looking to build Dynaco

[tony Ma]

Hi All,

I am looking to build dynaco ST70

Here is my plans
1. Buy this one Dynaco Dyna ST70 ST-70 NEW Amp Fully BOARD PC3 PC-3A (Dynaco Dyna ST70 ST-70 NEW Amp Fully BOARD PC3 PC-3A - eBay (item 290524489320 end time Mar-19-11 10:46:53 PDT))
2. Dynaco Dyna ST70 ST-70 NEW Amp Fully BOARD PC3 PC-3A (Upgrade Power Supply Capacitor Boards for Dynaco & Fisher Amplifiers)
3. buy the Transformer
4. Buy the tubes

Am I right??? sorry I am still new bee here.
I tried to save some money to build this thing up
Any suggestions?

 

[ArtG]

That certainly appears to be a fairly good price for an item that looks to be of good quality. You may want to consider a more modern driver board that should improve the quality of the sound. Triode offers these boards either "bare" or "ready to go", as do a number of other people, including at least one that is a contributor to this forum. If you use the search feature of this forum, typing in "Stereo 70", or "ST70", you will find a lot of information here.

 

[indianajo]

The first pcb has two sockets, indicating perhaps 7199, totally original. 7199's are like hen's teeth, neither triodeelectronics.com nor tubesandmore.com sell them. No russian plant makes 7199, neither does JJ. My original ST70 with the RCA 7199's from 1970, and new JJ output tubes, sounds honkier on middle octave piano than either my djoffe mod ST120 (transistor) or Peavey CS800s. I'm thinking of buying this pcb designed by kevinkr, Classic Valve Design - Dynaco Clone and Original Design Boards and Repair Kits - Dynaco ST-70 Modifications which uses three 12AX7's for less distortion. 12AX7's are in production at 3 or 4 factories. The triodeelectronics ST70 driver pcb uses three tubes, two of them EF86. There is at least one russian souce of new EF86 copies that is not ****, according to Eli Duttman although the 6pi "EF86" from winged C St Petersburg that triodeelectronics sells as their generic EF86 is " harsh" according to some users. True GEC 70's production EF86 were extremely linear, according to the "what tube is this" thread. The 6DH8 conversion of 7199, looking at the spec sheet, I don't think so.
Eli Duttman said this about EF86's on the triodeelectronics driver pcb. "Yes, St. Petersburg made EF86 equivalents are not good. However, you are not restricted to COSTLY NOS to get good sounding EF86s. New Sensor's Saratov, Russia, plant currently makes 2 good varieties: the EH labeled tube is good and not especially expensive. The TungSol labeled tube is not inexpensive, but still remains affordable, and it's very good.
FWIW, I'm fond of the TE replacement ST70 driver board populated with 2X EF86s and an ECC99, as it retains Dyna's small signal topology, while using better pentodes and triodes than those found in 2X 7199s." I think Mr Duttman means ElectroHarmonix" for EH, and triodeelectronics.com for TE.
For good discussion of the driver pcb's for triodes, search zigzagflux, and gordonw along with ST70 upgrade.
Your chinese driver pcb doesn't specify (on base sheet) what dielectric cap, and what technology resistors. The resistors over 100k should really be metal film since newark has them so cheap. I am pleased with the generic "multicomp" newark.com resistors I redid my PAS2 preamp with recently. Polyprophylene caps are supposed to be the best according to J Curl, although the polyethylene caps I used were improvement on 50 year old paper caps in the PAS2.
The power supply pcb you're looking at from triode has a lot of capacitance, more than the 5AR4 rectifier is designed for. The 40-20-20-20@520V FP can from either triode or tunesandmore (I forget which) is only $35. Or you can buy 40 and three 20 uf axials or radials in the 450 V version from newark or mouser, and put them under the chassis on phenolic terminal strips from tuibesandmore.com or electronicsurplus.com. About $12 for all. (I like surplus's price on teflon wire, also). 450 VAC caps won't last 30 years, but then FP's might not either, they don't publish expected hours data. I used sprague atomlytic 450 v axial caps in the ST70 from 1971 until 1982 and used them a lot of hours, wore out the output tubes with them. If you do use that much capacitance put a 75-100 ohm 3 W resistor between the rectifier cathode (8) and the choke input, to cut the inrush current and prevent arcover on non-US made tubes. Put it on a phenolic terminal strip. Some people install a solid state 1n4007 rectifier, but there is enough noise about "cathode stripping" pro and con, that I'm staying with the time delay 5AR4 rectifier. They are in production in russia and china.

 

[indianajo]

To answer Mr Ma's query about a cheap ST70,
tony Ma

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diyAudio Member

Join Date: Feb 2011

Can you pls direct me to the path building dynaco
Hi there ,

Can you pls direct me to build the DYNACO ST120?
I am in very limited budget and wants to try DIY. Bob Latino price is around $1100 it's kind of out range
Can you pls help me?
tks so much
Start with the triodeelectronics ST70 kit without tubes, $700. To delete the chassis, $119, buy a hammond M1,2,3 organ, $50, bolt two of those chassis together and salvage the 12ax7 tubes. rectifier tube is almost good enough, also, until you have more money. Sell the organ bench on CL, -$40, 6BQ5 tubes -$20, AO28 amp -$40/ buy triodeelectronics (TE) EC99 from JJ, $28. JJ EL34 matched quad from TE, $69. 2 ea tungsram EF86, $64. Shipping $10. Forget the EC99 and use a 12AX7 from the organ, delete $28. There may be better deals on the Electroharmonix EF86, look around as Ed Dutton says. You'll need to drill some holes, hand crank stanley drill is okay up to 1/4" dia, use 5/32" and 1/4" titanium bits from Irwin (the HS steel is ****) saw big holes with Stanley carbide rod hacksaw blade, $8 plus $3 chinese saw frame. Package of 6--32x3/8" screws, 6-32 elastic stop nuts, $4. 8 gang (saw up) 3AG Fuse clip $3, teflon wire for newbies $8, terminal strips in 5 term and 3 terminal varieties, $4, 5 terminal phenolic screw boards for output $2, electronicsurplus.com $10 shipping. WP35 weller iron, chisel tip for same, 68 ohm 5 W resistor $3,5 ea MDL3 fuses $5, through panel fuse holder $2, 1 lb 60/40 tin lead rosin core solder, $30, newark.com. $10 shipping. You'll need safety glasses for soldering, and a meter to determine voltages are below 25 V before touching anything, Sears $35. Also diagonal cutters, wire stripper (miller) needle nose pliers, slip joint pliers, 6 way screwdriver, rat tail file to wallow out 3/8" hole to 7/16" D for fuse holder, hardware store, $60. Buy american tools (except slip joint pliers indian okay,and 6 way screwdriver, taiwan ok) and keep them, Harbor Freight only has 3 good tools. (One is diamond dust hole enlarger bits for electric drill, $15) Read HV for newbies at top of tube amp thread before plugging anything in.

 

[indianajo]

From the TE $700 kit, you can delete the driver board -$60, delete the EC99 -$28, delete the EF86 pair -$64. Add the KTA triple triode driver board from classicvalve.ca Classic Valve Design - Dynaco Clone and Original Design Boards and Repair Kits - Dynaco ST-70 Modifications $28 plus 3 sockets $6 +$12 mail from canada, and use the 12AX7 tubes you salvage from the Hammond organ. Buy only an M or an L organ, T models are transistorized, also organs with names instead of letters are transistorized. The electrolytic capacitors will be dried up but any red caps of the right value can be salvaged, and any you cannot reuse flogged off on E-bay also. I find the polyester caps from Illinois cap sold by newark.com (made in Korea) sound fine in the PAS2 preamp, are pretty cheap, but are stupidly marked. Buy multicomp metal film resistors in the 2 W or bigger size from newark.com along with the iron to save a shipping charge. Get your cap and resistor list from the KTA parts list. Subtract off anything you find in the organ, only don't use old resistors over 47k, they are noisier in carbon comp (old) instead of metal film (new). Don't use any old capacitor with a + on one end or a minus on the other end, it is electrolytic.

 

[indianajo]

Oh, cheap ST`120 is only available used. Transformer condition unknown used. Without the djoffe PC15 bias modification, they have distortion at low levels. They also tend to cook the output transistors due to poor heatsinking. Djoffe is selling 2 chip amp PC's for the original chassis, not the PC15 with the bias modification. The chip amp he chose is LM3886 with insulated tab.

 

[indianajo]

Well, live and learn. The bob latino "ST120" is a tube amp with bigger transformers and KT88 output tubes instead of 6CA7's. No wonder it is $1010 plus shipping. tubes4hifi.com I can already drive myself out of the room on music with the ST70 with 101 db@1W Peavey SP2-XT speakers. Save your money, buy the TE kit with the KTA board for $500, spend $400 on some used Peavey SP2 speakers. The Dynakit ST120 was a transistor amp, tended to overheat, had a distortion problem, but almost never blew expensive speakers because it hat a 3300 uf cap in series with the speaker. Very stupid proof on speakers, very stupid on heat management. I run mine 18 hours a day now, but have 2 3" fans blowing right on the heatsinks at all times. The CS800s sounds great, better than ST70, has 3 levels of protection on the speakers to compensate for the danger of the split power supply, but my $200 bargain has blown it's circuit breaker, so I'm going to have to learn to fix switching power supplies - fun fun. The ST120 & ST70 require less thought.

 

[rmyauck]

Instead of ST-70 output transformers maybe try Dynaco Mark III (A-431) new production or old as they have a little more bandwidth at full power, the same primary impedence, and I would think a fair bit greater bandwith running them at 35 Watts instead of 60. It gives you the option for more power any way if wanted later.