• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hybrid amplifier help...

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
i just put the amp back together and there is deffinatley somethign wrong with it today, very faint output gets louder when i power down the output stage.. or bypass the output stage alltogether.. so must be a fault in the output stage somewhere, mabey the output transistors were damaged aswell when i had the power the wrong way?
Owen
 
Administrator
Joined 2004
Paid Member
Yuk!

Diode test time on all the transistors. Do you have a variac or adjustable power supply you can use for these experiments? I generally use a bench supply until I know the circuit doesn't do bad things, then it's on to the intended supply through a variac. I watch the current when bringing the power up.

-Chris
 
unfortunatley not, i guess i just have to rip all the transistors out and test them. Hopefully the output transistors are ok otherwise i have to order new ones..
I will test all the transistors and add the CSS and Current sink and see how i get along. probably one of the transistors have gone down on the board again. I dont mind burning a few out whilst expermineting, there arent any expencive components on the board so it's not that bad.
Owen
Forgot to mention how amazed i was that the tube stage could power an 8Ohm speaker all by itself.. lol
 
Administrator
Joined 2004
Paid Member
Hi Owen,
Interesting result. I would have expected the drivers to turn into fancy looking wires too. Somehow I knew your bias transistor would be a piece of wire.

Count both outputs as toast. I'd be very wary of the driver transistors. If it were me, into the garbage can they go.

Put fuses or something in there to help out. Variac??

-Chris
 
Hi Chris, i will just binm these output boards i think, easier to start from scratch and just built it all on a breadboard, Since i am going to order new output transistors any sugestions what i should get (preferably cheap) or should i stick with what i was useing?
Many Thanks,
Owen
 
are 2SC2579 and 2SA1104 any use? Just found 2 of each on an old heatsink under my bed and they test good, must have been pulled from an old amplifier or something.
EDIT: Just found what appears to be part of an old PA amp aswell, 2N3055 output transistors, Power supply and some other stuff, i have stripped it back down to power supply and output transistors.. should be good because i have about 50 spare 2N3055's lying around and the power supply is really beefy on it.
Owen
 
Administrator
Joined 2004
Paid Member
Hi Owen,
2SC2579 and 2SA1104 are rated at 120V, 8A, 80W and a bogey gain of 80. They should work very well. You need driver transistors too. 2SD667A and 2SB647A might still be available. The first stage could be 2SC2240 and 2SA970. Clamp the input(s) to the rails with diodes so you can't drive the inputs into zener breakdown. Use whatever transistor is easy to work with for bias. 2SC3421 would work well here.

Use whatever you can to not blow up this set. 10 ohm resistors, 1 ohm resistors, fuses - whatever you can.

-Chris
 
i will have a look on ebay for a variac. and see if i can find any of the driver transistors and stuff i need for them.
Hopefully i wont blow this set up, they look liek they have hardly been used if atall, they are mounted on a heatsink with thermal pads and thermal paste but the legs are all clean with no signs of solder or anything on them. Cant remember where they came from..lol.
Many Thanks,
Owen
 
Administrator
Joined 2004
Paid Member
I've been following this thread with interest over the last few days, and I have a couple of comments.

In addition to a variac wire up a light bulb in series with a receptacle and plug and use this between your variac and the amplifier output stage power transformer mains input - use a 60W bulb to start, you will be amazed at how many power transistors this gambit will save. This is a lamp ballast, at low currents there is little voltage drop across the lamp, dead short almost all of the voltage drops across the lamp and the mains current is limited to about 0.25A. Change to higher wattage bulb as needed.

Fuse one of the supplies and the speaker output with fast blow fuses, (put a cap across the fuse 1uF/50V film should suffice) I have generally found fuses in both supply legs to be ineffective. Make sure that if both fuses open that there is no place for fault current to flow from the other supply leg.

Finally take a look at the 5687 as an SRPP driver stage - it will comfortably drive impedances below 10K if you run it at about 10mA.

Diode clamps from the coupling caps (vbe mult side) to the supply rails (recommend making a set of reference supplies using zeners to drive the clamp diodes) along with an output relay will keep things simple and avoid speaker killing output transients.
 
Thanks for the advice Kevin, The light bulb is a good idea and ive heard of it before, i don't know why i didnt think of that before.
I'm currently trying to get hold of all the transistors for a new redesigned output stage, goign to go have a rummage in my parts box later and see if i happen to have any of them in there.
I am useing the ECC88 because i have quite a lot of them sitting around doing nothing, but i will have a look and see if i have any 5687's in my boxes. :)
Thanks,
Owen
 
Administrator
Joined 2004
Paid Member
Hi Owen,
Ha! I used to sell those, and repair the kits people messed up. :(

They are an entire amplifier channel, not just the output stage. You will see input and ground pins, an output pin and power supply pins. They had a PC board you could use, or many people just went point to point.

Sorry, you can't use them as an output stage as they have too much gain. They are also easy to blow up sadly.

-Chris
 
no problem, i'll leave them in their box for another day. mabey i will eventually get round to makeing some amplified speakers witht hem like i had originally planned.

I am haveing problems sourcing the transistors you suggested i should use, the only ones i have managed to source are the 2SC2240 and 2SA970, I have e-mailed a few suppliers to see if they could get them in for me but they all either said no or didnt reply and one of them said that they were all discontinued by their supplier..
Sooo..
Any suggestions on replacements or shall i just look through the datasheets and see what i can come up with? :)
Many thanks,
OWen
 
Administrator
Joined 2004
Paid Member
Hi Owen,
Find some TO-220 parts you like for drivers and then use the outputs you have. You might be able to get away with a TO-126 size part. Right now it's a concept you want working. I look for a flat gain vs current curve and good bandwidth.

-Chris
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.