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A Very Compact Hybrid Amp - Page 113

post #1681 of 2213
with a 50k ALPS pot..47k works very nicely.
post #1682 of 2213
That's how mine is set too up except mine has 52k resistors (that's what was on hand) and it is a good improvement over a standard cheap pot.
post #1683 of 2213
Thread Starter 
Quote:
Originally Posted by Ged View Post
I have just finished replacing all the transistors and still have problems
The V on R18R is still wrong currently 23.5V
I believe one of the parts was 40 class
The second phase of the repair were from bills I am sure they are alright
Ged, are the voltages at OG and R18L still ok (with respect to ground)?

Edit to say R18L not R19L.
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 #1684 of 2213
all other volages are still ok
could it be the wrong class (40) part in Q8R be the problem I have ordered a repacement
post #1685 of 2213
Another one lives!
Sounds swell, using the 6BQ7A that Bill shipped with the kit. Can't wait to get rolling!
The regulator gets quit hot, can't put my finger on it for more than 2 seconds. Using a 1.6 amp power supply. Is this ok?
post #1686 of 2213
Thread Starter 
Congratulations!!! Enjoy the tube rolling.

It's hard to say exactly how things will go but it is true that the 1.6A wall wart is likely to put a higher voltage across the regulator and to increase its power dissipation. If you plan to stay with this setup I would recommend putting a small finned heatsink onto the regulator. You'll have to bend the fins a bit, but others have done this.

Others here can suggest a good part number to use.
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 #1687 of 2213
runeight do you think that replacing Q8R with the correct class of transistor will solve the voltage problem
post #1688 of 2213
Thread Starter 
It IS possible depending on how different the hfe in the class 40 device is from the other class 25 output transistor in the buffer. It should be done anyway so that, if there is still a problem, we can elminate this as a source of the problem.

This eventually has to work. Don't give up!!!
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 #1689 of 2213
I will keep on going but I find your replies reassuring THANKS
post #1690 of 2213
I does have a heatsink on it that is why I am concerned. It is about 28vac unloaded so I am sure the reg is working harder than it has to and the extra has to go somewhere. The specs state 24VAC 1.6 amp. I'll have to find something closer to the ideal or figure out how to sink it better. I have a variac, maybe that will help.
Sounds great but I have begun to notice that the left channel will distort the bass if turned up to much. It is not noticeable generally but definitely there. I only had the one tube with me so will check that first. Any idea where I should start if it is not the tube?
post #1691 of 2213
Thread Starter 
28VAC will yield close to 38V at the first filter cap and slighly less than that into the regulator. The design spec is for 32VDC into regulator. You are likely to have aroune 36V. This is quite a lot more power and it is probably correct that the small heatsink there now cannot do the job.

It's hard to say about the distortion. I don't recall hearing about this type of issue before, but a tube change is a good thing to try. However, I suspect that this will not fix the problem I wonder if this could be related to some of the issues you had earlier with hum, etc?

If not the tube, we'll have to inspect the buffers to see what their operating points are.

To make a start, measure the voltages on all of the tube pins with respect to SG. And measure all of IC1 (buffer opamp) pins with respect to SG too. This will tell us a lot. Be careful not to short anything. As you know replacing parts is a tight squeeze.
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 #1692 of 2213
OK to start I have a more sane supply. 24.4VAC unloaded. It is spec'd at 20v 1amp. Things are cooler for sure I can hold my finger on the reg heatsink now and it's moderately hot just a bit more than warm, so I think things are good there. I changed the tube out, the plate voltages changed from pin1 90v pin 6 81.6v to what I put down below. I also changed the output jack and wiring. I know I changed a lot at once, not the best for troubleshooting I cannot recreate the bass distortion and e-12 activation... Hopefully things are better!!
My measurements as requested.
Tube:
1- 78
2- 0
3- 1.7
4- 6.0
5- 0
6- 78.4
7- 0
8- 1.6
9- 0

IC1
1- 7.5
2- 11.9
3- 11.9
4- 0
5- 11.9
6- 11.9
7- 5.4
8- 24

Tube = 6BQ7A
post #1693 of 2213
I just discovered as I was swapping out tubes. It is the tube causing the problem. The others I tried did not cause any trouble, put the original one back in and there it was!
post #1694 of 2213
Thread Starter 

Field Upgrade - For All CTH Builds

There have been occasional reports of failures to Q1P. After some investigation we are pretty sure that this happens in builds where the area around the 24V regulator (where Q1P is) is not ventilated well enough. Because the regulator gets really hot the ambient temperature in the area around the reg gets fairly high.

If any of you experience Q1P failures replace this transistors (BC550C) with one of the BC337-25 transistors as used in the ouput stage. The BC337 is a higher current, higher wattage transistor that should be more comfortable in the hot spot around the regulator. This will not, however, make the LV ps immune to direct shorts at the output of the regulator. In this case Q1P will blow protecting the other components in the system.

If you're not having problems there is no need to make this change.

The Cavalli Audio website has been updated as well as www.sprucecanyonlabs.com.

All kits from Spruce Canyon will now come with the BC337 instead of BC550C for Q1P.

Thanks to those builders who helped to locate this problem and to cfcubed for helping to diagnose it.
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 #1695 of 2213
All kits will now come with Q1P = BC337, and two complete sets of BC550/560s and BC327/337s
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