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SOHA II Builders Thread - Page 83

post #1231 of 1687
Thread Starter 
Quote:
Originally Posted by JamesL View Post
I haven't tested the amp, but shouldn't there be approximately 19v drop from TB+ (r/l) to TP(r/l)?

Right now, TP is reading almost the same as TB+.
Yes, you're right. Did you use the right transistors??? If so, then one or both Q1/Q2 are still bad.

You are getting the correct voltage on Pin 1.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #1232 of 1687
Quote:
Originally Posted by runeight View Post
Yes, you're right. Did you use the right transistors??? If so, then one or both Q1/Q2 are still bad.

You are getting the correct voltage on Pin 1.
Both Q1 and Q2 on each channel read BC560C737
Mouser #863-BC560CG

I'll go ahead and put a few more of those on my next order.
Are there any other possible reasons, or is the symptom pretty well isolated to that part of the circuit?

I was checking the resistor values on my board, and I came across a discrepancy. The power supply values on your schematics and parts list are different across the board. Which one is correct?
post #1233 of 1687
Thread Starter 
Interesting catch on the parts page. Amazing that it has been wrong for so long, but I think the excel BoM is correct to the schematic.

I must have done something when editing to damage the parts page. I'll try to fix it soon. Thanks.

It is hard to see any other cause for this problem. It's either the tube or the transistors assuming that certain things are true.

1. The tail current is 2mA
2. Pin 1 is at around 40-45V.
3. The triodes are reasonably matched
4. They are both conducting
5. There are not other unseen problems like solder bridges, etc.

If the first triode is passing 1mA and if its plate is at 45V and if their cathodes are both connected to a CCS set for 2mA then the second triode has to bias very close to 45V. It has no choice.

Maybe you could measure the voltages on all the pins of both tubes. Lets see if there is anything else weird going on.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #1234 of 1687
1. I read 400mV across R6(200r)
2. Pin 1 reads 40.8v
3. I'm unsure if the triodes are reasonably matched
4. Again, its hard to be sure
5. I can't see anything that remotely resembles a solder bridge or bad joint.

Here are the measurements on each of the pins.

1, 40.8v
2, 0v
3, 2.0v
4, -12.8v
5, 0v
6, 60.6v
7, 0v
8, 2.0v
9, -6.35v

I love your sig btw =p
post #1235 of 1687
Thread Starter 
The voltages are near perfect except for pin 6, the second plate. So it just has to be the transistors. I can't think of any other problem.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #1236 of 1687
Thread Starter 
Meant to ask you what tube you are using. Is it a 12AU7?
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #1237 of 1687
Thanks Alex, I'll try replace them again let you know how they turn out.

Yup. EH 12AU7
post #1238 of 1687
I have been running my SOHA II for over a year with no problems.
I recently did Regal's mod to increase the plate current on my SOHA II and now have an interesting problem.
I had a small amount of hum/noise with Grados once I had it back up and running. The noise was only present with tubes in place. This noise would last for a couple of minutes, then stop. I checked the usual suspects, grounds, source, xformer and swapped tubes but found nothing to account for it. Once stopped, it would stay gone as long as amp was warm.
I started probing with a scope and DMM and found that touching a meter lead to the heater circuit, especially at pin 4 of the left tube would make the noise disappear. The meter lead didn't even have to be connected to anything to have this effect. A non conductive probe had no effect, so the pressure of the probe was not flexing the board and causing the change.
I replaced the heater regulator and C5P and resoldered everything in heater circuit, pulled the tube sockets and resoldered sockets and everything related to input circuit.
At this point I checked for oscillation and found a 10 MHz signal on the cathode and the output plate of about 100mv. This signal disappears when the noise goes away.
So it appears the amp is oscillating and the heater circuit is involved in the feedback path. Some characteristic is changing when the amp warms up which kills the oscillation, but I don't know what. I could probably go back to 2ma tail current, but I would prefer to make it behave at 5.5ma. Any ideas?
post #1239 of 1687
Quote:
Originally Posted by grendel23 View Post
I have been running my SOHA II for over a year with no problems.
I recently did Regal's mod to increase the plate current on my SOHA II and now have an interesting problem.
I had a small amount of hum/noise with Grados once I had it back up and running. The noise was only present with tubes in place. This noise would last for a couple of minutes, then stop. ?
I think it is normal for tubes to oscillate until fuly warmed up, remember this isn't tube rectified so we are flipping a switch and turning on 5.5mA of current thru the tube before it gets any heat. I don't have this issue with mine, my guess is if you try more tubes you will find a pair robust enough to handle without squeaking.
post #1240 of 1687
Would this sort of issue be prevented by having the heater turned on for a little while before the main power is turned on, ala the EHHA? I suppose since the current levels being used in the EHHA is considerably higher, the situation could be much worse, and thus the "pre-heat" requirement of the EHHA..
post #1241 of 1687
Quote:
Originally Posted by MrSlim View Post
Would this sort of issue be prevented by having the heater turned on for a little while before the main power is turned on, ala the EHHA? I suppose since the current levels being used in the EHHA is considerably higher, the situation could be much worse, and thus the "pre-heat" requirement of the EHHA..
Yes but I don't think we are hurting anything here as opposed to the EHHA, I just consider it standard practice to turn on a tube amp at least 30 minutes before use. Never heard a tube amp sound good during the first few minutes after powering up.
post #1242 of 1687
It is mentioned that Antek uses unloaded voltage on its labels. Anyone using Antek transformers? 14.5V (50VA) secondaries at 1.6A load still seems fine.
post #1243 of 1687
There was some talk about it on the prototype thread
post #1244 of 1687
Thanks for pointing out holland's post. But with the 2.5x CW multiplier, I should still get a B+ of 98V.

I'll get one anyway since its real cheap.

Edit: Wait, at 14V the LV supply would fall out of regulation.

I could use their 20-0-20 instead, secondaries drop to 19.7V at 0.9A load. I'll get 137V B+ (still under TL783's 150V max Vin) and use 7818/7918 for the LV supply. Would I need a bigger heatsink? The LM337 needs to drop more voltage.
post #1245 of 1687
Quote:
Originally Posted by Eokboy View Post
Thanks for pointing out holland's post. But with the 2.5x CW multiplier, I should still get a B+ of 98V.

I'll get one anyway since its real cheap.

Edit: Wait, at 14V the LV supply would fall out of regulation.

I could use their 20-0-20 instead, secondaries drop to 19.7V at 0.9A load. I'll get 137V B+ (still under TL783's 150V max Vin) and use 7818/7918 for the LV supply. Would I need a bigger heatsink? The LM337 needs to drop more voltage.
Just increase the resistor (and wattage) in front of the LM337 to take the addional load off.
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