Wanting some specs for Vibroverb project plans

[goldstache]

Hello,
 
I am wanting to put together a 6G16 vibroverb or a derivative.  Around this Transformer:


350-0-350 @120MA


5V @3A


6.3V @4.5A


My first question is how overrated is the HT current on a stock amps transformer?  I think the modern replacement transformers are around 160MA.  I am concerned that there wont be sufficient current headroom under the demands with 120MA spec'd on the transformer I have.




Pretty sure I have everything but an OT to build it, even a prepunched chassis!  Hoping to only have to buy 1 iron and not two ;)


   

[Willabe]

You have plenty of heater current for that amp.

6x12A_ 7 @ 300mA= 1.8A
2X6L6GC @900mA= 1.8A

1.8x2=3.6A

120mA for B+ is a little low for a 2x6L6GC amp. I think you want/need ~180mA for B+?


             Brad 

[goldstache]

Thanks for the reply.  That was precisely what i was worried about.  I know I always try to have overage in the B+ current spec.  Just kinda wondering if anyone off the top knew, actual cumulative current draw of the revered fender.


I guess I will have to do the math.  Let me take a crack at this.
V1 - V6's plate current calculations added together??? 
Are there any other things to factor into the equation, or is that the it?????
noob

[PRR]

That's a 50 Watt power amp.

The input power to the final stage will be close to 100 Watts.

The B+ is shown to be +440V. (Not sure the 350-0-350 is quite right.)

100W/440V= 0.227 Amps or 227mA.

Add a few for preamps.

Add/subtract for high/low B+ according you use Silicon or Vacuum rectifier.

I'd say this is half the iron you need.

[tompagan123]

Have you built the Vibroverb?  Awesome versatile amp.  I have a 63 reissue Vibroverb and it definitely has its own signature voice, due to the unique tone stack and reverb mixer (no 3.3M / 10pf). To my ears it the 'verb has more high mid content and less high treble content compared to the AB763.  It can get real trebly without inflicting pain.  The value of this pot,  treble cap and treble pot tail resistor vary slightly between the 63 reissue and the original 63.  I'd thought about experimenting with this tone stack but haven't settled on where to source the tapped pot.   
My particular Vibroverb has 2 x 10" eminence legend 1028 alnicos which are really cool speakers for this amp.
I found this quote from the reissue service manual interesting:

"The VIBROVERB is unique in Fender amplifier history by the
implementation of its tone circuit. The VIBROVERB was the
first and ONLY Fender amplifier to use this type of tone circuit.
The treble control is “tapped,” meaning it responds
more like an active tone control allowing boost AND cut of
high frequencies. Conventional Fender tone circuits provide
a small amount of tone shaping even in the “0” position,
but if the VIBROVERB Treble Control is set to about
“5”, the response is flat."

It would be cool to get an update on this build.

[Willabe]

"The VIBROVERB is unique in Fender amplifier history by the
implementation of its tone circuit. The VIBROVERB was the
first and ONLY Fender amplifier to use this type of tone circuit.

Not true, most brown face 6G.... amps have the same tapped treble pot.


               Brad     

[tompagan123]

Hi Brad,  I'm looking at the schematics for the brown '63 reissue  and the original.  The *reissue* has a 250k tapped pot and 250pf treble cap while the original has a 350k tapped pot with a 330pf treble cap.  And I know for sure that my reissue matches the schematic.  With all due respect, whats not true about that?

[Willabe]

"The VIBROVERB is unique in Fender amplifier history by the
implementation of its tone circuit. The VIBROVERB was the
first and ONLY Fender amplifier to use this type of tone circuit.
The treble control is “tapped,” meaning it responds
more like an active tone control allowing boost AND cut of high frequencies.

It's not the only Fender amp to use that type of tone circuit. A number of brown face Fenders used a 'tapped treble control'. It doesn't matter what the values of the pots/caps in the tone circuit it's still the 'same type of tone circuit.'


                       Brad     

[HotBluePlates]

Maybe Fender bent the truth by meaning the 6G16 was the first circuit to use that setup, and it's the only Fender amplifier (that we still make today) which uses it.

Of course, they stopped making the 6G16 reissue quite a while ago.    But Willabe's right: except for the Champ (which stayed in tweed format), Princeton and Deluxe, all the early 60's Fender amps had the tapped-Treble pot somewhere in their tone circuit.

Bassman
Pro
Showman
Super
Tremolux
Twin
Vibrasonic

That one feature is probably why so few copies/homebrews exist of the early-60's Fenders; until relatively recently, there was just no source for the tapped Treble pot.

[tompagan123]

I agree, they did bend the truth.  I also noticed that the 6G12-A Concert has it, but the 6G12 does not.  perhaps they were still perfecting that tone stack when the Concert was initially produced.   Playing around  with my re-issue Vibroverb, it's interesting to listen to the transition where the tap is when turning up the treble pot. You can hear it "kick in".   I'm not so much impressed with the treble control in of itself, but just the overall voice of the amp.    That may also have to do with the different reverb mixer.  The 2nd triode on the reverb channel has a "tapped" plate load resistor.   82K in series with a 22K.   The dry signal is level shifted down by tapping it off the 22K into a 470K to the grid of the 3rd stage.   This level shifting obviates the 3.3M / 10pf.   I tried this on one of my builds and it sounds way smoother.    The 10pf seems to define the blackface sound which can get into some harsh territory frequency wise.  Fender may have transitioned to the blackface scheme because level shifting the dry signal takes up more real estate, OR they thought it sounds better (not to my ears :-). 

[HotBluePlates]

When I tried out the Vibrverb reissue in the late 90's, overall it sounded the same as my '67 Princeton Reverb (as far as overall tonal quality). Seemed like a great reissue at the time.

[Ed_Chambley]

I have built a few Brown fenders with the 350 with the tap.  Unless original, the pot Doug sells here and the one Weber have are the same.  They are smaller and measure inconsistent.  I have a few with the tap reading as low as 44ohm.  Anyway, I have simply bought a few.  Some say your can use the 250K pot and adjust the treble cap and you can get almost the same thing.  I found this not to be the case.  Anyway, it is simple to add a series resistor with the tap to get 70ohm.  Just getting close is all that is needed.


I used a Super Reverb/Bassman replacement I had on a 63 Viborverb. 


Mike Marsh sells some parts for Brown Fenders.
http://www.marshamps.com/


Also, Doug's Super Reverb/Bassman Replacement should work well.