diytube.com

Hey guys,

I am finishing purchasing all the parts for a stereo Poseidon. I'm building the Mark III version in the beginning of the manual. As I was reading through the manual, I noticed that the letters on page one say EL34, but the other pages say KT88. Can you use EL34 with this configuration. I was under the assumption I would have to use KT88, KT90, and 6550s right?

Thanks,

Blair

Hi Blair -

The amp is intended for 6550's, KT-88's and KT-90's.

I built mine with the ElectroHarmonix KT-90s and it was great.

... tom

Hi Blair,

Any amp that's designed to work with 6550s will work with EL34s; you just have to set the bias for a cathode current of somewhere between 45 and 60 mA.

The reverse, however, is not always true. Some manufacturers will skimp on the bias and power supplies so there's not enough oomph to run 6550s. That's not the case with the poseidon, as you can make the power supplies as beefy as you want by selecting the appropriate transformer.

Hey Eric,

Since I am using a 550ma 800vct tranny, there should be plenty of oomph....So you are saying I can buy a quad of cheap el34s to test the amp out before investing in a good quad of 6550s or kt88s? I can get a cheap quad of el34s for 50 bucks. The reason I want to test it cheaply is that I changed a few things in order to use a different tranny and SS rectification. Will the bias pots fluctuate that much?

Thanks Eric,

Blair

Hi Blair,

Since the maximum ratings, excepting plate dissipation, for an EL34 actually exceed those of the 6550, you can drop 'em in with no circuit mods. Just make sure pins 1 and 8 are connected together at the socket. The bias voltage is already there--enough to totally cut off an EL34--so that's not a problem, either.

I ran original Svetlana EL34s in my 50-50 watt amps for years, because they sounded better than the Russian 6550s and KT88s available. When I got talked into trying the JJ KT88s by Bud Wyatt, there was no looking back.

The Posie driver boards can be set up for either EL34 (MKIV ) or 6550/KT88 (MKIII) operation, there are a few resistor differences between the two.

I built mine up as the MKIV version, with the intention of driving PPP EL84s. All I need is some "free" time... Yellow_Light_Colorz_PDT_02

/ed B in NH

I only had 1 pair of Sovtek KT88 and 1 pair of Sylvania 6550's for my MK-3's, so I stuck in some Electro- Harmonix 6CA7's. The E-H tubes are very good, but when I find another pair of Sylvania 6550's, I'll be happy!

FWIW, the large 6CA7 biases about the same as a 6550/ KT88, and the innards look real similar. The large 6CA7 and the skinny EL34 are really two different tubes.

As Ed says, the manual has Mark III and Mark IV parts lists, so that's what it's all about. Definitely go for the big bottles if you want to go to eleven.

So, to summarize:

I could build a Poseidon using the Mark III power and output transformers, following the schematic in the manual for the Mark III. Based on Terry's experience, the Electro-harmonix 6CA7 "big bottle" will drop right in, and run just fine. I should probably keep an eye on idle current and overall dissipation. When funds allow, the 6CA7 can be pulled out and replaced with KT88 for more power.

Been a while since I posted this, but yeah you can do that. the bias pots allow enough voltage change to drop it to the level needed for EL34s. I have done this once in my amp. but have 6550s and KT88s for it so it only lasted a few minutes plus they were cheap EL34 tubes, and my SED kt88s are much better IMO.

Blair

TerrySmith wrote:FWIW, the large 6CA7 biases about the same as a 6550/ KT88, and the innards look real similar.

Terry, do you recall your bias voltage, and how you built the bias circuit? Did you use the 4.75K / 10K pot / 18K divider, as per the manual?