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on Sat May 09, 2009 12:54 pm

Tube Nube

Bob, Roy,

You guys are gonna go crazy from insomnia, and / or obsessive compulsive disorder from your meticulous attention to your customers, potential customers, and forum-community members.

Don't get me wrong here, I really appreciate your generosity here, and other members too, who follow suit with friendly, well informed support.

But in the interests of ensuring you guys are still gonna be here for me over the next 20 years (since that's apparently how long I keep my amplifiers), I do hope you're taking time for whatever takes the stress out of life for you.

If not, I insist you get to it. Doctor's orders. And I don't want to hear any excuses.

;-)

Brenton

on Sat May 09, 2009 4:12 am

tubes4hifi

Hi Jonathan, Roy here, caught your question . . .
I started this hobby/biz over 20 years ago, and during all that time, have bought, sold, and rebuilt probably 100 hi-fi tube amps, and I know that Bob has probably done even more than that.
Of course it's my opinion, and someone who doesn't know me may think I'm biased, but IMHO
I would have to say that the amps Bob and I sell would beat out 80-90% of what's on the market, and that includes the $5000+ spread. I'd say Bob's kit (with my VTA board in it) is an absolute steal at any price from $600-800, and I've been told by several dozen customers that it for sure beats out anything available for under $2500. I have a neighbor who's been thru several AudioResearch tube amps the past half-dozen years and I even bought one for myself to make a fair comparision, and it PALED!!! It was a ten year old, not a new $4000 one, but I sold it fast for $1000 and haven't turned back. Bob is also correct in saying a new one is a bargain compared to an older used one. Several years ago I even offered a money back guarantee, never had anyone take me up on it. That's hard to do on a complete amp with a low profit margin, but on the kit's I was selling it helped to open up alot of disbelieving eyes and minds.

on Fri May 08, 2009 9:40 pm

Tube Nube

Johnathan,

Sent you a PM.

B.

on Fri May 08, 2009 11:00 am

Bob Latino

Tube Nube wrote:There's some of that expert advice I was talkin' about.

Correct me if I'm wrong, Bob -- and I know I'm inviting you to offer a self serving opinion here -- but fun as it may be to get an old Dynaco and rebuild it, wouldn't the quickest and perhaps cost-effectivest (sic) route to a state of the art ST 70 be to get one of your kits?

John, I'm interested to hear that the tube jobs really smooth out the Kan speakers. I drooled over the Kans back in the early 90's.

I'm interested to know what you built with the Fostex drivers.

Brenton


Brenton,

As I have mentioned a few times on some of these posts about the only thing you can be fairly sure of when you buy an old original ST-70 is that the two output transformers are probably as good as what can be produced today. The rest of the parts, although they were probably "good" for that era, in light of what is available today, are only of mediocre quality.

1. The power transformer was undersized.
2. The quad cap only has 20 uF to run the two output transformers - way low ..
3. The driver board was made of an inexpensive phenolic material that would turn black from the heat under and around the 7199 driver tubes.
4. The carbon composition resistors throughout the amp by now have probably drifted from their original value. Carbon composition resistors also tend to get "noisy" after a while. What you think is "tube hiss" in your amp may be coming from one or more resistors.
5. The tube sockets were a cheap plastic material.
6. The bias system is crude by today's standards.
7. The driver circuitry with the 7199 tubes was not linear at all drive levels. As you ran the volume control UP on your preamp increasing the amp's drive level > the amp's high frequency response would go DOWN.
8. The stereo/mono switch only worked correctly if you ran a wire from one output transformer to the other. (16 ohm to 16 ohm if you had 8 ohm speakers and 8 ohm to 8 ohm if you had 4 ohm speakers) People would flip the switch to "mono" and think the amp was in mono but it wasn't - what they had now was two 35 watt channels carrying the same signal. You have to wire the two output transformers together to actually have the entire ST-70 run in mono.

If you want an ST-70 to have around for old times sake then get one off Ebay but IMHO this shouldn't be the amp you play on a daily basis unless you take care to restore or rebuild the amp.

Minimum things to do for reliability in order of importance ..

1. If your amp has an original quad cap - replace it. A bad B+ high voltage system can take out the rectifier and the output tubes. If your amp has a low level hum that does not change with the volume control - the quad cap is probably statring to go bad.
2. Replace the selenium rectifier with a diode and change out the two capacitors and resistors on lugs 1-4 of the 7 lug terminal strip. Your bias system is now much more reliable. A bad or unreliable bias sytem can eat up output tubes.
3. Replace the 6 coupling caps on the original driver board.

Optional for reliability and better sound ...

1. Replace the driver board with a modern driver board with all triode circuitry. The amp will now SOUND better.
2. Replace the octal tube sockets with modern octal sockets.
3. Replace the input jacks with modern gold plated input jacks.
4. Replace the output terminal strip with modern gold plated binding posts
5. (And this will cost you some money > $125) Replace the power transformer with a new one if the old one mechanically vibrates and/or has a buzz.

If you don't have an ST-70 and you want to get one and PLAY THE AMP ON A DAILY BASIS it probably would be more cost effective to build one yourself NEW from a kit. All the parts are new and most kit vendors will warranty the parts for a year. The ST-70 in kit form is an easy build. From 1959 to the mid '70's about 350,000 audio amateurs built the ST-70 from a kit. Many had never soldered anything before. Dynaco reported years ago that only about 2 % of the kits were shipped back to them because the kit builder couldn't get it to work correctly.

Bob

on Fri May 08, 2009 9:30 am

summilux

I used the Fostex FE127 PAWO plans at planet_10 hi fi. The not only sound good but have a very good wife acceptance factor.