5F10 Build and Bias Question

[Amp vonRorklander]

Hey folks, this is my first post on here. I’ve been a lurker for a while but I thought I’d join the fun…
I’m modding a 5F2a chassis into a 5F10 Harvard and I have a question about the adjustable bias. Basically, the question is: did I do it right?
I used a 50K linear pot with a 33K resistor heading up to ground. The wiper is jumpered over to the diode and the typical bias circuit. I also jumpered the wiper to the unused lug on the pot. Was this correct?
Pictures are below for clarity… Heaters still need to be run along with the power cord.
I used a Hammond 290HAX PT and a Mojo768 OT. I used shielded coax from the input jacks and the 68K’s down to the 6AT6 and from the volume pot down to the preamp tube as well. All other components are to spec.

 

[King Fan]

That's a nice tidy build. Lemme blow up your bias pic and add the schematic voltage details in case that helps anyone.

 

[Ten Over]

Hey folks, this is my first post on here. I’ve been a lurker for a while but I thought I’d join the fun…
I’m modding a 5F2a chassis into a 5F10 Harvard and I have a question about the adjustable bias. Basically, the question is: did I do it right?
I used a 50K linear pot with a 33K resistor heading up to ground. The wiper is jumpered over to the diode and the typical bias circuit. I also jumpered the wiper to the unused lug on the pot. Was this correct?
Pictures are below for clarity… Heaters still need to be run along with the power cord.
I used a Hammond 290HAX PT and a Mojo768 OT. I used shielded coax from the input jacks and the 68K’s down to the 6AT6 and from the volume pot down to the preamp tube as well. All other components are to spec.

The 290HAX has a 50V bias tap and the original PT had about half that voltage. This will cause your bias to be way too cold with the values you are using.

 

[Intubator]

You could probably get rid of that 33K resistor all together and you'll have a 0-50K range. I'd estimate that you'd need to drop the resistance to 25-30K to reach a -27 bias V.
I believe the connection from the wiper to the second (upper in the photo) leg should not be connected, your just looking for a resistance sweep on one side. Someone correct me if I'm wrong.
That's a beautiful build there. I am just now in the planning stages to do one myself. Do you have a cab for it yet? Whats your speaker selection?

 

[King Fan]

FWIW, folks often find they need a smaller "tail" resistor -- given your big 50K pot, you might try something like 10k instead of the 33k.

 

[Ten Over]

Change the 6.8k resistor to 15k and change the 33k resistor to 20k. Should give you a range around -18V to -35V.

 

[Ten Over]

You could probably get rid of that 33K resistor all together and you'll have a 0-50K range.

Don't do that. The output tubes get real hot when the unsuspecting end user turns the bias to 0V.

 

[Ten Over]

I believe the connection from the wiper to the second (upper in the photo) leg should not be connected, your just looking for a resistance sweep on one side. Someone correct me if I'm wrong.

The difference is what happens when the wiper fails. With the wiper connected to the unused lug the bias will default to the coldest setting. With the lug left open, the bias will rise to the peak AC voltage from the bias tap. This is frequently way over the voltage rating of the smoothing capacitor.

 

[Intubator]

Don't do that. The output tubes get real hot when the unsuspecting end user turns the bias to 0V.

Right, one should be aware of what they're doing when biasing an amp..

 

[Intubator]

The difference is what happens when the wiper fails. With the wiper connected to the unused lug the bias will default to the coldest setting. With the lug left open, the bias will rise to the peak AC voltage from the bias tap. This is frequently way over the voltage rating of the smoothing capacitor.

That makes great sense. Failsafe mechanism!

 

[King Fan]

BTW, @Amp vonRorklander , welcome to TDPRI. As you've found, despite our biases, bias is one interest we all share (wink, wink). Let me give a big nod in the direction of @Ten Over -- he's helped me understand bias a ton better.

FWIW, Merlin on bias is well worth a read (you don't have to use his bias scheme -- as he notes, many will work). He suggests a tail resistor (his R2) as a backstop to a bias pot (P1): "It is good design practice to limit the range of voltage adjustment so it is impossible to reduce the bias to nothing (0V). This at least gives a modicum of protection against accidental or unskilled adjustment."

This bias stuff isn't always easy or obvious. You might enjoy a recent thread on a closely-related bias scheme.

https://www.tdpri.com/threads/mercury-ftvp-240m-50v-bias-tap-putting-out-28v.1127745/#post-11862654

 

[Amp vonRorklander]

The difference is what happens when the wiper fails. With the wiper connected to the unused lug the bias will default to the coldest setting. With the lug left open, the bias will rise to the peak AC voltage from the bias tap. This is frequently way over the voltage rating of the smoothing capacitor.

Thank you Ten Over. That’s exactly the analysis I was hoping for. I’ll change out that 33k to get a better range.

 

[Amp vonRorklander]

BTW, @Amp vonRorklander , welcome to TDPRI. As you've found, despite our biases, bias is one interest we all share (wink, wink). Let me give a big nod in the direction of @Ten Over -- he's helped me understand bias a ton better.

FWIW, Merlin on bias is well worth a read (you don't have to use his bias scheme -- as he notes, many will work). He suggests a tail resistor (his R2) as a backstop to a bias pot (P1): "It is good design practice to limit the range of voltage adjustment so it is impossible to reduce the bias to nothing (0V). This at least gives a modicum of protection against accidental or unskilled adjustment."

This bias stuff isn't always easy or obvious. You might enjoy a recent thread on a closely-related bias scheme.

https://www.tdpri.com/threads/mercury-ftvp-240m-50v-bias-tap-putting-out-28v.1127745/#post-11862654

Thanks, King Fan. I’ve checked out most of those threads in looking for an answer. I finally just decided to post my own pics and get a specific answer to my build. You guys did not disappoint. Thanks again!

 

[Amp vonRorklander]

You could probably get rid of that 33K resistor all together and you'll have a 0-50K range. I'd estimate that you'd need to drop the resistance to 25-30K to reach a -27 bias V.
I believe the connection from the wiper to the second (upper in the photo) leg should not be connected, your just looking for a resistance sweep on one side. Someone correct me if I'm wrong.
That's a beautiful build there. I am just now in the planning stages to do one myself. Do you have a cab for it yet? Whats your speaker selection?

I used the Weber 5F2A chassis and drilled the extra holes for the additional 6V6 and the 6AT6 as well as the third input. This was not that easy. I used a step bit but chassis punches probably would have been better for the two larger holes (someone on here may have better advice). The 3/8” hole for the input was no problem with a regular bit. Just take your time centering those holes.

For the cabinet I’m using the Weber 5F2A cab. No modification necessary here. I shellacked it for a bit of an aged/not so new look.

I got the 5F2A small parts kit from Mojotone And substituted out the capacitors. I’ve used the F&T’s and Sozos on previous builds (5F1, 5E3, and Princeton Reverb) with good results.

As far as the speaker goes, I’m using a Weber 10A100T. I’ll give my review of that when I finally get to play through it.

Thanks for the comments and good luck on your build.

 

[gabasa]

I went down this rabbit hole a while ago. Because I used Victoria chassis and transformers, I asked Mark Baier what values he uses in their Ivy League amp and this was what he told me:

5Y3 Pin 4 -> 100k resistor -> diode -> 25k pot -> 15k resistor to ground

I know you have the bias tap on your PT, but I thought I'd throw in this info in case it helps at all. After rotating many 10" and 12" speakers in and out of my Harvard, the 10A100T was the one that stayed in for the long haul. After a bunch of loud band rehearsals and gigs, it finally broke-in really nicely and it suits the amp really well. I'd love to hear your opinion on it, especially after a good break-in period!

 

[mrfitz98]

Using that transformer (290HAX) I ended up with 17k resistor after the 10k pot that I used, 68K -> diode -> 10K pot -> 17K. You'll probably find that 50k pot to be pretty touchy.

 

[Amp vonRorklander]

Using that transformer (290HAX) I ended up with 17k resistor after the 10k pot that I used, 68K -> diode -> 10K pot -> 17K. You'll probably find that 50k pot to be pretty touchy.

Thanks for the info. Was that a 68k or a 6.8k before the diode?

 

[Amp vonRorklander]

I went down this rabbit hole a while ago. Because I used Victoria chassis and transformers, I asked Mark Baier what values he uses in their Ivy League amp and this was what he told me:

5Y3 Pin 4 -> 100k resistor -> diode -> 25k pot -> 15k resistor to ground

I know you have the bias tap on your PT, but I thought I'd throw in this info in case it helps at all. After rotating many 10" and 12" speakers in and out of my Harvard, the 10A100T was the one that stayed in for the long haul. After a bunch of loud band rehearsals and gigs, it finally broke-in really nicely and it suits the amp really well. I'd love to hear your opinion on it, especially after a good break-in period!

Thanks! I’ll let you know.

 

[King Fan]

My name is King Fan and I have a new addiction, simulating these 10,000 maniac bias circuits. Here's the nice solution @Ten Over mentioned above -- as always, if I did it right.



As he predicted, the bias range runs about –19 to -35V. These sims aren't perfect -- but a range anything like that should work well.

 

[Amp vonRorklander]

My name is King Fan and I have a new addiction, simulating these 10,000 maniac bias circuits. Here's the nice solution @Ten Over mentioned above -- as always, if I did it right.



As he predicted, the bias range runs about –19 to -35V. These sims aren't perfect -- but a range anything like that should work well.

That’s awesome! Thank you.