SUNN Concert Bass

[e_c_mccollum]

Have a good question for you guys........

I need a schematic for a Sunn Cocert Bass amp. This amp is basic having no dual distort and footpedal.

The only schematic I could find has a dual distort with a footpedal. This schematic has some variations in the pre-amp / output stage that are different from what I have. Same wattage, but different circuit design.

Until I find one, maybe you can shed some light on why Q4 (2N3055) is heating up quickly upon turning amp on.

Transistor tests good, checked / replaced emitter resisitor, -34 volt power supply is good, driver transformer has proper voltage on primary side, driver transformer test good (current schematic lists dc resistance on primary and secondary), wires test good, fixed soldering. Nothing currently tests bad.

I am going to replace Q4 socket. Looks good, but never know.

[bbartilson]

Until I find one, maybe you can shed some light on why Q4 (2N3055) is heating up quickly upon turning amp on.

Transistor tests good, checked / replaced emitter resisitor, -34 volt power supply is good, driver transformer has proper voltage on primary side, driver transformer test good (current schematic lists dc resistance on primary and secondary), wires test good, fixed soldering. Nothing currently tests bad.

I am going to replace Q4 socket. Looks good, but never know.

You say the -34 supply is good, but you say nothing of the + supply. The fact that Q4 is drawing excessive current and heating and the supply being dragged down are likely very much related.

With no load/speaker, and no signal, check and report the DC voltages on all legs of Q4. (or at the closest convenient, but directly connected point. Turn the thing off in between measurements if you need to.

Is there DC at the output jack? If so, how much?

I'm going to guess that with no load, your heating of Q4 should disappear. If it does not, remove it and swap Q3 and Q4. Does the problem stay at the site of Q4, or does it move with the transistor? (This presumes you can't simply replace the transistor with one from your stock - which would be the best choice to verify the part isn't bad.)


Report back with the voltages around Q4 and at the speaker output, and the result of your swap/replacement of Q4, and we'll move on.

-B

[Enzo]

Each of the four output transistors has the same little circuit around it. COmpare readings to Q5.

This output stage is transformer driven, so make sure the transformer winding for that transistor is OK. For each transistor there is a voltage divider comprising a 1.5k on top and a 15 ohm under it. The transformer winding is wired from the junction of those to the base of the Q. In the case of Q4, there is a 0.27ohm resistor from the emotter to -34. The 15 ohm resistor runs to the -34 as well. So all this means is that there should be about 15 ohms between emitter and base while in circuit. If it does not measure 15 ohms but is a lot higher, then either the 0.68 is open, the 15 ohm is open, or hte transformer winding is open or its wire broken.

When transistors fail or have difficulty always check the resistors associated with them.

This amp should function well enough for testing with Q4 removed.

I will post a partial drawing on www.ampix.org

[e_c_mccollum]

Thanks,

I have already exchanged the transistors and it does'nt migrate with the original transistor. No dc on output. I checked the .22uf coupling capacitor.

And yes, I have checked the positive power supply resulting in a + 34 volt reading where it should be. The + and - voltage does'nt drop.

I replaced Q4 socket with some success. Q5 socket is next. My new ones have much better contacts.

I will let you know the voltages around Q4.

Recently, I had to remove the 3055's to remove the heatsink. The heatsink screws came loose and dirt had accumlated in the gap. I like maximum efficiency. After all, it is a SUNN. Anyway, when I put the 3055's back in I was hoping the socket contacts would be okay. Never know with old equipment.

I love to rebuild vintage equipment, but sometimes it can be a pain.

I am on top of it thanks to my new schematic.

[e_c_mccollum]

I was wondering if I could exchange Q4 driver tranformer leads with Q5 to see if the problem follows to Q5. This amp had a blowout before (R124, 22ohm 5watt burned). Not suprised if the transformer was affected in some way. (bought at pawn shop)

Where could I get another driver transformer if bad?

The main filter caps don't test bad but are old. I am going to exchange the two to see if it affects it in any way.

I always test my amps on smaller fuses to protect them.

[Enzo]

Filter caps would make the amp hummy, they would not cause ONE transistor to overheat.

New transformer? HAH, that's funny.

DO that resistance reading I mentioned base to emitter on Q4. WHat does it read?

[e_c_mccollum]

Q5: 28.3 ohms
Q4: 26.4 ohms
Q3: 26.0 ohms
Q2: 26.8 ohms

Sounds way off.............

The schematic and your last posts conflict. The voltage divider does have a 1.5kohm on top but uses a 22ohm 2 watt resistor on the bottom connecting to the -34v. Mine uses 24ohm 2watt. The 15ohm resistor does connect to the -34v, but is in series with a diode connecting to the base of the Q.

The schematic you sent me is a pretty good representation on my amp. Some IN4007 diodes exist that are not on your schematic. It must be a newer model.

My next post will include voltage readings.

[Enzo]

Sorry, my post #3 refers to 15 ohm resistors when I should have said 22 as in the schematic. I was thinking conceptually and lost focus on the actual drawing. The 15 ohms caught my eye - I should have said 22.

So your resistance readings look fine, one reads a couple ohms higher, but I bet a resistor is off value there, check them. 22 ohms + 0.27 ohms = a couple ohms for the winding = 26 ohms looks reasonable.

In answer to your earlier question, yes, there is no reason you would not be able to swap transformer wires between Q4 and Q5. And that would show us that the transformer is involved or not. MAke sure to keep the phase relation correct.

Hey there is a thought, is the phase relation for Q4 correct as it is now?

Extra 1N4007s? I would not be surprised if they added a couple from the output line to each power rail.

[e_c_mccollum]

Phase relation should be correct but I will check it again. Each transformer connection is marked (color) on the board. Since it has been worked on before, no harm in checking the other connections. Might as well clean them too.

I am wondering if there is a tempermental resistor somewhere. I have had it in the past where they check good but start failing when heated.

I will post the outcome of the transformer lead swap.

[e_c_mccollum]

The transformer lead swap said alot.

The problem stays with Q4 no matter what. It's getting better but still there. Q5 might be the problem because originally the E-B ohmage was 33ohms. It got better when I cleaned the connectins.

Today is Peavey day. I need to get out two old (30+ years) peavey amps.

[e_c_mccollum]

Finally got the output transistors happy again. Some of the components had thermal issuses. Anyway, I am trying to figure out why the input preamp is very "treblelly". Any ideas?

[e_c_mccollum]

Anyone have a schematic of the input pre-amp? Looking for one without the footpedal and dual distort function.

[e_c_mccollum]

Need to find the 2n4304 transistors used in the input preamp. I hear they are hard to find. Any ideas?

[Enzo]

I put the preamp on www.ampix.org in the Enzo gallery.

Motorola shows the 2N5458 as close to the 2N4304. But really most any N-channel JFET ought to work fine. J202, 2N5457, etc.

[e_c_mccollum]

Got an engineering question for you, Enzo. Orginally 100k ohms bias pot was used. The replacement bias pot measures 80kohms, while company states is is 100k ohms. It is hard for me to find another pot the correct physical size.

On top of the pot is a 100k resistor. The bottom has a 33k ohm resistor leading to ground. What values can I use to compensate for the difference in value for the new pot?