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FDP Forum / Amp Mods, Repairs, and Projects / 67 voltage ?

cedarchoper58

62 Strat Man

Feb 13th, 2019 02:33 PM   Edit   Profile  

I have a 67 blackface pro reverb that runs 2-6L6 tubes and the voltage on the plates is 488 and 490 it use to run around 480v when my wall voltage was 123V AC now my wall voltage is 124.2 V AC. Is the plate voltage to high and is so what can i do thks

Leftee
Contributing Member
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VA

Toxic Humility
Feb 13th, 2019 05:07 PM   Edit   Profile  

I usually use the heater voltage to judge how an old amp is running on modern wall voltage.

cedarchoper58

62 Strat Man

Feb 14th, 2019 02:01 PM   Edit   Profile  

I dont really wan to pull the chasis i got readings with my bias probe. How much plat voltage can Tungsol 6L6's take and what would be the heater range if I pull it and what can be done to lower it thks

pdf64

UK

Feb 14th, 2019 04:53 PM   Edit   Profile  

See link for the best way to correct for modern high mains voltage.
Brownbox is a commercial example of such a mains bucker.

bucker

cedarchoper58

62 Strat Man

Feb 14th, 2019 05:13 PM   Edit   Profile  

I use a Ultimatte Attenuator that has a lower voltage tap that drops the voltage from 124.4V to 105V would that be the way to go or be two low,? I tried it on my 68 Marshall and my heaters were bellow tolereance

Leftee
Contributing Member
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VA

Toxic Humility
Feb 14th, 2019 05:29 PM   Edit   Profile  

Here’s the spec sheet for those tubes. Assuming they’re new production.

External link

cedarchoper58

62 Strat Man

Feb 14th, 2019 06:07 PM   Edit   Profile  

it says max plate is 500v does that mean 501v will blow the tubes or is 490 to close thks

pdf64

UK

Feb 16th, 2019 11:40 AM   Edit   Profile  

Tubes have various limiting values, plate voltage is one. If the tube is operated above any (or worse, several at once) those then its useful life may reduce, perhaps drastically.
Your mains voltage may rise to 127 (or fall to 104) yet still remain compliant to ANSI C84.1-2016.
Hence you may find it beneficial to monitor it and take appropriate mitigation as required.

What do you mean by 'my heaters were bellow tolereance'?

slider313

NC

Feb 16th, 2019 01:02 PM   Edit   Profile  

Try using a 5R4GB rectifier in place of the 5AR4. This should bring your B+ voltage down to more acceptable levels.

cedarchoper58

62 Strat Man

Feb 16th, 2019 06:04 PM   Edit   Profile  

Heater voltage low bellow tolerance

pdf64

UK

Feb 17th, 2019 04:28 AM   Edit   Profile  

'Heater voltage low bellow tolerance'
Please detail the measurements you have made and the info you are referencing, so that I can work out how you have come to that conclusion.

Note that use of a 5R4 leaves the issue of excessive heater voltage unaddressed.

cedarchoper58

62 Strat Man

Feb 17th, 2019 06:13 PM   Edit   Profile  

Its been a long time since i measured the heater voltage but it was out of range low on the 105v tap but in range on the wall voltage.

SoK66

USA

We had the hit but Van got the money
Feb 18th, 2019 03:24 PM   Edit   Profile  

"I use a Ultimatte Attenuator that has a lower voltage tap that drops the voltage from 124.4V to 105V would that be the way to go or be two low,? I tried it on my 68 Marshall and my heaters were bellow tolereance"

Nah, that'll render way too low B+. The tubes will be fine below 500v, but the amp won't sing like it's supposed to. You want to see something on the order of 440vdc B+ with the amp biased at arund 60%. To get there the best way is to reduce the line voltage ahead of the amp variac-style, but by using something simpler called a "buck transformer." That's a transfomer that lowers, rather than raises the voltage. You want your line voltage to be in the 117vdc range, which is about what it was designed around back in the day. You can make one with stuff from an electronics store following the schematic link below, OR, buy a nice commercial one like The Amp Preserver or similar. I run all my vintage Fenders through one and set them up accordingly.

Vintage Voltage Circuit

FDP Forum / Amp Mods, Repairs, and Projects / 67 voltage ?




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