Tweedle Dee bias concern

andresound · Post by **andresound** » Mon Apr 09, 2018 6:52 am

Just completed a Tweedle Dee with parts lying around. Amp sounds great and is both quiet and hum free.

Old 60’s combo chassis (dont laugh). Was a President combo. Could not find info on it anywhere!!
Custom PT
Unknown Hi-Fi OT (7-8k)
Used 716 and 6PS

Voltages are spot on (+- a volt or 2) with the layout in the files section. I have 171.2 and 171.6 on v1 and v2. Strangely, I had to use a 5Y3 (NOS RCA), to get the voltages right. With a GZ34, voltages were too high. However... I am concerned that 15.2W per tube is HOT!!! (Old Hultron 6V6 GT)

Also, heaters are 6.8v!! (Will not order PT from same outfit again). Any suggestions on how to lower them? In-line resistor on both legs?

Any help would be appreciated.

andresound · Post by **andresound** » Mon Apr 09, 2018 6:56 am

Bias results!!

martin manning · Post by **martin manning** » Mon Apr 09, 2018 10:17 am

You could try a different set of output tubes (find a pair that run cooler), or maybe try JJ 6V6S, which are rated at 14W. Otherwise, you'll have to increase the cathode resistor value.

andresound · Post by **andresound** » Mon Apr 09, 2018 10:22 am

Thanks Martin. Think I will order a set of JJ’s. And what about the heater voltage? I have the same problem on my rocket build with the heaters (same PT supplier

)

martin manning · Post by **martin manning** » Mon Apr 09, 2018 10:43 am

Technically 6.8V should be ok, since it's within the +/-10% rule. Is your line voltage running at the level expected by the PT?

andresound · Post by **andresound** » Mon Apr 09, 2018 3:28 pm

Line voltage is correct and (235-240v). Did some googling, like I should and sorted. The ADD in me put .1ohm 5w resistors in line.. and now I get 6.22v

. While I was at it.. sorted the Rocket as well.Thats why I like hanging out here, you learn something everyday

PaisleyTube · Post by **PaisleyTube** » Tue Apr 10, 2018 6:02 pm

try lowering the voltage by using a bucking tranfsormer

andresound · Post by **andresound** » Mon Apr 30, 2018 7:19 am

martin manning wrote: ↑Mon Apr 09, 2018 10:17 am You could try a different set of output tubes (find a pair that run cooler), or maybe try JJ 6V6S, which are rated at 14W. Otherwise, you'll have to increase the cathode resistor value.

Found a set of GEC 6v6 that run at 14w. Amps sound really good. In a band (old school rock) setting, this thing is loud enough. I am waiting on the arrival of the JJ’s. Might then try it with thre GZ34 as per Charlie’s post. Thanks for all the help guys.

drew · Post by **drew** » Mon Apr 30, 2018 4:03 pm

http://www.valvewizard.co.uk/heater.html
Scroll down to the part that begins with “A less obvious option is to use a pair of diodes in anti-parallel.”

andresound · Post by **andresound** » Tue May 01, 2018 4:21 pm

Thanks Drew.. nice read

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