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-   -   Michaelson and Austin TVA1 revB (http://www.diyaudio.com/forums/tubes-valves/208483-michaelson-austin-tva1-revb.html)

Broncobillyo 9th March 2012 11:36 PM

Michaelson and Austin TVA1 revB
 
1 Attachment(s)
Hi. This my first post.
I have a TVA1 that I used to run with GE 6550s with no problems. It took out a few MO KT88s until I switched to the US tube.
Anyhow, in all the photos I have ever seen, there is one small circuit board for the triodes with the octal bases visible. The amp I have has a large board that covers all tube bases. It is marked Soundlease Services Issue B. I will try post a photo. Does anyone know anything about this version? I have just bought 4 matched EH KT88s for the beast and a variac to help recharge the caps as it has been in storage for 11 years. Thanks.

kevinkr 9th March 2012 11:52 PM

Welcome to the forum broncobillyo! I don't know anything about this amplifier directly, but we have plenty of UK members who can probably lend a hand. :D

Broncobillyo 10th March 2012 12:00 AM

Thanks Kevin! It is a great sounding amp. I would like to rebuild it someday but the circuit has a bad rap and I wonder if it is worth the investment.

Broncobillyo 11th March 2012 08:01 PM

TVA1 running at half voltage
 
OK, so I have brought it up to half voltage - full on the variac but still config for 220V. I have yet to change the tags to 120V. I need some help with the bias. I have read that the bias resistors should be decreased in value to stop runaway with newer KT88s. Does anyone have specific info on which resistors? I assume they are on the bias board.
Thanks

Broncobillyo 28th March 2012 03:36 PM

Up and running
 
This might help someone else. The bias resistors in question are the 4 220k ones on the bias board. They are the orange ones visible in the photo. They need to be dropped to 100k to raise the -V on the grid to limit current thru the tube.
I haven't done that yet but I did replace the old black plate Brimar ECC83s and ECC81s with a new set of Sovtek 12AX7LPS (I know the drivers should be 12AT7s but I have run it before with ECC83s and it sounds leaner and more transparent in this config) and brought it up to 85V on the variac. It sounds astonishingly good.

John Koval 11th February 2013 12:27 AM

M&A TVA-1
 
Hi,

Saw your questions on the TVA-1. I've got one a week ago to start up (w variac) just like yours with the big ckt board in the bottom. I haven't found much info on this one. I've seen a later version?? without the big ckt bd and a number of what apparently are copies by some Japanese Audiophiles. This apparently was a highly thought of Amp.

Wondered if you have found any more data such as cathode current, rated power out, correct ckt diag,etc. The Ckt Diagram that I've seen only generally represents this amp. Most values are different.

It has taken some time to actually get the amp running stably. At first all the outputs glowed bright red (obviously suggested a bias problem). I finally determined that the main problem was the 47uf/100v caps on the bias board. They would not reform and therefore were dragging the bias voltages down. You do not need (or want) to reduce the 220Ks. I strongly suspect you caps are bad also.

Based on the tube data, I selected 60 ma cathode current. The bias voltage needed is about -58Volts with 33ohm cathode resistors in my amp. With the caps replaced, the bias controls give a nice range and readily give the -58V. (the amp will run without the caps)

It looks like the amp may be able to handle much more cathode current.

At the 60ma, the output swings roughly 60W into 8ohms but rounded on the Positive swing.Thinking the 2 pots on the monster ckt board might adjust symmetry but they didn't do anything. Do you know how the pots are hooked into the circuit - what they are supposed to do?? It would be a major project to raise the ckt board to see how the pots are hooked in - I started but gave up!!

John

Broncobillyo 3rd March 2013 04:54 AM

John,
PM sent.
Can you post some photos?
B.

Broncobillyo 20th April 2013 11:47 PM

Rev B on Ebay
 
one for sale on Ebay, same version as mine.
This is not mine however.
Michaelson Austin TVA 1 Tube Power Amplifier Serviced Tested Works Great Clean | eBay


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