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Old 2nd June 2009, 09:47 PM   #1
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Question Blue Voodoo Solid State Rects to Tube Rects?

I know I'm new here but the basis for the join is I've about racked my brain out on this probably because I'm overlooking something very basic...

Let me start off by saying this isn't my first project with amps but it is my first with tube amps. I own 3 tube amps plus this and want to mod it to see if the overall effect and appearance is worth modding my real amps.

The current amp I'm modding for fun is the Crate Blue Voodoo 120w. It's by no mean a top of the line amp but it's my test mod amp and I'd rather destroy this than my other 3 amps...

BV120_2 BV120_1

You can see the 3 full wave rectifier circuits and maybe I'm a little confused here but would each of those need an individual tube? I was under the impression 1 tube or 2 tubes could drive all 3 circuits... I am looking at installing 5AR4's... Anybody have input?
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Old 2nd June 2009, 09:57 PM   #2
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BV120 comes up ok but BV120 1 seems to be a bad file.
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Old 2nd June 2009, 11:09 PM   #3
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That was my bad I'll re-up the files...
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Old 2nd June 2009, 11:09 PM   #4
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Default Re-Upped Files

The new files are listed at this HTML:

Blue Voodoo
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Old 2nd June 2009, 11:30 PM   #5
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Hi, what sort of change to the sound are you seeking?
Looking at schematic of power supply, I cannot see an easy way of replacing SS bridge with a 5AR4, the HT winding has no centre tap, and there is no 5V heater supply winding.
If slow start is required, may add a TV damper diode such as 6AU4 or 6AX4 in series with HT after first filter capacitor. Another idea might to replace the 1N4007 rectifiers with diodes that have a more benign switching characteristics, there are other on this forum that can suggest something, I have kept to tube rectification in my own designs, so cannot really suggest specific types.
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Old 3rd June 2009, 06:36 AM   #6
Enzo is offline Enzo  United States
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Right. You just say you want to "mod" these, but what are you trying to achieve? By inference you seem to want to make the power suply rectifiers tube based, but you don;t state why. I can see that a tube rectifier in the B+ circuit might add some sag dynamics, but hte rest is a mystery.

Unless you just want the challenge of making just EVERYTHING a tube, I don;t see the purpose. You have three bridge circuits. I don;t know what you mean by one tube could drive all the circuits.

One bridge provides the B+ - the high voltage for the tubes. But as radiotron points out, there is no center tap on the HV winding, and no 5v winding for the rectifier heater, so that makes a tube rectifier a lot harder to use. I suppose you could add a 5v transformer for the heater, and you could replace half the bridge with a tube maybe. But just sticking a tube rectifier in there in place of the diodes will drop your B+ voltage considerably.

Then ther are the other two bridges. Did you sort out what each does? The lower one make DC voltage for the preamp tube heaters. Why on earth would you want to try to run that off a tube rectifier? It's not in the signal path.

The bridge in the center makes +/-9.5VDC for the 5532 effects loop op amp IC, and +/-10VDC for the reverb driver IC as well as the LEDs and the transistor switching circuits. Again, I can;t imagine why you;d want to make the power supply for that tube based. Those op amps are in the signal path, but they are all working class A, there would be no sag dynamics to play with anyway.

Adding all these tubes would certainly add weight and a lot of heat to your amp.

Regardless of what you think of the tone, the amp as it exists now does work, so I don;t think. a soft start is an issue.

SO what are your overall goals in this project? I see plenty of possibilities to modify the signal circuits, massage the tone, tweak the gain structure, ets.
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Old 3rd June 2009, 03:27 PM   #7
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Your both absolutely correct, the point or inspiration is really just for the challenge to see if I could get it done, I'm not looking for an overall tonal change since sag only occurs when the amp is played at a rather loud volume. The whole change would be just to add tubes to make it an "all" tube amp. I'd already thought about the the transformer and you are correct I'd be adding another one to the two transformers already in the unit.
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Old 4th June 2009, 12:28 AM   #8
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Quote:
Originally posted by Enzo
SO what are your overall goals in this project? I see plenty of possibilities to modify the signal circuits, massage the tone, tweak the gain structure, ets.
If you have other thoughts aside from toying around with the rectifier I'd be interested in hearing them. After allm, all I do this for is fun and for the learning experience I am not an "audiophile" I just like finding new and better ways to modify some things. I am still searching for the best tone out of each amplifier but I like doing things one step at a time...
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Old 4th June 2009, 02:48 AM   #9
Enzo is offline Enzo  United States
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I don;t have specific ideas for you, that isn;t my thing. I run a pro audio maintenance and repair operation. I am more interested i getting things back to normal.

But all the things people do to amps could be done to yours. Changing tone stack values, changing voicing in the stages. Massaging the NFB, altering the gain structure, trying different tubes. (Get yourself a 5751 and sub it for each 12AX7 one at a time and listen for the differences sometime) And so on.

There are some great resources. Under the software downloads - they are free - at DUncan amps is a tone stack calculator. It allows you to select a number of tone stack circuit types and then vary the component values and see the response curve changes in real time graphically. Very cool. Plenty of other good stuff there.

www.duncanamps.com

Check out the tech info section at Aiken's site

www.aikenamps.com

kevin O'Connor's site has good stuff too

www.londonpower.com

And while the forum here is generally aimed more at hifi, there are other forums that are focused on guitar amps, you might look around. I like the MEF

www.music-electronics-forum.com
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Old 4th June 2009, 08:13 AM   #10
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Stryker11487,
Tube rectifiers for Guitar Amps are generally not a good thing.

It depends a bit on what you want to achieve - for example, modern metal tones are produced by overdriving the preamp and running a very clean almost HiFi'ish power amp which needs a clean fast and have a reasonably hard "little sag" power supply. SS is the go for this.

The ONLY sound that "requires" a tube rectifier is that 60's and early 70's Dirty Blues sound which is achieved by a reasonably clean preamp sound and driving the power amp hard so that you get power supply sag on the attack.

This sound can be achieved in various ways. A current limit in the high voltage supply is one way to do it (a MOSFET and 1 transistor circuit). If it is just Power Amp "compression" that you want then you can actually induce the Power Section to do this at a lot lower volume by replacing those 470 Ohm Screen resistors with 2K2 or even higher.

Cheers,
Ian
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