You´re right. Unfortunately this is only a mistake in my sloppy layout. In real life, I have them orientated correctly. The problem is, that I´m only able to dial in negative voltage in the mv-range
Hi
Referring to the quoted message from MelB, if your transformer has no center tap on the HT winding - yes, the bridge rectifier is the solution for B+.
You will find certain amps that have a grounded center tap - you will connect your bias circuit, like on your .pdf, to one of the HT secondary leads, and it would work.
With the bridge involved, it does not permit usable current flow in the bias diode - so it develops no appreciable voltage.
If you trace your current flow, the bias circuit is shunted by the bridge and cannot develop bias voltage.
If your transformer has no bias winding in the secondary, you will need a separate transformer. It wouldn't have to be very big or large capacity for bias duty.
carry on
Deric
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Hi djcgtr
It has a bias winding, but it only provides 20v, so it is not quite going to cut it. Maybe I should just buy stuff that works, but it is so expensive and less fun😉
It has a bias winding, but it only provides 20v, so it is not quite going to cut it. Maybe I should just buy stuff that works, but it is so expensive and less fun😉
If your existing transformer has only 20 volts AC on the winding, that is a bit odd if is a JCM 900 transformer...
I agree about the fun part of seeking to use what you have to build.
One thing you might try is building a little voltage doubler circuit off that winding. Then it is completely away from your HT bridge.
You would rework the bias resistances in the supply since you would have much less initial AC voltage.
Just an idea.
carry on
Deric
Thanks
To be honest, i do not know if the 20 v Winding is for bias originally. The jcm 900 is full of diodes and ic's. But your idea is a good one. I will look into it.
Best regards
Kasper
To be honest, i do not know if the 20 v Winding is for bias originally. The jcm 900 is full of diodes and ic's. But your idea is a good one. I will look into it.
Best regards
Kasper
I´ve tried to incorporate a voltage doubler cirquit on my board. But I´m confused, when it comes to connecting it to the rest of the bias cirquit? I´m guessing it would produce around 40 v dc, but I will have to invert it to negative voltage somehow. Can this be done by connecting it to the existing bias cirquit, as in the attached layout?
Hello
I seen your update in my email so thought I would drop in. I looked at your .pdf of your doubler.
Before I went to post here, I looked up a couple things. An example of a 50 watt JCM 900 (which like you said, has IC's) - the Marshall schematic for 50 watt version has a 40 volt - center tapped winding. 20 volts each side of the tap.
Also I went to Magnetic Components site (I am about done with building a stereo 50 watt guitar amp with their iron, project on hold at the moment...) for their offerings for JCM 900. Their 50 watt also shows a 40 volt center tap. (Their 100 W tranny is 43 volt CT)
If you do have the JCM 900 tranny, you should see or could measure for the full winding instead of 20 volt (possibly 1/2 of the winding). Even though it was intended for supplying the +/-15 volt DC bipolar voltages for the ICs - 40 volt gives you a good chance of around 56 volts (rectified and first filter) without going with a doubler...
Which is grounding one side of the full winding and simply using one diode for your negative bias voltage. Leave the CT alone, tape it off)
Whether you need to use a doubler - or - find the full winding, you can experiment with your circuit to achieve a decent range of adjustment for biasing your outputs with the bias pot. I would even try feeding your original bias diode with the full 40 volts AC (or the doubler from 20 volts) and see what your range of voltage adjustment ends up being.
If using the doubler, you can leave the "single" bias diode out of your circuit...
And for negative voltage out of the doubler (if going that way) simply reference the + side of your C1 to ground and take the negative voltage from the minus side of C2 to the rest of the bias circuit...
And not ground any of the LT winding leads if using the doubler.
I have created bias doubler circuits for some repairs on Hiwatt amps that simply didn't have enough negative voltage from the factory. (known problem)
Hope that helps.
carry on
Deric
I found the other half of the 40 v ac winding. I cut it off months ago, but was able to make it work with wire and heat shrink tubing. Grounded one side, taped off the center tap and hooked the remaining side to the reversed diode...messed around with a few resistors and 🙂 Succes! It gives me a range from - 36 to -29 v dc. Needs a little tweaking, but The B+ is also too high. The bridged rectifier raises it to 460 v dc...that's too much. Will need to find a way to lower that a tad.(Can I use a string of 15 v zeners from the bridge rectifier to chassis?)
Thank you so much. Your posts has been extremely helpful.
The best part is that the amp works flawlessly. Every control does what it´s supposed to, and there is no oscillation, even with presence maxed. Of course there is hum, but I think this will improve when I close of the chassis with a metal plate, or copper or something like that.
Thank you so much. Your posts has been extremely helpful.
The best part is that the amp works flawlessly. Every control does what it´s supposed to, and there is no oscillation, even with presence maxed. Of course there is hum, but I think this will improve when I close of the chassis with a metal plate, or copper or something like that.
Hi
It was good to hear you found the other lead on the 40 volt winding. And it looks like you will be able to play with some resistor values to dial in bias.
Is the B+ you measured with output tubes installed and operating ? Or is it "no load" on the HT ?
I referred back to your .pdf of the amp's component layout and I see choosing EL34 or 6V6. EL34 is fine with 460 volts. The amp I built has about 490 volts on the EL34 plates, for me, I am good with that figure. A run of the mill 6V6, with 460 v plate, is pushing it like you say. There are a couple 6V6 tube types being sold that claim a higher than "normal" max plate B+. I believe one of them is the JJ 6v6S. Even that one on the JJ data sheet is a bit less than 460 volt for "max"...
If you do not wish to eliminate 6V6 from the option list on your build, then yes, a lower B+ would be advisable.
Shunting the B+ with zeners (and if so, maybe a switch installed to take the zeners out of circuit when using EL34 type outputs), in my thoughts, is wasting B+ secondary milliamps, and tends to eliminate B+ "sag" until the zeners would quit conducting as the voltage lowers, with more output current draw...
All that, besides adding series resistance to permit the zeners to operate in a workable range of current according to their wattage rating.
I have not tried the zener method on B+ like what you propose. Perhaps another forum member could weigh in and give their experience trying this idea.
There are members who have experimented I am sure, with changing output tube voltage by series regulation with a variety of ways, to be able to adjust power output. I have read some discussions on it. Doing a search on the forum should bring it up.
That adds a measurable bit of complication to your amp circuitry. It is up to you how much you wish to dive into.
The EL34 and 6V6 types ought to be biased differently, not just a "swap" at a moments notice, to keep it in mind...
It is good to hear that your amp is working and the hum issue can be a whole lot of different possibilities. Once you sort out your B+ and tube type decisions, there is a lot of reading available on sources of hum.
On my stereo gtr amp I built, I have no noticeable hum or hiss, I used a star ground scheme besides careful location of the circuitry. I am fortunate it came out the way it has in that regard. I will get that one wrapped up when things slow down...
carry on
Deric
Hi again
The measured B+ is with all tubes installed and operating. It´s not absolutely nessecary to swap for 6v6 tubes, but I would think that the amp would behave a little different with a higher B+ Would it not give more clean headroom? Maybe this is a good thing. A Trainwreck is supposed to overdrive 😉
Wasting milliamps on the HV will happen with the zener approach, but Since the Powertransformer is meant for 4 el34 I would think this wouldn´t be a problem? The B+ sag you´re talking about, I´m not so sure of? I´ve heard about this "Sag" phenomenon before, but I must admit that I don´t really understand it?
The measured B+ is with all tubes installed and operating. It´s not absolutely nessecary to swap for 6v6 tubes, but I would think that the amp would behave a little different with a higher B+ Would it not give more clean headroom? Maybe this is a good thing. A Trainwreck is supposed to overdrive 😉
Wasting milliamps on the HV will happen with the zener approach, but Since the Powertransformer is meant for 4 el34 I would think this wouldn´t be a problem? The B+ sag you´re talking about, I´m not so sure of? I´ve heard about this "Sag" phenomenon before, but I must admit that I don´t really understand it?
Hi,
I'm not a tube guy but I know that sometime in power supply for tubes are used big coil in series with the supply positive line for filtering. If you use one coil like this but in front of diode bridge (on AC), you can find a value of the coil that will alow a voltage drop enought for your need.
I'm not a tube guy but I know that sometime in power supply for tubes are used big coil in series with the supply positive line for filtering. If you use one coil like this but in front of diode bridge (on AC), you can find a value of the coil that will alow a voltage drop enought for your need.
I Think perhaps, what you are reffering to, is called a choke? And that should work, yes. A bit more expensive than zeners, but maybe a better solution.
Sag happens due to series resistance in the B+ rail. As the power tubes demand more current, the voltage drop in any series component will go up according to ohm's law. It happens much more if a tube rectifier is used. The plates of the tube have internal resistance, so since they are in series with the B+ rail, the internal voltage drop will go up according to ohm's law due to the increase in current. When this happens, there is less B+ output and sag results. Think of tube rectifiers as a voltage source with series resistance.
There is less sag in a SS rectifier because the voltage drop across the diodes stays constant at about .7v per diode. Sag can be mimicked in an SS rectifier by adding a series resistor at the output of the bridge.
Sag will happen to some extent on the series dropping resistors, again due to ohm's law. Increase in current equals larger voltage drop on the resistor. This causes lower B+ at any node after that resistor.
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This won't work because by lowering the AC in, you lower the raw DC voltage out. Sure, you will lower the plate voltage but this also will lower your DC in to your bias circuit, so you will lose bias voltage as well. You just fought so hard to gain more.
Try hooking the zeners in series with the wire going to your center tap of your output transformer. This way, only the plate voltage will go down. Remember to keep track of your screen voltage, I don't recall seeing what this reading is. Make sure to keep it at least a couple of volts lower than the plates. You might have to increase the screen resistors to do this.You can buy 5 watt zeners and string them together to add up to the desired voltage drop. These are fairly cheap.
Review your comments with schematic in front.
If lowering voltage with zener will work, lowering with choke will work to...
From what I read, the bias is negative and it is made with another ac winding, but as I said I'm not a tube guy and it is possible to understood wrong.
However the best solution is to use a series adjustable linear regulator, and this will help a lot to solve other usuale problems : hum
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You refer to lower the AC at the input of transformer?
I'm referring to lower the ac voltage at the input of the bridge rectifier, after transformer.
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I misunderstood where you wanted to lower the voltage. Apologies. The zener solution will provide an exact amount of voltage dropped. Do you have a way to know what size choke to buy so you will know how much voltage will be dropped? I must admit I have never heard of this solution before. It will be interesting to see it in action.
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Been a busy day and not over yet, stopping in to comment.
Anyway - I am glad to see some others weigh in on their thoughts.
I was not sure if you had a "50" or "100" watt power transformer, so now we know. Yes, additional current is available from the secondaries in your 100 w tranny.
I will say a reduced B+ will lower the clean output wattage capability. And then if you are seeking "earlier" output tube clip, it would occur, with lower B+.
Keep in mind regarding how much volume reduction will happen in consideration of lower power output rating - one half power is only 3 dB SPL lower...In case you were looking to saturate outputs at a significant lower level volume.
For my amps and how I wish to use them, most of my sound and processing is before the amp. I do not use "all in one" devices, but I achieve my OD before the amp also, it is a pedal with a dual triode I designed around 1990.
Many players wish output clipping over "preamp" clipping. I respect that. I choose to run my outputs clean and last longer for me. Output tubes in clip do wear faster but if that is what you wish for sound, by all means - do it.
You asked about sag.
If it is ok to post it, I have a link here from Aiken amps web site, and much more on sag is on the 'net to find.
What is "Sag"?
carry on
Deric
Thanks for the replies everyone. I think I will tie a string of 15v 5w zeners to ground from the rectifier. My amp is already made from old parts. It works, but it´s not a beauty, so I will add a switch to bypass the zeners, so I have an option for higher B+. This way I will have the intended "dirty-but-loud" Trainwreck + an option with a little more headroom. I will put the switch inside the chassis, so you don´t accidentally flip it, but only reach it when you can set the bias at the same time.
Thank you very much for all the help and input
Thank you very much for all the help and input
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