Hello All,
I was wondering if some of the tube gurues out there could help me with a project I have in mind.
I am looking to build a dual mono tube amplifier to use with my Johnson J-Station guitar pre-amp and a pair of EV 12L speakers in individual cabinets. I want to go dual mono so I can maintain the stereo effects that the J-Station produces. I don't need any more small combo amps or high powered rock amps, as I have these already.
I recently came across an old Stromberg Carlson PA amp, and I would like to salvage as many parts from this as possible. Namely the power transformer, rectifier, 12ax7's and 6l6's. I might even be able to use the chassie.
What I was thinking was, half of a 12ax7 to drive a single 6L6 running class A single ended. As apealing as an SET layout would be, I don't think this will provide the power I am looking for. For guitar amp duties, the higher distortion of pentode operation will be acceptable.
The reason I am asking for help is, I haven't really come accross any schematics for doing this with a 6L6. I've found plenty of examples using the 6V6, but I would like to use a 6L6 to get a little bit more power, and I have a few around already. I have combed the pages of the Aspen-Pittman book for ideas, but have only found 6L6's run in push-pull mode. Is there any reason why musical instrument amplifiers haven't used single ended 6L6 configuations over the years?
Anyway, any help would be appreciated. I hope to hear from you soon.
Cheers,
Zach
I was wondering if some of the tube gurues out there could help me with a project I have in mind.
I am looking to build a dual mono tube amplifier to use with my Johnson J-Station guitar pre-amp and a pair of EV 12L speakers in individual cabinets. I want to go dual mono so I can maintain the stereo effects that the J-Station produces. I don't need any more small combo amps or high powered rock amps, as I have these already.
I recently came across an old Stromberg Carlson PA amp, and I would like to salvage as many parts from this as possible. Namely the power transformer, rectifier, 12ax7's and 6l6's. I might even be able to use the chassie.
What I was thinking was, half of a 12ax7 to drive a single 6L6 running class A single ended. As apealing as an SET layout would be, I don't think this will provide the power I am looking for. For guitar amp duties, the higher distortion of pentode operation will be acceptable.
The reason I am asking for help is, I haven't really come accross any schematics for doing this with a 6L6. I've found plenty of examples using the 6V6, but I would like to use a 6L6 to get a little bit more power, and I have a few around already. I have combed the pages of the Aspen-Pittman book for ideas, but have only found 6L6's run in push-pull mode. Is there any reason why musical instrument amplifiers haven't used single ended 6L6 configuations over the years?
Anyway, any help would be appreciated. I hope to hear from you soon.
Cheers,
Zach
Take a look at the Electar Tube 10 schematic at musicyo.com.
http://www.musicyo.com/planet/graphics/tube10scmtc.gif
It uses a 12AX7 and single ended 6L6. The output stage is very much like a Fender Champ. You may need to adjust the 470 ohm cathode resistor depending on available B+ voltage and output transfomer impedance.
Ususally the single-ended offerings from the amp manufacturers were meant to be entry level practice amps, and were made as cheap as possible, 5 watts or so, usually with 6V6 or EL84. They could use just about any old piece of junk transformer and it would work ok. As you go above this wattage, the SE transformer has to carry more DC current and has to be designed better and is thus more expensive. Thus it is cheaper to go push-pull with a couple of 6V6 or EL84 when you get into the 12-15 watt range.
On a subjective note: I like the sound of push-pull better for guitar amps. I spent a lot of time diddling with SE configurations including 6L6 and EL34. But to me, a pair of 6V6 push-pull, driven by a long tailed pair, sounds better than a SE 6L6. I do like about 6 - 10 db negative feedback as well, and I like pentode operation vs triode. In a lot of ways, Leo Fender had this whole guitar amp thing nailed pretty well by 1963!
Regards,
Mike
http://www.musicyo.com/planet/graphics/tube10scmtc.gif
It uses a 12AX7 and single ended 6L6. The output stage is very much like a Fender Champ. You may need to adjust the 470 ohm cathode resistor depending on available B+ voltage and output transfomer impedance.
Ususally the single-ended offerings from the amp manufacturers were meant to be entry level practice amps, and were made as cheap as possible, 5 watts or so, usually with 6V6 or EL84. They could use just about any old piece of junk transformer and it would work ok. As you go above this wattage, the SE transformer has to carry more DC current and has to be designed better and is thus more expensive. Thus it is cheaper to go push-pull with a couple of 6V6 or EL84 when you get into the 12-15 watt range.
On a subjective note: I like the sound of push-pull better for guitar amps. I spent a lot of time diddling with SE configurations including 6L6 and EL34. But to me, a pair of 6V6 push-pull, driven by a long tailed pair, sounds better than a SE 6L6. I do like about 6 - 10 db negative feedback as well, and I like pentode operation vs triode. In a lot of ways, Leo Fender had this whole guitar amp thing nailed pretty well by 1963!
Regards,
Mike
mjd_tech said:
That Gibson amp looks pretty good. Now, if I were to run a stereo line-level signal in to this, would I insert it right after the volume pot between the first and second gain stages of the 12AX7? i.e. feed the grid directly from a line level source? Any possible problems doing this?
I like the push-pull class A stuff myself. They only reason why I mentioned SE is, that's what I've got right now. A push pull EL84 might do a pretty good job at what I'm after too. One thing, will I have to use half of a 12AX7 for a phase splitter and run feedback to the plate? That's what I think I'm seeing in some of the higher power Fenders.
Cheers,
Zach
Just a couple of comments on the Gibson amp. First of all, there is a negative feedback loop from the secondary of the output transformer to the cathode of V1b, which cannot possibly work as the cathode resistor is bypassed. Either the bypass cap must go or another resistor should be inserted between the cathode and the current resistor and cap to provide a point for feedback injection. (Possibly a typographical error?). Secondly, if the feedback does work your bass, treble and master volume controls are within the loop, so the lift and cut of the tone controls will be limited by the effect of feedback, and the setting of the master volume will affect the level of feedback. While some control of feedback might be desirable in a guitar amp, here it can only be affected by overall volume level - in other amps which have variable feedback I've seen this is usually accomplished by a variable resistor in the feedback circuit itself or in the cathode of the valve where feedback is taken to.
In answer to your question, if you feed a signal straight to the grid of V1b after the gain pot, the signal could still be affected by the setting of the pot and if your signal is of low impedence, it would provide a mismatch from the feed from V1a which is higher impedence, and you'd notice the effect if you were trying to mix sources. Why not fit another gain control? Your signal could go to the top of the new control and both this control, and the existing gain control, would each individually have their wipers connected to the grid of V1B via a resistor (say, two 470k types), which would allow each gain control to be relatively independent, and not effect, each others operations. Not a hifi solution, but in practice it seems to work OK.
In answer to your question, if you feed a signal straight to the grid of V1b after the gain pot, the signal could still be affected by the setting of the pot and if your signal is of low impedence, it would provide a mismatch from the feed from V1a which is higher impedence, and you'd notice the effect if you were trying to mix sources. Why not fit another gain control? Your signal could go to the top of the new control and both this control, and the existing gain control, would each individually have their wipers connected to the grid of V1B via a resistor (say, two 470k types), which would allow each gain control to be relatively independent, and not effect, each others operations. Not a hifi solution, but in practice it seems to work OK.
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