ST-70 clone complete, but plagued by buzz :(

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ST-70 clone complete, but plagued by buzz :(

Postby mugsy » Mon Jul 21, 2008 2:17 pm

Hi folks.

After a year of collecting parts (not much cash alas!) and 'little by little' piecing this classic together, I finally got to switch on point this morning. All parts are brand new apart from a pair of original output trannies with a mixture of cloth and PVC covered leads.

Driver board is the Dynakits restoration module (same as original). Quad cap is the Dynakits 525V 80/40/30/20 type. Bias caps are Black Gate standard 100uF/100V types. The power transformer is a 240V type from Triode (P-521-S). This vibrates/buzzes quite a bit - quite disappointing as I was led to believe from their website that these are meant to be quiet compared with original items. The mains here in Basingstoke, England, measures around 247V so that should be well within the specs of this power trannie...

I made sure I sure followed the original Dynakit wiring diagram to the letter, so am quite surprised I have hum issues.

I tried plugging just the ST70 in, disconnected the earth wire (I live in the UK), and used a pair of cheating RCAs. There was still a low level buzz coming from the speakers so - alas - the problem rests somewhere inside the amp. The unit seems to bias OK.

Music sounds absolutely stunning through the amp, so I really want to try and get rid of this hum or I'll have to go back to solid(boring)-state - eeek!

I'll post a pic once I've worked out how this is done Yellow_Light_Colorz_PDT_21

All help gratefully received!

- John

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Last edited by mugsy on Wed Jul 23, 2008 1:49 pm, edited 3 times in total.
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Postby mugsy » Mon Jul 21, 2008 3:31 pm

Replying to my own post... how sad! hahaha! Yellow_Light_Colorz_PDT_09

Just noticed that if I stick the red probe (on the continuity setting) on ANY of the Dynaco speaker terminals, and touch the black probe to the chassis, EACH of them causes a buzz from the multimeter showing there's a circuit.

Surely only the common terminal should be referenced to earth/chassis, or have I got this all wrong?!? (more than likely!!!!).

Ta,

- John
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Postby mesherm » Mon Jul 21, 2008 4:14 pm

Surely only the common terminal should be referenced to earth/chassis, or have I got this all wrong?!? (more than likely!!!!).


You don't have it wrong. The positive speaker terminals are naturally connected to the negative terminals through a coil of wire (the tranny outputs). The DC resistance will be low, about 2-5 ohms, so it will appear as a short in a continuity test.

First thing to do is short the RCA input jacks to ground. The center RCA conductor needs to be shorted to ground on both channels. Then see if you have hummmm.
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Postby Ty_Bower » Mon Jul 21, 2008 5:06 pm

Nice looking job on the wiring. It looks really neat and clean in there.
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Postby Bob01605 » Tue Jul 22, 2008 8:17 am

mesherm wrote:
Surely only the common terminal should be referenced to earth/chassis, or have I got this all wrong?!? (more than likely!!!!).


You don't have it wrong. The positive speaker terminals are naturally connected to the negative terminals through a coil of wire (the tranny outputs). The DC resistance will be low, about 2-5 ohms, so it will appear as a short in a continuity test.

First thing to do is short the RCA input jacks to ground. The center RCA conductor needs to be shorted to ground on both channels. Then see if you have hummmm.


The BLACK NEGATIVE speaker terminal must be grounded to the chassis. Each black speaker terminal (binding post) should have TWO wires connected to it.
1. One connection from the BLACK output transformer wire
2. And one wire that connects that BLACK binding post to the main ground lug erminal near the quad cap.

It does appear that you have each output transformer BLACK lead connected to chassis ground. You could check with a continuity tester if one ground lead somewhere else is loose or not properly soldered.

Try shorting the input jacks as was mentioned above - If the buzz goes away the problem is with your associtaed equipment.

Bob Latino
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Postby mugsy » Tue Jul 22, 2008 12:23 pm

Thanks for the comments everyone.

Perhaps I didn't make it clear in the first post, but I did try the RCA cheater plugs and the amp hummed, so the problem is purely with the ST70.

I also found out that not only are the Common (black) speaker sockets grounded (as they should be) but the others (4 / 8 / 16-ohm) are ALSO all connected to ground according to my multimeter test.

I think the problem is the large solder tags I attached to each speaker transformer lead to facilitate easy doing/undoing of these from the speaker terminals for future upgrades/etc. Think these were lightly touched the chassis, and the speaker sockets I used where also very close to the chassis. Some may have been touching there as well.

Anyway I will redo all this and see if the problem is solved.

Thanks,

- John

P.S. I also noticed that the bias would suddenly shoot up when using the cheater plugs... again suggesting a ground related problem which then affects the bias.
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Postby mesherm » Tue Jul 22, 2008 7:56 pm

Although it is not easy to see in the wiring photo, a wire runs from the bias supply positive to the center of the two 0.02 caps on the filament leads and then on to the main star ground by the filter cap. The original ST70 grounded the 15.6 ohm resistors to the ground lugs of the octals using the chassis as a ground return. Not the best of practices. I use 2 or 3 termial solder lug strip fastened at one socket screw to solder the end of the cathode resistor and then run a wire from each to the main star ground. I do that with the negative speaker terminals to ground also. That way no part of the chassis is used as a ground return. It means a lot of wires at the star ground but its worth it.
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Postby mugsy » Wed Jul 23, 2008 1:45 pm

Hi Mesherm.

I may try your suggestion if I have no luck, though the stubburn part of me wants to do this exactly to Dynaco's original instructions, since the unit is meant to have a -90db noise figure. There must be something I haven't done correctly...

Managed to get a vast improvement on the hum today by re-doing the speaker socket connections. Hum levels went down considerably, and most notable was that the mains transformer stopped making such a racket - I'd say it was approx. 70% quieter. I humbly take back what I said about the Triode power trannie being so noisey - now it's just a low level mechanical hum which I'm sure can be solved with rubber gromets or something.

Taking the earth off my DAC (Peter Daniel NOS unit) and the pre-amp (KingRex Pre) but leaving the CD player and ST70 earthed really improved things. Still get a slight bit of hum (NOT buzz) when the ST70 has nothing connected to the inputs (or RCA cheating plugs) so I guess the next step is triple checking my wiring to make sure it's the same as the Dynaco manual pictorials. I'm thinking that the blue negative return wire on the right hand side is right next to the mains transformer (which doesn't have a bottom plate) - perhaps this is introducing some hum into the circuit?...

Nearly there I think Yellow_Light_Colorz_PDT_06

- John.


P.S. Used - and loved! - the Electro Harmonix EL34, and am currently trying out the JJ KT77. One of the KT77s flashed momentaryily accompanied by a pop from the speakers. Biasing down to 1.5V DC rather than 1.56V seemed to cure this and they've been working OK for the past hour. Tried a quad of Golden Dragon 'Retro' KT66s but even on the lowest bias settings they still peaked at 1.65-1.7V DC so I thought I'd better play it safe and remove them.

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Postby mesherm » Wed Jul 23, 2008 4:51 pm

One of the KT77s flashed momentaryily accompanied by a pop from the speakers.


I had one JJ KT77 of a quad do the exact same thing and I couldn't set the bias on it as it would swing wildly. Since I got them from Eurotubes I assume they were tested so I assumed it was something in my amp. I bought a replacement KT77 from AES but it baised so different from the 3 remaining KT77s that I was forced to put the wonky one back in. It eventually settled down and acted right. That experience and others with JJ tubes has since made me avoid buying them.

Yes, move that blue wire further away from the tranny. The original ST70 PTs were shielded on the bottom so they could get by with routing wires close to the PT.
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Postby burnedfingers » Wed Jul 23, 2008 5:44 pm

Does the driver board make a ground contact with the chassis? If so this is a common error with aftermarket boards. In the event an aftermarket board has a ground track running around the outside of the board and also a ground point on the board that is to be returned to the star ground on the chassis you must consider not letting the board make a ground contact with the chassis. If the board has a ground track around the outside of the board it must be raised so that there is only ONE ground point. This will also mean using non-conductive mounting hardware.
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Postby mugsy » Wed Jul 23, 2008 5:48 pm

Thanks for that suggestion Burnedfingers - will check ASAP.

Mersherm, sorry to hear you had the same experience as well. Mine were provided by Watford Valves here in the UK who usually do a sterling job with testing the valves first. Like you I'd think twice about buying JJ in the future alas.

Still concerned why - at the very minimum setting on the 10k bias pots - I could only get 1.6-1.7V min on the KT66s. Acording to the manual they can be used in place of EL34s? Had no probs biasing either the KT77 or EH-EL34s. Perhaps the newer KT66s require slightly lower bias than the originals?

Double checked the bias resistors and they are spot on 10k.

By the way do I have the neutral and Live wires connected around the correct way? Have heard transformer hum (mechanical hum) can increase if not corrected correctly... even though we are meant to be dealing with AC...
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Postby kt88pppamp » Thu Jul 24, 2008 1:07 am

Is it normal for a power amplifier to hum and/or buzz when nothing is connected?
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Postby TomMcNally » Thu Jul 24, 2008 7:22 am

Yes, an amp should either be:

1) turned off
2) have a pair of shorting plugs on the input for testing
3) be connected to a source

Tubes amps should always have a load on the
speaker terminals also.
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Postby mugsy » Thu Jul 24, 2008 8:50 am

I have to say this hum issue is driving me absolutely nuts! The unit is pretty quiet using the RCA shorting plugs - just a wee bit of hum with my head next to the speaker, but nothing dramatic.

However it seems that any source I use increases the hum. I tried moving that blue negative return wire and it didn't do much - if anything. Have checked and re-checked the earthing connections inside the amp and all is as it should be (as per the Dynaco manual).

If one of those 0.02uF ceramic disc caps was faulty could that create these symptoms?

I've tried taking the earth off EVERYTHING and that didn't seem to help either.

HELP!!!
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Postby Ty_Bower » Thu Jul 24, 2008 11:26 am

Try it with an iPod, or some battery powered portable CD player.
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