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..
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
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!
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.
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?
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