Chinese 6SN7 tubes

sKiZo wrote:

arledgsc wrote:
Looking good there Skizo.   How much heat is coming off the power transformer?   Seems you have a lot of heater power demand.

Haven't gone gung ho with the temp probe yet, but I have checked a time or two after an ... "energetic" session, with no seemingly significant increase in btu's. At least it passes the "putting a finger on the iron and not leaving a couple layers of skin" test.

All the tubes used present a heavier load than normal, but no issues so far. Probably helps that I run the thing at around 116vac courtesy of my lil bucker ...



... and I'd think the additional ventilation on the chassis plate helps a bunch.

Also worth mentioning, I ran the KT120's for a long time at 60mV each, and have backed that down to a more typical 55mV. At one point, the higher bias provided more bottom end punch, but that doesn't seem to be the case now that the tubes are well broken in.

On the flip side, I don't expect that my usual heaping helping of dbx bass synth and expansion help the heat situation any, but the amp should be used to that by now ... ;-}

BTW ... is it just me, or does the adapter make this tube look like a bowling trophy?

Skizo - I was wondering why the need to use a bucking tranny down to 116 volts? Isn't Bob's transformer designed around 120VAC? That is only +3 volts away from your source. BTW that amp with the GZ37 & KT-120 look awesome.

Sorry for diverting the topic away from the OP.

deepee99 wrote:Back 'em off to about .45 or .50 and see what happens. That seems to be the magic number for KT-88s.

Nah ... been there, done that.

Bias is an interesting thing in that it drives a tube differently at different SPLs. The harder you crank the volume, the more prone a low bias setting is to clipping. I like it loud, and the higher bias settings give me a whole lot more headroom before that happens. And again, using bass synth and expansion has to to enter into the equation here.

FOOD FOR THOUGHT

- A tube works best in AB1 mode at around 70% dissipation.
- Max output for a KT120 is 60 watts
- Plate voltage on the ST120 is 500v (give or take)

Run the numbers, and the bias should be at around 85mV?

(would you like fries with that?)  

Run the same numbers for a KT88, and we're talking around 50mV, which is right in the ballpark for Bob's recommendation. I assUme the extra 5mV compensates for power leaching from the supporting electronics?

The fly in the ointment here is that the limiting factor isn't the max output of a KT120, but what the amps can deliver. With an ST120, I'd say I'm pushing my luck going over 75mV per tube on the bias. What the math shows is that can't hurt the tube itself, and may well improve it's overall performance.

Ergo, epso, facto, urp ('scuse me) ... wonder how much LESS time the amp spends in pure A mode at the "lower" bias settings using KT120's?

BNR_1 wrote:Skizo - I was wondering why the need to use a bucking tranny down to 116 volts?  Isn't Bob's transformer designed around 120VAC?  That is only +3 volts away from your source.

Way I figure it, the iron is built to handle 120vac, but these classic amp designs and tubes tend to be most comfortable between 110-117vac historically. The only potential drawback I see is if your B+ drops below spec. That's still at least in the low end of the recommended range.

Besides, maybe due to my possibly psychoacoustically active imagination, but the amp sounds "pinched" at the higher wall outlet vac ... more relaxed and refined with the bucker in place.

A bucker is on my bucket list. 
But with our local power company getting things straightened out with the up- and down-voltages required by our silver-lead-zinc mines, each of which pulls several thousand horsepower off and on, we get a pretty straight 120 VAC feed. Doesn't seem to bother the toobs or the iron.

Just took a quickie peek at the wall power here ...
(Gotta love that Kill-A-Watt)

... and current reading is 123.8vac ...

I'd rate that as  on a scale of

... for most any tube equipment. Solid state, not so much - that's more sensitive to brownout than over voltage anyway.

Solid state? What is that? Is it made out of ice or something?

deepee99 wrote:Solid state? What is that? Is it made out of ice or something?

It's Alaska.

Oh, I see. (Oh, so icy)