Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on June 19, 2011, 12:03:02 pm

Title: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: kagliostro on June 19, 2011, 12:03:02 pm

I'm looking aroud about LS50 GU-50 tube amps

I've find

here http://www.jogis-roehrenbude.de/Verstaerker/GU50-Grommes/GU50.htm (http://www.jogis-roehrenbude.de/Verstaerker/GU50-Grommes/GU50.htm)

this strange and not comprehensible (to me) PS schematic

there is a rectified voltage that is less than the AC voltage using a transformer without the CT

is this schematic right or wrong ?

is this a way to obtain a low voltage for grids of tubes like the el500 & 6bq6 ????

Thanks

 Kagliostro

Title: Re: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: plexi50 on June 19, 2011, 07:19:34 pm

Cant say myself but it does look like it could have interesting possibilities

Title: Re: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: Tiny_Daddy on June 19, 2011, 10:25:44 pm

Ain't gonna happen.

Title: Re: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: andrew_k on June 20, 2011, 12:51:05 am

Get three 120V secondaries (3 cheap isolating xformers?) and stack the full wave bridge rectified outputs -- ground from the top goes to + of the next. Top one will be ~380, middle on will get ~240 and bottom one will give you ~120 if you need it.

Also, the PCB on that page you linked to is terrifying. Panel-mount sockets for power valves with the solder lugs stood up and soldered onto a PCB with zero mechanical connection? Would be mighty interesting if/when a couple of those pins desolder themselves.

Title: Re: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: PRR on June 20, 2011, 01:29:50 am

> is this schematic right or wrong ?

Can not say without values for the chokes and the loads.

It *appears* that L2 = zero and L1 = very-large. Then C2 charges to the peak of the AC sine-wave, 1.414 times 270V or 381V, and C3 charges to the *average* of the AC sine-wave, 0.9 times 270V or 243V.

The "problem" is that if the load is low current the choke must have LARGE inductance. For no-load the output rises to 1.414*Vac unless the choke is infinite.

http://www.paleoelectronics.com/RDH4/CHAPTR30.PDF (http://www.paleoelectronics.com/RDH4/CHAPTR30.PDF)
Page 1182 (page 22 of this PDF)

> {screen} grids of tubes like the el500

EL500 wants about 150V at current of say 3mA to 30mA (and zero mA until the tubes warm up).

The equivalent of 150V at 30mA is 150V/30mA= 5K; the equivalent of 150V at 3mA is 150V/3mA= 50K.

The formula in RDH suggests a choke of 5H to 50H (and infinite Henries at no-load).

For an inexpensive 1-pound (half-KG) choke, in Hammond the nearest seems to be model 157G - 30H 40mA 595 ohms, about US$32. The 30H goes back in the formula and says the minimum load is 30H*940= 28,200 ohms. At 150V this is 5.3mA. We could add a 5mA bleeder to ensure the choke stays loaded. It would dissipate near a Watt, perhaps acceptable. The full-roar current is now 30mA in the screen grids plus the 5mA bleeder; 35mA is within the choke's 40mA rating. It will work.

But for $10 you could buy a 230V:115V 10VA transformer to get 160V 30mA without a choke.

Title: Re: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: kagliostro on June 20, 2011, 06:28:01 am

Many Thaks Andrew_k

PRR as allways your explanation are comprehensive

Many thaks for your pearls of wisdom

Franco

Title: Re: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: Tiny_Daddy on June 20, 2011, 08:18:36 pm

If you set up a capacitor-input filter (no choke) for the higher voltage (1.4 X Vin), and then add a diode-isolated choke-input filter as shown (.9 x Vin), it could work. I am thinking you would need only one diode in series with the bridge to do this. The choke would, as stated above, needs to be selected depending on current draw.

Title: Re: I've foud a strange PS schematic - is this a good way to obtain low grid volt ??
Post by: DummyLoad on June 20, 2011, 11:08:35 pm

paleoelectronics.com

love the URL name - reference to electronics that are ancient, early, prehistoric, primitive.

does that mean we're circuit paleontologists? should we rename the glo-champ amplisourus-rex?

 :icon_biggrin: