brosenau said:
I got me an Aztec -- and an ST-70 -- nuttin' cheaper than a twin or a boat these days.
Cheers!
Its worse than Heroin.
Picture of latest amp.
Unit was a recent purchase off eblay with a GSI board. The GSI board was pulled and I put this Mapletree driver board on today.
Only thing left to do is put a SDS cap board on it. With 6SJ7 input tube and 6SU7 long tail it sounds quite good to my old ears.😀
I like the Chrome chassis
Picture of latest amp.
Unit was a recent purchase off eblay with a GSI board. The GSI board was pulled and I put this Mapletree driver board on today.
Only thing left to do is put a SDS cap board on it. With 6SJ7 input tube and 6SU7 long tail it sounds quite good to my old ears.😀
I like the Chrome chassis
Attachments
I found my last ST-70 in a dumpster about 2 years ago. It's a completely un-gomered unit. The accompanying PAS had a bad line cord and all the tubes were either weak or shot -- so it had some surgery which included a new RIAA section.
A Major Award indeed...along with that Leggy Lamp!
I wish I could get lucky enough to find a treasure like that - it would be like hitting the lottery -
I built mine from indivual parts that I bought off of E-bay. First, it was the chassis, then the PC-3 (original) board, then the power transformer, the EL-34s, the driver tubes with adapters, then finally the piece de'la resistance' - the output transformers.
That was a mere three months ago. Then I bought a new can to start upgrades...and the rest is history.
Now, I'm looking for those cool orange-drop capacitors... None around here... Any clue as to who has them at the best price?
Brian
I wish I could get lucky enough to find a treasure like that - it would be like hitting the lottery -
I built mine from indivual parts that I bought off of E-bay. First, it was the chassis, then the PC-3 (original) board, then the power transformer, the EL-34s, the driver tubes with adapters, then finally the piece de'la resistance' - the output transformers.
That was a mere three months ago. Then I bought a new can to start upgrades...and the rest is history.
Now, I'm looking for those cool orange-drop capacitors... None around here... Any clue as to who has them at the best price?
Brian
A quicky post - I just E-bay scored some orangedrop caps for a few bucks. No need to respond to previous request on those items.
Everyone have a great weekend!
Brian
Everyone have a great weekend!
Brian
Seems with tube amps, the older and dirtier the better they sound when finally resurrected...🙂
cheers,
Douglas
cheers,
Douglas
Feedback tap
Any advantage in taking the feedback off the 4 ohm tap for example if one has 4 ohm speakers hooked up verses the 16 ohm tap?
If so how does one calculate the difference?
Any advantage in taking the feedback off the 4 ohm tap for example if one has 4 ohm speakers hooked up verses the 16 ohm tap?
If so how does one calculate the difference?
Well, I think you would need to restate your question slightly. Are you just talking about removing the other output taps or just connecting, say, a 4 ohm speaker up to a tap other than the 4 ohm?
If you connect a specific load (4 ohms for example) up to the 16 ohm tap, the impedance mismatch would be noticable in the sound output. I believe it would ultimately affect the output linearity curve of the output tube as well.
Am I way off course here, or are you just talking about disconnecting the tap all together...?
Brian
If you connect a specific load (4 ohms for example) up to the 16 ohm tap, the impedance mismatch would be noticable in the sound output. I believe it would ultimately affect the output linearity curve of the output tube as well.
Am I way off course here, or are you just talking about disconnecting the tap all together...?
Brian
Domino Cap on the PC-3
Jim (& gang), what's the specific characteristic of the .390pf cap on the PC-3 that looks like a domino? Why would the developers choose that particular style?
Could it be replaced by a 716P Orange with the same rating without doing any harm to the sound characteristics?
Just curious...
Brian
Jim (& gang), what's the specific characteristic of the .390pf cap on the PC-3 that looks like a domino? Why would the developers choose that particular style?
Could it be replaced by a 716P Orange with the same rating without doing any harm to the sound characteristics?
Just curious...
Brian
That's a mica capacitor and must be a 1000 volt cap. It has over 400 VDC on it plus a few hundred volts of signal. I don't think "Orange Drops" are made in the pf range so that option is out. If it is still good use it or get a new silvered mica from Mouser or Allied, they also have the 82pf if you are sticking with the stock circuit. The 390pf is part of the feedback network and probably should be left alone with the stock circuit.
Craig
Craig
I was wondering since I have only 4 ohms speakers that I drive with my Dynaco would there be any advantage to soldering the feedback line to the 4 ohm tap? I have been told there would be an advantage but I do not see how since the feedback would end up being a the same if taken off the 4 ohm tap with different sized resistor and cap.
I realize the resistor and cap size would change if the feedback were off the 4 ohm tap instead of the 16 ohm tap.
I realize the resistor and cap size would change if the feedback were off the 4 ohm tap instead of the 16 ohm tap.
burnedfingers said:
Hold on there... The feedback line comes off the primary side of the output transformer...and has a purpose all of its own. The segmented taps (4, 8, & 16 ohms) all come off the other side - the output side - of the output transformers. Don't link the two sides together.......that would be disasterous.
Now, if I have not clearly understood what you are identifying as a "feedback line" then I still don't understand. My guess is that you connected the 4 ohm speaker positive terminal to the 4 ohm tap on the output side of the tranny (as designed) and you connected the negative side of the speaker to chassis ground (the black lug). Is this correct? If so, no change is needed. If not, then I'm still confused...
Brian
Look at the schematic
You will see the feedback taken off the 16 ohm tap on the secondary side of the transformer.
My question is.... If I re-calculated the value of the feedback resistor which is 1K in the case of the Dynaco Stereo 70 driver board to a value of 500 ohms and re-soldered the feedback line to the 4 ohm tap would this be more correct than using the 16 ohm tap? I have 4 ohm speakers.
I also have a different amplifier which has a resistor and cap in the feedback loop and both would have to be re-sized.
Sorry for your misunderstanding as I did not mention anything about changing a thing on the primary side of the transformer.
I hope that you can see where I am coming from now.🙁
You will see the feedback taken off the 16 ohm tap on the secondary side of the transformer.
My question is.... If I re-calculated the value of the feedback resistor which is 1K in the case of the Dynaco Stereo 70 driver board to a value of 500 ohms and re-soldered the feedback line to the 4 ohm tap would this be more correct than using the 16 ohm tap? I have 4 ohm speakers.
I also have a different amplifier which has a resistor and cap in the feedback loop and both would have to be re-sized.
Sorry for your misunderstanding as I did not mention anything about changing a thing on the primary side of the transformer.
I hope that you can see where I am coming from now.🙁
Attachments
The stock ST70 has two feedback paths, one from the primary (UL tap) and one from the secondary (16 Ohm tap). What Burnedfingers is asking is if he moves the feedback from 16 to 4 what size resistor to use to get the same amount of feedback as the stock circuit. The signal on the 4 Ohm tap is much lower in level that the 16 Ohm tap, so a smaller resistor is in order. The whole idea of this is to get the speaker connected directly to the feedback. This may or may not sound better, but no harm in trying. I would get a variable resistor, adjust it until the output is the same (Watts wise) as the 16 Ohm, 1KOhm combo. Then read the resistance and replace with a resistor that is close.
Craig
Craig
In theory, it's better, but in practice, I haven't found a difference. Anyway, resizing is easy- the 4 ohm tap is actually in the center of the winding, so just cut the resistor in half. I think it was 1k, so use a 500R (470R or 510R will be fine, as will be two 1k in parallel).
New to ST-70
Hi Folks - I used to design, build and sell amps for a living many moons ago. Now I'd like to try my hand at building a tube amp - after 25 years!! I want to build an excellent sounding amp, at reasonable cost. The one that looks interesting is TriodeElectronics ST-70 Tube Amp Kit - retails for ~$600 including tubes. The website is http://store.triodestore.com/st70buildkit.html.
What is your view on if I should build the ST-70 and if so, is this kit the one you would also choose? If not then which one?
Thanks a bunch!
NM
Hi Folks - I used to design, build and sell amps for a living many moons ago. Now I'd like to try my hand at building a tube amp - after 25 years!! I want to build an excellent sounding amp, at reasonable cost. The one that looks interesting is TriodeElectronics ST-70 Tube Amp Kit - retails for ~$600 including tubes. The website is http://store.triodestore.com/st70buildkit.html.
What is your view on if I should build the ST-70 and if so, is this kit the one you would also choose? If not then which one?
Thanks a bunch!
NM
I looked on their site and the information says
You may want to reconsider and look for a used one to play with.
You may want to reconsider and look for a used one to play with.
hey-Hey!!!,
I'd get an original and restore it, then on to mods. That $600 price tag will cover an original and lots of interesting stuff later on as you're more skilled.
cheers,
Douglas
I'd get an original and restore it, then on to mods. That $600 price tag will cover an original and lots of interesting stuff later on as you're more skilled.
cheers,
Douglas
There is a large upside to a new kit - the improved power transformer.
The downside is a less than studly driver circuit, and the reliance on a rectifier that is undersized, and nowhere near the quality of the old Mullard's.
The downside is a less than studly driver circuit, and the reliance on a rectifier that is undersized, and nowhere near the quality of the old Mullard's.
Re: New to ST-70
NM, you know, I bought my ST-70 in pieces all over E-bay. First, I bought the chassis, then the PC-3 board, tubes, power trans, and the output trannies...pieces at a time. I had a total blast researching all of the DIY forums and putting it together that way. When I finally plugged it in...it didn't work. A day later, I looked at the schematic and the drawing and discovered that I missed a wire on the bias P/S lugs. Then it fired right up. I belive I purchased all of the parts I needed for less than $500 over a 45 day period. If you don't have the patience, then by all means, buy the kit. If you have the time, take a slower approach, save some money, and spread your joy out over a couple of weeks.
Now, I've been busy tweaking and modifying all I can...until I break it again. Well, it's all worth the time...and the ST-70 is a pretty nice little amp. It's not the best, but it's very popular and you can find parts for it easily. That's my 2 cents worth...
Brian
nmehta0 said:
NM, you know, I bought my ST-70 in pieces all over E-bay. First, I bought the chassis, then the PC-3 board, tubes, power trans, and the output trannies...pieces at a time. I had a total blast researching all of the DIY forums and putting it together that way. When I finally plugged it in...it didn't work. A day later, I looked at the schematic and the drawing and discovered that I missed a wire on the bias P/S lugs. Then it fired right up. I belive I purchased all of the parts I needed for less than $500 over a 45 day period. If you don't have the patience, then by all means, buy the kit. If you have the time, take a slower approach, save some money, and spread your joy out over a couple of weeks.
Now, I've been busy tweaking and modifying all I can...until I break it again. Well, it's all worth the time...and the ST-70 is a pretty nice little amp. It's not the best, but it's very popular and you can find parts for it easily. That's my 2 cents worth...
Brian
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