liverpool 4 EL84 output impedence

[goodtone77]

does any one know what the impedence of the 4xel84 liverpool is? the trainwreck transformers from toneslut is 6.6k and 5.2k. I thought 4xel84 took 4k so do you use 5.2k or cut the 6.6k in half with the speaker combination? i am a little confused?

[Mark]

Dear Goodtone 77

It is my understanding that the Rocket's output transformer has a 4.2K primary impedance and the Liverpool uses the same transformer as the Express, which would mean it's transformer has a 6.6K primary impedance.

I haven't built a Liverpool amplifier using a 6.6K transformer, so I can't comment on whether a transformer with 4.2K or 6.6K primary impedance sounds better.

I do believe that Chris G has built such an amp and would be the best person to talk to on the matter.

[doctord02]

Moose/Toneslut (Chris Moore) has also built amps using the 6k6/5k2 OT that he sells; you can ask him as well.

The standard AC30 OT is a 4k2 impedance, and it's my understanding that what Mark sez is true - the Rocket uses 4k2 and the Liverpool and Express use the same 6k6/5k2 iron...

I'm building an AC30/Rocket like amp and I already had a 4k OT so I'm using it.

[Moose]

Just to clear this up -- the "standard" express iron is not what I sell.

The standard iron had only one primary impedance. However, some trainwrecks were 5.2K, some 6.5K, and this has been verified by folks who went to great lengths to poke around in original trainwrecks. The folks who did the research on these trannies before I stocked them preferred the 6.5K, but 5K seems to be about perfect for 4xEL84s if you extrapolate from the datasheet.

Also, 10K is a much better value than 13K or 6.5K for a pair of 6V6, and mismatching the secondary can give you this with the lower impedance primary.

We also know KF used a variety of iron winders, but that's an aside.

Anyway, with all this in mind, I decided to go dual tap so folks could decide for themselves. Heyboer was accomodating.

As for the liverpool issue, if you look at the datasheets, 4K in an AC30 was extrapolated from the reccomendations for an AB1 amp on the 6BQ5 datasheet. However, those datasheets also say 300V max and if you run the curves that 4K is only close to optimal at the 250V example in the datasheet. AC30s run 325V I think, and only 100ohms on the screens so they're a good way outside the example data. And, they are notoriously hard on tubes. Looking at the curves for 6BQ5 at over 300V, 5K actually has a little less distortion and almost the same power, and that added to the higher value screen resistors in the Liverpool should relieve some of the stress on those power tubes.

Personally, I can't decide which value I like better. My 4K expriement just finished its third national tour with no problems and after 200 shows the owner is still stoked. I am making my next amp with the trainwreck iron at 5200 primary, though, as I like it and am happy with the results there, too.

[fusionbear]

What about a quad of 6v6's. Is the 6.6k primary appropriate?

[skyboltone]

What about a quad of 6v6's. Is the 6.6k primary appropriate?

Look at the tube data sheets, tube manual. It's all in there. It depends on plate voltage, whether it's cathode or fixed bias and what the bias point is. It's not a simple question.

I've built a two hole liverpool with a 10500ohm primary transformer. The owner is stoked. Twice the 5.2K express transformer lower primary. Ginger (a KF built liverpool) was built with the 6.6K tap. To roughly approximate the tone you'ld be thinking about doubleing that with a pair of tubes. So parallel push pull you want to half the primary right? At 300 volts a pair of 6V6 (from memory) would like to see 8000 at about -16V fixed bias. At 250v maybe 10000 ohms? At 375V that's gonna drop. If you cathode bias it it goes up. So if you bias a 6V6 at -30v? 6K6 is a good guess. I think Mister Fisher called the 6V6 an american EL-84.

[fusionbear]

Thank you!

[Curbdog]

I would like to use a 16 ohm cab with an Express with 2 - 6V6s, built with TW style OT, using the 5.2K tap of the OT primary. What is the optimum selection on the impedance selector switch for the OT secondary, 4, 8, or 16 ohms? Thanks for your help.

[CaseyJones]

What about a quad of 6v6's. Is the 6.6k primary appropriate?

6V6s and EL84s "like" the same load impedance using similar voltages.

You can split hairs, look up the educated technical dissertation... then toss it all out the window.

Anything from around 4k on up to 6.6k will work. 6.6k is a good choice if you're going to use a simple "half power" switch to cut out two tubes, 6.6k is high enough for two tubes but low enough for all four. It's a good compromise value. 4k or 5k works if you switch the speaker load as well as take two tubes out of circuit.

With 4 x 6V6s it's not an AC30 derived circuit anymore, is it?

I think Mister Fisher called the 6V6 an american EL-84.

If you've ever tried mixed 6V6s and EL84s you'll find they're not even close. 6V6s are pretty mellow compared to EL84s and they require a relatively high drive level for a given output. EL84s require much less drive, big fat grid blockers tend to level the playing field slightly. Still it's not that difficult to make those skinny little EL84s walk all over 6V6s.

6V6s = easier to get tight bass

EL84s = chimey top end without equal

6V6 = beam power tube

EL84 = power pentode

[skyboltone]

If you've ever tried mixed 6V6s and EL84s you'll find they're not even close. 6V6s are pretty mellow compared to EL84s and they require a relatively high drive level for a given output. EL84s require much less drive, big fat grid blockers tend to level the playing field slightly. Still it's not that difficult to make those skinny little EL84s walk all over 6V6s.

6V6s = easier to get tight bass

EL84s = chimey top end without equal

6V6 = beam power tube

EL84 = power pentode

I like it. Thanks.