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Swapping tubes in a hybrid amp

#41
Ewo said:
Hey, Andy--I want to get in on all the education you're so generously providing!

A few posts ago you made mention of transformers saturating. I've not heard of that before. 'Splain, please.
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An output transformer does 2 things simultaneously.

First, it couples the signal across an air gap. Current flowing in the primary, which is wrapped around an iron/steel core, creates a magnetic field in the core that is then picked up by the secondary winding that is wrapped around the same core. This goes back to a fundamental electromagnetic rule of current being induced in wires intersecting a varying magnetic field (AC).

The second function of the output transformer is to match the plate impedance of the output tubes to the speaker impedance. Since the plate impedance may be on the order of 2000 Ohm's and the speaker impedance being 8 Ohm's, there is an impedance ratio of about 200. This principle is exactly the same as a mechanical block and tackle pulley, in the electrical model you trade off voltage for current and in the mechanical model you trade off force for distance.

The saturation issue I mentioned is caused by inadequate core material to maintain the magnetic field (which is what bucks infinite current flow in the primary coil). In transformer design (pardon the lack of precision as I am writing this on an I-device off the top of my head) the total magnetic field in the core due to the current in the primary coil is the magnetic flux. The core's crossection must be able to support the maximum flux required, and each core material has a different maximum flux density. So, total flux divided by core area = flux density, thus total flux and core material govern core size. The other factor governing core crossection is the fact that the material must support this flux at low frequencies. In practice, the core area increases as frequency decreases.

When the core saturates, the resisting field (that keeps the core from looking like a short circuit) collapses. The primary impedance drops and now the output tube plates are driving a much lower effective impedance. Since the transformer is no longer matching impedances properly, gross clipping occurs, along with a rapid decrease in damping factor (because of loss of transformer matching AND loss of any global feedback control)

So while it sounds like transformer saturation would make a good marketing term, in practice it makes for poor engineering and poor real world performance. That doesn't stop folks from attributing a magic tone to something totally technically unrelated...this actually happens all the time which creates urban myths and old wives tales.
 
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#43
sratas said:
yeah, will do. Could you tell me what the end effect should be and wich tube is the V1? The one near the heat sink or the one nearer to the toroid transformer?
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On the Walkabout, it's the tube closest to the input jack and heatsink.

I'll let you decide what the end result is and what your impressions is.
 
#44
rekesbass said:
All this info is mind boggling..haha...I have a Big Block 750...I thought about doing some rolling and wondering if there would be a noticeable difference,specifically on the o.d. channel..and perhaps even on the main(normal) channel.. agedhorse, if you could, shed a little light on the subject for me since you are in the know with Mesa gear..is the BB750 sensitive to tube changes ? and do you have any recommendations on tubes (or any combinations of tubes) to try in this particular amp
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The Big Block 750 will show some changes, IMO the most significant change will be swapping a 12AT7 or 12AU7 tube into the V3 position. (yeah, I know that the "internet" says the V1 position will result in the most change for ALL tube amps, but in this case I might know a little more than the internet ;) ).

I think you might fins a little more interesting dynamics if that's what you like.
 
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#45
A nice example what diferent tubes can do to the sound performance is the 70's svt power amp.
I think the pentodes 6550 need no comment cause it's well known what the difference means with different 6550 pentodes.

More of interest is the PI stage cause this tube can be replaced with any 12DW7 or 12AX7 without any audible difference.

While most overlooked are the 12BH7 driver tubes cause tube rolling on this tube set has an similar effect to the total sound performance like different 6550's pentodes
It makes me wonder a little bit that forum discussions refere mainly to 6550 choice while the driver tubes can effect the sound just the same way. I think the meaning of the driver stage tube to the sound performance for the svt is totally underrated (or overlooked).

Btw IIRC for the preamp V1 and V3 show some tonal "change" to different tubes while V2 seems to be rather "steady" in this regard.

I claim that a preamp tube stage that does nothing but gain the signal with less THD as possible can be substituded by any SS design without any audible difference.
 
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#46
I always had the preconception that any 12ax7 could be substituted with at7, ay7 obtaining mainly less gain, but after the suggestion of agedhorse I searched the internet founding contrasting believings. Some even say that doing this can possibly blow the amp. This is new to me but now I want to learn more. Please agedhorse, could you clarity this aspect?
 
#47
@ sratas, I think your preconception is largely correct as the 12A?7 tubes are all pin equivalent. But 12AT7s are a high(er) current tube and need the power supply to be able to cope (thus the chance of blowing up the amp).
Interesting the use of "Hybrid" here. To me a tube/ss hybrid is when they are combined in the same preamp OR poweramp circuit. E.g. it seems many bass amps include a tube in their ss preamp stage to add some "tube" warmth, as with my Ivory TLA 5001 mic preamp, it says it's a 4 channel tube preamp, it's not, it's a ss preamp with a tube you can dial in for colour/warmth. And ADA made 2 "hybrid" poweramps (MT100/200) where the tubes control the mosfet output transistors. Having a tube preamp with a ss poweramp is just that (even in the same box), not (to me) a hybrid.

But back to the original post/question re rolling tubes in a Bugera "all tube" preamp stage, and noting that this is for bass (newish for me as I've played guitar for over 40 years and just started to play bass) so the paradigm is quite different than for guitar (where we chase gain/distortion etc). All 12AX7s sound different (some differences are more pronounced than others and again as agedhorse pointed out, some circuits "care" more than others e.g. soldano tube preamaps only sound good with particular tubes).

So the Bugera has 2 input tubes (V1 and V2), trying a few different 12AX7 tubes (either the same or mix and match) will sound different, Some differences will be more pronounced than others (hence some of my suggestions re what to try and why). Understanding that a 12AU7 has less gain than a 12AX7 and that long plates are more 3D and better (and smoother/warmer) bottom end than short plates can help you decide what to try. It's very subjective and depends entirely on what you like in your tone. Probably the closest you'll find as an equivalent to good quality Chinese bottles (used in lots of OEM gear over the years and IIRC what the Bugera came with) are TAD short plates. In my previous post I included a link to the adadepot forum tube topic where we have discussed various tubes adnausium. While this is predominantly for guitar preamps, many different tubes get discussed to help inform a decision of which ones to try. Also don't forget that tubes wear out over time (modern tubes much faster than the tubes of old) and also take a little time to "burn in". Because they wear slowly you don't normally notice. With normal use (in a guitar preamp) 12 months is around when a tube replacement comes due (although some change their tubes every couple of months, others a couple of years), check you manual for manufacturer recommendation.

@ThisBass, the PI (Phase Spliter) stage etc (output transformer saturation, output tubes) is tube poweramp territory, not what this thread was initially about, albeit a very interesting topic (and many thanks to agedhorse for the info) and I agree the phase spliter isn't often included in those discussions and should be. However, there isn't a "tube" PI stage in MrCashs Bugera, it has a ss poweramp. Phase spliter tubes you generally want clean and transparent with balanced triodes (When the plate current draw of each triode is the same), and if you have a guitar amp, high current (e.g. 12AT7) to chase some output stage distortion (probably no so much for bass).

(I claim that a preamp tube stage that does nothing but gain the signal with less THD as possible can be substituded by any SS design without any audible difference) Sorry, I don't agree, high end SS designs maybe, but not "any", although I get where you are coming from.

@ paulraphel, (there's no solid state anything in the signal path), you probably have to go back to the early 70s' or before for this to be the case (e.g. 1972 Marshall (no PCB just a breadboard)). Most tube circuits include ss IC opp amps etc
 
#48
Thanks rnolan...lot if knowkedge here. But i just do not agree that preamp tubes wear out in a few months or a a year. This can be true if we talk of power tubes being pushed hard all the time tough. My tube pres usually don'don't need tube change because of time dependent failure, maybe I'm just lucky
 
#49
Hey sratas, they tend to wear faster in high gain guitar preamps as we push them very hard all the time and a new set do tend to sparkle (some like that new edge). That said, the original chinese bottles in my ADA guitar preamps (MP1 & MP2) lasted years of constant gigging. Bass preamp tubes should last much longer I would expect as they aren't working so hard (depending..) And you can get lucky with tubes (and unlucky). Also, as the change is so subtle over time it's hard to hear until you put in a new set (unless of course they have really died, that you will hear (big gain loss, microphonic etc not pretty), this is a common issue in ADA MP1 guitar preamps with 25+year old tubes).
 
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#50
I recently did a big rolling test in my walkabout, wich has a 15 yo set in it. Contenders were tung sol, sylvania, philips, national, jj, groive tubes, sovtek. New and old...I recorded every set and was able to compare things better. I ended up using the stock old tubes, midel russian 2.
Played guitar for 30 yrs, mainly old marshall all tube, never had to service a pre tube. Sometimes a faulty one happened to be evident from the first use.
 
#51
agedhorse said:
The Big Block 750 will show some changes, IMO the most significant change will be swapping a 12AT7 or 12AU7 tube into the V3 position. (yeah, I know that the "internet" says the V1 position will result in the most change for ALL tube amps, but in this case I might know a little more than the internet ;) ).

I think you might fins a little more interesting dynamics if that's what you like.
Click to expand...

Thank you for the quick response...I'm willing to try anything..especially when it comes from the(aged)horse's mouth...(ok, I'm sorry about that..lol)..
Is the V3 position the one for the o.d. channel ?..Dynamics is what I'm looking for on the o.d. channel, to make it more usable for me...I like the o.d. but, it's pretty over the top(uncontrollable) even at low gain settings..I'd rather use it as a dynamic o.d.and have it break up when I dig in(at low gain settings) and be able to have it more agressive at higher gain settings...also, now(out of the blue) it has a horrific POP when I engage it. I have to mute the amp(via the pull mute or footswitch),engage the o.d. then un-mute the amp,very annoying..EDIT: which tube would best do what I described above..12AT or 12AU ? and would that help tame(or eliminate) the POP ?
 
#54
Th
rekesbass said:
Thank you for the quick response...I'm willing to try anything..especially when it comes from the(aged)horse's mouth...(ok, I'm sorry about that..lol)..
Is the V3 position the one for the o.d. channel ?..Dynamics is what I'm looking for on the o.d. channel, to make it more usable for me...I like the o.d. but, it's pretty over the top(uncontrollable) even at low gain settings..I'd rather use it as a dynamic o.d.and have it break up when I dig in(at low gain settings) and be able to have it more agressive at higher gain settings...also, now(out of the blue) it has a horrific POP when I engage it. I have to mute the amp(via the pull mute or footswitch),engage the o.d. then un-mute the amp,very annoying..EDIT: which tube would best do what I described above..12AT or 12AU ? and would that help tame(or eliminate) the POP ?
Click to expand...
V3 is the tone stack driver, a 12AT or AU scales the linearity differently. I have no idea about the pop.
 
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#55
@rnolan - If a circuit wears a tube out in a year, it is a grossly terrible circuit from an engineering and customer service point of view. Every preamp tube circuit I design has an expected life of 10 years at average usage (say 2 hours a day, 365 days a year). In practice, from the crossection of amps I see coming in from the field, 10 years is being conservative and 20+ years is more likely. This is with quality new production tubes. And, it doesn't matter how hard you drive the tube as there is plenty adequate margin for doing so continuously.

You can not say that a 12AX7 has more "gain" without considering the circuit it's being used in and the impedances surrounding the tube. I have designed circuits where a 12AU7 provided far more gain because of limitations in the 12AX7. Circuit gain comes in 3 different flavors, voltage, current and power. All 3 are interactive, all 3 can be used in a preamp design.

Replacing a 12AX with an AT or AU will not generally damage anything unless the product is so marginally designed as to be useless. A 12AU can provide greater current but only if the circuit is designed to allow it.
 
#57
RHA III said:
Any suggestions for replacing the tube in my shuttlemax9.2?
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Is it not working? Unless there are problems with it, there's no reason why it shouldn't last 20 years without significant degradation of performance. If there is a problem, I generally recommend any of the 12AX7's except the long plates (for current density reasons), the circuit tends to minimize the majority of wildly varying parameters so they will all sound pretty close (though not "identical"). The circuit is also somewhat resistant to microphonics, though is can still be a problem with really marginal tubes.
 
#58
agedhorse said:
Is it not working? Unless there are problems with it, there's no reason why it shouldn't last 20 years without significant degradation of performance. If there is a problem, I generally recommend any of the 12AX7's except the long plates (for current density reasons), the circuit tends to minimize the majority of wildly varying parameters so they will all sound pretty close (though not "identical"). The circuit is also somewhat resistant to microphonics, though is can still be a problem with really marginal tubes.
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So I'm not going to improve my tone or change it much? If it ain't broke...
 
#59
ok...got some serious mod fever after all this talk about hybrids and tubes.

re-listened to my samples with various tubes and combinations of them.
had a hard time picking a clear favorite or drastic changes in tone. Think about subleties, even with reference headphones and good ears :) Much more easy to detect a change in the overall loudness of the sample, maybe indicating different gain in my particular circuit. The most particular ones, compared to the stock russian 2 tubes, proved to be two Tung-sol (very clear, transparent, extended, defined, with as much gain), and the Sovteks LPS (almost as much clarity as the Tung-sol, with a little bit more gain and colouration).
Pumped by all this tube talk, kept the two Tungsol 12ax7, and after a good two hours playing, I can say they can be a very good compliment. They sound a lot clearer and now I can keep the sublows flat without too much boominess, where the stock tubes forced me to cut drastically the 30 Hz frequency point on the parametric.
Let's see with the band, but I'm quite pleased.
Unfortunately, I have not any 12AT7 on hand at the moment, but I am intentioned to try one (maybe the Siemens ECC801S or a Gold Lion) when I happen to buy it.
 

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