Voltage Doubler for tube preamp

Tube or solid-state, this section goes to eleven!
Post Reply
User avatar
phase396
Information
Posts: 7
Joined: 24 Mar 2017, 04:07
Has thanked: 1 time

Post by phase396 »

I want to use two 12 volt filament transformers back to back followed by a voltage doubler to power a marshall style tube preamp. The voltage doubler is a full wave bridge rectifier with two of the diodes replaced with two capacitors. (You all probably allready know this.) My question is how large and what voltage should the capacators be? I haven't found many schematics on this but the ones I've found range from 10uf to 330uf. I'm using two 12ax7s. The voltage doubler is converting 120 volts to about 340volts. Any help would be appreciated Thanks..

User avatar
ppluis0
Diode Debunker
Information
Posts: 912
Joined: 14 Jul 2010, 18:33
Location: Argentina
Has thanked: 127 times
Been thanked: 299 times

Post by ppluis0 »

Hi phase396,

Usually a 12AX7 draws 1 mA per triode section, so you need to supply a small current (but well filtered) to feed your preamp.

The voltage doubler you describe can be implemented with two electrolytics of 33 or 47uF rated at 250 Vcc or more.

But take into account that a voltage doubler is equivalent to a half wave rectifier regarding the ripple content so you need several RC sections after the doubler to have a cleaner supply source to your tubes.
As the current involved is so small you can employ also 33 or 47 uF -with adecuate voltage rating this time- and several Kohms in each RC section without much voltage drop.

BTW the filament supply taken from the intermediate connection between transformers can't be floating and must be referred to ground. This is valid if you are planned to use AC or DC to feed the heaters.

Good luck with your project and keep us posted (schematics please !) about your work 8)

Cheers,
Jose

User avatar
phase396
Information
Posts: 7
Joined: 24 Mar 2017, 04:07
Has thanked: 1 time

Post by phase396 »

Thanks for the info ppluis0 ! The preamp is a Jose style modded Marshall preamp, which to me, is somewhat like aJCM800. But there are differences. I got some info on it from a you tube video from Ossie Ahsen from 3Monkeys amplifiers. It's titled "My real Jose modded 1979 Marshall 100 watt head." In it he goes through the amp's circuit in great detail. The most clever thing about it is perhaps the master volume which can be placed before the tone stack with clipping diodes or after the tonestack without the diodes. My preamp is silmpler, with one channel, and with a conventional post tonestack master volume and no clipping diodes. The preamp I've built has a fair amount of noise, but the underlying tone is quite good!! I have it on a breadboard with long wire leads. So that could be contributing to the noise. I'm using four 22uf/ 500v filter caps plus three 10k dropping resistors between them. Do you think that's enough filtering? Oh, and I'm using two 10uf/250v caps for the voltage doubler. If you check out a you tube video titled " Marshall 1959 HW-stealth Jose mod. tones & DIY guide" by Headfirst Amplification you can see pretty much the exact schematic of the preamp I'm working on (minus the clipping diodes and switchable bright caps.) Thanks again ppluis0, just thought you might be interested in some more detail.

User avatar
ppluis0
Diode Debunker
Information
Posts: 912
Joined: 14 Jul 2010, 18:33
Location: Argentina
Has thanked: 127 times
Been thanked: 299 times

Post by ppluis0 »

Hi phase396,

I will see the videos you mentioned to know about this mod on Marshall amps.

(If the person who modified these amps is called Jose like me, then he must be a great guy... :mrgreen: )

Cheers,
Jose

User avatar
dorkthrone_
Information
Posts: 1
Joined: 01 Dec 2023, 11:19

Post by dorkthrone_ »

Hey phase396, did you ever try the voltage doubler? I had the same idea after seeing ol' uncle doug's video about it on YouTube and wanted to give it a try. Two transformers with a voltage doubler circuit. Did it work? I'm also doing a Marshall style preamp. The JMP. Always loved the sound of that amp.

User avatar
Yazoo55
Breadboard Brother
Information
Posts: 128
Joined: 27 Apr 2008, 11:35
Location: Morpeth, Northumberland, U.K.
Has thanked: 11 times
Been thanked: 27 times

Post by Yazoo55 »

I also saw the Uncle Doug video and used the doubler in a Fender Champ build. It worked fine.

User avatar
mozz
Breadboard Brother
Information
Posts: 90
Joined: 26 Jan 2016, 23:07
Location: Pennsylvania
Has thanked: 52 times
Been thanked: 22 times

Post by mozz »

If you can find a old vtvm for parts, they usually have a small transformer operating the dual triode in there.

User avatar
Tassieviking
Breadboard Brother
Information
Posts: 67
Joined: 25 Nov 2018, 12:09
Location: Tassie
Has thanked: 39 times
Been thanked: 25 times

Post by Tassieviking »

Why use a voltage doubler ?
If you are using 2 transformers then get the correct voltage out of the second transformer, I saw a circuit where Moz did this a long time ago.
Attachments
BOM m_pre.pdf
(27.67 KiB) Downloaded 40 times
m_pre_pcb_bw.pdf
(64.63 KiB) Downloaded 34 times
m_pre_pcb.pdf
(49.64 KiB) Downloaded 33 times
m_pre_5.pdf
(73.22 KiB) Downloaded 47 times
Building a JCM800 preamp for recording.pdf
(944.44 KiB) Downloaded 47 times

User avatar
Tassieviking
Breadboard Brother
Information
Posts: 67
Joined: 25 Nov 2018, 12:09
Location: Tassie
Has thanked: 39 times
Been thanked: 25 times

Post by Tassieviking »

Another way to power tube amps is using "HV nixie power supplies", there are plenty of DIY ones around on the net, and also on sale from all the usual cheap places like Ali, Ebay etc.

NIXIE tubes are the really old school lit up numbers in a tube casing, like they have on the really old scifi movies.
It was the firs type of electric numeral display that I know off, they are still popular for the nostalgic old school look.
https://en.wikipedia.org/wiki/Nixie_tube

The nixie power supplies have been used on several DIY tube pedal builds, as many run from low DC voltages.
https://www.instructables.com/High-Volt ... alve-Tube/

User avatar
Yazoo55
Breadboard Brother
Information
Posts: 128
Joined: 27 Apr 2008, 11:35
Location: Morpeth, Northumberland, U.K.
Has thanked: 11 times
Been thanked: 27 times

Post by Yazoo55 »

I have used back-to-back transformers a few times in the UK, using transformers with 12V secondaries to power the heaters in series. The problem is that with 240V, you end up with less than 200V under load for the HT supply, not enough for something like a Fender Champ. This is where a voltage doubler comes in very useful.

User avatar
Tassieviking
Breadboard Brother
Information
Posts: 67
Joined: 25 Nov 2018, 12:09
Location: Tassie
Has thanked: 39 times
Been thanked: 25 times

Post by Tassieviking »

You should not use 2 transformers that are the same, In the build I posted above by Moz he used 2 different transformers.
Moz used them to drop the voltage, but you can also boost the voltage.
You can try a 240 to 15v-15v and then connect to a 9v-9v to 240 and see what happens.

You have to realize that there are always losses in a transformer. that's why you end up with less if you use the same voltages.
If you have some transformers laying around do some tests and see what happens, I have no idea how to work out the losses on paper, only the voltage if there were no losses.

The other way is to use one heavy transformer for 240v to 12v-12v, and then 2 transformers at 12v-12v to 240v, you would end up with 2 transformers putting out about 200v.
That will give you 200v-0v-200v if you connect them in series, 400v CT AC is more then enough to drive most preamp circuits.
There are lots of small PCB mount transformers around you could use for this power supply.

Post Reply