donvan wrote: ↑Tue Jan 29, 2019 10:16 pm Why is it necessary to tie together the cathodes of the power tubes that are in phase? Is that required on all of the methods?
Yes. You want to sense the total current in each side.
donvan wrote: ↑Tue Jan 29, 2019 10:16 pmMartin, you have an interesting method where you compare the *AC* difference with an oscilloscope. I was wondering why you do that at the 1 ohm cathode resister? That would be a small (millivolts) AC voltage at least for my lower resolution scope. Why not compare the AC voltages at the power tube grids and then do an A + B on the scope, then set the trim pot for a flat line?
Signal level shouldn’t be a problem for your scope. An A+B line would be even smaller signal, no? It’s nice to see the waveform, and it’s the top half that matters.
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Signal level shouldn’t be a problem for your scope. An A+B line would be even smaller signal, no? It’s nice to see the waveform, and it’s the top half that matters.
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I'm glad you pointed out that it is the top of the waveform that matters. I was seeing distortion on the bottom half and was thinking that I was driving something too hard even though the input didn't seem too hot. I forgot that it is only the top half that matters.
Would it be wrong to look at the signal at the power tube grids rather than the cathodes?
My amp is set up and sounding good. I don't have the FET circuit working yet. I mentioned earlier that I am using a 2N5486. Months ago, before I installed the FET board in the amp, I used Martin's procedure to set the bias using 2, 9 volt batteries. My circuit needed Rs = 8.8K. I know that the next step, now that I am hooked up to B+5, is to change one of the resistors in the voltage divider so that the voltage at the top of Rd is ~20V. I have not done that yet. Right now, that voltage is at 21V. Shouldn't that be close enough to at least get some signal out of the FET?
Other than the values and components that I have already mentioned, the FET circuit that I am using is from the 183 layout thread in the files section - the mdroberts schematic.
Here are my voltages:
B+5 = 302V
Top of Rd = 21V
Drain = 18V
Source = 2.68V
Even though VRd is a volt high, I still expected Rd to drop half of that voltage yet it's only dropping 3 volts. In other words, with the top of Rd currently being 21V, I would expect to have 10.5V at the drain. Am I wrong? Any tips on what I should look at next?
Seems like your source resistor is too big. You could try clipping a pot across it and see if you can get Vd to Vdd/2. Start with the pot at 50k or so and slowly reduce it. The Vdd will come down as the FET current increases.
I now have the FET circuit biased (Martin was right - Rs was too big) so my amp is officially fully functional. Yay! I really appreciate all of the folks here who answered my questions along the way and all of the great past posts, layouts, and schematics! Thank you! I have learned so much.
The next step is focusing on how to get the tone that I want. I am still getting acquainted with the amp controls as well as tinkering with the FET circuit output trimmer and overdrive trimmer. I'm happy with the bias and PI settings. I'm getting nice rich harmonic content. I have a strat, tele, and ES-335. So far I've been pretty unorganized going back and forth between each and they all produce very different results as you would expect. It might help if I focus on one at a time. I love the clean Fender tones and over-driven ES-335 tones that I am getting. Now, I want to work on things that I don't like. I guess I will start with working on overdrive tone with single coil pickups. What I really don't like are the buzzy low note sounds that I sometimes get. I found a great example of what I am talking about on a real Dumble amp. In the following clip, at around 6:02, a little after Mick Taylor plugs his strat into the FET input, he plays a low note 3 times in a row. Those notes have a buzziness that I just don't care for. I get that same buzziness with my amp when I play low notes with a strat in overdrive mode. Maybe this is inherent with a Dumble amp and a strat or maybe the amp in this clip was not voiced for a strat?
Any tips or ideas how warm that up? I don't really get that with the 335.
I realize that there will be trade-offs for whatever I do but it will be fun learning what those trade-offs are and how to voice or set the amp for different guitars.
donvan wrote: ↑Tue Feb 12, 2019 5:47 pmThose notes have a buzziness that I just don't care for.
What speaker and cab are you using?
That's a great question/point. That could be my issue. I am using speakers from existing amps that I own - a Lab Series L5 with 2 stock 12's and a MusicMan RD 112. My understanding is that the MusicMan has an EV speaker in it but I don't see any label on the speaker. It is from around 1981.
My plan is to try out various cabs that my friends own before deciding what cab/speakers that I want to go with.
But, your question implies some good advice - I may need to have speakers and cab that are closer to what I will eventually end up with before jacking with tone modifications.
Check out that EV, if that's what you've got. EVM 12L is commonly used in D-style cabs. If it's in good shape, it will give you a very clear picture of the amp's voice.
I build and repair tube amps. http://amps.monkeymatic.com
xtian wrote: ↑Tue Feb 12, 2019 6:44 pm
Check out that EV, if that's what you've got. EVM 12L is commonly used in D-style cabs. If it's in good shape, it will give you a very clear picture of the amp's voice.
I agree, the EV is very neutral tonally and also it doesn't break up so it will really show you what your amp sounds like. Having said that you may prefer a speaker that breaks up or has a tonal quality of it's own.
there's no right and wrong with this stuff.
personally I like the tone of the strat at that point in the video, in fact I think the amp sounds lovely the whole way through.
