Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: silat on July 22, 2021, 08:54:04 am
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What is the best way to add one 12 Ax7 to 5C1 Champ? I am Using Bogen CH8 Iron which shows it will run a 5y3 rectifier a 6L6, 6SJ7 and a 6AV6. I do not have the 6AV6. I have the board for the 5c1 Champ and it is mpstly populated. I figured it might not take too much to change 6AV6.
Thanks in advance for your help. I know you all are always eager to pitch in. I am still trying to learn but it has been really slow for me.
Silat.~
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P.S. I cannibalized the Bogen but left the Iron and the Chassis. I do not see a layout or a Schematic for what i want but close.
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Here's a little help for anyone else who is playing along:
Bogen Challenger CH8 - rev A.pdf (https://el34world.com/Forum/file:///C:/Users/MW/Downloads/Bogen%20Challenger%20CH8%20-%20rev%20A.pdf)
Fender_champ_5c1_schem.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5c1_schem.pdf)
You might find this informative:
How to convert a bogen 6L6 Challenger to Fender champ deluxe tube amp Tweed sound - YouTube (https://www.youtube.com/watch?v=sMfYSLZj4hc)
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Thanks for answering Silvergun. I did see that schematic, but since the amp already had the 6L6 I would like to keep it instead of the 6v6 .
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Thanks for answering Silvergun. I did see that schematic, but since the amp already had the 6L6 I would like to keep it instead of the 6v6 .
Wel, that's OK. Since you know the transformer supports it, you are good to go. The values from the 6V6 are also an OK place to start, so your schematic is 50% complete.
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I gutted the thing so I am not going point to point. I think the Iron will support at least half of the 12AX7. I was wondering where I could put that in? Thanks for the Information on the schematic BTW SG.
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Here's a quick mock up.
Lots of signal voltage available out of that second stage. I would consider paralleling the 2 halves instead.
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I gutted the thing so I am not going point to point. I think the Iron will support at least half of the 12AX7. I was wondering where I could put that in? Thanks for the Information on the schematic BTW SG.
The iron should easily support both halves. I wouldn't sweat that.
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Awesome SG! I appreciate it. I thought it would, but I wanted to make sure.
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I have played a bit with a similar concept.
I started with parallel triodes into the pentode.
At the moment I have a TB tone stack (can’t remember which one) between the triodes then into the pentode. It sounds good, but is not quite stable—too much gain. This has been a total experiment and is a real hot mess on the inside due to reworked point to point prototype kind of build.
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Lots of signal voltage available out of that second stage.
Only if you put a plate load resistor in the circuit. :icon_biggrin:
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Ooops...sorry silat.
Thank goodness for other sets of eyes.
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Thank You Both for the catch SG and Sluckey! Will a 470 work on the cathode on the 6L6 or do I actually need to put a 500 ohm resistor?
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Thank You Both for the catch SG and Sluckey! Will a 470 work on the cathode on the 6L6 or do I actually need to put a 500 ohm resistor?
470 is close enough for rock and roll.
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Also, no need to search for a 5MEG grid leak on the 6SJ7.
1M will be just fine.
OR, if you leave the 12AX7s' triodes cascaded, you might want to add a resistor and make it a voltage divider.
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> best way to add one 12 AX7 to a 5C1 Champ?
I was not aware the 5C1 needed an added 12AX7.
While the 5C1 has some gain-strain with just the only preamp stage, both sections of a 12AX7 adds gain of 600 to 2500. On top of the stock 5C1 sensitivity of about 100mV, the sensitivity is like 0.1mV. Hot enough to hear a gnat think.
It makes some sense to toss-out the 6SJ7 and just use a 12AX7; that sold well for many decades.
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Also, no need to search for a 5MEG grid leak on the 6SJ7.
1M will be just fine.
That 5MEG is necessary. Look closely. That 6SJ7 is operating with grid leak bias (cathode grounded).
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> best way to add one 12 AX7 to a 5C1 Champ?
I was not aware the 5C1 needed an added 12AX7.
While the 5C1 has some gain-strain with just the only preamp stage, both sections of a 12AX7 adds gain of 600 to 2500. On top of the stock 5C1 sensitivity of about 100mV, the sensitivity is like 0.1mV. Hot enough to hear a gnat think.
It makes some sense to toss-out the 6SJ7 and just use a 12AX7; that sold well for many decades.
PRR, it is not actually 5C1.... I am re-purposing the Power Transformer and the Ouput Trans from a Bogen CH-8 which has a 5y3 Rectifier, a 6l6, 6SJ7 and a 6Av6 I wanted to add the 12AX7 instead of the 6AV6. It seems like it would handle it alright. I gutted the Amp and I have all the parts for the modified champ.
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not assuming to speak for him, but i think his point is that two triodes into a pentode is a LOT of gain, most of it not even usable. unless you're using voltage dividers to tame the signal the extra tube will not likely be a welcome addition
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thetragichero, Yeah so I have scrapped the idea.
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Personally, I would just leave the 6SJ7 out of the circuit. I built a single ended 6L6 amp from the iron from an old Stromberg Carlson amp that had a 6L6 output. I just wired a 12AX7 in like the Champ, same resistor values, it pushes the 6L6 just fine. I figure that it has close to 10 watts output to the speaker.
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There's another single ended amp you could build if you want to get carried away.
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There are voltage dividers, both fixed and variable in my amp—it’s still unstable at high settings. I think poor evolving layout is contributing.
By the way, I was mistaken and forgot about changing the arrangement in the amp I described to have the tone stack after the second triode and before the pentode.
Also, the more complicated my experimenting gets, the more I appreciate a simple less is more approach.
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I still don't think your initial question/plan is that far out of the realm of reality.
I've done dumber stuff. Revising the plan so you have 2 gain stages into a 6L6 is not a bad or crazy idea. Change up the 6SJ7 to cathode bias and add a variable voltage divider ahead of it.
The "easiest" way to add a full 12AX7 package would be to tie the socket pins together and run it paralleled.
So I submit this for review:
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Parallel triodes with the first tube is an inspiring idea. That would really help with excessive gain...and noise too. Probably still too much gain at the bass frequencies would cause the farty distortion. If so, that could be helped by reducing the first coupling cap to about .0047. And reducing the 22uf cathode bypass cap to a tiny 0.47 The master volume pot could be a 250ka to better fit the needs of the power tube and give finer control of the volume.
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Considering the mods of the components around the pentode proposed by SILVERGUN
(https://el34world.com/Forum/index.php?action=dlattach;topic=27752.0;attach=92945;image)
one other way may be to use a triode of the 12ax7 as input of the amp, the second triode configured as Catode Follower which will drive a Tonestack, the exit of the Tone Stack to the input of the pentode
EDIT: One other way will be to use a triode as input then gain pot then pentode followed by the otrer triode as Catode Follower driving a Tone Stack
Franco