Dynaco PAM hum one side

RE: Dynaco PAM hum one side, posted on June 17, 2019 at 20:44:30
petercapo Audiophile Posts: 665 Joined: December 29, 2012	Compare wire routing between your two PAM-1s. Could be the filament electrolytics. If you can get a hold of a PAM-1 manual, look up the voltage check points and compare with readings you take in yours. There's also the hum pot on the back panel that might need adjusting.

RE: Dynaco PAM hum one side, posted on June 18, 2019 at 09:16:19
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	test

RE: Dynaco PAM hum one side, posted on June 18, 2019 at 12:17:37
petercapo Audiophile Posts: 665 Joined: December 29, 2012	It could be the can, yes, but the voltage readings are the next most important thing to check. You'd also want to take readings of your AC mains, as the voltage values in the manual are referenced to a certain value of AC mains.

RE: Dynaco PAM hum one side, posted on June 18, 2019 at 21:18:57
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	OK Bias

RE: Dynaco PAM hum one side, posted on June 18, 2019 at 21:33:37
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	test

RE: Dynaco PAM hum one side, posted on June 18, 2019 at 21:48:19

petercapo
Audiophile

Posts: 665
Joined: December 29, 2012

This is a little difficult. The humming PAM voltages are lower than the quiet PAM. But, I am not sure if the differences are accounted-for by the can.

I would hate to see you go to the trouble and expense of changing the can capacitor and still have the problem. Did you try different tubes in the humming PAM?

Otherwise, changing the can is not unreasonable. If you change the can, be careful not to damage the wafer under its bottom.

RE: Dynaco PAM hum one side, posted on June 18, 2019 at 21:50:37
petercapo Audiophile Posts: 665 Joined: December 29, 2012	Maybe someone tried shielded wire to get rid of the hum. It would also be good to review all the wring against the manual. Are all the grounds well soldered, for example?

RE: Dynaco PAM hum one side, posted on June 18, 2019 at 22:34:24
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	test

RE: Dynaco PAM hum one side, posted on June 19, 2019 at 05:15:25
petercapo Audiophile Posts: 665 Joined: December 29, 2012	Yes, but it can be other things, as well.

RE: Dynaco PAM hum one side, posted on June 19, 2019 at 08:57:34
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	test

RE: Dynaco PAM hum one side, posted on June 19, 2019 at 09:56:04

petercapo
Audiophile

Posts: 665
Joined: December 29, 2012

In the schematic I see the 20uF section connected to: a 10kΩ resistor, a 470kΩ resistor, eyelet 11, and another 10kΩ resistor.

There's a copy of the manual with pictorial diagrams at this link:

http://akdatabase.com/AKview/albums/userpics/10004/Dynaco%20

RE: Dynaco PAM hum one side, posted on June 19, 2019 at 11:32:05
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	test

RE: Dynaco PAM hum one side, posted on June 19, 2019 at 13:03:58
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	test

RE: Dynaco PAM hum one side, posted on June 19, 2019 at 13:18:36
petercapo Audiophile Posts: 665 Joined: December 29, 2012	We'll see what happens with changing the multi-section can.

RE: Dynaco PAM hum one side, posted on June 19, 2019 at 22:39:30
omnibusspresss@yahoo.com Audiophile Posts: 18 Joined: February 17, 2008	test

Look at this discussion on hum , posted on June 20, 2019 at 08:15:36

petercapo
Audiophile

Posts: 665
Joined: December 29, 2012

Hence my reluctance to immediately replace the can. Hum can be a vexing problem with multiple possible causes.

http://dynacotubeaudio.forumotion.com/t4119-hum-and-its-caus

RE: Dynaco PAM hum one side, posted on June 20, 2019 at 08:18:47
petercapo Audiophile Posts: 665 Joined: December 29, 2012	Glad I could be of at least some help. You know, this forum used to be a hive of activity. IMO, the archives in this forum are the best on the Internet.

"the archives in this forum are the best on the Internet", posted on June 24, 2019 at 06:01:57
1973shovel Audiophile Posts: 10117 Location: Greenville SC Joined: February 25, 2007	I agree with you Peter. There's a wealth of accurate Dynaco information contained in these Archives, which is not always the case elsewhere. Even though Mr. Curico seems to be missing in action and this forum is often a ghost-town, I hope Rod never decides to shut it down.

Couldn't agree more!, posted on June 24, 2019 at 06:38:03
petercapo Audiophile Posts: 665 Joined: December 29, 2012	Let's please keep it going!

I hope my Asylum membership $..., posted on June 27, 2019 at 07:06:55

1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007

...helps keep these archives open.

But as a back-up, I try to search information on what I'm interested in (or might be interested in, in the future) and copy and paste the information into a Word document.

I'm guilty of posting Dynaco information elsewhere, simply because of the tumbleweeds on this forum. If you haven't already seen them, here are two of my recent posts:

The first one is a link to the 1958 David Hafler article describing Dynaco's new MK-III amplifier, which can be found here. (link)

The second is to dispute the often repeated error that if a SCA-35 is to be used as a power amp only, simply bypass C17, the 0.1 uF input cap
with a piece of wire, as it's only there to block DC. It turns out that's NOT why the cap is there, but the uninformed mod suggestion persists. (link below)

I neglected to ask Dave Gillespie about C1, the series input cap on the ST-35, but suspect the answer on that cap will be the same. Dynaco was not noted for adding unnecessary parts, yet more than a few "experts" often suggesting bypassing that cap too.

And by the way, thanks for helping keep this forum active!

SCA-35 info

RE: I hope my Asylum membership $..., posted on June 28, 2019 at 08:20:13
petercapo Audiophile Posts: 665 Joined: December 29, 2012	This is great information. You have convinced me to use the original Dynaco Biaset scheme with the one cathode resistor per output tube pair, for my next Stereo 70. Also, to use the input coupling capacitor in my Stereo 35. A while back I posted about it, but nobody knew what it was there for.

"great information", posted on June 28, 2019 at 13:10:41

1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007

I have a pair of MK-III amps which are long overdue for a rebuild. I've decided to return them to single cathode resistor and one bias pot, when the time comes.

As to C1 in the the ST-35, after I replied to your post, the need for the input capacitor was on my mind. I wrote Dave Gillespie this morning, asking him about it. Below, in bold, is his reply.

"C1 in the ST-35 does allow a small amount of contact bias to form across R2, but nothing like the amount developed in the SCA-35 design, and in this case, it's really not needed for the purpose used in the SCA, because small signal triode tubes are inherently more consistent devices from one tube to the next than the tri-pent tubes are, so performance is inherently more stable, which is always important in direct coupled designs.

In the ST design, the AF Amplifier portion of the 7247 tube is biased closer to the Eg=0 end of its load line, caused by minimizing the cathode bias resistor used to its lowest practical value. This, in conjunction with the large value plate resistor used, then ensures maximum practical gain from the stage. Less bias voltage increases gain, with the reduced resistor value also allowing less negative current feedback voltage to develop across the bias resistor under dynamic conditions, which also helps to ensure maximum gain from the stage as well.

Now the generation of contact bias is a natural occurrence in all vacuum tubes employing a control grid, the amount generated being dependent on the design of the tube, as well as current flow through it. Sometimes it is maximized and exploited as it is in the SCA, other times, circuit conditions minimize it, as in the ST (the grid return resistor in the ST is 1/10th the value used in the SCA). When C1 is installed in the ST, contact bias (however small) can form, allowing the tube to be further stabilized by isolating it from the DC resistance presented at the input jacks by the source driving the amplifier.

By far however, the bigger effect of C1 is to isolate the input stage from any DC present at the output of the preamp, which the early PAS preamplifiers in fact had. Therefore, its presence is most likely a protective measure more than anything else. And, its value is large enough that when tested under honest, controlled conditions, it would be exceedingly hard to know when the cap was in, or out of the circuit.

Best-

Dave Gillespie"

So my assumption was wrong, and it does seem that C1, the 0.1 uF cap could be "optional", if one is certain that their source has no DC present, according to how I interpret Dave Gillespie's explanation. However, the "contact bias" stabilization from leaving it in place, perhaps using a film cap instead of the original ceramic, seems worthwhile.

That said, when I scratch build a ST-35 on a larger chassis, I'll try the circuit with, and without C1, and if I can't hear any difference, then I'll leave it in.

What started all this for me is that I bought a couple of Dave's circuit boards, to convert my SCA-35 to use the 6GH8A tube, in place of the unavailable 7199. I asked Dave about omitting C17 (as one popular mod for the SCA-35 suggests) and you read his reply I linked to about why it needs to be there.

Another thing I learned from Dave, and might as well be put into the Dynaco archives here, is regarding C18, listed as 0.1 uF in the SCA-35 parts list. The value of the cap Dynaco used is actually 1 uF, and apparently the 0.1 uF was a typo.

Thanks again for helping keep this forum alive!

RE: "great information", posted on June 28, 2019 at 15:15:59
petercapo Audiophile Posts: 665 Joined: December 29, 2012	Dave Gillespie and George Ronnenkamp are treasures to the hobby. I'd love to see another Dynaco forum run by them and Kevin Devaney...

Yes, so would I! (nt), posted on June 29, 2019 at 04:39:09
1973shovel Audiophile Posts: 10117 Location: Greenville SC Joined: February 25, 2007

Dynaco-Doctor Forum

Dynaco PAM hum one side

184.69.65.66