swapping Vibro Champ & MM Bass power sections

[Groover]

I have this same post in the Conversions section but have gotten no hits there

I want to put the power amp section of my MM Bass amp into my Vibro-Champ chassis to make it a 12 watt VC, and convert the MM Bass to a 5F2 single ended, except maybe with a EL34 instead of a 6V6.

So I will have to swap output trasformers, and take the interstage transformer from the MMB to use in the VC.

Here's the question: Can I leave the original power transformers in their respective chassis? The PT in the VC is giving approx. 390 volts B+, the PT in the MMB is giving about 290 volts . I assume I can use the hotter B+ in the VC with the MMB power amp section. Can I build a 5F2 in the MMB chassis using the lower voltage MMB transformer? I don't mind sticking with a solid state rectifier since SE class A amps don't "sag" anyway.

(If the voltage is too low for an EL34 I can stick with a 6v6 in the 5F2, I just thought for the purpose I have in mind I'd like to try a EL34 to see what that sounds like.)

[catnine]

I have this same post in the Conversions section but have gotten no hits there

I want to put the power amp section of my MM Bass amp into my Vibro-Champ chassis to make it a 12 watt VC, and convert the MM Bass to a 5F2 single ended, except maybe with a EL34 instead of a 6V6.

So I will have to swap output trasformers, and take the interstage transformer from the MMB to use in the VC.

Here's the question: Can I leave the original power transformers in their respective chassis? The PT in the VC is giving approx. 390 volts B+, the PT in the MMB is giving about 290 volts . I assume I can use the hotter B+ in the VC with the MMB power amp section. Can I build a 5F2 in the MMB chassis using the lower voltage MMB transformer? I don't mind sticking with a solid state rectifier since SE class A amps don't "sag" anyway.

(If the voltage is too low for an EL34 I can stick with a 6v6 in the 5F2, I just thought for the purpose I have in mind I'd like to try a EL34 to see what that sounds like.)

I don't see what you will gain by doing this.

The MM bass amp PT runs 2 6V6's so it should run a EL34 . However the transformer PI in the MM bass amp does not connect to the output tubes the same way as most push/pull amps and then you have an issue with the virbrato working after .

You would be turning the Champ into a push/pull setup whcih you already have in the MMbass amp .

Seems like a lot of work and change and might render two amps that have issues.

The champ would end up close to a 6G2 princeton with a BF tone stack with cathode bias . The MM bass amp is sort of a one of a kind circuit the way it is setup with the tranformer PI ,it's not wired at all like a tube PI or split load .

You might be better off building what you want .

[Groover]

What I would gain is basically a Vibro-Champ sized amp with more power and fidelity (kind of a mini Princeton), which I would like to have for theater pit band gigs, recording and the occasional church gigs and such, and hopefully a nice SE class A amp with a more "Marshally" type voice for recording.

Those are my goals. I could also covert the VC to a tube PI and tremolo like in a Princeton non-rev, but that seems like more work.

The VC tremolo works on the 2nd pre-amp stage, so that can stay as-is and should still work. I know the interstage transformer connects to the power tubes differently than a tube stage PI, but I don't see any reason it wouldn't work with the VC preamp.

[catnine]

What I would gain is basically a Vibro-Champ sized amp with more power and fidelity (kind of a mini Princeton), which I would like to have for theater pit band gigs, recording and the occasional church gigs and such, and hopefully a nice SE class A amp with a more "Marshally" type voice for recording.

Those are my goals. I could also covert the VC to a tube PI and tremolo like in a Princeton non-rev, but that seems like more work.

The VC tremolo works on the 2nd pre-amp stage, so that can stay as-is and should still work. I know the interstage transformer connects to the power tubes differently than a tube stage PI, but I don't see any reason it wouldn't work with the VC preamp.

I think it would be easier to add a 9 pin for the PI simply because for me anyway it's difficult to tell how the transfromer PI would work .

Perhaps someone else knows , there are not the two 220K ohm resistors after the PI before the output tubes and you would have a cathode biased amp which is fine .

[Groover]

I don't see any reason the transformer PI would work any differently (ie be any more problematic) than it would in the MMB amp now.

[Groover]

I'm forging ahead with this project. I have swapped the PTs & OTs, I have the PT all wired up in the VC chassis. I have wired up the SS rectifier with a NTC inrush current limiter on a terminal strip, and have a good ground-buss bar soldered to the chassis.

I had to remove the cap can to clear the speaker but I have 3 small radial filter caps that I will silicone to the chassis.

Still dicking around with the fitment of the IS trans. It would fit between the 12AX7s except for the socket mounting screws. So I may drill new mounting holes for the 9-pin sockets in such a way as to be able to use the same mounting hdwr to mount the IS trans, or I may move the LFO tube to the hole where the cap can was, using an adapter plate.

It's good to make actual progress, after obsessing about and researching this project for so long.

[catnine]

I'm forging ahead with this project. I have swapped the PTs & OTs, I have the PT all wired up in the VC chassis. I have wired up the SS rectifier with a NTC inrush current limiter on a terminal strip, and have a good ground-buss bar soldered to the chassis.

I had to remove the cap can to clear the speaker but I have 3 small radial filter caps that I will silicone to the chassis.

Still dicking around with the fitment of the IS trans. It would fit between the 12AX7s except for the socket mounting screws. So I may drill new mounting holes for the 9-pin sockets in such a way as to be able to use the same mounting hdwr to mount the IS trans, or I may move the LFO tube to the hole where the cap can was, using an adapter plate.

It's good to make actual progress, after obsessing about and researching this project for so long.

Thing is the champ has a more powerful PT than the MM bass amp .

What I did in my MM bass amp chassis was gut it and add the tube PI and built the amp as a princeton 6G2 front end and a 5E3 power section. I don't have the tremlo.

With the champ chassis you have a tube rectifier , two 9 pin sockets that could be used for the pre amp and PI and all you would need in on more Octal for the second 6V6 but you would not have tremlo . Or you could use the SS rect and the rect tube socket for the other 6V6 .

The champ and the MM bass amp use the same chassis with different punchings .

[Groover]

I'm going to build a 5F2a in the MMbass chassis initially, but after I get it up and running I plan to modify it for an EL34 SE power section.

I'm aware that the MMbass PT doesn't have as high a voltage on the HT secondary as the VC, but the power section should be fine, since I am duplicating what was in the MMbass, and hopefully the lower voltage won't affect the preamp stages and LFO much.

[catnine]

I'm going to build a 5F2a in the MMbass chassis initially, but after I get it up and running I plan to modify it for an EL34 SE power section.

I'm aware that the MMbass PT doesn't have as high a voltage on the HT secondary as the VC, but the power section should be fine, since I am duplicating what was in the MMbass, and hopefully the lower voltage won't affect the preamp stages and LFO much.

The PT secondary voltage will affect the entire amp .

My point is the champ is a fine SE amp as it is . Put a Champ PT and push pull OT in the MM bass amp and you will have a great amp just by adding the 9 pin PI right where the PI transformer is located and you only have to work on one amp and you end up with more in the end. This way all you need is the tone and volume pots the MM already has wired just like the 6G2 and the power section of the 5E3 and it will kick the stock MM out of this world.

What I am saying is I don't understand why you want to mess with two amps when you want more power. The MM bass amp is alread stock like a 12 watt champ with a tone and volume stack.

They are your amps to do what you want with , I must be missing something here. I just don't see what you will gain.

[Groover]

What I gain is Vibrato[sic] and full tone stack.

I suppose I could have drilled out the MMB for 3 more pots and another tube, and I could have put a 5F2A front end in the VC and disconnected the Vibrato circuit.

I also could have done a scratch build of a BFPrinceton non-rev in an Allen mini-project chassis. But that would have been a lot more costly and would have somehow defeated this possibly warped creative vision I had.

So this is how I decided to do it. It makes sense to me. I think I'll still end up with two cool amps.

[catnine]

I'm not certain stiil what your goal is. You have very little info to go on.

Tone stacks due change the amps sound. The champ only has a vol, treb and bass and the MM has one tone and a volume. The MM is already a push /pull 12 watt with a transformer for a PI and a low power PT.

It seems to me you are using the MM bass amp to build a single ended 5F2 witha vol and tone and changing the Champ into a push pull with a full treb bass mid stack. . Tone stacks suck out tone . Is this push/pull champ going to have a diode rectifier and are you using the old spot where the cap can was for the extra octal for the 6V6 ? Then use the same champ cab and run an 8 inch speaker. Both amps will need reworked eyelet boards and so much more.

[Groover]

Catnine, I don't suppose it matters that you don't understand my motives. It is something I have wanted to do for a long time and I am finally doing it. I thought it might be of interest to others. I will provide further updates when I have the amps up and running, or if I have more questions.