DIY Bass Amp

[Fan O' Zakk]

Tube rectification generally used a centre tap secondary on the power transformer allowing full wave rectification with only two tube rectifiers. Which saved quite a bit of money in the old days!

Keith

Some diode configurations, depending upon the voltage produced across each half of the secondary, even used the center taps! Still needed 4 diodes in that case, though...

[truerussian558]

explain why you need 4 diodes

the way i see it, is that the tube rectifier is just basically 2 vacuum diodes in one tube (or at least the one for my princeton is), it works by when the ac is switching back and forth, it only lets positive voltage values through, and then the humps are flattened out by the use of a inductor and caps, thus if you replace the tube rectifier with 2 diodes, it does the same principle thing, unless i have the whole concept wrong (from my grnadpa's electronics lectures over the summer), so if i do can you please elaborate.

Edited September 7, 2004 by truerussian558

[lovekraft]

You don't need 4 diodes! You're simply going to get a higher voltage from the output with silicon diodes because the tube rectifier isn't nearly as efficient, and drops a fair amount of voltage, generally 20-50 volts, whereas a silicon diode drops about .70 volts. In the case of the Princeton, it simply involves either using a 300-0-300 volt power transformer instead of the stock Princeton 340-0-340 volt, or using a voltage divider to get the stock transformer back down to the 420 volts DC that the circuit needs. Using a Hammond 272BX in place of the Fender transfomer pretty much takes care of the problem.

[Fan O' Zakk]

explain why you need 4 diodes

the way i see it, is that the tube rectifier is just basically 2 vacuum diodes in one tube (or at least the one for my princeton is), it works by when the ac is switching back and forth, it only lets positive voltage values through, and then the humps are flattened out by the use of a inductor and caps, thus if you replace the tube rectifier with 2 diodes, it does the same principle thing, unless i have the whole concept wrong (from my grnadpa's electronics lectures over the summer), so if i do can you please elaborate.

In place of the resistor, in this diagram, imagine a filter cap.

I can't explain exactly why you do it, but I've always been taught that. 4 diodes always. I've seen either two diodes on each side of the secondary, in series, or else set into a full-bridge configuration, as above.

Your concept is correct, in that it limits the two-way AC current flow to one way, and by each end of the PT's secondary being 180* out of phase with the other side, it fills in the "gaps" and creates a DC current flow.

As was stated above, silicone diodes aren't a direct replacement for tube rectifiers, for two reasons. One, the voltage drop is significantly less, and two, you DO NEED to add in a standby switch. Most circuits that run a tube rectifier don't need one, because the rectifier tube itself brings the current up slowly, and cathodes are not stripped.

Now, I can't explain offhand why 4 diodes are used, I just know that in every amp I've seen, that's how it's been. It might be a noise filtering feature, it might be a safety feature, it could be a lot of things...but don't you think that if it weren't necessary, they wouldn't do it??

[truerussian558]

...but don't you think that if it weren't necessary, they wouldn't do it??

youll find that a lot of stuff people do that is very common isnt neccesary at all.

but i still dont see why yuou need 4 diodes, maybe its so you wont have to use the inductance and/or caps and resistors , but can anybody please explain the purpose of the 4 diodes.

Edited September 8, 2004 by truerussian558

[lovekraft]

The diode bridge (shown above) is used with transformers that don't have a center tap - it is not generally used in classical tube amplifier circuits, ie, Fender, Marshall, Vox etc. A lot of modern amps use it because it allows the manufacturer to use a cheaper transformer with no center tap - at the time most of the amps we know and love were being designed, transformers were very cheap and rectifiers were expensive, but those costs have changed radically since the 50s, and since transformers are now fairly expensive while silicon diodes are less than a penny apiece in quantity, the tradeoff is obvious to any company wanting to maximize its profit. There is nothing inherently better about using a bridge over a regular center-tapped full wave rectifier (two diodes), or vice-versa. The one nice thing about using a traditional two diode setup is being able to use the center tap to control the high-voltage, making a standby switch, a soft-start circuit, and a reduced voltage output extremely easy to implement without any serious modification to the original tube rectifier circuit.

You could easily work a bridge into the Princeton power supply circuit - you could also use a MOSFET source follower to improve the frequency response of the tone stack, change the tremelo circuit to a solid-state VCA with an opamp LFO , and even replace the tubes with high-voltage MOSFETs if you wanted to, but if you want a Princeton, I'd build it as close to the original design as possible. Switching to SS rectifers will change the sound a bit, but not a lot, and adding a standby switch is trivial if you use the standard power supply circuit, so why rework a successful design?

[KTLguitars]

Back to the bass tube power amp!

There is no substitute to cubic inches... nor tube power!

Why not try this:

If you want a high power tube amp - build two push pull output sections like e.g. Marshall 100W power stage, complete with output transformers, inputs coupled in parallell, - series coupling of the two secondary windings of the output transformers gives a bridged ouput - that will give you the double output voltage - and the the square of this, = 4 times, of power ( P=U2/R ) giving 400W. This will give the need for two standard output transformers and e.g 8 EL34 tubes.

PS

Safety trick - before starting the trial and error sequecies - put an indicator light built of a LED, series coupled with a resistor over the ca 450V Power supply output - dimension the resistor to give you the right current for the LED - calculate the power rating for the resistor. When you turn off the amp this circuit will aid the draining of the capacitors and giving you a warning light thats goes out at about 40- 50 volts - when the circuit is safe!

When the LED lights - put one hand in your back pocket and leave it there as long as you do work inside the amp with the other one!

[lovekraft]

...build two push pull output sections like e.g. Marshall 100W power stage, complete with output transformers, inputs coupled in parallell...

Wouldn't that be even more expensive and complicated than using a single higher-powered output section, like a Marshall Major? And wouldn't you have to double the load impedance to match the output transformer's primary impedance, effectively eliminating any increase in power? At the power levels we're talking about, an impedance mismatch could very easily have catastrophic consequences for those expensive transformers and power tubes.

I can't help thinking that there's a good reason why nobody manufactures a huge tube power amp for bass guitar - if it was in any way practical, Randall Smith would be doing it, instead of using MOSFETs in his high-powered bass amps. But that's just my opinion - try it, and let us know.

[bassman]

I have also been verry interested in bilding a high powered tube bass amp...... I ended up buying parts for a 100 watt SS Hi FI amp... I ha eyet to build it because I find myself to deep into bass building ideas right now.

anyways... why not use a tube preamp with a ss power section? I have an AMpeg SVT Pro4.. it has a tube preamp 12ax7's... with a 1600 watt ss power section- you get "most" of the benefits of the tube sound this way yet you do not get the problems that are often associated with them..

so jsut build a large ss power section and a tube preamp and then go to town with your "own sound"... nothing beats your own hand made bass, preamp, power ampe and cabinets set up

peace

RYan

[KTLguitars]

lovekraft:

Of course it woudn't be easy, and yes, you have to impedance mach the output, but normally the output transformers have an often un-used 4 ohm speaker connection - so it could be done. Economically you save 8 tubes (if using EL34s), and you can actually build one driver circuit (instead of two) with some more powerfull drivers than the normal ECC83 types. And the std. 100W output transformer are fairly priced due to high production numbers...

The reason why someone don't....it is anyway far more expensive than ss - it produces a considerable amount of heat, requiring large open chassies, and the reliability isn't the same, you have to service it, it will be heavy....but it would have been damn cool to build one, wouldn't it?

Other reason: -the tube amp business is quite conservative. It is only the last years it has been normal to have bridged power amps for ss hi-fi systems, inherited from PA systems, and this is still new in the tube amp business, - and it seems quite accepted to use ss amps for high-power bass systems.

Or you can do as some of the more extreme japanese hi-fi builders, using radio transmitter tubes....EXPENSIVE is the word I think!

Pity I only play the guitar - could be funny to try it out - but hey, I save the money!

KTL

[lovekraft]

You need to take a look at Ken Gilbert's BAGA - EL84s direct-driving 6 KT90s at 620 volts for an earth-shaking 600w monster guitar amp! He proves that it can be done, but looking over his schematic, I'm back to my original point - for what he spent, you could build a really nice 100 watt bass amp and mike it through about 2 kilowatts of solid state power and subwoofers. Nice for those occasions when simply moving peoples emotions isn't enough, and you want to move them physically!

Edited September 9, 2004 by lovekraft

[KTLguitars]

For the price of that one you could probably build yourself a 5kW ss amp with tube preamp....and save a lot of heat-, hum-, and other problems!