Dynaco ST-70: Base Line Testing

Yes -- but hardly to the extent that it occurs in the ST-70. Under a worst case scenario, the two power amplifiers in the 70 could pull as much as .5 A from the rectifier tube if it and the power transformer were capable. Under the same scenario in the MK III (full power 20 Hz sine waves), a stock unit draws just over 300 ma, producing about 55 watts RMS. Also, where as the stock ST-70 pulls about 220 ma just waiting for the needle to drop on the record, the MK III only pulls about 150 ma in the same scenario. Other than the higher PIV the rectifier tube must withstand in the MK III, this all adds up to a much kinder environment for the tube to operate in in that unit.

The performance of the MK III would no doubt improve as rectifier capacity increases. However, it does not have the history of rectifier tube failure that the ST-70 does, and in my experience, stands up to the worst conditions quite admirably. Back in the 60s, a near exact copy of the Dynaco MK III circuit was used as the basis of the power amplifier section in a particular model Sunn guitar amplifier (including the use of authentic Dynaco transformers as well). It was known to be a dependable model even under the rather extreme conditions that kind of service requires, such that I have always considered the MK III a "band rated" amp (my term) dependability wise. I was in a pretty good garage band during those times, and it was well known throughout the bands what worked, and what didn't. I am rather certain that given the same scenario, a stock ST-70 would not have garnered the same reputation.

Dave
 
Not to muddy the water on this even further.....but since the comparison to the Mk III has come up, I can't help but wonder if a pair of Mk IVs might have been Dynaco's answer to getting all the performance a ST-70 'might' have had if the PS were able to 'deliver'? I'm making an assumption that the Mk IV driver stage has all the attributes of that of the ST-70. Just thinkin'........ WC
 
To the best of my knowledge the MK IV power supply uses the same small choke and small filtering capacitors as ST-70. So any advantage of the MK IV over the ST-70 would have come from having separate power supplies with presumably greater reserve. But this advantage comes would be at far greater cost than an improved ST-70 since there is a lot of duplication with two chassis.

Hafler could have improved the ST-70 with a second rectifier and choke, but that would have increased the cost. The high-end amplifiers of the day used dual rectifiers but, again, they cost more. The MK-IV was a modest improvement over the MK-III with slightly more power, and I think that was the goal not to kill the ST-70, which was a decided moneymaker.
 
Ret, in referring to the Mk 4.... I believe it is still significantly behind the Mk 3 in terms of power. I think the Mk II (2) was rated at 50 watts by Dynaco....still using 2 EL34s, where the Mk III (3) had the power rating 'upped' to 60 watts and used the two 6550s. I think the Mk IV (4), which also used two EL34s, was considered more of a 'mono' version of the ST-70.....or so I believe. Back in it's day there were still a lot of people who'd not yet 'switched' to 'stereo' I think it was the Mk VI (6) where Dynaco went really nutz with power....and that amp (again, if I recall correctly) was a mono-block at 120 watts.....and really designed more for 'commercial' use as I think it was a 'rack-mount' type face plate unit. What somewhat escapes me at the moment is any specific recollection of a Mk V. I know there was a later version of the ST-70...... that was considered by some to be a significantly improved version which, I think, used a solid-state rectifier, came with a black chassis, chrome transformer covers, and had some LED lights on the front panel. I think that amp had some more 'advanced' circuits in it for bias, a B+ timer (maybe) and a few other things. That may have been the Mk V....but I don't specifically recall. But as to the Mk IV (4)....seems to me that with the ST70 being at about 500ma when 'topped out'...... if the GZ34 is good for 250 ma and the choke is rated for about the same....then the Mk 4 should be 'close' to being OK in the PS department. Assuming then that the driver stage follows all the ST70s design attributes.....seems to me like a pair of those amps would offer all the benefits of an ST-70 but not have the same level of limitations.
 
Power output (from the Dynaco catalog):
MK III: 60 watts continuous, 140 watts peak
MK IV: 40 watts continuous, 90 watts peak​

But, again, I believe the IV basicaly uses the same power supply as the ST-70, except maybe the power transformer is different. The chokes and capacitors are the same. Roughly the same power supply, just driving one channel instead of two, so that's why the MK IV it has "more" power than the ST-70.

The ST-70 Series II was produced by Panor Corporation after Dynaco was sold. The major differences were solid-state rectification for the power supply, roughly treble the capacitance in the power supply, Ultralinear used the 8 Ω tap instead of 16 Ω, and the front-end changed from the discontinued 7199 to the 6GH8A.

I don't think that either is incredibly superior to an ST-70 with suitable modifications. Either a second rectifier or switching to solid-state with a B+ delay and regulation to ensure the B+ doesn't climb because of the elimination of voltage drop from the rectifier tube.

So if you want to hotrod your ST-70, I'd say go with a second rectifier tube (limiting factor) or a solid-state rectifier with B+ delay and regulation.
 
One more thing. The Dynaco ST-70 Series II had a global filter to limit frequency response to prevent oscillation. Very similar to the AVA modification from what I remember.
 
Never heard of a Mark V. The Mark VI was the 120 watt monster. Believe it or not, I still have two original, unbuilt Mark VI kits from the day.

Dunn's history of Dynaco doesn't list a MK V. I don't think it existed. The Mk VI was Laurent's last Dynaco design. Very impressive @ 120 WPC.
 
Dave-I had a Sunn Sonic 1 (Dynaco transformers) in high school.Bulletproof,absolutely bulletproof. I restored/resurrected two Sunn's for a group long ago,a 2000S and a 1200S.Both models used twin 5AR4's.Again,bulletproof.It was pretty cool to see a band with matching (and rare) Sunn rigs.Boy,were those guys LOUD!I still have one of their 45's,pressed on green vinyl:rolleyes:

I believe West amplifiers (Grand Funk Railroad & Mountain) also used Dynaco transformers and circuits.

Art
 
I had a pair of the Mk Vis.......and never really got much out of them. They were just 'too big'.....not to mention the thought of 're-tubing' the amp scared the crap out of my bank account! Those 8 output tubes were running about $1200 for a set at the time....... maybe more! Don't know what they are today!

I have a friend who just acquired that 'Series II' ST-70. He's not got it up and running yet.......but when he does, I'm going to try to 'steal' it from him.

I want the set of KITs'!!........lol
 
Last edited:
I had a Sunn Sonic 1 (Dynaco transformers) in high school.Bulletproof,absolutely bulletproof. I restored/resurrected two Sunn's for a group long ago,a 2000S and a 1200S.

For those of you who don't play guitar or bass, Sunn made guitar/bass amplifiers using Dynaco kits. The company was founded by Norm and Conrad Sundholm. (The name "Sunn" had an extra "n" to avoid trademark issues with the "Sun Electronics" which sold tachometers.) Norm was the bass player for The Kingsmen, an obscure band of no particular historical importance[1]. After some minor success Norm asked his brother Conrad to build him a bass amp that wouldn't get lost in the larger venues. And what else would Conrad use but a Dynaco PAM-1 preamplifier and Dynaco MK III amplifier[2]?

Here's the story on the amps:
https://books.google.com/books?id=8wAVHBFxl5cC&pg=PA189&lpg=PA189#v=onepage&q&f=false
While the early Sunn amps used a PAM-1 and MK III, but later ones just used clones of the Dynaco transformers. The early one's with a smiley-face logo are built from dual chassis Dynaco.

[1] Just in case someone doesn't understand just how completely and utterly unimportant the Kingsmen were in the history of music:

[2] Confirmation of this dual-chassis can be found here: music-electronics-forum.com/t24090/
 
I gotta say,my all-time favorite bass tone is Mel Schacher through his West amps.I've always tried to recreate the bass tone of GFR's first studio and live albums.
I have also played through many models of Sunn amps,and have always loved them.

My research has always come up that the West's were Dynaco based,and since I have 4 MKIII's lying around,perhaps a little experimenting is in order:)
 
My research has always come up that the West's were Dynaco based,and since I have 4 MKIII's lying around,perhaps a little experimenting is in order:)

My reading was that the West Fillmore was built using Dynaco transformers to the Dynaco circuit, but not using an actual Dynaco unit.
 
A further comment. The Dynaco frequency response was a lot better than most of the existing amplifiers. No shock there.

I recall reading that with some tweaks the Dynaco-derived bass amplifiers (I think this was for Sunn but don't remember) would go down to 10 Hz. My guess is the cathode-bias bypass capacitors were dramatically increased to avoid the treble boost and loss of LF response, and maybe the coupling capacitors were reduced to improve LF response. (For those of you who don't play or design for guitar/bass amplifiers, HF response isn't an issue in a guitar or bass amp.)

I don't think anyone was doing DC restoration using diodes, which would have solved the interstage coupling issue.
 
Geezz -- and I was unsure if anybody would even believe me about the Sunn/Dynaco connection!o_O

Dave

I do not believe that your credibility will ever be an issue Dave:)

When I think back to how many Dynaco's I scrapped just to salvage the OPT's from them to build musical instrument amps I feel really horrible:(
 
had a pair of the Mk IVs.......and never really got much out of them. They were just 'too big'.....not to mention the thought of 're-tubing' the amp scared the crap out of my bank account! Those 8 output tubes were running about $1200 for a set at the time....... maybe more! Don't know what they are today!

I think you may be thinking of the Mark VI. Thats the one with the 4 8417 tubes per amplifier. The Mark IV is half an ST70, one 7199 and a pair of EL34.

My big Bogen amps started life with a quad of 8417's each. I got them tubeless, and they sat for quite a while as door stops once I realized how expensive that would be to fix. Luckily Dave was generous enough to post his work on that amp so they now live with 2 more commonly available tubes each instead of 4 you have to sell a kidney to obtain.
 
Dave-I had a Sunn Sonic 1 (Dynaco transformers) in high school.Bulletproof,absolutely bulletproof. I restored/resurrected two Sunn's for a group long ago,a 2000S and a 1200S.Both models used twin 5AR4's.Again,bulletproof.It was pretty cool to see a band with matching (and rare) Sunn rigs.Boy,were those guys LOUD!I still have one of their 45's,pressed on green vinyl:rolleyes:

I believe West amplifiers (Grand Funk Railroad & Mountain) also used Dynaco transformers and circuits.

Art

I gotta say,my all-time favorite bass tone is Mel Schacher through his West amps.I've always tried to recreate the bass tone of GFR's first studio and live albums.
I have also played through many models of Sunn amps,and have always loved them.

My research has always come up that the West's were Dynaco based,and since I have 4 MKIII's lying around,perhaps a little experimenting is in order:)

My reading was that the West Fillmore was built using Dynaco transformers to the Dynaco circuit, but not using an actual Dynaco unit.

Sadly, Dave West died a couple of years ago, and the resurrection of West Labs died with him. I met him in 1982 when he was running his shop in Lansing, Michigan and was closing it down due to financial troubles. I bought a number of Dynaco transformers from him, including some A450's and a few West marked outputs as well. I have a number of his letters and invoices from Dynaco / Hafler on Dave buying parts from them. There are schematics of the West Amps still existing on the Internet wayback machine, I can find the links if anyone is interested.

Of course, the best thing I got from Dave was a complete box of 24 NIB blue boxed MOV Genalex KT88's at the steep price of $12 per. It was all I could afford at the time on a medical student's meager budget. Happily or unhappily, I sold them a few years later to a fellow from Detroit (from Audio Mart mail correspondence) for double my money, $25 per. How I wish I had held on to them.

Dave went on to do computer programming for street paving machines and other stuff, only coming back to guitar amps in the late 2000's. Sad to hear of his passing.
 
I hope I'm not off base posting here. Inspired by this thread (after much delay) I happened into an st70 for this build. Aside from some tweaks in the bias circuits' fixed resistors, I believe it follows the recommendations faithfully.
I was running start up testing last night and was somewhat surprised,
With bias all the way down
PS voltages were a good deal higher (6%)
With bias set at (or near) 1.56v, PS a little low (5%?)
(Variac @ 117vac, filaments @ 6.42v)
It's not so much I didn't expect a swing, I was surprised at how much.
For sake of accuracy, this was a mix of old 6CA7s
It came with.
I'm not sure how much that would effect it ( I would tend to doubt but what do I know. It's why I asked)
 
Last edited:
Back
Top Bottom