Screen voltage too high in amp for new tubes?

[Ampzone]

Is there any way to get screen voltages on a seventies Hiwatt 100w DR103 circuit down in a range that new production output tubes can be happy? We'ere talking 503vdc on screens, plates around 508vdc biased around an average 28ma. I can remove the 100ohm 1w stock screens to a 1k 5w, but I'm thinking of going as high as 2k - 2.2k 5w.


It would be nice to the screens in a 460-470ish range.

[catalin gramada]

Hi, I have the same Partridge PT into almost the same circuit. It shows that voltage without load. As time stb switch is engaged voltages drop to 480v around for average 60percent biasing at iddle and more sag 50 v around to 440-430v full throttle. Wanna drop more? Run it more hot...till a point. The screens will stay 390v around dimmed. I used new tung sol el34 edition. One tube from a set blows away but have one set which perform steady. How much you expect to use the tubes into guitar amp? One suggestion is to drop voltage from mains selector (e.g. use 240v selector for 230v on mains). The amp will work with 440v around at iddle. Check the heaters, mine drop to 5.9v around...still usable
And No, you cannot drop more voltage no matter how big resistor use, if don.t run some semnificative current through it.That.s it, if wanna drop more voltage you have to run more current, but not from the screens, but plate instead, the PT is flexible enough (read loose). Make a proper bias circuit, bias you tubes into 70-80 percent range and the screen voltage will slide to 460-470v range at iddle as you looking for

[J M Fahey]

Looks like running the car with parking brakes engaged to me.

I would explore lowering screen voltages to sensible values not through resistors or overloading PT to death (in exchange of nothing, and besides plates are happy with high voltage) using a MosFet regulator.

It´s not the 70´s any more.
For one side we have "weaker tubes" although I am not 100% happy with that definition,on the other hand we have robust and inexpensive MosFets.

Search this very Forum, that topic has already been covered in detail.

Also "R G Keen´s Mosfet follies" with many cool ideas.
http://www.geofex.com/Article_Folder...osfetfolly.htm

Some practical examples:
https://el34world.com/Forum/index.php?topic=21818.0

Adapt to what you have and need:


or adjustable versions:

[Ampzone]

Looks like running the car with parking brakes engaged to me.

I would explore lowering screen voltages to sensible values not through resistors or overloading PT to death (in exchange of nothing, and besides plates are happy with high voltage) using a MosFet regulator.

It´s not the 70´s any more.
For one side we have "weaker tubes" although I am not 100% happy with that definition,on the other hand we have robust and inexpensive MosFets.

Search this very Forum, that topic has already been covered in detail.

Also "R G Keen´s Mosfet follies" with many cool ideas.
http://www.geofex.com/Article_Folder...osfetfolly.htm

Some practical examples:
https://el34world.com/Forum/index.php?topic=21818.0

Adapt to what you have and need:


or adjustable versions:

Thanks for the links

It sounds like your saying with new tubes under load the screen voltage will drop into a safer range, hopefully the new 1k 5w screens will help, upping to the 2k resistors WILL not help. And bias a little hotter and hope for the best?

Mod the circuit with a mosfet and your ok.

Or buy some old tubes that can handle the voltage?

[catalin gramada]

No el34 is rated for more than 410v into screens,period. Some tubes are from manufacture rugged than other. I run tung sol at 480v without problems, one tube was happened to broke, that is, a lottery when you overpass the specs. What I trying to say is you transformer will put low voltage if you draw more current. 26mA meant pretty cold bias for You amp. Biasing around 35-40mA will draw more current from power supply and in consequence will sag a little from over 500 to ca. 480-470 v region. a little bit safer but is possible to not like how the amp perform biased 70percent around. If You cannot deal with bias to get voltage you want you can try to undervoltage you amp selecting from mains voltage selector the next value up in respect with you mains have. I did an example just put the selector in 240v position and supplied from 230v mains.
Or stuck with solid state blasphemy and make an screen regulator as JMF states (Best option)
I did some sugestions cause I know how this power transformer perform. If was happened to have a PT which not sag more than few percents under full load, like I usually use, this considerations can not be applied

[Helmholtz]

No el34 is rated for more than 410v into screens,period.

Most original manufacturers (Mullard, Philips, Siemens) allowed for Vs = 425V. The 1969 Telefunken datasheet specifies Vs = 500V (design center rating).
Screen voltage limits hold for any non-zero screen current situation, including idle.

[Ampzone]

No el34 is rated for more than 410v into screens,period. Some tubes are from manufacture rugged than other. I run tung sol at 480v without problems, one tube was happened to broke, that is, a lottery when you overpass the specs. What I trying to say is you transformer will put low voltage if you draw more current. 26mA meant pretty cold bias for You amp. Biasing around 35-40mA will draw more current from power supply and in consequence will sag a little from over 500 to ca. 480-470 v region. a little bit safer but is possible to not like how the amp perform biased 70percent around. If You cannot deal with bias to get voltage you want you can try to undervoltage you amp selecting from mains voltage selector the next value up in respect with you mains have. I did an example just put the selector in 240v position and supplied from 230v mains.
Or stuck with solid state blasphemy and make an screen regulator as JMF states (Best option)
I did some sugestions cause I know how this power transformer perform. If was happened to have a PT which not sag more than 10percent under full load, like I usually use, this considerations can not be applied

Thanks for the tips here.

Reading around I see some people including Marshall in the 90's used 2k -2.2k on screens. I just tried it. At 26ma per tube except one is at 33ma, its reading at idle 490 plates 482 screens. It looks like I can try and get another tube to match the other three at 26ma and raise up to 35ma overall, I think it will get the screens down in the 470ish range and the plates around 485ish at idle. That could maybe work?

Now this Hiwatt has 105v 115v 220v 240v taps. Our ac wall voltage is 120 here. Not sure what region the 105v tap was for.

[catalin gramada]

Screen resistors are there to limit power dissipation into the screens which meant different thing. Anyhow you have, or should, a common 470ohm screen resistor which is almost 2k individuals equivalent.(effective not quite cause is shared between rails)
Yeah, you have not option to use a mains tap, but can use an autotransformer, variac instead

[Ampzone]

Screen resistors are there to limit power dissipation into the screens which meant different thing. Anyhow you have, or should, a common 470ohm screen resistor which is almost 2k individuals equivalent.(effective not quite cause is shared between rails)
Yeah, you have not option to use a mains tap, but can use an autotransformer, variac instead

Yep there is a 470ohm 10w resistor they all share. Maybe it be ok with a hotter bias setting putting the screens around 470ish at idle. Can the new JJ brand el34's hold up to that environment? Any experiences with them?

[catalin gramada]

JJ are good tubes and sound very nice at high voltages. Are very afordable replacement too.At regular street price I didn.t bother to ask. Go ahead!

[J M Fahey]

Thanks for the links

It sounds like your saying with new tubes under load the screen voltage will drop into a safer range NO , hopefully the new 1k 5w screens will help NO , upping to the 2k resistors WILL not help NO . And bias a little hotter and hope for the best? NO

Mod the circuit with a mosfet and your ok. YES . Set screen voltage to 400/410V

Or buy some old tubes that can handle the voltage? NO

Please read what was actually suggested.

[TimmyP1955]

With 480 on the plates of EL34s, I bias about 31ma (in the hope of getting maximum life, since we have a lot of amps to feed around here).