Official Luthiers Forum!
http://luthiersforum.com/forum/

Dynaco based STA-70 tube amplifier
http://luthiersforum.com/forum/viewtopic.php?f=10101&t=41599		 Page 1 of 2
Author:  David LaPlante [ Mon Oct 07, 2013 8:22 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
I have two Dynaco amplifiers that my dad made back in the 1960s (one in current use and one I keep as a spare). He had used them in a bi-amplified system that he had set up years ago..........they're great and nearly indestructable I think.

Best!
Author:  Michael.N. [ Mon Oct 07, 2013 9:12 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Oh No!! I used to build valve (that's the real name!) amps for fun. I finally cured myself after I'd paid over $300 for a turntable cartridge and then 3 days later looking at one that cost over $500. Understand that this was over 15 years ago.
I finally realised it was a bottomless pit!
Author:  James Orr [ Mon Oct 07, 2013 10:29 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Very cool! I'm looking forward to seeing it in it's enclosure. Sounds attractive.
Author:  Linus [ Mon Oct 07, 2013 10:49 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
I have recurring fantasies about McIntosh MC275s. All this gear list is related.
Author:  klooker [ Mon Oct 07, 2013 11:33 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Very cool.

Did the chassis & boards come in a kit? Diyaudio forum?

Kevin Looker
Author:  James Orr [ Mon Oct 07, 2013 1:05 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Filippo Morelli wrote:
By the way, to do an excellent job requires real work. This was not like wiring up an electric guitar :-). I probably had about 25 solid hours into the whole thing. Someone that does this stuff all the time would do it in well less time. I was very thorough in soldering clean joints and doubling back to check my work, which slowed me down but assured minimal headaches in a big bang kind of way.


Not to mention the time doing research.
Author:  nyazzip [ Mon Oct 07, 2013 1:23 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Quote:
This sucker puts out a fair bit of heat.

i put a "computer fan" with an aluminum heat sink on top of my power transformer which really cooled it down; it was running so hot i couldn't touch it....although the experts assured me it was normal i figure dumping some excess heat might help prolong the life of the other components in the chassis. of course the tubes have to be hot...
i built an st-35 kit, a clone of Dynaco's little brother to the 70, half the power
Author:  Bri [ Mon Oct 07, 2013 8:51 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Awesome! I have been researching the exact kit myself.
Love the look of the kt 88 over the el 84.
Did you ever look into fitting a selector switch for 2 or more inputs?
Why no wooden case?

Brian
Author:  dzsmith [ Wed Oct 09, 2013 6:13 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Thanks for the info Mr. Morelli, sweet looking build!
I'm looking for a tube amp kit.
I want an old-style amp without all of the digital modeling front end stuff.
Is the stepped attenuator on the input gain side, or the power side?
I'll want some way to distort the tubes but at low volume output.
I used two giant rheostats on the speaker on an old amp I had, but there's probably a better way.
I will definitely research this amp.
Author:  James Orr [ Wed Oct 09, 2013 7:47 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
dzsmith wrote:
I'll want some way to distort the tubes but at low volume output.


Filippo will correct me if I'm wrong, but I believe this is a hi-fi amp for home audio, which is designed to have very little distortion. If you're looking for overdrive at low volume, I'm presuming it's for guitar. I'd look for a classic Fender 5E3 kit, but even those are pretty loud.
Author:  nyazzip [ Thu Oct 10, 2013 12:11 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
...the attenuator/volume control(in a hi-fi amp) is the first thing the incoming signal goes through. stepped attenuators are very accurate, and very pricey, and at low volumes may not be able to "dial in" the perfect desired volume levels; a decent stereo pot(or two monos) will work pretty well too, unless you get a lemon, and do so for a good hundred(or more) bucks less
Author:  Michael.N. [ Thu Oct 10, 2013 5:46 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
I once built a single tube (power) amplifier using just an ECL 86. Not much power but it was a sweet little thing. The one I was most fond of was a Push/Pull 6V6 - or at least that one lasted the longest before it got scavenged for parts.
Author:  jfmckenna [ Thu Oct 10, 2013 8:56 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
That's a lovely looking think Filippo. Forgive my ignorance but what will this power a guitar? Stereo?
Author:  Greg B [ Thu Oct 10, 2013 1:55 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Welcome to your new addicting and time sucking hobby. ;)

A hot rodded ST70 is a great choice, and even if you move on to other projects later, it will always be an enjoyable amp. One tip: keep an open mind RE coupling caps. They are easy to swap out and make a huge difference. I've heard Sprague 716P outperform certain $25+ "tone fairy approved" PIO caps, and not by an insignificant margin. PIO vary a lot. Maybe those Russian PIO are great. Can't say myself - I've only used them for guitar tone controls. My main amp these days is transformer coupled, and thus I avoid the issue entirely.

Prepare to get sucked into high efficiency speakers. That's the usual next step, and it involves woodworking. Eat Drink I got into tube hifi and high efficiency speakers just after making my first DIY instruments, which was more than 20 years ago now. On the plus side, some of the acoustics knowledge involved is extremely helpful for guitar/instrument building. BTW, you don't really need super high efficiency with an ST70, unless you like to hear music over the power tools.
Author:  jfmckenna [ Thu Oct 10, 2013 2:17 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Ok so it's for listening to music, how do power a radio, CD, Turntable ect... to it?

I just moved into a new house and they left an old beat up record player in the basement with a bunch of records. It's a GE record player built into a closed cabinet and powered by a tube amp. I didn't think much of it but we decided to give it a try and wow O.o that little sucker sounds great!
Author:  nyazzip [ Thu Oct 10, 2013 3:57 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Shannon Parks who runs DiyTube has excellent amp and preamp boards/schematics/BOMs/and tons of free, no-BS advice. worth checking out. i used his ST-35 printed circuit board. preamp here:
http://www.diytube.com/phpBB2/viewtopic.php?f=18&t=4709
Author:  nyazzip [ Thu Oct 10, 2013 7:30 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
...are you sure you really need tone controls, or is it just habit...? it felt odd to me at first too, but if your amp+speakers are good, and you are listening to commercially produced/recorded/mixed material, it ought to sound fine straight up. i never even think about EQ anymore, been in the "tube club" about 5 years now with regards to hifi playback. speaker placement/room is quite important. for awhile i was a bit disappointed with my ST-35's bass response, but i moved in to a different place and now i have no complaints.
Dynaco did make preamps with tone controls also, the well regarded PAS 2 and 3(not sure if there was a "PAS 1").
i use Klipsch RF-52 speakers, nothing fancy by goldenear standards. i think they are rated at 96dB efficiency...
Author:  nyazzip [ Fri Oct 11, 2013 12:32 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
the forum members at Diytube will gladly walk you through anything, very helpful/friendly and extremely knowledgable group of people there. there is a section dedicated to the st-70, probably the most popular amp in the forum
Author:  ballbanjos [ Fri Oct 11, 2013 5:08 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
ST 70 is a good way to go if you have relatively inefficient speakers, and there's no doubt that the user community of builders out there is bigger than for probably any other DIY HiFi amp out there. I've built a couple, and they're good amps.

But, if you have efficient speakers--I'd say 95 or 96 db minimum, my horns are more like 106--let me recommend trying a direct heated, single end triode amp of some sort. I'm currently running one pair of amps with type 45 triodes and another with type 2A3. There are plenty of schematics out there for Western Electric or Loftin White based designs for 1 to 5 watt amps that, when coupled to the right speakers, will knock any PP amp out of the park IMO. No negative feedback in these amps, so dynamics (especially through good horns) is unbelievable. Although on paper the distortion figures are horrid, in real life, the single ended amp produces only even order distortion, which is not perceived as distortion by the human ear. PP on the other hand, produces both even and odd order harmonic distortion, and odd order sounds like distortion to us.

The biggest difference in the sound of the two is that where with big PP amps, as well as the high end transistor stuff, the focus has been primarily on making the recording sound like you are in the concert hall. With SE triodes, the sound is more like the musicians are in YOUR room. Different strokes--I prefer the second effect, but not everyone will.

From a DIY perspective, SE amps are much simpler, so can be much easier first projects. But, due to the very small number of parts, the quality of each part makes a huge impact on tone. Good parts typically = $$, just like with tonewood.

For musical instrument amps, I'd recommend trying a 5F1 Fender clone for starters. They are simple, cheap to build, easy to get parts for and sound great. They're also single ended, for what it's worth. You can add tone stacks if that's your thing, or leave it as is with nothing but a volume control. You can easily add/subtract negative feedback to taste and have 4 or 5 watts of real great sounding fun.

Dave
Author:  Greg B [ Fri Oct 11, 2013 12:55 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Did you test with speakers, or measure AC voltage at the outputs with a dummy load? It could be a speaker issue rather than an amp issue. I've heard some Klipsch from that era that were pretty bass limited. Yeah, posting on one of the audio forums should get you some good help.

AFA tone circuits, I usually bypass them in my vintage amps. Some are better than others. Why not try a PAS 3 or something? Many long time DIY'ers prefer active preamps to 'purist' passive volume controls. The debate rages on about that. I find it depends on your particular system.
Author:  nyazzip [ Fri Oct 11, 2013 2:13 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
impedance mismatch affects volume, but as far as the sound, i have never noticed. re: the iPod, while MP3s and MP4s aren't super ideal, again, the "correct"(studio-intended) tone ought to be there, except for ultra high frequencies. i'm thinking a wrong(too small) coupling capcitor value somewhere- the smaller the value, the less bass that gets through, up to about 25mfd if i recall correctly....anything larger than that should pass any low audible to the human ear
Author:  Greg B [ Fri Oct 11, 2013 2:18 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Yeah, could also be impedance tracking issue, which can do funny things to the freq response. Also, double check speaker phase. If one is backwards there'll be no bass. Make sure there are no cold solder joints.
Author:  ballbanjos [ Fri Oct 11, 2013 7:21 pm ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
Filippo Morelli wrote:
I was driving the speakers before with another amp and they sounded great. The current recommendations I have is to test with a CD player instead of an iPod and secondly to change over to a 4 ohm versus 8 ohm configuration on the amp. The KG4s are rated at 6 ohms.

Filippo

I have a pair of KG4s that I've driven with a ST-35 (the ST-70's EL84 driven little brother) wired at 8 ohms with very good results. I'd be checking connections/phasing etc. before messing around with output impedance. Source will make some difference, but from my experience unless you go analog, there isn't a night and day difference between input devices. Sure, CD sounds better than iPod and is probably better for testing, but not as much better as good analog sounds over ANY digital input. Coupling cap value will definitely have an impact on bass.

My KG4s sound best with a good transistor amp (Heresy!), but they sounded good with the ST-35 too. I'd bet there's something other than output impedance and input source going on here.

Dave
Author:  nyazzip [ Sat Oct 12, 2013 1:03 am ]
Post subject:  Re: Dynaco based STA-70 tube amplifier
if you do install a toggle to switch output impedances, i would check to see if it is kosher to do so during operation...not sure, but it doesn't sound like a good idea. rather, i'd wait when the amp is powered off to switch it. that load impedance is reflected back at the output tubes, so it might cause arc-over within the tubes, or maybe smoke the output tranny
Page 1 of 2 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/