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I was reading bias instructions on the HF-89, and it read that it is normal to have red spots on the plates! :angryfire: Seems like EL34's would not last very long.

Anyone have any thoughts about this?

TerrySmith wrote:I was reading bias instructions on the HF-89, and it read that it is normal to have red spots on the plates! :angryfire: Seems like EL34's would not last very long.

Anyone have any thoughts about this?

But that's why they sound soooo good. :onfire:

Looking at the schematic and bias chart for a HF89, we see .63V at the cathodes with a 10 ohm resistor and 475V on the plates. So they're biased at 63mA*475V= 29.925W -- let's just say 30W dissipation, which would be even higher with today's line voltages! I suggest 55mA idle bias in the manual, but actually I run my Ikes at 60mA*480V. All the more reason to use 6550's or KT88's - or even KT90's when using the Ikes.

I've read where some folks bias their EL34 PP amp at 50mA (regardless of their B+), to prolong the tube life. The difference in sound between a conservative setting and ~28W dissipation is pretty huge in my opinion. I would suggest folks who want to spare no expense for the ultimate sounding amp run their outputs aggressively and spend their money on a new quad of tubes every thousand hours or two. This is a <very> measurable difference, compared to such things as electrolytic bypassing and fancy coupling caps.

Shannon

This may be a good place to try some 6BG6s, which can be considered either a 6L6GC on steroids, or an 807 with an octal base. These can replace the EL34/6L6GC/6550/KT88 series, with a swap of a couple pins and a plate cap. Then nice things about 6BG6s are that they are as tough as nails, and they are dirt cheap, between $4 and $8 for NOS/NIB. I have four of the old ST bulb ones, the newer ones are in a somewhat smaller and shorter cylindrical bottle. Check my 6BG6 note under the DIY folder.
Or just click here: http://www.vacuumtubes.com/6BG6.html

Those new ones he has are a real bargain, but they just don't have t hat classic "ST" bottle, like the old ones have.

Those "cherry red" glowing plates looks good along with the blue glow inside the glass :twisted:

/ed

Hmmm, that might be a good idea, Ed. SND tube sells Sylvania 6BG6-GA's real cheap. And adapters also to replace 6L6, 7027, and other similar types. I wonder if they have the same blue glow?

Hi All,

A couple of footnotes...

Shannon, I found that the EL34s in my prototypes of the (very slightly modified versions) HF-89, were happiest at 60 mA, also. Taking them up to the max didn't improve the sonics, and just made everything run hot. I also found (through some hints from Bob Neil and Bud Wyatt) that 72 mA was the magic number for 6550s. And I concur that bias contributes more to how an amplifier sounds than any candyassed coupling caps or titanium oxide resistors.

Bud and I got to talking about the evolution of the 6L6 one evening. We're both hams, so we started comparing the 6L6 to the 6BG6 and the 807, its big brothers. I mentioned that it seemed that all the early (1948~1953) televisions with CRTs larger than 7" used 6BG6s as the horizontal output tube. Bud said that the RCA engineers originally tried to use a 6L6 as the HO, but had internal arcing problems. So they made an octal version of the 807, which was a highly regarded transmitting tube based on the 6L6, and used extensively in WWII equipment, along with its 12 volt counterpart, the 1625. Well, turns out the added current clout the 807 provided didn't help with the arcing problem. It was a voltage-power thing, not a current-power one. So they made some minor mods to the 807 (bumped the EPmax from 600 to 700V; the 6L6 was a paltry 360V) and beefed up the internal insulating properties so the tube could withstand 6.6 KV for 10 µS of every horizontal line generated. That was pretty impressive engineering, but we all know that things electronic kinda went nuts in general in that era.

Frank McIntosh used the 6BG6 as the output tube in the first generation of the MC-30, and, in my opinion, did not improve the amplifier by going to the 6L6 in its later iterations. I'm not disparaging the 6L6 by any means; it's a noble tube. The 6BG6 just kicks its ass.

Actually, I was planning to use KT88's and bias them around 70 ma each. Hope the power transformer can keep up!

TerrySmith wrote:Actually, I was planning to use KT88's and bias them around 70 ma each. Hope the power transformer can keep up!

Thermion was over to test his modded MKIII's a few weeks back. Since they both used a similar front end topology, for comparsion I threw in a pair of Sovtek KT88's in an Ike and biased them up around 70mA. I plugged them into the wall (not the variac) and didn't check the B+, but I got 69W ouput at 1%THD+N at 1kHz. Those KT88's smoke.

Shannon