EF86 is giving me grief...

[timjimbaker]

Hey guys... I modded a couple of modules from an old roberts 770 tube tape machine as per Rod C.'s instructions on beyondsanityproductions.com (Thanks Rod!). The first preamp works great. I am having a problem with the mic preamp section based around the EF86/6267 pentode in the second preamp...

http://www.beyondsanityproductions.com/misc/Akai%20M7%20Right%20Schematic%20_MOD_BorisModel.jpg

First, why is C3 connecting the screen grid to the cathode resistor(R6) and bypass cap (C2)? Should C3 be passing a voltage through to the cathode? My working pre is reading about 30VDC at the junction between C2 and C3 as opposed to 1.3VDC on the non-working unit.  I am using new tubes, I've tried replacing components and i can't seem to get the second unit to pass signal. I am starting to think that maybe a small voltage (roughly 1.3VDC) is normal for the cathode... Any help explaining what's supposed to be happening in this circuit would be really appreciated. Best regards, Tim

 

[ruffrecords]

The screen grid needs to be at the same ac potential as the cathode which is why C3 is connected from the screen grid to the cathode.

The cathode voltage should not be at 30V. C3 is supposed to block dc not pass it. If you have 30V on the cathode I suspect C3 is leaky and should be changed. Make sure it is rated at a high enough voltage.

About 1.3V is about right for the cathode voltage in this circuit.

Cheers

Ian

 

[PRR]

> I am starting to think that maybe a small voltage (roughly 1.3VDC) is normal for the cathode...

Grid-cathode voltage is "an input"; plate-cathode voltage is "an output". In a HIGH-gain circuit the input is much-much smaller than the output. In a tube the output voltage is typically a hundred volts or so. "Much-much smaller" is probably closer to 1V than to 30V.

Also: study tube sales-sheets. The Philips EF86 sheet is exceptionally good, it has pages of suggested conditions.

http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EF86.pdf

You want "Pentode" with 250K plate, 1Meg Screen, 2K cathode. Top of page 3 (attached) shows 220K-1Meg-2.2K..... close enough for fault-finding. We don't know(?) the Roberts supply voltage, but I bet it is in the 200V-250V range.

Cathode current is given as 0.75mA or 0.9mA. In your 2K this gives 1.5V to 1.8V across the cathode resistor. You can also compute the approximate drop in the 250K plate resistor. (Plate current is a little less than cathode current.)

 

[timjimbaker]

PRR and Ruffrecords, sorry for my late reply. It is quite inconsiderate not to take the time for a response when you guys, who I have a lot of respect for, took time out of your busy days to help me.... I've been quite busy in the studio trying to wrap up a few projects. As well, the birth of my third child (1st daughter) has things running full tilt.

As luck would have it, my "working" 770 has the wrong voltages in the EF86 circuit and the one that won't pass signal looks ok...  I need a bit of time to study this information and get both pres working nicely... I'll post an update as soon as I do so.

PRR...  I don't understand why the cathode is considered an input when the + side of the mic transformer secondary is connected to the control grid... I need to learn more tube theory.

I appreciate both of your help and expertise. Thank you.

 

[Sredna]

PRR...  I don't understand why the cathode is considered an input when the + side of the mic transformer secondary is connected to the control grid... I need to learn more tube theory.

He wrote "Grid-cathode voltage" which means the voltage between grid and cathode (ground).

Input is between grid and cathode (ground), output is between plate and cathode (ground).

 

[timjimbaker]

Sredna... Thanks, apparently I need to work on my reading skills as well... "Grid"-cathode, that makes more sense. Best regards...

 

[AZ999]

> I am starting to think that maybe a small voltage (roughly 1.3VDC) is normal for the cathode...

Grid-cathode voltage is "an input"; plate-cathode voltage is "an output". In a HIGH-gain circuit the input is much-much smaller than the output. In a tube the output voltage is typically a hundred volts or so. "Much-much smaller" is probably closer to 1V than to 30V.

Also: study tube sales-sheets. The Philips EF86 sheet is exceptionally good, it has pages of suggested conditions.

http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EF86.pdf

You want "Pentode" with 250K plate, 1Meg Screen, 2K cathode. Top of page 3 (attached) shows 220K-1Meg-2.2K..... close enough for fault-finding. We don't know(?) the Roberts supply voltage, but I bet it is in the 200V-250V range.

Cathode current is given as 0.75mA or 0.9mA. In your 2K this gives 1.5V to 1.8V across the cathode resistor. You can also compute the approximate drop in the 250K plate resistor. (Plate current is a little less than cathode current.)

Hi,

i have a question about voltage rating of caps in circuits similar to this; on most schematics C3 would have 400VDC max rating or more when there are around 100V DC on G2. I don't remember exact meassured AC on G2, but i think it should be even lower than 100V. Anode has little higher voltages, maybe half HT (120V DC) with less than 100Vpp AC. And coupling cap (here marked as .22) would tipicaly have 400-630VDC rating. It is similar with triodes, max DC rating of caps is often like 2-3 times higher than meassured DC or peak AC voltages.
I know cap's max voltage should be rated at about 1/3 higher than actual voltage (and max AC is lower than max DC rating), so i'm wondering why schematics show them at much higher voltage ratings. Can you please explain what i don't understand here? To me it seems like expensive overkill.

 

[ruffrecords]

Think about what happens when the circuit is first turned on and the heater is cold. Or what happens if the heater fails or someone pulls the tube out.

Basically, as well as normal operating conditions, you have to think about worst case ones too.

Cheers

Ian

 

[lassoharp]

> I am starting to think that maybe a small voltage (roughly 1.3VDC) is normal for the cathode...

Grid-cathode voltage is "an input"; plate-cathode voltage is "an output". In a HIGH-gain circuit the input is much-much smaller than the output. In a tube the output voltage is typically a hundred volts or so. "Much-much smaller" is probably closer to 1V than to 30V.

Also: study tube sales-sheets. The Philips EF86 sheet is exceptionally good, it has pages of suggested conditions.

http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EF86.pdf

You want "Pentode" with 250K plate, 1Meg Screen, 2K cathode. Top of page 3 (attached) shows 220K-1Meg-2.2K..... close enough for fault-finding. We don't know(?) the Roberts supply voltage, but I bet it is in the 200V-250V range.

Cathode current is given as 0.75mA or 0.9mA. In your 2K this gives 1.5V to 1.8V across the cathode resistor. You can also compute the approximate drop in the 250K plate resistor. (Plate current is a little less than cathode current.)

Hi,

i have a question about voltage rating of caps in circuits similar to this; on most schematics C3 would have 400VDC max rating or more when there are around 100V DC on G2. I don't remember exact meassured AC on G2, but i think it should be even lower than 100V. Anode has little higher voltages, maybe half HT (120V DC) with less than 100Vpp AC. And coupling cap (here marked as .22) would tipicaly have 400-630VDC rating. It is similar with triodes, max DC rating of caps is often like 2-3 times higher than meassured DC or peak AC voltages.
I know cap's max voltage should be rated at about 1/3 higher than actual voltage (and max AC is lower than max DC rating), so i'm wondering why schematics show them at much higher voltage ratings. Can you please explain what i don't understand here? To me it seems like expensive overkill.

As Ian said, you want to rate the critical caps based on what happens at cold conditions.  Assume they will see the full supply voltage at turn on.  I believe the Roberts PT is 250-0-250.  Peak voltage is 1.4 times 250VDC.  Rate above that value to be on the safe side.

 

[AZ999]

Hey, thanks!
When tube is cold it doesn't conduct and voltage at the psu is at highest point because tube doesn't pull current. I know caps at the psu are working hard, i use 450V rating or more. But i find one thing interesting; Morgan Jones (Valve Amplifiers, chapter about practical regulators)  suggests to use 385V rated caps made for SMPS when secondary tr. voltages are 230V. I didn't check if this caps are really much cheaper because i still have some nice Epcos, but i might in the future. Do you agree with him or should this caps have typical 450V if placed after gretz?
I also think about using NTC thermistors for next projects, i find it surprising there are not much schems with them when it's easy to use them and should work well.
Well, i calculated what happens if tube is pulled and i get full HT at anode socket, is this correct calculation? But can't imagine what happens if tube is cold and not conducting, do i still get full HT until current rises?

 

[lassoharp]

If I'm reading the info correctly, 230 X peak = 325VDC.  Should be ok for a 350V cap.

When no current is being drawn you will generally see the full supply voltage in all branches, give or take a few volts here and there.

 

[ruffrecords]

If I'm reading the info correctly, 230 X peak = 325VDC.  Should be ok for a 350V cap.

When no current is being drawn you will generally see the full supply voltage in all branches, give or take a few volts here and there.

But don't forget about mains voltage tolerance too. In the UK our mains voltage is still 240V most of the time even though it is meant to be 'harmonised' with Europe at 230V (it is still within the allowed tolerance) which gives a peak voltage of 350V. I generally use 400V or 450V caps in my HT supplies. They seem to be commonly available voltages. Can't honestly say I have seen any 385V rated ones.

Cheers

Ian

 

[lassoharp]

We have a similar situation in my local area.  The lines at my residence are often 110 to 113V.  The main grid in the nearest city usually stays solid around 120 or up to a few volts higher.  I have to take that into account when figuring out circuit voltages and parts ratings on DIY tube builds and modifications.