AC107 (or AC125/2N406) transistors for MCR21 (BTTT)

[Pamphonica]

We are restoring this wonderful old 1963 Outside Broadcast truck - MCR21, and I am trying to get the original sound desk working. It uses a LOT of germanium transistors! At present I am trying to find some PNP AC107 for the microphone amplifier modules. Quite a few seem to have died in the first stage of these circuits. Alternatives are apparently AC125 and 2N406, although Towers quotes quite different characteristics for the alternatives. Before I just put in whatever is to hand, I would like to try some original types. The originals have the metal can.

I know that germanium transistors (including these devices) are now highly valued by the fuzz-box brigade, and hence apparently costly! But if anyone here has some stock (any quantity) that they would sell or donate to our endeavour we would be very grateful indeed.
The obvious sources are many ££ each and that is not feasible when we may need dozens.

Does anyone have some lurking in their spares boxes? or a good alternative?
many thanks
-Jeremy

[paulsherwin]

An AC126 should be OK and easier to find, but may be a bit noisy for a mic input even by early 60s standards.

An AC107 is really just a renumbered OC70, which in turn is an OC71 selected for low noise.

[Radio Wrangler]

It's not low voltage battery powered stuff, so there might be enough compliance in the bias circuitry to tolerate silicon PNP. If so, although it's not original to the fullest extent, it would get things going, not starve funds and give you reliability to not distract from future activities. A shufti at the diagrams would give clues.

David

[paulsherwin]

I suppose it depends on how the equipment is actually going to be used. Obviously nobody is going to use a 1963 scanner to produce broadcast quality programming today, so the mic preamps just have to work well enough for a demonstration rather than be particularly high performance.

[Ed_Dinning]

Hi Jeremy, I may have some AC107's here if I can find them, but I've certainly got some OC70's as well as lots of other early Ge types in the OC series, NKT, MAT and 2G types

PM me your address and I'll put a couple of OC70's in the post for you to try

Ed

[MotorBikeLes]

I have a single AC125, plus about 3 unmarked (illegible) similar looking devices. Pulls, not tested, FOC if you want them.
I also have about a dozen each of OC77 and OC81, likewise FOC. Are any of those suitable.
If they are NOT suitable, somebody tell me if these last two types are worth silly money from gullible people instead.
I have loads of assorted Ge pnps of different sorts. No doubt I could set up a go/no go tester to sell them off.
Les.

[paulsherwin]

They aren't a good choice for a low noise input stage, but will probably work after a fashion. Sadly they aren't greatly sought after by the guitar brigade - they insist on historically correct transistor types for their repro pedals, even though the originals just used whatever was cheap at the time. That's why the OC series are so popular - they were obsolescent when rock was taking off in the mid 60s and were cheap.

[Ed_Dinning]

Hi Paul, I have one of the fabled Ediswan transistor testers that it was rumoured were used to select the transistors for fuzz pedals.
Basically a very crude gain sorted !

Ed

[G.Castle]

Jeremy.
Three points.
It sometimes doesn't take much to graft in silicon, if BC109 was available to the designer at the time, they would have used them.

Every so often RFT manufacture GS 109C (germanium PNP, similar to OC45) come up cheaply, they don't seem to have the same stomp box cred as old Mullard soot.

Send a nice letter to Langrex. I've recently been dealing with them for some valves and noted they have S02 cased Mullard AC107's: over 17,000 in stock. Not stupid money but a big outlay if you need lots, you may get a bulk discount though...

https://www.langrex.co.uk/products/a...w-black-glass/

[Pamphonica]

Well gents, thanks for that very helpful set of replies.
In order:

Paul: many thanks for that extra substitution info, I had found AC125 but not OC70.
David: Yes, Si would be an option, but we prefer to err on the side of originality if we can, as long as it works!
Ed: As ever your spares box amazes. Yes please to some AC107s or OC70s to try. I will PM you.
Les: very kind offer. In fact I have quite a few misc Ge to try if I need them, but l'll try near-equivalents if I can.
Mr Castle: Yes, I have lots of BC109 but prefer more direct substitution if possible. I'll definitely keep an eye on the GS109C. I already sent Langrex a message yesterday. They do have a huge AC107 stock!

With Ed's help I think I should be able to get a few more of the mic amps going now. We need to get 5 working mic channels if possible. I have 6 modules (only 1 working currently) to work with, and I am hoping nothing irreplaceable like input transformers is dead.
I will report back elsewhere once I make some progress.

Huge thanks all. Maybe see some of you at Biggleswade on Sunday...
-Jeremy

[Pamphonica]

Update:
Langrex have very kindly offered to supply us as a donation, to support our restoration. What lovely people!
-Jeremy

[paulsherwin]

If you do end up doing a silicon conversion, a low noise PNP type like a BC214 or BC559 would probably be your best bet. I can let you have a few dozen BC214s FOC if you take that route.

(Just seen #11, great news.)

[Craig Sawyers]

Originally Posted by Pamphonica

Update:
Langrex have very kindly offered to supply us as a donation, to support our restoration. What lovely people!
-Jeremy

Wow. That is impressive! Hats off to Langrex.

Craig

[Pamphonica]

Mods, I suggest we close this very helpful thread. It has certainly served its purpose.
thanks
-Jeremy