Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tony321owen on March 20, 2020, 05:43:06 pm
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Recently I have built a 5f2a inspired amplifier which has reminded me how much I love the sound of low wattage single ended amps! Many have pondered how to get a higher powered version of that tonal bliss and have come up with parallel single ended designs as well as using larger transformers/power tubes to simply build a bigger single ended amp. Lately I have been interested in some of the old integrated high-fi tube amps that use two Output transformers which made me wonder: Could someone build 2 champ circuits in one combo amp chassis/ cabinet fed by the same input, with seperate vol and tone controls, separate OTs and a speaker for each channel?
I started sketching out a schematic drawing on some of the forum threads where people have built similar stereo guitar amps and then while reading Jack Darr's book, I found that he has a schematic for a circuit that uses a phase splitter after a tremolo oscillator and inserts the out of phase signals to the grids of two separate preamp channels to create true stereo tremolo! In the end I got down a rough draft without any component values.
My list of concerns for actualizing this thing:
1. The power supply. I figured if I used a simple preamp circuit with modest current demands that maybe I can use a simple, single-rectifier power rail design as opposed to a separate power supply for each circuit like on some stereo amp designs?
2. Transformer specs. would a normal PT and single ended OTs work?
3. Input signal strength. I know using a passive AB boxes cut you signal (in half?) when you use two amps at the same time.. Im guessing it would be the same if the input signal was separated to two different input stages.
Would it be as simple as adjusting the input impedance by increasing grid leak values on input stage?
4. tremolo function/ insertion. In Jack Darrs book he inserts the tremolo into the grids of two separate preamp channels using a phase splitter. On a typical vibrochamp the insertion point is the cathode of the second stage.. I have read that a cathode follower is used as a buffer to provide a low impedance signal which matches the cathode of the tube being inserted into.. I imagine the higher impedance plate signal from the splitter could cause problems and perhaps there are other problems I am probably not aware of.
I was thinking of maybe building it in a deluxe reverb sized cabinet or something similar with a 10" and 8" speaker.. It seems to have almost exactly the right number of holes for tone controls and the size seems about right to fit the speakers.
Anyone point me to schematics of people building something similar?
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I think your idea works. ASn alternative is tremolo in one channel, maybe reverb in the other.
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It would be so cool If it had reverb in one of the channels so that there was a dry and a wet channel! I hear there's some old Gibsons that do something like that if i'm not mistaken..
I haven't seen very many single ended amps with reverb. Know of any? One of the first amps I ever built was a parallel single ended vibrochamp reverb but the reverb never sounded right so I converted it to a Princeton reverb and sold it to my friend.
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Search this Forum for adding such reverb circuits.
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Know of any
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga1rvt.pdf
I'm not holding your hand while you debug it though :icon_biggrin:
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I have a GA1-RVT.. It's been sitting for 5 years because I don't want to deal with it (yet)! :l2:
You might want to look at how Swart went about doing their stereo amps (both the SE & PP). They have slightly different implementations from each other and the PP version seems closer to what you are planning.
http://swartamps.com/swart_st-stereo.htm
http://swartamps.com/swart_stereo_master_20.htm
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Here's my 2nd draft with added reverb.
Does this design look possible assuming someone knew how to tweak component values to make it work or are there fundamental things wrong with it that makes it so I should just throw it in the bin and redesign?
(especially curious about the Stereo tremolo and reverb circuits)
Thanks for your insights/ honest criticism.
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Around RvBin/RvbOut...
No grid-leak on that stage.
Injecting reverb INside a NFB loop seems problematic. Good NFB would cancel the reverb. I don't think this small-NFB plan could kill reverb but it could do odd things.
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Thanks for the reply PRR!
Good call on the negative feedback situation and noticing the lack of grid leak resistor..Maybe if I inject negative feedback into the third stage instead of the second that would solve the issue..
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I would suggest to stick to the Vibro Champ schematic. It's a proven circuit and the tremolo and NFB work very well. Then all you gotta do is shoe in the reverb circuit and debug.
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Here's a cleaned up, easier to read 3rd draft with some changes.
I was searching for "champ reverb" through the forums and found a few examples of people injecting reverb right after the Volume control and before the 2nd stage. I have also found versions where people have added a 3rd gain stage which is what I based my last schematic on. This one seems truer to the original vibrochamp circuit so I modified the design..
Does anyone know if I need to double the grid leak values at the input stage to make up for one signal divided by two channels?