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Thread: Urgent...6SN7 bias Question

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    Forum Member Cygnus X1's Avatar
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    Urgent...6SN7 bias Question

    I have this posted at several places, no responses yet.

    I am working on trying to put a 6SN7 in my single ended lead amp.
    Right now it has a 6V6. The schematic shows a 1K/2W pot for the bias.

    I don't have one. I want to replace that stage with a fixed resistor to get this started. Do I simply use a 1K resistor?

    ...The amp is cathode biased, single power tube.
    Schematic for the power stage is here:
    http://www.ax84.com/static/corepower..._Schematic.pdf

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    I have this question out on several forums.

    It must be a real head scratcher...the only answer I got was some guy trying to redesign the entire power stage.

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    Forum Member stevo's Avatar
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    Re: Urgent...6SN7 bias Question

    No, you can't just sub a 1K resistor there. The correct Rk value for a 6SN7 is much lower. Take a look here for a similar idea, without the pot: http://boozhoundlabs.com/12sn7-geetar/

    steven

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Thank you, stevo.

    The design I am using is single ended, no pi...just straight off the master volume. No center tap on the OT. Check the schematic.

    I've been using this amp with either a 6V6 or a 6550 for almost two months now. Just wanting to try the 6SN7.

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    Forum Member stevo's Avatar
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    Re: Urgent...6SN7 bias Question

    Ah, yeah, sorry, usually people do self-split push-pull in these things. You want to use half the Rk that would normally be used for a single section. Start with 680, and work down until you get the tone you want.

    steven

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Excellent.
    I'll give it a try.

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    Forum Member cdw2000's Avatar
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    Re: Urgent...6SN7 bias Question

    I was not able to find a 6SN7 datasheet, but assuming the 6SN7GT has the same characteristics:

    The maximum combined plate dissipation should not exceed 7.5W with both triodes operating. For cathode biased SE amps you should not exceed 90% of the rating. So 0.9 x 7.5W = 6.75W.

    At 250V, the corresponding plate current is then 6.75W/250V = 27mA. You should measure the plate voltage and adjust the result accordingly to get the correct bias current.

    So ... I would start with 1K ohm resistor, measure the voltage between TP1 and TP2 and use successively smaller standard resistor values (820, 680, 560, 470, etc.) until you get a voltage reading of close to 27mV (or whatever valure you come up with based on the actual plate voltage). Also since this fixed resistor will be in series with R11 you could eliminate R11 and just use one resistor. It will need to be rated at 2W or greater.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Thank you, CDW. I will try that.
    The project became not-quite-so-hot as the workweek started. (Real life intervened). The amp is now upside down on the bench and hasn't seen a soldering iron since Sunday!
    Back to it Friday (I'm off), unless the honey-do list gets out of hand!
    Amp widows are tough to pacify sometimes.

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    Forum Member cdw2000's Avatar
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    Re: Urgent...6SN7 bias Question

    Quote Originally Posted by Cygnus X1 View Post
    Back to it Friday (I'm off), unless the honey-do list gets out of hand!
    Amp widows are tough to pacify sometimes.
    I hear ya there!
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

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    Forum Member yankeerob's Avatar
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    Re: Urgent...6SN7 bias Question

    Yer trying to replace a pentode with a dual triode - which has two cathodes, two grids and two anodes - there's no supression or bias grid. However - in our little 1W wonder (based on this - thank you Guido) we use a 6SN7 in P/P - produces a very loud and ballsy 1.1W RMS (and plenty of saturation/overdrive and sustain after that) into an 8K/8Ω OT (so the OT - provided it's around 5.5K primary - shouldn't be too badly matched for a 6SN7 run single ended in parallel though something around 4K would be better) - BTW - the EH 6SN7's sound every bit as good in an output tube role as many NOS's we tried - NOS is preferable in many cases but not - it would seem - in this one... have a look at the circuit though - it might give you a clue as to what values you might be looking for
    Last edited by yankeerob; 10-18-2008 at 04:22 AM.
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    Forum Member yankeerob's Avatar
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    Re: Urgent...6SN7 bias Question

    I've had a think about this over the weekend and am dubious about using both triodes in parallel in the same bottle - perhaps some of the more experienced guys could comment but my engineering instinct says that using both triodes in an 'on' state at the same time might be a problem - I suspect that using it in P/P where one triode is off whilst the other is on is OK whereas unwanted oscillations might occur otherwise... I haven't done the experiment myself and that'd be the only to find out for me - any thoughts guys?
    If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

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    Forum Member cdw2000's Avatar
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    Re: Urgent...6SN7 bias Question

    The data sheet says it can be done, but you have to derate the total power dissipation. The 7.5 Watts I quoted is for both plates being used simultaneously. Individually each triode is capable of 5 Watts.

    I think he'll be fine if he is using the AX84 schematic that he linked to.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    OK...I did it, and have been playing this for a week.
    The 6SN7 power tube is running cold...literally cold enough to touch it when it has been running for several hours.

    It sounds virtually identical to running the power stage at 15 some watts with the big bottle 6550. I detect no power tube distortion. The volume characteristic has been scaled back to about 1/5 of the original design. So..I'm on the right track.

    I need to grab the actual dissipation (I'm lazy, I plugged a equivalent resistor instead of using a pot for the cathode bias).
    So I can start bumping it up until I'm happy with it. It will get into "mushy" territory pretty fast, so that is the limit I'm looking for. Just short of that, I should be able to get the "full" sound I'm looking for.

    The more I tweak, the pickier I get!
    I will say this...the background noise is almost nonexistent...it is whisper quiet with the gains dimed....none of that characteristic hiss that comes out of most Marshalls I've played.

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    Forum Member yankeerob's Avatar
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    Re: Urgent...6SN7 bias Question

    Funny that - we too get very little noise even though we're pushing the 6SN7 way into saturation - OK our earthing scheme is pretty thorough but we've got it screamin' it's t*ts off with no GNF and it's like flicking a switch noise-wise - glad it's working out for you - fun ain't it!!
    If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Yes!
    Thank you for the inspiration, Rob.
    You suggested this a couple of months ago for a low powered amp...I just put two and two together, and found a plan at AX84 to try.

    Love it when a plan comes together.
    LOL.

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    So I have a 1 ohm, 10 watt resistor in linebetween the cathodes (tied together) and the bias pot.
    The meter is reading "3" on the 200m DC range.

    This reading doesn't change if I turn the pot.

    I attached the meter from ground to the cathode, and I get a reading of 12VDC. This does reflect change when I dial the pot.

    So,what the heck am I looking at here?

    My B+ is showing 345 volts at the plate.

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Quote Originally Posted by cdw2000 View Post
    I was not able to find a 6SN7 datasheet, but assuming the 6SN7GT has the same characteristics:

    The maximum combined plate dissipation should not exceed 7.5W with both triodes operating. For cathode biased SE amps you should not exceed 90% of the rating. So 0.9 x 7.5W = 6.75W.

    At 250V, the corresponding plate current is then 6.75W/250V = 27mA. You should measure the plate voltage and adjust the result accordingly to get the correct bias current.

    So ... I would start with 1K ohm resistor, measure the voltage between TP1 and TP2 and use successively smaller standard resistor values (820, 680, 560, 470, etc.) until you get a voltage reading of close to 27mV (or whatever valure you come up with based on the actual plate voltage). Also since this fixed resistor will be in series with R11 you could eliminate R11 and just use one resistor. It will need to be rated at 2W or greater.

    ...So I suppose this "3" I am seeing across the 1 0hm resistor is equivalent to the "27mV"? Perhaps my meter isn't accurate enough to read this number correctly?

    At 345V on the plate, I am adjusting the cathode anywhere between 9V and 14V. Dropping this makes it sound fuller, etc.
    I haven't reached anything close to red-plating. Things overall seem to be running cool. Any additional help would be appreciated.

    I have this question out at several amp sites...and it seems running a 6SN7 as a cathode biased single ended power tube is a rare enough phenomenon that it has the experts scratching their collective heads. (Probable wondering why I'm not running it push-pull).

  18. #18
    Forum Member cdw2000's Avatar
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    Re: Urgent...6SN7 bias Question

    The fact that you have 345V on the plate tells me you do not have exactly that AX84 schematic, since it indicates about 250V plate voltage. Can you post a sketch of your schematic?

    I also have a concern that you maybe exceeding the maximum plate voltage rating. What is the exact part number on your tube? If it is a 6SN7GTA or 6SN7GTB you are OK. If not you should somehow lower the plate voltage to 250V. This can be done with a 4.7K, 5 Watt dropping resistor (or by changing out the power transformer).

    That being said, at 345V plate you should not have more than 20mA for the bias current to get the 90% of max plate dissipation.

    A reading of "3" on the 200mV scale would only be 3mV which equates to 3mA through a 1 ohm resistor.

    When you measured the 9V to 14V what is the total Cathode Resistance?

    To measure, turn off the power, discharge the HV caps and remove the 6SN7 tube. Adjust to one extreme of your variable bias resistor and do an ohms measurement across all resistors going from the 6SN7 cathodes to ground. Now adjust to the other extreme of your bias resistor and re-measure.

    At 14V you should have a total cathode resistance of 700 ohms (ohms law: E=IR, therefore R = E/I = 14V/20mA = 700 ohms).

    At 9V you should have a total cathode resistance of 450 ohms.

    (Both voltage and current are varying as you adjust the bias resistance so these are just ballpark figures)

    Let me know the measured resistance values and which measured cathode voltage they correspond to. I'll try to help you figure out what's going on.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Thank you, cdw.
    I currently have a Sylvania 6SN7GTB in it right now.
    Someone...somewhere stated that 345VDC would be safe to run on those plates. I'll measure the total resistance in a bit.

  20. #20
    Forum Member cdw2000's Avatar
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    Re: Urgent...6SN7 bias Question

    A 6SN7GTB is OK for that plate voltage. It has a rating of 450V.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    http://www.triodeel.com/6sn7_p4.gif

    Tube chart for a GTA

    Another one for GTB:
    http://scottbecker.net/tube/sheets/127/6/6SN7GTB.pdf

    And yet another one...
    http://scottbecker.net/tube/sheets/093/6/6SN7GTB.pdf

    How this applies to parallel operation is what I don't know how to calculate.
    It does seem to be doing ok as it is.
    I still need to disconnect the temporary pot and measure the total resistance.
    Last edited by Cygnus X1; 10-24-2008 at 07:53 AM.

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    Forum Member cdw2000's Avatar
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    Re: Urgent...6SN7 bias Question

    Because you are running the triodes in parallel the impedance should be 1/2 and the current should be double (through the cathode resistor after the cathodes are tied together).

    So if you look at the "Typical Operation for a Class A Amplifier" shown at the top of page 3 on the GE datasheet, you will see that for a 250V plate voltage, each triode wants a 7700 ohm load and 9 mA bias current. For parallel operation, the pair will ideally want a 3850 ohm load and the combined bias current will be 18mA (the data sheet maybe rating things more conservatively).

    Assuming you have the same output transformer as in the AX84 schematic, you have a 4000 ohm load, which is close enough.

    The only other controllable element is the cathode bias. For your application, you simply need to find the correct cathode resistance so that the bias current is around 20mA. This will be trial and error unless you want to get into calculating load lines and such, but based on your previous measurements, I think it should be somewhere between 450 ohms and 700 ohms.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
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    Forum Member Offshore Angler's Avatar
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    Re: Urgent...6SN7 bias Question

    Quote Originally Posted by yankeerob View Post
    Yer trying to replace a pentode with a dual triode - which has two cathodes, two grids and two anodes - there's no supression or bias grid. However - in our little 1W wonder (based on this - thank you Guido) we use a 6SN7 in P/P - produces a very loud and ballsy 1.1W RMS (and plenty of saturation/overdrive and sustain after that) into an 8K/8Ω OT (so the OT - provided it's around 5.5K primary - shouldn't be too badly matched for a 6SN7 run single ended in parallel though something around 4K would be better) - BTW - the EH 6SN7's sound every bit as good in an output tube role as many NOS's we tried - NOS is preferable in many cases but not - it would seem - in this one... have a look at the circuit though - it might give you a clue as to what values you might be looking for

    Rob, looking at that schematic - I'm just a dumb aerospace engineer so if this is a stupid question... but it looks like there is ony a single transformer. Ther eshould be an input transformer too for the heaters an +300V, correct? Looks like a fun build I may try.

    Chuck
    "No harmonic knowledge, no sense of time, a ghastly tone, unskilled vibrato, and so on. Chuck is one of the worst guitar players I know" -Gravity Jim

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    Forum Member ziess's Avatar
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    Re: Urgent...6SN7 bias Question

    OSA, yup, that's right. There's no power supply there but you could just tag one one that'll provide 300VDC and take the 6.3VAC from PT.

    Tommy.

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Well, I didn't go exactly by that schematic.
    Using a Weber WGTGP universal PT, with a full wave bridge rectifier for the center. It's good to 125ma...plenty for this app.
    The OT is a Hammond universal, 125ESE...6K primary.

    I noodled with the rigged up bias pot and it sounded best and ran
    a bit warm, but not too hot at 9VDC on the cathode. That ended up at 240 ohms on that pot, so I subbed in a 250 and ended up with 9.5
    VDC.

    I still am not sure what exactly the ma (or mv) reading is...as I indicated, it might be a problem with either the meter, or the operator.

    All in all, it seems just fine. Keep in mind this is a Marshall inspired high gain, low volume circuit. So too much power tube distortion would sound muddy at best. Just a little, not much. Clean is ok (much better than your average Marshall Super Lead). But that wasn't the goal. Turning the gains down gives off a nice emulation of a cranked Tweed Deluxe.

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    Forum Member cdw2000's Avatar
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    Re: Urgent...6SN7 bias Question

    Quote Originally Posted by Cygnus X1 View Post
    I still am not sure what exactly the ma (or mv) reading is...as I indicated, it might be a problem with either the meter, or the operator.
    Is it possible maybe that what you think is a 1 ohm resistor could be a 0.1 ohm resistor? This would result in 1/10 the voltage across it which could account for the low mV reading.

    Probably measuring the overall cathode resistance is the only way to know for sure. However if it sounds good, the only worry is that the current isn't so high it shortens the life of the tube.

    Anyway, from you are are saying, you have 9.5V across 100 ohms + 250 ohms = 350 ohms. So 9.5V/350 ohms = 27mA. If my theory about the 0.1 ohm resistor is correct this would read as 3mV on a 200mV DMM.

    That sounds like you are running a bit hot to me if your plate voltage is 354V Plate dissipation = 354V x 27mA = 9.6 watts.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

  27. #27
    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    No doubt, it was a 1 ohm resistor...it is out of the circuit right now.
    I dimed the amp for about 20 minutes this afternoon. (Loud enough to keep up with a drummer). No issues yet.

    Running an old Tung-Sol GTB.
    I'm still shocked at how quiet the amp is!

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    Forum Member Offshore Angler's Avatar
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    Re: Urgent...6SN7 bias Question

    I'm scrounging now. I've been looking for a first amp build and this looks like a good candidate.
    "No harmonic knowledge, no sense of time, a ghastly tone, unskilled vibrato, and so on. Chuck is one of the worst guitar players I know" -Gravity Jim

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Quote Originally Posted by Offshore Angler View Post
    I'm scrounging now. I've been looking for a first amp build and this looks like a good candidate.

    I would not recommend this particular one as a first build.
    Perhaps going strictly by the AX84...but even then one is in risky territory. Not speaking against them at all.

    I built the SEL circuit as my third (actually 4th), after a champ-type Magna rebuild, a 5E3 build, and many eons ago fooling with other amps such as Ampeg V-4 mods. I also have EQ's, preamps, and pedals in my experience.

    I came to this particular point after building the stock SEL and tweaking it to my taste. It took several weeks of swapping out caps and resistors to get it right. The 6SN7 version of it was frankensteined out of the fact that I had plenty of them to play with, and seeing the unfinished 2 Watt schem at AX84. I have no idea what it would sound like with their values. Their plans sound too "buzzy" and artificial. I like the distortion, the drive, but I like a good clean tone also. So I go from there.

    Edit to add...
    I have constructed the AX84 drawings that I use into Autocad drawings.
    I don't know why they do it, but the layout drawings don't contain the component values. And though pretty and color coded, they are quite hard to read. The schems are ok, but you have combine the schematic, the parts list,a nd the layout drawing to put it all together. I simply drew up the layout and plugged in the schematic values.
    It did take a while, as there are some spots where the parts list and the schematic don't match.

    I will update my cad drawing to what I actually have installed in the amp.
    As mentioned, I am using a different (beefier) power supply and output transformer.
    I use a star grounding system that I discovered at the Tone Lounge (Tone Lizard).
    No hum, no buzz, and only once did it pick up a radio station. I am armed with the tools to deal with that if it comes up again.
    Last edited by Cygnus X1; 10-24-2008 at 04:50 PM.

  30. #30
    Forum Member yankeerob's Avatar
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    Re: Urgent...6SN7 bias Question

    Quote Originally Posted by Offshore Angler View Post
    Rob, looking at that schematic - I'm just a dumb aerospace engineer so if this is a stupid question... but it looks like there is ony a single transformer. Ther eshould be an input transformer too for the heaters an +300V, correct? Looks like a fun build I may try.

    Chuck
    Sorry to highjack the thread but I'll be brief - I had a 220V 100mA/6.3V 1.5A PT wound for it (which we can now get at a very good price) - I also added 47uF of smoothing on the first stage filtering (B+) and used the 150uF shown on the schemo as second stage filtering - that worked very well though this an area you can play around with - you might be able to drop either of those values - we just haven't tried it yet but it might be beneficial as I find the tone just a bit too EL84 for my liking - I also added an 82K resistor to ground on the tone pot to bleed a little bass response off but I'm also gonna mess around with that as we've just had an 8K/8Ω OT wound with an additional 600Ω secondary - how this will behave and how we actually derive a clean quiet signal from it remains to be seen but if you don't push the envelope... but yer I'm building another prototype next week - I'm happy to email the current schematic to anyone who'd like to see where we're up to but it's pretty cool and drives a 25-30W 12" no probs
    If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

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    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Not a hijack at all!
    Whatever we can design for this very cool tube is on topic.

    I got here, kind of blindly...but I am really happy with the result.
    Rob...how would you describe the sound of your design?
    Vox...Fender tweed...or what?

    Mine is a fuller version of a Marshall. A lot of the gain, but not that "snotty mid" that I don't like.

  32. #32
    Forum Member Offshore Angler's Avatar
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    Re: Urgent...6SN7 bias Question

    Thanks Rob. Should I run the heaters DC?
    "No harmonic knowledge, no sense of time, a ghastly tone, unskilled vibrato, and so on. Chuck is one of the worst guitar players I know" -Gravity Jim

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    Forum Member yankeerob's Avatar
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    Re: Urgent...6SN7 bias Question

    I've come to the conclusion that heaters (along with most folk I hasten to add) - apart from very extreme high gain apps - don't need to be DC as long as your wiring is tidy and they're twisted reasonably tight - the sound so far is pretty 'AC30' though 'loosening up' the smoothing might have an effect - 150uF seems a little overkill - swapping out for NOS Tungsols, JAN GE and JAN RCA's are just slightly different responses but pretty much the same overall sound - it's a cute little amp that does what it says on the tin - I'm looking forward to stickin' it in a slightly larger chassis so I can really muck about with it...
    If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

  34. #34
    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    The tidy wiring on mine went out the window a while ago.

    Long story, but the faceplate was made reversed from the way
    I already had the amp wired!

  35. #35
    Forum Member yankeerob's Avatar
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    Re: Urgent...6SN7 bias Question

    Hey - it's my Hank Aaron post...

    Quote Originally Posted by Cygnus X1 View Post
    The tidy wiring on mine went out the window a while ago.

    Long story, but the faceplate was made reversed from the way
    I already had the amp wired!
    D'oh!!

    Quote Originally Posted by ziess View Post
    OSA, yup, that's right. There's no power supply there but you could just tag one one that'll provide 300VDC and take the 6.3VAC from PT.

    Tommy.
    The 220V PT I had made by Majestic produced 320V under load with a straight 4 x 1N4007 FWBR with no CT - it's probably a little strident and the sound is just that - it's an interesting approach though - using a dual triode - that's classically employed in a pre-amp role - as a push-pull power stage - and driving it with a clean front end to get natural power amp saturation... curiously it's not far off the 5E3 in terms of where the vol and tone controls wind up when you start getting natural cream that still has bite - it only takes a coupla minutes of messing around with a coupla knobs to get it to do something that you like - the tone's scooped at 12 o'clock - the mids rise either side - it's still not quiet enough for most apartments when you hit the sweet spot - that's why we're developing the OT with a 600Ω secondary for phones/line out - it's early days - but we figure it'll be even more fun in front of a bit more power...
    If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

  36. #36
    Forum Member Cygnus X1's Avatar
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    Re: Urgent...6SN7 bias Question

    Without quoting it all, Rob, that is just about what I discovered.
    Even though our approach is different, the characteristics of this tube remain similar. It is a territory to be exploited!

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