AC15 clone cathode resistor question

[Big_Mike]

I am working on building an AC15 based on the Hoffman AC30 layout.  I am only using two EL84s as the output section.  I have AC15 power transformer, choke, and output transformer.  I think the only other modification to the layout will need to be a change in the cathode resistor value.

On the Hoffman AC30 layout, it shows 100 ohm 3 watt cathode resistors for each of the four EL84s.  The question is this - for a two EL84 version, what should the cathode resistor be?  I have seen some people say maybe 130 ohms, while others have said for a two EL84 output version of an AC30, you should double the cathode resistor value, which would be 200 ohms.  Does anyone have a suggestion for a cathode resistor value for a 2 EL84 version of the Hoffman AC30?  And are there any other modifications to the layout that needs to be done for a 2 EL84 version of the Hoffman AC30?  Thanks.

[Adrien]

Those four 100ohm/3 watt resistors are the screen resistors. Just use two of them, one for each tube.

The cathode resistor is the 50ohm/10 watt to the right. You will need to double that to 100ohm for the AC15, but I would have a 120 and 150 ohm on hand as well in case the bias runs too hot with the 100.

[SEL49]

On the Hoffman AC30 layout, it shows 100 ohm 3 watt cathode resistors for each of the four EL84s.

Not so. Those four 100Ω resistors are the screen resistors. Leave them be. The cathode resistor is 50Ω/10W. This is the only resistor that needs to be doubled when using only two EL84s. But 100Ω would run the EL84s very hot. Better to go with 130Ω which is still very hot. I prefer to use 150Ω.

EDIT... Oops! Sorry for just parroting what Adrien said.

[Big_Mike]

Thanks, sorry I read that incorrectly on the layout!  I see Sluckey on his AC15 used a 130 ohm resistor in that spot, so I might try that out.  I don't want it to run super hot.

Does the electrolytic cap in parallel with the cathode resistor have to be different on an AC15 versus the Hoffman AC30 layout?  On the AC30, it is 220uf versus Sluckey used a 50uf.  I didn't know if that component also needed to be changed along with the cathode resistor.

[Adrien]

IME it will make a little bit of difference to the bass output of the amp (220=more and 50=a bit less) but it’s more a matter of taste than anything. It does not strictly need to be changed. If you plan to play it overdriven I would think about changing it to 50.

[kagliostro]

Values of the cathode resistor on the AC15 vary from 100R, 120R and 130R, give a look to the schematics on the library of the forum

If you want to arrange someting of different, look to the cathode switch on the AC30CC2, on that amp the value can be switched betwen 50R (Hot - really around 53R) and 82R (Warm), for an AC15 values will be 100R (for Hot - really around 106R) and 164R (for Warm)

Franco

[Merlin]

FWIW the single shared cathode resistor in the Vox has always been a dodgy design choice. In the AC15 is was originally 130 ohms, taken straight from the 1950s datasheet. But a shared bias resistor relies on matched tubes, and if one dies or is pulled from its socket, the other one will cook. It's much better to use individual cathode resistors on each tube (twice the value, e.g. 270 ohms works fine for AC15 and AC30) and your tubes will last longer. Even the datasheet was updated to recommend this in the 1960s.

[pdf64]

FWIW the single shared cathode resistor in the Vox has always been a dodgy design choice. In the AC15 is was originally 130 ohms, taken straight from the 1950s datasheet. But a shared bias resistor relies on matched tubes, and if one dies or is pulled from its socket, the other one will cook. It's much better to use individual cathode resistors on each tube (twice the value, e.g. 270 ohms works fine for AC15 and AC30) and your tubes will last longer. Even the datasheet was updated to recommend this in the 1960s.

I like to make amps resilient to valve failure, and using a dedicated cathode bias resistor per cathode seems like it should help with that.
But I'm a bit conflicted, because on your push pull webpage, you mention that a shared cathode bias resistor can help to promote second harmonic distortion.
Could you expand on how that 2nd harmonic thing works, and whether there's a way to achieve that with a cathode resistor per cathode?
eg perhaps AC coupling the cathodes with a suitable cap, as per some late 60s Fenders with mixed bias?
https://www.valvewizard.co.uk/pp.html
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab568.pdf

[kagliostro]

To avoid the loss of the generation of the 2nd armonic, what about to use a resistor for each catode and connect it to ground via a single (bypassed) resistor ?

Say a common resistor of about the 50% (or it will be better a less % ?) of the necessary R

Not the same 2nd armonic like with shared resistor but some of it will be saved

EDIT: And what about to have a fuse for each catode then connect it to a single (bypassed) resistor ?

Will this maintain the 2nd armonic of the shared resistor and be safe if one tube melts ?

Franco

[Merlin]

Could you expand on how that 2nd harmonic thing works, and whether there's a way to achieve that with a cathode resistor per cathode?

Ugh that page is so old, I should really rewrite it. A PP stage will cancel even harmonics when both sides of the circuit are perfectly matched. If it is unbalanced, even harmonics will be less suppressed.A single shared bias resistor cannot compensate for tube mismatch, whereas individual bias resistors help to even-out mismatch between tubes a bit. Therefore a single shared bias resistor will produce a more unbalanced output stage than individual bias resistors, assuming the same tubes.It's a moot point really; there is so much distortion in a guitar amp that fussing about fractions of a percent of this or that harmonic in the output stage are irrelevant.