New build - 5E7 - Bias Question

[fdesalvo]

Hello, all.

I've just completed a 5E7 Chassis using a Weber PT and am using some NOS JAN Phil 6L6WGBs. Bias circuit is like the stock fender, but subbing a 50KL pot for the 56K resistor. pV is a bit low @ 388V, but the preamp voltages are spot on. I initially read ~64mA across each 1 Ohm rK to ground with my 50K pot maxed out. Experimenting with a decade box, it took a 100K resistor in series with the bias pot to get the reading down to 30mA across each resistor. At this point, the bias pot was maxed in the other direction. It seems that a ~70K resistor would allow the pot to sweep 20-40mA. I've never had to use such a large resistor here. Any insight?

Thanks!

[pdf64]

See http://music-electronics-forum.com/t40368/#post396679
A link to the exact schematic used would be helpful.
As a general principle, with the saggy B+ supply in most tube guitar amps, the particular B+ voltage is dependant on the current being drawn as the time of measurement.
As your power tubes were drawing a lot of current, their plate voltage was being pulled down.

[g1]

Also the specific Weber PT.

[fdesalvo]

I will review the suggested link - thanks!

Layout and PT specs attached.

[lowell]

I think we all want a schematic not a layout. Personally I wouldnt be concerned if you had to add that resistance. All that matters is that the bias is on point. I'm not a fan of depending on 1ohm resistors for bias measurement. It's too important. Please make sure those 1ohms are spot on. It's hard to do with a basic meter. Better yet ditch em and measure bias current via the transformer shunt method.

[fdesalvo]

I think we all want a schematic not a layout. Personally I wouldnt be concerned if you had to add that resistance. All that matters is that the bias is on point. I'm not a fan of depending on 1ohm resistors for bias measurement. It's too important. Please make sure those 1ohms are spot on. It's hard to do with a basic meter.

The 1ohm resistors are rated @ 1% tolerance. I have a very nice meter. I was expecting to have to add a 30K at most. Since the other measurements are within range in the circuit, I suppose I can rest easy with a larger value resistor here. Thanks.


bandmaster_5e7_schem.pdf

[lowell]

Yep I wouldnt sweat it as long as bias current is in range.

[fdesalvo]

Thanks again, Lowell.

[pdf64]

...measure bias current via the transformer shunt method.

Come on, please don't advise that pointlessly hazardous / Darwin award contender shunt method!
Especially when g1 kindly documented the much safer alternative within the past 24 hrs http://music-electronics-forum.com/t40348-2/#post396718

[g1]

Come on, please don't advise that pointlessly hazardous / Darwin award contender shunt method!

While I won't deny having used it, I agree it's extremely dangerous and something it's better to not mention on a public forum where novices may not be aware of the hazard.

Lowell: When you do use it, you should be aware of the burden voltage spec for the various DC current ranges on your meter. It can very much affect the accuracy when shunting such low resistances.

If you really want to be precise, you would use the current meter in series with the plate, rather than relying on shunt.

[g1]

I've never had to use such a large resistor here. Any insight?

You can also reduce the size of the other resistor that is before the bias rectifier. Looks like it's 4800 or is that 6800 on the schematic?
What do you have there?

[fdesalvo]

That's a 6.8k, my friend.

[g1]

Then decreasing it should have the same effect as increasing the 56K.

[fdesalvo]

I'll give it a whirl and report back. TY

[fdesalvo]

Found this on ye olde ampage:

"Another thing I forgot to mention was that with Ted's PT, I found that the stock 5E7 fixed bias supply resistor values were too low and it made my tubes run too hot. I replaced the 56K with a 100K and added a 50K trim pot to get it set right (approx. 35ma across my cathode resistors)."

#notalone