Stereo 70

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Stereo 70

Postby FRE » Sat Jan 03, 2009 12:00 am

Way back in about 1962 I bought the Dynakit Stereo 70 amplifier in kit form. I used it 'til in about 1974 I replaced it with the Dynakit solid state equivalent. I also had the Dynakit FM tuner and pre-amplifier.

The only 2 changes I ever made to the Stereo 70 were to replace the vacuum tube rectifier with a plug-in solid state rectifier and add a device called a surgister; probably they are unknown now. The reason I replaced the rectifier tube was to reduce heat output, but it probably slightly increased the power output also. The surgister was connected in series with the primary of the power transformer. It added resistance to the primary for a few seconds before applying the full voltage. The theory was that it would increase the life of the vacuum tubes.

In all the years I had the amplifier, I never had any trouble with it; I can't even recall that I ever had to replace any tubes. I eventually gave it to my sister and some years later, she gave it to my brother who still has it. However, it's now sitting on a shelf and not being used. Whether it would work now, I don't know. It could be that some capacitors would now require replacing.

I'm a bit skeptical when people say that one amplifier sounds better than another, or that some minor change improved the sound. All high quality amplifiers sound alike unless they are over-driven to the point that they produce distortion.

From what I've read, tubes may last longer if power is applied to the filaments for a while before applying the B+. Whether there would be a convenient and neat way to do that with the Stereo 70 I don't know. However, considering that vacuum tubes can be hard to get and expensive, it might be worth considering.

Incindently, before I got my degree in Business Administration, I had a trade school certificate in electronics and have a pretty good background in Physics. I'm retired now and am toying with the idea of making a stereo amplifier using push-pull 6V6s to produce about 20 watts per channel, AFTER I move into my new house. I currently have no vacuum tube equipment and would really like something to show people what vacuum tubes are; many people have never even heard of vacuum tubes.

Here's a tip to reduce the likelihood of getting smoke or blowing fuses when testing equipment; perhaps others here already do it. Connect the equipment in series with a light bulb of appropriate size. A tungsten filament, when cold, has about 1/20 the resistance it has when hot. If there is something wrong with the equipment that causes it to draw excessive current, the light bulb will light up and limit the current and probably prevent damage or blowing the fuse. If there is nothing wrong, the equipment may operate almost normally even with the light bulb in series with it. That works better than replacing fuses several times before finding a problem.
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Postby burnedfingers » Sat Jan 03, 2009 6:51 am

[quote] The theory was that it would increase the life of the vacuum tubes.

There is a lot of discussion over the slow B+ theory. Some think it does some good and yet others like myself feel it does not contribute to cathode stripping unless there is signal present at the same time. One must also remember that the early electronics did not posess slow B+ turn on. A couple of classic examples would be my 1928 RCA Radiola model 62 or my 1928 Atwater Kent Model 60 radios or my Wurlitzer 6L6 amplifiers from 1940. Note there are current amplifiers being made today with SS rectification and no slow B+ turn on.
With respect to the SS rectifier adding more power is would be minimal at best at 1 watt or 2 and not worth consideration since power would need to double before you get a 3 db increase in sound level. There have been a number of papers written about the horors of fast B+ turn on but yet NO hard proof to say that this is the only factor contributing to cathode stripping. I have over 5K hours on some of my output tubes with no change in the bias setting.

[quote]I'm a bit skeptical when people say that one amplifier sounds better than another, or that some minor change improved the sound. All high quality amplifiers sound alike unless they are over-driven to the point that they produce distortion.

First of all your comment about distortion... since I own 3) distortion analyzers. We get so carried away trying to strive for a .001 reading when we cannot hear a difference until it gets down to around .1 for some people and even 1% for others. Next we have the difference between even distortion and odd order. Different high quality amplifier will have different levels of even and or odd order 2nd,3rd,4th,5th and so on and this is dependant upon the circuit.

While I tend to agree to a degree. I don't suscribe to the idea that a magical power cord is going to make a drastic difference nor do I put much faith in changing a $5 capacitor in favor of a $75 making a drastic difference. Some minor difference may be heard and I can say I have experienced some of that. Your point of high quality amplifiers sounding alike I cannot agree with. Different topologies/circuits sound differently. Different tubes sound differently. Cathode resistors verses LED's verses CCS's all have different sonic signatures. I could show you a night and day difference between a Dynaco Stereo 70 with a stock driver board verses one that has been modified. A difference between that driver board and say a Blackburn board with 3) 6SN7's and a difference between that and a Mapletree driver board that uses a 6SL7 and 6SL7 per channel. I could go further and compare a Quicksilver 8417 and a KT88 mono block amplifier as having different sonic qualities. Furthermore this carries into SS amplifiers also because in the past almost 40 years I have owned, modified, and repaired most of the high dollar amplifiers money can buy. Going back to your thinking along this line you will probably suscribe to the idea that all preamps and line stages will sound the same.
You would be mistaken also.

Please don't mistake my comments as me comming down on you because I am not. I am stating a few of my ideas based on my close to 40 years of playing with my mistress. If anyone were to dismiss the claims of the magical components making a difference I would probably be the first. The long line of "Snake Oil" components will probably continue as long as there are people with an abundance of money thinking they can buy a difference. Hell, there is a vast amount of money being made on everything from RCA gold plated chassis mount connectors to interconnect cables to high dollar speaker cables to freeze dried tubes.

Your tip on the use of the light bulb may help some here on this board. Some of us have been using the poor mans variac when the real thing hasn't been available for many years.
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Re: Stereo 70

Postby Ty_Bower » Sat Jan 03, 2009 7:35 am

FRE wrote:I'm a bit skeptical when people say that one amplifier sounds better than another, or that some minor change improved the sound. All high quality amplifiers sound alike unless they are over-driven to the point that they produce distortion.


I've more tube amps around than I ought. I find that each one has a subtly different sonic character. Putting different tubes into the same amp also results in minor changes to the amp's sound. Whether I could say that one sounds better is another question, but I have little doubt that each one sounds different.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Slartibartfast » Sat Jan 03, 2009 10:51 am

The SS rectification may help clean up the "muddy" bass of the ST-70, that some people criticize it for. I too have a Curcio SS rectifier/can cap replacement board in my ST-70, and there is no muddy bass to be heard.

As for the surgeistor, today it is called a CL90, CL for current limiter. I have one in my ST-70 and it keeps the power transformer from jumping out of the chassis, when the power is turned on. :o This is suppose to be nicer to the power tranny and extend its life. This tranny is already working very hard as it is...
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Postby TerrySmith » Sat Jan 03, 2009 11:26 am

I would recomend a total electronic overhaul, EXCEPT the tubes. If you still have your original Mullard EL34's, keep them if still good.

The main sore spots are:
Bias supply - replace selenium rect with a diode.
Quad cap - replace with a new can or cap board.
Tube sockets - especially replace the EL34 sockets
Capacitors - replace ALL other capacitors!

Also the DIY-70 board is a nice upgrade, and is the most stock looking board you can buy. I also like to replace the hookup wire with 22ga 7-strand silver plated teflon wire.
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Postby SDS-PAGE » Sat Jan 03, 2009 11:56 am

The surgister was connected in series with the primary of the power transformer. It added resistance to the primary for a few seconds before applying the full voltage. The theory was that it would increase the life of the vacuum tubes.


I think those are thermistors or "in-rush limiters". They are cheaply available at Mouser or elsewhere. If the GZ-34 used, it will act as a HT delay.

From what I've read, tubes may last longer if power is applied to the filaments for a while before applying the B+. Whether there would be a convenient and neat way to do that with the Stereo 70 I don't know. However, considering that vacuum tubes can be hard to get and expensive, it might be worth considering.


Yes, there are couple of ways to do it: 1) Sophia Electric sells a time delay circuit that delays HT for 30 sec, during which time it allows tubes to warm up, 2) you can install a pair of DPDT toggle switches so that you can switch filament irons first before switching on the HT. Spacing might an issue in the ST-70.


I'm a bit skeptical when people say that one amplifier sounds better than another, or that some minor change improved the sound. All high quality amplifiers sound alike unless they are over-driven to the point that they produce distortion.


I tend to disagree with your comments about all high quality amplifiers sounding alike. For one, the sound is percieved very subjectively by ears, and what sounds good to one person may sound otherwise to another. This is the case with on-going debate with SE and PP amps. I likewise don't give any credibility to those products that targets audiophools. However, I do think that certain types/brands of caps (to certain extent resistors) and OPTs can make some positive difference within the same schematic. Certainly, the quality of tubes can contribute as well. Just my personal experience after having built 25+ amps for kicks last year.

Good luck with the new build! -Min

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ST-70, etc.

Postby FRE » Sat Jan 03, 2009 12:50 pm

I hadn't known about the CL90 and I doubt that it existed in the 1960s. Probably it's better than the device that I wired into my ST-70. The surgister was actually a simple resistor and a pair of mechanical contacts which shunted the resistor after a few seconds. Whether it would extend the life of the transformer is unclear. Transformers are very heavy devices and most can withstand horrible overloads briefly with no damage. However, at the time I installed the surgister, it was commonly believed that tubes have a shorter life if the filaments are powered on an off frequently and that avoiding applying full power suddenly extends their life. From the controversy over this sort of thing it appears that no carefully controlled studies have been done to determine the effects of suddenly applying full power to filaments or applying B+ before the filaments have warmed up or, if such a study has been done, the results are not widely known.

One problem with power transformers is that, depending on precisely where the 60 Hz waveform is when the power is turned off and back on, the core can saturate when the power is turned on, resulting in very high current for about 1/2 cycle. In that case, the current would be limited only by the DC resistance of the primary winding. If a fuse is chosen so that it won't blow during that surge, it may be too large to provide adequate protection, even if it is a slow blow fuse. A capacitor input filter would exacerbate the problem. From my work with magnetic amplifiers, I learned about core saturation.

I prefer choke input filters because they have better regulation over the range of current normally used. However, with absolutely no load and no bleeder, the voltage can really soar. I had to deal with that when I designed a DC dynamemeter control system for testing engines when I worked for an engine and generator manufacturer. That was before switching power supplies existed.

I never found the bass of the ST-70 to be "muddy," but I suppose that if the speakers were in a large room and the amplifier were working very hard, the power supply voltage would sag when the 32 foot stop on an organ is used causing clipping. I'd expect that unbalance in the EL34s would also effect the bass, partly by causing core saturation in the output transformers. If I do build an amplifier using 6V6s, I may use adjustable bias on each 6V6 separately to assure accurate balance. Ideally there'd be a built-in milliammeter in series each cathode, but probably that would be more trouble than it would be worth. The Dynaco method of having a low value cathode resistor to enable determining current by measuring the voltage across the cathode resistor is more practical, but the ST-70 used only one resistor for each pair of EL34s and there was no balancing provision. In theory the EL34s were matched, but who knows how well.

I've noticed that guys are putting all sorts of solid state devices into tube amplifiers now. My inclination would be to avoid doing that with the exception of simple diodes or power supply components, even though the solid state devices no doubt work very well.

Slartibartfast wrote:The SS rectification may help clean up the "muddy" bass of the ST-70, that some people criticize it for. I too have a Curcio SS rectifier/can cap replacement board in my ST-70, and there is no muddy bass to be heard.

As for the surgeistor, today it is called a CL90, CL for current limiter. I have one in my ST-70 and it keeps the power transformer from jumping out of the chassis, when the power is turned on. :o This is suppose to be nicer to the power tranny and extend its life. This tranny is already working very hard as it is...
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ST-70, etc.

Postby FRE » Sat Jan 03, 2009 1:17 pm

I'm sure you're right, but it's my brother's amplifier now. For normal use, I have a totally solid state integrated amplifier for my home cinema system; a high-powered vacuum tube amplifier with 4 channels would be a bit unwieldy and tax the air conditioning system. But after I move into my new house, I'll want a much smaller system for the master bedroom. That's where a simple stereo amplifier using 6V6s would come in. Probably I'll use a self-powered sub-wolfer for the base, so a 20 W per channel tube amp should be more than adequate. It wouldn't even have tone controls; it would have only a volume control and an input selector switch. Actually, it would be mostly a nostalgia trip and a way to show younger guys that we had good sound available before transistors and integrated circuits were available.

Probably the only 6 tubes required would be 2 pairs of 6V6s and 2 combinations of pentode and triode, the triode being the phase splitter. Magnetic phono pickups are seldom used now, so a high-gain pre-amp should not be required. At one time, I had a fair amount of tube experience, but it's been decades.

The new house should be finished by the end of the month, I hope. AFTER the move, I'll start looking for parts and the necessary test instruments. An oscilloscope would be helpful, but it would be hard to justify the cost. However, I think that I could get by with my digital multimeter if I buy an audio signal generator.

TerrySmith wrote:I would recomend a total electronic overhaul, EXCEPT the tubes. If you still have your original Mullard EL34's, keep them if still good.

The main sore spots are:
Bias supply - replace selenium rect with a diode.
Quad cap - replace with a new can or cap board.
Tube sockets - especially replace the EL34 sockets
Capacitors - replace ALL other capacitors!

Also the DIY-70 board is a nice upgrade, and is the most stock looking board you can buy. I also like to replace the hookup wire with 22ga 7-strand silver plated teflon wire.
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Postby burnedfingers » Sat Jan 03, 2009 5:13 pm

[quote]The SS rectification may help clean up the "muddy" bass of the ST-70, that some people criticize it for. I too have a Curcio SS rectifier/can cap replacement board in my ST-70, and there is no muddy bass to be heard. [quote]

Actually I believe the stock driver board to be at fault for the muddy bass.
When the stock board is modified and or replaced the muddy sloppy bass is gone.
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Postby Slartibartfast » Sat Jan 03, 2009 5:31 pm

burnedfingers wrote:

Actually I believe the stock driver board to be at fault for the muddy bass.
When the stock board is modified and or replaced the muddy sloppy bass is gone.



My amp has a reproduction stock circuit and SS rectification. There is no muddiness in the bass. Maybe the muddiness comes in as they age...
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Postby FRE » Sat Jan 03, 2009 10:51 pm

burnedfingers wrote:
The SS rectification may help clean up the "muddy" bass of the ST-70, that some people criticize it for. I too have a Curcio SS rectifier/can cap replacement board in my ST-70, and there is no muddy bass to be heard.

Actually I believe the stock driver board to be at fault for the muddy bass.
When the stock board is modified and or replaced the muddy sloppy bass is gone.


I would think that with adequate instruments, it would be possible to determine exactly what the source of the muddy bass is without guesswork. On the other hand, the necessary instruments can be extremely expensive.
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Postby burnedfingers » Sun Jan 04, 2009 6:09 am

One good modification that you should consider if using the stock Dynaco driver board. This was taken after doing a Google.

Install Frank Van Alstine's Input Bandpass Filter

OK, here's a controversial move. I read Frank's Stereo 70 treatment and what he claims made sense to me, and I figured the rolloff he suggests would be almost unnoticeable anyway, and you know what--it is. So I figured if it would unburden the feedback loop and the rest of the amp with passing frequencies it couldn't handle, all the better,and it allows an upping of the coupling caps to the output tubes and doesn't adversely affect the sound. At all--in fact, it tightens it up in the bass a small bit. I liked it so much, I did it to my Mark III's. It consists of a .02mfd@50vdc film cap in series with a 10K resistor to the input grid, which has a 475K (I used a 470K) resistor in parallel with a 1000pf (.001mfd to you) 50v film cap to ground. Neat. As I eliminated the Stereo/Mono switch, a future mod might be a switch to select the input filter or bypass it, but on the other hand, perhaps not.


I would think that with adequate instruments, it would be possible to determine exactly what the source of the muddy bass is without guesswork. On the other hand, the necessary instruments can be extremely expensive.


It all depends in my opinion. Some things/problems can be seen on a scope or distortion analyzer and some cannot. When we look back the Dynaco was marketed as a $99 kit and it made quite an impact when it hit. The marketing was at a perfect time before the on set of the watt wars where the power you had sitting on your shelf was a status symbol. I've always thought that if a capacitor made a sonic difference that difference should show up as a decrease in harmonic distortion and my measurements have proven that is not the case. Blind listening tests have proven there have been slight improvements in the sonic character of an amplifier after certain modifications. The Frank Van Alstine's Input Bandpass Filter in conjunction with resistor matching and replacement does get rid of the Dynaco's muddy bass response. Having a few Dynaco's allows me to make side by side tests to prove or disprove the modifications. There is also considerable written text on the internet that agrees with my findings. I also have to consider there are those incapable of being able to judge the difference and those people would be happy listening to a $150 boom box thinking it had a wonderful sound.

When you audition the Stock Dynaco next to a Dynaco with a Roy Mottrum Driver board or a Mapletree Driver board by Lloyd Peppard there is no comparison. The sonic character of the amplifier changes completely and you start to get the beginning of something that sounds wonderful. When you start with the power supply modifications the amplifier goes into a different class where it will hold its own with amplifiers costing a few thousand dollars.

Circuit design,components, layout, power supply and a number of factors influence the sound and performance of amplifiers.

I will concede that there is a slight difference between in the bass when using a SS rectifier verses a tube rectifier. The differences start to become smaller after modifications have been done. I tend to appreciate using a SS rectifier with the 5AR4 tube. The tube gives the slow warm up and takes the nasties away from the SS rectification.
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Postby Slartibartfast » Sun Jan 04, 2009 8:54 am

burnedfingers wrote:Install Frank Van Alstine's Input Bandpass Filter

OK, here's a controversial move. I read Frank's Stereo 70 treatment and what he claims made sense to me, and I figured the rolloff he suggests would be almost unnoticeable anyway, and you know what--it is. So I figured if it would unburden the feedback loop and the rest of the amp with passing frequencies it couldn't handle,



Can the ST-70 run without a feedback loop?
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Postby burnedfingers » Sun Jan 04, 2009 9:07 am

Can the ST-70 run without a feedback loop?


Why would you want to?

Changing the feedback will change the amount of gain the amplifier has. I personally believe that amplifiers do not sound as good without some feedback.

The Van alstine modification changes the amplifiers response. It had ultra high response before and it was way above what was needed thus taxing the amplifier. After the modification the amplifier lives an easier life. Thus no need to change or eliminate the amount of feedback in my opinion.
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Postby Slartibartfast » Sun Jan 04, 2009 9:33 am

burnedfingers wrote:
Can the ST-70 run without a feedback loop?


Why would you want to?
[/quote]


I have a nice SET amp that does not use feedback. Also in Radiotron 3rd edition, it states that PP Class A1 triode operation is regarded as providing the highest standard of fidelity. There is no mention of feedback in that reference. Triode strapping the EL34 tube is a small task. Of course this assumes efficient speakers to work with lower power output.

There are PP designs that do not use feedback, and I for one see feedback, especially large GLOBAL feedback as a bandaid fix to the problem of distortion. feedback has been shown to only cancel out 2nd order harmonics. It does little or nothing to the bigger offenders in odd order harmonics.
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