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Old 7th July 2011, 02:41 AM   #471
TheGimp is offline TheGimp  United States
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It will be interesting to see how people approach the distortion aspect.

I've got mine to the point where one can dial the power from below 0.1W up to 3W with the distortion staying at about the same level, although the noise floor comes up with power.
First is distortion mode at 0.1W, second is same at 3W before clipping and the third is in clean mode at 1W.
Attached Images
File Type: jpg pt_1W.JPG (20.8 KB, 208 views)
File Type: jpg 3W.JPG (23.8 KB, 207 views)
File Type: jpg 1W_clean.JPG (22.7 KB, 206 views)
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Old 7th July 2011, 03:01 AM   #472
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Originally Posted by TheGimp View Post
It will be interesting to see how people approach the distortion aspect.

I've got mine to the point where one can dial the power from below 0.1W up to 3W with the distortion staying at about the same level, although the noise floor comes up with power.
First is distortion mode at 0.1W, second is same at 3W before clipping and the third is in clean mode at 1W.
Oh yeah? Take this!


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Old 7th July 2011, 03:18 AM   #473
PB2 is offline PB2  United States
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Hmmm, blowing things up fast here. Thought we lost a 5881 but it was actually a cap to the grid that turned into an 8K resistor. It sounded a bit odd on the first try, lots of gain and distortion difficult to get a clean sound. I suspected oscillation which I confirmed on the bench with a guitar as input and dummy/speaker load - about 100KHz. Then one 5881 red plated and I shut it down. Suspected the tube for a while then measured with the tubes out and found about 200V on the grid due to the bad cap. Measured 200 mA in the tube that red plated.

Changed the cap, pulled the driver tube so there'd be no chance of oscillation, so I thought, and was too lazy to hook up a the dummy load just to check the bias - big mistake. I believe that one output tube went into oscillation and arced pin 3 to 2 then held an 1/8" wide arc until I hit the power switch. Nearly desoldered pin 3 and fried one of the 100 ohm heater to ground resistors. Fixed it all tested again with a dummy load and it biased up fine - one step forward 2 steps back! The transformer is rated for 200mA on the HV so it states.

I have a good supply of NOS Allen Bradley carbon comp resistors that I'm using, not that I like them. I've been checking values before using them and many 5% are 10% off. These were given to me when a friend emptied out his dad's electronics lab that had not been used in at least 15 years maybe 20-25. I used a string of 2W resistors for the cathode bias and one marked 100 ohms measured 180 after the overload.

I worked a good amount with tubes as a very young kid, shifted to SS by fourth grade, but continued with tube repairs for a while longer. Never saw an arc like that or an interstage cap fail like that.
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Last edited by PB2; 7th July 2011 at 03:21 AM.
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Old 7th July 2011, 03:20 AM   #474
Cassiel is offline Cassiel  Libya
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Originally Posted by wdcw View Post
Thanks bst,

I'm in & have mine designed & priced already, total cost around $80.00 not including speaker or box, ....
Time to build the little sucker & create some smoke ha!

Cheers
Wow, guess Tubelab was wrong, this thread is a success. So many people building amps. Feeling a bit guilty because of my latest trolling.

Gonna shut my mouth. If only the rules would let me in. Damn.
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Old 7th July 2011, 03:43 AM   #475
TheGimp is offline TheGimp  United States
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He..he.... I finally Red-Plated an output tube. 400V and 4X rating watts didn't do it, but accidentally shorting the grid to 45V sure will do the job.

The tube still works!
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Old 7th July 2011, 03:58 AM   #476
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Originally Posted by Cassiel View Post
Wow, guess Tubelab was wrong, this thread is a success. So many people building amps. Feeling a bit guilty because of my latest trolling.

Gonna shut my mouth. If only the rules would let me in. Damn.
Come on in, nothing saying we can't throw you out later.
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Old 7th July 2011, 04:57 AM   #477
PB2 is offline PB2  United States
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Pulled the output tubes and still had the oscillation. Turning the mid control on the tone stack killed the oscillation. Moved a ground off the mid pot to a better position and it is gone. Added a gain switch to simulate the 1 vs. 2 input on a Fender and now have excellent clean or distorted tones. The tubes all survived the upsets.
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Old 7th July 2011, 09:36 AM   #478
wdcw is offline wdcw  Australia
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Originally Posted by TheGimp View Post
It will be interesting to see how people approach the distortion aspect.

I've got mine to the point where one can dial the power from below 0.1W up to 3W with the distortion staying at about the same level, although the noise floor comes up with power.
First is distortion mode at 0.1W, second is same at 3W before clipping and the third is in clean mode at 1W.

I noticed your fundamental frequency is 1kHz, you may find it useful for a guitar amp to set your fundamental test frequency at around 400Hz.

Many will not agree but 1kHz is the upper limit for guitar note frequencies. Above this all the frequencies are Harmonics--Overtones & not fundamental frequencies of the notes played.

I find it useful to test at around 400Hz.

I have attached a fret note frequency chart to have a look at & you'll see what I mean.

For Audio 1kHz is the test std but for guitar amps I don't agree with that, just my thought's & it may help out.


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File Type: jpg Fret Note Frequencies.jpg (195.1 KB, 177 views)
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Old 7th July 2011, 10:55 AM   #479
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I test at 200 Hz, far less annoying, mind you I drive a speaker.
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Old 7th July 2011, 11:03 AM   #480
wdcw is offline wdcw  Australia
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Gday Printer2,

Yep 200Hz to 400Hz is where the money is!

Has anyone done any AC analysis sims on their circuits, I just ran one on mine--attached.

Cheers
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