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Topic: Tube amp prototype |
Steve Sycamore
From: Sweden
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Posted 6 Jul 2021 8:52 pm
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It's underway in earnest now. Many years ago I bought an inexpensive solid state guitar amplifier made in Hong Kong for the specific purpose to gut the electronics and replace them with a circuit of my own design. I've breadboarded critical parts of the circuit and am satisfied the design is working properly. This weekend I cut the necessary holes in the chassis for the transformers, tube sockets, switches, pots and jacks. The front panel will require an overlay since each channel will have 12 or 13 control pots rather than the usual 6 or 7. This is a very radical circuit design.
Since this prototype is intended to be a lightweight "studio" specific variant of a possible eventual commercial offering, it will put out only 10 watts of class A single-ended power. But an effects out signal will allow bypassing the power amp section to send the signal to a more powerful amplifier such as a Nashville 400. The real magic will be in the preamplifier.
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Bill Sinclair
From: Waynesboro, PA, USA
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Posted 7 Jul 2021 5:56 am
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I'm looking forward to seeing more on this thread. I've never seen toroid transformers in a tube amp before. I'm assuming the square transformer is the Output? What output tube will you use - a single 6L6GC? (I see a couple octal sockets back there). Will it be a 2 channel amp? What are all those control pots gonna do? Staying tuned. |
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Steve Sycamore
From: Sweden
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Posted 7 Jul 2021 7:25 am
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Hi Bill, Yes, there is usually a worry that too much current can rush in at times from a toroidal power transformer. The square transformer is a choke that directly follows the rectifier diodes of the high voltage power transformer in (a somewhat old fashioned) input choke filter configuration. It has a high Henry value with a bleed resistor following that smooths and limits current spikes well enough I believe.
I'm using 2 6V6 output tubes. With a single-ended output circuit it is important to have a large, robust output transformer, especially since the one I've chosen is also a toroidal transformer. That one is specifically designed for single-ended tube output and has quite an overkill in dimensions in relation to the mere 10 watts the 6V6's will deliver.
I plan to build in 2 channels, yes. Until I've submitted a patent application I can't go into too much detail about the preamp circuit. Maybe it can be revealed that some of the control pots will allow you to adjust the bias of the preamp tubes without needing to compensate the output volume while you are making changes. But that's a comparatively minor feature of the circuit. |
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Greg Lambert
From: Illinois, USA
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Posted 7 Jul 2021 2:09 pm
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Did those transformers come out a nuclear reactor ? LOL |
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Cappone dAngelo
From: Vancouver, BC, Canada
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Posted 8 Jul 2021 9:47 am
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That's a lot of triodes - I look forward to seeing the final product!
I'm currently experimenting with tube preamp designs for pedal steel - I'm very new to pedal steel, but have been DIY-ing pedals and amps for several years. For PSG I am currently using a high voltage tube preamp that I initially built for my 6-string - it has a blackface-inspired channel, with a 'tweed' mode that bypasses the tone stack and has separate gain and master controls (i.e., it can get quite dirty), as well as an EF86 channel. I'm planning to replace it with something that is designed to have more clean headroom on the blackface channel, and will omit the EF86 (I don't feel like I need multiple dirty tones available for PSG ...). |
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Steve Sycamore
From: Sweden
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Posted 8 Jul 2021 10:18 am
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Sounds like an interesting way to go Cappone. It feels to me like the PSG sometimes wants a bit of compression. Not so much, probably, compression of all dynamic levels but maybe more sustain in the low end dynamics and more limiting of the top end dynamics. But then I play only vintage models with pickups in the 10k to 12k ohm range. They can kick a bit when you play hard |
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Cappone dAngelo
From: Vancouver, BC, Canada
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Posted 9 Jul 2021 12:41 pm
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thanks - re compression, what I'm planning to try on my prototyping rig is a blackface front end (very similar to the alembic bass preamp) but then add a 3rd triode for some additional clean gain and then a cathode follower - which in my experience with other circuits can add compression. That said, I've never tried it with a blackface-based circuit ... |
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Steve Sycamore
From: Sweden
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Posted 11 Jul 2021 12:47 am
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I modified my Red Knob The Twin so that the clean channel is pretty much a Blackface circuit but did use another triode for the clean channel that was available with a master volume control for it. I really liked having the option to add more gain. Squeaky clean is not allows the best sound for any particular situation. Even with 2 power tubes removed and with the 40% power switch engaged that amp is just overwhelmingly loud. |
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