Hum Problems and Input Transformer Choices for Altec 436 Clone

[xfmr]

I have built an Altec 436 clone of sorts into an Akai Roberts Tape Deck Monoblock and I am having hum problems. I am using Edcor XSM 10k:10k for the input and I have traced it as the source of my hum issues. I am using Edcor XPP 600:10k as my output by the way. I've got everything working, including several of the RS124 mods and the tone sounds great, but this hum is driving me crazy. I have ordered some Ultraperm 80 and Mu-Metal foil to give this a try, but I am not exactly sure how to go about applying this material to this open frame transformer. I imagine that I have to first build a little box to surround the transformer, then wrap it in the shielding. So, my questions are:

1. How exactly should I go about adding a shield the Edcor to eliminate this hum?

2. Would moving or reorienting the transformers make more of a difference than shielding?

Which leads me to the second part of my post. I am aware of the Sowter used in the Drip clone, but I don't think there is enough room for that guy in this in this application. I'd prefer something I can order from ebay or another USA manufacturer like Cinemag or Jensen, however I can't seem to find anything they make that will work.

3. Is there a better transformer that I should just use?

I have been looking at UTC A-18 and the Ouncer version O-6, so...

4. Will either of these UTCs work for this? (I know the impedance will work, but I am worried that the ouncer can't handle the levels or mu-line voltage). Are there better transformers that are relatively easy to find?

 

[CJ]

you built the compressor From the AKAI, right?

here is a link for the schemo>

http://www.sowter.co.uk/schematics/alt436c.pdf

30 db is the required level capability, do not know if you need the 1:2 , 1:1 might work,

this would open up the flood gates as far as xfmr choice, you do need a CT on sec,

level should not be a problem as the turns count is very high for a bridging= less flux,

most of the 600:600 bridging i have torn apart have an e shield,

so i would think that a 10K would need a shield also

if you have a lot of hum, relocating probably will not get it all,

try the shielding if you already bought it,

then go for a transformer with a shield if that does not work,

check UTC catalog for 10K:10:, then Peerless or Triad,

Stancor is also good and also always overlooked which means they are cheap,

 

[xfmr]

Yeah, I use these Akai monoblocks for all kinds of stuff. They are a good cheap platform. I tear them down to bare chassis and then can install whatever I want. Great power transformer, nice choke, decent tag strip, usually has plenty of room...

The problem I am running into looking for other transformers is finding suitable units with a dual secondary or center tap where I can apply the mu-line. I have built the same design with the same parts in other chassis before, but they weren't as cramped and the transformers were oriented differently. Since I have already built the whole blasted thing, there is little I can do as far as rearrangement.

I have the input transformer all the way to the left side of the chassis, as far away from the power tranny and choke as possible. However it is only about 5/8" of an inch from the output transformer. I can move the output about half an inch further away from the input, but this moves it closer to the power transformer and choke. I have attached a picture to show the layout.

 

[CJ]

an easy thing to try would be to flip the input wires out of phase just for a hum check,

low probability but if it does work then you do not have to do any metal work,

yes 30 db with center tap seems to be a problem,

Triad HS-25 has CT but only about 20 db,  ???

time to check evilbay for ideas,

 

[xfmr]

Yeah, I have been scouring ebay with no luck, that is why I came here.

This one looks promising, but it doesn't say what the impedance or step-up is. Looking at its application in the Fender Musicmaster Bass amp, it looks like it might work. Any ideas?

http://store.triodestore.com/40-18093.html

What will flipping the phase do? I don't see how that would make any difference. It is not that I doubt you CJ, but I am curious what that will accomplish.

 

[MatthisD]

Look at an old Lundahl LL1521B (no electrostatic shield). With parallel primaries it would be a 1:2.28 stepup and 140:690Henries, centre tapping the secondaries 140H:182H+182H. Very similar to LL1540 which is 1+1:1+1 and does have an electrostatic shield.
  I suppose for max compression you need 1:2CT but to achieve the distortion spec given by Lundahl you'd have to series connect the primaries, 1:1CT.

I have a UTC O-6(rated +8dBm) but have never tried it in anything...I'll check the inductance.

 

[CJ]

you could also try grounding or un-grounding the input signal,

and also grounding or un grounding one of the pri leads,

if you swap the phase, you are swapping the start and finish,

one might be less prone to hum pickup than the other.

Lundahl uses 2 coils most of the time for their inputs,

this means humbucking, so the shield is not as vital as with a single coil EI core,

 

[xfmr]

I just head back from Triode Electronics on this Musicmaster interstage transformer clone. It sounds like it will be perfect as an input for this 436 since it is already somewhat shielded and will be easier than the Edcor to wrap with mu-metal because of how it is constructed. I just don't know how much current the mu-line is on the 436. This guy says it will handle 20-25mA, which sounds like it will be fine, but I can't find that info anywhere. Anyone have any idea what the current level is on the mu-line?? Anyway, here is what they told me about this 40-18093 transformer. If anyone thinks that this guy will work, please give me the thumbs up so I can order a pair of these. Thanks in advance. OK, here is what I heard from Triode...

"This is a very unusual part !  Basically we replicated a vintage part. The impedances are unknown. In the amp, the Musicmaster Bass, the load on the secondary is effectively open, since it drives the grids of the output tubes. Therefore, I can't calculate the primary impedance. The primary is used to keep the DC current out of the secondary side and the secondary is used to as a phase inverter. It does step up the voltage about 40%. It has LOTS of turns of very fine wire. The DC resistances are usually high for an audio transformer; 1400 Ohms for the primary and 2600 Ohms for the secondary. This has a major impact on calculating the reflected impedance, as this causes it to vary drastically with different loads.

In normal use, the primary sees a little under 3mA DC unbalanced. The secondary has no DC current. If this guy wants to feed balanced DC into the center tap, the limiting factor is the wire size and not the transformer or core. For this unit, it will handle 20-25mA DC. The turns ratio is 1 : 1.41, primary to secondary. "

Also, for the sake of completeness, here is the link and data sheet for this transformer of which I write:

http://store.triodestore.com/40-18093.html