[Alexdad54]

Hi all, a tube noob here...I wanted to ask the collected wisdom here for info. I have just bought a used Assemblage ST-40 (which I understand is esentially an Anthem 1) and the seller recently rewired it from a 4 ohm to the 8 ohm tap. I need to change it back to 4 ohm for my speakers (Salk SongTowers)  and the manual  does tell you how to do it, basically a simple soldering job. My problem is that I have no soldering skills so can anyone recommend a tech/store in the Philly area who could do it for me?  All sugggestions/offers appreciated!

[andrewbee]

Hi,

I also have a ST-40, an excellent amp. For the change over on the output taps I suggest you contact Chris Johnson - partsconnexion.com

Chris is the man or one of them anyway behind what was Sonic Frontiers and their kit line - Assemblage.

There were reports of earlier ST-40's with some sort of defect on the 4R tap, I don't remember where I got the info though, again I am sure Chris can help.

Andrew

[Alexdad54]

Thanks Andrew,
I had JoeC here in the Pilly area check the amp over after it arrived last week and he said that everything seems OK except that there is a strange oscillation when the amp is pushed a bit hard. He is not sure if it is solely due to one of the tubes needing replacing or if there is something going on in the circuitry. He showed me on his oscilloscope (there was a thickening at the top of the curve at what would be high volume) but was not sure if I will be able to actually hear it. (Caveat:Not being a techie, I might be misquoting him here...)

I will call Joe and then e-mail Chris Johnson to see if he can provide any info.

Now I have to find a replacement for the bad input tube, it's a 6CA7 EH, made in Russia. Won't bias lower than 43 but is supposed to be biased at 40. Can anyone recommend a source?

[andrewbee]

The 6CA7 or EL34 same thing for all intents and purposes is actually an output tube and two are used per channel. Try tubesandmore.com.

I have tried a few different EL34's in mine and even KT88's which are not really recommended for the amp. I presently use JJ EL34S tubes, which of about three different brands I have tried I like the best, seem to have a little more low end presence but any of them should work.

The input tube is a 12AX7 (the middle of the three small tubes in the front and flanking the 12AX7 are 12AU7 tubes which are the driver / phase splitter tubes.

Not sure about the abnormality on the scope, maybe it is simply just a weak tube.

I have had mine since they first came out and it has been trouble free. If you have the manual the schematic is on the last page, its a more than decent design.


Andrew

[Alexdad54]

I stand corrected! 
I have since done the homework I should have started with (like reading this circle's sticky on tube dealers!  ) and am in contact with Jim McShane and will get a matched quad of EH 6CA7's to start. I still have to get my speaker placement right and then break-in the whole system, speakers, peamp and amp before I go any further with the fine tuning. Thanks for all your help!
Dave

[andrewbee]

I have never dealt with Jim McShane but from what I have read of him over the years he is as straight as an arrow and not one much for "fluff", we need more like him.
Hope you enjoy the amp when you get it re-tubed.
Glad to be able to help!

Andrew

[Alexdad54]

Thanks Andrew. The tubes arrived from Jim (after a bit of a runaround with USPS), beautifully packed.
I also bought a new Tenma multimeter from MCM and am ready to do my first biasing!
My tech recently showed me how so it shouldn't be a problem but I do have a couple of questions: Once I install the new tubes how long should I let them warm up before biasing, an hour or so? I also understand that I should have the amp connected to the pre and the speakers but without any music playing. Is this correct?
thanks,
Dave

[andrewbee]

As soon as they are installed they should be biased, iirc its 40mA. There is a little test socket beside each el34 for the positive lead of the meter and the negative lead you can connect to the negative speaker connector. The bias adjustment pots are beside the sockets for each tube.

The bias is going to drift a couple mA one way or the other over time so don't panic. After the first 15 minutes or so make a quick check to see if they are still around 40mA, a few mA( say 5 or so) one way or the other is not critical at this point. After half an hour check and adjust and then again after one hour.

You can play music if you wish, just stop the source e.g. cd or whatever and then adjust the bias and then play again, you only need to stop the music to adjust the bias.

Keep an eye on the amp and also the nose, e.g. if you see a tube "glowing" red or smell something untoward then shut it down, but this is usually not a problem unless you have a defective tube.

After the first hour I would check it again after the next hour and perhaps a couple more times.

The tubes should have settled more or less by now. Try to adjust each tube so the bias is the same.

Once the tubes are holding the bias setting and not drifting then depending upon how often you use the amp you could check every month or every few months.

If you are using JJ EL34L tubes you may want to adjust to ~ 45mA.

Just a note, the power transformer runs quite hot in this amp so if it seems hot its because it is.

Follow your techs advice re the connections, particularly the connections to the speakers.

It is important that whenever you turn your amp on you have speakers connected.

Good luck.

[Alexdad54]

Thanks Andrew, MUCH appreciated! I've printed out your advice and will be implementing it tomorrow.
I have another question, if I may. I was told that my tubed preamp, the Mapletree Ultra 4SE "inverts" the signal which means I should switch the positive and negative speaker cables when plugging them into the speaker. I did so and the sound seems fine. My question is, does the ST-40 have any role to play in this, i.e. does it perhaps "re-invert" the signal after it comes out of the Mapletree? Do I just leave the speaker cables as is?
Forgive my ignorance but I'm still on a steep learning curve and apologies if this belongs on a different circle.....
thanks,
Dave

[andrewbee]

This whole signal inversion issue is to me a non issue. There is no way to easily tell / know if a signal is "inverted". If you have "golden ears" and have practiced then you may be able to tell the difference but whats the point? The whole idea of the equipment is to listen to music and not dissect every sound (unless thats your thing).

The signal begins with your source and who knows how it was recorded so while there may be a signal inversion at your linestage, there may also be at the source, and what about if you have several stages in an amplifer, depending on the topology, each stage may also invert.
My advice is not to get too caught up into the "audiophool" aspect and just enjoy the music.

Forgive my ignorance ...

Dave, you show no ignorance, you are smart enough to ask

We all started at the begining and I certainly do not consider myself an expert. No matter how much we may think or actually do know on any subject, there is always someone who knows more.

Your preamp is from Dr. Lloyd Peppard. His website is here http://hollowstate.netfirms.com/

A little about him is here http://www.ece.queensu.ca/alum/news/peppard.html

Andrew