Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: waldner on April 07, 2015, 01:35:12 pm
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Hello,
Rookie here. I have a Schedule 40 that I build a few years ago and it's a neat little amp. The only thing I wish I could change is that in the lower frequencies it sounds a little farty and worble-y. It also doesn't sustain very well. Meaning notes tend to weakly fade off into the distance with a slight warble as they go. I know that's how these amps are supposed to sound given hearing lots of them over the years. BTW I do have a switch I installed to select with/without NFB already.
What can I do that would make the bass sound smoother (a little less farty) and improve sustain. I love the tweed sound of my 5E3 by comparison, so it's not that I don't like the character of tweed amps.
I've heard of some things:
1. install grid stopper
2. change cap values
3. increase filter capacitance for power supply
4. change OT?
Or is all of this a waste of time because the amp will never be much different than it is?
Any advice would be appreciated.
Thanks
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You can lower the bypass cap value on V1A and reduce the resistor value. Try 1.5K with a 10uf/50v. Don't be afraid of going even lower like 4.7uf. I don't believe a new Output Trans is in order.
Think about it this way. If the Output Transformer does not have to reproduce lower frequencies there is no need to change it which with small watts it becomes more difficult.
After, I may add another bypass cap on v1b if you want it to grind more. Increasing filtration can help, but sometimes it makes the amp feel stiff.
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Thanks Ed.
I've circled in red what I believe you are referring to as "the bypass cap and resistor value for V1A" Is this correct?
I'm not sure where V1B is exactly? What is the A and B referencing? It's a miracle I've successfully built 3 amps:)
Much appreciate your help.
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I've circled in red what I believe you are referring to as "the bypass cap and resistor value for V1A" Is this correct?
Yes, you are correct.
I'm not sure where V1B is exactly? What is the A and B referencing?
A 12AX7 has 2 triode's in 1 bottle. Pins 1,2,3 are V1a and pins 6,7,8 are V1b.
Brad :icon_biggrin:
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Thanks Brad. That makes perfect sense.
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It's a miracle I've successfully built 3 amps
With the guidance, knowledge, skill, and work put in by the host and moderators here, you, me and a lot of others get to start a few rungs up the ladder :icon_biggrin:
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In my case, even a blind squirrel finds an acorn once in awhile. Thank God for the knowledge imparted by the generous people here on the forum!!
:worthy1:
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Ok, I just tried changing out the 22uf cap going to V1a to a 10uf cap and you can hear that it reduced the bass extension and bass output. It makes me realize that what I am trying to get rid of is not just in the bass but everywhere only it's more noticeable in the lower to middle frequencies. Again it's kind of a subtle rough warble. I think it may just be the character of this kind of amplifier circuit.
Just for the sake of experimentation I went the other way and added back the 22uf cap on top of the 10uf and increased the bottom end and I found I like it better this way. That's how I realized that it's really the rough warble character of the sound that I was trying to get rid of and it's infused more into everything that comes our of this amp.
A good chance to try an experiment and learn nonetheless.
Thanks.