# Push Pull Biasing (shared resistor)

P Rails
July 20th, 2012, 03:43 PM
I just built a Push-Pull, 2 X EL84 build. This is my first PP build and the schematic I followed doesn't show any cathode voltages. My question is:

Top of cathode = 5v
EL84 (1) cathode =5v
EL84 (2) cathode = 5v

would this be (1)5v + (2)5V = 10v total

I am using the Weber Bias Calculator to figure dissipation

kleuck
July 20th, 2012, 05:45 PM
No, the resistor is shared, the voltage is the same, and pretty low in your case.

printer2
July 20th, 2012, 06:21 PM
This should help

http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/EL84/8BQ5.pdf

I would assume you are running Class AB not A. On the sheet it gives a resistor value of 130 ohms for the given voltage. The bias voltage on the tubes would be the total current through the tube at zero signal (72mA + 8mA) multiplied by the resistance of 130 ohms = 10.4V. With a higher or lower voltage you would have less or more current.

P Rails
July 20th, 2012, 06:54 PM
The schematic I am following http://www.ax84.com/static/daisycutter/18w_daisycutter_100422.jpg calls for higher voltage than the PT I have on hand. My PT puts out 310v at secondaries and needs to be full wave rectified. The schematic shows a shared 150R/5w and I figured that with my PT voltage I will eventually lower it to 120 to 130R but I wired in the 150R to be on the safe side.

Plugging in my current numbers (5v cathode) in the weber bias calculator I am definitely at a cold bias. It reflects in the sound too because from 3/4 to full volume it doesn't get any louder and gets mushy.

printer2
July 20th, 2012, 08:25 PM
At 310V you will have to drop some voltage to get down to 340-365V on the schematic.

P Rails
July 21st, 2012, 08:28 AM
As in 310 X 1.4 (FWBR) = 434 before PSU. Readings at PA plates/0T are 365

printer2
July 21st, 2012, 08:56 AM
Then you are good. I would start with a 150 ohm resistor and if I had to bring up the current I would parallel a higher value resistor across it.

tubeswell
July 21st, 2012, 09:20 AM
Firstly a word on Ohms Law E=IxR. Out of this we can see that, the higher the resistance, the less current will flow for a given voltage. If you have a 150R resistor that is dropping 5V, then that is 33mA flowing through that resistor. If that is shared amongst 2 tubes, that is 16.5mA per tube. The less tube current, the higher the plate voltage(s) will idle at, and the more sensitive the stage will be to input swing on the signal grid. However, the cathode voltage and resistance alone doesn't tell you whether the tube is biased optimally without also knowing what the plate and screen voltages are. With a cathode-biased stage, you want to make sure that the idle voltage doesn't exceed 100% of the maximum recommended plate dissipation (even in PP).

The maximum rated plate dissipation for an EL84 is 12W. So if the plate voltage is around 250V, with say -7V of bias voltage, that is 243V plate-to-cathode @ (say) 48mA = 11.6W, which is in the ballpark. You will get 48mA at 7V with 145R (150R is the closest standard value, which should put you at 7.2V with 48mA tube current). For 2 tubes that would be 2 x 48mA, so the voltage drop across a shared 150R would be 14.4V.

Now if your plate voltage is 365, and you're dropping 5V across a shared 150R for a pair of EL84s, that is a plate dissipation of 6W, which is wayyy to low. Have you got any screen voltage? (If the screen voltage looks normal, then try lowering the cathode resistance to 130R and seeing where that gets you)

P Rails
July 21st, 2012, 09:35 AM
Then you are good. I would start with a 150 ohm resistor and if I had to bring up the current I would parallel a higher value resistor across it.

Sounds like a plan. What I probably should do is get a bias pot and a couple 100R's to make it adjustable. The amp is scratchy/noisy and very loud with a 150R in it, from my limited experience that's usually a sign the tubes are running hot, but there's no red plating. I am using Russian 6p15p-EV (SV83 equiv) branched PSU to keep screen voltage at 195. These tubes are as tough as Russian winters and can probably withstand higher than 365 plates voltage without breaking a sweat, so read plating may not determine actual sign of excessive dissipation.

I am not a fan of a EL84 in SE single output, but in push pull digging the clarity and treble. Unlike the schematic I haven't added a zobel network, but after hearing it I now know it won't dull the sound too much.

kleuck
July 21st, 2012, 10:08 AM
Sounds like a plan. What I probably should do is get a bias pot and a couple 100R's to make it adjustable. The amp is scratchy/noisy and very loud with a 150R in it, from my limited experience that's usually a sign the tubes are running hot, but there's no red plating. I am using Russian 6p15p-EV (SV83 equiv) branched PSU to keep screen voltage at 195. These tubes are as tough as Russian winters and can probably withstand higher than 365 plates voltage without breaking a sweat, so read plating may not determine actual sign of excessive dissipation.

I am not a fan of a EL84 in SE single output, but in push pull digging the clarity and treble. Unlike the schematic I haven't added a zobel network, but after hearing it I now know it won't dull the sound too much.
6P14P-EV can stand 14 watts of anode dissipation if i remember well the datasheets, more than the E84L or 7819 can.
But 6P15P are meant for lower dissipation, 9 watts i think, and more important a lower screen voltage.
You will fry them.

P Rails
July 21st, 2012, 10:35 AM
6P14P-EV can stand 14 watts of anode dissipation if i remember well the datasheets, more than the E84L or 7819 can.
But 6P15P are meant for lower dissipation, 9 watts i think, and more important a lower screen voltage.
You will fry them.

The screens are at 195v. At first start-up they were at 265v, so I branched and dropped more PSU voltage coupled with higher value screen resistors. These tubes handled 265v screen and didn't break like Rocky against Ivan in the first match. I do plan to get some JJ El84's eventually though and rearrange the PSU chain accordingly.

P Rails
July 21st, 2012, 11:00 AM
Firstly a word on Ohms Law E=IxR. Out of this we can see that, the higher the resistance, the less current will flow for a given voltage. If you have a 150R resistor that is dropping 5V, then that is 33mA flowing through that resistor. If that is shared amongst 2 tubes, that is 16.5mA per tube. The less tube current, the higher the plate voltage(s) will idle at, and the more sensitive the stage will be to input swing on the signal grid. However, the cathode voltage and resistance alone doesn't tell you whether the tube is biased optimally without also knowing what the plate and screen voltages are. With a cathode-biased stage, you want to make sure that the idle voltage doesn't exceed 100% of the maximum recommended plate dissipation (even in PP).

The maximum rated plate dissipation for an EL84 is 12W. So if the plate voltage is around 250V, with say -7V of bias voltage, that is 243V plate-to-cathode @ (say) 48mA = 11.6W, which is in the ballpark. You will get 48mA at 7V with 145R (150R is the closest standard value, which should put you at 7.2V with 48mA tube current). For 2 tubes that would be 2 x 48mA, so the voltage drop across a shared 150R would be 14.4V.

Now if your plate voltage is 365, and you're dropping 5V across a shared 150R for a pair of EL84s, that is a plate dissipation of 6W, which is wayyy to low. Have you got any screen voltage? (If the screen voltage looks normal, then try lowering the cathode resistance to 130R and seeing where that gets you)

Thanks Tubeswell I will try it, and that was my initial thinking. Maybe the scratchiness is coming from a dirty socket or one of the output tubes isn't 100%.

It's using a Triad Magnetics PT (China) and a un-branded OT from Antique Electronic Supply. This is a rebuild from a previous 5e3 type amp I purchased a few years ago from a builder. Couldn't pass on the \$175 price fully built with tubes included, but it was built like this (is this) http://www.lh-electric.net/projects/17gita.html, and sucks for cabinet mounting. The amp sounds punchy, but not necessarily in a good way. Even in it's original 5e3 build it sounded so, I am using a 33uf reservoir cap, but I wouldn't say it's a stiff supply 33uf OT/ plates, 22uF screens, 22uF preamp,10uF preamp.

I know a lot of boutique builders tout Mercury Magnetics PT and OT's as a selling point,in your experience would you say my "cheaper" PT and OT may cause a poorer sound.

P Rails
July 22nd, 2012, 10:27 AM
I swapped my shared bias cathode resistor from 150R to 130R and the cathode voltage reading went from 5 to 4.
According to this link http://www.tubes.ru/techinfo/HiFiAudio/6p15p.html the 6p15p-ev calls for a 70R cathode resistor.

I think my best course of action is swap the russian sv83 equivs for some JJ-EL84's. I've read a few post that doing so solves issues in amps that use 6p15p's. So, now my dilemma is do I order on-line and wait for shipping or drive 1.5 hours roundtrip to purchase some at Watts Tube Audio.

tubeswell
July 22nd, 2012, 10:57 AM
So you are using 6P15P? Nothing wrong with those, but you should run them accordingly (not as an EL84 or a 6P14P).

P Rails
July 22nd, 2012, 01:12 PM
[QUOTE=tubeswell;4316191]So you are using 6P15P? Nothing wrong with those, but you should run them accordingly (not as an EL84 or a 6P14P).[/QUOTE

I am using 6p15p-EV (extended life) and I know they are SV83 equivalents.
I have the screens at 180v. I used them once before in a P1 type amp I built a few years back, and recall having issues that were straightened out by using a 6p14p russian (EL84 equivalent).
I only used the 6p15p's cause I had them on hand and figured if the don't work or sound right I'll spend a few beans and get some EL84's.

I am getting squealing near full volume and crackles with the 6p15p's. This may be due to biasing or may be due to bad output tubes or other issues. I chop-sticked it and no change. I have a 22k grid stopper on first preamp V1a so I'll start by upping that to a higher value.
I will say that Randy's preamps design sounds damn good (up until my oscillations) notes are tight and chimey.