Hello I am new to this forum. I previously built a Soldano SLO 100 Clone from scratch and a 4x12 cabinet to go with my guitar rig. Now, I decided that getting into the hi-fi world of tube amps is the next step I want to take. I am no noob to tubes, but I am not an expert either. I do, however, lack the theory that goes behind making a tube amp (I know how tubes work but how switching components changes the sound I am clueless). This leads me to my first project. I've always been a Mcintosh fan in a sense that I can appreciate the amplifier and listen to it at my local store but unfortunately the budget does not allow for that. Can anybody recommend a tube audio amplifier I can first start with?
Than you
Than you
Tube amps have the advantages that 1. If they go bad they won't blow up your speaker 2. Lightning hits don't hurt them, at least the kind we have in Indiana, and before that, Kansas. I've lost a couple of power switches and turn-off pop caps is all. Hifi amps are not supposed to distort, so all the "sound" lore that guitar amp people talk about tubes is irrelevant. If you do play them too loud, crossing the clipping threshhold is not painful to hear like a BJT transistor amp.
The base amp I started with, and there are thousands of them, is the Dynakit ST70. Triodeelectronics.com near Chicago sells a clone kit. The disadvantage of the ST70 is it takes a 100 db@1W speaker to blow you out of the room, but my SP2-XP's are 101 db@1W and sound great on tube or transistor. I'm using a disco mixer to drive the St70 right now(op amp based) so a tube preamp is not necessary.
The base ST70 amp had two 7199 driver tubes, which are unavailable and produced about 1% harmonic distortion on a good day. Great for 1961 but not now. Triode has a 3 tube driver PCB kit involving EF86's that some members say sounds better. Triode's 6Pi tube from WingedC (Russia) they sell for the EF86 is not too linear; those same members reccommend a Russian Tung-Sol EF86 tube I haven't found yet. I just put JJ output tubes in mine (6CA7), an original 1961 kit. Plenty of power, but a little honkier than the CS800s amp in the midrange on 1970 7199 tubes. The ST70 is a lot easier to repair than the CS800s though, which I bought at a bargain and which recently blew the breaker, which explains the 80% off price. Switching power supplies, scratch scratch - - -
The ST70 only has about 100 parts, so it not too cluttered. As far as improvements go, the multicomp metal film 3 W resistors I put in the PAS2 tube preamp helped eliminate the hiss, and plastic caps over paper polished up the highs a little. About $.07 ea for the resistors and about $.25 ea for illlinois cap brand plastic caps (really Korean). I can't believe how much triode charges for the discrete parts of their kit, and nobody likes Xicor much. My ST70 is pretty quiet now with no input, but I am laying in some metal film resistors to replace the carbon AB brand ones at least over 100000 ohms. I also fooled around with the negative grid bias circuit (fixed resistor values) so that if the pot loses contact the tubes don't run away and burn holes in their plates. As I was doing that I shorted the grids bias to ground and had the 6CA7 tubes drawing 400 ma apiece before I found the problem and it blew the fuse twice. No damage. Tubes are pretty stupid proof as long as you don't electrocute yourself on the high voltage.
The base amp I started with, and there are thousands of them, is the Dynakit ST70. Triodeelectronics.com near Chicago sells a clone kit. The disadvantage of the ST70 is it takes a 100 db@1W speaker to blow you out of the room, but my SP2-XP's are 101 db@1W and sound great on tube or transistor. I'm using a disco mixer to drive the St70 right now(op amp based) so a tube preamp is not necessary.
The base ST70 amp had two 7199 driver tubes, which are unavailable and produced about 1% harmonic distortion on a good day. Great for 1961 but not now. Triode has a 3 tube driver PCB kit involving EF86's that some members say sounds better. Triode's 6Pi tube from WingedC (Russia) they sell for the EF86 is not too linear; those same members reccommend a Russian Tung-Sol EF86 tube I haven't found yet. I just put JJ output tubes in mine (6CA7), an original 1961 kit. Plenty of power, but a little honkier than the CS800s amp in the midrange on 1970 7199 tubes. The ST70 is a lot easier to repair than the CS800s though, which I bought at a bargain and which recently blew the breaker, which explains the 80% off price. Switching power supplies, scratch scratch - - -
The ST70 only has about 100 parts, so it not too cluttered. As far as improvements go, the multicomp metal film 3 W resistors I put in the PAS2 tube preamp helped eliminate the hiss, and plastic caps over paper polished up the highs a little. About $.07 ea for the resistors and about $.25 ea for illlinois cap brand plastic caps (really Korean). I can't believe how much triode charges for the discrete parts of their kit, and nobody likes Xicor much. My ST70 is pretty quiet now with no input, but I am laying in some metal film resistors to replace the carbon AB brand ones at least over 100000 ohms. I also fooled around with the negative grid bias circuit (fixed resistor values) so that if the pot loses contact the tubes don't run away and burn holes in their plates. As I was doing that I shorted the grids bias to ground and had the 6CA7 tubes drawing 400 ma apiece before I found the problem and it blew the fuse twice. No damage. Tubes are pretty stupid proof as long as you don't electrocute yourself on the high voltage.
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Once you start tweaking the design it is no longer an ST70 
But there are lots of PP EL34 amplifiers, many of them very good. Mine uses ST70 iron with Gregg The Geek's DynaMutt front end (Classic Valve design) -- sounds great. I am prepping to do one loosly based on Allen Wright's PP1c (same front end, more versatile output stage with O'Netics OPTs)
dave
But there are lots of PP EL34 amplifiers, many of them very good. Mine uses ST70 iron with Gregg The Geek's DynaMutt front end (Classic Valve design) -- sounds great. I am prepping to do one loosly based on Allen Wright's PP1c (same front end, more versatile output stage with O'Netics OPTs)
dave
dynaco-doctor.com has got the ST70 manual on line which has pictures and build instructions. Classic Valve Design - Dynaco Clone and Original Design Boards and Repair Kits - Dynaco ST-70 Modifications has got both the dynamutt and KTA driver boards which are both supposed to sound better than the OEM one (which they and triode also have).
Triode's steel box has the socket holes already punched and a nice perforated cover, which is a big time saver. If you search or scan back a year on tube amp forum here, there are a lot of different St70 tweaks, including a 120 W kit with chassis Bob Latino is selling in NJ that uses different output transformers and tubes, but costing $1200. tubes4hifi.com Here is a thread http://www.diyaudio.com/forums/tubes-valves/175952-hello-another-st-70-project.html Kevin the last poster is the designer of the KTA driver PCB. Since you're a member you can see all the schematics & pictures immediately. Have fun.
Triode's steel box has the socket holes already punched and a nice perforated cover, which is a big time saver. If you search or scan back a year on tube amp forum here, there are a lot of different St70 tweaks, including a 120 W kit with chassis Bob Latino is selling in NJ that uses different output transformers and tubes, but costing $1200. tubes4hifi.com Here is a thread http://www.diyaudio.com/forums/tubes-valves/175952-hello-another-st-70-project.html Kevin the last poster is the designer of the KTA driver PCB. Since you're a member you can see all the schematics & pictures immediately. Have fun.
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I am a big fan of Bud's transformers (o'netics). I used them in my guitar amplifier and it's the best sounding I've heard. Thank you for all your help. This is very helpful getting a start on a project. From my reading the st70 is considered a poor man's Mcintosh. Does how does it compare sonically? Any tube amp, I assume, is better than a solid state after all.
The ST 70 is only a power amp correct? That means I can't plug a cd player directly and need a preamplifier too?
Thank you once again.
The ST 70 is only a power amp correct? That means I can't plug a cd player directly and need a preamplifier too?
Thank you once again.
With the OEM 7199 driver board, my ST70 is honkier on the midrange than either the Peavey CS800S (BJTransistor complementary design) or the djoffe bias mod dynakit ST120(a NPN quasi comp 4 output transistor amp). This correlates to the 1% THD of the original design, compared to .03% @ 240 W/ch (8 ohm) of the CS800s. Any of the driver PCB's listed above are supposed to be an improvement. Note the ST70 transformers are better on 8 ohm speakers, will handle 16 ohm speakers, and are barely capable at 4 ohms, whereas a lot of modern speakers are 4 ohm and 2 ohm. My Peavey SP2's are an exception at 8 ohm.
The ST70 is a power amp, but responds to 1 Vpp fine with a few picofarads of load. A CD player or an FM radio will drive it fine. You need a mini-stereo-phone plug to dual RCA plug adapter. The one be careful warning, is that the base PCB connects the input directly to a tube grid. If the input tube is good, fine, but they have been known to short grid to plate (400 VDC), so putting a .1 uf 600 V cap between the tube grid and the input jack may save your CD player some day from burning up. A PAS2 preamp wouldn't care, it has an output cap that size.
While they sound better, both of my transistor amps have issues right now, the ST120 an ittermittant channel (cold solder joint?) and the CS800s a bad power supply, while the ST70 soldier's on. We just had a big lightning storm where I unplugged the TV from the antenna, the computer from the phone line, and the ST70 plowed on right through the flashes and bangs playing classical FM music - no risk, my 41 years of ownership tells me. I would have unplugged the CS800s if it had been working, the power supply doesn't have a lot of lightning protection, and it has about 1000 parts to go bad if struck. The running cost of tube amp- new power and rectifier tubes every 8-10 years, new electrolytic cap every 10 years, 10x idle current from the AC line. I've upgraded my B+ (high voltage) cap recently from 30-20-20-20uf to 47-33-33-33 by replacing the tall can with under deck radial caps on terminal strips. Tons of capacitance like the triode cap PCB can damage the rectifier tube IMHO. I put a 47 uf 3 W resistor in series between the tube and the cap to cut turn-on surge. A bigger choke would be more elegant, but expensive.
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