5881 to 6V6gt conversion

[blown240]

Is there anything I need to take into consideration when replacing 5881 with 6V6gt tubes?  or are they a direct swap?

Thanks!

[HotBluePlates]

You need to consider the B+ applied to the tube, and the bias.

Cathode bias should be fairly "automatic" in that often same/similar resistor values are used for 6L6 and 6V6. You'll want to verify how much current is passing through the tube to be sure you don't exceed dissipation.

For fixed-bias, you're likely safe initially, as 6L6's often have higher bias voltage present than typical in a similar 6V6 amp. That means the 6V6 is likely to be biased cold. But it really depends on the specific amp. Either way, you'll again want to verify tube current, and make sure you don't exceed dissipation. You'll probably need to adjust down the amount of 6V6 bias voltage, to get a little more idle current.

And please do the math in advance, know what current you need. A 6V6 with ~400v on the plate is often biased around 21mA, give or take.

[kagliostro]

5881 & 6L6 can be swapped

5881 to 6V6 is a conversion

Kagliostro

edit: I posted near the same time with HBP, he has already replied to you

[blown240]

Its in a copy of a 6G6B bassman.  I built the original schematic, but I swapped in 6V6GT tubes and I am getting no output.  I hooked up a bias meter to the tubes and they dont read anything.  Seems like I should change to a lower value resistor.  The tubes are for sure good, they were just tested tonight.

Does that make sense?

[PRR]

> I hooked up a bias meter to the tubes and they dont read anything.

Turn it over and read ALL the pin voltages.

Forgotten G2 connection (zero instead of +350V) will tend to "zero" cathode current and insignificant sound.

[Colas LeGrippa]

why don't you solder a 1 OHM resistor in series between bias resistor and gnd so that you could just know the current with your voltmeter by taking the voltage accross it ( 60 mV meaning 60 mA ) .in fixed bias, the 1 OHM resistor between cathode and grnd.......IMO it is much more easy to take the bias reading this way

Colas

[blown240]

> I hooked up a bias meter to the tubes and they dont read anything.

Turn it over and read ALL the pin voltages.

Forgotten G2 connection (zero instead of +350V) will tend to "zero" cathode current and insignificant sound.

Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:

Pin 1- 0
Pin 2- 29vac
Pin 3- 401vdc
Pin 4- 401vdc
Pin 5-  -43 vdc
Pin 6- 400vdc
Pin 7- 33vac
Pin 8- 0

And just for reference

5Y3 Pin 8- 402vdc

Bias Connections:

rec pin4 to 100k - 416vac
100k to diode - 193vac
diode to cap - -51
cap to gnd - 0

[HotBluePlates]

Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:

...
Pin 2- 29vac
...
Pin 7- 33vac
...

I hope you really mean 2.9vac and 3.3vac for these pins. The nominal filament voltage is 6.3vac, but when measuring to ground, you'll see half of that at each pin, or 3.15vac. Each would be within tolerance if the decimal is in the wrong place. If not, your filament voltage is way wrong.

Pin 5-  -43 vdc

That may be too much bias voltage for 6V6's. That's roughly 1/10th of screen voltage, which should bias a 6V6 near cutoff. You'll want to experiment with reducing the resistance to ground across the bias cap to reduce the negative voltage and increase tube current.

The "easy" way to do that is to use a 100-250k pot, wired as a variable resistor, across the resistor to ground. Start at the position of maximum resistance, and slowly turn the shaft while monitoring tube current. You'll eventually land on your desired current. At that point, shut off the amp. Measure resistance across the parallel combination of stock resistor and pot, get the nearest standard-value resistor, and replace the stock resistor and pot with this new resistor to ground. Bias done.

[blown240]

Thanks!  Ya I left the decimal point out.  Sorry.  Its odd.  I had the bias set at exactly -35 a couple days ago.   The only thing that has changed is the resistor on the center tap of the PT to bring voltage down.

I was just going over the schematic and layout for the 10th time and I found something I missed.  There is a jumper from pin 3 to pin 8 on the 3rd 12ax7.  Well I missed that jumper!  

Putting the jumper in gives me lots of output, but also shows that there are some other BIG problems, like heater voltage really being that high.  At least there is some progress being made.  

[stratele52]

The only "easy" way to reduce   6V6 plate voltage is with some zener diodes in series with the center tap of your power transformer.

If you put bigger resistor in power supply you will have too much sag.

[blown240]

Would I just use a couple zener diodes in series on the center tap?  What can too much sag do?