ic-racer wrote:Tweaking Summary:
2) 4.7uf cap on V2 (instead of 1.0 Ceriatone value)
Can you elaborate more on this tweak?
What is different now from the 1.0 value?
Thanks
Sure.
Raising the 1.0 uf to 4.7uf came from a post by dogears about someone else's amp. 4.7uf is the actual from amp #124.
While checking the effect of the 4.7uf cap (on the cathode bypass of the second OD stage) I just soldered it over the 1uf (giving 5.7uf) and I could not hear any difference. So I un-soldered half and then I could push it down a little to make intermittent contact and see if I could hear a difference while playing...no change.
Then I realized that if the OD input trimmer is too high, the first OD stage overpowers the second stage. After turning down the OD trimmer even more, I was then able to hear the effect of the capacitor as I made intermittent co contact.
The effect of the was quite noticable after quelling the first stage OD gain, but both 5.7uf and 1uf sounded good. Just differnet shades of good. However, the most important thing I learned here was that it was very easy to have the OD trimmer set for too much gain!
ic-racer wrote:At first I wasn't sure which three tube holes to use. I chose the three that were closest to the power amp section, and this seemed to work out OK.
Here is a picture I got off the internet. I think it shows a build on the same or a similar chassis with 4 preamp tube holes. In this case they put the 'extra' hole between the preamp and PI stage. Also, looks like he filled the 'extra' hole with a non-functional socket (probably fitted with a tube that would glow )
Hello ic-racer.
I did put the 'extra' hole between the preamp and PI stage.
Because I will add dumblerator in chassis. Thanks
PS.Nice job!!
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I did put the 'extra' hole between the preamp and PI stage.
Because I will add dumblerator in chassis. Thanks
PS.Nice job!!
Thanks for posting and welcome to the forum. I saw your amp pictures and traslated your web pages to English a few months ago and that was the inspiration for my project!
I noticed more posts by Gregor Hilden on YouTube and enjoyed hearing his Dumble with various guitars. His amp seems to sound good with all the guitars. And it seemed to sound better than mine, so on to some more tweaking...
I was curious how a 220k feeding the 350k pot in the OD entrance would sound, so I tried this and it caused a buzzy distortion, so I went back to the 150k (Would have done 120K, but I don't have one laying around). I also swapped out the OD tube again and things sounded good. I did not go back and try the 220k with the new tube, though. What I think happened was the 220k was bringing out some buzzyness to the tube I had in there.
Last edited by ic-racer on Sun Apr 13, 2008 3:50 am, edited 1 time in total.
Next wanted to fiddle with the PI trimmer. To my ear, this trimmer does nothing. This 5k trimmer does not seem to have enough range to cause a real audible effect TO MY EAR. I can't really check the AC signal after the coupling caps (other than to measure it with an AC RMS meter) but I did check the DC voltages before the coupling caps. I noticed about 308v to 296v, and the 5k trimmer could not balance these out. So I went through the tube box until I found a tube that was pretty balanced already, then I could get the two sides of the tube to match within .1v with the trimmer.
Now, with the matched tube halves in the PI I was getting a very smooth sound. What I don't know is if this is because the tube sides were closely matched, or if the one tube is just 'better sounding' than the other. Again, the 5k trimmer could not 'un-match' the tube enough to make an audible difference; so I could not really test to see which was the case.
Anyway, the end result was a new OD tube and a perfectly DC balanced PI tube. I re-set the OD trimmer 'by ear' and it came out at about 50k from ground (which is where it was when I started).
Last edited by ic-racer on Sun Apr 13, 2008 3:49 am, edited 1 time in total.
I like a 10K trimmer here. You definately can hear and feel the difference when sweeping the trimmer. I like to hit low notes to test. The feel gets hard and edgy when the trimmer is set "wrong". I tested my entire stash of NOS tubes this weekend with a VTV tester. It checks for current draw balance as well as gain. I used my most perfectly balanced tubes in the PI of two of my amps yesterday. In both cases, the best tone occurred with about 14K between the plate loads. The voltage in both cases was about 8V lower on the higher plate side. Ie, 265/257 in one of my amps (Bluesmaster PI although this won't matter). In other words, the voltage won't be the same if you use a balanced tube.
ic-racer wrote:Next wanted to fiddle with the PI trimmer. To my ear, this trimmer does nothing. This 5k trimmer does not seem to have enough range to cause a real audible effect TO MY EAR. I can't really check the AC signal after the coupling caps (other than to measure it with an AC RMS meter) but I did check the DC voltages before the coupling caps. I noticed about 308v to 296v, and the 5k trimmer could not balance these out. So I went through the tube box until I found a tube that was pretty balanced already, then I could get the two sides of the tube to match within .1v with the trimmer.
Now, with the matched tube halves in the PI I was getting a very smooth sound. What I don't know is if this is because the tube sides were closely matched, or if the one tube is just 'better sounding' than the other. Again, the 5k trimmer could not 'un-match' the tube enough to make an audible difference; so I could not really test to see which was the case.
Anyway, the end result was a new OD tube and a perfectly DC balanced PI tube. I re-set the OD trimmer 'by ear' and it came out at about 50k from ground (which is where it was when I started).
Thanks for this comment. Prior to your post I had just today been reviewing Gil's original post on #124 and saw that he documented 280v and 290v on the respective PI plates and was re-thinking my PI trimming. I was not sure if someone had just popped any old 12AX7 in #124's PI spot during the amps lifetime, or if that was the way it left HADs shop.
Thanks for those tips. Knowing those voltage differences may let me select a tube to get into a range where I can hear something change with the 5k trimmer. Otherwise I'll go with a 10k.
When I was building the amp I was hoping the PI trimmer would have a sonic effect like Fender's "output tubes matching" trimmer, which is located just on the other side of the PI coupling caps, and is a bias voltage balance.
Also by purposely unbalancing the PI slightly you can bring out the 2nd order harmonics (octave higher). Find a note that wants to "hang" on the octave and adjust to make it even louder. You have to restrike the note each time you adjust, there is a point that gets obviously louder on the 2nd harmonic(not drastic but enough to hear). It also helps if you have the speaker lead polarity oriented for the easiest sustain. It's usually a range between 3 and 6 volts difference but it is also dependent on the output tubes matching so it can be a much larger range. Don't do this adjustment until the amp has been on for awhile and the output section is stable.
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