Best choice of ST70 output transformers

[justinsweber]

Im building a ST70.
I intend to use Triode PA060 S Dynaco ST70 Upgrade Power Transformer.
El34 tubes, 5ar4 rectifer.

So the question is which to use. All are close enough in price I want the best chioce... performace and response ... I want this setup to last for a looooooonnnngggg time so over build under rated is important.

Choices as I see it are:
Hammond 1660K $115 per
50watts
3400ohms Primary impedance
Max DC 318

Hammond 1660N $104 per
60watts
4300ohms Primary impedance
Max DC 318

Edcor CXPP60-MS-4.2K $71
60 watts
4200ohms Primary impedance

Triode Electronics a470s $95
35watts/70watts
4300ohms



Thanx everyone.


justin

[justinsweber]

importance on primary impedance when chosing transformers?
Can some one explain.


justin

[EWBrown]

If you are building this amp on an ST-70 chassis, then the Triode A470s are the best choice, as they are virtual "clones" of the original Dynacos, and will fit the chassis perfectly. The other OPTs are most likely somewhat larger and heavier, and won't fit onto the ST70 chassis.

The original A470s were 4300 ohms, CT, and that impedance is a good target, 3400 ohms may be a bit too low, though they may deliver a bit more power.

Using the 60 watt OPTs would be overkill, in this application, as the ST70 was 35 watts per channel.

The Triode PA060 has more current handling capability and will not run as hot as the vintage Dyna power trannies.

The ST70 driver board from DIYtube and the VTA-70 driver board are both excellent circuits, and the boards will fit onto th ST70 chassis perfectly. I'd also go with one of the SDS or C Chong cap boards, instead of the multi-section "can" cap.

The cap boards fit under the optput transformer, inside the chassis. Some of these also have the bias circuitry on them, and the VTA-70 driver board has the bias circuitry and four independent bias adjustment pots on board.

HTH

/ed B

[Ty_Bower]

Im building a ST70.

Triode Electronics a470s $95
35watts/70watts
4300ohms

Go with the transformers for which the ST70 circuit was designed. The A470 is a good transformer, and I expect the clones which Triode is selling are just as good as the original. You may need to fiddle the values of the cap and resistor in the feedback circuit if you opt for a different output transformer.

Primary impedance is important, but not critical. Keep in mind the specified primary impedance is only valid when the secondary is loaded with a constant impedance as well. Ever see an impedance curve for a speaker? It's not flat. In general, as the primary impedance goes lower, you'll get more power and higher distortion out of the amplifier.

[kheper]

Triode Electronics a470s $95
35watts/70watts
4300ohms

The A-470s are the best of the lot.

I second EWBrown's preference for a capboard. A multi-cap went out on me, recently. It was only 1 1/2 years old.

[justinsweber]

Going under rated... IE big OPT is not a good idea? The Hammond 1660K seemed very close to origonal, but have more a grater watts rating. I intend to use EL34 tubes... and consider the tubes expendible... Id like to build this once.... over build it once...


I hear you that Triodes OPT are great... Ive bought alot from them... just wondering if there were better options if one wanted to over build.

[EWBrown]

If cost is no object, then Magnequest has some suitable "Peerless" OPTs, but be prepared for "sticker shock" :o

http://www.magnequest.us/

Mikey can wind just about anything, so "custom jobs" are available.

/ed B

[justinsweber]

ok.. I get it... in MY PRICE range the A470 from triode is best choice?

[burnedfingers]

I have used the Hammonds 1665K on a Quicksilver chassis I have and they sound good. They sound good with KT88's,KT90's, 6550's, 6BG6GA's and EL34's. Not bad bang for the buck.

The next Dynaco clone I put together my have the Edcors on it. Granted the Dynaco transformers are very good transformers but you have to realize that transformer technology has improved in the last 30 yrs over what it was before. I have used other Edcor transformers and the quality is excellent.

[Ty_Bower]

The next Dynaco clone I put together may have the Edcors on it.

Mike once built a "not quite a ST70" using the Edcor CXPP25-8-7.6K. It works well and sounds rather good, actually. There's a thread on it here: http://www.diytube.com/phpBB2/viewtopic.php?t=880

[burnedfingers]

Very nice looking amplifier

[justinsweber]

I guess my next question would be... if using Shannons ST70 board and a Cap board... coudl adjust values if needed... and intending to use EL34 tubes... would you chose the Hammonds 1665K or Triodes... Triodes have signif lower impedance, but output is lower too... The Hammonds have less Freq. response range... but output is signif higher. I want to really use the crap out of the amp and never worry Im over working the Iron.


Thoughts?

[EWBrown]

WIth a pair of EL34s and the usual ST70 operating voltages, your power output is going to be 35-40 watts, regardless of the maximum power rating of the output trannie.

THe 3400 ohm OPTs will deliver around 40W, with somewhat higher THD, the 4200-4300 ohms closer to 35W, with lower THD. The actual perceived sound level difference between 35 and 40 watts will be minimal.

The Triode A470s can handle higher power levels but the lower and higher frequency response will be affected.

HTH

/ed B

[Brinkman]

Also, check out dynakitparts.com. They offer replica Dynaco kits that are compatible with the online suport you find for vintage dyancos. I built an ST-70 kit from them and while I do not know if it is lesser, superior or equal to the triode clones, I know the build support from the gentleman who owns the company is fast and generous. This is crucial to a first-time builder and cannot be underestimated. Additonally, the power transformer has some upgrades allowing for a more diverse pick of output tubes.

I would suggest purchasing a chassis, power tranny, and a pair OPTs (they throw in a choke) from them and getting a driver from elsewhere as suggested. After all, it is the output trannies that make it an ST-70, so paying for higher-quality transformers suggests you want something "better" than an ST70.

If you want to open that door, "better" means a lot of different things to a lot of different folks.

[burnedfingers]

orginally posted

Posted: Sat Mar 07, 2009 6:13 am Post subject:

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I have used the Hammonds 1665K on a Quicksilver chassis I have and they sound good. They sound good with KT88's,KT90's, 6550's, 6BG6GA's and EL34's. Not bad bang for the buck.

The next Dynaco clone I put together my have the Edcors on it. Granted the Dynaco transformers are very good transformers but you have to realize that transformer technology has improved in the last 30 yrs over what it was before. I have used other Edcor transformers and the quality is excellent.

The transformers I used were 1650K and not 1665K.