New Project - Sunn 2000s

[pbman1953]

I just picked up a 2000s that needs work. I sent you pictures
 First, I tested the tubes with my Sencor. Even though it’s  a basic tester, at least it can tell me if a tube is good or bad.
 The unit had 4 vintage Tung Sol 6550’s- 3 tested good and one bad. But I have a GE 6550 that could possibly take its place. Both the TS’s and GE are 42 watts.
 The IEC 6An8 tested bad , but I have a GE . The Amperex 12ax7 tested good.
 The first electrical test , using my Weber Bias rite, was to test each power tube socket. I wanted to see if it read voltage. They were al  around 569 volts. I guess that’s a good sign that the OT may be ok.
 I plugged the tubes in and all there was a loud hum. I had sound from either 4 or 8 ohm jack. I figured that it would be the caps, hopefully not more. . Then I started to measure.
 Here’s the test measurements-
----------------------------------------------------------------------------------------------------
Resistors-
Carbon comp -
Silver (10%)-
 Mounted to the can cap -
15k @ 17.3k
33k@   36.2k
Power tubes – half were CC resistors, half stone
47 ohm- 50.7,50.5, 51, 50.8
The stones were marked 50ohm  and they were spot on
 5% (gold) resistors-
 2 @ 47ohm read 51.4 ohms
---------------------------------------------------------------------------------------------------------------------
 Large Non- polar caps
2- .25mf (10%) @ .28
 Electrolytics-
 2 @ 50uf ( silver body Mallory’s) -  1 @ 132 and the other measured “OL” no measurement
 Sprague caps-
 20uf@ 55
10uf@ 11.57
10uf@  38
 Can cap- which is a 30, 20, 20, 20
 Only the 30 and 1-20 was used
30 @ 41
20@ 31
20 @32.4
20 @ 32




At a first look, this amp looks untouched. It still has the 2 prong plug but some of the values are under the schematic
.
There’s a scenario I don't understand. The first filter cap is 20uf/600v off the standby to ground. That should of had at least a 30uf/600v value.  The next section, for "A" after the chokes, are 2- 10uf's , in parallel , to make 20uf @ 600v. Then the "B" section is using the 30uf post . off the can cap.  The "C" connection is using a 20uf post. The  2n'd and 3rd -20uf are not used. One should of been used for the "C" leg.
I'd appreciate any replacement value and filter cap suggestions .
 
 Thanks for looking!

[triode]

Are you measuring these electrolytic caps in circuit?

You note that one of them is 132 or something, but IIRC, the first gain stage has a 250uf/15v on it and not 22uf.

The open circuit cap sounds like it is open circuit. Again, I would pull one lead and check it out of circuit, if you have a real capacitance meter. 

I have worked on a few Sunn 2000S amps. Even for their age, I have a tendency to replace the small electrolytics in the cathodes, there are not many of them.

There’s a scenario I don't understand.

I am not sure I understand the question, are you asking if you should go back to factory and put a 30uf as the first cap. Again, IIRC, the 2000S has a real PI filter before anything (e.g. the output transformer tap is after the chokes in the PI filter in the power supply), so 30uf vs 20uf is not a  huge deal, unless you are designinga bucking system, which you should not be. The tolerance on the 20uf or 30uf caps is huge, but if you want to be "as specified when built" I think it was 30.

You say the second and third filters in the power supply are not used, but you also say the amp looks untouched, are you sure they are not used? I see a number of wires coming from the cap cans in your photos.I also see a broken negative lead on one of your 10/600s in parallel, I am guessing you did that (it matters not if you are changing them, and I have a tendency to change those old cardboard Spragues no matter what they are reading).

[pbman1953]

A couple issues have changed but I'll address your question first. The cut wire on the 10Uf Sprague was to get a better reading. I went back to isolate the 50uf's and one was in the low 90's and the other one still "OL" on the meter. I had a 47uf /160v so I took out the bad 50 cap and now it's super quiet and playable. I ordered the new cap parts.

On this one I'm going for stock values so I'm using:

Stage 1- 2@68uf/450v (in series)= 34/900v- with 220k resistors, one on each cap

A-  2@ 47uf/450v  (in series)= 23.5uf/900-  with 220k resistors, one on each cap

B- 22uf/ 450

C- 22uf/450

I'm having the tubes tested today to see how much life is left

[pbman1953]

"You say the second and third filters in the power supply are not used, but you also say the amp looks untouched, are you sure they are not used? I see a number of wires coming from the cap cans in your photos.I also see a broken negative lead on one of your 10/600s in parallel, I am guessing you did that (it matters not if you are changing them, and I have a tendency to change those old cardboard Spragues no matter what they are reading)."

The can cap is a 30/20/20/20. Only 2 posts of the can were used. The 30 and the 20.

The first section had the Sprague 20uf

A was taken care of from the 30 uf post

B was using 0ne 20 post

C was using a second post. BUT, they didn't add another 20uf post to make the 40uf like in the schematic

[shaun]

Just to throw in my 2c worth...

You say "I'd appreciate any replacement value and filter cap suggestions ."

Here are some suggestions. Those orange Atom power caps look way past their use-by date, especially if they have the old cardboard casings, so they should be changed. You have a lot of real-estate to spare, so you can replace them with much higher uF values and probably still use caps that are physically smaller, thus making the amp a better amp. Modern caps offer greater filtering in a smaller package. Make sure replacements are at least the same voltage rating or higher than what's in there now. So instead of a 20uF 500v cap, install a 40uF 500v or similar.

If you're worried about "vintage authenticity," keep in mind that E-caps have a limited shelf life, and if you don't have good power filtering caps, nothing else will make much sense.

Not sure if that's any help, but I hope so.

[pbman1953]

Thanks for the input !


Not sure if you saw my list above  but here's the revised list:


Stage 1- 2@80uf/5000v (in series)= 40/1000v- with 220k resistors, (one on each cap)A & B-  1- 32uf/32uf/ 560 volt JJ can cap- one leg for "A- then  one leg for "B" with all voltages above the stated minimum


C- 40uf/450-    updated

[Balticnoiseforge]

Also got a sunny 2000s, 20uF is actually factory for alot of the amplifiers. But it is 30uF on the schematic - but you can choose either value, or even higher since your has silicone dioder rectification.

I went for 30uF for the first filter capacitator with a 630v SCR solen - they are quite huge though.
Q
Also changed the 47ohm sg2 resistors to 470ohm, can be a good idea to do either 470ohm to 1K due to the high sg2 voltage.

Very clean looking amp you got either way!

[pbman1953]

Also got a sunny 2000s, 20uF is actually factory for alot of the amplifiers. But it is 30uF on the schematic - but you can choose either value, or even higher since your has silicone dioder rectification.

I went for 30uF for the first filter capacitator with a 630v SCR solen - they are quite huge though.
Q
Also changed the 47ohm sg2 resistors to 470ohm, can be a good idea to do either 470ohm to 1K due to the high sg2 voltage.

Very clean looking amp you got either way!

I changed out the 47ohm carbon comps to 1k's

[sluckey]

I changed out the 47ohm carbon comps to 1k's

That amp is an Ultra-Linear design! I recommend you put the 47Ω screen resistors back in.

[pbman1953]

I changed out the 47ohm carbon comps to 1k's

That amp is an Ultra-Linear design! I recommend you put the 47Ω screen resistors back in.

Good catch, thanks!
My question,  since I still have the original cc resistors- Should I put those back in or buy metal film?


After, a out of chassis metering, each 47 ohm is 50-51 ohms.

[sluckey]

My question,  since I still have the original cc resistors- Should I put those back in or buy metal film?

That depends on how boogered the leads are. If you can reinstall them just as neatly as they were originally installed, go for it. Otherwise, replace with 47Ω 3W metal oxide resistors.

BTW, the originals are 2 watt carbon comp and they look just as good as they did 55 years ago. Absolutely no sign of stress.

After, a out of chassis metering, each 47 ohm is 50-51 ohms.

Not an issue. Those resistors are 10% tolerance.

BTW, this is what Google has to say about those resistors.

[pbman1953]

My question,  since I still have the original cc resistors- Should I put those back in or buy metal film?

That depends on how boogered the leads are. If you can reinstall them just as neatly as they were originally installed, go for it. Otherwise, replace with 47Ω 3W metal oxide resistors. The leads are unchanged after the removal

BTW, the originals are 2 watt carbon comp and they look just as good as they did 55 years ago. Absolutely no sign of stress.- they are in very good shape with no physical abnormalities.

After, a out of chassis metering, each 47 ohm is 50-51 ohms.

Not an issue. Those resistors are 10% tolerance.

BTW, this is what Google has to say about those resistors.

Thanks!

[Balticnoiseforge]

I changed out the 47ohm carbon comps to 1k's

That amp is an Ultra-Linear design! I recommend you put the 47Ω screen resistors back in.

Are several good arguments to increase the value up from 47ohm also with the amp being run quite abit above 500vdc, in later sunn amp designs they even went for 1.5K for 6550s.

Its not any audible difference, but if you want to keep it all original then sure.

Using 47ohm or similar value makes sense for a hifi pa design, but with a guitar/bass amplifiers wouldnt you also have more current spikes when playing at high volume? If you know any article please share. 👍

[sluckey]

Using 47ohm or similar value makes sense for a hifi pa design

The original Sunn bass amp actually used a Dynaco Mark III hifi power amp mono block. They stuck with that circuit for all their first generation amps. Later amps like the Model T did use 1.5K screen resistors with a U/L OT, but the entire power amp circuit had changed too. I'm sure there were reasons.

But we're talking about a Sunn 2000S circuit which was still following the original Dynaco circuit. And the 47Ω 2W screen resistors in this amp showed no signs of stress. The resistors were used to prevent parasitic oscillations in the four tube U/L circuits (no screen resistors were used in the two tube circuits). They were not used to limit screen current. That's my only reasoning for staying with the original design. This is the circuit that made Sunn amps famous. The Sunn name was already fading from glory when the Model T came along.

[Balticnoiseforge]

Sluckey:
Yeah they are based on the dynacos, but also from a time when they were using gec 6550s - and from what ive understood many complaints against new production 6550 tubes is especially that their screen grid isnt as forgiving to voltage spikes or even as high vdc as their predecessors.

But i dont know what tubes OP are using, or how much vdc he has on the B+, i know mine which is tube rectified ( albeit parallel gz34 ) is upwards 520vdc - and if it is the diode rectified version that is still using the older power transformers it might run even higher, which most would consider high for most new production tubes