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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Thank you very much ,I will change them and also the power supply and bias .I don't want to buy valves at today's prices to watch them burn again,Could EL34 valves be substituted for a period unroll mods prove reliable.?Regards Darryl
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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Working as a turtle, but keeping pace ...
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