Altec 438c repair and mods - work in progress

There is a nice pair of old Altec 438c tube compressors sitting on my bench. I'd like to show some of the crimes I have committed :grin: and ask for a bit of advice. First a pic with clothes on:



The units are in decent shape and there has been some work done on these by the studio tech before I got my hands on these (added an attack pot). Still there are some issues which I started to work on, after reading all the great information here provided by NewYorkDave and others. :thumb:

In agreement with the owner we decided to ditch the mic preamp section, which gave me a chance to add an input pad to replace the volume pot on the front. These units have tons of gain, even on the line input, so the pad is a stepped U-pad with 3dB steps going from -12dB to -27dB. In addition to that I built a little "outboard" output pad which is a 600 ohm bridged-T pad with 0dB / -6dB / -12dB / -18dB. This gives good control over the levels.



Next the worn out selenium rectifier was replaced with two 1N4007 diodes. This knocked up the voltages a bit, maybe by 10% or so, which I presume should still be ok.

The PSU filter caps will be replaced, I just got the replacent caps in the mail today.

I'd like to add a three pronged American mains connector, but they're hard to find in Germany, if somebody knows a place that carries them let me know!

still to do: check out the attack and release times, recap the signal path caps and add a meter adjustment pot.



I'd also like to add a balance control to the 6BC8 variable-gain stage, as suggested by NYD - is it best to use a mulit-turn pot for this? How many Watts does it need to handle?

Last question: Can I use the mic input transformer for the one-bottle-preamp? :grin: It is a type K308D, I couldn't find any specs on this though...

I'll post further progress of this project, I'd be happy for any suggestions!
cheers, Marten

Not to spoil the fun, but before more lots of effort are put into this, I recall the somewhat sobering comments here:

http://www.electrical.com/item.php?page=225&pic=pictures/225-0.jpg

FWIW, YMMV etc etc, I was actually a bit surprised a bit by those words there....

[quote author="martthie_08"] Can I use the mic input transformer for the one-bottle-preamp? :grin: It is a type K308D, I couldn't find any specs on this though...
[/quote]

Of course.

That's funny about the Electrical Audio link. I think the harder its driven the better it sounds. Not transparent, but very cool.

...got to work a bit more on the Altecs, the attack and release times are pretty good now (and have nice knobs to them). I wish the threshold pot would go "down" a bit more, but it's ok with the stepped input pad. The meter is now fully adjustable with a screwdriver just under the right side of the meter. The signal path caps are replaced with orange drops, sounds fine to my ears.





I hope to get a balance control in place of the left screw under the meter, but still need to understand how to calculate the maximum Watts for this pot. I'd rather not spend big bucks on an expensive 2 or 3 Watt helicap if it isn't necessary. The document on the EMI mods that was posted here calls for a 2 Watt pot on the plates of the 6BC8 and Dave used a [quote author="NewYorkDave"]250-ohm trimpot wired between the cathodes, with the wiper going to the GR meter[/quote] with unspecified Wattage. Not sure which way to go :roll:

Some more infos: The input transformer will find a new home in NYD one-bottle land :grin:. I got rid of the input pot completely, the input signal goes to a stepped u-pad and then straight to the interstage transformer, this seems to work fine.

there is a ton of pdf files of Altec history, tech notes etc that i came across the other day, are you familiar with that?

nope, I just have the 436c manual with EMI mods, NYD's advice from his posts here and the 438a stuff from Purple. When I get some projects off my workbench it'll be time to hit some textbooks. btw, I just restored a busted Siemens w295b, not a bad EQ, number 9 for me, they used to be dirt cheap. I should build a small console out of those :roll:

some cool pics here:

http://www.leesound.com/bw_photos/

here's Keef, in the early days, pimpin his wife off for a lift back to the airport because the buggy lost the distributor pigtail. again.:



BTW, on my vacation, i helped a stranded motorist, the only one I saw in ten days.
What kind of car?
Never mind.
(float bowl overflowing onto the exaust, what else is new...)

holy crap, there are nice pics at leesound
:thumb:

Having used lots of 288 / 1005 tar horns in the 70's and 80's

I really love this one!!! :shock: :green:



Thanks cj for the link :thumb:

GARY

ok, a bit more progress on this. The balance pot now is in place, I got a 10-turn linear 100K "Wendelpoti" on the plates of the 6BC8, as described in the "pseudo-emi" mod paper.

The thing is I now have the pot going in parallel to the 47k plate resistors which in effect halves the resistance and thus the idle voltage goes way up, of course. At the moment the plates are sitting at 120V DC with no compression going on, this can't be healthy for the poor 6BC8 :roll: Any comments?

I did the EMI mods on my Altec 436C and I didn't want to drill the original faceplate so I did a custom FPD panel for it. Turned out well. I mounted the Balance pot on an L bracket on the inside and have an adjustment hole on the panel like an 1176.

I'm happy to share it. Just PM your email address.

Mike
Westend Recording Studios

For what it's worth, the Collins 26-J Limiter manual/schematic has a great "circuit description" walkthrough of the working of a very similar vari-mu circuit:

http://www.collinsradio.org/Manuals/26J-1(5-60)Limiting%20Amp/26J-1(5-60)Limiting%20Amp.pdf

Also very helpful is a column in the parts list which notes what the function of each part in the circuit is. For example, a resistor will have its function listed as "cathode bias for V1", or "part of attack-and-release time circuit."

I wish I had found this Collins schematic earlier in my Altec 436 troubleshooting efforts.

Leigh

PS: sorry, you'll have to copy and paste that entire URL - it's getting screwed up because of the parentheses in the pathname...

[quote author="martthie_08"]I hope to get a balance control in place of the left screw under the meter, but still need to understand how to calculate the maximum Watts for this pot. I'd rather not spend big bucks on an expensive 2 or 3 Watt helicap if it isn't necessary. The document on the EMI mods that was posted here calls for a 2 Watt pot on the plates of the 6BC8 and Dave used a [quote author="NewYorkDave"]250-ohm trimpot wired between the cathodes, with the wiper going to the GR meter[/quote] with unspecified Wattage. Not sure which way to go :roll: [/quote]

Would anyone care to comment on the pros and cons of implementing a balance control on the plate vs on the cathode?

I went with a cathode balancing pot on mine, since a tiny 250-ohm trimpot is cheaper and easier to source and work with than a large resistance, multi-turn pot. I'm not sure if there's a tradeoff in performance, however... anyone?

Thanks,
Leigh

Id also like to hear info about cathode Vs Plate balancing ...........anyone...............

gary o said:

Id also like to hear info about cathode Vs Plate balancing ...........anyone...............

Click to expand...

me too