Get a nicely restored Dynaco ST70. Plenty of dynamic power compared to a typical SET amp. Agon member autospec has one for sale here.
Compact ~20WPC Tube Amp
I should say 20WPC or more....
I’ve been trying to use a 8WPC 300B SET (Sun Audio) with some vintage bass reflex studio monitors from the 70s (96dB) and it’s clear that I need a bit more power. Everything sounds really awesome but the highs will distort at anything beyond low-medium volume which has been fine but I want to play a little louder sometimes!
Space is limited so I need a stereo amp or maybe small monos are possible.
57 responses Add your response
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With your budget you leave yourself with good flexibility in choices. I agree that Quicksilver amplifiers merit attention. I’d give Aric Audio a call/email. I believe that he could build a really nice quality amplifier within your price range. You’re in a good position with new and certainly used possibilities. Charles |
If you can find a Dynaco ST35, its a lot more linear amp than its bigger brother. But it makes about 17 Watts/channel. SETs tend to have a limited amount of what I call 'usable power'. By that I mean its usually not a good idea to push them past 20-25% of full power. Based on your comments your speakers are nowhere near efficient enough to work with your SET. If that 96dB number is real, your speakers are similar to mine which are 98dB. I find that I need a bit of power; at least 30 Watts. I have an average room, so I suspect you'll need at least that much. So the amps you are looking for (if you stay with tubes) will be PushPull. They tend to have a much higher amount of 'usable power'; usually close to 95%. The ST70 is not a bad suggestion in this range; the RM10 is another good one. Either should be properly refurbished. Another amp that might suit is the Harmon Kardon Citation 2 if you can find one refurbished. They make about 60 Watt/ch. |
agree with jasonbourne52 seek out a ST-70 properly refurbished. I bought a new custom build recently from retroaudio on US audiomart for peanuts just to have a tube amp to rotate and it's more enjoyable in my system than my previous RM-10. He's restored hundreds of Dynacos for decades and knows his craft. He does ST-35's as well. |
Which leads me to suspect that the 96 db sensitivity rating is exaggerated or (More possibly) the speaker’s impedance curve and load just isn’t suitable for an SET 300b. So, much depends upon speaker design and intended use.Push pull tube amplifiers will usually have lower output impedance and thus higher DF. Also generally they utilize some degree of NFB which increases DF. Most SET amplifiers are zero NFB (Or very low) and will require different/compatible speaker loads. OP’s speakers just need a good quality push pull amplifier and they’ll do well. Charles |
@charles1dad That wouldn't surprise me at all. The impedance thing is why I wrote this article since so many speakers that are rated 8 Ohms actually have 4 Ohm woofer arrays and simply aren't meant to be driven by zero feedback tube amps! That was part of why I convinced Israel to try making speakers with higher impedances. It doesn't help that speaker manufacturers seem to often overstate their numbers as well. |
The Sun Audio 300B amp, if I’m not mistaken, uses two 6SN7 stages to drive the 300Bs and is the main issue with the amplifier distorting prematurely. The 300B and many other DHTs will draw grid current long before their power rating is reached and will "slew" causing distortion (the coupling cap is discharged faster than the driver can charge it leading to rolled off highs and sloppy bass). This is really common with all of the 2 stage 6SN7/300B amps out there. The 300B really needs a stout drive capable of kicking it in the @$$ to really allow it to open up. While a push pull tube amplifier will certainly give you the merits of more power, if you’re looking to stick with the overall sonic character of a single-ended amplifier, there are other options out there. My single-ended KT120 for example, with a solid 6SN7 SRPP driver will sound more like a 35 WPC amplifier, but can still produce 18 watts per channel with excellent cone control/damping. However it does use NFB as non DHT tubes really do not sound good without it unless they are "triode-strapped", which is an inherent form of local feedback. With most SETs the driver is the amplifier’s downfall- not the power rating of the amp, or the 300B.
Best regards, Aric |
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Aric this is precisely what the builder of my Coincident SET (And speakers) explained years ago. The main limitation with some SET amplifiers is the choice of weak/inadequate driver tube. Just as you say, this driver needs to be robust. The DHT output tube demands this. He chose the 6EM7 rather than the 6SN7 for that reason. Good advice you gave to Israel Blume (Coincident). I’m happy that he took note of that wise counsel,😊 As you’ve pointed out before, higher speaker impedance means less amplifier stress/work and less distortion. Charles |
@charles1dad Exactly! The 6EM7 has two different triodes inside, one has high gain and low Gm so is used aptly as a voltage amp where its only job is to make the input signal larger, which is then cap-coupled to the higher Gm, lower gain and lower output impedance section of the tube. Then that is coupled to the 300Bs grid by a 2:1 interstage transformer so it makes a dog's lunch out of the 300Bs grid current and allows the amplifier to realize the full power of the 300B. My 300B is similar, but takes a slightly different road to get there (same principle however).
Regarding speakers, another trick is to use an output transformer with a higher primary impedance to drive a lower impedance speaker. I've built amplifiers for several customers where their speaker's impedance profile dipped down heavily into 2 ohms, and the SET was able to drive them just fine, or more so surprisingly fine!
All the best, Aric |
compact ________? We really know what we own or have known, My Cayin AT88 is not compact, a ’regular’ sized tube amp, 45 wpc. It had to sound as good as my Fisher 80az mono blocks made in 1958, it does, and gave me remote volume, which allowed me to use a vintage tube preamp with no remote anything. If interested, get a later model with external bias, I needed 16 ohm taps, only m1 has those. later have 4ohm and 8 ohm taps 6 moons review convinced me to try it, glad I did
https://6moons.com/audioreviews/cayin/a88t.html
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I will have to throw a few votes in there for the "talk to Aric" comment. Ive spent the last hour after getting home from work. Listening to a nice mix of mellow male and female vocal/jazz. Playing my Innuos Zen thru a Pontus II To a Aric Audio 6sn7 preamp (currently running Linlai tubes) and Aric Audio Super KT88/120 SET amplifier (currently running Westinghouse 6sn7gtb tall bottles and KT170's in triode). At the end of the line I have a pair of Spatial M3TM's that you would never know were even in the room! Arics pieces are just amazing you should honestly reach out to him. I don't disagree with previous recommendations as I have no experience with most. I do know that my experience with Aric is nothing short of amazing, a good honest man! J.F. |
I've had my Quicksilver Minimite monoblocks for the past 17 years and still truly enjoy them with some of my British monitors. Linn Kan I and KEF Reference Series Model I (similar to the LS3/5A BBC monitor). If you can find a used pair in nice shape for around $750 I think you'd really like them. If you want to purchase new, then the Quicksilver horn monoblocks at around $1600 the pair might make sense. They're 25 watts per channel but sound much more powerful. Good luck! 😀
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Check out the Unison Research Triode 25. It is an EL34 based Push Pull that is 25w. It’s a nice amp and should address most of your issues. At $4500 retail it is on target in terms of pricing. It delivers the quality that you want. They have good support in the US as the distributor is strong. PrimaLuna, Decware, and Line Magnetic would all be viable alternatives in your price range. You could also look around to see if you could find a used, Art Audio Carissa. Hell of an amp. Full disclosure...I am a Unison dealer.
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@charles1dad The 6SN7 has no worries driving a single DHT! That's not the problem- the coupling cap and the timing constants around it is. We ran into the same problem in our OTLs, but solved it by direct-coupling. We have a single 6SN7 thus driving 8 power triode grids with no distortion problems even when drawing substantial grid current. But to do this properly, you need a negative power supply and most SET designers aren't interested in the additional cost. |
I am not an electrical engineer. I do listen intently when people who are successful at what they do offer their perspective and experiences. Israel Blume is very accomplished with what he does and so is Aric. They both share the same opinion with regard to appropriate driver tubes for DHT tubes (Specifically 300b). Either of them (And other amplifier builders who have similar view) are far more qualified than me to debate this point with you. I can say that their comments make plenty of sense and are backed up by their SET amplifier performance results. I just listen and read in an attempt to learn. Charles |
What they are doing is a brute force thing by running lots of current through the tube so they can have lots of current drive on the grid. Of course by simply paralleling elements in the 6SN7 they could do the same thing... but at any rate, there is a series impedance (the coupling cap) which limits how much current can be delivered to the power tube grid. So they are going for overkill. But if you direct couple you solve this problem, although to do it effectively you have to run the driver tube with no voltage gain. That’s exactly how we’ve done it in our OTLs. The advantage of this approach is there is no overload recovery time- its instantaneous as there are no timing constants present between the driver and power tube grid. But it does make for what appears to be a more complex circuit, something that seems anathema to SET designers. However, there is another advantage: the coupling cap that you do have to have to couple the signal to the driver tube can be a considerably lower value, usually by at least an order of magnitude, possibly more. This means that no matter what the coupling cap, the smaller cap will always sound better. So you have a double whammy. The downside is the driver tube isn’t available for gain. But that’s OK- being a lower powered amp, an SET doesn’t need much gain (certainly less than we have in our OTLs). The funny thing is you could execute this with the same number of tube sections as used in a traditional SET right now (three total, including the power tube). The reason you don’t see this is the fact that you have to build a negative power supply. But if you did, you could build an SET that was inherently lower distortion which means it would be more transparent. |
Interesting you say “more transparent “. ironic then, one of the first things I noticed when I heard my 300b SET mono blocks, superb transparency and startling nuance/inner detail. As if the speakers weren’t even in the room! I do not possess the technical background or expertise to explain why this is exactly so. I can only rely on what I hear. That initial listening encounter was striking! Charles
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+1 Will Vincent. Good value for money, and nice industrial design. His piece powers nicely a relatively insensitive set of Harbeths (albeit I’m not listening to rock @ 90 db). Even if I change tube amps, I will keep his, just to look at - it’s nice to own something made by an actual person in this day and age (if you go that route, I’d ask him to put an LED in the bias meter for cosmetic purposes). I have worked with Aric but he is next on my list Good luck!
Jonathan |
@charles1dad The best amp I've ever heard uses 6sn7 to drive 6c33c. I just picked up an amp to play with, partly because it uses 6sn7 to drive many choices of power tubes. I'm eager to experiment with it. |
Alot of solid recommendations and yet, not a mention of the speakers in question. The OP hasn’t mentioned what speakers he is using that are admittedly somewhere around 45-50 years old. He hasn’t mentiontioned impedence, brand, whether the crossovers have been updated to specification and whether the drivers are in proper working order. If it was my money, before I spent a dime on anything further, I would think it prudent to have the speakers tested by someone competent. It’s likely there is an amp/speaker mismatch but how do you know? And how can we make appropriate recommendations? |
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Horses for courses. I just want to clarify for the OP, that sure- the 6SN7 can be used as a driver with good results to a DHT like the 300B, such as SRPP, DC coupled cathode follower with a negative supply at the cathode, or grounded cathode with local feedback (anode follower). However my point was only that in the Sun Amp (which is very similar, if not identical to the JE Labs 300B SET and also the design posted by Morrison on DIY Audio projects), it is simply using the first section of the 6SN7 as a grounded cathode volt-amp DC coupled to the second triode and then capacitor coupled to the 300B grid. This amp does not do the 300B justice (I’ve built it before). There are many other ways to drive a 300B as well- a C3M pentode, a triode strapped 6V6, a 6EM7, etc. That is not to say that I do not LOVE the 6SN7 (I use it in many of my products) but used properly and not typically being asked to provide large amounts of current sounds best to me. Again, with all engineering there are trade offs and there is no one way to implement an amplifier circuit. I don’t believe economics are the key driven point with a negative supply either, if so, then many designers wouldn’t bother using an inter-stage transformer. As they say, there is more than one way to skin a cat. |
As you know a circuit like this will result in a higher output impedance so wouldn't be the best application for driving a grid that has high capacitance and needs a lot of swing. The 300b has nearly the same mu as the power tubes we use in our OTLs. We also built an OTL using 300bs for a demonstration so I'm not speaking idly. We don't run the tube with a lot of current either, despite that, direct coupled to the grid of a 300b will have no problem driving it to saturation. At that point the 6SN7 section will saturate too; when replaced by a much gutsier tube the difference was less than a 1% change so its very obvious that the 6SN7 can do this job quite well. We use the 6SN7 driving the 6AS7G; if you compare specs to a 300b you see that the former has slightly higher grid capacitance and slightly lower mu; the 300b is easier to drive. Despite that we can get one 6SN7 section to drive as many a 14 6AS7G grids with no worries. One grid of a 300b would be like a walk in the park. If you like the 6SN7 that much, if I were you I'd be looking a better topologies to make it happen. |
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Sounds as if a compact, integrated EL-84 amp might just fit the bill for around 20 w/ch. There’s a newer Quicksilver and or older, refurbished Scott, Heathkit and Eico integrateds. EL-84’s were often the tube of choice back "when"....they still sound great! - Still, choose the right speakers, that you like & fit the room size, then choose your suitable amp (tubes generally like 8 ohms or more).
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Are you going to share the specifics on your speakers? I’m a huge fan of the EL84 tube but depending on your speakers, they could be wrong. It also depends on the amp topology. A single ended el84 amp begins to distort at 5ish watts with max power 9-10 watts per channel. Push pull begins to distort around 6-9 watts and tops out around 12-15 wpc. Some of the old Scott’s that use the 7189 can be pushed a little further to achieve high teens realistically. I believe the EAR, quicksilver, Manley and some others use 8 rather than four el84’s so those numbers can double up.
Once again though, if you would share your speakers with us we might be able to provide better advice. 4ohm? All bets are off. Crossovers with 50 year old caps? Ditto. |
I Have the Cabin CS-55A commented by @henry53 I bought it for $2220 from a New jersey distributor. I spent about $800 on good tubes. It is a great amp, dead quiet, plenty of power with my Monitor Audio Silver 500 speakers and REL T/5X sub. |