5E3 bias

[Leevi]

I got a new 5E3 wired and I'm wondering the big cathode current
which is 48mA/tube. The amp is one to one copy of the original 5E3.
The plate voltage is 425v and cathode resistor 250 Ohm. According
to Weber bias calculator the tubes are running hot.

http://www.webervst.com/tubes1/calcbias.htm

/Leevi

[phsyconoodler]

Well to start with a 5E3 should be around 350v-380v,no more.
 And it's cathode biased so it depends on what the numbers are.
What is the voltage across the cathode resistor and the actual plate voltage from cathode to plate?

 425v!!!! I bet it is running WAY hot.

[Leevi]

Voltage between plate and cathode is 395v and over the cathode 24v (common resistor for both tubes).
I'm using Hammond PT (291 BEX) which offers 330-0-330 secondary and which is recommended
by Hammond to be used as a replacement in Deluxe. That means you will get the B+ on more than 400v level.
/Leevi

[Fresh_Start]

Are you using a new 5Y3-GT rectifier tube?  If so, you need to know that they aren't built the same way as a real 5Y3-GT and do not drop as much voltage.  In a Tweed Princeton (5F2-A), my voltage dropped by a bit more than 25 volts when I used a real, NOS 5Y3-GT.  Not cheap, but not horrible either in terms of cost.  Add in higher wall voltages today and you get excessive B+ voltage.

Once you get a real 5Y3 rectifier tube in there, tell us what the power tube plate voltage is.  

330 volts x 1.20 = 396
(rough guestimate of effect of real 5Y3)

Knowing how much more voltage you need to drop will help us figure out what to do next.

I forgot to ask - Does the amp make any noise at all?  I don't recommend playing it right now because those poor 6V6s are smokin' hot.  Just wondering if something else isn't drawing current which should be.  Heater voltages all OK?  12AY7 in V1 spot?

BTW JJ 6V6s supposedly can handle signficantly higher plate voltages than most.  I wonder if some 5E3 clones aren't counting on that.  The Trinity Amps schematic actually specifies a 350-0-350 HV secondary (with same 250 ohm shared cathode resistor)

Cheers,

Chip

[Leevi]

The amp is one to one copy of the original 5E3

Actually it's not one to one copy because there is an SS rectifier inside which probably increases
the voltage by some ten volts. When the amp is warm the plate voltage drops to 415v.

The amp is not noisy and the sound is excellent.

Heater voltages all OK?  12AY7 in V1 spot

Yes


/Leevi

[phsyconoodler]

The 291BEX is for the Brown Deluxe and Deluxe Reverb.The one for the 'Tweed Deluxe' is one up from that.

But......330v x 1.09 for an NOS 5Y3 only brings you to 359v. So what rectifier are you using and what wall voltage?Hmm...European voltage,all bets are off there.
  You may have to use a Zener diode to drop the voltage to a more acceptable level.

With a diode rectifier you will get 330 x 1.414 = 466.6v  and with a 5AR4  300v x 1.3= 429v  Some Sovtek and New production 5Y3's are almost as much as a 5AR4.Get an NOS 5Y3 if you can.

[Leevi]

The 291BEX is for the Brown Deluxe and Deluxe Reverb.The one for the 'Tweed Deluxe' is one up from that.

Yes, I know the 290BEX would have been better but it was sold out. But the voltage is only 10 volts higer in the 291BEX.
Phsyconoodler, please read my previous post. The root cause is the SS rectifier. I should either change it to tube or
change bigger resistor to the cathode. I don't want to use zener which of course could be one solution. Some volts can be dropped by adding a power resistor after the rectifier.
/Leevi

[phsyconoodler]

I posted while those other guys answered.

You could up the cathode resistor and the screen resistor.

[Leevi]

I changed the cathode to 480 Ohm. The current per tube is now 30mA which is still
hot but acceptable.
/Leevi

[tubeswell]

IMHO it'll sound more like a 5E3 if you can get the plates to around 350, and keeping the 250R cathode resistor in.

[Fresh_Start]

IMHO it'll sound more like a 5E3 if you can get the plates to around 350, and keeping the 250R cathode resistor in.

+1 on that.  I've experimented a lot with plate voltages on a Tweed Princeton, and even 25 volts can make a difference in how the amp sounds depending on your tubes and the absolute voltage level.

The tube rectifier also contributes to the feel of the amp - more compression.  If you want it to sound like a 5E3, I honestly think a 5Y3 rectifier tube is essential.  YMMV.

Chip

[Leevi]

If you want it to sound like a 5E3, I honestly think a 5Y3 rectifier tube is essential.

I agree with you, but the idea with the SS rectifier was to find a little bit different and cleaner sound for the amp. There is place reserved for a 5Y3 in the chassis if we don't get this work.
/Leevi

[Fresh_Start]

If you want it to sound like a 5E3, I honestly think a 5Y3 rectifier tube is essential.

I agree with you, but the idea with the SS rectifier was to find a little bit different and cleaner sound for the amp. There is place reserved for a 5Y3 in the chassis if we don't get this work.
/Leevi

Do you have a Standby switch on this amp?  If so, there's a way to use a DP/DT On-Off-On switch to go from diode rectification to tube rectification.  I've also seen a direct tube-diode rectification switch but worry about switching noise and the jolt on the whole power rail.

Respectfully,

Chip

[Leevi]

Yes, this is one option of course.
One question related to the circuit: It seems that the rectifiers are parallel
if the SS side is selected. Is that the idea or should the tube rectifier be disconnected
when SS is connected?
/Leevi

[phsyconoodler]

The tube rectifier does not need to be disconnected when using the DPDT switch.It simply idles when not completing the circuit.

I have built a couple of higher voltage 5E3's and I like the extra punchiness for some stuff.I also lowered the screen voltage to keep the tubes alive longer.

[Leevi]

Second question related to the circuit:
How do you fix the bias when switching from one type rectifier to another?
There can be a huge difference in the B+ which means the cathode resistors should be switched too?
/Leevi

[tubeswell]

Second question related to the circuit:
How do you fix the bias when switching from one type rectifier to another?
There can be a huge difference in the B+ which means the cathode resistors should be switched too?
/Leevi

Remember that a 5E3 output stage is cathode-biased, so you don't need to worry about changes in B+ voltage (to a point) because cathode-biased stages are 'auto-biasing'.

[phsyconoodler]

It becomes a 'compromise' bias that way.You have to bias colder for the tube so it doesn't fry them when using diodes.Or try using a large resistor that's mounted solidly to the chassis and has heat transfer paste on the back.It does work.I use the finned aluminum jacketed resistors and have not had any issues.
  That way you won't have to use a tube.Simply switch the resistor in and out of the circuit.The only drawback is where to put it in a tight chassis.I have put components on the outside of chassis on occasion as long as it gets well protected from hands.

[Leevi]

Remember that a 5E3 output stage is cathode-biased, so you don't need to worry about changes in B+ voltage (to a point) because cathode-biased stages are 'auto-biasing'.

I would say that if you increase the plate voltage the cathode voltage increases as well which means that
the cathode current is higher.
/Leevi

[sluckey]

I would say that if you increase the plate voltage the cathode voltage increases as well which means that
the cathode current is higher.

That's true. And the higher cathode current causes a higher voltage drop across the cathode resistor. The plate voltage should be measured between plate and cathode, so now the plate voltage is a little less. But also, that higher voltage on the cathode will cause the tube to conduct less because the grid/cathode bias voltage has increased. Everything kinda slugs it out until the tube reaches a stable static operation. That's why the cathode bias circuit is also called self bias or maybe auto bias.

Using diodes versus a tube rectifier will cause the B+ to increase, but the effect on a cathode biased output tube will not be as great as you may first think. Fixed bias would be a different story.

 

[Leevi]

But you have to keep the cathode current under the specified maximum limit?
/Leevi

[tubeswell]

But you have to keep the cathode current under the specified maximum limit?
/Leevi

But as more current flows through rk, the voltage drop across rk increases (E = I x R), which amounts to an increase the bias (-ve grid-to-cathode) voltage. So your increase in tube current doesn't result in an increase in plate dissipation in the same way as it would if you had the same amount of tube current increase in a fixed-bias stage.

i.e.: e.g. I have measured changes on a cathode-biased SE 6V6 with the screen sitting at ~60V below the plate and with a 470R cathode resistor. At 340 on the plate the cathode voltage is around 17.7 (37.6mA tube current, minus about 3mA for the screen = 34.6mA plate current, x 322Va-k =  11.1W). When I increase the B+ so that the plate is then at 406, the cathode voltage is now around 19.8V, which is 42.1mA tube current minus in this case 5.3mA (measured) screen current = 36.8mA x 390Va-k = 14.4W, which isn't going to kill a decent 6V6 biased in Class A, because that average plate current is going to stay the same.  No matter how much voltage swing goes though the plate, the tube is going to stay at 14.4W

Whereas if you had a fixed bias 6V6 stage at 406 and 36.8mA plate current that's 14.9W at idle!, and when you hit it with a big signal, the plate dissipation is going to jump to around 19W, which is not going to do your 6V6 much good running under that condition for very long.