Dear sir can I ask you some questions, how long have you been using the amp now?is it still behaving well.and what valves are you using I have a TVA 1 that has 6550a valves that were biased @40ma this would last perhaps .4months or a year or two but always the same valve would red plate No -3 has yours 100k resistors in the bias circuit or 220k?'have you deployed any other mods. I would be gratefull for any tips you might have on how to cure this ,I've read all the threads but none are clear on this subject I even spoke to ToP some years ago with no joy. Your comments would be welcome.regards Darryl.
Hi biffo24,
As was mentioned before, I have used almost every weekend and have had no problems.
Moreover, as the prototype did not show any problem and the sound pleased me a lot, I lost a bit of motivation to complete the initial project. However, I am returning to it and still trying to finalize this year.
I'm using JJ KT88 (blue). I chose these only by the visual effect and prices.
I think the big problem is really the capacitors. If at any time the amplifier is switched on without tubes, probably some component suffered from overvoltage. I used coupling capacitors of 0.47 / 1000v.
In my project, the entire circuit is oversized (withstand voltages for components above the original project) and also has independent channels feed. It''s really two completely separate channels mounted on one chassis.
I will check the squematic and post bias values here...
As was mentioned before, I have used almost every weekend and have had no problems.
Moreover, as the prototype did not show any problem and the sound pleased me a lot, I lost a bit of motivation to complete the initial project. However, I am returning to it and still trying to finalize this year.
I'm using JJ KT88 (blue). I chose these only by the visual effect and prices.
I think the big problem is really the capacitors. If at any time the amplifier is switched on without tubes, probably some component suffered from overvoltage. I used coupling capacitors of 0.47 / 1000v.
In my project, the entire circuit is oversized (withstand voltages for components above the original project) and also has independent channels feed. It''s really two completely separate channels mounted on one chassis.
I will check the squematic and post bias values here...
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I don't think that is a good choice (replace KT88 valve for EL34), since the last one has considerable lesser plate voltage admissible. Anyway, adjusting bias is possible, but I do not recommend.
Hi
The circuit has a very high resistance between ground and g1 on the output tubes, well above 360K. Many, many years ago I repaired a TVA1 for my brother. It had destroyed several output tubes. I replaced the bias pots, lowered the g1-ground resistance to approximately 100K and set the bias at 45-50 mA. The amp has worked without trouble ever since (more than 25 years).
JJ KT88´s have a recommended g1-ground resistance of 220k maximum when running fixed bias. For Sovtek KT88´s the maximum resistance is 51K if I read the datasheet correctly. I would not use those 330K resistors between g1 and ground like in your schematic and I suggest lowering it to 200K or 100K.
Torben
The circuit has a very high resistance between ground and g1 on the output tubes, well above 360K. Many, many years ago I repaired a TVA1 for my brother. It had destroyed several output tubes. I replaced the bias pots, lowered the g1-ground resistance to approximately 100K and set the bias at 45-50 mA. The amp has worked without trouble ever since (more than 25 years).
JJ KT88´s have a recommended g1-ground resistance of 220k maximum when running fixed bias. For Sovtek KT88´s the maximum resistance is 51K if I read the datasheet correctly. I would not use those 330K resistors between g1 and ground like in your schematic and I suggest lowering it to 200K or 100K.
Torben
Hi Herrmann,
I think your are right. That explained why my amp works fine. I used 220k, not 330k as drawn in the schematic. As I said, it needs a revision. In the link below, gorlando shows a schematic with 100k resistor:
http://www.diyaudio.com/forums/tube...ng-michaelson-austin-tva-1-a.html#post2738986
I will try a 100k resistor.
I think your are right. That explained why my amp works fine. I used 220k, not 330k as drawn in the schematic. As I said, it needs a revision. In the link below, gorlando shows a schematic with 100k resistor:
http://www.diyaudio.com/forums/tube...ng-michaelson-austin-tva-1-a.html#post2738986
I will try a 100k resistor.
Maybe, if I can update the pcb. There are some components with duplicate labels. You will also need a list updated components and assembly instructions. I'll see if I can prepare this stuff for next year. Some friends here from Brazil also requested a GB.
Meanwhile, I'm working on changes values for NFB to suit my OT (there isn't a 0-16 ohms secondary windings) ...
Here are some pictures of the assembly of the final project ....
Meanwhile, I'm working on changes values for NFB to suit my OT (there isn't a 0-16 ohms secondary windings) ...
Here are some pictures of the assembly of the final project ....
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