• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dynaco ST-70 Triode Conversion Techniques

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#21
I would humbly suggest you not "cheap out" on the switch. I use coin silver contact C&K DPDT (on-on) switches for this purpose, one per channel to minimize the extra wiring often needed with just one 4PDT switch for both channels. Not cheap, but I don't want failures.
Click to expand...

While I don't believe this switch is rated any higher for either current or voltage I must agree with Jim on the quality. I use C&K switches in commercial applications and you cannot beat the quality. I have had failures with cheaper switches of both the mechanical and electrical nature.
 
#22
OK, I received enough encouragement to tackle this thing again. I'm now on the hunt for a good C&K DPDT 9221. Ebay is lacking, so I'll check with my local shops around San Antonio. Thanks for the input.

BTW Burnedfingers, I drew up the same picture a few minutes before opening up the attached picture you had...

I've been staring at the schematics for so long now, I think I have them memorized.

- B

😀
 
#23
brosenau said:
OK, I received enough encouragement to tackle this thing again. I'm now on the hunt for a good C&K DPDT 9221. Ebay is lacking, so I'll check with my local shops around San Antonio. Thanks for the input.

BTW Burnedfingers, I drew up the same picture a few minutes before opening up the attached picture you had...

I've been staring at the schematics for so long now, I think I have them memorized.

- B

😀
Click to expand...

I have the miniature toggle panel mount version of those on hand if that will help.
 
#25
brosenau said:



Jim, I'm still a little new in this DIY Forum. What company do you represent? Do you have a web site?

- B

😉
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Rather than post it all here, could you drop me a note at:

jimmcshane AT prodigy DOT net (replace AT and DOT with the proper symbols)

I think that's more appropriate, this isn't just MY forum after all, and I want to be a good citizen. Thanks.
 
#29
Well, I was all set to bench check my ST-70 for the reasons why the bias voltage shot off the charts after my UL/Triode conversion, then I sprained my ankle in moving about in the garage. That's Murphy's Law for you...

Anyway, I drempt in my sleep that I might have a bad coupling cap or two on the PC-3. Since the right channel nearly always bolted out of control (momentarily until the left channel followed), I suspect that the one of the right channel .1mf or 390 caps is bad, but I'd just rather replace all of them to be safe - they are all very old originals. In fact, I've never seen these black with the yellow stripe types before now. Does the yellow stripe indicate neg or pos on these puppies? Of course, this is only a theory since I'm at work and not at home testing the unit...but hay, I need to start somewhere - right?

Most of the upgrades that I've seen include the orange drop caps that perform and look all-so-much better (from hear-say) may be difficult to find...at least at a reasonable price. Even my local vendor doesn't carry them.

Who actually has the best price on these orange gems?

- Brian

PS - Jim, thanks for the quick response on the E-mail!
 
#30
I just cruised E-bay to find some of those OrangeDrop caps for about $2.50 each. I guess they aren't as rare as I thought. A music store in NJ had a bunch of the .1uf in orange-drop and the 390pf caps in a black-drop.

Yup, life is good. I'm off to see what the forum has to offer on building my next project...a tube pre-amp.

Brian
 
#32
But I keep posting this and people still don't seem to understand - there are a wide variety of "orange drops", and they are not all equal in performance.

The only "orange drops" I use are the 716P Vishay/Sprague. They are film/foil polypropylene with copper leads. They are the only cap in the line with those features. The 715P uses steel leads, and there are many polyester caps too - such as the 6PS and PVC series Mallory, the 225P, etc.

Be careful what you are buying.
 
#33
Ah Jim, I just got lucky I guess. The ones I purchased were indeed the 716P versions. Since I'm still rather new to the DIYAudio forum, I have not seen much on the orange drop phenomena.

Just to ease your mind a little, I actually DID do a search to see what people were saying in retrospect.

I certainly acknowledge your spirit and experience, and I take your words with pride when I enter into a discussion with others. While here though, I'm the new guy with little to offer. So, I'm the proverbial sponge...for now...

Brian
:apathic:
 
#34
Jim McShane said:
But I keep posting this and people still don't seem to understand - there are a wide variety of "orange drops", and they are not all equal in performance.

The only "orange drops" I use are the 716P Vishay/Sprague. They are film/foil polypropylene with copper leads. They are the only cap in the line with those features. The 715P uses steel leads, and there are many polyester caps too - such as the 6PS and PVC series Mallory, the 225P, etc.

Be careful what you are buying.
Click to expand...

Jim, I installed the orange caps, and they really added some new life - it was like adding new tubes [or so I'm told].

Here's a new question for you though - when I make the initial bias adjustment, it's with the input muted. After I let it sit idle for a while I watched the meters carefully. Then, I added some music and watched the meters some more. As I added some volume, the meters remained stable until the volume was pretty healthy - then they began to move...upward ever so slightly...from the 1.56 to about 1.62 or 64. Is that normal, or is that the reason why some people talk about using zeners to control the bias so much? What do you think?

Brian
 
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