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portlandlay 25th November 2005 09:36 PM

Transistor specialist needed
 
Ok I am not too good (bad) with transistor amps and I want to resurrect my Sugden A21 series II amp.

When I bought this old amp the output transistors did not match and I want to right this poor beast before I put new PS caps in and fire it up.

The not to unusual problems is the original power transistor is a BDY38 and these are not available. I emailed Sugden and I was told that a TIP 35c or MJE2955 could be used.

Now I am confused. Totally. The original is an NPN type in a T0-3 case and while the TIP35C is NPN the MJE2955 is PNP and neither is T0-3 form. Now a MJ2955 is the right case but it is still PNP and the Sugden gentleman said that "PNP is Spec"? But wait the original iBDY38 is NPN :xeye: Then I read about the MJ1500X's being a good replacement in the JLH amps. HELP ME!

Perhaps someone is familiar with this Class A amp and I chose to put this request in this forum after reading about Nelson Pass working on the JLH (PLH).

Stephen

Netlist 25th November 2005 09:57 PM

Stephen,
In the old days we would have replaced a BDY38 with 2N3055 which is also TO3.
These are however outdated transistors and today I'd probably try something like MJ15xxx. Other members, more familiar with that amp will probably point you to a more precise device.

/Hugo :)

PB2 25th November 2005 10:15 PM

How many BDY38s do you need?
Do you know who manufactured the originals?
Phillips or ?

Was it common for them to fail in this amp?

Pete B.

portlandlay 25th November 2005 10:30 PM

Only one is missing. It does not have a manufactuer designation on the outside of the can it just says BDY38 and under that 3K50.
The can has a little more depth (thicker) than other T0-3 types I have seen.

I do not know too much about the transistors in this amp failing but it is from the early 70's so... Perhaps bad caps caused it?

PB2 25th November 2005 11:40 PM

Quote:

Originally posted by portlandlay
Only one is missing. It does not have a manufactuer designation on the outside of the can it just says BDY38 and under that 3K50.
The can has a little more depth (thicker) than other T0-3 types I have seen.

I do not know too much about the transistors in this amp failing but it is from the early 70's so... Perhaps bad caps caused it?


I have a friend who can get NOS semis but that's a small quantity and replacements would probably be from much newer stock so you'd probably want to replace all of them.

Pete B.

anatech 26th November 2005 12:03 AM

Hi portlandlay,
I think you'd be further ahead picking up some MJ15022's, a set. At least the amp has new stock. You may need to kill some oscillation, but it may anyway (oscillate) if supply and supply decoupling caps are bad.

-Chris

portlandlay 26th November 2005 12:03 AM

Hey that would be great. If you can help I would appreciate it. There are only four total. I would take two sets for the future.

I still would like to know what others would be good replacements since the Sugden guy really confused me...

PB2 26th November 2005 01:00 AM

Quote:

Originally posted by portlandlay
Hey that would be great. If you can help I would appreciate it. There are only four total. I would take two sets for the future.

I still would like to know what others would be good replacements since the Sugden guy really confused me...


Do you have a schematic and complete specs on the BDY38?

Do you know if parts were matched or selected?

I'd suggest, over the 2N3055A, the 2N3772 or even better 2N3773 which is a preferred part at On Semi.
But I have no experience with that amp.
These parts originated around the same time as the BDY38 but have much better SOA characteristics, and are probably better in every other way.
I've done many substitutions without problems, but I usually choose good designs to work with. One with marginal stability could easily oscillate with substituted parts.

Pete B.

portlandlay 26th November 2005 04:23 AM

I am not sure if the transistors were matched or selected but I do not think they were.

The schematic can be found in here http://www.tcaas.btinternet.co.uk/ navigate to "other class A amplifiers" and then to the Sugden 10w.

The ouput stage is a bit different but that can be found on page 6 of the associated article found at the same web address.

anatech 26th November 2005 06:29 PM

Hi PB2,
2N3773 is a much better sub. That's the one to use. I didn't know it was still in the catalog.

Matching them is always a good idea, this will improve current sharing. When the amp was build, the devices would have been close from the manufacturer anyway.

-Chris


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