a 20H choke" "why is it a 50mA choke"... etc. Paradoxically, you will more easily find 50mA 20H chokes than 20H 100mA chokes, since the latter would need quantities of EI laminations and wire that are difficult to market... If I recall correctly, Elvis used two 50mA 20H chokes scavenged from some military equipment (one for each channel, just like the RH807 he built).

If I build an amp exactly to the schematic, and it doesn't work right, are you going to fix it? I don't think so.

Well this is not OK as an attitude. I am not selling anything and you are not buying. Therefore there is no money back guarantee. For that one you will have to wait for the RH amps to become kits (who knows, maybe I might get wiser, start selling kits and leave the forums altogether, like Broskie?).

On the other hand, you were interested in building it because you heard a lot of good about it, and saw how many other diyers built it without problems and are quite happy with it, some at their second or third build, constantly improving the quality of the parts or craftsmanship. That probably means something, doesn't it?