Schematic for 6CW5/EL86 amp?

[AmberB]

Does anyone have a schematic or 2 for an amp that uses 6CW5/EL-86 power tubes?
I have a few...I tried to build an amp for them but ran into problems.  I'm not sure if it was the way I wired it up, or if it was the new output transformer that I bought for the tubes.  The amp chassis has been sitting for a couple years now...

[tubeswell]

The stuff you need to know is on this data sheet


https://frank.pocnet.net/sheets/129/e/EL86.pdf

[AmberB]

Thanks!  That does help somewhat.
My RCA tube manual doesn't give me any information on OT plate loading and at what voltage.  The information that I found in the past wasn't all that helpful either. 
The output transformer that I have in circuit is a 3.3K load, so that's close to the recommended 3.5k at 250 volts on the plate.  I guess I need to dig back into this thing.
The OT tests good with the multimeter, so I'm guessing it's ok and it's something in the circuitry.

[tubeswell]

For parallel, you should halve the reflected load of a single tube, so to get maximum power for Center-bias Class A with 2 parallel EL86:



Zout = [Vp/(Pmax/Vp)]/2


[250/(12/250)]/2 = 2k6


3k2 will work but you probably want to lower Vg2 to about 190 or 200. The load line will be well below the knee of Vg=0 and so you risk excessive screen current under big signal conditions. (So you won’t get full possible output power)

[AmberB]

The output transformer that I have is a push pull transformer...

[tubeswell]

The output transformer that I have is a push pull transformer...

Sorry, I initially assumed it was PP and then something made me think you meant Parallel SE (can't recall what?) and so I changed what I was posting to SE, but go figure. Middle of the night over here when I looked at that


So in that case you want 5k2 plate-to-plate for 250V to be able to run decently in Class AB1

However if you double the speaker impedance on your PP OT, you have 6k6, which is also good enough - run the screens at 200V or so

[PRR]

> The output transformer that I have

More important for this tube: do you have 250v-300V of DC to feed them? These are not high-voltage tubes. They are highly optimized for the lowish DC you can get directly (no PT) from European wall voltages. (This can be safe for radio/TV; it is NOT safe for hands-on guitar, so we must have a PT.)

[AmberB]

> The output transformer that I have

More important for this tube: do you have 250v-300V of DC to feed them? These are not high-voltage tubes. They are highly optimized for the lowish DC you can get directly (no PT) from European wall voltages. (This can be safe for radio/TV; it is NOT safe for hands-on guitar, so we must have a PT.)

The power transformer that I was using for this build is an original Bogan from a CHB35A.  That amp used a voltage doubler to get close to the 475-500 volts that was on the primary B+.  Without the voltage doubler it puts out less than 300 volts at the first filter cap.  It's been a while since I looked at it, but I think it was around 250 or so.

I was using the PT and chassis for this build because the output transformer on it was fried, so I had to replace it with something anyway.  Also, I had been wanting to try using the EL86s because you can get a fair amount of power from a pair of them at 250 volts on the plates.

[AmberB]

So in that case you want 5k2 plate-to-plate for 250V to be able to run decently in Class AB1

However if you double the speaker impedance on your PP OT, you have 6k6, which is also good enough - run the screens at 200V or so

That may explain part of the problem that I was having, way too low of OT impedance for the setup.  The OT has 2 primary side impedances, 3.3k and 4.3k.  The secondary side has 4, 8, and 16 ohm taps.  I could use the 8 ohm tap for 4 ohms and the 16 ohm tap for 8 ohms.

The mismatch in the output probably isn't the primary cause of the problem I was having, but it could be a contributing factor...

[HotBluePlates]

... My RCA tube manual doesn't give me any information on OT plate loading and at what voltage.  ...

The designer (you) supply that by looking at the sheet to understand the tube's peak current limits/capability for the supply voltage you've chosen (and that is within the tube's limits).

PRR addressed the issue of voltage limitation.  "Ideal load" is not too critical; lots of wiggle room, and is about maximum clean output power.  If you can tolerate a little more distortion and a little less than max clean output power other loads work fine.  There's a bit of a different process for picking your Class AB load than a Class A load (though the real difference comes in other tubes able to work at much higher voltages).

... That may explain part of the problem that I was having, way too low of OT impedance for the setup. ... The mismatch in the output probably isn't the primary cause of the problem I was having, but it could be a contributing factor...

If you want to get to good understanding of power amp operation, you'd be wise to drop the whole "mismatch" notion (I've been fighting that battle on another forum).  Other-than-ideal loading simply gives more distortion, less clean output power, or both.  And the "less power" is often measurable but not (obviously) audible.

[PRR]

> Bogan from a CHB35A.  That amp used a voltage doubler to get close to the 475-500 volts that was on the primary B+.  Without the voltage doubler it puts out less than 300 volts at the first filter cap.  ....I think it was around 250 or so.

Plan shows 450V nominal, and you are not aiming for 35 Watts, so 225V or a bit more. This lies near specs for 15 Watts in either 3k or 5.5k load.

[AmberB]

I have 3 of the CHB35A amplifiers that I've worked on.  2 of them had bad output transformers when I got them, the third one got the output tube sockets changed to octals and a pair of 7591s put it it, along with some mods for guitar use.  That one works pretty well.
This one with the EL86s, not so much...
I think I need to just start on it again and see what happens.  It's probably something that I missed somewhere...
I did see a spec for the tube somewhere when I was looking at the stuff a couple years ago that talked about getting 20 watts from a pair of them at 250 volts on the plates, and 200 on the screen grids.  I've also read that the screen grid voltage can be more than 200 volts as long as it's somewhat lower than the plate voltage.  I can't say if that's true or not...

[tubeswell]

... getting 20 watts from a pair of them at 250 volts on the plates, and 200 on the screen grids.  I've also read that the screen grid voltage can be more than 200 volts as long as it's somewhat lower than the plate voltage.  I can't say if that's true or not...

EL86 was designed for SRPP directly driving a high impedance (200R) speaker running from a lowish (250) plate voltage with high tube current.
Screen current goes up to a whopping 13mA (for each tube) under the published data sheet conditions for 18W Pout from a pair in Class AB1 PP. If screen voltage is too high, then screen current will shoot right up under big signal conditions, and risks screen meltdown.

[PRR]

> I did see a spec for the tube somewhere ..... I've also read that .... somewhat lower ....  I can't say if that's true or not...

Hardly anybody knows more than the factory.

https://frank.pocnet.net/sheets/129/e/EL86.pdf
https://frank.pocnet.net/sheets/010/e/EL86.pdf

[DummyLoad]

I have 3 of the CHB35A amplifiers that I've worked on.  2 of them had bad output transformers when I got them, the third one got the output tube sockets changed to octals and a pair of 7591s put it it, along with some mods for guitar use.  That one works pretty well.
This one with the EL86s, not so much...
I think I need to just start on it again and see what happens.  It's probably something that I missed somewhere...
I did see a spec for the tube somewhere when I was looking at the stuff a couple years ago that talked about getting 20 watts from a pair of them at 250 volts on the plates, and 200 on the screen grids.  I've also read that the screen grid voltage can be more than 200 volts as long as it's somewhat lower than the plate voltage.  I can't say if that's true or not...

GE published data on the 6CW5 - US EL86 equivalent. in reality most of the US marked are euro tubes. sylvania seemed to be the exception.

25W claimed in class AB1, fixed bias (-18.5V to g1) with 250V Vp, 200V VG2, 3K Ra-a load. see page 2 of the attached datasheet.

see attached. i designed my dime store 5 amp based on the GE datasheet.

--pete

[AmberB]

I believe that the GE data sheet is probably where I got the information for the 25 watts when I was first trying to build the amp several years ago.  It's probably also why I chose the 3.3k OT load.
Apparently I didn't download that info to my computer then, or else it didn't get transferred to the computer that I'm using now...
Thanks for the link!