Buildmasters...Help w 5F11 Bias Issue plz?

[Pappy58]

Greetings,

I am currently at the conclusion of a 5F11 build using a mojotone kit and spec. I finished the chassis build last night and started powering her up today. We got to fully powered with all tubes equipped but no load, all controls at 0. My bias voltage is like -100vdc vice the -31vdc the spec calls for. The PT is putting out 52vac for the bias control, so it's good. The mojo kit included a 10k pot and a 33k resistor (see below). the diode seems to test ok.

Any thoughts idea's on why this is so high? See chart and chassis pics below. Thanks in advance for any idea's and/or assistance.

 

[Cjsinla]

This is how I have mine wired. I have -51 VDC at the board, too. Between the resistor and diode I have +7.76 VDC. I have -31.9 VDC from the diode to the center lug of the pot. Are you sure that you have a 10K resistor between the -51 VDC lead and the diode?

I also have -31.9 VDC at the left lug of the depth control. Hope that helps.

 

[Pappy58]

This is how I have mine wired. I have -51 VDC at the board, too. Between the resistor and diode I have +7.76 VDC. I have -31.9 VDC from the diode to the center lug of the pot. Are you sure that you have a 10K resistor between the -51 VDC lead and the diode?

I also have -31.9 VDC at the left lug of the depth control. Hope that helps.

View attachment 323539

Thanks man! I was hoping you would jump in. I have the 10k in place and everything here looks schematically the same. I have a theory that my grounding scheme may be introducing a floating ground, as I did a Robinette variation with the PT center tap connected to the filters as 0vr reference. I tied the bias circuit ground to the same bus, hence there is no direct shunt to chassis ground. I'm going to make that change and try again.

I see you added some grid resistors? Any specific reason for that? Grounding pins 1&8 on the power tubes wigged me out a little. Are you resistors shunting that?

Thanks again for chiming in. Much appreciated.

 

[Cjsinla]

Thanks man! I was hoping you would jump in. I have the 10k in place and everything here looks schematically the same. I have a theory that my grounding scheme may be introducing a floating ground, as I did a Robinette variation with the PT center tap connected to the filters as 0vr reference. I tied the bias circuit ground to the same bus, hence there is no direct shunt to chassis ground. I'm going to make that change and try again.

I see you added some grid resistors? Any specific reason for that? Grounding pins 1&8 on the power tubes wigged me out a little. Are you resistors shunting that?

Thanks again for chiming in. Much appreciated.

I added 1 ohm resistors to help with checking bias.

 

[Pappy58]

This is how I have mine wired. I have -51 VDC at the board, too. Between the resistor and diode I have +7.76 VDC. I have -31.9 VDC from the diode to the center lug of the pot. Are you sure that you have a 10K resistor between the -51 VDC lead and the diode?

I also have -31.9 VDC at the left lug of the depth control. Hope that helps.

View attachment 323539

Ok, Changed the ground and now I'm down to min -68vdc. However I have a question about your reading above: "Between the resistor and diode I have +7.76 VDC."

You sure about that one? That should still be AC on pos side of the diode yeah? I have
52vac from the PT
52vac on the drop side of the resistor at the pos side of the diode (No Drop)
Results in -68vdc on the neg side.

So I think my 10k resistor is not resisting much!

Back at it after supper!

 

[Cjsinla]

Ok, Changed the ground and now I'm down to min -68vdc. However I have a question about your reading above: "Between the resistor and diode I have +7.76 VDC."

You sure about that one? That should still be AC on pos side of the diode yeah? I have
52vac from the PT
52vac on the drop side of the resistor at the pos side of the diode (No Drop)
Results in -68vdc on the neg side.

So I think my 10k resistor is not resisting much!

Back at it after supper!

That resistor (or wiring) is suspect. I show 40 vac, good call on the voltage type.

 

[Pappy58]

That resistor (or wiring) is suspect. I show 40 vac, good call on the voltage type.

So it seems your's truly installed a 100ohm vice the 10k....and of course there is not a 10k to be found. I just did an order to Mojo early last week for common spare caps and resistors. So do you think I bought any 10k's??? NOOOOO! As luck would have it I have to go to the Valley tomorrow to see my daughter and I know where there is still a radio shack in Strasburg. ...now just hope they have a 10k....or it's another wait for more mojomail!

 

[Cjsinla]

So it seems your's truly installed a 100ohm vice the 10k....and of course there is not a 10k to be found. I just did an order to Mojo early last week for common spare caps and resistors. So do you think I bought any 10k's??? NOOOOO! As luck would have it I have to go to the Valley tomorrow to see my daughter and I know where there is still a radio shack in Strasburg. ...now just hope they have a 10k....or it's another wait for more mojomail!

Don’t forget that I mixed up the heater virtual ct 100 ohm resistors with the 100k plate resistors in my Vibrolux build and nothing worked right. Glad to hear that you on track to getting it fixed. It’s a truly sweet sounding amp and you will be pleased when you get it working.

Btw are you sure that you didn’t do what I did? Check the two resistors in your virtual center tap, they are the only 100 ohm resistors in the kit. Make sure that you didn’t put the 10k there.

 

[Cjsinla]

@Pappy58 So? Did you ever get that worked out? I’m anxious to get your review of the working amp. I love mine.

 

[Pappy58]

Still waiting on 10ks via pony express. Really hoping they show today. I used a Allen PT and it has center tap for the heaters so the 100s for that are not installed. Believe me, I want to hear it too! This one is actually being sold and the buyer is anxious to

 

[Pappy58]

@Cjsinla Ok I got the 10k in and my bias voltage range is now within spec. However my plate voltage is 400 and Cath @ ~27.5ma and thats the best I can get with the bias pot at ~8. There is no more room to work with. So I think I need to go with a bigger resistor on the bias pot and increase my headroom? Thoughts?

Again, as discussed in previous build threads the voltages are hot coming off the PT. This one is an Allen TP-25 and it's pushing ~350vac on the secondary when their spec States 310vac. I suspect this is driving the restrictions in the bias circuit. But getting the cath down to a reasonable ~20-22ma is going to raise the plate voltage considerably.

I'm getting pretty frustrated with these PT's. Seems all of them out too hot on the secondary, especially with a 125vac input.

TIA for any idea's you might have!

 

[Cjsinla]

@Cjsinla Ok I got the 10k in and my bias voltage range is now within spec. However my plate voltage is 400 and Cath @ ~27.5ma and thats the best I can get with the bias pot at ~8. There is no more room to work with. So I think I need to go with a bigger resistor on the bias pot and increase my headroom? Thoughts?

Again, as discussed in previous build threads the voltages are hot coming off the PT. This one is an Allen TP-25 and it's pushing ~350vac on the secondary when their spec States 310vac. I suspect this is driving the restrictions in the bias circuit. But getting the cath down to a reasonable ~20-22ma is going to raise the plate voltage considerably.

I'm getting pretty frustrated with these PT's. Seems all of them out too hot on the secondary, especially with a 125vac input.

TIA for any idea's you might have!

Just saw your post, let me check and get back to you tomorrow.

 

[Cjsinla]

@Cjsinla Ok I got the 10k in and my bias voltage range is now within spec. However my plate voltage is 400 and Cath @ ~27.5ma and thats the best I can get with the bias pot at ~8. There is no more room to work with. So I think I need to go with a bigger resistor on the bias pot and increase my headroom? Thoughts?

Again, as discussed in previous build threads the voltages are hot coming off the PT. This one is an Allen TP-25 and it's pushing ~350vac on the secondary when their spec States 310vac. I suspect this is driving the restrictions in the bias circuit. But getting the cath down to a reasonable ~20-22ma is going to raise the plate voltage considerably.

I'm getting pretty frustrated with these PT's. Seems all of them out too hot on the secondary, especially with a 125vac input.

TIA for any idea's you might have!

Ok, I have 120 at the socket, 343 coming off my PT, 384 coming off the rectifier to the Board. I have 375 at the plates. It looks like I have 300 hitting the resistors at the PI and V1.

As before, 50vac hitting my 10k and 40v leaving.
There’s -32 hitting my 25k bias pot and -28 leaving the pot. It looks like I have a 33k resistor from the pot to ground and nothing between that resistor to ground.

Not sure if that helps, what resistor were you thinking about changing?

 

[Pappy58]

Ok, I have 120 at the socket, 343 coming off my PT, 384 coming off the rectifier to the Board. I have 375 at the plates. It looks like I have 300 hitting the resistors at the PI and V1.

As before, 50vac hitting my 10k and 40v leaving.
There’s -32 hitting my 25k bias pot and -28 leaving the pot. It looks like I have a 33k resistor from the pot to ground and nothing between that resistor to ground.

Not sure if that helps, what resistor were you thinking about changing?

Thanks for taking the time to do these measures. My wall is hot..125vac. My bias circuit is in spec voltage wise now, but I read that a larger resistor on the adjustment pot would increase the range. But I'm still concerned that if I do that and dial the cathode current down to the 20-24ma they want in the tdsl, the plate voltage is going to be in the area of ~430vdc. I'm not sure there is way out of this mousetrap sans installing a step down, or just running it hot. Some folks seem to be ok with that on the various forums.

I also have a problem with the depth pot it appears. I have to turn it up to 10 to get the vibrato to kick in... trying to sort thru that as well...

 

[Cjsinla]

Thanks for taking the time to do these measures. My wall is hot..125vac. My bias circuit is in spec voltage wise now, but I read that a larger resistor on the adjustment pot would increase the range. But I'm still concerned that if I do that and dial the cathode current down to the 20-24ma they want in the tdsl, the plate voltage is going to be in the area of ~430vdc. I'm not sure there is way out of this mousetrap sans installing a step down, or just running it hot. Some folks seem to be ok with that on the various forums.

I also have a problem with the depth pot it appears. I have to turn it up to 10 to get the vibrato to kick in... trying to sort thru that as well...

That depth pot works directly with the bias circuit, that might part of the problem or might be due to the bias not set right in the first place.

 

[Cjsinla]

@Pappy58 Hey, did you get this figured out. I was playing mine yesterday and it made me think about you.

 

[Pappy58]

@Pappy58 Hey, did you get this figured out. I was playing mine yesterday and it made me think about you.

Unfortunately fate interjected and I had to go to FL for extended time to deal with family things. .. it's all working just that depth control is still a little wonky. Hoping to get back at it this week and try a different POT.

 

[Pappy58]

Just to close this out, I squirted some deoxit in that pot and re-soldered the connections and it's all working now....

I also did some voltage testing with my variac. Both this one and my recent Deluxe build are much happier with ~115vac at the input. It's amazing how much 5-10volts changes the internal voltages. Both the Allen and the Classic-Tone PT's put out ~20% higher secondary voltage than their spec states @120vac....it doesn't seem to bother the amp, it's maybe just a little more brash on wall power, but from the measurements the power tubes are cooking.....with glass!

Anyway this one was delivered to it's customer today,,,,,the complaint line is open.

Next another tweed deluxe, then a JTM45. Thanks everyone for your inputs, and especially @Cjsinla