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Discussion in 'Amp Central Station' started by 8urchevy, Mar 13, 2019.
Yes it measures .473k
Is V3 with the presence pot rewired?
Did you repeat this to confirm the fix to the wiring.
I just read through your other thread. Did you ever try the smaller OT?
I would recommend installing the fixed bias circuit. You are running your cathode bias 6l6gc30w at 67%. Pretty cool for cathode bias. Looking at the tweed pro is misleading. Doing the math with (PRo)6l6g19w,250r,26v bias.... this is running right at 100% where it should be.
I haven't tried the other ot. I ordered a couple lower resistors to try to bring the bias up. Cathode biasing should be higher than fixed bias?
Cathode bias will decrease as tube is driven
Fixed bias will increase as tube is driven
This is from robrobs site
Well shoot. I used his bias calculator and managed to overlook that . Well, the resistors should be here by Saturday. Must be the issue then because I don't think I am getting much power tube break up. Even with the master volume removed and diming the volume.
Thanks for all your help. If the bias correction doesn't do it I will get some new tubes
Fixed and cathode bias are going to give different results.
D’tar, are you sure the cathode biased 5E5 tweed pro with 26 volt bias on the power tubes is biased any hotter than urchevy’s amp? He has a higher plate voltage, 416 v versus 395 v, and a higher bias voltage, 29 v versus 26 volts, which is more current for more idle power. How is it less?
If I wasn’t getting power tube break up, I would measure the signal on the grids of the power tubes under playing conditions. If the tubes aren’t being driven hard enough, the signal hitting those tubes needs to be raised.
6l6g 19w tube
6l6gc 30w tube
I see what you mean.
In my opinion, this is an example how the bias being off by 20% may change some tone but it’s not going to make a dirty amp clean. Take an early tweed pro, put whatever tubes in it you want, play it with the stock bias and it’s going to have killer distortion. I’ve played thru a TV front pro. They rock.
Wow, yes they recommended that I start out with something a little more simple for my first build as well, this is the circuit I wanted but I shy’d away after I had some feedback about the build
Maybe I'm reading the handwriting wrong and if so I'm sorry for muddying things up, but those voltages for V3 look way off to me. I wouldn't chase down issues of overdrive voice or power tube bias or any other subtlety until the basic amp circuit works.
You should be measuring:
the 'top' of the cathode resistor (where pin 3 and 8 are tied together)
the intersection of the cathode resistor (470R) and the tail resistor (10k)
I am familiar with schematics as I deal with daily at work so it wasn't too much for me. Obviously I am human and made a couple errors .lol. After the wonderful help from this crew and a ton of prior research I dove in. I agree of you aren't familiar with electricity and how to read schematics this circuit may not be the best first build choice.
That is the all important detail i gave no thought when I converted the schematic to cathode bias . I would consider 11w difference to be considerable.
I would agree, but keep in mind I have both halves of V1 in circuit, one half on a switch, the grid input.
But I dont2 have an oscilloscope so nit sure what voltage you would consider to be high enough?
This is an area I find very interesting. You can have whatever tubes and circuitry in there but if it’s going to sound like what you want the power tubes have to be running like they were meant to be run.
To signal test the power section, you don’t need an O scope, it’s just AC voltage you are measuring. You will get good steady AC measurements if you use a signal generator, and these are available on your mobile device but I don’t recommend using a signal generator although you could. The signal generator signal is a weird thing. It’s great for measuring gain but it’s not like your guitar trying to make your amp sound off. Just clip your multimeter between a grid and ground, plug in and play. See how high you can make the signal go. Pay attention to the highest number you can see, and note a number less than that. One that is easily repeatable. Is it high enough? It should be half of the bias or more but probably should not exceed the bias. Right in there.
I agree with Snfoilhat here. I didn't mean to redirect the thread to power tube bias issues. I was just commenting after reading through the other threads related to this build. I really don't think your LTPI is where it needs to be. If it is wired to 5f6a specs, something is wrong there. I can only suggest again to repeat the ohm readings mentioned earlier and give us numbers to digest. Remember to disconnect the NFB. Are you using a 12ax7?