Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: martinman on August 02, 2024, 07:18:26 am
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Hi Everyone,
I'm thinking of adding a Mojotone style (https://www.mojotone.com/Mojotone-Discrete-Hi-Voltage-Series-Effects-Loop-for-Vacuum-Tube-Amplifiers (https://www.mojotone.com/Mojotone-Discrete-Hi-Voltage-Series-Effects-Loop-for-Vacuum-Tube-Amplifiers)?) effects loop to my 6v6 plexi. Wondering if anyone has done this and can point me in the right direction? I added a PPIMV along the way too (which btw completely transformed the amp!). I've seen a few diagrams where the master volume is integrated to the effects loop (though I guess technically it's not a PPI MV anymore). Anyway, just looking to get some discussion going.
Thanks!
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Nice and useful mod
Instruction sheet say all you need to know.
In my opinion, the only difficulty is where to locate this circuit in the chassis.
Best is to post schematic and inside pictures of your Plexi 6V6 to start discussion.
You should look here too ; https://el34world.com/Forum/index.php?topic=10208.0#google_vignette
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Heres the layout schematic I followed (standard Hoffmann build):
https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf
Then, i added a PPIMV here:
(http://)
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I've installed a similar loop (Metro Zero loss) into a Marshall 2204-style build and other Plexi-style builds. According to the schematic, the Hoffman Plexi 6V6 has a pre-PI master volume. I'd recommend to insert the effects loop between the treble pot wiper and the input into the pre-PIMV pot, rather then between the pre-PIMV and the PI input as shown in the Mojotone instructions. I've never seen one inserted AFTER the pre-PIMV and before the PI.
If your amp does NOT have a pre-PI master volume, then the loop will go between the Treble wiper and the input to the phase inverter.
For B+ to power the board, you can tap off of the phase inverter node. You may need to re-adjust (lower) the dropping resistor for this node after you install the FX board to preserve the voltages you had prior to the board installation. When I did my board installation, the change in voltage at this node was minimal so I did not have to adjust the dropping resistor value.
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I was looking to build Doug's Plexi 6V6 with an effects loop and was wondering how your build is going as I really need some guidance in particular regarding the effects loop. Any help or advice you could provide would be appreciated.
Thanks.
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I've installed a similar loop (Metro Zero loss) into a Marshall 2204-style build and other Plexi-style builds. According to the schematic, the Hoffman Plexi 6V6 has a pre-PI master volume. I'd recommend to insert the effects loop between the treble pot wiper and the input into the pre-PIMV pot, rather then between the pre-PIMV and the PI input as shown in the Mojotone instructions. I've never seen one inserted AFTER the pre-PIMV and before the PI.
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Just to note, that's where Marshall insert the fx loop on the reissues https://music-electronics-forum.com/filedata/fetch?id=1004326
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I've installed a similar loop (Metro Zero loss) into a Marshall 2204-style build and other Plexi-style builds. According to the schematic, the Hoffman Plexi 6V6 has a pre-PI master volume. I'd recommend to insert the effects loop between the treble pot wiper and the input into the pre-PIMV pot, rather then between the pre-PIMV and the PI input as shown in the Mojotone instructions. I've never seen one inserted AFTER the pre-PIMV and before the PI.
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Just to note, that's where Marshall insert the fx loop on the reissues https://music-electronics-forum.com/filedata/fetch?id=1004326
Thank Kindly for your response. Am I assuming correctly then that the Mojotone or other Metro Zero Loss boards would tap into the Plexi 6V6 somewhere to get their power? I haven't seen any instructions for either one but would think so. Doesn't look like either board would fit in the chassis that Doug has specifically for the Plexi 6V6 build.
Thanks Again.
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Am I assuming correctly then that the Mojotone or other Metro Zero Loss boards would tap into the Plexi 6V6 somewhere to get their power?
As I recall, most of these loop boards take their power supply feed from the phase splitter HT node.
Sorry, I've no idea what will physically fit.
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Sadly they will not fit in a predrilled PR chassis. The Rev/Trem holes are too close together.. These are made for a blank chassis or retrofit into a chassis that has the space and room to drill the mounting holes.
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As I recall, most of these loop boards take their power supply feed from the phase splitter HT node.
Sorry, I've no idea what will physically fit.
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Thanks for the info. I had the feeling those boards need to go in a bigger chassis.
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I'll toss a few ideas out for your consideration. I am NOT saying you should do this.
I've used a mosfet CF with a psuedo active FX loop behind it on 4 amps. Every delay or reverb pedal I've tried (maybe 7 different pedals) has sounded good to me using this approach. The mosfet cathode follower does not change the tone of the signal, in my experience. I've done both a tube CF and a mosfet CF on the same amp as a comparison and experiment. I hear no difference.
I enjoy having a 5879 V1 tube. Years ago on the forum, DaGeezer developed the HoSo56. I've done quite a few versions of this and have liked all of them. I currently have this concept with 6BM8 tubes & I designed it to be a reasonably clean tone for playing jazz and rhythm guitar. I enjoy the amp alot. It's called "The Minimalist" and I've played it at open mics with 7-9 watts of power and it's done great.
So, one could approach a Plexi 25w amp design with a 5879 in V1 ............. OR simply parallel a 12AX7 or 5751 in V1.
Just adding some ideas to the mix to consider .............
With respect, Tubenit
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OR cathode biased (which I prefer) with a PPIMV master volume. Please note with the FX pots and the PPIMV, there is a great deal of tone shaping one can do. I put the FX send pot on the back of my amp chassis as a "set and forget" adjustment.
I typically build my chassis boards as a paralleled turret board which makes modifications (if needed) easier to do, IMO.
With respect, Tubenit
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Great stuff tubenit. 😁
I'm curious about the 5879 in V1. Are there some particular characteristics it has over a12AX7? You also mentoned using a 12AX7 in V1 and running it in parallel mode. Could you elaborate on this a bit more? Would a 16" chassis be big enough to accomodate these circuits?
Thanks Kindly.
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A paralleled 12AX7 dual triode gives about 30% more gain without increase floor noise.
A 5879 has sort of a warm full chimey tone to my ears. I like that tube and have a lifetime stash of them for my amps.
It sounds different than a 12A_7 tube, IMO.
With respect, Tubenit
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A paralleled 12AX7 dual triode gives about 30% more gain without increase floor noise.
A 5879 has sort of a warm full chimey tone to my ears. I like that tube and have a lifetime stash of them for my amps.
It sounds different than a 12A_7 tube, IMO.
With respect, Tubenit
Thank You Tubenit for answering my question. I'm contemplating a new build and my interest in the 5789 has been piqued.
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I'm contemplating a new build and my interest in the 5789 has been piqued.
Watch out; it's 5879 not 5789 which is a whole other thing.
Also - I think a parallel 12ax7 has 30% more output than 1 of the triodes in a dual triode 12ax7, but less gain than a 12ax7 with the two triodes wired in series. Just trying to clarify - I think I have that right.
TBone, you will note that the Clubman you were looking at uses a parallel wired dual triode in first position driving a pentode in the second position - 5879, ef86, 6Sh7, 6Sj7 are all pentodes that have similar characteristics.
I agree with Tubenit on the sound of the 5879. I will add that it is very flexible and can be set up in many ways to change it's character. I'm pretty sure that Tubenit put together some typical 5879 schematics. Most famously they were used in the 60s version of the Gibson GA-40 Les Paul amp. In that amp they were setup for a lot of gain. I've used them in two Tubenit designs, a single channel GA-40, and a modified version of a Dr. Z Z-28. All sound good. There is a rare Valco/Supro amp with a 5879 - can't seem to find a schematic for that amp.
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I'm pretty sure that Tubenit put together some typical 5879 schematics.
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I think a parallel 12ax7 has 30% more output than 1 of the triodes in a dual triode 12ax7, but less gain than a 12ax7 with the two triodes wired in series.
That is my understanding also regarding 30% more than a single 12Ax7 triode.
With respect, Tubenit
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Heres the layout schematic I followed (standard Hoffmann build):
https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf (https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf)
Then, i added a PPIMV here:
Maybe you should have used a dual 100k pot for the PPIMV since you used 100k grids..?
https://el34world.com/Forum/index.php?action=dlattach;topic=31906.0;attach=114949 (https://el34world.com/Forum/index.php?action=dlattach;topic=31906.0;attach=114949)
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I'm contemplating a new build and my interest in the 5789 has been piqued.
Watch out; it's 5879 not 5789 which is a whole other thing.
Also - I think a parallel 12ax7 has 30% more output than 1 of the triodes in a dual triode 12ax7, but less gain than a 12ax7 with the two triodes wired in series. Just trying to clarify - I think I have that right.
TBone, you will note that the Clubman you were looking at uses a parallel wired dual triode in first position driving a pentode in the second position - 5879, ef86, 6Sh7, 6Sj7 are all pentodes that have similar characteristics.
I agree with Tubenit on the sound of the 5879. I will add that it is very flexible and can be set up in many ways to change it's character. I'm pretty sure that Tubenit put together some typical 5879 schematics. Most famously they were used in the 60s version of the Gibson GA-40 Les Paul amp. In that amp they were setup for a lot of gain. I've used them in two Tubenit designs, a single channel GA-40, and a modified version of a Dr. Z Z-28. All sound good. There is a rare Valco/Supro amp with a 5879 - can't seem to find a schematic for that amp.
Oops! Sorry about the tube number mix up. Have to be more careful. I am looking to build something different to get a different sound and feel so going with the 5879 might be the ticket. I did have a look at the Clubman again and saw the parallel wired 12AX7 in V1 followed by the 5879.
Thanks for input.
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I'm pretty sure that Tubenit put together some typical 5879 schematics.
Thanks for posting these tubenit.
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I came across this video and thought it would be worthwhile sharing. It's a build very similar to the Plexi 6V6 but what interested me the most was the FX Loop design.
There's a link to the builders web site where you can get the plans to build his version for free. It includes templates for the chassis, turret board, schematic and layout. You can purchase the FX loop from him.
The video is quite long as he takes you through his build so be prepared to spend some time.
Here's the link i=jHlKBncIjWgEsupv
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One tube you can try on V1 for a clean amp is the 6AK5 or the similar 5654
You can also try it as V2 (Mr Merlin say that pentodes on V2 give their better result)
To see an example of guitar use in V1 look at Sluckey's
Smoky
https://sluckeyamps.com/smoky/smoky.htm (https://sluckeyamps.com/smoky/smoky.htm)
K
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One tube you can try on V1 for a clean amp is the 6AK5 or the similar 5654
You can also try it as V2 (Mr Merlin say that pentodes on V2 give their better result)
To see an example of guitar use in V1 look at Sluckey's
Smoky
https://sluckeyamps.com/smoky/smoky.htm (https://sluckeyamps.com/smoky/smoky.htm)
K
Thanks for the info about Smoky. I'll check out the link.