Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on January 17, 2021, 10:03:22 am

Title: Harvard 5F10 bias pot
Post by: Diverted on January 17, 2021, 10:03:22 am

Hi all,

Working on laying out a Fender Harvard 5F10 for a build. This amp has a dedicated bias supply and the transformer has a tap so I want to add a 50K bias pot to ease biasing.
I have installed many bias pots but always did it by just tapping the HV secondary. I want to use the bias tap.
I read that what's needed to switch to adjustable fixed bias in an amp with a dedicated tap is to reduce the first resistor off the tap in half, then replace the second resistor with the 50K pot.

I was wondering if anyone could look at the layout I'm working on and let me know if I have it right. Thanks!

Title: Re: Harvard 5F10 bias pot
Post by: thetragichero on January 17, 2021, 10:20:34 am

put a resistor between bias pot lug 3 and ground. 22k? 33k? something like that
also connect the wiper to the unused lug

don't forget to add 1 ohm 1% bias sense resistor(s) between power tube cathodes and ground

Title: Re: Harvard 5F10 bias pot
Post by: sluckey on January 17, 2021, 10:23:50 am

You need a resistor between the pot and ground to prevent turning the bias voltage to zero and destroying the output tubes. I used a 50K pot and 24K resistor to replace the original 56K resistor. No need to reduce the original 6.8K. See pages 3 and 4...

     http://sluckeyamps.com/harvard/harvard_5f10_schem.pdf

Title: Re: Harvard 5F10 bias pot
Post by: Diverted on January 17, 2021, 08:59:40 pm

Thank you. Definitely planning on putting 1 ohm resistors in as well.

Title: Re: Harvard 5F10 bias pot
Post by: Diverted on January 18, 2021, 12:14:35 pm

Follow-up, and this question might even be stupider than my usual variety :laugh:
I've been looking at a lot of 5F10 chassis pix online, both originals and clones. In every one I've seen, the power/output transformer orientation is different than most amps I've seen. The transformers are lined up without rotating one 90 degrees to the other.
Any ideas why this was/is? I was just going to lay out the transformers in the standard orientation (opposed at 90 degrees) but am just wondering why the other arrangement seems common in this amp.
Thanks.

Title: Re: Harvard 5F10 bias pot
Post by: 2deaf on January 18, 2021, 12:29:31 pm

Quote from: Diverted on January 18, 2021, 12:14:35 pm

Follow-up, and this question might even be stupider than my usual variety :laugh:
I've been looking at a lot of 5F10 chassis pix online, both originals and clones. In every one I've seen, the power/output transformer orientation is different than most amps I've seen. The transformers are lined up without rotating one 90 degrees to the other.
Any ideas why this was/is? I was just going to lay out the transformers in the standard orientation (opposed at 90 degrees) but am just wondering why the other arrangement seems common in this amp.
Thanks.

Doesn't the 5F10 have a lay-down power transformer?

Title: Re: Harvard 5F10 bias pot
Post by: sluckey on January 18, 2021, 12:35:18 pm

The transformers are ***ALWAYS*** 90° to each other, just like most all Fender amps. If your PT is a lay-down style (big rectangular hole) just install yours exactly as all the pics you see. Here's my real one...

     http://sluckeyamps.com/harvard/harvard.htm

Title: Re: Harvard 5F10 bias pot
Post by: Diverted on January 18, 2021, 12:44:41 pm

Thanks. OK, that clarifies the whole orientation thing for me. Thanks for the photos.