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Swapping tubes in a hybrid amp

#21
Your Tubeworks and Demeter amps use totally different circuit topolgies, the Tubeworks topology was chosen to provide a fairly gnarly and highly compressed overdrive, whereas the Demeter is pretty far from that territory. The number of tubes has little to do with the amount of change you are likely to hear.

For example, there are tube based operational amp platforms (topologies) that use 3 tubes but by design have almost no difference in sound no matter what tube is used... it's characteristic of that particular topology. There are also single tube topologies that are highly dependent of different tube parameters, and while a small sample of tubes may sound excellent, a large percentage may sound very poor as well.
 
#23
MrCash said:
I have a question.
How much does a transistor based power amp colour the tube pre? Or do they do a good job putting out what the tubes give out?
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They CAN color the sound IF that's what they were designed to do or they can not color the sound. It's all about the designer's intent. In general, most (but not all) are reasonably transparent.
 
#24
agedhorse said:
They CAN color the sound IF that's what they were designed to do or they can not color the sound. It's all about the designer's intent. In general, most (but not all) are reasonably transparent.
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Indeed. Is there any loss of the "tube" sound with having a transistor power amp? Say if your driving the valves hard and your getting that nice valve clipping can the transistor power amp faithfully put that out?
 
#25
MrCash said:
Indeed. Is there any loss of the "tube" sound with having a transistor power amp? Say if your driving the valves hard and your getting that nice valve clipping can the transistor power amp faithfully put that out?
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If the preamp overdrive is what you are looking for, than most solid state power amps can reproduce this faithfully. There are solid state power amps that incorporate all kinds of adjunct circuitry that enhances or creates additional overdriven tone and especially feel, which is why I mentioned this.
 
#26
MrCash said:
Indeed. Is there any loss of the "tube" sound with having a transistor power amp? Say if your driving the valves hard and your getting that nice valve clipping can the transistor power amp faithfully put that out?
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There is preamp tube tone and poweramp tube tone. When driven close to its limits, the saturated transformer and power tubes develop a tone all their own. Some call it 'heft' and 'grunt'. But again, how much depends on the amp designers goals. The Ampeg SVT has the most recognizeable power tube tone, while other tube heads like the Reeves may exhibit very little of that type of character. As far as an ss power section having character, the Mesa Walkabout is often talked about as having power tube-like character when driven hard. Not sure if that effect was by-design, or a happy coincidental byproduct. Mosfets perhaps?
 
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#27
Coolhandjjl said:
There is preamp tube tone and poweramp tube tone. When driven close to its limits, the power tubes develop a compressed tone all their own. Some call it 'heft' and 'grunt'. But again, how much depends on the amp designers goals. The Ampeg SVT has the most recognizeable power tube tone, while other tube heads like the Reeves may exhibit very little of that type of character. As far as an ss power section having character, the Mesa Walkabout is often talked about as having power tube-like character when driven hard. Not sure if that effect was by-design, or a happy coincidental byproduct. Mosfets perhaps?
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Yes I understand pre and power amp tube tone.
Its just I've never quite heard a hybrid amp sound as open as an all tube amp. And I'm not talking about high volume levels where you would have the power tubes "kicking in" so to say. I'm talking about just driving the front end hard while keeping the master down. I thought maybe the transistor power section might blunt the tube pre tone.
But as agedhorse says transistor based power amps can faithfully put out what the tube pre is producing then Im guessing there's other factors involved.

As to my original question I think I will try some tube rolling guess that's the only way to find out how much a difference it will make.
 
#29
In general, output transformer saturation sounds pretty horrible, especially when the transition into saturation is steep. When an output transformer saturated, it does so first at the lowest frequency. As the transformer saturated, the primary impedance falls and loads the output tube plates down (additional mismatch).

This is one big reason why (for identical core materials) a bass guitar transformer is so large... To avoid saturation. Otherwise, a smaller transformer would saturate earlier and be a lot cheaper too.
 
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#30
Tube choice makes a "big" difference that you will hear, they all sound different. All the 12A?7 (?=AX, AU, AT, AY) tubes variants will be pin compatible although higher current tubes like 12AT7s need the right amount of power supply.. I wouldn't put a 12AT7 in a preamp stage (lower gain, high current tube, good for a phase spliter in a tube poweramp stage or an input buffer tube in a tube poweramp) and 12AY7s sound crap in general, I'd stay away from them.
In practical terms, these days tubes come from 3 places, Slovakia (JJs formerly Tesla), Russia (lots of choices, New sensor get re-issue tubes of various "brands" made there, Mullard, Tungsol etc.) and Chinese tubes (again lots of choices, some are great some are crap). NOS tubes (when you can find them) can be very good but are very expensive. Most of the brands you know (Boogie, GT, EV etc) are re-badged tested and selected tubes (e.g. Boogie tubes are mostly JJs, but not always), they don't make their own. Moreover, they are built to not last too long these days (the OEMs need something to sell you). Unless you want to reduce the gain in your (all tube) preamp stage, then 12AX7s are the go, they are the highest gain of the 12A?7s. But this makes more sense for distorted guitar, not so much for bass. If you want to clean up the gain/distortion a bit then 12AU7s are a good choice as they are a lower gain tube.

There are short and long plate tube designs. Short plate are a little less microphonic but generally very 2D. Long plates are very 3D but show off all your mistakes LoL. Some tubes suit particular circuits better than others (for all the reasons/variables agedhorse has posted). You can also mix and match different brand 12AX7s (and/or 12AU7s) in the V1 and V2 positions. V1 tends to make your tone, V2 tends to amplify it more. If you want lovely 3D depth in your tone (where even the slightest change of pressure of your fretting is clearly defined and audible) then the New Sensor Mullard re-issue Long plates are the go (these suit more accomplished players who want to get "more" from their tone). These are made in Russia, great tubes, I use them now in all my ADA tube preamps (4 x ADA MP2s for guitar and 2 x ADA MB1s for bass). But a good combination is also a NS re-issue Mullard short plate in V1 and a JJ in V2. This is a long and "very" subjective topic. See here for some good discussion (Invalid Link Removed) albeit focused on guitar and primarily ADA MP1 and MP2 preamps. Preamp tubes should cost you around $20 to $30 each (depends on the country you live in a bit, European tube prices seem to be higher), buy them from a good source e.g. Dougs Tubes (Vacuum Tubes For Guitar Amps and Hifi Audio | Dougstubes.com) where they are well tested etc. I'm in Australia so I get mine from (Mullard 12AX7 / ECC83). For your application you don't need to worry about matched pairs and/or balanced/matched triodes etc.

Good clean/transparent SS power amps work well with tube preamps (ADA did this for years and I still do..) although some SS amps work/sound better than others. The SS poweramp in your Bugera will, no doubt, be well suited for the job, they make reasonable gear. Clean/transparent tube poweramps add a little warmth. Also the nature of tube amps is they have allot of "stored power" (partly why 50 tube watts seems much louder than 50 SS watts). This is partly what the $30,000 hand made SS hifi amps chase... they have huge power transformers and weigh a tonne.
 
#31
Coolhandjjl said:
There is preamp tube tone and poweramp tube tone. When driven close to its limits, the saturated transformer and power tubes develop a tone all their own. Some call it 'heft' and 'grunt'. But again, how much depends on the amp designers goals. The Ampeg SVT has the most recognizeable power tube tone, while other tube heads like the Reeves may exhibit very little of that type of character. As far as an ss power section having character, the Mesa Walkabout is often talked about as having power tube-like character when driven hard. Not sure if that effect was by-design, or a happy coincidental byproduct. Mosfets perhaps?
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Owning and loving a Walkabout. It has two 12ax7 in it, but I hear a very complex gain structure that involves the power stage too. Pumping up the master introduces a middy harmonic content and a lot of grind on the attack, an finally wind it up and it adds to the preamp drive building a full blast distortion. The effect of the power stage is totally different from what I may expect from a SS unit. It sounds wonderful and easy to the ear. If Agedhorse could possibly explain why it sounds so tubey i would be thankful. There is a tube involved or is a completely SS path in the poweramp?
 
#32
One reason tube circuits tend to sound more interesting (in general, not always) is because they tend not to have large amounts of global feedback (a byproduct of not having large amounts of open loop gain). Other things include the often excessive loading of high output impedance stages

Now regarding rnolan's broad comments about 12AX7's, in fact for many circuits in audio, 12AT7's and 12AU7's will end up with net gain figures close to 12AX7's in circuits with heavy loads. Both are some of my favorite tube types with some awesome properties (when properly exploited).

Sratas - try a 12AT7 in the V1 position of your Walkabout and let me know what you think ;)
 
#33
I experimented early on replacing the EH 12AX7 in my Demeter HBP-1 preamp with a NOS GE 12AX7. It was many years ago, but as I recall, I didn't detect a huge change in it's tone.

So, to my ears, the HBP-1's circuitry must be as agedhorse has posted, a design that isn't greatly affected by a tube swap.
 
#36
MrCash said:
How much difference does swapping out tubes on a hybrid (tube pre) amp make?
I know it would vary from one amp design to another and how they use the tubes. But in general does it make much difference?
My amp has 4 cheap generic Chinese 12ax7a's in.
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I put these in a Demeter amp.
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They are pretty sweet.
 
#37
agedhorse said:
One reason tube circuits tend to sound more interesting (in general, not always) is because they tend not to have large amounts of global feedback (a byproduct of not having large amounts of open loop gain). Other things include the often excessive loading of high output impedance stages

Now regarding rnolan's broad comments about 12AX7's, in fact for many circuits in audio, 12AT7's and 12AU7's will end up with net gain figures close to 12AX7's in circuits with heavy loads. Both are some of my favorite tube types with some awesome properties (when properly exploited).

Sratas - try a 12AT7 in the V1 position of your Walkabout and let me know what you think ;)
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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
 
#39
I remember when I had a MusicMan HD150 bass head, which was a tube power section, with an SS preamp. It had some Electro Harmonix , 6L6 tubes in it when I got it. I liked the way it sounded, but I luckily found some SED Winged C tubes, before the company went down, and, oh man, what a difference in tone ! The Winged C tubes gave me a more chunky tone, especially in the upper bass, low mids, and just a more ballsy kind of tone overall. Best hybrid bass amp I ever played, except for the Walkabout. To me, it sounded more like a Bassman on steroids.
 
#40
agedhorse said:
Your Tubeworks and Demeter amps use totally different circuit topolgies, the Tubeworks topology was chosen to provide a fairly gnarly and highly compressed overdrive, whereas the Demeter is pretty far from that territory. The number of tubes has little to do with the amount of change you are likely to hear.
Click to expand...

I didn't assume that the difference had to do with the number of tubes ... just that since the tubes were doing basically everything in that circuit (there's no solid state anything in the signal path) tube choice would probably make a difference. Is this a reasonable assumption?

In the Tubeworks head (mine didn't provide useable overdrive, fwiw), there was a lot of solid state circuitry in the signal path, so I had no idea what the tube was actually doing in there. This particular amp is probably a bad example, since mine never really worked right, but I've heard people say the same thing about some eden and SWR heads ... they'd throw in an expensive, famously lush-sounding NOS tube and it wouldn't make any difference.

Someone earlier mentioned that some of these designs use the tube as a cathode follower, and that it in this application the tube doesn't make much difference. What exactly (besides marketing) is the point of using a tube with circuits like this?
 

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