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Approx. Cost to Rebuild a Dynaco ST70 and PAS

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Posted on January 20, 2016 at 06:50:37
Charlie8521
Audiophile

Posts: 914
Location: South East Michigan
Joined: October 2, 2004



To do a good quality job on an original stock Dynaco ST70 and PAS preamp, what is the approximate cost to rebuild excluding tubes. Both the approx. cost of parts for DIY, and than having someone else do the whole job? Objective is top performance from units, not just making them safe to operate.

 

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How high is "up"?, posted on January 20, 2016 at 07:43:56
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 Your question, unfortunately, is too vague.

Are you planning on keeping the circuit completely stock, using the 7199s, etc?

It would be easy for someone to say "$300", but lets say you felt that V-Cap Teflon capacitors would give you the "top performance" you seek from these units. If that's the case, you could easily spend more than $300 on coupling caps alone.

Your picture shows the squatty original power transformer, which many claim is under-powered, even for the stock circuit. There's another (approximately ) $130, just for a power transformer.

You'll get clearer answers once you decide what you want to do with your Dyna find.

Oh, and please don't succumb to temptation and just plug them in, if you have no way to variac or other "soft-start" method. What might be perfectly good could be ruined by such haste.

Those ST-70 tubes alone are worth quite a bit. Don't tempt fate! Nice find, by the way.

 

Mullard?!!, posted on January 20, 2016 at 08:12:45
Crazy Dave
Audiophile

Posts: 14371
Location: East Coast
Joined: October 4, 2001
 You are not kidding about the tubes, assuming they still work!

Dave

 

RE: How high is "up"?, posted on January 20, 2016 at 08:39:09
Charlie8521
Audiophile

Posts: 914
Location: South East Michigan
Joined: October 2, 2004



So I stumbled across these items with the tuner buried in a new acquaintance basement yesterday. I'm not a collect, accumulator, or buyer and seller. They just wanted to get rid of them, but I advised someone out there would be interested in them, but not me. So I offered to try to sell the items for them. They also have a tube Fisher Counsel, but it is buried beneath stuff for the moment.

My objective is to reach out to personal friends to see if any of them would be interested. That is where I need a cost estimate so as to give them some rough guidance as to what they would be getting into ( I assume a possible fire hazard if run as if, even if they power up).

I don't know the tubes resale value, but I assumed the old Mullard EL84's and others alone have value. Bring 70 up on a Variac and everything looks great. Attached is a pic of inside of the PAS - pristine.

Thank you for the input.

 

Bare Minimum, posted on January 20, 2016 at 09:45:48
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 I would replace (or suggest the buyer replace) the electrolytic caps in all three pieces. If the stock look is desired, then you're going to have to go with multi-section can caps for the B+. I've linked to one place below where you can get them (scroll down for the can caps). There may be other places which are cheaper. This is just to give you a ballpark figure of the costs.

The filament caps for the PAS, and the bias supply caps for the ST-70 can be purchased from DigiKey, Mouser, etc.

One other thing I'd consider a "must do" is the selenium rectifiers in the PAS and ST-70. Replacement diodes for those could go anywhere from pennies to a few dollars, depending on which diodes were used.

Be sure the correct value fuses are inserted in the FM-3 and ST-70 (no fuse was used in the PAS, but one should be added). While fire is always a possibility, I'd be more concerned about wrecking the output transformers in the ST-70. Next to the Mullard EL-34s and 5AR4, the output transformers are the most valuable part of the amp. That's why you want to replace the power supply caps (i.e. the can cap) as soon as possible.

Quite often the FM-3 and PAS used smooth plate Telefunken 12AX7s. Be aware that they can be quite valuable as well, so treat them gently, and, as you already voiced a concern, don't wipe off the stenciling!

 

Dynaco/Mullard GZ34/5AR4, posted on January 20, 2016 at 09:49:43
Charlie8521
Audiophile

Posts: 914
Location: South East Michigan
Joined: October 2, 2004



It also has a Mullard GZ34, I don't know it's value. And their are two 7199 marked Dynaco Made in the USA. These items are all original.

As far as working I'm first bringing the ST70 up on a Variac over say 48 hours (I hope that is OK). Than the PAS Preamp and Tuner. I'll than plug them together and see what I have in sound.

They were basically one step from the dumpster.

Anyway to clean the dirt off of the tubes without damaging the stenciling.

 

RE: Approx. Cost to Rebuild a Dynaco ST70 and PAS, posted on January 20, 2016 at 15:55:04
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



The other posters have given you some good hints. Definitely heed the warning about selenium rectifiers being ticking, toxic, time bombs.

Selling those valuable old tubes is an excellent way to raise cash, to fund the refurbishment effort. IMO, the "reissue" GEC KT77 is the O/P tube to use. Contact AA sponsor Jim McShane for your tube needs and quite a few of the necessary new parts.

Spend the money and put a new power trafo in. L_RD knows lots of the OEM trafos failed and, as was stated, the original is not quite up to the job it has to do.

The 7199 small signal tube has gone the way of the dodo. A few specimens are "floating" around at uncomfortable prices. Any number of replacement driver boards are available for the ST70. All of them have strengths and weaknesses. FWIW, I favor the Triode Electronics board populated with 2X EF86s and an ECC99. Those tubes are in production and allow the OEM topology to be retained, while improving the pentodes and triodes used.

A low cost, current production, Sovtek 5AR4 will work very well, provided the uploaded SS diode tweak is installed. Use the quieter UF4007, instead of the 1N4007 shown.

If an adventurous spirit is present, I have a method for safely introducing SS B+ rectification, which eliminates the problems associated with 5AR4s. I'll go into detail, should interest be shown.

BTW, the C354 filter choke is another part not quite up to the job it has do and frequently fails.


Eli D.

 

RE: Approx. Cost to Rebuild a Dynaco ST70 and PAS, posted on January 20, 2016 at 16:03:54
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 Look over at tubes4hifi.com

they have a large number of parts and boards. Replace the obvious - electrolytic caps, clean the sockets, resolder all the joints, replace the power resistors with a beefier metal oxide.

Keep as much original as you can. However the driver board really is a soft spot on that unit. You can replace the 7199 with 6gh8 and an adaptor now. But the board really is past a usable date. Do get a replacement board.

The PAS - pretty much the same. Replace the power supply parts. Otherwise a good cleaning is all it needs.

And a fresh set of rectifier tubes isn't a bad idea.

but check the tubes4hifi sit. it seems to be down at this time but check back later.

charles

 

RE: Dynaco/Mullard GZ34/5AR4, posted on January 21, 2016 at 05:56:17
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 There is no safe way to clean the tubes and spare the silkscreening. Just try to clean as much as possible around the print.

Even if you keep the tubes I would have them checked. Rare or not they may not be any good.

How are the sockets? Obviously clean the crap out of them. And remove and reflow all the solder joints. Consider replacing the important cathode resistors and just redo the bias circuit. It's too old, important and simple to just leave.

Download some of the trouble shooting tips from Joe Curcio's page on how to bring it up and test things like transformers before retubing it.

Also again, I would consider a new driver board. Those originals did tend to rot.

 

link, posted on January 21, 2016 at 06:07:06
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 /

 

RE: Bare Minimum, posted on January 21, 2016 at 06:16:23
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 How did the PAS stay so clean and the ST-70 get so dirty? Just because of the cover?


I would have the tubes tested first. Just because they are rare and valuable does not mean they may still be any good. And again, i would consider new rectifier tubes and do all the upgrades you mentioned.

But test test test first before blowing out a pack of mullards!

 

RE: link, posted on January 21, 2016 at 07:36:09
Charlie8521
Audiophile

Posts: 914
Location: South East Michigan
Joined: October 2, 2004
 Excellent link. Thank you.

 

RE: Bare Minimum, posted on January 21, 2016 at 08:41:57
Crazy Dave
Audiophile

Posts: 14371
Location: East Coast
Joined: October 4, 2001
 If I had a quad of Mullard EL34's (well actually I do!) I would buy some modern EL34 tubes to use while I sorted things out.

Dave

 

RE: Dynaco/Mullard GZ34/5AR4, posted on January 21, 2016 at 08:45:01
Crazy Dave
Audiophile

Posts: 14371
Location: East Coast
Joined: October 4, 2001
 I would change the electrolytic capacitors in everything before I brought them up on a variac.

Dave

 

RE: Bare Minimum, posted on January 21, 2016 at 08:57:36
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 Mullards are a legendary tube. But truth be told there are several REALLY good new production EL34s now available. I was given a set of Shuguands (sp?) and was SHOCKED at how good these tubes are.

Personally I would store those vintage tubes for limited use. And get a good set for heavy duty listening use. I'm funny that way. I like to spare those oldies.

 

RE: Bare Minimum, posted on January 22, 2016 at 09:47:48
Crazy Dave
Audiophile

Posts: 14371
Location: East Coast
Joined: October 4, 2001
 You give sound advice. I am planning on building an EL34 amp and will start with modern tubes, but I have to give the Mullards a try, just to see!

Dave

 

What Airtime said! ST70, posted on January 22, 2016 at 10:29:36
Mossback
Audiophile

Posts: 1871
Location: Washington, the State
Joined: November 17, 2001
 I rebuilt two ST70 amps for friends using the 12AU7 board from them. I only kept the chassis and outputs. Everything else was replaced. The photos and instructions made this a fairly easy job. While the amp will no longer be stock it is much improved.
I also completely agree with Eli, sell those vintage tubes buy new ones Jim can suggest replacements

 

RE: Approx. Cost to Rebuild a Dynaco ST70 and PAS, posted on January 22, 2016 at 11:01:58
DannyR
Audiophile

Posts: 611
Joined: January 21, 2001
 Read the document in the link below. It's a in depth analysis of the stock circuit and includes some small things that can be done to address small issues. You can do all of em or pick and choose.

http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

I'd keep the stock board, replace the quad cap, bias capcitors and selenium rectifier, clean/tighten the sockets also clean the bias pots, replace the four coupling capacitors that go the output tubes with something 630v, do the small modifications outlined in the above document maybe skipping the replacement of the GZ34, and it will turn out to be one hell of a amplifier and should be the best price/performance option. I did the above and liked the results more than the VTA70, KTA mods etc. The only board that I liked more was DIYTube's which ironically was the only one that included measurements to back up his work etc in it's instructions. The others you have to take on faith.

On the PAS, clean switch/jacks/sockets, maybe just replace the selenium rectifier and power supply capacitors. If you don't like the results you can bypass the tone controls and even do Curcio's RIAA mod.

Below are some very detailed analysis of the PAS's performance.

http://www.audioregenesis.com/documents/PAS_Line.pdf


http://www.audioregenesis.com/documents/PAS_Phono.pdf

Please post what you end up doing and if you need any help post or drop me a message and I'd be happy to assist.

Good luck.

 

Please read this before you start, posted on January 22, 2016 at 11:06:10
junkwaffel
Audiophile

Posts: 163
Joined: August 15, 2002
 David Gillespie and George Ronnenkamp wrote a detailed analysis of the Dynaco ST70 amp (Dave) and the PAS preamp (George). It is definitely worth your while to read these articles before starting.

Eli, I would be interested in your method of replacing the GZ34 with ss diodes for the ST70.

 

RE: Please read this before you start, posted on January 22, 2016 at 11:37:39
DannyR
Audiophile

Posts: 611
Joined: January 21, 2001
 Great minds think alike. I posted the same advice probably at the same time you did.

There's also a great thread detailing Dave's ST70 reports along with questions and answers over at Audiokarma.

http://audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/

 

RE: Please read this before you start, posted on January 22, 2016 at 11:51:59
junkwaffel
Audiophile

Posts: 163
Joined: August 15, 2002
 Looks like you beat me by 4 min and 2 sec although I didn't see you response when I replied. The great minds are Dave and George as these articles demonstrate. Am anxiously waiting for the PAS power supply and suggested PAS mods articles.

 

RE: Please read this before you start, posted on January 22, 2016 at 12:06:05
DannyR
Audiophile

Posts: 611
Joined: January 21, 2001
 Agreed.

I've done mods to a Fisher 400 and two Dynaco ST70's that Dave had a hand in and both turned out fantastic and were huge learning opportunities. It's amazing the work that he and George put into the articles. They're so well written and informative.

I'm looking forward to the final PAS articles also. I even have a old PAS 2 ready for whatever is recommended.

 

SS B+ Rectification in ST70s, posted on January 22, 2016 at 22:33:17
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 "Eli, I would be interested in your method of replacing the GZ34 with ss diodes for the ST70."

Use 2X Cree C4D02120A 2 A./1200 PIV Schottky diodes. Insert a CL90 inrush current limiter between the rectifying diodes and the PSU filter. Use only enough capacitance in the 1st position of the CLC filter to keep the B+ rail voltage up. 10 to 15 μF. should be quite ample. Unless the available space demon stirs things up, a 600 WVDC film part is best here. As stated many times, the OEM C354 choke is a POS. Squeeze a 300 mA./2.5 H. Hammond 159T in. Pile the energy storage up in the reservoir position.

The rail voltage will come in higher than OEM and that fact must be dealt with. Adjust the resistance value in the decoupling network to bring things into line for the small signal circuitry. Last and definitely not least, switch the O/P tube pairs to combination bias. "Stand" each pair on a shared 1% tolerance 100 Ω/470 μF. network. The OEM single bias trim pot./channel is OK. Notice the very convenient "idle" current test point. O/P tube matching requirements are slightly relaxed. As is always the case, gm must be close, but a small amount of cathode current imbalance is OK. The shared RC network will correct small differences in cathode current.


Eli D.

 

RE: Approx. Cost to Rebuild a Dynaco ST70 and PAS, posted on January 23, 2016 at 11:35:20
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 I did this years ago, read what I could on PAS upgrades. Parts were about $30, some value changes, no new circuit boards. The result was sound that blew away the CJ tube preamplifier I was using.

 

RE: Approx. Cost to Rebuild a Dynaco ST70 and PAS, posted on January 26, 2016 at 08:27:38
Crazy Dave
Audiophile

Posts: 14371
Location: East Coast
Joined: October 4, 2001
 The CJ most likely had a cathode follower, unless it was a PV-3. If your situation does not require one, there are those who feel they hurt the sound.

Dave

 

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