Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JeffAlanAmps on November 09, 2020, 10:04:58 am
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Hey Team,
I'm about to dive into a 5F1 build using a traditional 5F1 chassis. Was cruising youtube land and found this interesting video from D-lab (see minute 1:19):
https://youtu.be/XWqhx7XL0ik?t=79 (https://youtu.be/XWqhx7XL0ik?t=79)
The recommendation is to move the speaker output jack (and thus the OT secondary wiring) away from the 12ax7 (V1) and reposition it between the 6V6 (V2) and the 5Y3 (V3).
Seems like a good idea, so I'm considering doing this - If I do, I'm thinking of using that leftover pre-drilled hole between V1 and V2 to install a mod consisting of a adjustable negative feedback pot, following the concept in Uncle Doug's video (starting at 8:11)
https://youtu.be/PtoPma2-N1I?t=491 (https://youtu.be/PtoPma2-N1I?t=491)
QUESTION FOR YOU GUYS: When moving the OT secondary wiring to come through the chassis between V2 and V3, what (if any) considerations do I need to bear in mind regarding potential interference with the B+ or filament wiring?
Thanks!
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The recommendation is to move the speaker output jack (and thus the OT secondary wiring) away from the 12ax7 (V1) and reposition it between the 6V6 (V2) and the 5Y3 (V3).
That's a good idea.
Seems like a good idea, so I'm considering doing this - If I do, I'm thinking of using that leftover pre-drilled hole between V1 and V2 to install a mod consisting of a adjustable negative feedback pot
Bad idea. You just moved the speaker output away from the preamp and now you're gonna wire it to a pot that's near the preamp? I suggest locating the NFB pot far away from the preamp tube.
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I guess I'm a little confused though regarding why the negative feedback signal being close to V1 would be a problem. On the Hoffman 5F1 turret board, that negative feedback wire is coming back onto the board pretty darn close to V1 anyway isn't it?
Thanks!
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I guess I'm a little confused though regarding why the negative feedback signal being close to V1 would be a problem. On the Hoffman 5F1 turret board, that negative feedback wire is coming back onto the board pretty darn close to V1 anyway isn't it?
The NFB signal ***IS*** the speaker signal. You want to move the speaker jack to get it far away from the preamp tube. But then you want to put a pot in the vacated hole and run a wire from the relocated speaker jack to that pot. At this point you have the speaker wire right back in the original location where it can squeal just like the original. May as well never move it!
Hoffman's layout is not to scale. Inside a chassis, maybe that NFB wire is close to the preamp tube, maybe not. But it doesn't matter if you use a traditional board which is quite different from the Hoffman board.
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Fair enough, that makes sense and thanks for the explanation. Follow-up question for a newbie. In that d-lab video link, when he relocates the jack between V2 and V3, he also re-runs both the primary and secondary wires of the OT through the same hole in the chassis. Check it out here:
https://youtu.be/XWqhx7XL0ik?t=275 (https://youtu.be/XWqhx7XL0ik?t=275)
Is there any issue in having primary and secondary OT wires that close to one another? I guess one is carrying AC and the other is HV DC so maybe not? My question is probably broader around avoiding signal interference. As far as interference with the amplified signal path is concerned, when you're over in the area of V2 and V3, what wires do you need to keep as far away as possible? Is it the green filament wires? Real estate is super limited in that small 5F1 chassis, so to the extent possible I want to avoid any big no-no's. Thanks!
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...he also re-runs both the primary and secondary wires of the OT through the same hole in the chassis.
Is there any issue in having primary and secondary OT wires that close to one another?
No problem. Those wires are very close together inside the OT.
As far as interference with the amplified signal path is concerned, when you're over in the area of V2 and V3, what wires do you need to keep as far away as possible?
Just do a neat wiring job and don't "snake" the 6V6 pin 5 grid wire anywhere near the rectifier tube. The guitar signal is very big by the time it gets to the 6V6 grid and is not nearly as susceptible to interference as the low level signal at the preamp.