Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TurboGuitarMelton on February 09, 2019, 04:33:26 am
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Hey guys! Finished wiring my build (as topic implies its a 5f1 champ with reverb)
It powers up fine and sounds the part. I used the 1 tube reverb that tubenit posted a while ago. When the volume of the amp is down to 2/3 it is perfect and i feel like I have enough adjustment range for it to feel right. But when I turn the amp up the reverb seems to get "left behind". Is this a mixing resistor issue? Right now im using a 100k resistor.
Few notes:
I have no NFB loop.
It is also motorboating when I get it up to 7 on the volume knob and it slows down when I turn it up to 10.
I have a 1amp fuse in there because thats all i had. Is that causing the motorboating?
I dont have a schematic yet. But I will make one tomorrow morning and post pictures too.
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Schematic
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The reverb driver must have a resistor from grid to ground. Typical value is 1M. Also, I suggest you move the reverb transformer B+ lead to the same filter cap that feeds the output tube screen.
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some minor suggestions...
5e1 was cleaner - less buzzzzzzzzz, as it had a C-L-C filtered plate supply. the 5f1 was cheaper to build - no choke. the 5f1 is more popular since it had a much longer production run than the 5e1.
schema attached is how i'd take a shot at it.
good luck & happy hacking!
--pete
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I can't thank you guys enough! Totally fixed both issues. I was hesistant to put the reverb high voltage on the screen tap because i am seeing about 350v there. And i know the 12ax7 is max at 330 on plates. But it works for now. I am going to put a 5r4 rectifier in there to drop voltages a bit to be on the safe side.
This little amp is killer! Nice grit and reverb. Washy/twangy/gritty sound like jim campilongo. Love it! 5f1 sounds good with no NFB.
*just checked the plate voltage on the 12ax7 and its @335.5 with the 5y3 rectifier.
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Could you also post a layout in addition to the schematic? What transformers did you use?
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Sure thing, give me some time and ill make them. Ill try to make them look nice instead of hand drawn. :icon_biggrin:
Transformers:
Classictone 40-18019 PT
Old RCA radio output transformer (10k - 4ohm)
Standard fender style reverb transformer.
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I think I would like to build this amp. Any possibility for some photos of the turret board and the top of the chassis for layout.
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Is there enough reverb available with the one tube
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Papa Jim,
Sorry for the delayed response. Sure, here are some photos of the build. I just used eyelet board material and a eyelet tool and eyelets from Lowe's. Lol Worked okay for me.
I have done some tweaking and experimenting since I finished it. Ive changed a few values and added a switchable NFB loop, a three way "tone" control for bright, normal, and dark. I also tweaked the values in the reverb circuit a bit. I went back to a 1.5k cathode resistor on the reverb driver and changed the 1M reverb pot to 250k. (What I had on hand) I also removed the cathode bypass cap on the 2nd stage of the V1 tube. I found that that gave the amp waaay too much gain and sounded pretty awful dimed.
As far as the quality of the reverb, I feel like it sounds great. Not as crazy as the typical fender reverb but for one 12ax7 it's pretty darn convincing. My tweaks to the circuit made it a little more prominent and helped to minimize distortion in the driver. At first I had a lot of weird clangy ambient distortion going on. For my purposes I like it set to around 7 (which to me sounds like a bigger fender reverb on 3 or 4)
It may be the cheaper speaker, the smaller cab, or adding the reverb circuit but I really prefer my stock 5f1 clone to this "5f1r". This one sounds slightly anemic to me. Who knows. I am keeping this amp at work to teach with so it works for that just fine!
* transformer pic is from the start of the build but it accurately shows how they are mounted. middle Transformer is the OT
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Here is a messy sketch of how I did the layout. I think it works pretty well. Everything fit perfect. The amp runs dead quiet aside from normal se idle hum.
*I am using 12ay7 in V1. It helps to balance the reverb mix better. The dry mix seemed to overpower the wet mix more when I had a 12ax7 in there.
*added a finished build pic. Turned out nice!
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If you want less dry signal in the mix, try attenuating the dry signal more by increasing the 100k to (say) 150k to 220k, and see where that leaves the dry signal level. Might be a bit of trial and error to suit your taste.
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Thanks for the suggestion! I actually tried that and messed with that resistor for a long while. Haha. I settled on the 100k again in the end. It seemed to adversely effect the tone of the amp with a 150k or 220k. That may have been all in my head though. Changing the v1 tube seemed to have a slightly different effect than changing the resistor.
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Thanks for the pictures and other ideas. Your amp really looks nice. Doesn't even resemble mine. Lol Next time I'm gonna use a turret board I believe. Gonna add the reverb and try a few of those other ideas. I am using a Celestion G12M Greenback, but have heard a lot of good things about the Weber speakers. A 10" weber would be nice. Do you prefer the ceramic.
http://www.tdpri.com/media/users/rolloman.103491/
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Thanks for the suggestion! I actually tried that and messed with that resistor for a long while. Haha. I settled on the 100k again in the end. It seemed to adversely effect the tone of the amp with a 150k or 220k. That may have been all in my head though. Changing the v1 tube seemed to have a slightly different effect than changing the resistor.
Yes it may affect tone due to interaction with Miller capacitance of the following gain stage. You could counteract this with a smallish value bypass cap in parallel with the resistor in the range of 100pF to 1nF
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Man my amp really started sounding thick and great to my ears when I removed the NFB Loop. I know that you used a switchable NFB Loop. Do you find yourself switched to the no NFB Loop position mostly?
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Haha yeah, I mostly leave it with no NFB at normal volume levels (3-6). I do like the sound with it too though. The 5f1 champ can be noisy (as lots of people have said) so it makes it run slightly quieter with the NFB. I also prefer the sound of the amp dimed when the NFB loop is engaged. it sounds harsher without it.