68 Fender Vibrolux Reverb homemade power soak.

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GMaq
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68 Fender Vibrolux Reverb homemade power soak.

Post by GMaq »

Hi,

I have 68 Silverface Fender Vibrolux Reverb reissue guitar amp and I love it.. It's 30 watts of Fender power from two 6L6 Tubes and it is in the middle of the classic Fender Silverface line between the 22 watt Deluxe Reverb and the big Fender Twin... 30 watts may sound like not very much for a home stereo or a solid state guitar amp but it's killer loud in a tube amp and therein lies the problem.. For small gigs and rehearsing this amp is too loud and with the volume turned down you miss all the crunchy tube fun so I did some reading and it seemed like a 'Power Soak' (a resistor equipped device to soak up some of the power going to the speakers) was what I needed to get that classic tube tone at a lower volume..

After some reading and a few YT Videos it seemed like this was the best option:
https://www.bugera-amps.com/product?modelCode=P0BV1

So I bought one and in the midst of the pandemic they were pretty scarce and I ended up paying more than list just to get one.. after a few rehearsals I sadly discovered the Bugera PS1 wasn't all it was cracked up to be... It was rated to be used with a 100 watt amp and here I was using it with a 30 watt amp and the resistors were getting so hot that I could smell them heating up and the sound started cutting out due to built-in thermal protection... I did some more reading about speaker impedances etc. just to be certain and I had all my settings correct so it wasn't pilot error.. I noted when I just ran my guitar straight into the amp without any pedals the PS1 worked fine but if I switched on my overdrive or octave fuzz pedals within 5 minutes it would overheat and cut out. I don't know a rock guitarist anywhere that isn't going to expect to run pedals into their amp so I had to face up to the fact that the PS1 was a lightweight fail.. :x

My bandmate Peter Cox does industrial electronics for a living and when I told him about what was happening he said "let's try some industrial strength resistors and see how that works". So the next week at band practice he showed up with a bag full of goodies and he temporarily hooked up the new resistors and after a 3 hours rehearsal of playing hard the industrial resistors were barely warm to the touch... the issue was solved, it was just a matter of looking at the amp chassis and figuring out where the resistors would fit and how to wire things so I could switch between full power and half power depending on my power needs. We got them installed pretty neatly and it turned out great!

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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by glowrak guy »

A little too far north to be 'yankee ingenuity', but a very nice solution! Hope you can resell the gizmo that didn't work out. I wonder if an external cabinet with 4X10's would be another possibility? And might bring some tonal benefits for some songs?
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by GMoon »

Hey, that looks pretty capable. Did your bandmate wire it as an L-Pad?

I have an old Altair attenuator. They don't get much respect now-a-days because of all the other fancy inductive attenuators, but IMHO a simple resistor L-Pad works very well.
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by autostatic »

Now that's cool! The Bugera PS1 looks like a copy of the Marshall PB100 but then Music Tribe quality apparently, bummer :( But luckily you could solve it yourself, looks great! Any chance you could post what parts were used and maybe some schematics (or links)? I'm attempting to build a Deluxe at the moment and adding such an attenuator might be a great option.
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by GMaq »

autostatic wrote: Tue May 31, 2022 11:47 am Now that's cool! The Bugera PS1 looks like a copy of the Marshall PB100 but then Music Tribe quality apparently, bummer :( But luckily you could solve it yourself, looks great! Any chance you could post what parts were used and maybe some schematics (or links)? I'm attempting to build a Deluxe at the moment and adding such an attenuator might be a great option.
I'll take a look and ask Pete, the schematic was written in his brain and we don't have a hardcopy but I'll see what I can do..

My son has a Deluxe, great amps! So many hit records recorded with Deluxes! 8)
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by sunrat »

Looks like the resistors are simply wired in series between the amp and speakers with the switch to bypass them to go direct. I couldn't tell exactly how many ohms the resistors are though.
Excellent wiring work BTW! :wink:
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by autostatic »

GMaq wrote: Wed Jun 01, 2022 1:17 pm My son has a Deluxe, great amps! So many hit records recorded with Deluxes! 8)
Couldn't agree more. For me it's Neil Young that made me want to build this amp.
sunrat wrote: Wed Jun 01, 2022 2:29 pm Excellent wiring work BTW! :wink:
True that too!!
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by merlyn »

The resistors are 5 Ohms. There's an identifier C300K5R0E. The 5R0 is 5 Ohms. I would think it's wired like this :

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Although it looks like a pad that would be used to half voltage in a signal circuit, this is reducing the current in the speaker (by more than half), and using two 5 Ohm resistors is a fluke of the numbers. 5 Ohms in parallel with 8 Ohms gives 3 Ohms, then the series 5 Ohm resistor brings the impedance back to 8 Ohms, so the amp sees the same load whether the power soak is in or out. Nice design!
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by folderol »

Yep nice job. The simple solutions are usually the best ones :D

I would however question the switch wiring. You shouldn't need a two pole one. There is a method using the a single pole one where at the time of switch over the amp is never completely disconnected.
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by GMaq »

Hi everyone, sorry... spread a bit thin these days..

@autostatic @merlyn @folderol

Here is the schematic, it's not exactly rocket science and I think everyone has guessed the wiring already but for the record here it is..
power soak.pdf
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by folderol »

That's what I expected but as I said, for a valve amp there is the risk of the output being briefly disconnected as you switch over. This could the fatal for the transformer if the amp is being driven at the time.

Code: Select all

---------------
      |       |
      R       LS
      \       |
       \-------
              |
      |       R
      |       |
---------------
The above is my attempt at ASCII art showing the alternative. The switch is in the low power position. Although at switchover the load is a higher impedance, it is not dangerously so, and would be safe even if the switch failed completely.
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Re: 68 Fender Vibrolux Reverb homemade power soak.

Post by GMaq »

folderol wrote: Wed Jun 08, 2022 7:47 am That's what I expected but as I said, for a valve amp there is the risk of the output being briefly disconnected as you switch over. This could the fatal for the transformer if the amp is being driven at the time.

Code: Select all

---------------
      |       |
      R       LS
      \       |
       \-------
              |
      |       R
      |       |
---------------
The above is my attempt at ASCII art showing the alternative. The switch is in the low power position. Although at switchover the load is a higher impedance, it is not dangerously so, and would be safe even if the switch failed completely.
Interesting!

To be clear the switch is never used while performing (ie it's not intended as a volume attenuation effect)it's simply a small gig/rehearsal set low and leave it, large gig set it high and leave it kind of thing, Volume boosts in a finer sense are handled by the guitar volume control and pedal settings etc..
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