Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Morse-Electric on December 25, 2020, 10:59:05 am
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Hey folks, having a head scratcher here. You’ve perhaps seen this schematic here before and it’s what I’ve used for my first amp build. I’ve checking my work over a few times and get the feeling I am missing something. General info: I am currently waiting on a SE O.T. so in the mean time I subbed in a 5K primary PP O.T. I had laying about and cappped the center tap. I realize that’s entirely wrong but I wanted to start checking rail voltages and I’m an impatient bastard. Powered up And although tone signal was getting through the chain, it was terrible and it began to oscillate at about half volume. So I started checking voltages at the tubes. Black lead to chassis and red lead to either pin 3 on the 6V6’s and suddenly clean loud clear 440Hz! Second I took the red lead off the pin, back to garbage. I checked wiring again with no “aha!” I moved onto looking at the sockets and tubes. At some point I powered up with only one tube installed and WHAM clear powerful tone. Moved that tube to the other socket and same result. Tried the other tube and same result. Both tubes back in place and garbage. So I’m open to any suggestions or “Hey dummy, you forgot to...”.
Thanks
Mike
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so according to your schematic your power tubes share a grid resistor, between them; and share a grid leak resistor, sort of?
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Yes. I looked at a couple other DSE circuits and they did as well. The schematic is labeled as a 1948 Supro I found on the Hoffman site
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try snipping out the 50pf
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I'd think a more conventional setup of preamp -> grid leak + grid stopper (shared or individual) -> power tubes would give better results than preamp -> power tube -> grid stopper -> power tube
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I'd think a more conventional setup of preamp -> grid leak + grid stopper (shared or individual) -> power tubes would give better results than preamp -> power tube -> grid stopper -> power tube
That very well may be. As I go along I hope to learn why and how.
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try snipping out the 50pf
Will do and report back
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built one of those - added a 6SJ7 front-end. killer amp. we had issues with the layout, attached is what worked for us.
--pete
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... resized & cropped to fit forum.
--pete
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Lifted the 50p and no change. @Dummyload All I can say is wow! Trying to resize a pic but frickin’ iStuff won’t let me! I’ll keep trying and keep re-checking my work.
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About the best I can get for a photo. All criticism, suggestions or just good banter is welcome. So much I will do differently next time. The filters I slapped on that board for now as I haven’t figured out the dog house yet.
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http://dbl.musicweb.ch/Schematics/Music%20Instruments/Amplifier/Other%20Homebrews/Angela%20Instruments%20Angela%20Super%20Single%20Ended%206V6%20Amplifier.htm
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^^^^^^ that came up in my quick google searching and i like that output stage better
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Low and behold progress. Maybe? I went back to check voltages on C at the plates and WAY too high. Dialed down with the variac til I had 225 there. This put B at 176 and A at 140. Plugged in a strat and it was...everything I hoped for (it worked and didn’t sound terrible).Not a lot of headroom but man does it break up nice and warm. Amazingly quiet with no signal and turned up. Then it occurred to me perhaps I did not get the right PT. This one is a 550V. When I checked the rail quite a while back (no tubes except rectifier) I remember thinking it seemed high but once everything was cooking it would come down. Wrong.
So do I buck down? I could experiment with some 10W sandbars for C and adjust B and A accordingly? OR do I not monkey around and step down with a different PT. I get my stuff from AES and the lowest I can go with needed filaments is 480V. One thing I learned, I need to SLOW DOWN and think a little further.
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SLOW DOWN and think a little further.
been there, learn'nt, now
The right part for the right job
and you're playing instead of fixin
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SLOW DOWN and think a little further.
The right part for the right job
and you're playing instead of fixin
Yep. I’ll find a use for this one :icon_biggrin:
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In my SE builds with twin power tubes , every power tubes is independant. Only the signal is sent to both control grids. More, I always feed the power tubes plates after the PI filter. In your schem you re sending the B+ immediately to the plates which calls for troubles.
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what is the PT make and model #?
see attached - i built with a 700VCT @ 90mA NOS stancore PT, coupled to a RCA 5Y3GT and it delivers ~370VDC for the plate supply.
--pete
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i'd start over. you have the preamp sandwiched between the power supply and output stage.
nearly all successful builds follow a common plan input > preamps > phase splitter/driver > output stage > power supply. keep the power supply and output stage as far away as possible from the preamp section.
grid stoppers and grid leak resistors are ideally connected to the pins of the sockets, whenever possible, and at the very least the grid stoppers should be. also, that includes the screen stoppers and any screen decoupling filters.
--pete
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nearly all successful builds follow a common plan input > preamps > phase splitter/driver > output stage > power supply. keep the power supply and output stage as far away as possible from the preamp section.
Your advice is well taken and appreciated Pete. Is this “common layout” strictly to reduce hum or are there other factors at play? As my first amp I laid it out basically based on a tube transmitter I built which was PS-OSC-RF OUT. I realize this was a RF circuit so it’s apple to oranges.
Regardless my new guy errors will be mitigated on the next version. Also thanks for the schematic you threw in there.
Mike
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...I laid it out basically based on a tube transmitter I built which was PS-OSC-RF OUT.....
The RF Oscillator was many-volts output. Guitar pickup input is 0.020 Volts level. So 100X more sensitive to wall-power hum/buzz. Also a simple radio transmitter is in MAX clipping all the way through, guitar amps sometimes play less than MAX.