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Old 6th March 2012, 12:04 AM   #41
SandyG is offline SandyG  United States
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I have a few things to try. Easy enough to drop the voltage a bit on that stage, will try also a relocation of the parts.

My gut feeling tells me the layout is OK, but still don't have my 'Vacuum Tube' legs, and will be a long long time before I do

The funny thing is that I chose that schematic for the project as the schematic was using a different topology. Next time will show the schematic and get some feedback.

All in all it does sound very good, and I'm sure with one of the fixes will be an excellent amp.

Sandy
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Old 6th March 2012, 01:56 AM   #42
kevinkr is offline kevinkr  United States
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Nothing's easy, you're learning all about applying diagnostic tools and reasoning to the problem, and I am confident you will fix it. Given the care in construction I think you will end up quite pleased with your efforts.
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Old 8th March 2012, 02:48 AM   #43
SandyG is offline SandyG  United States
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Thanks Kevin and DF and all the other for the help, it's moving along. I have only a few minutes to play with it today and took a couple of voltage measurements to see if things were out of whack with what I had on an old SAMS PHOTOFACT for the Grommes 260A.

Tube Pin My Amp Photofact
------------------------------
12AU7 Pin 1 49 42
12AU7 Pin 6 80 75
12BH7 Pin 1 201 195
12BH7 Pin 6 199 190

Voltage into the OPT center tap was 468v and the Photofact shows 450. I think this difference accounts for the different voltage as seen above. Voltages taken with a load and no signal on input.

Clipping happens at about 18.3vrms (at 1kHz) on the meter which clicks in about 41 watts into an 8 ohm load. This is with the 10meg (approx) resistor Swamping C15 on the input stage for a nice clean signal. I think power is limited by the input stage, but not going to mess with that other then trying a pair of GZ34's as the rectifiers to see if it's worth it for any additional power and to see if the increased overall voltage does anything to stability. I think both power and opt are capable of much more then 40 watts, but will save some of the extra iron for the next project...

I also took off the Feedback from the transformer to the board and noticed that the amp was pretty stable unlike before where it seemed to run away a bit, but was now quiet.

I'm going to try to reduce the voltage of the 12AU7 for grins as suggested as that's a pretty easy change.

Then put it back on a speaker with the 'clean signal mod' and see what it sounds like.

Sandy

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Old 8th March 2012, 03:55 AM   #44
kevinkr is offline kevinkr  United States
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Hi Sandy,
Reducing the voltage is probably a good idea. One problem I have had with Sams is that the voltages are always measured with an analog meter with something like 20K per volt input resistance which means the meter loads the measurement point a good deal more than a modern DVM - this can account for differences of several % or more. Just thought I should mention it as it trips me up from time to time..
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Old 5th April 2012, 02:07 AM   #45
SandyG is offline SandyG  United States
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Been a bit since I had a chance to mess with the amp, but with the 10meg 'Cheese-sistor' (or 'Hack-sistor') across the cap, the amp is very well behaved. I checked no load stability and it's also good. I still have to mess with the dropping resistors to the 12AU7A to see if that does anything, but for the most part I think it is looking pretty good.

I also have a couple of KT120's that I might pop in just for the heck of it, but don't see much value since it seems that the amp has limitations of driving a pair of KT88 flat out, but might do it for grins if anyone is interested.

I did swap out the 5U4's for GZ34's and re-biased the tubes and got a few more watts out with still with good stability, but I like the larger 5U4's and don't really need to squeeze a few extra watts out of the amp so back to the original 5U4's. Waiting for a couple of Hammond boxes to build 2 units up and will give them the shake down as a pair.

Already looking at 6SN7GTB's and quad KT88 for the next project... but will wait until a pair of these are buttoned up and playing!

Will get some pictures as they come together.

Sandy
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Old 5th April 2012, 05:52 PM   #46
kevinkr is offline kevinkr  United States
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Looking forward to seeing those pix!
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