Original Dynakit ST70 - Quad cap is vibrating with the music

Hello All,
I have been enjoying the vintage audio hobby for close to 20 years. I have always taken my tube gear to techs for repair but my favorites have either moved or unfortunately passed. I figured that an ST70 would be a good start to learning how to do some of this work myself.

I recently purchased a vintage system from an estate sale. Dynakit PAS2, Dynakit ST70 and a pair of AR2a speakers.

The Pas2 and st70 appear to have been serviced sometime after 2001 judging by the date on the CE Quad cap. Lots of new yellow caps on the boards along with many new resistors. No selenium. All original boards.

ST70 has GZ34 rectifier, two Electro Harmonics EL34s, two EL34s (no white lettering but I found XF1 on one dual Halos - Mullards?) and two unknown 7199 tubes. The Mullards are in the right channel.

I powered up the amp and preamp with a variac over the period of a few hours. Adjusted the bias. The Left channel seemed like it was running out of adjustment on the bias pot Everything otherwise seemed fine and sounded good. Then after a few days of listening I saw and heard that the Right channel was motorboating. While I was looking up the causes for this, the Quad cap let go with lots of smoke and noise. Thankfully I was sitting in front of it and I cut the power.

I did some checking and found that my main voltage is 123VAC. This is pretty consistent. So when I had my variac set to 110 it was actually putting out 120VAC - Lesson learned and I will be adding a meter to my variac. Setting the variac at about 108 gets me 115VAC

I ordered a new 525 volt 80/40/30/20 quad cap. While I was waiting for it to arrive I removed the resistors from the quad cap and checked them. The 22K was at 20K and the 6.8K resistor read more than 13K. I checked all of the 1K resistors on the tube sockets and the one on V2 was 901 ohms. The others were very close to 1K. The Bias resistors were spot on. I picked up all new 1% resistors for the amp.

I wired up the quad cap with the diagram provided including new 22K and 6.8K resistors. I brought the power up slowly with the variac making sure to stop at 115VAC. I biased the tubes to 1.56. This time there was more adjustment available with the left channel. The amp sounds really good.

I touched the Quad cap to see if it was getting warm and was surprised to feel it vibrating. If the volume is increased, the vibration seems to modulate with the music. No vibration can be felt anywhere on the chassis or transformers. Maybe some on the output tubes. If I use a pen to touch the quad cap or the tubes I can feel the vibration but not as much. Is this normal? Is it just that the quad cap is tall and the chassis or transformer vibrations are being mechanically amplified?

I have been checking all of the forum posts and do not find anything about a quad cap vibrating. I did see in several pictures that the 6.8K ohm resistor is replaced by a straight wire. Not sure if this is only for VTA-70 amps. Should I delete the 6.8k ohm resistor?


Thanks for reading this long post.
Michael

Bob Latino wrote:The straight wire on the quad cap is just for the VTA amps .. You do need a 6.8K resistor in that position for original Dynaco ST-70 amps.

If the CE quad cap you have on the amp now is of 2001 vintage, you are correct in replacing this quad cap with a new one ..

Bob

Hello Bob, Thank you for the reply. The 2001 quad cap is the one that blew up. I replaced it with one from Dynakitparts.

So is the vibration I feel in the quad cap and a little on the tubes normal? I'm still uneasy after having the CE cap blow up in front of me on the dining room table.


Going a little off topic in my own post.
I did also just get an older VTA-70. It sounds great! I think that it is circa 2012 and has an autobias board installed. Based on some of your posts should I remove that board? It seems to be working but I do not want to risk another failure.

I like the way that the VTA-70 drives the Yamaha ns1000m's. I can't go as loud as I can with the 200+ watts of the p2200 amp but it sounds very good at lower volume levels.

Best Regards,
Michael

.

My amp is sitting close to a speaker and it does vibrate my output tubes if the music is at listening level, and the ringing of the tubes can be heard through the speakers. I put damping rings on the tubes as well as setting the amp on a more stable shelf. My ST-70 series II doesn't have the quad cap but I wonder if you have a similar situation.

I could not add the quote from Peter since it has links and I'm new to the forum.

Thank you Peter. If I touch the power transformer or other parts of the chassis I do not feel any vibration. The rectifier is a Mullard GZ34. I also have a 5UGB rectifier from the VTA-70 I also just picked up. It works fine in the VTA-70 but not in the ST-70. The Bias was way off. I will check my box of misc tubes to see if I have another GZ34.

I have a healthy respect for high voltage learned when I was a kid playing with an old oscilloscope. I pulled the high voltage lead off of the CRT and touched the "button". Quickly added that to my "do not do again" list.

The ST-70 is still connected to the variac so that I have 115VAC supplied to the PAS2 and ST-70. I'll check the voltages per the OG manual this afternoon. Then if all looks good I will try the amp without the variac and check the voltages again.

The variac is an ancient Superior Electronics Powerstat. Maybe it is affecting the amp somehow.

MechEngVic wrote:My amp is sitting close to a speaker and it does vibrate my output tubes if the music is at listening level, and the ringing of the tubes can be heard through the speakers. I put damping rings on the tubes as well as setting the amp on a more stable shelf. My ST-70 series II doesn't have the quad cap but I wonder if you have a similar situation.

Thank you for the tip. I have had other amps that had microphonic tubes. I used damper rings on them.

The vibration is there even at zero volume.

Whoa! Even with no signal... Then it makes more sense that it's coming from the power supply side. I guess voltage checks are in order.

.

Hello All,

I know this is going to be a "dumb" question but better safe than sorry.

In the ST 70 manual there is a chart for checking voltages.

Is it correct that any voltage that does not have AC next to it should be a DC voltage measured from the Pin to the main ground point in the amp?
Clip the negative lead to the ground lug and then with only one hand use the positive lead to touch the reference pins?

For the voltages that do have AC indicated, I check the AC between the two pins indicated? Pins 2&7 on the EL34s, Pins 4&6 on the GZ34 and Pins 4&5 on the 7199s? The chart does not show the brackets linking pins 4&6 on the GZ34 like it does for the EL34s and 7199s

The vibration in the quad cap seems less now. Deleting the Variac and using 123VAC main power did not affect the vibration. I got side tracked with repairing one of the AR2a speakers. Now it's time to return to the st70.

https://i.servimg.com/u/f74/16/95/26/06/screen10.png

Thank you!

Potato_Head wrote:Hello All,

I know this is going to be a "dumb" question but better safe than sorry.

In the ST 70 manual there is a chart for checking voltages.

Is it correct that any voltage that does not have AC next to it should be a DC voltage measured from the Pin to the main ground point in the amp?
   Clip the negative lead to the ground lug and then with only one hand use the positive lead to touch the reference pins?

For the voltages that do have AC indicated, I check the AC between the two pins indicated? Pins 2&7 on the EL34s, Pins 4&6 on the GZ34 and Pins 4&5 on the 7199s? The chart does not show the brackets linking pins 4&6 on the GZ34 like it does for the EL34s and 7199s

The vibration in the quad cap seems less now. Deleting the Variac and using 123VAC main power did not affect the vibration. I got side tracked with repairing one of the AR2a speakers. Now it's time to return to the st70.

https://i.servimg.com/u/f74/16/95/26/06/screen10.png

Thank you!

Yes you are right about the voltage chart, it's DC relative chassies.
The interesting values are DC.

.

I first tested the amp with the variac dialed and measured to 115VAC

I did not let the amp warm up for a long period of time. Just 5 minutes. My readings are in parenthesis.

Quad cap - 80/40/30/20 525vdc
A 305 (402)
B 375 (265)
C 415 (336)
D 435 (390)

EL34
3 410 (382)
4 415 (389)
5 -32 (-24)
6 -32 (-24)

GZ-34
2 435 (400)
4 with 6 360vac (700) is this because it is doubling the 360 value? Should I be using a different test point for each pin or measuring them together?
8 435 (400)

Circuit Board
#3 370 (327)
#18 370 (331)

I did not know where to test the selenium rectifier since it has been removed. Do I test each side of the diode to ground?

Overall the voltages look lower than specified with the amp supplied with 115VAC. The GZ34 pins 4 & 6 are way off unless I measured them incorrectly.

I will do a test at 123VAC next. I have been doing spot tests and our main voltage does not appear to fluctuate more than .5 volts AC.

Thank you,
Michael

I let the amp warm up for 10-15 minutes playing music.
Then I stopped the input and turned the volume down on the PAS 2

Quad cap - 80/40/30/20 525vdc
A 305 (436)
B 375 (287)
C 415 (365)
D 435 (423)

EL34
2+7 6.4vac (6.4vac)
3 410 (419)
4 415 (422)
5 -32 (-25.6)
6 -32 (-25.6)

GZ-34
2 435 (438)
4 with 6 360vac (755) is this because it is doubling the 360 value? Should I be using a different test point for each pin or measuring them together?
8 435 (438)

Circuit Board
#3 370 (357)
#18 370 (356)

Thank you for your help.

Michael

[quote="Potato_Head"]I let the amp warm up for 10-15 minutes playing music.
Then I stopped the input and turned the volume down on the PAS 2

Quad cap - 80/40/30/20 525vdc
A 305 (436)
B 375 (287)
C 415 (365)
D 435 (423)

EL34
2+7 6.4vac (6.4vac)
3 410 (419)
4 415 (422)
5 -32 (-25.6)
6 -32 (-25.6)

GZ-34
2 435 (438)
4 with 6 360vac (755) is this because it is doubling the 360 value? Should I be using a different test point for each pin or measuring them together?
8 435 (438)

Circuit Board
#3 370 (357)
#18 370 (356)

Thank you for your help.


The A B C D connections on the can cap seems twisted, but that is all right as they
are all similar. Apart from that it seems ok to me.

.

Hello Peter,

This appears to be an original Dynakit. The Power transformer is PA 060.

I will do my next tests at 117VAC instead of 115VAC. I actually measured the Variac Voltage at it's output. I will measure at the transformer primary leads next time.

I understand how to measure the Diode voltage now - Also how to properly read GZ34 pins 4&6. Thank you.

It does seem like there is an issue with wiring the Quad cap. I followed a picture and post by Bob Latino where he said put the 80 section (half circle) towards the rear of the amp. The diagram from Dynakit parts showed the same.

The label on the quad cap shows
Half circle - 80
Square - 40
Triangle - 30
Blank - 20

Petercapo wrote:You apparently have the 80/40/30/20 quad cap. The order of installation should be 40/80/30/20. 40µF on node "D," 80µF on "C," 30µF on "B" and 20µF on "A," all referenced to the schematic.

If I am reading the OG schematic correctly and I have the amp positioned so that the transformers are at the bottom, the nodes and values on the quad cap are.


Top Lug #3 = A = Square - schematic value 20 As Installed Top Lug #3= A = Triangle - cap value 30 Needs to be #3 A = Blank = 20
Right Lug #4= B = Triangle - schematic value 20. As Installed Right Lug #4 = B = Blank - cap value 20 Needs to be #4 B = Triangle = 30
Bottom Lug #1 = C = Blank - schematic value 20. As Installed Bottom Lug #1 = C = Half Circle - cap value 80. Needs to be #1 C = Half circle = 80
Left Lug #2 = D = half circle schematic value 30 As Installed Left Lug #2 = D = Square - cap value 40 Needs to be #2 D = Square = 40

There does not appear to be a way to rotate the quad cap to get the Blank, Triangle, Half Circle and Square orientation. I could switch the positions of the 22K and 6.8K resistors and also switch wires 19 and 21. Lets see if the 10MFD change and resistance change has any effect on the voltages.

The meter I am using is a Sperry DM-4100A. Thank you for the heads up on the leads. The meter goes to 1000VDC and 500VAC. Not sure how I could have read 755VAC.

Thank you again for your help!

.

Hello - Maybe these numbers will look better?

Petercapo wrote:Is the 6.8KΩ resistor (blue-gray-red) rated at 1-watt? Kind of hard to tell from the image.

The image of the bottom of your quad cap is, I think, looking correct. Hope someone else will also confirm.

It doesn't matter very much if the 20µF and 30µF sections are swapped. I cannot see where the other ends of the wires from the four lugs go, however. Can you trace them out and compare to your pictorial diagram? Where do the two wires connected to the semicircle go, for instance? I'd check the other wires to the quad cap lugs, too.

Yes - the 6.8K is 1 watt Flameproof and 2%. With a fresh battery in my meter it tested at 6.78K Ohms.

I swapped the sections anyways. Just in case the 10 extra MFD would make a difference. (switched 22K and 6.8 resistors and leads 19 and 20 so that Nodes A and B on quad cap are switched to B and A)

Lug 1 (C) Half circle - Red(faded) from each of the output transformers, one choke lead, 22Kohm resistor going to Lug 4 (now B).
Lug 2 (D) Square - other choke lead, lead connected to Pin 8 of rectifier tube.
Lug 3 (now B) Triangle - 22K ohm from Lug 1, 6.8K resistor and Pin 20 from circuit board
Lug 4 (now A) Blank - 6.8K from Lug 3 and Pin 19 from circuit board.

Quad cap - 80/40/30/20 525vdc
Lug 4 A 305 (235) 20MFD
Lug 3 B 375 (253) 30MFD
Lug 2 C 415 (401) 40MFD
Lug 1 D 435 (415) 80MFD

EL34
2+7 6.4vac (6.4vac)
3 410 (380)
4 415 (384)
5 -32 (-22)
6 -32 (-22)

GZ-34
2 435 (408)
4 360VAC (354VAC)
6 360 VAC (348VAC)
8 435 (408)

Circuit Board
#3 370 (242)
#18 370 (247)

Bias Diode
Cathode 50.1VAC
Anode -68.7
I am guessing that the rectifier tube may be getting worn?

The amp still sounds good!

.

Petercapo wrote:Node "D" should have the 40µF section, not the 80µF section. 80µF is a bit much to have immediately following the 5AR4 and could shorten the rectifier tube's life, though it should not affect the readings. The 80µF section should be on node "C."

My Bad, Going crosseyed and too many distractions at home.

Quad cap - 80/40/30/20 525vdc
Lug 4 blank Node A 305 (235) 20MFD
Lug 3 triangle Node B 375 (253) 30MFD
Lug 2 square Node D 415 (401) 40MFD
Lug 1 half circle Node C = 435 (415) 80MFD



I hope to have a lot of overtime next month. I think that I will just order new tube sockets, a pair of GZ34s, wiring and a VTA board. Maybe splurge on the updated power transformer too. There are repaired solder traces on the driver board and 7199s are probably always going to be an issue. Full tear down and rebuild. I will keep the OG parts so that the next owner can have them.

Then I will have two mostly matched ST70s (updated ST70 and VTA-70). The main difference will be that the VTA-70 has an AB board.

Thank you again for all of your assistance.
M