Applications for UL84 tubes

[Rontone]

I got some scrap amps for parts, have some UL84's that are in ok condition, is there any good guitar amp uses before I put them on eBay?

Wish they were EL84's as there's plenty of projects with those, would like a little VOX build or a small Marshall type...

I have seen they run on lower voltages and like lower OT impedances, but I think they have 45v 100mA heaters, so kind of difficult, would need another transformer or more complicated circuit to get a DC supply to them

In this context they had an EF86 and ECC83 in front of them, so I could see that a little amp could be made with them but its maybe not worth the effort,

[kagliostro]

Some time ago I recovered (from an old radio ? don't remember) one of those tube and the OT (PT was an autotransformer, so, no good to be repurpose)

I was thinking that the tube will sound similar to the EL84 (so reusable in circuit that use that tube) but I'm not sure about

as you I'm curious to read something about from our skilled friends

--

Something about el84/ul84 here

http://www.r-type.org/exhib/aai0038.htm

http://www.r-type.org/exhib/aaa0028.htm

Franco

[Rontone]

Yes, they seem to mostly be in radios, some old portable vinyl record players,

These were in a film projector amplifier, one pair are used for the power supply directly with no mains input transformer

Found this online

"In this machine a pair of UL84 pentodes form a power oscillator which runs directly off of the rectified 110V, and via a high frequency transformer supply the power to run the rest of the amplifier - a forerunner of today's switch mode PSUs. This solution is brilliant and achieves several desired features all at once:

A high frequency source is available to run the exciter lamp to eliminate mains hum, it can also supply the erase head and record bias. The pre-amp valve heaters are also run at HF to eliminate mains hum. No conventional mains transformer is required eliminating weight and a potential hum source. Although some parts of the amp are connected direct to the rectified 110V supply the inputs and outputs remain isolated giving the safety of a conventional AC design. A further pair of UL84's in push-pull give the amplifier an output power of approx 15W."

[AmberB]

One way to use the tubes would be to use a series heater string for the tubes.  There's a lot of older amps from the 1950s and 60s that had a series heater string.  Also, many radios used a series heater string.  You would just need an isolation transformer for safety.  There's information in the RCA tube manual about the use of series heater strings.

[thetragichero]

so it looks like the limiting factor with these would be that the 100ma heater current would need the preamp tubes to also have 100ma heaters for the series string to work, as far as i understand series heaters
something like the PL84 with its 300ma heaters would allow the use of 12ax7 and similarly common preamp tubes

[kagliostro]

Merlin on his PS book explain how to solve the matter

when the tubes to be put in series didn't have the same current, using resistor the problem can be solved



to me the problem is to have under hand an adequate transformer or the game won't be worth the candle (too much money involved to compensate the value of the el84 tube)

EDIT:

I was thinking about your wish to use the UL84 tube you recovered

Assuming you want to build someting like a 5F1 or 5F2 (so an UL84 plus a 12ax7)

To me the better way will be to use a 12V 800mA / 1A illumination Transformer

(you can find on the cheap buying at a flea market a desk lamp that is rated for 10W @ 12V, usually I pay 5 € or less for one)

to obtain 45V starting from 12V you must use a quadrupler and feed the UL84 heaters in DC, the 12ax7 tube heaters will be connected to the 12V AC

Why an 800mA / 1A @ 12V if the 12ax7 connected for 12 V consumption is of 100mA and the UL84 will be 100mA?

Because to obtain the 45V you use the quadrupler and using it the voltage rises and the disposable current drop (nothing is created, nothing is destroyed, everything is transformed), using a quadrupler you must divide by 4 the AC disposable current, so

800mA (available at the Transformer) - 150mA (12ax7 AC current consumption) = 650mA

650mA (current that remains available in AC): 4 (divided for four as you use a quadrupler) = 162.5 mA

162.5mA (DC current available) - 100mA (heater current consumption of the UL84) = 62.5mA

Those 62.5mA are an extra that will allow your quadrupled voltage don't drop excessively under load and preserve the life of the Transformer

ps: I don't think you will need to use, but if under load the UL heaters result be feed with a voltage a bit high , a series resistor will solve the problem, remember to measure with the heaters of the UL tube connected, not at the exit of the quadrupler without the load

Franco