1974 Sunn Model A No Sound!

[plexi50]

Can someone please explain to me what the Standby switch in this amp is doing? Master Volume what??? It sure isn't any standby switch i have ever seen before. I have a perfect power amp and bias @ -57vdc on a pair of 6550's. The plates of the preamp tubes are very unbalanced. For example V1A 66vdc / V1B & 69vdc.  V3A 242vdc & V3B 42vdc. I have had to get away from this yesterday as it is really becoming a PITA.

[plexi50]

Channel 1 board & Channel 2 board. All electrolytics recapped/ Polorized and NP

[plexi50]

Schematic.

[sluckey]

That STBY switch simply grounds/kills the signal out of the MV pot, same as turning the mv to zero. No voltages inside the amp are switched.

[thetragichero]

time to get out the scope and see where the signal dies

[plexi50]

That STBY switch simply grounds/kills the signal out of the MV pot, same as turning the mv to zero. No voltages inside the amp are switched.

Why do they call the master grounding a standby switch?

[sluckey]

That STBY switch simply grounds/kills the signal out of the MV pot, same as turning the mv to zero. No voltages inside the amp are switched.

Why do they call the master grounding a standby switch?

Why do we call you plexi50?

[plexi50]

That STBY switch simply grounds/kills the signal out of the MV pot, same as turning the mv to zero. No voltages inside the amp are switched.

Why do they call the master grounding a standby switch?

Why do we call you plexi50?

Cause me and Sunn made up a name from thin air?  Also cause i'm not suppose to be messing with this d*m thing!

[sluckey]

Yep.   

[PRR]

Why do they call the master grounding a standby switch?

It shuts-up the amp while the preacher speaks. It isn't really necessary to power-off the amp to "stand by". It is nicer to the amp to leave it running for short breaks.

[plexi50]

The Preacher has spoken! I'm putting triple overtime in this trying to get it up and running. It's mine so i'm not under too much pressure to get it working right. Just a little depressed that i have tested every resistor,replaced most caps and voltage checked it to death. I will find it! I could rebuild the preamp boards to more of a Marshall circuit,but then i wouldn't have an original 1974 Model A either. All tubes are used but test good. (that means nothing). All sockets tight and re-soldered. I do get crackle noise if i rotate the volume pot's back and forth. Coupling caps? I will probably replace them next. I don't like just replacing parts because i want to know the exact cause, but being this old i suppose that's the only way to go. I could not believe the cost of all the electrolytics. Those 20uf / 600vdc caps alone were $29.00 each! The dual 80uf / 500vdc cap can was very reasonable. I had to use a 100uf-100uf 500vdc cap can. Couldn't find a dual 80uf.

[sluckey]

I don't like just replacing parts because i want to know the exact cause, but being this old i suppose that's the only way to go.

Well, stop throwing parts at it. Connect a sig-gen to the input and use a scope to trace the signal through the amp.

[PRR]

...20uf / 600vdc caps alone were $29.00 each!

Two 47uFd 350V caps, twice (a 4-pack) might be $25 total?

(And the "600V" cap is surely two in one sleeve.)

[j_bruce]

Two 47uFd 350V caps, twice (a 4-pack) might be $25 total?

(And the "600V" cap is surely two in one sleeve.)

So just to be clear can I ask: placing two caps in series adds the capacity inversely but adds the voltage rating directly?

i.e. 2 x 47 uF @350 in series = 23.5 uF at 700?

Thanks,
Jerry

[thetragichero]

yes but parallel current balancing resistors (the "totem pole" in the bigger fenders) is prudent

[PRR]

....adds the voltage rating directly? ...

Almost. Round down for safety. I would take two 350V, of the same type and age, as 600V to be safe.

Some of those old "600V" caps do turn out to be two 300V caps, but literally twins from the same vat, so they balance well.

[plexi50]

Ok. After throwing all new electrolytics and coupling caps at it, Polar & Non Polar i have 6 volts sitting on the channel 1 input jack and minimal sound maxed out. If i strum a guitar and turn the amp off, i get full volume and a slow fade out of sound 4 seconds worth. I have run a tone generator but that's no good until i can figure out how -6 volts dc is sitting on Channel 1 input jack. No voltage on channel 2 input jack. This is the weirdest preamp and how it runs through both channels. I was thinking a cap is installed in the wrong polarity but that's been confirmed a dozen time's not to be the problem. The damest thing is when i use the tone generator i have full volume and tone stack function on both channels. I know it's going to be something real stupid and simple by the time this get's figured out. Output transformer? I don't like ultra linear output transformers. All my voltages in the power section and bias are as the schematic. Time for a straight jacket!!!

[plexi50]

Pic's

[PRR]

...This is the weirdest preamp and how it runs through both channels....

"Both channels"? No, but it IS confusing. Apparently it was begun as one channel all on one PCB, and then another channel was added. But there are just two similar/different preamps, and a mixer with recovery stage. And more confusion around the switchable reverb path, right.

Let us see the six voltages around V101. That's an uncommon way to feed a tube. It may be more stable than usual, but it has fault-modes we never find in other amps.

[plexi50]

Yes PRR it probably is the most confusing amp i have ever seen or worked on. All my tubes are good. I have seen amp's when shut off produce full volume on bleed down before. Thanks for clarifying the schematic. That makes a lot more sense as to why i have been jumping back and forth with the push on connectors.  I'm still thinking it's a capacitor. ZERO voltage documentation or any other chassis pic's online. It's a Ghost!

[plexi50]

...This is the weirdest preamp and how it runs through both channels....

"Both channels"? No, but it IS confusing. Apparently it was begun as one channel all on one PCB, and then another channel was added. But there are just two similar/different preamps, and a mixer with recovery stage. And more confusion around the switchable reverb path, right.

Let us see the six voltages around V101. That's an uncommon way to feed a tube. It may be more stable than usual, but it has fault-modes we never find in other amps.

I also have 275vdc on the B+ preamp feed rail feeding V1 120K plate resistors. Fine. But only 68 volts dc on the plates of V1 pin #1 & pin #6. And similar for 2 other the other preamp tubes as well. Something is drawing the voltage down bad. Like some kind of ground through a misplaced electrolytic cap. NO! I marked the boards with polarity and capacitor values and placement and they follow the schematic. Look at the cathode resistor values and then look at the cathode bypass cap as well. These preamp boards are both totally from another planet in component values. I have had this happen in the past where an amp would have full volume as you shut it down (off) like bleed down. It is getting grounded some where at power up. The power supply section i have 100% faith in. Though that is generally where in the past i have found a bad PS capacitor that did the same thing this one is doing on shut down giving full loud volume and fade away. I have to walk away from this for a bit and think. I'm all thinked out!

[sluckey]

Notice that the preamp cathodes do not connect thru a resistor to ground. They connect to -22V. Measure that -22v where it connects to each board (pin 6 for ch.1, pin 27 for ch.2. Also verify that C105 positive terminal connects to ground. Report back.

[plexi50]

Notice that the preamp cathodes do not connect thru a resistor to ground. They connect to -22V. Measure that -22v where it connects to each board (pin 6 for ch.1, pin 27 for ch.2. Also verify that C105 positive terminal connects to ground. Report back.

C105 Pos terminal is going to ground. -28.5 volts on neg side of C105. Only one +22 volt wire coming from power board feeding channel 2 board at the very end. (Same wire from power board) CR302 is wrong and has +42 volts. No -22 volts on channel 1 board. Q201 and CR201 missing from channel 2 board. Some one added a diode under the board in that area. Picture channel 2 board where Q201 is missing i believe.

[plexi50]

Ok. Here is channel 2 underside of the board. A diode and trans should be on the top side. Why do people do stuff like this? I'm sure there was a reason. I could gut it and save time by building another marshall type amp. But i am not a quitter and i want to understand this. I know it's more problem than it's worth but such is life.  Pic's

[thetragichero]

looks like a component may have gotten shot enough to start lifting traces OR poor soldering/desoldering technique resulted in lifted traces. on some boards it's unavoidable, as they were constructed so cheaply they can only be sucessfully soldered once

[plexi50]

Ok i have deciphered the path and components under the board as the schematic. Don't know why they were relocated. All soldering and traces are good. Cleaned and got off all old flux from traces. But i am still getting -43 volts on CR302. I did use a 1N4007 diode if that makes any difference.  Diode and cap polarity is correct.Let me see if i can read the original diode i removed from CR302.

[plexi50]

Removed 1N4007 diode and cap. Replaced with original ZBC228 diode and cap. -22 VDC. FULLY FUNCTIONAL AMP!!!!!!!!!!!!!!!!!! My mistake on the diode value. This is what it's all about! Never giving up! Now im going to play this thing and report back on the good's and or bad of the tones. YEAH!

[sluckey]

CR302 IS A 22 VOLT ZENER!!! 1N4007 is not a substitute!

[plexi50]

CR302 IS A 22 VOLT ZENER!!! 1N4007 is not a substitute!

I rushed a bit when changing out CR301 which was shorted. My brain missed the symbol for zener on the schematic. Is 1N4007 ok for CR301 4004? I have (1) 6550 power tube that want's to red plate. I swapped sockets and it followed. Otherwise the amp sounds great! Decent good distortion and very good cleans. Like ice pick clean but warm and full sounding  using the mid frequency select switch and high/low boost switch.

[sluckey]

CR301 is the bias diode. 1N4007 will work just fine. But that shorted diode probably stressed the 6550s quite a bit. May want to replace the pair.

Does the reverb work?

[plexi50]

CR301 is the bias diode. 1N4007 will work just fine. But that shorted diode probably stressed the 6550s quite a bit. May want to replace the pair.

Does the reverb work?

Yes going to replace both 6550 tubes. The reverb does not have a signal sound as i touch the output cable. But i do not have a reverb tank for it. I have one that i plugged in and did hear the irritating spring sound through the speakers but it dulled the volume of the amp right away. Wrong tank. Do you have a tank in mind that would work? 29-0006 On the schematic. Fat chance that number still exists as for that manufacturer. Impedance info next to figure out. Maybe most any solid state reverb tank may be close and work.

[plexi50]

Just finished wiring up the reverb wires for the foot switch that were missing. Tested and have output using a funky unknown brand X reverb tank. Grounding foot switch shuts off the reverb. Ran the wires the best way i could from power supply area to reverb transistor & ground terminal. Another day has passed with much success. Thanks for all your help! When in doubt,Shout! If i can dig up a black wire long enough i will change the white ground out to black.

[plexi50]

Steve will IN4748A 22 volt 1 watt Zener Diode work here? Wattage is my question.

[thetragichero]

But i do not have a reverb tank for it,,, Do you have a tank in mind that would work? 29-0006 On the schematic. Fat chance that number still exists as for that manufacturer. Impedance info next to figure out. Maybe most any solid state reverb tank may be close and work.

from https://forum.sunnstillshines.online/index.php?topic=9988.0

the reverb tank model, like yours, is O.C. Electronics P/N 29.0006 Type 51.  The input resistance was about 175Ω and the output resistance was about 173Ω.

Upon further review Accutronics (belton) told me "The OC Type 51 is same as accutronics 8FB3A1B"

https://www.tubesandmore.com/products/reverb-tank-mod-8fb3c1b-long-decay-3-spring
https://www.tubeampdoctor.com/en/accutronics-reverb-can-8fb3c1b

[plexi50]

But i do not have a reverb tank for it,,, Do you have a tank in mind that would work? 29-0006 On the schematic. Fat chance that number still exists as for that manufacturer. Impedance info next to figure out. Maybe most any solid state reverb tank may be close and work.

from https://forum.sunnstillshines.online/index.php?topic=9988.0

the reverb tank model, like yours, is O.C. Electronics P/N 29.0006 Type 51.  The input resistance was about 175Ω and the output resistance was about 173Ω.

Upon further review Accutronics (belton) told me "The OC Type 51 is same as accutronics 8FB3A1B"

https://www.tubesandmore.com/products/reverb-tank-mod-8fb3c1b-long-decay-3-spring
https://www.tubeampdoctor.com/en/accutronics-reverb-can-8fb3c1b

Thanks so much for the research and info! This thing sounds really good! The cleans are unreal. The distortion is good as well. Im surprised i stayed up till 1:00 a.m. last night going over and over the -22 volts feed from C302 and figured it out this morning. Now to top off a good reverb tank! Thanks for all your help!

[plexi50]

The price of tubes has gotten bad. I don't have anymore 6550 Sylvania tube's. And from what i have read these 8417 tubes spook me. You can look at the bottom of the tube and see how close the elements are in relation to each other. Scarry! Im just going to have to fork out for a new pair of 6550 tubes.

[plexi50]

CR302 Factory Diode ZBC22B is Referenced as:
NTE NTE5136A, 22V @ 5W Zener Diode

[plexi50]

Update: Got the reverb tank thetragichero suggested and it sounds wonderful. Had to lift the board again and do some re-soldering of the push on terminal wire post's. The amp sound amazing with the frequency select switch giving a choice of three different tones. Thanks for all the help and info pointing me in the right direction. I'm in disbelief as to how good this amp sounds. I cant play very well anymore. I cut my left middle index finger off just below the nail 2 years ago working on an air compressor. (Got to close to the spinning stainless fan blades) St.Luke's told me it would never grow back and there was nothing they could do for me. Well unbelievably it grew back all the way. BUT" I have no feeling on the tip of that finger to run scales and be fluid enough to sound good. So i will eventually put it on Reverb or some where. Don't mean to gross out the thread with my story. Thanks again all!

[thetragichero]

my dad had a friend who only had one hand who was dynamite blues guitarist. sometimes i think about that when i have trouble playing something

[plexi50]

It's really irritating. Some times it complies, most times not. There are some amazing people in this world!

[PRR]

... only had one hand who was dynamite blues guitarist. sometimes i think about that when i have trouble playing something

Garcia and Iommi are the best-known short-handed guitarists. Django too.
https://www.pastemagazine.com/music/five-musicians-with-missing-and-damaged-fingers/

https://www.thetoptens.com/music/musicians-with-missing-or-damaged-fingers/

Andrés Godoy lost his right arm.

Roy Thackerson (The Fingerless Fiddle-Player).

Les not forget Les Paul: car crash, had his right elbow set for guitar, and only ring and pinkie on the left. He was playing killer way into his 90s.