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Discussion in 'Amp Tech Center' started by 8urchevy, Feb 14, 2019.
Thank you. I don't have one. Wish I did fir resistors and another for capacitors .
Well, last night I checked every stinking resistor in that amp and they are all within 5% which in my book is phenomenal. I have no clue as to why my voltages are reading what they do. I am using a nice Fluke meter so they should be fairly accurate.
However, upon even closer scrutiny I found I completely botched the tone controls wiring. Must have been half asleep I guess. Haven't examined the schematic close enough to determine if that could have caused the readings, nor have I rechecked them yet. The wiring has been corrected, but I am yet to play it due to having to leave for work. Can't wait to get home and try it out.
Tone controls work as they should, but still tight. Clean straight into od. Gonna change the resistors out to bring the voltage downm hopefully tonight .
This is where the magic happens. Have fun and be safe.
The amp was rebiased from below 30% to normal levels and the amp still sounds tight.
Something else is wrong.
Did you get the bias on the pre amp figured out? There was a 12 volt bias in the preamp. Has that been righted?
An important number to see if the power section is operating normally is the fixed bias voltage on the power tubes.
In order to get you bias in the normal range, what did the fixed bias end up being set to?
And once that’s known, what kind of signal is your pre amp putting up on the grids of the power tubes? Is it enough to drive the tubes?
Ok here are the updated numbers and sound sample. I am quite happy now. Still need to spend some more time with it to dial it in exactly the way I want it, but it is very responsive, plenty of gain, loud as hell, and the cleans are great. The sample doesn't contain clean tones, but I think you can tell they would be great and they are. The master volume isn't up much and the recording is from my cell phone mic. The compressed, tight, and overly punchy tones are gone. Thank you all for your help. One happy camper here.
The output tubes are biased at 63,7 % which is 19.1 watts for 6l6GC. and they are well matched
plate voltage 410 411 resistance to CT of OT 112 and 121 voltage drop 5.76 and 6.20 cathode bias resistor actual reading 254.6 ohms cathode bias resistor voltage drop 28.6
The V1 tube plates are at 170 volts now. Which I am finding to be quite nice.
Looks like your in target from what I can tell. Really glad you’re satisfied with the overall tone!
I am indeed. Still need to spend some real time with it to get it exactly the way I want it, but that sample isn't too far off. Thank you so much for bring up those plate volts. I have learned quite a bit building this.
I am trying to decide if I am going to hook the presence and or negative feedback back up. I may split the VI cathode bias resistor and cap in order to bias the second half a little less gain. It adds a tad too much and creates a weird sizzle sound. It overdrives very smooth if I turn the gain knob up a touch more.
One other thing I would like to achieve is more smooth master volume. It is very touchy. Goes from bedroom volume to holy hell moving it a hair.
Thanks for providing the bias numbers. It looks like they’re set to a good level.
The 5E5 Pro schematic shows a 26 volt bias on the power tubes from a combined 250 ohm cathode resistor.
Yours is very close to that.
What made the difference for the sound?
Lower pre amp plate voltages?
I used that schematic to cathode bias this build because the transformer that came with the PA originally doesn't have a bias tap. Lowering the V1 plate volts did the trick for the tone. What a difference it made. It breaks up much sooner and actually has a transition from clean to mild break up to overdriven. No pedals needed with this amp for sure. May add reverb some day. There are two more available preamp sockets so it is possible. Also, I thought about adding an effects loop because sometimes I like to use delay.
Peteb is def more technical than I am. I just know a bit and how to season.
I think I’ve got a type 4 mv on mine and it’s very linear and smooth. Took it from robrob’s website iirc.
I’d def put the nfb on a pot! Adding some back into the mix can quiet things down and take the raspy edge off the overdrive. I’d still be tempted to get the preamps’s plates to ~160v. That’s a horse I can’t quit beating!
I’m a little surprised.
8urchevy says they sound better at 170 volts. I would think lowering them from 190 to 170 would not make much difference. I just looked at my own voltage charts. I don’t think I saw a pre amp plate under 200. Mostly they are 230, 250, even 270, 280. None of these sound tight.
That’s OK. I’m glad that you got the results that you were after. I can definitely hear more break up in the latest recording.
I forgot to ask.
Did you lower the pre amp plate voltages by increasing the plate resistor or increasing the power rail resistor?
I raised the plate resistors to 200k
That is a significant change to the circuit, more than dropping 20 volts at the plate.
That’s going to boost the signal at the plate, which is how much signal v1 hands off to v2.
That could be the difference to the break up.
Yeah I wasn't sure what the results would be in the sound, but it was worth a shot. Was a lot easier to swap those plate resistors than the voltage dividers on this one. So far it seems to like the change.