ST-70 Mods

ST-70 Mods

Postby mozilla » Tue Feb 11, 2003 11:55 am

Post your comments, ideas, experiences on restoration & mods for this amp.

To start off:

1. What is your preferred coupling cap? Jensens, Auricaps, Orange Drops?

2. What is your preferred restoration/mod for the quad cap? Original replace ment cap? SDS cap board? Curcio's cap board?

3. What are your preferred output tubes? NOS Mullards, I heard JJ's are great too, Svet's, EH's, Sovteks?

4. Preferred rectifier tubes?

5. Have you replaced the stock driver board with a new mil-spec board?

6. Power cables?

Etc., etc., discuss it all here!
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Re: ST-70 Mods

Postby rajan » Fri Feb 14, 2003 8:06 am

john poscablo cross referenced from the egroup wrote:ST-70 Owners,

I've done this upgrade on my ST-70.

1. Change the biasing to independent bias for each tube. You will
never find a perfectly much quartet of EL-34. If you are lucky
enough to source a quartet, Note that they will not deplete equally
on operation. Even a single twin triodes tubes will never have the
same parameters all the time. By doing this individual biasing of EL-
34, You are minimizing the crossover points distortion. Push pull
output stage has higher crossover distortion compared to a Single
Ended Triode Amp. In the future, you'll need to replace a broken
tube and not ending buying a new quad of EL-34.


2. Don't get it wired for triode operation!!! EL-34 is a PENTODE
tube. Gasoline engine is different from a diesel engine. They claim
to hear sonics increase, In expense for lesser tube life. Well,
their claim is being supported by their wallets and not by their
EARS.


3. Change the input RCA Jack to a gold plated one. You can never use
a good interconnect to the stocked one coz they are too closed to
each other.

4. Replace the speaker terminal with one that can hold a spade and
banana plug. This is for Biwiring options.

5. Replace the old power supply caps with Cerafine 47 mf 500 volts.
Solen Fast caps will also be best.


6. The stocked unit has a 5ar4 tube rectifier (Japanese brand) If
you can source a GZ-32 Mullard use it. I don't recommend 5U4,
They .will make the sound veiled.


7. Retain the original signal capacitor, But if you have a knack for
those fancy caps. Mit RTX is a good one 0.22 mf 630 volts


8. Keep a pair of NOS 7199. They are becoming scarce and expensive.
Modern production of 7199 will only give you hum problems and they
will never equall the NOS 7199.



My ST-70 is now in the listening room of a doctor friend, I'm into
triode amps now. Sometimes I am still missing the listening pleasure
from my Dynaco ST-70.

Treasure your ST-70's, The most loved tube amp in the world..


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Postby mozilla » Fri Feb 14, 2003 8:27 am

Raj,

Where did you find this? Any details on how us DIYers can do this ourselves?
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Post No. 1904

Postby rajan » Fri Feb 14, 2003 8:40 am

Moz,

It was posted by John Poscablo at the yahoo group. I don't see him posting anymore but Mickey normally corresponds with him.
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Postby arnoldc » Fri Feb 14, 2003 9:16 am

nice mod list! you can get some parts from Mickey -

a) MIT RTX .22uF 630V (P930)
b) Cerafine 47uF 500V (P2,500)

better yet, ask Mickey for his pricelist.
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Postby gaol » Fri Feb 14, 2003 11:40 am

Moz,

How did your stock st70 compare with vintage dog's st70 with the sound valves board?

Raj and I left just as you were setting up your ST70 so we weren't able to listen to the two...
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Postby mozilla » Fri Feb 14, 2003 12:14 pm

Sorry, we did not do a serious A/B of the two. We hooked it up and just played music w/o comparing. Chika time na & busog lahat..heheh
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Postby toobs » Mon Feb 17, 2003 10:16 am

Post your comments, ideas, experiences on restoration & mods for this amp.

Mozilla, here are some suggestions. My objective is to improve on the sound but preserve the original look of the ST70


To start off:

1. What is your preferred coupling cap? Jensens, Auricaps, Orange Drops?

I got good results using wondercap and MIT multicaps. You will need two pcs which are located on the board. Orange drops are also cost effective upgrade. Try them all in your ST70 to get a good match in your system. But I would like to warn you that you should be carefull in burning the copper traces of the board. You can break the traces by repeated soldering.
Keep the original black cat caps if you need to restore back the amps to its original state.

2. What is your preferred restoration/mod for the quad cap? Original replace ment cap? SDS cap board? Curcio's cap board?
As for the quad cap, i suggest that you keep the original quad cap and just add more capacitors inside the chassis. In this way you keep the original look of the ST70. Please note that do not add more caps in the 1st stage of the quad cap. This will cause undue stress on the rectifier tubes.

3. What are your preferred output tubes? NOS Mullards, I heard JJ's are great too, Svet's, EH's, Sovteks?
Mullard will always be better but since it is quite impossible already to acquire a good set, another good alternative is the Siemens skinny bottle which comes in different brands. Siemens was made in east Germany. It is still available at Angela.com if you need a match quad set. Locally, the Siemens are being sold under different brands like National and Westinghouse. As for new production brands, I prefer the Svetlana brands.


4. Preferred rectifier tubes?
Either a 5AR4 Mullard or Amperex. I do not recommend using new production tubes for the rectifier. Do not use 5U4 since it requires a 3ampere filament which will put unnecessary stress to your original power transformer.

5. Have you replaced the stock driver board with a new mil-spec board?
Rene has done this already. I also did this to my ST70 when I was able to buy a FR4 board from the US. I just transfered all the parts from the old to the new board.

6. Power cables?
I still used the original power cord to retain its original look.

7. Driver board circuit?
I had purchase the Van Alstine driver board but never had a chance to install it. I just kept enjoying the amp.
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Postby mozilla » Mon Feb 17, 2003 10:50 am

Ed,

Where'd you get your replacement board. Curcio sells them but seems too pricey at $40/board.

I see some at ebay goung for $25 though I don't know how reliable these are.

Thanks
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Postby vintage_dog » Sat Mar 01, 2003 6:57 pm

1. What is your preferred coupling cap? Jensens, Auricaps, Orange Drops?

i've tried so far:
oil caps: worse sounding- slow, rolled-off, siruppy sound
illinois caps: neutral, cheap, and surprisingly musical
orange drops: rolled off highs
solen: transparent, good sounstaging, extended highs (some find them bright)
stock caps:
auricaps: have not installed them yet but feedback from those who have tried them find them the best so far fpr st70. again, a matter of personal taste



2. What is your preferred restoration/mod for the quad cap? Original replace ment cap? SDS cap board? Curcio's cap board?

only tried replacing one unit with a NOS but added sprague atoms fo rthe B+
other unit has the welborne labs cap board with over 500uf capacitance (panasonics)...great bass, dynamic

3. What are your preferred output tubes? NOS Mullards, I heard JJ's are great too, Svet's, EH's, Sovteks?

forget the NOS. the are expensive. my fav is the svetlana. they are close to the mullards

4. Preferred rectifier tubes?

mullard and amperex

5. Have you replaced the stock driver board with a new mil-spec board?

have so far heard probably 6-8 differnet boards. i settled for the sound valves board. to me the best board is the welborne board, also teh most expensive and most difficult to install.
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Postby vintage_dog » Mon Mar 03, 2003 10:51 am

reposted, from PinoyDVD

--------------------------------------------------------------------------------


Quote from: levi on Today at 01:16:14am
Do you get tax if you bring in Vintage Tube amps like the ST70. Im planning to look for ST70 in the States this summer but I have no idea regarding customs rules regarding vintage gears. I only plan to bring 1 or 2 units and for personal use only. Any suggestion how can I bring it home. Thanks



levi,

todate, i may have brought in about 50 pieces of audio gear from the usa. most were shipped via balikbayan boxes. all arrived undamaged because they were properly packed.

the few that i brought home with my check-in cargo had some risks:

- i was held up at US customs twice already. last year, i checked in a vintage heathkit amp with the massive transformers. this showed up in their monitors and they had no clue what it was. so i was paged, brought to a secure area and my bag inspected. there were bomb detection procedures which took some time, and actually delayed the flight because of the amp ;-)

- i was back from the US again last month and brought home a DAC this time (used). bags were to be unlocked when checked-in. the bag arrived alright but with a note from US customs that they inspected the bag...the DAC must have shown again on their ray and they decided to open it up. net, you must be willing to take a risk in shipping gear in unlocked bags arriving at our airports. good luck.

- in general, our customs guys do not open bags unless you have more than 2 big bags. if they do, tell them you have some used equipment. not sure about tax rate, but if it's only one piece like a dynaco, they will probably not tax you.

- be careful about buying used gear in the US unless you are knowledgeable in testing them and you know the seller. in buying a dynaco, make sure the transformers are original AND working AND you have the orig version NOT the cheaper series 2. tubes are usually worn out even if seller says they are ok and working. you should get a good matched set while there.

hope this helps.

v 8) d
Last edited by vintage_dog on Mon Mar 03, 2003 11:08 am, edited 2 times in total.
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Postby vintage_dog » Mon Mar 03, 2003 10:52 am

Reposted from PinoyDVD:

------------------------------------------------------------------------------------

Quote from: mozilla on Today at 06:55:42am
When auditioning an unreconditioned "stock" St70, most people generally complain about the "muddy" bass and rolled off highs.

One must bear in mind that what he is hearing is NOT the ST70 sound but the sound of the 40 year old parts. Most of the resistors & caps are now out of tolerance due to age & heat.

All one has to to is to check the specs & replace these w/ new or NOS parts and whallah! you get one of the best sounding tube amps for the price.

Remember, there are basically 2 camps when it comes to st70's - those in the restoration camp and those in the modification camp.

Restoring means you replace the aged components with newer ones of basically the SAME SPECS. Some up the capacitance and voltage here & there, replace the RCA's, speaker posts and that's still considered restoration.

Some are into modifications: Uncle Ned's Triode Electronics board, Curcio, Welbourne, etc. Basically, all that's retained is the ST70's OPT's. Mods usually involve redesigned driver boards, power supplies, etc.

Most mod makers in the U.S. (esp. Joe Curcio with his 'premium board") claim -and their users swear by it too- that these modded amps can beat many high end, megabux tube amps (I won't name brands here). While some of those who belong to the classic restoration camp believe these mods do sound "better", it is not the classic ST70 sound anymore.

So take your pick. Restore & listen to the vintage ST70 sound; or modify and beat the hell out of your friend's mega bukol tube amp.

Cheers!



moz is right about mods and restoration.

i do both on my gear.

on the "upgrade boards" for st70, there are actually just 2 approaches:

- one, retain the basic driver (preamp) circuit topology of the st70, which is a penthode-triode. the orig board uses the 7199. they are rare, expensive and quite noisy at times. the triode electronics board and sound valves board use exactly the same circuit, but with different tubes, but still in the penthode-triode config. my sound valves board uses exactly the same circuit but have been repinned to take the 6gh8-6u8-6bl8-7687, etc tube config. advantage: dozens of permutations to tube roll while maintaining the SAME original design! well, i love this board big time ;-)

- the other is completely changing the driver circuit altogether. this is the welbourne labs. well, my welbourne labs st70 beats some of the modern amps costing 5-10x more. yes, i used to own one of these mega-branded amps, and it's now sold.

- and for pete's sake, NEVER replace your PS supply from tube (5AR4) to SS rectification, no matter what the other guy tells you...

- with a stock amp (orig board), the least you can do is replace offspecs parts - resistors and caps. the sound character of the ST70 now with all original parts is most likely NOT the sound of the ST70 as originally designed and played 40 years ago. why? resistor values have drifted or noisy, caps must be leaking if not deteriorated and open..what you get is a slightly muddy sound sometimes people call "tubey" or mataba. what to do?


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Postby vintage_dog » Sun Mar 16, 2003 9:12 am

mozzy,

so how's the recapping coming along?

new caps on the st70 need about 50-100 hrs to break-in and settle in. other vintage lovers have had good experience with the IC. one notices a more transparent presentation without the edginess of some cheap polyprop caps; extended lows and highs, but still musical...

v 8) d

i still have the 47uf...
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Postby vintage_dog » Sun Mar 16, 2003 9:12 am

mozzy,

so how's the recapping coming along on your st70?

new caps on the st70 need about 50-100 hrs to break-in and settle in. other vintage lovers have had good experience with the IC. one notices a more transparent presentation without the edginess of some cheap polyprop caps; extended lows and highs, but still musical...

v 8) d

i still have your 47uf for the B+ (after the choke)...
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Postby mozilla » Sun Mar 16, 2003 12:28 pm

Francis,

Thanks for your help last night. I was able to do the recap at about 1am already and was too tired to do much extened listening. Initial impresions were more extended lo's & hi's. It did sound like it needed more burning in to let the music flow. So I decided to burn it in some more before listening again. I am now logging 3 hours of burn-in so far. Still a long ways to go but very promising. Thanks.

You have to teach me how to hook up that 47uF. :wink:

For those following this thread,

I replaced the original Black Cat coupling caps with same-value Illinois Caps. Plan to roll them later with the popular Orange drops & post my listening notes here.

Other "upgrades" planned in the near future (not necessarily in this order):

1. replace the original driver phenolic board (same circuit design using 7199's), including tube sockets

2. Beef up power supply

3. Silver wiring for signal path

4. Modern resistors using same values but with better tolerance.

5. replace orig. power cord.

Upgrades to date are:

1. Replaced speakerterminals with gold-plated binding posts
2. Upgrade RCA plugs w/ gold-plated ones (c/o Hypertriode)

Otherwise, everything else is stock.

Note also, that I am planning to do a restoration - not a modification.

Those who have done mods or are planning to, pls share your notes here with the group.
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Postby mozilla » Wed Mar 26, 2003 8:36 am

Got to listen to the Illinois coupling caps last week after about 20 hrs of burn-in.

Right off the bat, the music was so much faster with more energy, esp in the highs. Cymbals & bells for example had more tonal colors showing variations in tone from the time they are struck to the time the sound decays.

The overall sound was that of a more modern amp with faster pace, more resolution & transparency. This however, seemed to be at the expense of a bit more bloom & air that the the original caps had. If that's a good thing or not, it would depend on one's personal preferences.

Normally, I would allow some weeks to familiarize myself with the sound of a new tweak or upgrade before proceeding with the next one, but spurred on by the sound of the Illinois caps, I decided to go ahead & replace the bias caps last night as well.

I replaced the original ones with Sprague Atoms, checked the bias & fired it up.

Without any burn in yet (I am told by a fellow St-70 user at AA that it takes about 2 weeks to get the full benefit of this change), I immediately sensed a more extended, more defined bass. Drumsets seemed to have more resolution defining a more precise & taut note. The plucks on acoustic bass also had more attack, rhythm & resonance. This also complemented the highs of the Illinois caps. The improvement was not on extension but rather on quality of the highs.

Listening more intently, I did find that there was a very slight drag on the overall sound which I hope will be recovered with more burn-in.
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Postby vintage_dog » Wed Mar 26, 2003 11:18 pm

one easy mod you can do is add some capacitance after the choke.

i have an illinois cap 47uf/450v you can use. this would tighten up the bass a bit and help improve dynamics.
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Postby mozilla » Thu Mar 27, 2003 6:51 am

vintage_dog wrote:one easy mod you can do is add some capacitance after the choke.

i have an illinois cap 47uf/450v you can use. this would tighten up the bass a bit and help improve dynamics.


Thanks VD,
You have to show me how to hook that up though. :lol:
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Postby arnoldc » Thu Mar 27, 2003 9:19 am

hmmm, interesting. is that observation (adding a 47uF after choke) is specific to ST-70 or could be applied in general? Adding extra capacitance in parallel, increases the actual capacitance, and may slightly increase B+ right? Or is it in series which is the opposite? Does that contribute to the added bass?
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Postby vintage_dog » Thu Mar 27, 2003 10:38 pm

arnoldc wrote:hmmm, interesting. is that observation (adding a 47uF after choke) is specific to ST-70 or could be applied in general? Adding extra capacitance in parallel, increases the actual capacitance, and may slightly increase B+ right? Or is it in series which is the opposite? Does that contribute to the added bass?


in the circuits (PP amps) where i've added extra capacitance, the B+ voltage didn't increase at all, BUT the ripple was reduced significantly.

you can add extra caps at the 2nd filter position BUT not the first one (after the rectifier tube). this 2nd bank gets connected to the output transformers usually (eg the center tab of an UL).

in some circuits, the rectifier tube is connected directly to the choke without a cap. adding even 10uf here (before the choke) increases the B+ voltage.

now again, i'll have the gurus explain the science behind that... :wink:
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