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SOHA II Builders Thread - Page 70

post #1036 of 1687
Two other things:

Not having the pot in the circuit maybe throwing things off (someone more knowledgeable than me can confirm this).

The jumper pins look like they are not soldered down?
post #1037 of 1687
Yes, that's true. I haven't decided if I wanted to wire a switch, so I ghetto-rigged the jumpers hairpin-style so they clip on tightly onto the pcb. I checked for continuity and it seemed to work out alright. In fact, they "grip" on so tightly that I have a hard time budging them loose.

From what I can figure out, shorting ground to signal-out of the potentiometer is equivalent to having a pot set to a fully counterclock position. Should I try it out?

I'm actually running on a very short supply of "hardware-store" wire; I'm trying to order more online but the new apartment I moved to can't accept packages larger than standard letter envelope, which has been giving me a lot of problems lately.
post #1038 of 1687
JamesL
It maynot relate to your problem, I just want to let you know you need to be awared of the connector for headphone. You soldered the screwconnector pin to the center 3 hole, of which lable is L,G, R, but the headphone from tips to sleeve are left/right and grond. I maynot explain well. Please look at my picture. I connect the socket to 3 holes close to relay.
post #1039 of 1687
Thanks, and yes I'm fully aware of that..
I haven't connected any load or source to the amp yet though. I don't want to proceed further until I find out why I'm not getting 19v drop across R4 =/
post #1040 of 1687
Thread Starter 
Quote:
Originally Posted by JamesL View Post
Hi all - I've started my soha II a couple days ago, but I'm not getting a 19v drop from TPR/L to TB+R/L.
Every other measurement has been spot on until I put the tubes in.

TB -108v
TH - -12.8v
TL+/- +14.9/-15.1v
220mV across R10 and 400mV across R6
(resistor values checked, for 10mA and 2mA across correspondingly)

65v at TB+
64.7v at TP
~300mV drop

I tested it with the tubes off, and I got ~7v at TPR, and <1v at TPL.. both values varying quite a bit. If I recall correctly, touching the test probe to TP consistently tripped the relay.


First batch of production boards
Jumper set to 12.6v @ 150mA, EH 12AU7, advertised as new.
BD139 transistors.. small signal transistors - BC550C & BC560C

Some observations:
I had a very difficult time getting a reading across R6 when I put my dmm to the mV setting; Even if I could consistently get 0.4v reading when set at the (auto)volt range, it would often read 'null' when using the mV range (it would sometimes read 400mV, but often times not.. I couldn't find a pattern)
The relay will click off when adjusting P1P with tube in.
The relay will click off randomly when touching multimeter probe to a resistor lead or less frequently when touching a test point.

I've tested many times over for several hours, so I'm pretty sure the above behaviors are fairly consistent and accurate.
JamesL it's not really possible to test with the tube out or off because then there is no current that can flow through the tail CCS and into the plate loads.

However, there is clearly a problem with the tubes in. I guess you are sure that the tubes actually light up???

If so, then they don't seem to be conducting very well. When you have the tubes in is that when you cannot adjust the tail current at R6. Or is when the tubes are out/off?
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 #1041 of 1687
Quote:
Originally Posted by runeight View Post
JamesL it's not really possible to test with the tube out or off because then there is no current that can flow through the tail CCS and into the plate loads.

However, there is clearly a problem with the tubes in. I guess you are sure that the tubes actually light up???

If so, then they don't seem to be conducting very well. When you have the tubes in is that when you cannot adjust the tail current at R6. Or is when the tubes are out/off?
Yes, the tubes light up, but it is quite faint/dim.
I am reading 400mV across R6 regardless of whether the tube is connected or not. It is about 10mV higher with the tubes plugged in, or 10mV lower with the tubes left out. I measured R6 to be 200ohms.

I wish I could tell you in more detail what exactly is happening, but I've little experience with tube circuits and can only describe to you the measurements I'm reading.
post #1042 of 1687
Thread Starter 
That's ok, we'll figure it out.

First, have you measured pins 4&5 of the tube socket. If you haven't try that to make sure that you have 12V on the tubes. Ifyou don't this means that the jumper settings are set for 6V tubes instead of 12V tubes.

The voltage across R6 cannot be the same when tubes are in or out. When the tubes are out there is not conduction path for the current in R6. If this is really what's happening then there is some other problem that we'll have to find. Are you absolutely certain that the voltage on R6 is the same with tube in and tubes out?
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 #1043 of 1687
Yup. 12.8v across pins 4&5 - Its the first thing I checked after rigging up the jumper pins.

The voltage across R6 reads the same with both the tubes in and out; 10mV higher with the tube in.
Double checked, triple checked, and then a half dozen more times after a good nights sleep.
I've checked the integrity of most of the parts, excluding mainly the IC's and vacuum tubes. Is there any way to check if they are functioning correctly without having to desolder them?
post #1044 of 1687
Thread Starter 
Yes, of course, because I was wrong. I was thinking of another amp. I should look at the schematics first.

The tube-in/tube-out readings on R6 are actually correct so measuring there won't help us.

Now, the TB+ and the TP readings would say that there is no current flowing in the tubes.

Have you measure the voltage across R4 on each channel with both tubes in? If not, try that first. And if you did can you please remind me of what you got?

Edit: And while you're at it, measure the drops across R2/R3 on both channels.
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 #1045 of 1687
Across R4, I'm getting 20.9v on the left, 22.1v on the right.
206mv R3, 233mv R2 on the right
204mV R3, 229mV R2 on the left
post #1046 of 1687
Thread Starter 
Hmmm. Those are the correct numbers.

What do you get if you measure pins 1 & 6 with respect to ground on both tubes?
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 #1047 of 1687
At pin 1, I'm getting 32.8v for the left, 41.6v on the right.
I'm getting the same numbers at pin 6 as I'm getting at the test point. Around 65v (~200mV under TB+ R/L)
post #1048 of 1687
Thread Starter 
OK. This is progress.

It seems to me that either Q1 or Q2 or both are bad on both channels. Make sure that they are BC560 (not BC550) and if they are, I hate to ask, but the only next step is to replace them. I would just do all four of them to be safe.
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 #1049 of 1687
Ah, alright.

I switched the tubes and measured pin 1 again and got 41.9 and 42.6
I don't know if that changes anything, but I'll go ahead and order some replacement parts.
Q1 and Q2 are both BC560s
I don't know when I'll get them because I have to send them to my friend's address, but I'll let you know when I do.
post #1050 of 1687
Thread Starter 
Those numbers on pin 1 are nearly perfect. But you still get 65V at pin 6??

Then it has to be the transistors.

Please let us know how you do. The amp will work when all the parts are right and working.
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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