ST-70 build combining kits, help needed

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Re: ST-70 build combining kits, help needed

Postby TomMcNally » Sun May 08, 2016 7:00 pm

One of the things I'm not seeing that may be the cause of your hum, is lack of a filament ground reference. In the original Dynaco design they grounded the center tap of the filament transformer(s) via .02 capacitors. I don't think you have centertaps, so you need to create them on each side by connecting 100 ohm resistors (2 watt) from each side to ground. I'd do it at the EL-34 socket where you have some room. Take a look at the ST-35 manual, page 13 and look at R40 and R41, you need that on each side. I think that might be your problem.

... tom
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Re: ST-70 build combining kits, help needed

Postby Aljaheejus » Mon May 09, 2016 12:28 am

Tom, Thanks for bearing with me and all the help!

Okay, an explicit search for R40 and ST35 turns up the Parks manual you must be referring to! - ST35REVE.pdf - Also found the TUBEZONE version of that which likes a 3W resistor rating for R40 and R41.

I'm not good enough with these amps/schematics to know which pin(s) that is on the EL34s, but I think it is either 2 or 7 (or both, but the ST35 schematic referred to only requires one (1) resistor per side). The #2s on each side are connected to each other, as are the #7s on each side.
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Re: ST-70 build combining kits, help needed

Postby TomMcNally » Mon May 09, 2016 3:16 am

OK, so add a 100 ohm 2 watt resistor from each yellow wire to ground, and each green wire to ground. Total of 4 resistors.
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Re: ST-70 build combining kits, help needed

Postby Aljaheejus » Thu May 12, 2016 9:14 pm

TomMcNally wrote:OK, so add a 100 ohm 2 watt resistor from each yellow wire to ground, and each green wire to ground. Total of 4 resistors.


As suggested above, I connected the resistors to the same ground on each side that the resistors from pins #8 are connected, and it plays no music. A little hiss/hum, but the input signal is not coming over the speakers anymore.

Any other suggestions, testing recommendations, etc.? I'm going to add a photo.
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Re: ST-70 build combining kits, help needed

Postby TomMcNally » Fri May 13, 2016 8:45 am

It looks like you have them in the right spots, pins 2 and 7 on each tube to ground, right ?
That would not make the amp stop working, double check everything, something must have gotten broken in the process.
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Re: ST-70 build combining kits, help needed

Postby Aljaheejus » Fri May 13, 2016 9:43 am

I grounded, with the resistors, pin2 on front EL34 socket and pin7 on rear EL34 socket, so b/c #2 pins are connected with wire and #7 pins are connected with wire, yes, I think I grounded all pins 2 and 7 on all four sockets.

First I have removed those connections last night and will re-test as previously configured ASAP. I should get music again, but if not then something else may have been broken.

I will double check the wiring connecting to the Triode Cap Board next. The power transformer has a lot of wires that tie to that, including the center taps you mentioned earlier. Maybe I got their terminals wrong, or something.

I'm also worried about the PT wires I have passing through a metal base fashioned from a lid of some sort. Openings in the lid/base might have cut the insulation and shorted something... Lots to double check. I also posted about this on the dynacoforum to see if anyone over there knows something about this particular combination of parts I have chosen to combine. Thanks again.
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Re: ST-70 build combining kits, help needed

Postby TomMcNally » Fri May 13, 2016 10:11 am

What's the make and model of your power transformer ? You'll find it, probably something simple, since it was working the other day. I think I'll fire my ST-70 up for my bedroom listening. The amp I was using is the one I pictured with the diytube ST-70 board, but it uses 6BG6G's which have plate caps with High Voltage on them, and my new kitten climbs on everything, so I need that ST-70 in the cage !

This one has an older Triode board in it and one of Geek's power supply boards. Original RCA jacks and speaker terminals. :P

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Re: ST-70 build combining kits, help needed

Postby Aljaheejus » Fri May 13, 2016 9:38 pm

I retested after disconnecting the resistors, and still no music, so thanks for your reply and suggestion something else broke. Yes, something simple. Not sure how I thought to try it, but replaced the center tube, the 12AX7, on the DIYtube driver board and voila - music! (Wonder if it was the tube gone bad or just the socket connection. The sockets are new, beige, Belkin brand, I think.

The PT is a Tomiko PT-ST-70 III.

With shorted inputs I hear a low level of noise, more of a rush than hum, and certainly not a buzz. When I power off it produces a little sound of flatulence over the speakers.

Next I will try again connecting the grounding resistors on the EL34 sockets, pins 2 and 7; to see if it runs a little more quiet (with the shorting input plugs.) Is that a good next step toward a quiet ST-70?
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Re: ST-70 build combining kits, help needed

Postby TomMcNally » Sun May 15, 2016 2:41 pm

Looking at the label on your power transformer, it looks like it might have filament center taps ? Are there yellow/black and green/black wires connecting to the terminals on the capacitor board labeled FCT ? I assume that means Filament Center Tap ... in which case you should be covered. One of the problems diagnosing "long distance" is we don't know how loud the hum or noise is. Is it unbearable, louder than the music, so loud you can hear it three rooms away ? Have you kept the RCA jacks insulated from the chassis and grounded the way they show in the docs ?
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Re: ST-70 build combining kits, help needed

Postby Aljaheejus » Wed May 18, 2016 9:21 pm

Thanks again, Tom. Yes it does appear to have filament center taps, and I connected them to the Triode Cap Board, as a standard PA060 would be connected. I still plan to loosen that board and double check wiring that is mostly underneath it. I set up this new one next to a very quiet ST70 to have all equal connections, and still a significant hum.

The hum noise is not louder than the music, but I am used to a quiet ST70 with original circuitry which I have with original board, repopulated, and original transformers. I seem to recall, in rebuilding that amp, I had to disconnected the power plug ground from anything in the amp to get it quiet. My house has a 1950, two-wire electrical system. Not much point in connecting a grounding wire!
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