Increasing the B+ ?

[pjd3]

Just curious,

Has anyone ever heard of a method of increasing the B+ voltage somewhat?

I'm aware of the "B+ voltage reducer" that is found on this site using the power MOSFET and zener diodes but, had to wonder if there were any methods of increasing the B+ voltage.

Reason I ask is that one of my current builds is for a 2204 amp using the chassis with transformers that came with the donor amp I was given. As is, the plate voltage would be around 400vdc on a pair of EL34's. I know that would work to its own degree but, have to wonder if the EL34's on this amp might benefit tone if it were a bit higher, closer to 440-460 maybe. There's not alot of room on the chassis so I"m not feeling inclined to replace the PT (unless I had to).

Thanks
Phil D

[martin manning]

JMP 2204 wants exactly that voltage. Go with it!

[sluckey]

Has anyone ever heard of a method of increasing the B+ voltage somewhat?

I've been hanging out on guitar amp forums (and AGA) for about 20 years. This is the first time I've seen someone ask how to increase B+ voltage. Some kind of award should be in order.

[R.G.]

As long as you have enough voltage and current, and electronics to do the conversion, there's always a way. It may not be practical, or reliable, but there is always a way.

By far the simplest and probably cheapest is to change to a different power transformer.

Quick and dirty solutions might be to use an extra 120V to 42V transformer to make 60Vdc floating and tie this on top of the existing 400V. It's kind of a redneck solution, but would work. It's probably the cheapest low tech way to do it.

Likewise, you might be able to buy a 48Vdc switching power supply with a floating output and stack that on top of the 400Vdc. Enter switching noise and EMI.

With today's electronics, it is >>conceptually<< reasonable to do a flyback boost circuit from the existing 400V. This would be a switching power supply that ate 400Vdc from the existing DC power supply and added another 50-60 V on top. The limitations are that you cannot pull more volts X amps from the existing power transformer/power supply than it can give. So if the 400V is rated for 0.5A, the hypothetical 450V supply would only have (400/450) * 0.5A = 0.444A of current available. This approach would carry some engineering risk and possibly some tears along the way.

There are other ways, these are just the ones that occur to me now.

[pjd3]

Ha OK! Thank you all for such a charged, funny and informative set of responses!

And for one thing, sure, I may end up being very happy with the 400vdc or so volts that would be present on the plates of a pair of EL34's. But, had to wonder "should" I wonder if a higher voltage would be in order for tone, would there be anyway to do it without replacing the PT. Just ask, why not, glad I did.

I'm aware of placing transformers in line with the AC input on the primary to lower voltage ( I assume you could go the other way with that) but, how to accomplish that gracefully with the HV coil was never something I could picture how to do and know its kosher. Had to wonder if there was any way of increasing the HV via the CT but, guess its not the same as raising say, referencing the 6.3vac heater by giving it a vdc reference voltage in the centertap but, this is the actual B+ and theres nothing around that is higher than that in the amp as is!

And once again R.G., thank you for your generous breakdown of possibilities with the risks and threats that might go with it. Well, if those 2204's have been living with a 400vdc plate, then there is really no reason I also wouldn't be happy with this for this amp. After all, this is going to be a dedicated "distortion" amp beginning with a classic 2204 with a couple of footswitchable gain mods (JEL primarily).

Thanks again good samaritans of the Ampgarage forum for your forthcoming offerings of information and insight.

And by the way Slucky, I"m gigging every weekend with your 22 watt Plexi. I have a 5751 in V1 and use the amp as a pedal platform. Still getting alot of compliments on my tone, so, thanks for putting all your cool amp ideas and projects up for us.

Best,
Phil d

[xtian]

You can use an autotransformer to try out higher voltages! My variac can be cranked up to 140vAC. So, imagine turning up the line voltage 10%. Your B+ goes from 400 to 440, your heaters from 6.3 to 6.93. Not terrible for long enough to try it out and listen.

[Roe]

around 400v is fine for a 2204. 460 is good for a 2203, not a 2204.

[bepone]

adding small transformer and connect its secondary in series with existing high voltage secondary ? cheap and simple?

[R.G.]

adding small transformer and connect its secondary in series with existing high voltage secondary ? cheap and simple?

That approach only works if the original rectifiers are full wave bridge. If the original is full wave center tap, you would need two secondaries to go in series, one on each half secondary.

An approach that hasn't been mentioned yet and that would work uses that a small additional transformer with a single 40Vac (or so) secondary plus full wave bridge on its secondary could be stuck in series between the junction of the existing rectifiers and the existing first filter cap.

[Phil_S]

If it has a tube rectifier, you can change to solid state. That will eliminate the voltage drop inherent in tube rectifiers. This is simple and easily reversible. If you remove the tube, you can build it on the socket pins.

[pjd3]

Thanks Phil S,

Thats a good point as I would expect that to bring the B+ up another 20-30 volts or so but the amp is already SS on the rectifier.

This amp was originally supplying V to a quartet of EL84's. I may have expected the 400vdc on an EL84 but was wary about that not being optimum on a pair of EL34's. One of the reasons I was intent on using EL34's was that the amp will have a "Zero loss FX loop" as the kind you might see at Metro or Headfirst amplification. My justification (whether warranted or not, don't really know yet) was that having FX in the amp might benefit somewhat by having more clean output available to keep the effects a bit cleaner. This amp was a Carvin V3M combo that was donated to me with alot of issues. I had no interest in having a Carvin combo so, I stripped it down to just the chassis and transformers. All the trannies checked out fully functional so I thought it good for a nice versatile Marshall based distortion combo with its own FX. At its base, it is a JCM800 2204 that will also have switchable features such as High V input FET boost, JEL switchable mods, DC heaters for the first couple of gain stages and relay switching to impliment all the gain mods. I purchased the boards from Jason at Headfirst. So, the intention was to never use the amp for any clean tones at all, just to make this amp into a very versatile Marshall amp for crunch/distortion tones to cover 60's, 70's, 80's and 90's type distortion tones and levels. Then, just use another amp with that for the clean tones with various pedals. Its sounding to me like based on this application I may not really need to bring a higher plate voltage to the plates of the EL34's. The other reason is that a buddy gave me an unused pair of Svetlana Winged C EL34's, plus, I got a small handful of old Mullard Blackburn EL34's from a deceased engineer. I have no idea of the condition those are in but, I'm all for letting themselves prove their condition to me when the time comes.

Thats my story and well, I should probably stick to it since I'm already far into the amp project. I have the 2204 turret board populated and installed, most of the rectification and power node is nearly in place. Now, its a matter of getting all the accessory boards installed and make sure my switchable gain ideas will function properly.

Thanks !
Phil Donovan

[sluckey]

Just a thought... Build your 2204 but use a quad of EL84s instead. Your transformers will be a perfect match and you'll have a unique amp. Could be a winner. Could be a weenie.

[pjd3]

Ya know Sluckey, I have actually thought about that, but, didn't dwell on it since I had the EL34's in hand.

But, I do still have a quad of JJ EL84s around. I really may have to consider that. And they might fit better than the EL34's. I actually had to protrude the speaker baffle forward 3/4" or so to even start feeling like the tubes would be an acceptable distance from the speaker magnet. I've had to consider using a NEO speaker for the reduction in magnet size, that there might be an issue if I didn't. There is definitely some indication that power tubes shouldn't be too too close a magnet as it can or will affect the motion of the electrons in the tube, and change /affect the sound.

Well, something to think about for sure. And it would be a unique amp as a result. I listened to a video one time that was comparing 6V6's to EL84's in a Marshall distortion-y amp. I preferred the 84's. More woody, or more character. Something there.

Thanks,
Phil d.