Hi,
I am new to the hobby and in the process of trying out and learning about various amps. So far I have tried a few kits (gainclone and class D). Now it is tube time
By coincidence I came across this Dynaco ST 70 kit:
Triode USA Dynaco ST-70 Tube DIY Amp Kit
I want to try it with various speakers, DIY Fostex FX120, DIY Jordan JX92s, DIY Wild Burro Betsy, commercial Vroemen Diva superiore. Frontend will be Bottlehead Quickie, 47 Treasure CD and vinyl.
What are your thoughts and recommendations about this? Any input is highly appreciated.
Thanks
Michael
___________________
Michael's Project Page
I am new to the hobby and in the process of trying out and learning about various amps. So far I have tried a few kits (gainclone and class D). Now it is tube time
By coincidence I came across this Dynaco ST 70 kit:
Triode USA Dynaco ST-70 Tube DIY Amp Kit
I want to try it with various speakers, DIY Fostex FX120, DIY Jordan JX92s, DIY Wild Burro Betsy, commercial Vroemen Diva superiore. Frontend will be Bottlehead Quickie, 47 Treasure CD and vinyl.
What are your thoughts and recommendations about this? Any input is highly appreciated.
Thanks
Michael
___________________
Michael's Project Page
I've got an original dynaco ST70 with the 7199 driver tubes. 1961 build with the fabric transformer insulation. It is a rugged beast with tens of thousands of hours on it, and decent sounding for 1961. I recently replaced the electrolytic caps (again, 3rd time) and the output tubes (third time) and it is back up to 35 W/ch and sounding original on my Peavey SP2-XT speakers. I've recently beaten the quallity of sound of the ST70 with 7199 drivers with a transistor amp modified for better cold bias current control, as does my road weary CS800s, so the 1% distortion Dynaco claimed for their circuit is about right. The triodeelectronics kit uses EF86 copy driver tubes, which might sound better than the unavailable 7199. The original design transformer had extra filament capacity to run two outboard two tube preamps, so the design has plenty of capability to run 4 driver tubes instead of two. The design has lots of aftermarket support, for example kevinkr has designed a 4 tube triode driver board sold by classicvalve.ca that he claims has a lot less distortion. My ST70 has been modified to derive the grid bias voltage from silicon rectifiers, not the original selenium one. I find the aftermarket high capacity cap boards a bit silly, as on classical music in my living room I'm running about 1.5 Vpp output max, so the semi-original power supply with 39 22 22 22 filter caps produces B+ just fine with no sags. This stays within the parameters of the 5AR4 rectifier tube, which I am still using due to the rumors about "cathode stripping" with silicon instant on B+ voltage. I've used the ST70 as a PA amp in a church occasionally, but didn't try to derive the high power out of it that you would want out of a bass guitar amp, for example. 35 W/ch works out to 17 Vac on 8 ohm, so the ST70 has plenty of power with fresh tubes and B+ cap for my 101 db@1W speakers. It was fine with my previous speakers, Peavey T-300 HF projectors, and KLH23 two ways. If you try to use acoustic suspension speakers like the old AR1's, the power out might be a little light for drum or cannon hits. I found over the years that if the ST70 sounded polite instead of real, the B+ voltage was sagging due to tired main caps, or the rectifier or output tubes were tired. I got about 8 years at 4 hr/night on the 5AR4, about 10 years on sets of 6CA7's, about 10 years per B+ cap.
One huge advantage of the tube design, mine has been struck by a lightning induced surge and only shorted across the power switch, and burned up the .01 uf turn off pop eliminator cap installed by me. No damage to the transformers. Try that with transistors without MOS supressors on the input. With the weather changing to be more extreme, you may soon have significant lightning storms in Europe.
I've bought parts form triodeelectronics.com, and find them helpful and nice to deal with. When they shipped me a mono volume pot instead of the stereo I ordered, they shipped the replacement to me at no charge.
So anyway, have fun.
One huge advantage of the tube design, mine has been struck by a lightning induced surge and only shorted across the power switch, and burned up the .01 uf turn off pop eliminator cap installed by me. No damage to the transformers. Try that with transistors without MOS supressors on the input. With the weather changing to be more extreme, you may soon have significant lightning storms in Europe.
I've bought parts form triodeelectronics.com, and find them helpful and nice to deal with. When they shipped me a mono volume pot instead of the stereo I ordered, they shipped the replacement to me at no charge.
So anyway, have fun.
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Do you have any goals in mind as to what sound you're after? You can't go wrong with a Dynaco ST70 or its variants as they are good amps. Personally, I prefer the EL84-based PP amps, and single-ended triode amps based on 45, 2A3, or 300B. They are very transparent and musically involving. Just giving you some more ideas.
Strap the outputs for triode.
Add more capacitance to the second main B+ filter, like 4x or more.
Reduce the cathode resistors by a factor of 10x. (you need a DVM to set the bias, it will be 10x lower than the stock value, put a sticker on the chassis to show that. This will improve the bass.
You might consider buying the parts and getting an "aftermarket" driver board that uses something OTHER than the 7199 or the Russian equivalent for the input stage. That is to say the kit without the driver board...
These are the basic things that can be done to improve the basic amp.
I'd go for the Teflon wire option, IF you can figure out how to strip it without nicking the wire and/or cutting it accidentally (unless they are precut and stripped, doubt that).
Add more capacitance to the second main B+ filter, like 4x or more.
Reduce the cathode resistors by a factor of 10x. (you need a DVM to set the bias, it will be 10x lower than the stock value, put a sticker on the chassis to show that. This will improve the bass.
You might consider buying the parts and getting an "aftermarket" driver board that uses something OTHER than the 7199 or the Russian equivalent for the input stage. That is to say the kit without the driver board...
These are the basic things that can be done to improve the basic amp.
I'd go for the Teflon wire option, IF you can figure out how to strip it without nicking the wire and/or cutting it accidentally (unless they are precut and stripped, doubt that).
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This kit sounds like fun for more experienced people who like to try many variations. A good learning tool for a newbie. I use an EF86 in a guitar amp I built, and the measured gain is about 240, which is way more than I needed. In this amp you would need to either attenuate it's output or use rather large amounts of negative feedback. More feedback isn't necessarily better sounding in tube amps.
I researched, designed and built a similar amp using 6SN7's for the front end and phase splitter, (EL34 outputs) and use only about 12dB of negative feedback, and it sounds great. The hard core tube guys I know all seem to think low or no feedback, and triode mode output stage is the best sounding. I tend to agree. Triode mode output means lower source impedance driving the speaker, so tighter control of the speaker, and less un-calibration of a passive crossover, when feedback in minimal. More feedback makes the issue of phase margin harder to predict (tendency to oscillate or ring). Some people say high feedback makes an amp sound "constipated"... Check out my amp (the "musicbox") if interested at
Bob's Website
http://www.spiritone.com/~rob_369
My front end tube is the earlier equivalent of the 6SN7 (I forget the number). I thought the older tube shape with the grid cap was cool looking, so used it instead. Although not shown on my website, I since replaced that front end tube with a regular 6SN7 to reduce hum, and replaced the plate resostor with a constant current source for better power supply hum rejection, better linearity, and more gain. Don't bother with silver wire or Teflon insulation wire. Speaker wire that costs more than $50 is a joke too. Use 14 or 16 AWG wire for the speakers. It's the connectors that could be a weak link. I use gold alloy plated banana connectors (gold doesn't oxidize over time), and I solder the wires to these connectors rather than using the set screw that comes loose over time.
I researched, designed and built a similar amp using 6SN7's for the front end and phase splitter, (EL34 outputs) and use only about 12dB of negative feedback, and it sounds great. The hard core tube guys I know all seem to think low or no feedback, and triode mode output stage is the best sounding. I tend to agree. Triode mode output means lower source impedance driving the speaker, so tighter control of the speaker, and less un-calibration of a passive crossover, when feedback in minimal. More feedback makes the issue of phase margin harder to predict (tendency to oscillate or ring). Some people say high feedback makes an amp sound "constipated"... Check out my amp (the "musicbox") if interested at
Bob's Website
http://www.spiritone.com/~rob_369
My front end tube is the earlier equivalent of the 6SN7 (I forget the number). I thought the older tube shape with the grid cap was cool looking, so used it instead. Although not shown on my website, I since replaced that front end tube with a regular 6SN7 to reduce hum, and replaced the plate resostor with a constant current source for better power supply hum rejection, better linearity, and more gain. Don't bother with silver wire or Teflon insulation wire. Speaker wire that costs more than $50 is a joke too. Use 14 or 16 AWG wire for the speakers. It's the connectors that could be a weak link. I use gold alloy plated banana connectors (gold doesn't oxidize over time), and I solder the wires to these connectors rather than using the set screw that comes loose over time.
Triode doesn't sell a 7199 driver board, because nobody makes them anymore. They sell a pentode board with the EF86 layout. Others sell straight triode triode driver boards. I bought an "EF86" from triode for my organ, it was actually a Russian 6 pi something tube.
Teflon insulated wire is great for newbies, keeps you from burning the insulation off when you move your eyes to the print or something and wave the soldering iron around. I'm not that new, but used a some 600V electrical tape in my H100 organ recap to repair damage to PVC insulation. I'm buying bulk teflon insulated wire from a suplus house, much cheaper than new silver plated stuff. Not legal in Euorpe, though.
I find the bass of my stock unit entirely adequate, actually. There are a million variations, the one I like the most adds two pots and splits the bias adjustment for the push-pull tubes and allows you to set them balanced again after they wear some, unevenly. Another thing I have done is run the B+ to a blank pin of the preamp power plug so I can monitor B+ while it is playing without taking it apart and tipping it on its back. That is now I know I don't have a B+ problem now at the listening levels I am using with 39-22-22-22 caps. Even when the cannon goes off in 1812 overture.
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Some comments, If I may. The Triode Electronics driver board gets populated with 2X EF86s and a single twin triode. That twin triode may be an ECC99, 12BH7, or 12AU7. I advise installing an ECC99, for its high gm. High gm in designs employing GNFB, like the ST70, provides resistance against slew limiting associated with the HF error correction signal.
As the ST70 uses "fixed" bias and an instant on, SS rectified, C- supply, even the slightest delay in B+ rise is sufficient to protect the O/P tubes against cathode stripping. A NTC inrush current limiting thermistor provides the short delay needed, if either SS diodes or a directly heated vacuum rectifier are/is employed in the B+ circuitry.
IMO, it's perfectly acceptable to SS rectify ST70 B+. You turn the excessively "tall" B+ rail from a liability into an asset, by employing combination bias. "Stand" the cathodes of each O/P tube pair on a 100 Ω/470 μF. network. A very convenient "idle" current test point is available and the original single bias adjustment for each channel is quite satisfactory. Matching requirements are reasonably relaxed. As is always the case, closeness in gm is a must for each pair. However, small differences in cathode current are acceptable.
An additional benefit of SS rectifying the B+ is the cooler running power trafo obtained by holding the value of the 1st filter cap. down to say 10 μF. Replace the overworked C354 choke with a Hammond 159T.
As the ST70 uses "fixed" bias and an instant on, SS rectified, C- supply, even the slightest delay in B+ rise is sufficient to protect the O/P tubes against cathode stripping. A NTC inrush current limiting thermistor provides the short delay needed, if either SS diodes or a directly heated vacuum rectifier are/is employed in the B+ circuitry.
IMO, it's perfectly acceptable to SS rectify ST70 B+. You turn the excessively "tall" B+ rail from a liability into an asset, by employing combination bias. "Stand" the cathodes of each O/P tube pair on a 100 Ω/470 μF. network. A very convenient "idle" current test point is available and the original single bias adjustment for each channel is quite satisfactory. Matching requirements are reasonably relaxed. As is always the case, closeness in gm is a must for each pair. However, small differences in cathode current are acceptable.
An additional benefit of SS rectifying the B+ is the cooler running power trafo obtained by holding the value of the 1st filter cap. down to say 10 μF. Replace the overworked C354 choke with a Hammond 159T.
Hi Eli -- I'm interested in adding those thermistors to my mutant amp in an ST70 chassis. I'm using SS diode rectification, getting about +460V. What specs to look for in the NTC thermistors? What resistance spec? Small in value, like 100R or less? Or more like 1k? Thanks for the idea.
Oh, and where do they go in the circuit for slow ramp up of B+? Perhaps in the center tap of the power transformer?
Oh, and where do they go in the circuit for slow ramp up of B+? Perhaps in the center tap of the power transformer?
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A single thermistor goes in the line between the diodes and the PSU filter. With a 5R4, I suggest a CL90. There's no reason to believe that selection will fare poorly in combination with SS diodes. If the part doesn't heat up enough to bring the cold resistance down a lot, you can try wrapping it in wool or acrylic yarn and encapsulating with heat shrink.
The data sheet for the NTC inrush current limiting thermistors is here.
The data sheet for the NTC inrush current limiting thermistors is here.
Michael
You have a ton of good stuff if you look around. I jump started with some Dynaco projects and all have had great results.
If you are looking for more of the original Dynaco start with this -
Welcome to DYNAKITPARTS.com
They have a good variety of dynaco kits and are pretty true to form. I bought a couple of trashed Dynaco MKIII and basically chucked out everything except the transformers, power cords and metal covers. All the rest was essentially the MKIII kit from them. I upgraded the driver which I got from Triode electronics.
The second path is Triodes kits, never built one but looks pretty good.
The last one is the Tubes4HiFi (Bob Latino). He has a few different amps, but the one that I really like is the ST70 well the ST120 version of it. The kit is well packaged with everything down to the wire for assembly and has a very good set of build instructions. It's my second favorite amp that I have and it's very powerful all in a compact Dynaco ST70 box. Has a ton of options too.
tubes4hifi home page
The last one to check out is the OddWatt series, I have not built one but they look like a nice package, don't have the link but you should be able to find one.
As a side not I had only thought I would build the ST120 and be done. 6 Monoblocks later... You get the idea, yes another pair in the works, a quad KT88/KT120 just to round out the collection.
Enjoy the addiction!
Sandy
You have a ton of good stuff if you look around. I jump started with some Dynaco projects and all have had great results.
If you are looking for more of the original Dynaco start with this -
Welcome to DYNAKITPARTS.com
They have a good variety of dynaco kits and are pretty true to form. I bought a couple of trashed Dynaco MKIII and basically chucked out everything except the transformers, power cords and metal covers. All the rest was essentially the MKIII kit from them. I upgraded the driver which I got from Triode electronics.
The second path is Triodes kits, never built one but looks pretty good.
The last one is the Tubes4HiFi (Bob Latino). He has a few different amps, but the one that I really like is the ST70 well the ST120 version of it. The kit is well packaged with everything down to the wire for assembly and has a very good set of build instructions. It's my second favorite amp that I have and it's very powerful all in a compact Dynaco ST70 box. Has a ton of options too.
tubes4hifi home page
The last one to check out is the OddWatt series, I have not built one but they look like a nice package, don't have the link but you should be able to find one.
As a side not I had only thought I would build the ST120 and be done. 6 Monoblocks later... You get the idea, yes another pair in the works, a quad KT88/KT120 just to round out the collection
.Enjoy the addiction!
Sandy
I have an ST-70, using the Classicvalve octal board, with their psu board and SS diodes. I have a C90 in the AC line before the transformer, which seems to be the common place to put it. It works well, but allow clearance because it does get quite hot.
Gents,
Thank you for your great input and feedback. I must admit that I am out of my depth as to all the detailed technical advise, which you provided.
I guess I will start off with one of the standard kits from either Tubes4hifi or the Triodes one. Once the stock kit has been built I will proceed applying the suggested improvements.
I will post my progress here in the forum.
Thanks a lot folks.
Greetings from Switzerland
Michael
Thank you for your great input and feedback. I must admit that I am out of my depth as to all the detailed technical advise, which you provided.
I guess I will start off with one of the standard kits from either Tubes4hifi or the Triodes one. Once the stock kit has been built I will proceed applying the suggested improvements.
I will post my progress here in the forum.
Thanks a lot folks.
Greetings from Switzerland
Michael
Just to briefly report back: the kit arrived yesterday safely here in Switzerland. Sales process, payment and shipment went flawlessly. Bob Latino of tubes4hifi is very helpful and responsive. I shall post pictures, once I completed the kit.
Thanks again
Michael
Michael's Project Page
Thanks again
Michael
Michael's Project Page
The kit is extremely nice. Every part is individually packed ina zipper bag and clearly labeled. The manual is good as well.
Michael
Michael's Project Page
Michael
Michael's Project Page
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