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Sag?
Sag?
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Tuckster
739 posts
Aug 31, 2010
8:04 AM
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In the "This Tone" thread sag was mentioned. Electronically,I know what sag is. I've only played through gear with S.S. rectifiers. So what does sag sound like?
Last Edited by on Aug 31, 2010 8:05 AM
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toddlgreene
1733 posts
Aug 31, 2010
8:13 AM
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I was wondering the same thing, specifically as relates to small amps. Here's what I found on a quick search, if it helps. Yeah, not a sound sample, but an explanation perhaps that it might not be able to acheive 'sag' on a small amp. The question is:at what wattage rating does an amp become a 'medium' or 'large' amp, and in what size amp would this sag begin to be noticeable?:
"Q-What about valve rectifiers - don’t they give a ‘better’ sound?' A- Some amplifiers exhibit what is known as “sag” when a note is struck due to a drop of voltage in the power supply. This changes the attack of the note and adds to the overall tone of the amp but not everyone will see it as an improved tone. The Valve Junior circuit and others like it (commonly referred to as “Champ” circuits after the Fender Champ) draw an almost constant current so fitting a tube rectifier will not induce SAG as there is not enough change in the current to cause sufficient voltage drop. " ----------

Crescent City Harmonica Club Todd L Greene, Co-Founder
Last Edited by on Aug 31, 2010 8:17 AM
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5F6H
288 posts
Aug 31, 2010
8:40 AM
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The answer posted above is incorrect, it is often mooted as the truth, but in fact is one of those commonly seen "lie repeated often enough becomes the truth scenarios". It is only true if the class A amp in question is being run in it's linear/clean portion of operation...generally not where it is going to be used in the real world.
If you fit a valve/tube rectifier to ANY amp 2 things will happen, the voltage supplied by the rectifier will drop (which changes the tone), the rectifier will have some internal resistance that will affect the current & voltage delivered to the amp, dynamically. Some amps have bridge rectifiers & the power transformer has no centre tap - in these cases a SS rectifier is a must.
Sag isn't particularly noticable on champ type amps though, usually because, even the cheapest champ style amp has a sufficiently rated power transformer, happily able to cope with the demands of one tube's plate (screen & preamp current draw are usually negligible). However, switching from tube to SS & back you will still hear a difference in tone.
Amps usually associated with a saggy sound are usually cathode biased, push-pull amps, like Masco, TV front Fenders etc. These draw high idle currents, sometimes exceeding the rating of the power transformer. If the current draw to the tubes goes up, then the voltage sags/drop, like when you play a note. If the amp in question has a brute force choke Pi filter (like a 5E7 Fender or a Masco MA17) then sag can be further encouraged.
However sag can be introduced in many ways, even if your amp is not cathode biased you may find a saggier rectifier, like 5U4G instead of a 5AR4/GZ34, or SS, gives a more pleasing compression. If current draw in your amp is low enough(1 or 2 EL84/6V6 power tubes?) a 5Y3 might even work, or a 5R4 (don't try these 2 in a bassman, 5Y3 can't supply enough current & the filter caps aren't wired right for a 5R4, though a tolex Fender can usually take a 5R4).
Certain brands of power tube compress more than others, e.g. a EH6V6, or Chinese 6V6 will compress better than a JJ6V6, a 6L6WGB/TAD6L6WGC will compress a little easier than a GE/JJ/7581a/Sovtek 5881.
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chromaticblues
166 posts
Aug 31, 2010
8:49 AM
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A tube rectifier does make a difference in the tone of an amp because of something called internal impedance! The higher the impedance of a rectifier the more it slows the current from flowing thru the power supply which causes sag. Now the values of resistors and capacitors have alot to do with how a power supply works, but if you keep those values constant and change rectifier tubes from a 5y3, 5v4, 5u4, 5ar4 and then a S.S. rectifier. The impedance will drop with change in that order. With each change the amp will get more responsive and louder with less sag and more headroom (that just means you can turn the amp up further before it starts to breakeup. Yes in a small amp if you remove the SS and put in a 5y3 it will make big difference!! This is the best Mod. you can do on a modern amp if you want that thicker vintage type sound!!
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toddlgreene
1734 posts
Aug 31, 2010
8:53 AM
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Thanks for the clarification, 5f and chromaticblues.
Now, the big test-if it is possible:Get two identical amps, one with a ss rectifier, the other with tube, and play both so the 'sag' can be noticeable on the tubed one. Or, play an amp with the tube rectifier, and purposely drive it to sag and point it out-can this be done, or is it more of an anomaly that happens sometimes?
----------

Crescent City Harmonica Club Todd L Greene, Co-Founder
Last Edited by on Aug 31, 2010 9:04 AM
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Greg Heumann
748 posts
Aug 31, 2010
9:04 AM
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On a kalamazoo, I have watched the preamp plate voltage SAG from 150 volts down to nearly 100 just by holding a loud chord. The energy stored in the capacitors is quickly depleted and the rectifier can't replenish it because it is (I presume) undersized to begin with.
Sag is a very real thing and causes break-up and natural compression. If you beef up the power supply capacitors in a Kalamazoo, which many people do, it sags less and loses a little of its mojo. That's why I use the original value caps when I rebuild them. ---------- /Greg
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kudzurunner
1819 posts
Aug 31, 2010
9:05 AM
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I'll defer on technical questions to people here who obviously know more about that than I do. My understanding of sag is strictly on the vernacular (everyday) level. When I bought Trip Henderson's 1956 Bassman back in 1991 or so, he used the term to describe a change in the tone--an advantageous change, from a harp player's perspective--that occurred when you removed one of the two rectifier tubes.
I thought the idea of running a big amp like that "one tube down" was cool; I couldn't believe it was OK for the amp, but he swore it was. In any case, he said that the amp with only one rectifier tube would have "more sag": a softer, fuller sound; a slightly slower, less crisp response, although not mushy by any means. A more three-dimensional sound.
Put in the second rectifier tube, by contrast, and you had a better amp for playing rhythm guitar: crisper, faster response. Tighter.
Replace the tube rectifier(s) with a solid state plug and you'd get an even tighter, crisper response.
That's how he explained it.
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5F6H
289 posts
Aug 31, 2010
9:20 AM
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@Todd "Now, the big test-if it is possible:Get two identical amps, one with a ss rectifier, the other with tube, and play both so the 'sag' can be noticeable on the tubed one. Or, play an amp with the tube rectifier, and purposely drive it to sag and point it out-can this be done, or is it more of an anomaly that happens sometimes?"
The 2 identical amps would have to be slightly un-identical, so that they ran comparable voltages with each other, despite the differing rectifiers...difficult for a "real world" test. But as Greg says, sag is real, for similar amps (same make & model) you will hear differences from one recto to another as Chromaticharp indicates, GZ34/5AR4 will sag "a little" more than a SS, maybejust soften the not front a little, brown the tone, next step down (for a big amp) is the 5U4G which has much higher internal resistance, thus more sag...this is all quite predictable.
How the amp is biased affects sag too, especially in a fixed bias amp, where bias can be adjusted over a huge margin (more plate current can mean more sag). Cathode biased amps normally always draw relatively high idle currents, but a fixed bias amps can be run anywhere from low (cold) to high (hot) as per owner's taste.
As Greg also says, filtration, especially the size of the first cap after the rectifier affects sag too, though you want a big enough cap as to eliminate hum (in SE amps hum can also come from the heaters, moreso than with a push-pull amp).
Another factor, especially regarding the V Jr is negative feedback - this amp lacks "negative feedback" which makes the amp "crisper" & compression at the front of the note less apparent. K'zoos also lack NFB, but run their EL84 at about a 100 volts lower than the V Jr, so the amp is typically warmer, less tight anyway.
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5F6H
290 posts
Aug 31, 2010
9:28 AM
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@ Adam - Yes Trip was perfectly right. Back when the 5E6 (two hole) bassman was built there were no GZ34 rectifiers available, nor solid state. Fender therefore built that amp (& the 5E8-A twin, Kendrick also did this with their 1210 harp amp) with 2 5U4G rectifiers, in parallel, halving the internal resistance, so they functioned more like a single GZ34/5AR4. When the 5AR4/GZ34 rectifier came available they went with the cheaper option & 1 rectifier tube (SS with respect to the twin) & a cheaper to build power transformer.
And yes the single 5U4G rectifier will take anything that 2x6L6 will throw at it.
This is a little different to pulling a power tube, though this can also be done (under advisement - check with your tech regarding each amp specifically!!!). Victoria, for example, will build amps (45410 bassman) with a switch to knock out a power tube.
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chromaticblues
167 posts
Aug 31, 2010
9:41 AM
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His description is absolutly correct! I think they used 5y3's when they had two rectifiers. You can do the same experiment that I stated above and you will hear the difference! I would only use one tube and probably not a 5y3. I don't think a single 5y3 would last very long in an amp that powerfull. I would try a 5v4. There is one one real NO NO with swapping rectifier tubes. Do not use a 5u4 tube unless the amp is designed for that tube. The heater draws 3 amps and the other tubes I mention draw between 1.8 and 2.2. You can burn up an amp real quick that way!
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barbequebob
1208 posts
Aug 31, 2010
10:33 AM
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@Kudzurunner --- The basic idea for putting in two rectifier tubes back then was to prevent sag (often known more often as distortion), and these amps were never designed for anything else but for bass players back then, and with bass, distorition was the last thing you ever wanted and many companies struggled to get a bass amp that could reproduced that bottom E note cleanly. ---------- Sincerely, Barbeque Bob Maglinte Boston, MA http://www.barbequebob.com CD available at http://www.cdbaby.com/cd/bbmaglinte
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Tuckster
740 posts
Aug 31, 2010
1:20 PM
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Thanks,guys. In the last 6 months or so,I've learned alot about tube amps as regards harp. We're blessed with some very knowledgeable tube amp people here. So-sag's a good thing,mostly? Just a matter of taste? More compression and a "softening" of the sound? Rounder? I think I get the idea.
ToddGreene- I think the Bassman RI lets you use either tube or S.S. If that's the case,I have a buddy with one. I'll see if he wants to experiment.
Last Edited by on Aug 31, 2010 1:21 PM
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