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In Reply to: Dynaco 70 mods posted by janos on April 4, 2007 at 20:32:30:
I guess it all depends what you want from an ST-70. I am trying to make a good looking amp, a good sounding amp and a dead reliable amp. I am also trying to address virtually every problem the old ST-70 had and correct them. Lets face it - the only GOOD things about the old ST-70 were the output transformers and the Mullard tubes. Everything else was a compromise of sorts so that Dynaco could sell the amp in a kit form for $100 or so.The power transformer was underpowered and really couldn't deliver the goods when the amp was played loudly. The quad cap was undersized. The driver board with those 7199 tubes was a poor design and Dynaco used a huge amount of feedback (20 db). The board made the amp sound "mellow" - mid bass heavy and a rolled off top end. It was polite but not accurate. The open solder traces on that cheap phenolic board would crack with the heat and a channel might cut out. Lets face it - the amp was not designed to last 40-50 years - it was designed to hit a price point
I see people all the time buy an old unmolested ST-70 on Ebay for $350 or so BUT what are you really getting for your money? The only good thing on the amp are the A-470 output transformers because those great Mullard tubes have probably long since been replaced. Some of the rest of the parts are serviceable but 90 % of the amp is junk - brittle 22 guage wiring, loose tube sockets, bad caps, junk driver board etc.
I have tried to bring the ST-70 into the 21st century with a modern sound and the general feedback I am getting from my customers is all positive.
To increase long term durability of the chassis frame, I have recently switched over to a type 304 STAINLESS STEEL chassis and will be selling all future amps with the stainless steel chassis.
Check the link below for a more in depth look at the Dynaco ST-70 issues.
Bob Latino
Follow Ups:
Hi Bob,Checked out your link; those are very good prices with all the work and parts you throw in. It's very rare for me to see a stereo 70 with such nice chassis - on the island here everything rusts in a few years....
Indeed, the st70 was designed to be a super-tight budget amp, and has tons of faults, which need to be corrected. However, as it is, it has a very unique sound that many like. With mods that correct these faults, we get better sound, but with different character. However, considering the excellent opts, and a couple of the parts that can be reused, one can rebuild it with sane amount of money to an amp that you'd like.
I think there was never an amp that was so popular for tweaking, and rightly so. I have found that one of its (many) weakest links is the bias supply. The bias voltage of the EL34s is super critical, yet in the st70 it is very crudely filtered. Using a high voltage schottky there, with beefed up caps (CRC filter with 2000uF - R- 100 uF instead of the original 50 uFs), it will sound way better. I have done a lot of PSUD simultions for the bias supply, and the first cap value determines the ripple rejection by far. There are rapid gains up to 2000 uF or so, above that you have to go way higher (10.0000uF) to have noticeable improvement, but that size cap would not fit under the chassis. Increasing the second cap size above 100 uF hardly lowers the ripple at all. I try to keep the cap values as low as possible, as doubling the cap size slows the cap down to half speed: it will take twice as long to recharge after a charge drain. With 100 uF second cap, the sound is very fast, and powerful. With 2000uF second cap the slound is bloated and sluggish.
I was trying to change the filter to RCRC, but any kind of resistance before the first cap slowed the sound of the amp tremendously.
Cheap tweak, done with a few $, and makes a huge difference. :))Keep on the good work!
Janos,Watch the size of those caps, especially the second one. The bias supply might take longer to come up to full voltage than the warmup time of the output tubes, especially if you use solid state diodes on the HV supply.
Hello Kyle,Thank you for the warning. It takes 1sec to reach 75% of the bias voltage with the 100uF as the second cap. (2sec - 90%)
I'm using tube rectification, so it's not a problem, but 2 sec should be safe with ss HV. I don't know how much the bias can limit the sudden hv rush on the cold cathode... but I'd guess it would be protetive.
Janos,You are right that the original bias supply is not a tight circuit. With the VTA driver board that I use on the new amps that I build the bias supply is ON THE BOARD and not centered around the 7 lug terminal strip. The original bias supply and those 10K potentiometers didn't really give enough range and as the tubes wore they didn't have enough adjustability. On the old amp with the old bias system the trick was to RAISE the value of R1 and LOWER the value of R2. Joe has some bias expanding instructions on his web site but what I have found is that if I parallel a 27K ohm resistor with the 10K on R1 you get about 7300 ohms and if you parallel a 7.5 K resistor with the original 10K on R2 you get about 4285 ohms. With an "effective" 7300 ohms for R1 and 4285 ohms for R2 you get a wider bias range and can more easily bias tubes that have some time on them. This is a little more range than the instructions on Joe's web site but works well for me on a customer's amp especially if the selenium rectifier has been replaced with a diode.
In it's day it was a hell of a good amp, almost as good as the best for half the money. By today's standards it could be improved but way back then neither the parts nor the market for that kind of thing existed. I think you are way too hard on one of the great classics in audio.
Hahax,Your point is well taken - It is hard to judge by todays standards an amp that was designed with standards used in 1959. Back in 1959 the major signal source was vinyl which had a much lower dynamic range than the digital sources we now use today. The lower dynamic range of vinyl (40-50 db ??) did not stress the power supply on those old tube amps as a CD will do today with its 90+ db dynamic range.
But the fact remains that Dynaco did not design the Stereo 70 to be "the best sounding 35 watts per channel amp you can buy" they designed the ST-70 to be " the best sounding 35 watts per channel amp kit you can buy for $100" and in doing that they made compromises that affected its sound quality and long term reliability.
There's a lot to think about here.I just bought a sort of DIY amp (sold commercially) that sounds very good, but i am having a ton of problems because the construction was shoddy and the builder made many stupid mistakes (like the wrong tube sockets). It comes in two parts and weighs about 40 kilos. I am a little nervous about going down the DIY route again, but this one looked like a supurb value.
Why not buy a commercial 50 watter for $2000, say Rogue Cronus?
cost? DIY can cut it by as much as half.
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