Cathod Bias Cap's "Volt Rating" in Power Section

[HiWaTTage]

Here's my question - where do y'all find 2200 uf caps?

[zaphod_phil]

2200uF caps at around 25V or 35V can be found just about anywhere that sells electronics components. They get used a lot in SS stuff.

[loyjdockery]

I know I should try to build it and try for myself, but I'd like to ask anyway. How much of a difference in sound is there between big cap cathode bias, normal cathode bias and fixed bias? For example, is it easy to tell which is which if you do an A/B comparison between big cap cathode bias and fixed bias?

And I was also wondering how much the bias voltage swings with normal cathode bias? Zaphod mentioned zooming up and down, so I assume it's reasonably large

[zaphod_phil]

The "large" is a relative thing. It's in comparison with an amp's idle cathode bias voltage level and affects different kinds of tubes to different extents, depending on their gain. Paul Ruby measured a 15V bias shift in one of his 18W amps when operating flat out, with a steady 200Hz sine wave, which is an awful lot for an EL84. His amp, which was running at somewhat lower voltages than a typical 18W, had an idle bias voltage of 7.5V. You would need a good storage scope to see some of the faster transients when using lower value cathode caps.

I'm now coming around to the view that, given that the typical bias voltage for many 18W amps is around 11 or 12V, this suggests that 25V may be on the margin for a cathode cap, and 35V or above may be a safer bet.

[planck2]

hmmm,
rjgtr and Z has two different opinion here.

rjgtr what value caps do you use for your amp?

Maybe I stay in between and use something like 470uF with 150 Ohm Resistor.

Ultimately I most likely will end up testing it myself.

Thanks

[zaphod_phil]

Maybe I stay in between and use something like 470uF with 150 Ohm Resistor.

I suspect that won't be very satisfying from either perspective. And a lot of people are now using the huge cap trick and swear they won't go back.

Actually Richard and I are in pretty close agreement that we don't want to completely lose the compression feel of a cathode-biased amp. But you're right that you need to try them for yourself.

[rjgtr]

hmmm,
rjgtr and Z has two different opinion here.

rjgtr what value caps do you use for your amp?

Maybe I stay in between and use something like 470uF with 150 Ohm Resistor.

Ultimately I most likely will end up testing it myself.

Thanks

In my 18 watt trem I'm using a 250uf. This controls the bass a little but otherwise sounds the same. This is pretty much the value I use for all of my 18 watt type cathode biased amps, but I've also used 100uf on occasion for even more low end control. When I want a more tight sound I generally use a fixed biased scheme.

I will warn you that I haven't tried anything over 1000uf. I'm planning on trying 2200uf because Phil's has got me interestted. You get the self-biasing of Cathode bias with a slighly tighter lows and faster attack. So this might just be the icing on the cake for some players.

I think "which is better" really depends on the feel you want from the amp. Do you want to accentuate the slower softer attack, say for blues, or do you want a more responsive harder edge for rock?

I don't think phil and I disagree too much, we're just having a conversation about the effects of using each of these methods.

I think that using a 50uf 250uf and 2200uf all have their merrits, but ultimately you'll need to hear them to decide what you want. I do think most people agree that the "stock" 470uf/500uf can be too woofy with most speakers.

[planck2]

but 1000uf will not be "woofy" then?

Intuitively it sounds like it might be.

Thanks for this interesting tip.

[kingedward]

I have the dockery 18 tmb w/gain mods and a pretty bright tmb channel.The 1000uf cap on the powertubes was the ticket for me.The low E string was mush w/ the original 470uf.This mushiness was worse after i boosted the gain,even with bright mods on the tmb channel.

[rjgtr]

but 1000uf will not be "woofy" then?

Intuitively it sounds like it might be.

Thanks for this interesting tip.

Changing the PA Cathode caps is like changing the preamp cathode caps. You're changing the low end cutoff and resonance point of the amp. So using 470uf just happens to accentuate some fairly whooly frequencies. With a 1000uf you're moving the cutoff frequency. Since the roll off is gradual, you hear the 1000uf as clearer but since the resonance peak is lower, you just hear a smoother low end.

[zaphod_phil]

but 1000uf will not be "woofy" then?

Intuitively it sounds like it might be.

Thanks for this interesting tip.

No, at this value it's now past the "woofy" point and instead works by stabilizing the EL84s' bias voltage against fluctuations caused by bass notes.

I think the moral of the story is to either use a smallish cathode cap (below 100uF) and bias fairly hot with a low cathode resistor value, or use a very large cathode cap (1000uF to 2200uF) and bias cooler with a larger resistor value. In between values will usually sound woofy.

[planck2]

alright, Z
I will try 150 Ohm with 1000uF/50V.

Thanks

[richon]

hopefully you would tell us the result on your test

[melodymaker]

What about 6V6's? I'm about to convert my LiteIIb to them. Would the same values apply? Thanks!

Bob

[zaphod_phil]

Generally, the same cap values work with 6V6s. In fact this particular mod AFAIK was first used in a 6V6 amp being worked on by an R&D engineer at Marshall. He mentioned to me that he had discovered this benefit of real large cathode caps, and after trying it myself, I spread the word around.

[planck2]

Z did you ever post a sound sample? How about you rjgtr?
I just wanted to listen to the sounds.
Thanks

[zaphod_phil]

I'm afraid I don't record sound samples. I just don't have the equipment nor the time. What I build I play out. That's what it's for. Sorry about that.

[richon]

i know that preamps cathode bypass caps can be switched... and how they can be switched


but, when talking about power cathode bypass caps i guess they can be only switched when the amp is OFF... isn't it?

[loyjdockery]

I haven't really read too much of the previous discussion, but I'm going to try and answer the question you just asked.

I think you could switch the cathode caps in the power amp by putting them in parallel and have a SPST switch one of the caps (let's say A) and the power tube cathode. That way, when the switch is open the cathode cap value is just the value of cap B and when the switch is closed the cathode cap value is A + B. That seems like the least stressful way to switch cap values in the power amp.

Disclaimer: I haven't tried this before and am just taking a stab at answering your question.

[richon]

planck2 did you already tried this?