# Ideas for a power supply?

#### RayRay1132

##### Member
I've been working with vacuum tubes lately and been wondering what would make a good permanent supply for a project. (other than the bench supply) Should I wind my own transformer and build a full bridge rectifier or should I go with a cheap switch mode power supply? The tube in question is a 6bm8/EZCL82. My mains Voltage is 120V 60Hz. What is a good voltage to run the ECL82 at? Any advice would be greatly appreciated.

#### rjenkinsgb

##### Well-Known Member
A 120V isolation transformer would give about 170V after rectification. Anything from around that up to a maximum of 200V is OK.

There are plenty of cheap transformers on ebay, around $30 -$40 - just search "tube amp transformer", assuming you are in the USA?
Many of those include the filament winding as well as HT.
eg. one of the first ones I found -

The direct secondary voltage is a bit high for the ECL82, but ideal for many other valves.

For yours you could connect the 33V winding in series out of phase with the main 170V secondary to bring that down to around 140V, so close to 200V after rectification and smoothing.

#### MacIntoshCZ

##### Active Member
Hi,
i would go with active forward topology (switching power supply).
IC is ucc2891.
if you want laboratory power supply...
If you want to smooth yours ac mains than definitely go for active pfc - ucc28180

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#### RayRay1132

##### Member
Hi,
i would go with active forward topology (switching power supply).
IC is ucc2891.
if you want laboratory power supply...
If you want to smooth yours ac mains than definitely go for active pfc - ucc28180
Im currently using a dc boost converter 'thing' from amazon. It has this screeching sound though that I can't seem to get rid of, any ideas? Should I add a smoothing cap across it?

#### MacIntoshCZ

##### Active Member
Im currently using a dc boost converter 'thing' from amazon. It has this screeching sound though that I can't seem to get rid of, any ideas? Should I add a smoothing cap across it?
I only experienced this when switching frequency of coil was too low. Under 15Khz. Its hardly removable. Can you measure switching frequency?

#### BobW

##### Active Member
My standard power supply for tube circuits uses an isolation transformer that came from an old bathroom shaver outlet. I found that I could get used ones dirt cheap from the local Habitat For Humanity restore, and other places that sell salvaged electrical parts. These are only rated 20VA, but that's usually enough for a one or two tube circuit. For more current you can use a Triad N-68X, rated 50VA and readily available from major electronics distributors.

For tube circuits you don't usually need a regulated supply as long as the ripple is reasonably well filtered out. I use a bridge rectifier followed by a pi ripple filter consisting of a 10μF electrolytic, a 1.5k 5W series resistor and then a 22μF electrolytic.

#### RayRay1132

##### Member
I only experienced this when switching frequency of coil was too low. Under 15Khz. Its hardly removable. Can you measure switching frequency?
I don't really know what/where to probe on the board. There is a transformer, a mosfet and a bunch of smt components. The output will fluctuate between 198 and 200 volts dc about once a second. The transistor also makes a clicking noise sometimes.

#### Nigel Goodwin

##### Super Moderator
For tube circuits you don't usually need a regulated supply as long as the ripple is reasonably well filtered out. I use a bridge rectifier followed by a pi ripple filter consisting of a 10μF electrolytic, a 1.5k 5W series resistor and then a 22μF electrolytic.
You're obviously far too young to be from the valve days

Smoothing was rarely that 'good', and almost all rectifiers were only half wave, either a valve rectifier, or a metal rectifier. Ripple was rarely 'well filtered out'

Commonly a smoothing choke was used as well, but you couldn't really get big electroytics back then.

They were crude days.

#### MacIntoshCZ

##### Active Member
Bigger than my hand?
I think igot bigger somewhere

#### BobW

##### Active Member
You're obviously far too young to be from the valve days

Smoothing was rarely that 'good', and almost all rectifiers were only half wave, either a valve rectifier, or a metal rectifier. Ripple was rarely 'well filtered out'
I've restored radios from the early 1930's. They all had full wave rectification using dual plate tubes (type 80) and centre tapped transformer secondaries. I recommended a bridge rectifier in this case to avoid the need for a centre tap. You are correct that the the power supply filters typically used a smoothing choke. In radios, it was the field coil for the speaker. So it served dual duty. Electrolytics came out in the early 1930's, and values up to at least 30μF were available. Almost no tube based consumer electronics used a regulated supply. Certain sections of tube colour TV's used voltage regulators, but that was the exception. High end lab equipment (eg., Hewlett Packard, Tektronix) were about the only things that used regulated supplies.

#### Nigel Goodwin

##### Super Moderator
I've restored radios from the early 1930's. They all had full wave rectification using dual plate tubes (type 80) and centre tapped transformer secondaries. I recommended a bridge rectifier in this case to avoid the need for a centre tap. You are correct that the the power supply filters typically used a smoothing choke. In radios, it was the field coil for the speaker. So it served dual duty. Electrolytics came out in the early 1930's, and values up to at least 30μF were available. Almost no tube based consumer electronics used a regulated supply. Certain sections of tube colour TV's used voltage regulators, but that was the exception. High end lab equipment (eg., Hewlett Packard, Tektronix) were about the only things that used regulated supplies.
I repaired radios professionally for decades, and back in my early days a LOT of them were valve based, including the battery powered valve models (D series miniature valves).

I don't recall ever seeing a radio with a full wave rectifier?, and a lot of them of course were live chassis as well, so didn't have a transformer to allow a full wave rectifier.

Perhaps it was a function of your low mains voltage?.

Likewise speakers with field coils, they were pretty rare - I've seen odd ones, and 'probably' seen one or two that used it as the filter choke - but I can't remember back that far

#### JimB

##### Super Moderator
I don't recall ever seeing a radio with a full wave rectifier?
Surely Nigel you came across the ubiquitous 5Z4G dual diode rectifier with a 5v heater?

JimB

#### BobW

##### Active Member
Power transformers were required in receivers in Canada until they relaxed the electrical code requirements due to material shortages during WW2. I don't think the lower mains voltage had much to do with it, because the US has the same 120V mains voltage and they made a lot of AC/DC 5 tube receivers using the 35Z5 (and later 35W4) half-wave rectifiers. After the war, the cheaper transformerless receivers became popular here. But the high end receivers continued to use power transformers and fullwave rectifiers such as the 5Y3, 5Y4 and 5U4. Almost all of these high end receivers also had push-pull output stages which were good for cancelling out hum.

I agree though, that the transformerless sets with halfwave rectifiers did hum unpleasantly. That was the tradeoff for a cheap radio.

I still build a lot of tube circuits for fun, and have never yet needed a regulated supply. I'm not saying that regulation is never required, but people just naturally assume that regulation is always required, when in many (if not most) cases, it's not.

#### Nigel Goodwin

##### Super Moderator
Power transformers were required in receivers in Canada until they relaxed the electrical code requirements due to material shortages during WW2. I don't think the lower mains voltage had much to do with it, because the US has the same 120V mains voltage and they made a lot of AC/DC 5 tube receivers using the 35Z5 (and later 35W4) half-wave rectifiers. After the war, the cheaper transformerless receivers became popular here. But the high end receivers continued to use power transformers and fullwave rectifiers such as the 5Y3, 5Y4 and 5U4. Almost all of these high end receivers also had push-pull output stages which were good for cancelling out hum.

I agree though, that the transformerless sets with halfwave rectifiers did hum unpleasantly. That was the tradeoff for a cheap radio.

I still build a lot of tube circuits for fun, and have never yet needed a regulated supply. I'm not saying that regulation is never required, but people just naturally assume that regulation is always required, when in many (if not most) cases, it's not.
Regulation was EXTREMELY rare on valve equipment.

And to be fair, VERY rare on early transistor equipment

Probably one of the most notable would have been the Quad 303 power amplifier, a device still highly rated today.

#### RayRay1132

##### Member
Well I've figured it out. I'm using two 120 to 12volt transformers back to back and a full wave rectifier.

#### BobW

##### Active Member
That's not a bad way to do it, especially if your tubes have 12V filaments. Then you can take the filament voltage from the 12V secondary of the input transformer. However, this is not a very efficient way of getting the HV, though it may not matter as long as you're not drawing a lot of current from the HV. The reason for the poor efficiency is that connecting the LV windings together gives a very low power factor. If it turns out to be a problem, you can correct for this by adding power a factor correction capacitor across the 12V windings.

An excellent explanation of the power factor problem and its correction can be seen here:

#### unclejed613

##### Well-Known Member
I don't recall ever seeing a radio with a full wave rectifier?,
i've repaired a lot of radios that used 5U4 rectifiers.... maybe it was more common on this side of the pond to use full wave rectifiers...
the AC/DC hot chassis radios were usually junk anyway, a single 4 or 5 inch speaker, and all plastic box and knobs (to isolate consumers from the hot chassis if it happened to be plugged into a non-polarized outlet). unfortunately for musicians, there were hot chassis guitar amplifiers too, and when i was in a band i kept a meter in the band toolkit, so nobody got zapped when they touched their guitar and something else, like a microphone...

#### Nigel Goodwin

##### Super Moderator
unfortunately for musicians, there were hot chassis guitar amplifiers too, and when i was in a band i kept a meter in the band toolkit, so nobody got zapped when they touched their guitar and something else, like a microphone...
As far as I'm aware, that wasn't so over here

#### RayRay1132

##### Member
That's not a bad way to do it, especially if your tubes have 12V filaments. Then you can take the filament voltage from the 12V secondary of the input transformer. However, this is not a very efficient way of getting the HV, though it may not matter as long as you're not drawing a lot of current from the HV. The reason for the poor efficiency is that connecting the LV windings together gives a very low power factor. If it turns out to be a problem, you can correct for this by adding power a factor correction capacitor across the 12V windings.

An excellent explanation of the power factor problem and its correction can be seen here:
No, actually I'm powering my tube's filament from a separate supply. It's an ecl82; so I believe the voltage is 6.3 volts for the filament winding.