ST-70 triode mod

organ

Dynaco btothers! I performed the triode mod a few hours ago right after work. I CAN'T BELIEVE WHAT I'M HEARING! Everything improved 110%. It only cost me 50 cents! I have to say this amp is the best investment I've made in my audio system.

Power is down to about 10w/ch but the amp is more dynamic so far in low level. It's 4:00am right now, I'll listen again before work and report back on how it does at higher volumes.

This is a tweak you guys shold try if your speakers are efficient enough. I had great results with the 800i in pentode mode so I guess the amp should still be able to push the RT speakers to moderate volume. It still sounds like there's more than enough power for the RF-35.

The electronic store had NOS Russian 5AR4. I bought one for $22. Haven't tried it but I doubt it will beat the Mullard's more powerful sound. I'll have to research that particular brand. It's labeled "International C" on the box and the tube is Labelled "5AR4/GZ34, IC, U.S.S.R."

I asked the guy about 7199 tubes but he said someone came in and purchased all of them. I bet it was an ST-70 owner:). He had a 5U4 from te 50's.

Maurice

dcarlson

Good to hear. What are you using as a pre right now?

organ

Believe it or not I'm using my NAD pre. Class A sand pre. I wonder how it will sound with a tube pre.

Maurice

gidrah

Please tell me more of this triode mod. Gotta link?


Glad to hear you're digging it so much.

organ

Gidrah,
You might want to try this. It shouldn't have a problem driving your VOT. What's their efficiency rating?

Here's the link: http://www.netaxs.com/~vkalia/st70-rebuild6.html

My nose was a little stuffed up in the morning and my ears felt clogged but I did listen at high volumes for almost an hour.

After a while I noticed something was missing. The warmth and sluggish bass which gave the amp a beautiful vintage sound. That sound gives me a warm fuzzy feeling and I really miss it. I'll give it a few more weeks to see if I should stick to triode or go back to pentode.

In triode the amp sounds more modern with tighter and 'faster' bass which you can clearly hear in drums. The biggest surprise was in the mids and highs. Smoooooooth as silk and so much more 'air'.

Let me know your results if you decide to try it. Make sure you buy about 10 of the 100ohms resistors. Measure all of them with a multimeter and use the 4 which have the closest tolerance level.

Maurice

gidrah

Thanks for the link. I'll check it out when I'm a little more sober.

My Altecs are in the high 90's sensitivity. I will be getting the bass horn cabs pretty soon, which should lift them over 100. I also have my eyes on some LaScala cabs, which are reputed to work real well with my woofers. Either way, I'm driving these with MacIntosh SS for now - no complaints. The tubes won't come into play until I find some rears at the earliest. I'm looking at Dyna ST-70 for the front and I've got a couple of SS Dyna ST-120s for the rears and center. I've got some tube amps to work with, but the speaker I plan on using for the center seems to be much less efficient (even though it has the same HF horn/driver config, has 2 - 15" woofers and is much larger ).

F1nut

Cool stuff!

I just wanted to add that you can buy resistors with a 1% tolerance rating.

gidrah

I used to sell drugs to my Physics/Electronics teacher. I'm sure I can get them tested even closer.

HBombToo

Originally posted by gidrah
I used to sell drugs to my Physics/Electronics teacher. I'm sure I can get them tested even closer.

***:D ROFLOL:D ***

madmax

Congrats on the triode mode mod! I did this for a friend on his Marshall guitar amp a few years ago and he is still talking about it. On the manleys I can hear the difference as well.

Let me say this, you only paid $0.50 but if you had some guru do it for you then the price would have been somewhere between $400 to $900. Don't forget that. Knowledge is worth a premium here!

madmax

organ

Gidrah,
That should work great. If you use the La Scala cabinets your eff should increase to around 104db/w/m. Could the difference in your center be in the x-over? Selling drugs to the physics teacher eh? Pimpin Mary Jane?;)

F1,
Do you know what color coding they use for 1%? I know silver is 10% and gold is 5%.

Madmax,
Wow!:eek: That's a lot of money for a 20min job. The triode is amazing but I need more time to let my ears adjust to the new sound I guess. I really miss the vintage sound. If I still miss that sound within a few weeks, I'll probably go back to pentode.

Did you get similar results as I did on the Manlies? Smoother sound, more air, depth, better imaging. I'll have to ask the guy who's going to work on my amp if he could use the mono/stereo switch on the amp and turn it into a pentode/triode switch. I'm sure it could be done.

Maurice

madmax

Yes,
I got the same results. It's almost as if you add a little confusion to the sound when you switch to pentode mode. Mine has a switch on the front from the factory. They say you shorten the tube life dramaticly if you switch while the amp is on but I've done it anyway. (Had to do the easy A-B comparison). I think you will settle on triode mode so long as that is enough power for your room and speakers. Manley suggests you may find different settings appropriate for different types of music and speakers (because of power needed). On the switch, I think all you have to do is switch in the two resistors so if it is a dpst switch or better the stereo switch should do it. If yours uses a separate resistor on each tube (4) then you would probably have to use a relay. No big deal either way.
madmax

organ

Thanks for the tip Madmax. The guys has worked on many Dyna equipments and have performed some mods. I'm sure he should be able to do it. I'll let him know about your advice on using relay switching.

So far I'm still in triode. I'm starting to fall in love with the smoothness and imaging/soundstage. I noticed that I had to turn the volume a little higer to get the tube 'bloom' compared to pentode operation. I'm sure the speakers are eff enough. 98db/w/m.

Maurice