Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

Hi there, I have just built a kit version of this amp. When I first powered it with my bench supply, it was short circuiting (current limiting). I discovered that I did not have insulating washers on the underside of the 2n3055 mounting screws. After adding washers, it now plays music, but very weak sounding and each of the stereo channels is only drawing 30mA. Heatsink is stone cold.....Have I blown up the 2N3055's?

Details of the circuit attached.

Re: Wireless 1969 Amp JLH. 2N3055 Transistors.
 

Hi Andy,
one of those ouch moments that we've all had at some point! Thinking it through, as the collector is brought out on the case this is what you will have shorted to the heatsink/chassis/ground. I doubt if that will have harmed the 2N3055 since it wouldn't be conducting in those circumstances, but it will have worried your power supply a bit!

I would check the 2N3055s with a DVM if you have one handy just for reassurance, but the PSU current limiting will have saved your bacon. There will probably be a fault elsewhere in the construction, since it has never actually been proven working at this point. Check through all the component values and their position on the PCBs just in case.

Use your DVM to check voltages on each transistor's terminals and see where that takes you?

BTW your circuit diagram seems a bit generic and the power transistors are not 2N3055s or showing as equivalent in this resource link, so component values could be critical.

https://alltransistors.com/crsearch....fe=10&caps=TO3

Good luck
Chris

Re: Wireless 1969 Amp JLH. 2N3055 Transistors.
 

I'm trying to understand the title of the thread. "Wireless 1969"? Do you mean a John Linsley Hood design in Wireless World? his class-A one?

David

Re: Wireless 1969 Amp JLH. 2N3055 Transistors.
 

It's a Kit supposedly based on the April 1969 JLH 10W class A amplifier.

https://worldradiohistory.com/UK/Wir...ld-1969-04.pdf

If anything's copped it most likely to be the driver transistor TR3 that's always assuming it would have worked in the first place :)

Cheers

Mike T

Re: Wireless 1969 Amp JLH. 2N3055 Transistors.
 

This is the diagram that seems to be floating around the net associated with the Kit.

Re: Wireless 1969 Amp JLH. 2N3055 Transistors.
 

Take some voltage reading across each tranny's base and emitter to see if they're on, Tr 3 & 4 should be at 0.6v ish, the 2N3055's at about 1.5v. Do you have a scope and sig gen? If so a probe at each trannys base and collector/emitter to see if you have a good signal is worth while too. Tr4 does most of the amplification, you should have a good signal there on its collector.

Andy.

Re: Wireless 1969 Amp JLH. 2N3055 Transistors.
 

Thread title updated.

Re: Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

I built this amplifier originally with 2S025 transistors to try out and it worked so well that I built a neater stereo version with 2N3055's which also worked well and was in service for several years.

If the gain of the output transistors is too low the distortion will increase rapidly, however.

Don't skimp on heatsinks, it gets very hot, over 35 watt dissipation per channel at all times.

Re: Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

Open and Honest Update: I managed to get the one channel working last night. I re-drilled the clearance holes in the heatsink (aluminium angle). so that it was isolated from the screw passing through it.

Well it worked for about 5 minutes, sounding crisp, powerful and concise, although not as loud as I would have liked when I cranked the source (mobile phone headphone output) up to full volume.

Current draw was about 1A at this point - on a single channel.

In the 6th minute I heard fizzle and a pop, then silence. I guess I have now cooked the 2N3055, due to insufficient heat sinking. I will order some replacement 2N3055's and a beefier finned heatsink.

How can I get more volume? I am using an 8ohm speaker. and running the Bench PSU at 24Vdc

Re: Wireless 1969 Amp JLH. 2N3055 Transistors.
 

I think I have posted the wrong circuit diagram in the original post.

Re: Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

The circuit you posted is from the original JLH article.

I suspect the circuit that Wave Solder posted is what the kit is actually uses.

Cheers

Mike T

Re: Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

Each output transistor needs to dissipate around 15 Watts, independent of signal level. You need really substantial heatsinking, just look at the ones Linley-Hood used as shown in the article. For a heatsink shared between two transistors, you need say 2 degrees C per watt or less.

Also, the amp is only 10 watts output so will not be “disco loud” especially with modern, relatively inefficient speakers.

I have used mine for many years, love them !

Re: Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

I'd echo Barry's point: if you're not used to Class A amplifiers, the heat sink will need to be much bigger than you might think. I can't remember the exact figures for this circuit's heatsinking, but you need to be aware of this requirement or you'll go round in circles replacing blown transistors.

Mike

[Sorry, my post crossed with Ambientnoise's]

Re: Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

Not all 2N3055's are created equal, before purchasing more 2N3055's this article could be a useful read.

https://www.vintage-radio.net/forum/...d.php?p=807991

Re: Wireless World April 1969 Amplifier Kit. 2N3055 Transistors.
 

A finger on the output transistor(s) will tell you if your heat-sink is up to it. If you can keep your finger on it for, say 2 seconds it's easily OK. You should be fine up to 70C case temperature (mild ouch!), even 80C or 90C (mild curse!) on a blisteringly hot day. You will need to allow time for it to get up to a working temperature in an environment similar to where you want it to finally reside (maybe 10mins minimum).

dc