Alternatives to AC187/AC188 O/P pair? - UK Vintage Radio Repair and Restoration Discussion Forum

paulsherwin · 8th Oct 2007, 3:03 pm

After reading John's writeup of his Hacker RP71 restoration here I'm inspired to look at mine again. John describes changing the O/P transistor pair to cure some distortion.

Does anybody have any suggestions of other transistor types to try in this position? I ask because, as usual, I don't have an AC187/188 pair in my stock of several hundred transistors.

Has anybody ever tried to sub a complementary Si pair such as a BC327/337?

I should explain that this set will never reach exhibition standard having numerous cosmetic problems and a replacement speaker, and my intention is simply to repair it. Originality isn't important.

Paul

plaka78 · 8th Oct 2007, 3:30 pm

If you are going to try a silicon O/P pair BD131/132 would be a good bet.
The resistor that connect between the to bases would need increasing to get over cross over distortion problems.

HG MICKE · 8th Oct 2007, 3:37 pm

AC128/AC176 pair will do just as well, If you use silicon transistors you will have to alter the Bias to suit.

Colin

jim_jobe · 8th Oct 2007, 6:10 pm

Hi Paul,
I have a spare pair of AC187/188 if you want them or you can have AC141/142, the type with the integral heatsink, if you prefer (or both!). PM me your address and I'll send them off when the strike is over!
Jim

paulsherwin · 8th Oct 2007, 6:22 pm

Thanks for that kind offer Jim. A couple of people have offered me parts, but that wasn't really the intention of this thread. I don't yet know if I even need the transistors

I was just pondering over the possibility of replacing the originals with something else. AC187/188s aren't all that rare yet, but it's useful to know about possible substitutions.

Paul

jim_jobe · 8th Oct 2007, 7:07 pm

No problem Paul. My own favourite complementary silicon pair is the BFY51/BFX88. When I did the first OU electronics course back in 1972 we had to design and build an amp around this pair. This worked a treat. I wish I still had my notes on it. The BFY51 should be available for ever more but I don't know about the BFX88. However there are plenty of substitutes for it.
Jim

Nickthedentist · 11th Oct 2007, 1:35 pm

Quote:

Originally Posted by paulsherwin

John describes changing the O/P transistor pair to cure some distortion.

Hacker claim 1.3W RMS with <1% distortion, as originally designed

paulsherwin · 11th Oct 2007, 3:07 pm

Quote:

Originally Posted by Nickthedentist

Hacker claim 1.3W RMS with <1% distortion, as originally designed

Mine sounds a lot worse than that, but that's a subjective assessment. I really ought to generate a sine wave on the computer, dig the scope out from under the stairs, and have a look what's going on. Thinking about it though, I'd better try aligning the audio stages first

Paul

Tim · 16th Oct 2007, 9:44 pm

Hi paul
CPC listed AC187/188 a couple of years ago. No idea if they still do. Both under a pound each.

Mike Phelan · 17th Oct 2007, 8:22 am

Yep - they are both still listed. Both 42p excl VAT.

8th Oct 2007, 3:03 pm	#1
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Alternatives to AC187/AC188 O/P pair? After reading John's writeup of his Hacker RP71 restoration here I'm inspired to look at mine again. John describes changing the O/P transistor pair to cure some distortion. Does anybody have any suggestions of other transistor types to try in this position? I ask because, as usual, I don't have an AC187/188 pair in my stock of several hundred transistors. Has anybody ever tried to sub a complementary Si pair such as a BC327/337? I should explain that this set will never reach exhibition standard having numerous cosmetic problems and a replacement speaker, and my intention is simply to repair it. Originality isn't important. Paul

8th Oct 2007, 3:30 pm	#2
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Alternatives to AC187/AC188 O/P pair? If you are going to try a silicon O/P pair BD131/132 would be a good bet. The resistor that connect between the to bases would need increasing to get over cross over distortion problems.

8th Oct 2007, 3:37 pm	#3
HG MICKE Pentode Join Date: Jul 2007 Location: London, UK. Posts: 165	Re: Alternatives to AC187/AC188 O/P pair? AC128/AC176 pair will do just as well, If you use silicon transistors you will have to alter the Bias to suit. Colin

8th Oct 2007, 6:10 pm	#4
jim_jobe Heptode Join Date: Nov 2004 Location: Ripon, N.Yorkshire, UK. Posts: 782	Re: Alternatives to AC187/AC188 O/P pair? Hi Paul, I have a spare pair of AC187/188 if you want them or you can have AC141/142, the type with the integral heatsink, if you prefer (or both!). PM me your address and I'll send them off when the strike is over! Jim

8th Oct 2007, 6:22 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Alternatives to AC187/AC188 O/P pair? Thanks for that kind offer Jim. A couple of people have offered me parts, but that wasn't really the intention of this thread. I don't yet know if I even need the transistors I was just pondering over the possibility of replacing the originals with something else. AC187/188s aren't all that rare yet, but it's useful to know about possible substitutions. Paul

8th Oct 2007, 7:07 pm	#6
jim_jobe Heptode Join Date: Nov 2004 Location: Ripon, N.Yorkshire, UK. Posts: 782	Re: Alternatives to AC187/AC188 O/P pair? No problem Paul. My own favourite complementary silicon pair is the BFY51/BFX88. When I did the first OU electronics course back in 1972 we had to design and build an amp around this pair. This worked a treat. I wish I still had my notes on it. The BFY51 should be available for ever more but I don't know about the BFX88. However there are plenty of substitutes for it. Jim

16th Oct 2007, 9:44 pm	#9
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Alternatives to AC187/AC188 O/P pair? Hi paul CPC listed AC187/188 a couple of years ago. No idea if they still do. Both under a pound each. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

17th Oct 2007, 8:22 am	#10
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Alternatives to AC187/AC188 O/P pair? Yep - they are both still listed. Both 42p excl VAT. __________________ Mike.