Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: casssax on August 17, 2015, 09:22:20 am
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This is regarding a mono block conversion project that I have been working on.
http://el34world.com/Forum/index.php?topic=18972.msg194758#msg194758 (http://el34world.com/Forum/index.php?topic=18972.msg194758#msg194758)
http://el34world.com/Forum/index.php?topic=19018.msg195340#msg195340 (http://el34world.com/Forum/index.php?topic=19018.msg195340#msg195340)
I converted the first two gain stages from a grid leak to a cathode bypass with 25uf/1500ohms.
The eq is modeled after a Princeton Reverb and I have a .022uf cap after the second gain stage.
The good news is that this seems to have solved the problem of outside interference. When turn my reverb pedal on I don't get a loud pop anymore.
The bad news is that this has made the amp extremely bassy. I have to turn the bass eq down to about 2 and the treble up to 7 or higher to get a not too bassy sound.
The ugly news is there is still a random 'bzzzzzt' sound coming from the amp, but I think I may know where this is coming from. While poking around in the amp with a stick while it was on I noticed this same sound if I wiggled the wire going from the input transformers 'E' (next to the 6.3V lugs) to ground. I need to check to see if there is a bad solder joint there.
As far as making it less bassy I was thinking that I should try a different coupling cap value after the second gain stage. This seems like the easiest fix. I thought I'd ask if this would be right place to start or if there are other/better options?
One other thing I noticed while looking at the schematic vs. the Princeton is that on the third gain stage on the Princeton they have the same 25uf/1500ohm bypass on this third stage (the same as on the first two) where as my amp has a 50uf/5000ohm bypass. I know that 50uf will pass more bass than 25uf, but I'm not sure how much.
Here is the schematic before I added the cathode bypass to it. (I'll update it soon to reflect the changes)
The only difference is that I removed the two 3M resistors and added the cap and resistor to pins 3 and 8.
Also the cap values in the two smaller caps in the tone stack are wrong on this. They should be .1uf and .047uf for the bass and treble respectively.
(http://www.unitedsound.us/images/AF-12-schematic2.0.jpg)
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Change the coupling cap after V1B from .02 to .01 (or .0047)
Change the cathode cap from 50uf to either 2.2uf or 5uf max
You can always add a bright cap to one of your volume/gain controls.
You can use the Duncan Tone Stack Calculator to see the "bass response" by changing out the bass and mid cap. Try looking at the tone pattern plugging in a .047 instead of the .022 bass cap. I realize the larger bass cap seems like it would increase bass ................. however look at what actually happens there when changing to a larger cap. You can carefully clip in a .022 in parallel with the .022 in the bass cap position using insulated alligator clipped wires and listen for yourself.
With respect, Tubenit
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Change the cathode cap from 50uf to either 2.2uf or 5uf max
Do the same for both of the 25µF caps you put on V1 pins 3 and 8.
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Great suggestions from Sluckey and Tubenit. They will work. Do you have a cap decade box? I have 2 and they are very handy at tweaking amps. All of our tastes in tone are different and while the suggestions are great, I have found sometimes I prefer a 10uf where others recommend a 4.7.
There are schematics for making them where you can use your preferred type of cap and there are some cheap prebuilt ones on ebay for less than the price a 4 Orange Drop caps.
In either case, the Duncan Tone Stack calculator is a great tool in addition to a decade box. I promise if you make one or 2 it will quickly become something you will not want to build without having it.