Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: nateflanigan on July 31, 2011, 02:45:23 pm

Title: Odd bias on twin re-issue
Post by: nateflanigan on July 31, 2011, 02:45:23 pm

I installed a new set of tubes in a friends TRRI and got some odd bias readings.  I didn't take the chassis out or anything I just used the eurotubes bias probe which reads the bias in mA's.  Looking at the service manual it gives a B+ of 440, the original schem has 460.  Using the online weber bias calculator I came up with a range of about 45-47 ma.  After installing the new tubes without adjusting anything the initial reading was 15 ma.  Turning the bias pot "up" as much as possible only got us to about 41 ma.  The amp sounds fine but those readings don't seem right do they?

Thanks!

Title: Re: Odd bias on twin re-issue
Post by: sluckey on July 31, 2011, 03:32:29 pm

You really need to know what the actual plate voltage is. Otherwise, if it sounds good, play it.

Title: Re: Odd bias on twin re-issue
Post by: nateflanigan on July 31, 2011, 04:05:00 pm

Out of curiosity if I pull one of the tubes and read the voltage on pin 3 will I be getting an accurate reading?  I know if there's no tubes the unloaded voltage will be a lot higher but I don't know if that's specific to one socket or more of an entire amp thing. 

Title: Re: Odd bias on twin re-issue
Post by: LooseChange on July 31, 2011, 05:02:23 pm

you're numbers look good. If you pull one tube and measure pin 3 you'll be close enough.

Title: Re: Odd bias on twin re-issue
Post by: nateflanigan on July 31, 2011, 05:17:35 pm

Thanks for the reassurance.

Title: Re: Odd bias on twin re-issue
Post by: jjasilli on July 31, 2011, 07:10:08 pm

Quote from: nateflanigan on July 31, 2011, 04:05:00 pm

Out of curiosity if I pull one of the tubes and read the voltage on pin 3 will I be getting an accurate reading?  I know if there's no tubes the unloaded voltage will be a lot higher but I don't know if that's specific to one socket or more of an entire amp thing. 

Ea tube is pulling current, and presents an impedance to the current flow.  Per Ohm's when current flows through an impedance, it cuase a voltage drop.  As you pull tubes, the voltage will rise, skewing your voltage readings readings. 

By what method are you deriving the current draw?

Title: Re: Odd bias on twin re-issue
Post by: nateflanigan on July 31, 2011, 07:33:10 pm

Quote

By what method are you deriving the current draw?

https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=108&sub_category_id=109

Title: Re: Odd bias on twin re-issue
Post by: HotBluePlates on July 31, 2011, 11:04:06 pm

Quote from: jjasilli on July 31, 2011, 07:10:08 pm

As you pull tubes, the voltage will rise, skewing your voltage readings readings. 

Yeah, but... In a Twin with 4 output tubes, yanking one at idle won't throw things off that far. Especially considering the solid-state rectifier.