Help With Triode Electronics / SDSLabs driver and caps board

knowledge base for the classic Dynaco ST70

Help With Triode Electronics / SDSLabs driver and caps board

Postby artclone » Wed Nov 12, 2008 6:38 pm

Hi all -

I emailed Triode Electronics a minute ago for help but I'm anxious to get working on this now, so I thought I'd ask here.

I installed the ST70 driver and caps boards in my Dynaco ST-70. The parts were well made, the components were all there and of high quality, and the instructions were pretty easy to follow. But I still might have gotten something wired wrong.

I have the boards in and am doing voltage checks as specified by the instructions with just the rectifier tube in.

I'm getting nearly 500V DC on pin one of the EF86 sockets when it says I should get closer to 80V.

Any ideas?

Thanks!
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Postby Ty_Bower » Wed Nov 12, 2008 7:34 pm

The voltages are dependent on the driver (EF86) and output (EL34) tubes being installed. If they aren't, there's no current draw across the power supply and anode resistors. No current draw = no voltage drop. You are seeing the raw unloaded voltage of the power supply.

If you want to do any kind of "pre-checks", I'd measure the bias voltage at the EL34 sockets. I would keep the rectifier tube out for this test. You may keep the other tubes in or out; it doesn't really matter.

I would not suggest running the amp with only the rectifier installed. If you must, make sure the power supply and coupling capacitors are rated for full unloaded voltage (in your case, 500 VDC).
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Instructions quote:

Postby artclone » Wed Nov 12, 2008 8:43 pm

Here's the instructions. I guess they mean to insert tubes between steps 10 and 11?



8. If the 5AR4 rectifier is still being used, insert it into its socket. Do not install any other tubes at this time.
9. Turn bias pots to minimum bias (fully counter-clockwise).
10. Plug amplifer in and turn on. Observe new circuit board for any signs of trouble, burnings smells etc.
11. With a DC voltmeter measure the voltages at the input sockets. They should be in the ranges shown below:



EF86

Pin
Description
Voltage

1
Screen
55-80V
2
Shield
0V
3
Cathode
1.2 - 1.4V
4
Filament
N/A
5
Filament
N/A
6
Plate (Anode)
80 - 110V
7
Shield
0V
8
Suppressor
0V
9
Grid
0V




12AU7

Pin
Description
Voltage

1
Plate (Anode) #1
235 – 265V
2
Grid #1
80 - 110V
3
Cathode #1
90 - 120V
4
Filament
N/A
5
Filament
N/A
6
Plate (Anode) #2
235 - 265V
7
Grid #2
80 - 110V
8
Cathode #2
90 - 120V
9
Filament CT
N/A
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Postby Ty_Bower » Wed Nov 12, 2008 9:34 pm

Those directions are curious, and only a little bit confusing. They do appear to have left out the part where you actually put the tubes in. I'll guarantee you the plate and cathode voltages around the EF86 won't be anything like those shown in the table if the EF86 tubes aren't installed.
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Success!

Postby artclone » Wed Nov 12, 2008 10:02 pm

Okay, I went for it. I brought it up slow variac because I was still a little bit scared. Bias was easy to set at 1.56V.

It works! Sounds good on the test bench, very clean compared to how it sounded stock. I just have an iPod plugged into it; when I listen to some LPs through the preamp I'll be able to hear if it really sounds any different.

Thanks for your advice!
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Success!

Postby artclone » Wed Nov 12, 2008 10:03 pm

Okay, I went for it. I brought it up slow on a variac because I was still a little bit scared. Bias was easy to set at 1.56V.

It works! Sounds good on the test bench, very clean compared to how it sounded stock. I just have an iPod plugged into it; when I listen to some LPs through the preamp I'll be able to hear if it really sounds any different.

Thanks for your advice!
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Postby soundmasterg » Fri Nov 14, 2008 5:41 pm

Would love to hear a report after you have it all dialed in. I've got these boards too but haven't installed them yet.

Greg

P.S. Which brands of EF86, 12AU7, and EL34 are you using?
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Triode Electronics cap board and driver board impressions.

Postby artclone » Wed Dec 03, 2008 11:39 am

I've been listening to my Dynaco ST70 after rebuilding it with the EF86 driver board and the cap board for about a month now.

The sound is excellent. Gone are the cracks and pops from loose sockets and ancient 7199 tubes. Gone is a general haze over all of its range.
The bass is solid, no longer flabby. The sound is clear and accurate to my ears, I can't think of how it could sound better.

In short, the amp no longer has a sound of its own, which is a good thing. I don't get tired of listening to it as I do my Kenwood solid state amp, but maybe that's just a psychosomatic preference for tubes. I've listened to the Kinks, Impressions, and Jimmy Reed on vinyl and Beck and The Strokes lossless tracks iPod and all sound excellent. Beck has very strong bass loops and the ST70 never falters.

I might replace the EF86 tubes just because I'm using budget new ones and I'd like to hear if some nice vintage variants (amperex, etc) sound different. I'm using a RCA 12au7 and Svetlana EL34s from bought about 6 years ago. The speakers currently are mid-range Polk three-ways.

Rebuilding the Eico HF-85 preamp is next.

Thanks again for your help and interest!

Coltrane
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Postby soundmasterg » Wed Dec 03, 2008 7:10 pm

Thanks for the report!

I've gotta get mine going so I can use it too.

You will hear a difference between modern Svetlana/Sovtek EF86's and vintage ones. I don't mind the sound of the Svets and they are reliable mostly, but the vintage ones are like taking a blanket off the sound. Granted I've only used them in guitar amps, but I'm sure the difference is there in audio too.

Greg
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