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ST-70 Tube Amplifier DIY Kit

dcgillespie said:
If you want to determine whether the oscillation is due to unstable NFB loop, simply disconnect it temporarily and see if you can bias the amplifier to 40 mA with the loop open. Then you’ll know.

Dave
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Thanks, but that should not be the problem. If it is there is other problems elsewhere being the original Dynaco uses a much smaller resistor 1K ohm in the feedback circuit with no problem, lots of feedback, whereas I am using the VTA board with way less FB using a 6.8K resistor. That and it is stable with slightly less bias, 30ma vs 40ma on the tubes. My thought if it were in the FB circuit it would oscillate no matter the bias setting, with some current naturally. For now, after fooling with this build for close to 2 months, I am ready to just listen to it for awhile.

I appreciate your suggestion though. Input is always welcome.
 
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Consider you're talking about completely different output transformers. The level of feedback these might tolerate is completely unrelated to how much the original Dynaco iron would accept. Also if no tuning was done to allow for the differences in response between the two its probably not going to work right. Probably time to break out the scope and see exactly what its doing.
 
Probably is room for improvements, most builds are but see no problem on scope or Arta distortion software at the wattages I tested it at. Seven watts was as far as I tested. Last week testing and looking at scope I saw some noise at 7 watts on the positive top part of the signal and reduced the voltage going to the driver board and solved that problem. Running stable, looks good on scope from 20hz to 20khz and sounding really good on my speakers right now. That is far as I am going to test it at the present time. Maybe later if I get some new test equipment to play with. I have a HP 334A distortion analyzer but I find Arta easier to use and accurate enough for my needs. It has been sitting on a shelf since I discovered Arta. I have been considering buying an IM tester. One of the old Heathkit ones. Roger Modjeski says they are good enough and cheap on Ebay.

If I had different speakers that required a lot of power I would test the amp right before clipping but I really to not have good enough 8ohm resistors for max out power. I never use more than 5 or 6 watts on my LaScalas. Probably a lot less than that.
 
My concern is less distortion than stability. Square waves tell a lot. If its a square its probably fine, if there is lots of overshoot and ringing, not so much.
 
Square wave test looked good as well. Looked really good with no ringing or overshoot, a square wave. This amplifier is working extremely well now after 2 months of fooling with it. I am going to recheck the bias and put the bottom plate on next week and call it good. Not surprising in that there have been many, many amplifiers built with the VTA driver board and similar to the original Dynaco output stage. Just had to get my voltages in line with the VTA Dynaco design using a different PS transformer other than the one that came with the kit and add a proven driver board. The only significant difference is the toroidal OPT's.

This build would have been a lot simpler if I had done my homework first. I was depending on Antek putting together a working kit.
 
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Ken Boyd said:
Figures I should have known better than to buy inferiour goods I have many good amps and preamps, but I thought it would be a good lesson and teaching project at that low price. But I should have kept to my original gut feelings to buy known good gear that has problems or needs restoration and get them for a low price and use restoring them for the lessons and learning curve. Mark it down as a lesson learned.
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Ken I hope I have not discouraged you about this build. Mine sounds good, really good. Clear clean sound, strong bass without losing the tube sound. I have many amplifiers to compare it to but am reluctant to quit listening to it to compare it to my other amplifiers. The amplifier on my table was started before receiving this kit and I put it aside to build the Antek. I was almost 1/2 finished with it before starting on the Antek. Got is working yesterday. It is a SE UL 6L6 build and I listened to it for a couple of songs before returning the Antek with the toroidal OPT's back into my main system.

I look forward to your impressions when you get it finished.
 
wdecho said:
Ken I hope I have not discouraged you about this build. Mine sounds good, really good. Clear clean sound, strong bass without losing the tube sound. I have many amplifiers to compare it to but am reluctant to quit listening to it to compare it to my other amplifiers. The amplifier on my table was started before receiving this kit and I put it aside to build the Antek. I was almost 1/2 finished with it before starting on the Antek. Got is working yesterday. It is a SE UL 6L6 build and I listened to it for a couple of songs before returning the Antek with the toroidal OPT's back into my main system.

I look forward to your impressions when you get it finished.
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I am using a rectifier tube for starters. And wiring the rectifier since I didn't get any drawings I was thinking of high voltage off the power transformer as two yellow wires and landing them on pins 4 and 6, and the blue wires off the transformer going to pins 2 and 8. It is a bit confusing as there are three yellow wires one of them is grouped with the orange wires. I haven't got to that wire yet to figure our where it goes.

WIN_20260821_17_46_03_Pro.jpg

Sometimes I just power up the primary side and than using my meter I can measure the voltage off the secondary wires to figure out which is which, mainly on old transformers that you can't tell what the color of the wires are anymore.
 
The yellow wire that goes with the filaments is close to the two orange wires if you line them up the way they come from the transformer. You can also check continuity with an ohm meter to verify which yellow wire goes to the filaments. That is the way I do it. I always check continuity with all my transformer wires checking for shorts before wiring it up and checking which wires go to which section. Doesn't take too long. If memory correct the center tap for the filaments are not used in this build.

If you are going to use that 300V transformer you will need to adjust the voltage dropping resistor for the VTA driver board. It wants 375vs going into it. You may not even need it. The 2.2K 3 watt resistor. I would not use tube rectification with that transformer either. You are going to need all the voltage that transformer makes to operate this design. You can try and always just take the tube out and install a couple of diodes across the rectifier tube socket if you are short on voltage.
 
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The yellow thats connected to the same winding as the one with the two orange wires is a 5v tap on a 6.3v winding. This won't be used.

Looks like the HV is yellow with a a white center tap and a gray bias tap at 55v. You'd need the two yellows to 4 and 6 on the rectifier socket, white to ground, and gray out to the bias supply.

The other connection would be the two blue wires to 2 and 8 on the rectifier.

if you're doing solid state rectification, the two blues won't get used. Yellows go to your silicon diodes. I'd go either a pair of 1N4007 in series, or something rated for 1500v or higher. Either works, but a 5AR4 is good for 1500v PIV, and many of the common silicon are not rated for more than 1000v.
 
I do not believe Ken is going to have enough voltage to correctly operate the VTA driver board which wants 375Vs in. I know for a fact I did not trying to use that transformer. The math does not add up either, 300V X 1.4 after rectification = 420V loosing at least 100Vs in output tubes when running only leaves 320Vs at most. You are going to loose more using a rectifier tube. Just saying. Someone can correct me if wrong in math.
 
well you just have to replace resistors with anti-resistors so it boosts the voltage. :)

I haven't had enough coffee to confirm the math but I don't think you're wrong. Could skip the filter stage after the output tube tap but thats likely to introduce a bunch of other issues.

Can probably modify the board so it will work with less voltage, or we have that bias supply. With enough hacking the ground connections for the tubes on the board can return to a negative supply which will give some extra room. Down side is that its half wave rectified, and it gets well into the land of undocumented homebrew witchcraft that nobody else is going to be able to help with.
 
I tried with the driver board sent with the kit adjusting bias but found myself trying to be a designer which I am not or care to be at this stage in my life, 77years old. The VTA driver board sounds and works so good as designed I would not try to change any of the circuit. My advice is still the same, swap that transformer for the one I am using. Same price. All it will cost is shipping fee. AS-3TD360 - 360V Tube Amp Transformer
No 5V for rectifier tube but other than increase in voltage everything else is the same. I added another 2.2K 3watt resistor in series with the other 2.2K 3 watt dropping resistor using SS rectification to lower the voltage into the VTA driver board and came out with exactly 375Vs into the driver board. Mostly luck with a little math used.
 
Not that I am aware of Ken. Sorry.

You really want around 400Vs on the plates of the EL34s when running. That 300V transformer is not going to get you that not even considering the driver board. Antek should not be putting that transformer with the kit. Not enough voltage.
 
wdecho said:
I tried with the driver board sent with the kit adjusting bias but found myself trying to be a designer which I am not or care to be at this stage in my life, 77years old. The VTA driver board sounds and works so good as designed I would not try to change any of the circuit. My advice is still the same, swap that transformer for the one I am using. Same price. All it will cost is shipping fee. AS-3TD360 - 360V Tube Amp Transformer
No 5V for rectifier tube but other than increase in voltage everything else is the same. I added another 2.2K 3watt resistor in series with the other 2.2K 3 watt dropping resistor using SS rectification to lower the voltage into the VTA driver board and came out with exactly 375Vs into the driver board. Mostly luck with a little math used.
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Ok have that transformer coming, that being said I take it this rectifier will be fine using the tube rectifier?
 
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I understand, mine will not see much either but if you want it to work correctly you need the proper voltage. Antek are good people to work with. John is who I talked with when trying to get mine to work with what is in the kit. Send an email to him and tell him you need the other transformer since you have not installed it yet.
 
Since I had already installed my 300V transformer I had to bite the bullet and buy the correct one needed. Still a good price for a PP EL-34 amplifier.
 
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