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Thread: Rectifier tubes and sag

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    Old Timer daz's Avatar
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    Rectifier tubes and sag

    Ok, so my amp i spent all these months perfecting is done and it's now going to belong to a friend of mine. I'm about to clone it for myself. But there are things i'm not sure i want to duplicate, one being the tube rectifier. I have it switchable from diode to 5ar4 and it also switches a resistor in the bias supply in for the diodes so that the bias stays they same with either diode or tube. Thing is, i can feel a tiny bit of sag or sponginess with the tube, but it's so subtle i'm not sure i even care. I'd probably rather build the new amp with only diode rectification. But the question is, how much more of that sag the 5ar4 has will i get with other tubes? As i've read i thing there are 2 or 3 others that are plug and play and they are supposed to offer various degrees of sag. Which ones would i want to consider to get a nice squishy tone thats considerably different that diodes without getting stupid squishy? (subjective, i know, but waddayathink anyways?)

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    Bent Member Chuck H's Avatar
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    Look up the data on all the common rectifier tubes. Find the ones that can handle your amps current draw. The ones that drop the most voltage will be the ones with the most sag.

    In considering this feature in your new build, you only need to ask yourself, 'While I was playing the amp, which rectifier did I most often use?'. If you were happy with the diode rectifier, thats it. Sometimes you have to stop scrutinizing an amp and just play through it. Thats the only way to really evaluate it.

    FWIW I recently did a survey at some local music stores to weed out unwanted features. The only feature that consistently ranked "NO" was a switchable tube / diode rectifier.

    Also, You don't need a tube to create sag. A big fat resistor will do just fine. You could use a tool like Duncan PSU to determine what value resistors simulate the rectifier tubes you want to emulate. There are some subtle differences between the tube and a resistor. But as you've already noted, the difference can be subtle anyway. I bet you could put the tube rectifier and a resistor on a switch and you'd have to toggle it back and forth many times to tell a difference. And then you'll still wonder. So why trouble with the tube. It means you need an extra socket, a 5 volt winding on the PT and of course the tube itself. As well as the occasional cost of replacing it. JM2C. On the flip side a tube does have a certain purist appeal and I do sometimes build an amp with a tube rectifier. And I think deep down I like it better for strictly aesthetic reasons. But for testing purposes at least you could use resistors. It might be a good idea to have an adjustable bias supply for this experiment.

    Chuck

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    In general a diode has no sag,5AR4 has a little more sag 5U4 a bit more,5V4 a bit more than that and a 5Y3 having the most sag.But be sure the tube you choose can handle the current demands of the amp.A 5Y3 can be marginal in most amps.

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    Old Timer daz's Avatar
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    I suppose it is nit picking and i should just use diodes. But i worry that down the road i'll start thinking about whether i should have gone tube tho. That and the fact that since it does feel a tiny bit nicer with the tube then maybe with a saggier tube i'd be in rectifier heaven. I just don't want to throw money away on a tube or 3 to find out. But as it sits right now i'm leaning towards sand only.

    I do have another question or 3 if you don't mind. I'll start a new thread if i have to, but maybe i can get answers in this one and save forum space. I'm considering verb in the new one but i'm not sure whether to punch the chassis for another 12A*7 or to just pass on the idea. I could just leave space for another socket and do it later if i feel inclined, but on the other hand if i don't i would prefer to be able to space the tubes out like i did in this amp to insure everything is the same and the tone is the same.
    So 1st question is what is the minimum tube spacing that insures no issues?
    Second would be, do you think adding tube reverb will be very problematic and i should just pass? i ask that one because i seem to see a lot of people who try this running into a lot of problems trying to get it to work right.
    And third, do most tube verb circuits eliminate any tone robbing 100% when the verb knob is all the way off?

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    Bent Member Chuck H's Avatar
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    IMHO adding verb to any "Marshall" type circuit is problematic. You really can't do it without adding an extra triode to the actual signal chain (which will invert phase, and other stuff), or at the very least, a large attenuating resistor in the signal chain (ick). Effects loops are hard too. All this depends on how you feel about changes to the original circuit effecting the tone (for better or worse). My personal Marshall has a "pre amp out" to "power amp in" off the treble pot. It works with most effects (but certainly not all). I usually just resolve to use floor pedals with Marshall style amps, and have a pedal board I assembled for just such use.

    Chuck

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    Old Timer daz's Avatar
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    I had a loop for a while thinking that would be good enough because i could use outboard verb/delay and whatever. But it did mess with the tone too much. I tried it off the master and the treble. I couldn't get the treble version to work but the master was ok. just changed the tone a bit too much.
    I don't know if i agree that it isn't possible because of the marshalls i've had, some had loops that were just fine and one (jcm800channel switcher) had what i felt was great verb. it also had a loop that was pretty transparent. Whether the amp would have sounded better w/o the verb circuit, i dunno. But as far as the loop at least i think it was passive and there was nothing that would be affecting tone when it wasn't in use. In fact, if not for the fact the circuit is so different from mine i'd have tried copying it. So i think it can be done probably. I just don't have the patience, and after all the hours i spent to get the tone right the last thing i want is to lose even a drop of that.

    As to going into the front end, thats not really an option with a high gain pre. Whatever level of verb or delay you use, when you turn you guitar down to clean up it goes away. Then turn it up and theres way too much. God forbid you use a clean boost....then your signal becomes 10% dry and 90% wet rendering your tone useless for anything but star trek sound effects. It works if the amp is always run clean and any dirt you get is from pedals that are before the effects, but i don't run that way.I prefer amp dist over pedals.

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  7. #7
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Chuck H View Post
    IMHO adding verb to any "Marshall" type circuit is problematic. You really can't do it without adding an extra triode to the actual signal chain (which will invert phase, and other stuff), or at the very least, a large attenuating resistor in the signal chain (ick). .... Chuck
    Not really true at all Chuck... it depends on whether or not you are trying to get black face Fender reverb or just some usable reverb.
    Many builders (myself included) have mixed a reverb signal into these types of topologies just by inserting the parallel, reverberated signal right into the grounded grid side of the phase inverter and with fairly decent sounding reverb FX.
    The presence control can effect the reverb in an odd way but it does work.

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    Bent Member Chuck H's Avatar
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    Bruce, wouldn't you have to lift the .1 bypass cap?

    Chuck

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Chuck H View Post
    Bruce, wouldn't you have to lift the .1 bypass cap?

    Chuck
    If you mean the presence cap, I can see where it would send quite a bit of the reverb signal to ground if set the or built the wrong way there.
    you use that normally grounded grid .1uf cap (with reverb injected phase inverters it is usually something smaller in value) to insert the reverberated signal from the proceeding reverb stage.
    Typically this could be a reverb level pot, etc... what ever it is, it needs to be at chassis ground so the PI's grounded grid is still referenced to ground there.

    ************************************
    Next message is a scribble showing what I mean.. ha ha.

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    Last edited by Bruce / Mission Amps; 08-24-2008 at 11:24 PM. Reason: added picture to next message
    Bruce

    Mission Amps
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    www.missionamps.com
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  10. #10
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    This is a seriously sloppy drawing with my mouse but maybe you'll see what I mean....
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	reverbinsertion.JPG 
Views:	310 
Size:	41.6 KB 
ID:	2964  

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    Bent Member Chuck H's Avatar
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    Thats been a consideration of mine for an EL84 proto I'm working on. But on a Marshall with a tail of 10k, doesn't it effect the gain of the PI more than, say, an EL84 amp with a 47k tail? Even using a very low value pot I think there may be a little difference in an EL34 Marshall...No?

    Chuck

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    In regards to sag I swapped the 5AR4 in my 5E3X2 w/ a 5Y3 and was extremely happy w/ the increased sag. I find 5AR4 tubes to be too tight, as with a ss rectifier. Why go through all the trouble of a rectifier tube (socket, 5v winding, expensive tube) when the difference b/t it and ss is marginal. IMHO if one wants to use a tube rectifier for sag, they should choose a tube w/ a fair amount of voltage drop/sag. This page has some good info:

    rectifiers

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    Lowell,

    I realize this is an ancient thread, but I also have a Weber 5E3X2 and I would also like to use a 5Y3 in mine. I was wondering if you bothered to remove the 100R/25W resistor (R32)? If not, does the amp run pretty hot?

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    I did not remove it...if anythin it helps keep the 5y3 alive. 40uf is too much for a res cap for a 5y3. I may have changed to a 30uf...but dont remember and dont own this amp anymore.

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    Okay then. I saw where Bruce here in another post somewhere suggested using a 5R4 instead, so I'm trying that first. I just ordered a NOS RCA 5R4GB. We'll see how that goes. I'm concerned that my 100R/25W resister may be shot too. I have absolutely no sag at all and very little breakup even with two 12AX7's. I believe the that resistor is supposed to add some sag to the GZ34. I gleaned this from another post:

    According to Merlin Blencowe's power supplies book, the anode resistance of a 5Y3 (high sag tube rectifier) is about 300 Ohms and the anode resistance of a GZ34 (low sag tube rectifier) is about 50 Ohms. So in the 5E3x2 you have the 50 Ohms from the GZ34 + the 100 Ohm resistor for 150 Ohms total series resistance. Since the 5E3x2 power amp uses twice as much current as the standard 5E3 Deluxe, that 150 Ohms series resistance should provide as much sag to the 5E3x2 as the 5Y3's 300 Ohms would provide to a 5E3.
    Well, if that resistor is supposed to add sag to my GZ34, it sure isn't doing it. The initial attack of the pick on the string is just crazy fast and makes the amp VERY hard sounding and difficult to play. I've played SS amps with much more sag than this.

    I'm also wondering why Weber is using 40-20-16 for the filter caps. Every Deluxe I've ever seen ran 16-16-16, and Victoria's Double Deluxe uses 16-16-16-16. I'm not good enough in theory to understand why this extra filtering is needed, and I'm wondering if it's contributing to the harsh sound of the amp.

    If neither a 5R4GB nor a 5Y3 help this amp, I may have to send the chassis to Bruce and see if he can figure it out. This is supposed to be my retirement amp darn it! I've got a little Super Champ XD sitting here to practice with that has tweed voices that'll blow this 5E3X2 out of the water as things are now.

    Thanks!

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    It may be that your operating conditions are very close to / at / past / class A; if so, there won't be much sag.
    What is the power tube cathode voltage at idle, and then when being overdriven hard?

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    Reduce your filter caps to 22uf/16uf etc.. Make sure that 100ohm is good. Possibly increase it's value a bit. Make sure your screen resistors on the 6v6 are good. For more sag increase the value of the screen resistors, then rebias to spec.

    Not sure why the Rscreen would need to be increased in this amp as it should have a good amount of sag.... but worth a try. Is the stock Weber output transformer in there? If it's an upgrade or bigger output transformer that will reduce sag as well.

    If you have little to no overdrive with the amp cranked, something is DEFINITELY wrong. Check all the voltages on all tubes. Try new tubes as well.

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    Supporting Member loudthud's Avatar
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    Check this thread: http://music-electronics-forum.com/t24269/

    Especially the last post by Bruce. He tells about a common wiring mistake that his customers sometimes make.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Just so you know where I'm at, I've tested all the 6V6 resisters, and they're fine. I've changed all the tubes, some of them more than once, and they were all apparently fine. I'm having a hard time getting my probes in there to test that 100K resister. I'll work on it more tomorrow and try to take some voltage readings as well.

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    Loudthud has a great suggestion there too. Definitely check that

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    Oh, I intend to check out everybody's suggestions. I just didn't get much time today. My only electronics training was in computer repair. There's a lot I don't know about amplifiers including some of the terminology. I do have the stock Weber output transformer by the way.

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    I still haven't taken any voltage readings, but I did find time to check every resistor in the amp except for the ones wired into the input jacks. That 100R/25W resistor (R32) always reads high, generally around 125K. I realize it needs to be changed, but do you think that's far enough off to be causing my problems?

    Also, it's a small thing, but I noticed that 64k was used for the R3, R4, R5, and R6 resistors instead of the specified 68k. I wouldn't think that would make a difference though.

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    Last edited by Yahoobuckaroo; 06-04-2016 at 01:33 AM.

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