But, if you use a high bright LED and throttle it down to a few mA or less than 1mA, the bipolar supply will not notice it.

The diode in reverse parallel with the LED protects it from too high reverse voltages, as you say. With the 1N4148 the maximim reverse voltage that the LED will see will be one diode drop. All LEDs that I know about can withstand this.

But adds in another LED current limiting resistor if I use two LEDs, unless I can run them in series - but what are the odds I'll like the brightness of each that way?

[Edited by Guitarsenal on 11-18-2005 at 11:44 AM.]

I thought this way i could get +-15V for buffer stage as well as 60V for plate and 12V(across +-6V for symmetry, or maybe i should eliminate 7906 and just put a 7812 instead of 7806) for heater supply.. which is much simpler in my case.

I am posting this b/c am not entirely sure whether this will work or not. will it?

[Edited by ryanjun on 11-19-2005 at 06:34 AM.]

I finished putting my power section together today, and when measuring the voltages, I get -16.5, +16.5 and +85!!!!

I've double checked my circuit and everything looks fine.

Please help

[edit] and my filament voltage is 41.5!!!!

[Edited by pho_boi on 11-19-2005 at 05:02 AM.]

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quote:

[By ryanjun] phoboi, runeight told me about the 'trafo regulation' problem; the trafo output voltage when unloaded maybe higher than its marked voltage because of trafo's internal resistence.. small trafos use thinner wires thus higher internal resistence and this could be the cause of ur problem. check the trafo output voltage to figure out if this be ur problem.

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But would that explain why the filament is 41.5v when it should only be 12.6? And 85v for the CSS?

[edit] heres a pic of the entire board so far, haven't dared to hook the power section to amp section yet

http://i31.phot...GP2887Large.jpg

[edit2] Ok I've fixed the opamp supply problem, I accidently put the 7912 in the 7812 position and the 7812 in the 7912 position. But just switched them around then i get a nice +12 and -12.

However the CSS voltage is still 84-85v instead of 55-60v and the filament is still 45v instead of 12.6

[Edited by pho_boi on 11-19-2005 at 08:15 AM.]

But would that explain why the filament is 41.5v
Yes.
If we assume 15-0-15 Vrms and NO load, then a voltmeter will show 42.42V for the filament (in theory).

Don't wire up the tube triode connections just yet, but *do* wire up the filament connections and tell us the voltage you see across the tube filament.
I think you'll find that it'll be much closer to 12.6V, but of course it's best to check before going further with the build.

The resistor values I settled on require the filament load to be present when you take readings to see expected voltages.

[Edited by mains_hum on 11-19-2005 at 08:54 AM.]

As mains suggest, try using some clip leads to wire up just the tube filament before putting the tube into the socket. Your heater voltage should come down to close to 12.6V and your B+ should come down to 55-60V.

If this happens then you can plug in your 12AU7 and adjust your trimpots for 40V on the plates. And listen.

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quote:

[By mains_hum] I finished putting my power section together today, and when measuring the voltages, I get -16.5, +16.5 and +85!!!!
That's fine.
Remember we are using 15-0-15 Vac (nominal) so 16.5 with no load says regulation is good, else, like me, you'd see 21V.
85V with no load is also just as expected, that's why we warned to be careful running the supply without the tube filament load, and to use capacitors with voltage ratings that meet this off-load condition.

But would that explain why the filament is 41.5v
Yes.
If we assume 15-0-15 Vrms and NO load, then a voltmeter will show 42.42V for the filament (in theory).

Don't wire up the tube triode connections just yet, but *do* wire up the filament connections and tell us the voltage you see across the tube filament.
I think you'll find that it'll be much closer to 12.6V, but of course it's best to check before going further with the build.

The resistor values I settled on require the filament load to be present when you take readings to see expected voltages.

---

Thanks so much!! I just connected the filament supply up and I got a reading of 10.8 , is this fine? or is it too low?

I also just measured the triode voltage and its sitting at 70v. Would it be safe to wire it up fully now?

Thanks again mains_hum

What is the AC voltage on the trafo secondary when the heaters are connected?

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quote:

[By runeight] 10.8V heater voltage is too low. Almost 2V too low.

What is the AC voltage on the trafo secondary when the heaters are connected?

---

The AC voltage on the transfo with the heaters is 33.5

Can you probe around more to give some clues ?

33.5-12.6 = 20.9V This the voltage we need to drop.

The resistance needed to drop this for 150mA is:

20.9 / 0.15 = 139R

Half of this number is about 69R, which is what 100R || 220R should be very close to.

Could you kindly check the resistor values in your heater dropping section?

[Edited by runeight on 11-19-2005 at 09:15 AM.]

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quote:

[By mains_hum] I just connected the filament supply up and I got a reading of 10.8
That's too low ... and I can't think why for the moment.

Can you probe around more to give some clues ?

---

I checked all the voltages and again and they are all fine.

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quote:

[By runeight] Something is not making sense. Let's calculate the dropping resistor value based on your secondary AC with the heaters connected.

33.5-12.6 = 20.9V This the voltage we need to drop.

The resistance needed to drop this for 150mA is:

20.9 / 0.15 = 139R

Half of this number is about 69R, which is what 100R || 220R should be very close to.

Could you kindly check the resistor values in your heater dropping section?

---

Would it matter that I'm not using a LED inbetween the dropping resistors?

Also, could it be the bad quality tube causing this? Because I just took it out of an old radio that was under my house, but it does work in my speaker amp though.

Looking at your picture, I see that the resistor values are correct. Have you measured the actual parallel resistance with the power turned off?

The lack of an LED is not the source of the problem.

[Edited by runeight on 11-19-2005 at 09:23 AM.]

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quote:

[By runeight] It could be a bad tube. Can you measure the voltage drop (AC) across both sets of dropping resistors please. This will tell us how much current is flowing into the filament.

Looking at your picture, I see that the resistor values are correct. Have you measured the actual parallel resistance with the power turned off?

The lack of an LED is not the source of the problem.

---

Voltage drop is 11.1 for both sets. I'm not sure if I measured it right, but I measured from one end of the parrallel resistors to the other.

Oh I just unsoldered the dropping resistors and measured there resistance and they are correct also.

[Edited by pho_boi on 11-19-2005 at 09:30 AM.]

I think that you can wire everything up and put the tube in.

But first, dial your trimpots so that they are in the maximum resistance position. This will put the maximum bias on the tube.

If nothing has fried then measure the voltage at one of the plates and adjust the trimpot until the voltage comes down to 40V. Do this with the other plate.

Then measure the voltage across the 1k2 resistors in each CCS. I would like to know what these are, but they should be around 1.2V.

After all of this adjustment, measure the B+ again. It should still be between 55-60V.

At some point you might need to replace the tube to get good results.

Have I mentioned that your board looks terrific.

[Edited by runeight on 11-19-2005 at 10:31 AM.]

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quote:

[By ryanjun] I thought this way i could get +-15V for buffer stage as well as 60V for plate and 12V(across +-6V for symmetry, or maybe i should eliminate 7906 and just put a 7812 instead of 7806) for heater supply.. which is much simpler in my case.

---

Yes, you can do that. You can even use a single 7812 regulator across the 30V for the heater.

---

quote:

[By ryanjun] Also, I have a stock of LM2577T which is a DC-to-DC step up regulator and I think it can supply the plate voltage i need.

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The 12AU7 only draws 2mA from the B+ supply. The LM2577ADJ might have a hard time with this. I tried to do the calulation for the inductor value from the National data sheet. The load current is so small that the resultant calculations lead to a region that is not even shown on their inductor chart. Maybe I got this wrong, but it would be a good exercise for you to go through to compare with my results.

Also, if you're having trouble finding the jfets, you can use these constant current diodes in place of them as shown.

Basic Stoopid Hybrid Amplifier 2

[Edited by runeight on 11-19-2005 at 02:17 PM.]

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quote:

[By Guitarsenal] When I saw that last post about the 1n5297 current regulating diodes I wondered why they weren't in the original spec. for the SOHA, but then I saw their price. $8.58 for two of them at Mouser!

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Exactly!!!!

J505s are cheaper, but still more expensive than four jfets and a resistor. J505 are 20% devices. The 1n5297s are 10%.

But, if you can't get anything else.

[Edited by runeight on 11-19-2005 at 03:05 PM.]

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quote:

[By runeight] This says that the tube is drawing 161mA. This is not impossible, but it is likely that the tube will draw even more current at 12.6V.

I think that you can wire everything up and put the tube in.

But first, dial your trimpots so that they are in the maximum resistance position. This will put the maximum bias on the tube.

If nothing has fried then measure the voltage at one of the plates and adjust the trimpot until the voltage comes down to 40V. Do this with the other plate.

Then measure the voltage across the 1k2 resistors in each CCS. I would like to know what these are, but they should be around 1.2V.

After all of this adjustment, measure the B+ again. It should still be between 55-60V.

At some point you might need to replace the tube to get good results.

Have I mentioned that your board looks terrific.

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Thanks for the comment

Ok, I just wired up a filament of a 12ax7 to test measure the voltage and i get 13.6!!!!

I'll wire everything up now with the 12au7 and see what happens.

[edit] just tried another 12au7 I had lieing around, it's some chinese one , and i get a measurement of 11.8

[Edited by pho_boi on 11-19-2005 at 05:38 PM.]