Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: silverfox on March 19, 2016, 02:37:21 am
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I first joined this Forum a little over 3 years ago, at the time in an effort to build a tube amplifier rather then continue to purchase used equipment only to become disappointed shortly after the purchase either because the condition of the amp was misrepresented or after playing through it, the amp didn't sound as good as the first time- For whatever reason. I believe now it takes at least over 40 hours of playing time before I can give an honest assessment of the tone as suited to my needs.
So having spent the past three years studying vacuum tube electronics and building several tube amps, each one coming closer to what I wanted in an amp, I may have finally arrived. Thanks to the contributions of many in the amp forum community.
Before returning to this forum I had considered this very topic and already decided that using my very first amp project parts, a Carvin MTS 3200, would be a great way to complete the circle. Short story on that- I checked out the Carvin and played if for about two hours. Decided to buy it but didn't have all the cash at the time so I put down some money and came back about two weeks later and picked up the amp. For whatever reason when I got it home it just didn't sound the way it had when I played through it. I don't know why. At first I was going to mod it but changed my mind since the only mods available at the time really didn't improve the tone so I ended up stripping it and trying to build a different amp. That was amp #1 and I didn't like it. And the rest is history. Three forums and three years later I'm ready to complete the circle. Every now and then my wife remarks at the various, "skeletons" I have laying around here without realizing that in this case, skeletons in the closet is a good thing.
And so without further ado I give you the: (Drum Roll), Full Circle Build, (Ta Da) A 70 Watt Dynaco Inspired Stereo Tube Amplifier.
I'm using the original Carvin PT and just purchased two Classic Tone TrainWreck style output transformers in the 35 watt range. I've been told the 35 watts are conservative.
And away we go.
silverfox.
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AWESOME!!!!!
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Pause.... I'm reviewing power supply design for stereo amps since I have a question on the channels interacting and transmitting transients between B+ outputs. While this is for guitar and some interaction and sag is desirable, there could easily be different signals on each channel as in effects such as echo and I may be using this with a mixer which could put vocals on the output too. I don't know if I need to use some sort of diode isolation between grounds, separate chokes or nothing. I've also considered floating the output section grounds off the chassis to keep the grounds separate.
silverfox.
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Been pretty busy lately but tonight I got a chance to do a mock up of the Iron. The choke is already mounted on the underside. I decided to move the preamp tubes so the holes get plugged. Everything fits with lots of room in the amp. The reverb tank won't be needed so at some point I'll pull that.
Really impressed with the "looks" of the Classic Tone Iron. I didn't expect it to be this large but that'll be great for field storage. Should get some time after tomorrow to get building.
silverfox.
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Finally got some time to work on the amp again. Filaments wired and the HV-Rectifier board built and tested. Unloaded voltage is @480 VDC. With all this iron on board this is one "Heavy Metal" amp!
silverfox.
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Looks good! Don't accidentally blow 450 volt caps testing the amp out with unloaded B+!
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eleventeen: Looks good! Don't accidentally blow 450 volt caps testing the amp out with unloaded B+!
I used the classic stacked caps and balance resistors scheme I learned from the Forum. 2 series 100uf 350 VDC caps with bleeder resistors to balance the charge. My plate supply is 100uf total and the screens are 50uf; both supply’s at 700 VDC working.
More work done last night with two blank perf boards cut and mounted in the drilled chassis.
silverfox.
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Good deal. I like doing that on the first cap rather than getting within 25 volts of the cap's rating.
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I'm finding a couple things here and there that may need to be changed. Other than that seems to going smoothly.
silverfox.
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Completed project. I have not tested it other than the power supply in an unloaded state but that comes next. I don't know that I'll post anything more on this due to the lack of responses. Since it is pretty much a copy of the Sunn S100 with some changes that have been tested in the past I do not anticipate any new problems. I did change the grid resistors on the output tubes to: 5.6K in an effort to address what may be grid blocking. It could also be severe overdrive of the driver stage since the input signals were too high. Anyway, some tweaking is to be expected once I get the bias set.
Thanks all for your past contributions to my learning experience. At this point I'm off to College in all likely-hood. All signed up and ready to go.
silverfox.
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"I don't know that I'll post anything more on this due to the lack of responses..."
Silverfox, don't think that a 'lack of responses' means that your work goes unnoticed. I for one notice most of the builds on this forum and just as often pick up (translate as 'plagerize') little things I can try in one my builds. As long whatever you are building is worthy of your time, feel free to share as much as you can with the community. That is what I think of when I hear the term 'full circle'.
There are few things as personally rewarding as finally building that amp. Regards
dennis
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Looks great! Keep the hard work ethic going for college, you'll ace it I'm sure. Thanks for sharing the build details, and I hope you end up with a perfect amp out of it.
~Phil
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First Bug! Bias resistor not correct value. I thought since I changed the bias design to a 4 port- One for each power tube, design, that I should use a larger size bias pot to ground, (ground lift) resistor. That resulted in an adjustable range of like, 10 volts and the upper value was 74 volts... Going back to the original 22k. This time I used the Marshall style bias power supply where the bias source is taken from one side of the AC HV tap. Very rewarding to see all the other voltages where they should be. Unloaded B+ is 475. I should also mention, I tried much harder this time to keep the wiring neat but in the end that sort of spun out of control. It is an improvement over the past projects.
I'm really looking forward to hearing these ClassicTone OT...
Ace College: Well, the instructor has let me into the DC Fundamentals class without the prerequisite math that I've got to take. I should be able to clear that part up by Fall. Always been weak in math as a result of not just, "Doing the Math". Welding is scheduled for the Summer, morning session before the heat is on for the day. That's kind of a review also but it's all been years ago. Seems like a nice college.
silverfox.