Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: timmyjoe on January 04, 2022, 05:02:07 am
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I'm moving my own response to my earlier post to a new topic. This drawing might be of interest to more people than just the part on dummy safety loads. And wasn't getting answers to questions. Not sure if this is right in that I haven't tested it yet. So looking for comments. This is for a 5E3.
*I'm having a really hard time finding a double pole three position switch with high enough amp rating. Thinking I'll just have to use double pole switch with two positions. Take out the 16ohm tap and resistor.
*I guess it is OK to leave taps unused in that diagrams I see have that very arrangement in terms of their electrical operation. I'm having to dod the same for the power transformer. It has an unused 50v tap.
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Maybe the 50v tap is for bias. Could switch between cathode and fixed bias :think1:
Negative feedback loop doesn't connect to the rectifier, usually it is taken from the secondary of the output transformer and injected back to the audio signal path.
https://robrobinette.com/images/Guitar/5E3P_Build/5E3_NFB_Mod_Schematic.jpg
(https://robrobinette.com/images/Guitar/5E3P_Build/5E3_NFB_Mod_Schematic.jpg)
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A schematic and explanation of what you’re trying to achieve would be beneficial.
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If the amp is cathode-biased, like the 5E3 shown, just use the shorting jack!! That's what it is for!!
No, shorting a cathode-bias amp will not burn it up.
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Thanks for your patience with me guys.
Sorry Uki. Yes the neg feedback is going to the signal path, not the rectifier.
PRR, I tried doing two output jacks, but this ends up with a dummy load running when you use the other output jack. So that won't work right? Too much draw on the amp I would think.
Here's the goal of this project:
*Create a 5E3 head unit.
*Protect the head unit from damage in the event it is left on without a speaker connected. Hence the dummy loads. I killed the filter caps in my first 5E3 project by accidentally turning it on without a speaker. Didn't realize it was plugged into the power strip I was using.
*Permit the user to use 8 ohm or 4 ohm output. The OT has 4, 8, and 16 ohm output.
*Use the Rob Robinette neg feedback mod with the footswitch
Questions I still have:
*What to do with the 16ohm output? Is it OK to just leave it unconnected?
*What to do with the PT 50V extra output? Someone recommended a Hammond BX 290 for this build in that it has a lower voltage. Their thinking was that it would give the amp a true 5E3 50s sound in that back then the power input from the wall was a lower voltage. I didn't realize there was an extra 50V line on this transformer. Is it OK to just leave it unconnected?
First drawing is what I think will work. The rotary switch idea is out due to the high cost. Second drawing has two output jacks with dummy loads. I don't think that will work because it would be like I was running two speakers all the time at different impedance.
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Questions I still have:
16ohm output Is it OK to just leave it unconnected? yes , but tape it
*What to do with the PT 50V extra output? Use for fixed bias
50V line on this transformer. Is it OK to just leave it unconnected? yes, but tape it
The 50v winding, it can be used for fixed bias, it is nice to have a dedicate winding for bias.
You could use a bias type switch (https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Bias_Switch), cathode/fixed, since the transformer have that winding, why not take advantage of it.
Here an amp with the bias type switch
(http://www.bcaudio.com/assets/images/bc-audio-bel-air-40-front-panel-b-1-1000x750.jpg)
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Thanks Uki!
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For the unused bias and 16R secondary leads, just trim the leads, heat shrink, and tie neatly out of the way. I've sometimes folded the end of the trimmed lead back on itself about 0.5-1" and heat-shrinked it that way.