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Thread: Preamp supply sag

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    Preamp supply sag

    Has anyone come up with a way to modulate the voltage supply to the preamp in a semi-automated manner via the audio signal? Obviously if the preamp is drawing it's B+ supply down the chain from the power amp supply, the supply voltage is going to bounce up and down as is typical to most amps as the power amp draws current. However, how about in a situation where the preamp is supplied from a different or very stiff source? The current draw of the preamp (LTPI through input stages) is at @ 8mA total, and doesn't really change one bit from idle to full bore with a signal injected. This is a later 60s plexi preamp, so it has a lot of capacitance which further keeps it very stiff. I may try to lower the capacitance somewhat, maybe to JTM45 level or even lower, but it may let some increased hum back in which I don't really want and frankly I doubt it will really affect the sag - or I should say, lack of sag, as the current draw just for the preamp really is not changing. What I need to do is figure out a way to cause the preamp supply to drop 20-30 V or so when the amp is being driven to emulate what will happen in a typical plexi circuit when the power amp gets cooking and the B+ sags because right now it is not sagging and the preamp is just too stiff. I suppose it could be done using a 2W pot as a variable resistor, maybe 10K or so, but this seems to typically be frowned upon and it would have to be adjusted manually. So I was thinking is there some way to use the variable audio signal as a "guide" and implement some way of causing the B+ to increase/decrease while tracking the audio signal? Or some other method? I guess I'm looking for something like one of the popular VVR controls, but tied only into the preamp supply and somehow running in an self-adjusting manner. Is this making sense?

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    I found this over on the AX84 board from just a couple of months ago, looks like someone was thinking along the same lines. I know nothing about mosfets though so I guess I'm going to have to read up to figure out if this could be implemented:

    "A crude overkill sort of scenario I had in mind was to take a mosfet like a IRF820 (or a valve stage), power it from the same power cap as the most critical drive stages in the preamp, and bias it just at cutoff. Feed it from the signal somewhere, but don't take any output from it. Then at idle it does nothing but as the signal increases it amplifies half the wave and starts to draw current, causing the B+ to sag and the valve stage on the same supply capacitor to lose headroom. "

    Is this something that would work?

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    So I was thinking is there some way to use the variable audio signal as a "guide" and implement some way of causing the B+ to increase/decrease while tracking the audio signal?
    I've seen two methods:

    1. Use a regulated supply. But instead of using "fixed" DC voltage reference for the regulator modulate the DC reference with signal coming from an envelope follower. Input to envelope follower is the signal you wish to "track" for simulated sag characteristics. Make the circuit operate so that higher amplitude output from the envelope follower decreases regulator's output voltage. (i.e. you may wish to invert the envelope follower output).
    This is the scheme used, for example, by Maven Peal, Fryette, Line 6 and Quilter. Maven Peal and Fryette have patented their circuitry.

    2. Add something that draws more current than the low current preamp stages as is. Roland's older Blues Cube series amps, for example, had a little "power amp" in the preamp section running to a dummy load. The operation is pretty self explanatory: The additional amp circuit driving a moderately low-ohmic load draws more current than the typical preamp stages. Since both are fed from same supply the current draw sags the voltage.

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    Bear in mind that the power tube screen grids tend to sag more than the plates, and in terms of the effect on things, it may be more accurate for the pre-amp HT sag to model the screen grid sag rather than (power tube) plate.

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    A typical supply will also have a "ripple" component in it because the DC voltage is created by means of rectifying and filtering a sinusoidal signal. The magnitude of this ripple component will increase dynamically with the current draw since the capacitance of the filter circuit can not be charged as effectively due to increased current draw. Basically, the greater the sag, the greater the magnitude of ripple.

    Like sagging, this ripple component will likewise modulate clipping thresholds, superimposing itself to the clipped signal. The audibility and "musicality" of this effect likely varies a great deal with each individual. Some hear obnoxious ghost notes due to superimposed ripple, some hear practically no effect from ripple modulation (that would be me) and some people sense that an essential ingredient of "tube-yness" is lost if there is only supply voltage sag but no associated ripple modulation.

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    Quote Originally Posted by pdf64 View Post
    Bear in mind that the power tube screen grids tend to sag more than the plates, and in terms of the effect on things, it may be more accurate for the pre-amp HT sag to model the screen grid sag rather than (power tube) plate.
    I believe you're correct. Just insofar as what I'm trying to do, however, I'm going by memory based upon the last time I had a Marshall here which was a early 70s metal panel I sold about a year ago. I know the supply voltage at the input stage feed was sagging about 20-30 V just going off what I recall when I measured it w/ a 1 kHz input signal and the amp 'cranked.'

    Quote Originally Posted by teemuk View Post
    2. Add something that draws more current than the low current preamp stages as is. Roland's older Blues Cube series amps, for example, had a little "power amp" in the preamp section running to a dummy load. The operation is pretty self explanatory: The additional amp circuit driving a moderately low-ohmic load draws more current than the typical preamp stages. Since both are fed from same supply the current draw sags the voltage.
    This is very interesting because I was thinking about a way to maybe string a pentode down at that end to do something like this, however a SS design would certainly be more discrete I suspect. Will have to look into this.

    Tried to read up on the Maven Peal design but I see it's patented.

    What I've been fiddling with here is a lower watt plexi type amp as I sure don't need 50W plus. I've got it down to 10W at full-on output into an inefficient speaker (single greenback), which for a true fixed bias pp power amp sounds awesome and a hugely noticeable volume reduction. I'm running the power amp at 240V plates/230 screens but I maintain a 500V supply to the LTPI/preamp which is filtered and dropped and then puts the pi/pre at typical 'black flag JTM' (50W) or later 60s preamp voltages. But, because the preamp supply is not being pulled down by the power amp when driven, it's just too stiff. For cleans of course it's awesome but when driven it's just a bit too much sledgehammer to the face.

    I may try to just lower the supply voltage via a simple/fixed resistive method and see how far I can go before it begins to break up too early to sound right but I thought I'd investigate a more complex solution first. Maybe this is backwards!

    Should probably try some capacitance changes too. Right now the supply for the pi/preamp is at 32 uF/15K/32uF, at which point it feeds the ltpi, then the standard 10K/32uF for the cathode follower then another 10K/32uF for V1. I measure only 9 mV of AC at the pi node and the amp is darn near dead silent insofar as hum.

    Thanks guys.

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    Last edited by EFK; 02-14-2016 at 07:40 PM.

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    Hmmmmm, very good point. And I actually like those ghost notes! The majority of my work is with trashy little amps like old Silvertones and Valcos, so I *expect* to hear them.

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    Quote Originally Posted by EFK View Post
    Tried to read up on the Maven Peal design but I see it's patented.
    That usually makes it much easier to read up on! A patent is just a license to sue anyone who makes too much money on an idea you claim, little people are largely exempt.

    The European Patent office is a better place to get nice pdf files of granted patents IME, while applications are best searched at the clunky USPTO site.

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    Last edited by tedmich; 02-14-2016 at 08:31 PM.

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    Quote Originally Posted by EFK View Post
    I believe you're correct. Just insofar as what I'm trying to do, however, I'm going by memory based upon the last time I had a Marshall here which was a early 70s metal panel I sold about a year ago. I know the supply voltage at the input stage feed was sagging about 20-30 V just going off what I recall when I measured it w/ a 1 kHz input signal and the amp 'cranked.'



    This is very interesting because I was thinking about a way to maybe string a pentode down at that end to do something like this, however a SS design would certainly be more discrete I suspect. Will have to look into this.

    Tried to read up on the Maven Peal design but I see it's patented.

    What I've been fiddling with here is a lower watt plexi type amp as I sure don't need 50W plus. I've got it down to 10W at full-on output into an inefficient speaker (single greenback), which for a true fixed bias pp power amp sounds awesome and a hugely noticeable volume reduction. I'm running the power amp at 240V plates/230 screens but I maintain a 500V supply to the LTPI/preamp which is filtered and dropped and then puts the pi/pre at typical 'black flag JTM' (50W) or later 60s preamp voltages. But, because the preamp supply is not being pulled down by the power amp when driven, it's just too stiff. For cleans of course it's awesome but when driven it's just a bit too much sledgehammer to the face.

    I may try to just lower the supply voltage via a simple/fixed resistive method and see how far I can go before it begins to break up too early to sound right but I thought I'd investigate a more complex solution first. Maybe this is backwards!

    Should probably try some capacitance changes too. Right now the supply for the pi/preamp is at 32 uF/15K/32uF, at which point it feeds the ltpi, then the standard 10K/32uF for the cathode follower then another 10K/32uF for V1. I measure only 9 mV of AC at the pi node and the amp is darn near dead silent insofar as hum.

    Thanks guys.
    What output tubes are you using? I am thinking of doing a Plexi with 6AQ5's as outputs.

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    Quote Originally Posted by EFK View Post
    What I've been fiddling with here is a lower watt plexi type amp as I sure don't need 50W plus. I've got it down to 10W at full-on output into an inefficient speaker (single greenback), which for a true fixed bias pp power amp sounds awesome and a hugely noticeable volume reduction. I'm running the power amp at 240V plates/230 screens but I maintain a 500V supply to the LTPI/preamp which is filtered and dropped and then puts the pi/pre at typical 'black flag JTM' (50W) or later 60s preamp voltages. But, because the preamp supply is not being pulled down by the power amp when driven, it's just too stiff. For cleans of course it's awesome but when driven it's just a bit too much sledgehammer to the face.
    Are you sure that it's a lack of preamp voltage sag that's causing the stiff sound?

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    Hi,

    Based on the assumption that the sag is the right solution against the stiffness of the preamp, my solution is:

    - the piloting system has to be the piloting system of a compressor: analyse the output swing;
    - the piloted system can directly be the power supply: rectify the ac coming from the secondary of the PT and go to a first cap, then to a variable voltage divider, then to the cap that supplies V2 of your preamp (or even better to V3, including the PI, and then to the preamp). The "variable voltage divider" will be a 1k resisor in series with the supply, plus a power transistor (piloted by the output swing of the preamp) and then a 1k resistor in series with the supply (the two resistors and the transistor form like a T on the schematic).

    This way you lower the voltage on the supply each time you increase the output signal swing (because you reduce the ratio of the voltage divider by decreasing the Resistance between collector and emitter of the transistor), obtaining the effect that you want. I'd put a resistor in series with the transistor, in order to reduce it's effectiveness.

    Pay attention to isolate the output of your preamp from the B+ of the transistor you are piloting.

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    Supporting Member loudthud's Avatar
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    Quote Originally Posted by ThermionicScott View Post
    Are you sure that it's a lack of preamp voltage sag that's causing the stiff sound?
    This (the stiffness) is generally recognized in amp setups that utilize a separate (perhaps rack mount) preamp where the preamp power supplies are rock steady. Not all players like the effect, especially super high gain metal players. It's more suited to Tweed, blackface, JTM45 and 5F6A style amps, not Triple Rectifier or SLO types.

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    I "solved" this issue once in a WDW setup by supplying the preamp from the last node of the dry PA.
    But it's something I can do for my rack, I won't feel safe to do it for someone else.

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    Quote Originally Posted by loudthud View Post
    This (the stiffness) is generally recognized in amp setups that utilize a separate (perhaps rack mount) preamp where the preamp power supplies are rock steady. Not all players like the effect, especially super high gain metal players. It's more suited to Tweed, blackface, JTM45 and 5F6A style amps, not Triple Rectifier or SLO types.
    Shh! I wanted to get EFK to walk us through his thought process.

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    Some more good stuff. Roberto, thank you - interesting approach and I will look into this. I've not ever done anything with transistors but your explanation makes sense and sounds theoretically like it would do what I want it to do.

    For anyone still interested, I set up the PS here in a roundabout way to get the voltages I wanted, and after some more experimentation I'm beginning to wonder if it may be contributing to the over-stiffness in some way that I don;t quite understand because when I take measurements at idle vs. under load, odd things happen which I am not used to seeing.

    I am running a 380 vdc, CT power transformer into a 6CA4 tube. The rectifier here is acting as the forward portion of a bridge I believe, then I am running two reversed diodes off the secondaries to ground (anodes to ground). This is giving me the rectified high (490-500 vdc) preamp voltage. I run this into a pair of stacked can caps, each a dual 32 uf. I run the PT center tap to the mid point of the totem setup, and so I get around 245-250 vdc there. This is where I'm supplying the plates and screens only. Bias is fixed, cap coupled off a 55 VAC tap on the PT.

    When I hit the amp with load, there is only a 9 vdc voltage drop from idle to "dimed." This is at the power tubes. The preamp does not sag at all, not one bit. I had the bright idea to work with a sag resistor between the tube rec and the top of the totem pole, but this is where it's getting weird. I went up to 400 ohms but the whole thing hardly sags at all. ??? the only thing that appears to be sagging is the preamp voltages taken from the top cap, and they are only dropping a minuscule fixed amount with no other change under load. I would think that despite the fact that the power amp supply is being pulled from the mid point of the totem pole, it should still pull down the whole chain with a big honking sag resistor right off the rectifier, but this is not the case.

    This setup is a first for me, so maybe something is going on of which I'm not aware?

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    I've also thought about setting up a fixed/cathode bias switch to see if cathode bias might get me a hint of a saggier feel, but I vaguely recall reading something relative to the cap-coupled bias setup I'm using indicating that it doesn't like to be run floating which it would be I suppose if put on a switch.

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    The lack of any sag seems normal. Preamp tubes are voltage amplifiers operating in class A. So they're not doing any significant work amplifying voltage and even if they were the amount of work wouldn't change because they swing from the center of their bias (or mostly anyway). So the preamp tubes are already doing as much work just sittin' there as when they are amplifying. This is also why your 400 ohm resistor doesn't create more sag. The current in the preamp is fairly constant. Adding resistance might drop a little voltage but it can't make the steady state respond to what isn't happening. Preamp voltage sag is a consequence of how the power amp affects the voltage supplied to it. Without a power amp to do that any voltage sag analogous to signal level needs to be artificially induced.

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    Right-o Chuck, thanks for responding. This I understand. But, this isn't a standalone preamp with it's own PS. The power amp is pulling off the midpoint of the totem filter cap setup and the preamp is pulling off the top for the higher (doubled) voltage. Neverthelss they're both operating off the same PT and the same tube rectifier. So as the power amp pulls current - idles @ 70 ma, rises to around 105 to 110 ma at max - and the voltage drops a little bit (maybe I need a saggier PT…), I would *expect* the preamp voltages to at the least sag the same voltage as the output is sagging. However, they're not. They're not sagging at all, and in fact, they actually bump up a few volts. This is what is not making sense to me and I'm wondering if the way I have the PS set up is the cause of this in a way that I don't understand. I also don;t understand why the power amp voltages were not affected basically at all by a big sag resistor, because whether the output is pulling off the midpoint or the top of the totem pole, I would think it all still has to come across that sag resistor. Maybe not? Maybe there is something going on with the PT center tap that I don't understand.

    Edit: if you look at the link I'm posting (completely unrelated topic, but the diagrams are what I'm referencing) and scroll down to the 4th drawing w/ the tube rectifier, this is the PS setup I'm using except I'm not running into two series cap/totem arrangements, just one as I didn't think two banks of series caps was necessary.

    Link: http://www.tubecad.com/2012/05/blog0230.htm

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    I've been doing that since forever (almost 40 years now), to inject some "tubeyness" in my otherwise very stiff and predictable Op Amp based preamps.
    This is a customer demoing an old FAHEY guitar amp from 1989 (but that sound had been developed since '75/'76) .
    Notice very audible sag and compression (yes, I also drive a compressor with voltage drop ... exactly what happens when pentode screens drop) :


    My SS version of a JCM800 . Single channel and not a tube in sight.
    Clipping stage is an overloaded mini power amp, fed from a very saggy separate supply.
    Time constant is critical, about the same as a real tube supply sagging so it's more believable, some smoke and mirrors are involved.
    Real power amp, of course, is based around a couple 2N3055 .
    Today I use TIP142/147 or IRFP250 (might even use a chipamp in a low power version) but in any case they do not change sound, just boost what's fed them.

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    Bent Member Chuck H's Avatar
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    It's hard (for me) to say why your power amp current is increasing significantly but the voltage only drops a little. I will say that the way the power supply is arranged is very stiff. You basically have totem poled Pi filters. I can only guess that because the preamp is on top of the totem instead of in series with the rail that any capacitive reserve (which is part of their job after all) is able to maintain voltages. Why the small voltage sag in the power amp isn't having any affect on the other operating voltage at all is some kind of "totem Pi filter with CT reference" juju I don't understand. So, since this is the actual question, I'm sorry I couldn't help. But I've been peeking in on this thread anyway so now I'm "subscribed"

    FWIW the signal gated SS regulation is my horse in this race.

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    Sag is for pants not amps!

    IMHO

    Click image for larger version. 

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    Peavey Triple XXX with 3x the PS filtering

    Don't tiny 20 uF filter caps and tube rectifiers give a nice sag?

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    Here's an off the wall idea. Put the 400 Ohm sag resistor in the ground leg of the bridge, between the SS diodes and ground. That way both supplies will sag.

    Yes, 20uF will create plenty of sag, although if the current is only going from 70mA to 110mA, it won't sag much. Like I said above, amps designed for Metal players have stiff supplies and lots of filtering.

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    Quiero uno de esos amplificadores de Fahey!

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    I'll try lifting the diodes ground side and putting a resistor there - will be interesting to see what will happen. In a 'typical' PS setup, i.e. tube rectified, ct grounded, I've used a resistor on the CT on other occasions to back bias and drop voltage in that manner.

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    Well, it works pretty much exactly as a resistor on the center tap of a typical fw/ct transformer setup. It develops a negative dc voltage, however it seems to be essentially 6 of one/half dozen of the other. I can put a resistor between the reversed diodes and ground or I can put one between the 6CA4 cathode and first cap, the voltage drops at idle and under load are identical save one is positive and one is negative. Using a 100 ohm resistor, I get about 7V idle to 12V loaded. Not much of a drop, would need to get into the 300-400 ohm range to get any appreciable sag between idle/load, at which point I'm really dropping too much idle voltage. *sigh* What I really wish I could come up with is something like the reverse of an inrush thermistor, something that was dropping very little when lightly loaded and dropping much more when the current increases. Maybe I could put a 240V incandescent bulb in the PT center tap line (it's at @ 240V) or on the center tap of the OT and experiment with different bulb wattages. I know the Samamp guy has some kind of patent with light bulbs but I think he's using them on the power tube cathodes because they're standard 120V bulbs. Hmmm. I wonder if a light bulb would run on negative DC? How about a 100 ohm resistor off the reversed diodes and ground and then run a 12V or 24V bulb between the resistor and ground…. might exponentially increase the resistance and buy me some more sag under loading. I'm probably getting silly here.

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    Also made a little plug in adapter so I could try a 6X4 as opposed to the 6CA4. Ought to get me maybe a few more volts of sag (especially loaded, as 6CA4 is quite stiff)? Nope. Literally no difference, idle or loaded.

    Aaaaaand, dropped the initial totem down from dual 32/32 stacked to dual 16/16 stacked, again no difference. Also lifted the 30 uf off the screen supply and popped in a 5 uF to see if I could get some sag there (don't want to increase the R value there yet) and of course, no difference at all.

    I think it just may be an overly stiff PT for this application. Darn thing doesn't even get warm. Maybe I should pop in an extra pair of power tubes, make it a mini Superlead instead, and pull down the supply that way!

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    Last edited by EFK; 03-15-2016 at 06:27 PM.

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    Quote Originally Posted by EFK View Post
    Also made a little plug in adapter so I could try a 6X4 as opposed to the 6CA4. Ought to get me maybe a few more volts of sag (especially loaded, as 6CA4 is quite stiff)? Nope. Literally no difference, idle or loaded.

    Aaaaaand, dropped the initial totem down from dual 32/32 stacked to dual 16/16 stacked, again no difference. Also lifted the 30 uf off the screen supply and popped in a 5 uF to see if I could get some sag there (don't want to increase the R value there yet) and of course, no difference at all.

    I think it just may be an overly stiff PT for this application. Darn thing doesn't even get warm. Maybe I should pop in an extra pair of power tubes, make it a mini Superlead instead, and pull down the supply that way!
    I'm "re" working a Champ on the bench right now. It has a PT secondary DCR of 450 ohms! I'm trying to use that to my advantage now. As long as it doesn't get too hot all is golden. But it sags with current like crazy.

    So I thought... What's your bias voltage relative to output? If you're using a hot bias it's possible that the amp will only sag a little before the tube just has no more to give?..

    if your HV DCR is low, and the mA rating is ample then a stiff power supply SHOULD result in very little sag. But that still doesn't explain why the PS sags for the power tubes with their node at the center of the totem, but the preamp doesn't get any of this sag. Like I alluded to above, perhaps it has something to do with the fact that there's a filter cap on top of the OT node in the totem arrangement and it's regulating voltage?

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    I wish I could figure out how to change the thread title, because I'd rather it reflect an attempt to cause the whole circuit to sag, not merely the preamp (as someone above questioned, and I believe he was correct). I put an extra big dropper to the preamp on a switch which essentially can toggle the preamp voltage between the early JTM45 voltages and early 70s 50W voltages (which are much lower), and frankly, aside from the bass getting a tiny bit mushier, you'd be very hard pressed to tell a difference because the damn PS is only sagging 8-10V max under load.

    Chuck, the bias is fixed, set at the old standard 70%. Under full loading, the tubes will run up to about 110% plate dissipation, but they can take it. So there is definitely a swing there.

    Most of my amps are 1950s types and you very often see a big 10K screen dropper used. So, I thought I'd give screen compression a try and see if - in this particular amp - it could give me some compression to help mask the lack of PS sag. I put it on a dpdt switch (I've got about 8 switches on this amp at this point) so that I could modify the bias ground resistor (off the adjustment pot) at the same time. Well, with a 10K dropper in there (and I tried 15K too), given the low voltage on the power tubes, I can actually run the bias up right to near 100% idling because even under load the current draw is so constricted I think I've basically biased it right into A. If I drop it back, I get some nasty sounding hash as the amp transitions from clean to drive that sounds like crossover distortion, which is why I biased it hotter in conjunction with the big screen dropper in the first place.

    Next up I'm going to try running a small cap, like maybe 1uF to 5uF, as a bypass to a 15K screen R and see if it creates - as mighty Steve Conner called it in a very old thread - a "ghetto vary-mu compressor" if I can get it "pumping" (as he also termed it).

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    Well I backed off the bias, temporarily wired in a second pot so I could play the amp and adjust by ear to see if I could back off and get some compression back without causing the nasty crap to rear its ugly head. Managed to find a pretty good sound, measured pot and replaced with resistor and it seems to work ok when biased at around 80% (with the big screen R switched in circuit). This is with a 15K (!!!) screen dropping resistor and a 1uF bypass cap, which given the small value may not really be doing anything but I didn't want to "waste" a nice new full-length-lead 5uF electrolytic and I have a big pile of these 1uF/630V plastic caps that I use for a cap coupled reverb circuit.

    This helps. It's not 100% what I'm after, but it's a help. Plus it knocked the volume back a touch more, and given my whole point has been to get a true Marshall 50W sound at a reduced volume, a bit more of a volume reduction is a good thing. So is having it on a switch.

    I'd equate the amp, at this point, to a brand new piece of reproduction 18th cent furniture. One of the huge attractions of antique woodwork is the wear, the evident 'softness' of the edges, variations in finish aging etc. That would be the old 60s/70s Marshall. This amp is like a really good copy, but there is no patina, age or wear on it. In other words, the edges are too sharp and precise. It's too 'hard.' I need to come up with creative ways to soften the edges because I can't seem to do it the old fashioned way with a saggy PT, and all those old Marshall PT's are saggy.

    Chuck I measured this one and it's only 50 ohms from each side of the secondary to the center tap.

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    Still here, still scratching my head. I've been playing with sag resistors again and something is seriously not making sense. I went up to a 450 ohm sag resistor right off the rectifier, which in any other amp would be a BIG mother of a sag resistor, but it only drops 4.6V. WTF? That works out to what, 10 or so mA? The power tubes alone are idling at around 35-36 mA each. How can this be? When I hit the amp with signal, the "sag" climbs to a whopping 8.2V total, so 18.2 mA. Meanwhile the tubes are pulling around 58-59 mA each at that point. It does not make any difference if I put the resistor right off the rectifier, or if I put it between the reversed diodes and ground. Same results either way.

    What seems even weirder is that, **given the same wall voltage** between tests, with the rectifier straight to the first cap I'm getting around 496-ish vdc at the first cap. With the 450 ohm resistor in place, this drops down to around 482-ish on the rectifier side of the resistor, with the aforementioned 4.6V drop across the resistor. So where the hell is that initial 14V going?

    Is my math wrong (would not be surprised) or is there something I'm missing here? It's not making sense to me.

    Is it possible I need to somehow 'balance' things, i.e. maybe two sag resistors, one off the forward rectifier and one off the reversed diodes? And maybe another on the center tap?

    I've not messed with this type of PS previously. I like what it does because it gives me perfect supply voltages for the preamp vs. power amp, but it doesn't quite seem to "act" the way I would expect.

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    Are you measuring AC? DC? Do you have a true RMS meter?
    Can you post the schematic of the circuit, it is not possible to tell from the thread which actual circuit is being discussed here anymore.

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    Yeah that sounds like a measuring problem. In the location you have it, the resistor wouldn't be expected to have a DC voltage across it. The diodes are off most of the time. They only open for a short time at the top of their cycle to slurp a quick pulse of electrons off the filter cap. That short current pulse will be higher than the average current draw of your amp, but then the current falls to zero until the next pulse comes through.

    It's very much like bailing out a boat with a slow leak. The boat is your filter cap, you want it to stay well drained of "water" (electrons). The slow leak is your power amp, it's pouring a continuous flow of electrons into your cap. The power supply diodes are the bucket. Every now and then they take a big scoop out of the water in the boat. That keeps the average level of water in the boat down, but they're not a continuous flow like a pump would be, only sudden scoops. Because of that there is no simple DC voltage across a resistor that's before the filter cap. Some meters can average all those current pulses out and tell you what an equivalent steady state voltage or current would be, some can't. It's not terribly helpful to you here even if it can because that average value isn't what's actually taking place.

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    Okee dokee, since I don't have any single 600V rated electrolytics here, I quick twisted up (4) 1uF/630 V plastic caps in parallel for a 4uF total, which ought to be enough just for testing purposes. I attached this *before* the 450 ohm resistor. Basically the same results (voltages a tiny bit higher but my wall voltage is higher today), the resistor is only dropping around 4-ish volts at idle and 8-ish when the amp is hit with signal and maxed. The resistor doesn't even get warm.

    I don't have a schematic because this is a prototype project I've been working on and I've been winging it as I go along. I'm using a Fluke true RMS meter for measurements.

    PT is 190/0/190, w/ a 6CA4 tube rectifier. Off each secondary is also a reversed diode to ground, and I'm using the center tap so I've essentially made a voltage doubler off the tube rec. The high (@ 500vdc) dc is running into a totem pole setup, currently dual 16uF cap cans each paralleled and in series w/ 100K sharing resistors. Preamp is running off the top of the totem, 500 vdc through an extra RC filter to drop and clean up the voltage feeding the ltpi and then on through the preamp. The PT center tap is running to the junction of the cap cans in series, and this is where I'm pulling my power amp dc (@250vdc) for plates and screens. the power tubes are running a total of @ 70 mA at idle, climb to @ 110-120 mA under full load. So, I would expect a 50 ohm resistor to initiate quite a bit of voltage sag. It isn't.

    I posted a link above to the rectification setup I'm using, I'll repost it here. When you follow the link, I am using a setup just like the 4th drawing down from the top of page, except I am using only a single totem/series cap filter, not two pairs (drawing shows two totem pole filters).

    Link: http://www.tubecad.com/2012/05/blog0230.htm

    I'm thinking the only thing left for me to do is to test my theory that I really need to split up the dropping resistor into three separate resistors to 'balance' out this setup: one off the tube rec, one between the reversed diodes and ground and one on the PT center tap. I'm pretty much out of ideas at this point, other than to try again with an even smaller and saggier PT. Have to say though, this one is not oversized to begin with.

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    Ooooh, took a gamble today and it definitely paid off. Two words - mixed bias. I know some people feel it doesn't make much of an audible difference but in this case it sure does. Very noticeable. Wanted to maintain a nice even number for easy bias readings so I popped a 100 ohm resistor on each cathode, *unbypassed* (I prefer an unbypassed cathode R in most amps). This seems a good balance and it is definitely adding some compression. I think I've got it set now and (for today anyway) I'm getting what I want out of it. It also sounds better as the amp transitions from clean to dirt, definitely smoother.

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    Just one question on something I missed. Your OT gets it's feed after your sag resistor, or before?
    From the following statement I get the impression the OT is before the sag resistor, right off the rectifier (so no sag provided by power tubes):
    Quote Originally Posted by EFK View Post
    The PT center tap is running to the junction of the cap cans in series, and this is where I'm pulling my power amp dc (@250vdc) for plates and screens. the power tubes are running a total of @ 70 mA at idle, climb to @ 110-120 mA under full load. So, I would expect a 50 ohm resistor to initiate quite a bit of voltage sag. It isn't.
    This is why it's so hard to discuss without a complete schematic.

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