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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products

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4 posters

almost finished my ST-70 VTA build

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°1

almost finished my ST-70 VTA build

by nerpissad Fri Sep 28, 2012 4:36 am

I'm very close to completing my first dynaco build (well, any build for that matter).
I'm looking to paint the output and power transformers but was unable to find out whether one can simply take the covers off and then spray-paint them?

and while I'm at it, anyone have any ideas on how to make a cover for the pcb on top of the amp? i.e. what materials to use and how to correctly and aesthetically do it?

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: almost finished my ST-70 VTA build

by Bob Latino Fri Sep 28, 2012 6:42 am

Hi,

You don't have to take the covers off. What I usually recommend is to remove the screws that came with the transformers and use other screws to hold the covers on. Two screws per transformer will work fine. Sand the plates and any varnish that may be present on the transformers. Get all the sanding dust off with alcohol on a paper towel. Cover the wires by placing them inside a plastic bag and spray on two light coats. I like Krylon #1618 which is a semi-gloss black heat resistant paint but you can use other paints. Below is a photo of an amp I did for someone about 5 years ago. These transformers were painted with the Krylon #1618 paint. At the time I was using red fiber washers like on the original Dynaco amps. You can get these washers from Kevin at Dynakitparts.

NOTE - 10/20/15 > I have found that Krylon Fusion semi-gloss black (Krylon #2421) is even a BETTER paint to use on audio transformers. #2421 is a satin black and has a harder and more durable surface that the Krylon #1618.

Check with Roy at the Tubes4hifi web site for a clear fiberglass custom cover for your ST-70. The only thing is > I think that the covers are only made for the VTA driver board and not a stock driver board. Below is a photo of how the covers look on a VTA boarded amp. NOTE - You will have to move C13 - C16 to the bottom of the board. On this particular board shown in the photo C13 - C16 had not as yet been installed.

Bob

Last edited by Bob Latino on Tue Oct 20, 2015 9:20 am; edited 1 time in total

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°3

Re: almost finished my ST-70 VTA build

by nerpissad Fri Sep 28, 2012 7:36 am

thanks for your elaborate answers, bob!
let me see if I fully understand: I thought it'd be easier to take the covers off since the transformers are already connected and cannot be taken off the chassis easily anymore.
do you mean I should leave the transformers connected to the amp and just put a plastic bag around the wires and the rest of the amp while spray-painting?

also, thanks for the tip regarding the fibreglass cover. I do indeed have the VTA driver board, so this one will fit perfectly after moving C13-C16 to the bottom of the board! I'm going to have a look at the tubes4hifi site and see if I can find it.

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°4

Re: almost finished my ST-70 VTA build

by Bob Latino Fri Sep 28, 2012 7:48 am

Hi,

I was assuming that you would remove the transformers from the amp when I gave my description above. You should remove them from the amp but you can paint them on the amp if you do remove the bolts and covers and then carefully mask any area you don't want to get paint on.

On the driver board cover, I forgot to mention that C1 , C2, C11 and C12 must also be moved to the bottom of the board.

Bob

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°5

Re: almost finished my ST-70 VTA build

by nerpissad Fri Sep 28, 2012 4:55 pm

As can be expected from a first time builder like myself, my shiny ST70 is not working yet.
I've gotten to the stage where I should put in the three 12AU7's and turn the amp on for the first time.
The 12AU7's do not light up, and the manual says to check the AC voltage where the twisted wires connect to the pcb. From what I understand, these are the FIL points on the VTA board. It measures 5.8v instead of the 6.1v - 6.5v range that is recommended. I'm unsure how big of a deal this difference is? Also, I'm hoping that it's something obvious I'm missing because I went through all the wiring a couple of times to look for loose ends.
Is there anything else I should be checking in order to figure out what the culprit could be?

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°6

Re: almost finished my ST-70 VTA build

by Bob Latino Fri Sep 28, 2012 5:07 pm

Hi,

It is possible that the tube filaments are on and they just don't show up well. ElectroHarmonix small signal tubes are like this. Turn the amp on, wait about 2 minutes and see if the tubes get warm to the touch. If they get warm you are probably OK.

If you measure 5.8 volts AC - that is a little low but should be OK. That AC filament line voltage depends on your incoming line voltage.

Bob

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°7

Re: almost finished my ST-70 VTA build

by nerpissad Fri Sep 28, 2012 6:43 pm

You were right. I proceeded installing the left EL34's and the rectifier tube 5AR4. I turned the amp on and initially only the left output tubes lit up nicely but not the 5AR4. Thinking I had incorrectly inserted the rectifier, I changed it into the right position in the socket and turned the amp on again. Immediately, I heard humming and the rectifier tube lit up really fast. I didn't manage to reach for the off switch in time, so the rectifier tube turned off or 'blew' I suppose. Now no tube lights up and I believe I may need a new rectifier tube..

Guest: Guest

Post n°8

Re: almost finished my ST-70 VTA build

by Guest Fri Sep 28, 2012 9:42 pm

I finished a ST-35 build a while ago and was troubled with a 5.8 VAC reading at the heaters. Wall voltage was 122 at the time. All other readings were good except for a couple of others. I couldn't figure out what may be causing this. I was using a new decent quality multimeter to check the readings. Just for fun, I tried a Fluke meter that I had borrowed from a guy in town. The heaters now read 6.25VAC. The other "suspect" readings were much better as well. Someone told me later that some of the cheaper multimeters don't do AC very well.

So, maybe try a top quality multimeter ( if you're not using one now).

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°9

Re: almost finished my ST-70 VTA build

by nerpissad Sat Sep 29, 2012 4:05 am

thanks for your reply captain coconut. I do have 3 different brands of multimeters. they're not expensive ones, but they do all indicate a 5.8 VAC reading.
it was already late last night, so I didn't check the most obvious place to look first: the fuse. it has blown Wink

Wink

fortunately, that means the 5AR4 tube didn't blow. at least I don't see it from the outside.
only this morning I noticed that tubes have numberings on them for correct placement, just goes to show that I shouldn't be working on the amp for so many hours straight...

I'm going to get some extra fuses now and see if I can make some progression today.

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°10

Re: almost finished my ST-70 VTA build

by nerpissad Sat Sep 29, 2012 6:31 pm

I have spent most of today figuring out what I did wrong. the current situation is as follows: when I only insert the three 12AU7's, the 12AU7 tubes warm up and no fuse blows. it's only when I insert the rectifier tube and the two left EL34's the fuse blows immediately. I went through the wiring many times, fixing some not so nice soldering here and there, but it hasn't solved the fuse from blowing yet. I bought 5 additional slo-blo fuses today. I got three left as of now Wink

Wink

if anyone can let me know what to look for I'd be very grateful. staring at my wiring and soldering for hours on end hasn't paid off so far Wink

Wink

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°11

Re: almost finished my ST-70 VTA build

by Bob Latino Sat Sep 29, 2012 7:42 pm

Hi,

See if you can take a photo of the internal wiring and upload the photo with your post. One of us on the forum may be able to pick up on an issue with your wiring that may not be readily apparent to you.

Check the stickys above on how to post a photo. You have to host the photo somewhere else on the internet. The forum here won't host a photo.

The fact that the fuse only blows when you have the rectifier in there tells us that there is some issue in the high voltage system of the amp. (rectifier tube, quad cap, Russian PIO caps, the 4 power supply caps on the driver board, or maybe just a bad solder connection somewhere in the high voltage system) The fact that the fuse does not blow with no rectifier in there tells us that the power transformer is OK.

You could also try just powering up the RIGHT channel with the other two output tubes and see if that blows a fuse? If it does then the problem is with the main high voltage system.

Bob

Last edited by Bob Latino on Sat Sep 29, 2012 10:23 pm; edited 1 time in total

nmchiefsfan: Posts : 77
Join date : 2012-03-21

Post n°12

Re: almost finished my ST-70 VTA build

by nmchiefsfan Sat Sep 29, 2012 9:24 pm

Maybe your rectifier tube was damaged when you plugged it in wrong initially. A bad rectifier will cause the fuse to blow at power up.

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°13

Re: almost finished my ST-70 VTA build

by nerpissad Sun Sep 30, 2012 4:28 am

I tried making some hi-res photos of the internal wiring of the ST70 as best as I could..
overview:
almost finished my ST-70 VTA build Pq1AX

almost finished my ST-70 VTA build Pq1AX

right channel:
almost finished my ST-70 VTA build GxOTl

almost finished my ST-70 VTA build GxOTl

center:

almost finished my ST-70 VTA build QGcWJ

left channel:
almost finished my ST-70 VTA build 176gY

almost finished my ST-70 VTA build 176gY

5AR4:

almost finished my ST-70 VTA build YlAw0

sorry for the huge size photos, but I figured it'd help when zooming in for details.
I'm going to try power up with the right output tubes now and see what that does and report back.
thanks for all your patience and support, I really appreciate it.

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°14

Re: almost finished my ST-70 VTA build

by Bob Latino Sun Sep 30, 2012 8:07 am

Hi,

On V3 you have a wire going from the triode pentode switch to pin # 5 instead of pin #4. Pin # 4 on V3 has no wire connected to it. Remove the wire from pin 5 and connect it to pin 4.

Nothing else looks out of place but there could be another error in your wiring?

Also (although this MAY not cause any harm). On the Russian PIO caps. The manual tells you to space the leads at 1 9/16 inches (about 39 mm). If you do this the leads, which are covered with black tubing, will stay away from the sides of the Russian PIO caps which are metal and conductive. You spaced the leads too close together. Each cap position has two solder pads. When spaced at 1 9/16 inch you should be using the two OUTER solder pads but you used the two INNER solder pads which causes the leads to touch the side of the case - BUT - the black tubing should prevent any electrical contact unless there is a slight hole in the tubing. I would pull the caps and then bend the leads outward and use the two outer solder pads on each cap.

Bob

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°15

Re: almost finished my ST-70 VTA build

by nerpissad Mon Oct 01, 2012 7:52 am

thanks for the tip, mr. hawk-eye Wink

Wink

I also realized, and I'm reluctant to say, that I had three of the tube socket installed upside down. that means that connections meant to be soldered on pin#6 for example, ended up on pin #3. it was just like the pictorial, yet with the socket turned half way.
during the process of reinstalling the three tube sockets, one of the 1000 Ohm 1 Watt resistors broke, so I'm waiting to go to the electronics store and buy one along with a few slo-blo fuses just to be sure Wink

Wink

.
I'll be able to check whether the amp works without blowing a fuse in a couple of hours!

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°16

Re: almost finished my ST-70 VTA build

by nerpissad Mon Oct 01, 2012 12:30 pm

almost finished my ST-70 VTA build UrnvD

that's right, that's a working ST70 right there.
only the right channel is working as of yet, but I'm assuming (hmmm, we know whose mother that is..) it's only a minor issue.

I'm so glad I got it working. I will dive in to try and solve the issue with the left channel not playing now. I think I've read something about that in the manual somewhere. but if anyone wants to give me pointers where I could look.

so far I couldn't be happier with the support on this forum and specifically Bob Latino. without that I just don't think I would have been able to finish this project. really appreciate the patience and customer service!

Last edited by nerpissad on Tue Oct 02, 2012 5:40 am; edited 1 time in total

danskman: Posts : 17
Join date : 2012-01-14
Age : 64
Location : Bière, Switzerland

Post n°17

Re: almost finished my ST-70 VTA build

by danskman Mon Oct 01, 2012 2:34 pm

Welcome aboard, nerpissad!
If I just add my opinion, I would say that you should try to improve your soldering skills, particularly to avoid solder blobs; when you do a new soldering, always let the iron 2-3 seconds on both the soldering point and the component lead, or wire, the solder will flow on both and will make a good contact. In a tube amp, there is many heat and after some time, bad solders will start to cause problems (buzz, cracks, and so on...).
on stranded wire (those coming out of the transformers, for example), you should twist the end and slightly solder it, to avoid arcing, bad solder or other future problems. Try to keep signal wires as short and neat as possible,
OK, these are just some guidances I can give to you; no bad intention or critics,I just wanted to help.
Good luck for the rest of your build, take your time and ENJOY it; these ST70 are gems!!
Best regards,
Daniel

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°18

Re: almost finished my ST-70 VTA build

by nerpissad Mon Oct 01, 2012 3:01 pm

thanks for your constructive criticism, daniel! I for sure am not taking it as bad intention on your behalf at all.
I do realize my soldering skills could be improved a lot. the last time I held a soldering iron was in primary school, I think.
a lot of practice is the only way to improve these skills and I have seen a few examples of wiring done right as well. some people are able to neatly wire the entire amp.
my primary goal was to finish the amp by following the guide step by step. that, and obviously my skill level, are the main reason the wiring and soldering could have been better/neater.

all in all, this has been (and still is) just a wonderful experience for someone like me with no electronic background at all. my next project will be making a phono preamp (actually 2 preamps, one for each turntable).
I was told a phono preamp is way easier to build than an amp, so I get another chance to work on my soldering & wiring skills Wink

Wink

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°19

Re: almost finished my ST-70 VTA build

by nerpissad Tue Oct 02, 2012 5:47 am

I have found this troubleshooting page:

13.No sound in one channel: First eliminate tubes as the source of the problem by swapping tubes between the sides (EL34’s in pairs), and turning on the amplifier to see if the problem follows the tube (s) between the channels. (The 12AU7/12BH7 on our board is common between both channels, try substituting it).
Next, check all connections: swap speaker & input connectors (avoid doing this while amplifier is powered up), between the two sides, see if the problem changes to opposite side (in which case you’ve got a problem with a cable, speaker or preamp).
Look at wires connecting between the board & tubes, make sure no leads going to the output terminals are disconnected o r touching the chassis (except the 0/common lead on each side). If you stuck a lead in a PC board pad or eyelet and either forgot to solder it, that can have the same effect.
Check all voltages on the board where the diagram indicates, voltages way off indicate a resistor is open/wrong value.
Check voltages on pins 3 & 4 of all EL34/6550 sockets (where leads from output transformers connect). They should have between 375 to 475 volts in an ST70, 425 to 475 in a Mk4 and 450 to 525V in a Mk3 or Mk2 depending on rectifier tubes used, biaset being set correctly, and your line voltage, and power transformer tolerances, but ALL of those pins should have similar voltages. Missing voltage on one indicates an broken wire or open in transformer.

I have already tried swapping the EL34 from the left to the right channel, but that didn't change anything.
it is my understanding that since all tubes light up, that I should be looking at the audio signal path to find faulty wiring/soldering. is this assumption correct?
when I get home from work I'll start measuring the voltage on the tube sockets and see if I can identify an issue there.

nerpissad: Posts : 58
Join date : 2012-09-16
Age : 50
Location : amsterdam

Post n°20

Re: almost finished my ST-70 VTA build

by nerpissad Thu Oct 04, 2012 5:17 pm

not sure what happened today. my amp, which I already loved playing only one channel, suddenly decided to open the second channel as well without me having touched the inside. I heard a slight crackling noise and the right channel came to life!
I suppose I shouldn't complain as long as it works, but it's weird just the same Wink

Wink

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°21

Re: almost finished my ST-70 VTA build

by Bob Latino Thu Oct 04, 2012 5:30 pm

Hi,

The fact that the channel came to life all of a sudden is probably due to a bad solder connection on the right channel somewhere. It may go out again? If it does go OUT again try tapping on that channel in various spots with maybe a plastic felt tip marker (with the cap ON the end you write with) or an all plastic ball point pen - something insulated anyhow and see if the channel comes to life again - OR crackles on for a second or two ...

You can also do a full voltage check as outlined in the assembly manual and compare the good left channel with the shaky right channel. If you see no voltage or a really low voltage sometimes that is a clue to a bad solder connection.

Bob

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Post n°22

Re: almost finished my ST-70 VTA build

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