DYNACO ST70 AMPLIFIER
 
HISTORY and MODS
  



AUDIOMODS

 
The original Dynaco ST-70 came with four 6CA7/EL34 power tubes, one 5AR4/GZ34 rectifier, and two 7199 preamp tubes. Sadly, production of the American 7199 ended in the 1980s while the Russian tube company Sovtek produced them until about 2007. NOS 7199s such as RCA, Sylvania, and Philips JAN (Joint Army Navy) versions remained available online up through the early 2000s. Today, with all supplies exhausted the solution is to substitute the more easily sourced 6U8A tube which some consider identical in sound quality. However, the 7199 was specifically designed as a low-noise HIFI tube while the 6U8A is a television/FM transmitter tube.


RCA APPLICATION NOTE AN-183
Applications of the RCA-7199 Triode-Pentode in High-Fidelity Audio Equipment

"The RCA-7199 is a 9-pin miniature type containing a medium-mu triode and a sharp-cutoff high-transconductance pentode, and having very low hum, leakage noise, and microphonism. It is specially designed and controlled for use in high-fidelity audio equipment in stages operating at signal levels in the order of 100 millivolts. The RCA-7199 utilizes a specially designed cage assembly and stem which make it much less susceptible than other triode-pentodes to micro-phonics. These features also provide a rugged tube structure and minimize leakage noise".
 

 
 

 

THE 1997 REBUILD
Below is a 1963 ST-70 I bought used in 1994 (modified in photo) as the 3rd owner. The 2nd owner had purchased it used in Alexandria, Va. on December 31, 1965 for $69.50. Instructions on the old invoice read “deliver after 3pm”. It sounds like it was intended for a New Years Eve party that night.

The amp had 1963 Dynaco 7199s, Mullard EL-34s, and a 1963 GE 5AR4 rectifier. It powered up fine and the tubes biased correctly. But when I saw the circuit board with ancient caps and grungy resistors I decided those parts had to go. Yet in 1994 no one was selling replacement 7199 boards. So I used an old-school method and traced and etched a new board. For modifications I used the easily sourced July 1982 issue of "Audio Basics" magazine which reveals some tricks to make the amp more linear and musical without altering the 7199 topology. One mod is to use a cap & resistor on the RCA input to filter out the extreme treble and extreme bass. This prevents the output transformers from becoming oversaturated with frequencies they can't handle, reducing distortion and increasing clarity in one step. It also recommends increasing the interstage coupling caps from .1uf to 1uf.

I selected Holco H4 resistors which were considered the best sonically in the 1990s. Four large polypropylene 1mf TRT Wondercaps and a yellow .047uf MIT PPMFX and 390/82pf mica rounded things out.

 

 
 

The original quad-cap worked but I was certain it was dried out after 33 years. However new quad caps sold in the mid 1990s were the wrong dimensions and didn't seem to offer any performance advantage. Then around 2003 I stumbled across the SDS Labs drop-in power supply. I immediately bought a bare board and selected my own parts: 82/150/150/100uf 400V caps and bypassed them with .1 polypropylene 400V caps underneath. The new power supply with the stock rectifier tube made a big difference. Less wooly, the sound took on a more focused character with a quieter soundstage. Dynamics were more punchy at higher volumes, especially the bass. Transients were more musical and accurate and popped out of the speakers. The best description is that the amp picked up the dynamics of a transistor amp while retaining the tubey smoothness of the old design. Another benefit is that I can detect no hum even with my ear close to the speaker.

NOTE: I replaced the power cord with a 3-prong but found that connecting a ground wire to the amp chassis created a ground loop and a hum from the speakers. Clipping the ground wire from the power cord removed the hum. I also installed a .01 film cap across the power switch to remove any snap at powerup. It has been said that only 5% of ST-70s were factory-wired the rest being kits. At some point in their production run Dynaco began supplying the factory-wired versions with a smaller manual (mine is a large 8x10), rivets instead of screws in places, spanner-screws on the bottom plate, a cage over the circuit board, and a sticker on the back. From what I have read these altered features didn't begin until about 1970 when Dynaco began calling the factory wired version the ST-70A (assembled). Mine is an original factory wired unit.

 
 

 
Even though my rebuilt ST-70 had been going strong since 1994 I decided the parts were getting long in the tooth. My plan was to maintain the 7199 circuit using the 1982 Audio Basic mods and simply upgrade the parts quality. STEP ONE was to get rid of my old handmade circuit board and replace it with a new 7199 version. STEP TWO was my favorite, the parts selection. I didn't want to use the same parts I see everyone else using, where's the fun of that? Lets get creative!

RESISTORS
I decided to use Takman REY metal films in all locations. Built specifically for sound quality they are well-regarded and I really like their look. I used REX carbon films and Dale CMFs in a couple of spots because Takman didn't make those values in REY.

FILM CAPACITORS
I have to admit I've never liked mica caps. I'm a parts snob and the shiny brown dipped look always seemed old-fashioned despite their good performance. While radial polypropylenes were available from Wima and Kemet I wanted to use axial film caps. After a search I was very pleased to find the perfect replacement: the Philips KP polypropylene. An axial epoxy-lacquered film/foil in cool blue I suspect they are produced by Vishay/BC Components. I was able to find 82 and 390pf in 1% online at low prices from overseas. The other stock film cap on the board is a .05uf which in the late 1950s would have been a 10% tolerance if lucky so I wasn't hung up on trying to locate an exact .05. Luckily in my parts box I had a pair of .047 630V Vishay MKP 1845s. They are rated 10% but my digital gauge showed them spot on at 47nf. Perfect.

For the tiny filter caps on the RCA jacks I went with Wima FKP2 film-foils in .022 and 1000pf 63V. Wima specifically markets the FKP2 for audio circuit sound quality.

For the four large coupling caps I went with a matched set of Russian K73-11 films. These caps are made with PETP (Polyethylene terephthalate also known as metallized polyester). Well known in the mod community these PETP caps are sonically well-regarded (I agree). I managed to find a set stamped with the coveted "OTK" Otdyel Technicheskovo Kontrolya (Bureau of Technical Control). This gives them a stamp of quality as well as an added mil-spec aura. Also printed on them is "СДЕЛАНО В СССР" which means "Made in USSR". Neat stuff.

WIRING
Underneath the chassis I tightened up the wiring and routed everything for lower noise. I had replaced much of the wiring years ago but still had original solid core wire in places. Not only can solid core can crack over the years but the quality of the copper wire used in 1963 was questionable. Tube amps have very little current in the circuit and don't require large gauge wire. So I decided on new Audience Auric 21g OFC wire in blue. Flexible, solders easily and fits the tube connectors properly.
 

 
 

 
DYNAKIT BBCU-1 BIAS CONTROL
Dynakit makes an interesting kit that replaces the coarse 15.6 ohm circuit and allows the bias for each tube to be dialed in individually. The concept is that while tubes get matched for plate voltage and transconductance each one may require slightly different voltages to bias properly. The kit locates a trimpot in the center of each preamp socket in the front, a great use of space. To use it you balance/zero out the voltage between tubes from the front trimpot, then set the master bias from the rear trimpot. I found the Bias Balance Kit did make an audible difference. Not sure what to expect I found that the soundstage was cleaner and more focused. Details seemed cleaner as though fuzz around the transients had been lifted. It had a sense of being "dialed in". I decided I didn't like the aluminum shafts peeking out of the center of each preamp socket so I used a black marker on each to mask them.
  

 

 
 

 
ABOVE & BELOW: the 2020 rebuild shows the new board with uncommon, superior quality parts. Note the new blue twisted-pair heater wires running between the EL34s. They carry 6.3V AC and their function is to heat the tube cathodes. While they don't touch the audio signal standard practice is that heater wiring be tightly wound to reduce hum. As you see from the interior this isn't a concours restoration. I enjoy upgrading vintage electronics while keeping the original rough edges.
 
   
   

 
TRANSFORMER MODS
Ever since I've owned the amp flipping the power switch has created a "bong" sound from the inrush of current, which is more obvious when the cover is on. I also noticed that if I lightly touched the tops of the 7199s I could feel a slight transformer hum vibrating through them. While not as worrisome on transistor equipment, microphonics on tube equipment is a HUGE deal. If the preamp tubes pick up hum it adds noise to the circuit, produces distortion, and wastes power even if it may not be audible. When used as a guitar amp it may not matter but for audio salon listening its not desirable.

To mod it I put rubber washers under the main transformer on all four screws. I also put tiny rubber washers between the main circuit board and chassis to isolate it. The result is no detectable hum when touching the 7199 tops, transformer bong is gone, and idle hum is nearly silent. A simple but effective fix. I replaced the transformer screws with stainless steel #8x32 2.5" versions, lock nuts, and new fiber washers. I decided to repaint the main transformer bell and found that semi-gloss black was the finish I liked. It matched the factory sheen better than satin in my opinion.
 

 
 

 

PREAMP TUBE COMPARISONS

RCA 7199
In the Fall of 2018 I stumbled across an ultra-rare pair of NOS RCA 7199s still in Dynaco boxes. Dating from 1968 they were found at the estate sale of a factory rep in the consumer electronics industry. They had been in storage for decades and with boxes labeled "Dynaco" were likely top-shelf RCA tubes. Pretty exciting stuff. I found them to have a dynamically textured musical sound. Treble is natural and cymbal sheen open and realistic. Bass is very good and soundstage deep.

DYNACO 7199
The age-old question is whether my original 1963 7199s contained some sonic magic from yesteryear. I found the 7199s had a slightly compressed soundstage. Acoustic guitar was clean but zips lacked the sparkling edge that make it sound live. Music was pleasant but soft and some detail seemed lost, I felt like the treble had been turned down slightly. Could be they're just old.

SOVTEK 7199
I used these for years. Very quiet, dynamic and open, details had a nice pop, great for acoustic guitar. It has been claimed that the Sovtek 7199 is actually an internally repinned 6U8A. But I squinted into both and the RCA and Sovtek 7199 and the cages and pinouts look identical to me. So is the Sovtek electrically a true 7199? Good question.

RECTIFIER COMPARISONS
1963 GE 5AR4 vs SOVTEK 5AR4
 My original GE 5AR4 dates to 1963 and supplies 425V. My Sovtek supplies 423V. In my mind this makes them pretty much identical but I prefer my 61 year old GE because of its legacy feel.

POWER TUBE COMPARISONS
As we know, the original EL34 (UK version) was identical to the 6CA7 (US version) which is why they have been direct substitutes over the decades. Both are power pentode tubes with 5 active electrodes and three grids.

ELECTRO-HARMONIX EL34
EH tubes are made in the Reflektor plant in Saratov, Russia. I have a matched quad of their EL34s in my ST70 and I like the look and feel of them. They sound articulate and clean and bias is rock solid. General consensus is that EH are best for sound quality rather than emphasizing breakup or distortion levels for guitar applications.

ELECTRO-HARMONIX 6CA7 "Big Bottle"
 The 6CA7EH is not a high-output version of the 6CA7. Its actually a different tube: a big-bottle tetrode with four electrodes and two grids. This is not worrisome as the 6CA7EH is a plug-in replacement for the EL34/6CA7 and has good reviews. Oddly, a key feature is stated as "high plate current" with little explanation. The 6CA7EH specs its plate current as 100 ± 20ma while a vintage 6CA7s is listed as 100-120ma. So where does this high plate current exist? Looking at the other voltage specs this tube doesn't appear electrically different than a standard 6CA7.

HOWEVER, on startup I found that my 1963 10K bias trimpots couldn't provide enough range and could only bring the bias down to about 57ma. A vital clue was found on a reputable tube vendor's site: "These are good for those who work on their own equipment and will make resistor adjustments in the bias circuit to bring them into the appropriate bias range, or those who already have amplifiers with a very wide range of bias available.

So yes the 6CA7EH draws more plate current than a generic EL34/6CA7. On guitar amps with fixed bias this means you're going to need to redesign the bias circuit or the tubes will run hot. And if you have adjustable bias your pots may not have enough range to bias them. My solution was to buy a pair of Uxcell 20K potentiometers to replace my stock pots. This let me easily able to bring their bias down to .50. I found the big bottles to have a musically dynamic, open sound. Treble was clean and extended without any etching. For critical listening I decided I like my standard Electro Harmonix EL34s better. I find them more detailed in transients, vocals are cleaner and centered more tightly in the soundstage. Personal preference of course, its a lot of fun doing comparisons.


  
 
 

 
1963 ST70 with 6CA7EHs, NOS RCA 7199s, 20K bias pots, modern board & parts, and OFC wiring. I find the amp to be musical, quiet, and articulate in my listening room. Acoustic guitar, vocals and live music are surprisingly open and realistic. Retaining and enhancing the 7199 topology was highly successful in my opinion.
 

 

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