ron
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Post by ron on Oct 1, 2015 13:50:48 GMT -5
I have a 100w Bogen CHB-100 with quad 7868 power tubes, 6EU7, 2-12AX7's, and a 6C4. My thought is to make 2 amps in one using the PT, OT, tubes and/or tube sockets that are already there. What I am thinking is a lower watt Fender style amp and a low watt Marshall-style in one chassis, using a switch or just different inputs to access either amp. Is this possible or been done before? I am able to follow schematics well but not at the level to design this myself Any thoughts?
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Post by newey on Oct 1, 2015 17:28:25 GMT -5
ron-
Hello and Welcome to G-Nutz2!
When it comes to amp questions, our resident guru is sumgai. He'll probably be along in a bit.
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Post by sumgai on Oct 2, 2015 19:45:38 GMT -5
How quick is a "bit"? ron, This has been done, although I have a question or two, before we begin... When you say "2 amps in one", do you mean two channels with one output, like most guitar amps? (Fender and Marshall, for instance.) Or did you think that you could end up with a stereo (dual outputs) amp in one chassis? Several years ago, a long unheard-from member by the name of Teleblooz did this with a 50 watt Bogen. The thread for that should be findable with the Search function, but if not, then ask for some help in that regard. I visited with him shortly after the completion of that project, and it really did sound like a Fender Princeton, or perhaps a Deluxe (probably dues to its 12" speaker). Getting that tone without laying out some serious cash is always a good thing! Not to mention the satisfaction of doing it one's own self. As to "any thoughts", I am forced to remind you that 7868 power tubes are not commonly found in most music stores, nor even online in every Tom, Dick and Harry tube store. When they are found, they are not cheap, to be certain. You might consider converting to another, more common power tube. Depending on your desired power level (and considering that the Bogen's OT was not rated for the continuous high output power demanded by a musical instrument), you don't necessarily need to use 6L6's, trying to throw out 100 watts. Conversely, you might use them, but dial back the actual power somewhat, both to "save" the wear-and-tear on the OT, and to find that magical sweet spot. In the long-run, you do have options, lots of them. If you're not in a hurry, take the time to consider your needs and desires, then list them out on a sheet of paper. Determine what has to stay, and what can go, if some kind of compromise arises. Setting goals like that will shorten your "design cycle", and get you to the workbench with more confidence in the final outcome. Trust me on that one. Keep us posted, and we'll be happy to help out wherever we can. HTH sumgai
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Post by reTrEaD on Oct 3, 2015 13:16:47 GMT -5
My thought is to make 2 amps in one using the PT, OT, tubes and/or tube sockets that are already there. What I am thinking is a lower watt Fender style amp and a low watt Marshall-style in one chassis, using a switch or just different inputs to access either amp. While "possible", it doesn't seem practical with that chassis. You have a limited amount of real estate on the front panel. Currently it hosts 6 pots and a power switch. You could have two preamps, each with a single volume, treble and bass controls. But that leaves no room for inputs or additional controls. The B+ supply of the CHB-100 is a rather hideous design. It uses a single secondary winding and a "voltage doubler". It's prone to a high level of ripple and strong limitations on current. Since you are interested in a "low watt" configuration, I'd suggest changing to a bridge rectifier. This would decrease the voltage to something in the 250v range but be much cleaner. Regarding the output tubes, I'd suggest using a pair rather than a quartet. This would further reduce the maximum output power. Between the PS change and using a pair of output tubes, you will be somewhere in the ballpark of 10W. That seems a rather desirable range for a guitar amp. Possible to get plenty dirty without being excessively loud. Since you will no longer have output tubes in parallel, it would be sensible to increase the primary impedance of the output transformer. If you connect an 8ohm load to the 4 ohm secondary tap, this will essentially double the primary impedance. I suggest evaluating the 7868s you have on a transconductance tube tester. If at least two of them are healthy, it would make sense to use what you have. If not, you'll need to change a pair of 6V6s or EL34s with Octal sockets or even a pair of EL84s with 9-pin miniature sockets. 6L6s would fit in the space you have, but just barely since they have a larger bulb. This would decrease the distance between envelopes if you use the existing holes in the chassis. Even though the power dissipation would be lower because of the new PS configuration, it's still poor practice have output tubes so close together. JMO. The CHB-100 is rather a mess below decks. Here's a gutshot of what appears to be a stock unit. Note the point-to-point wiring and ceramic caps. Ugh.
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ron
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Post by ron on Oct 3, 2015 23:36:32 GMT -5
What I was thinking of was a common pre-amp circuit that splits off (switchable) in the power section to either a pair of 6V6's (or a pair of the 7868's that I already have) or a pair of EL84's and then into the OT and out to speakers.
Here is an example of the power tube stage that was similar to what I was thinking.
Questions Would the transformers in my Bogen CHB-100 be able to handle this? Would there be enough of a sound difference to make this worthwhile?Attachments:
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Post by reTrEaD on Oct 5, 2015 16:52:26 GMT -5
What I was thinking of was a common pre-amp circuit that splits off (switchable) in the power section to either a pair of 6V6's (or a pair of the 7868's that I already have) or a pair of EL84's and then into the OT and out to speakers. Ron, that's a rather interesting configuration. I can't say that I've ever come across this sort of thinking. There will be an inherent problem or two. Those problems are manageable, although solving them does add some complexity. Would the transformers in my Bogen CHB-100 be able to handle this?You'll add some demands on the filament supply by adding an extra tube for the phase inverter. But the 6V6s take far less heater current than the stock 7868s. So all things considered, I don't think there will be an issue at all with the iron. Would there be enough of a sound difference to make this worthwhile?There won't be much difference between the sound of 6V6s and EL84s but I expect either will have noticeably different characteristics compared with the stock 7868s. A long-tailed pair vs a phase splitter should introduce a serious difference in sound when overdriven. Inherent problem #1:Since the LTP has significant gain, that means there needs to be an additional stage of amplification feeding the phase splitter or a voltage divider feeding the LTP to keep things equal when switching from one output stage to the other. Inherent problem #2:Tubes in a push-pull output stages draw a considerable amount of current at idle. When one side is driven to conduct harder, the other side is driven to conduct less. If the only change between which pair of tubes is being used is where the signal is applied, the inactive pair will continue to have both of its tubes drawing current through the primary of the OT. This will severely restrict the amount of power transferred to the output. Not good! I think it best to switch the grid bias on the inactive pair to a highly negative voltage (cutoff).
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ron
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Post by ron on Oct 13, 2015 16:13:24 GMT -5
Would having a separate OT for each set of power tubes solve this problem?
Can you please elaborate on what you mean "I think it best to switch the grid bias on the inactive pair to a highly negative (cutoff)."
Thanks, I am re-learning all of my electronic theory and this has been helpful.
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