K120 on 6550/KT88/KT90 anyone tried???

[salajazz]

Does anyone changed the stock 6550 with the Tung Sol KT120? Is that secure? what about the heater current increase?

[scott17]

Originally Posted by salajazz

Does anyone changed the stock 6550 with the Tung Sol KT120? Is that secure? what about the heater current increase?

You will have to provide more information such as type of amp, schematic, power supply amperage, etc.

There is a good chance that an amp designed for KT88/6550 will not have a sufficient power supply that you could just pop in KT120s.

[salajazz]

thx for the reply, the amps are:
AUDIO RESEARCH REFERENCE 610T MONO
CONRAD JOHNSON PREMIER ONE
HARMAN KARDON CITATION II
AUDIO RESEARCH VS-110

[HollowState]

Bob Carver just sold a Citation II with KT-120s in it. If you read the Q & A's you'll discover that he lowered the matching plate impedance by reassigning the 4-8-16 ohm speaker connections to 2-4-8 ohms. He also did his clamp diode modification trick and re-biased the KT-120s to 26mA idol current. My guess is that the other models you list would support the KT-120 as well without too much strain.
Harman Kardon Citation-II Stereo Tube Amplifier, Carver | eBay

[richwalters]

Only my opinion...The currently made KT88's and 6550's are not up to the sustained task anymore of unable to take 600V as they once could do...I'm often amazed to the numbers of variants around produced by vendors that end-users suck up alot of their rubbish.

I'm finding the EH KT90 a darned good tube in conventional fixed bias UL circuits with a 20% increase in throughput power with a good reduction in thd compared to KT88's and 6550's. The extra heater current has to taken into consideration. The EH KT90 requires a lower bias voltage too.
Where there is a top plate population problem esp with parallel p-p, the pencil type KT90 is ideal with far less radiated glass heat emitted and better air circulation when compared to fat envelope 88's.
I have my doubts that there is any performance advantage by swapping KT88's to
KT120's, in fact the opposite. I find the KT90 sits comfortably in between.
Many users have remarked on the hard tone of the KT90's but I don't find this. A very well controlled bass and a very clean mid-top range.

richy

[Wavebourn]

Grids of tubes are not virtual, they are real. They are made of thin wires around cathode, with gaps between them. Distance from wires to cathode varies, so fields right under them and between them have different strength. It causes "remote cut-off" effects in them, that is different for different geometries of tubes.

All tubes that are similar to 6L6 have similar curves, and their curves are not the best. If you look at 6L6 curves you may see that the higher is negative voltage on it's grid, the lower is it's transconductance. It is because wires in the control grid work like gates with variable width: the higher is negative voltage, the narrower that gates are. When control grid voltage is close to zero 6L6 clone works in most linear region; the further is it from zero the less linear it is. To be used as linear tube 6L6 must dissipate about 150W without change of geometry. But it can't. It is a generator tube, and made for RF generation, like remote cut-off tubes were made for gain controlled RF amplification.

Generator tube biases itself by rectified on the 1'st grid RF signal it generates, such a way it produces more stable signal with less distortions than an amplifying tube. However, it was pushed on audio market by RCA back in 1938.

KT-120 can dissipate more, but curves are still similar to 6L6. If to use it in place of smaller sisters it must be biased for the same current, or power supply and output transformer must be changed. With the same power supply and output transformer using KT-120 you have to bias for the same current on which the original tube was used (of course, if filament power supply can perform its duty). Now, draw the same load line on curves for both tubes KT-120 and 6550 and check, will be the result more linear, or less linear.

[tyu]

You say

He also did his clamp diode modification trick and re-biased the KT-120s to 26mA idol current


Can you Show this diode Setup Thanks

[Joshua_G]

Waveborn,
I assume that 6550/KT88 and KT120, when biased to the same current, will produce the same output power. Is that right?
Between 6550/KT88 and KT120, when biased to the same current, which will be more linear?

[HollowState]

Originally Posted by tyu

He also did his clamp diode modification trick and re-biased the KT-120s to 26mA idol current
Can you Show this diode Setup Thanks

If you use the link for the closed eBay auction I posted above, you will find your answer. However, since the link is only good for a month or less, I'll copy and paste part of it there.

"I intend to post it on the Carverfest website in a week or so. In the meantime, you can install the DC restorer by using a pair of 4148 diodes connected anode-to-anode, the cathode of each one connected to the grid of each output tube. Connect the common anode connection to the lowest (most negative) bias voltage(-55 VDC)through a 91K, 1/2 watt resistor. In other words, one end of the resistor goes to -55 volts, the other goes to the common anode connection of the 4148's. Set the idle current for 26 ma on each output tube. To obtain enough range with the existing controls, you will need to remove R30 and R69 (Sam's schematic), the optional 18K resistor shown on the schematic. Some factory-built units already have it removed, so check your unit to see how it came from the factory so long ago. A vacuum tube 6AL5 tube will work slightly better than the 4148, as it's smooth turn on characteristic does not induce the small non-linearities that the 4148 does."

[salajazz]

Originally Posted by HollowState

If you use the link for the closed eBay auction I posted above, you will find your answer. However, since the link is only good for a month or less, I'll copy and paste part of it there.

"I intend to post it on the Carverfest website in a week or so. In the meantime, you can install the DC restorer by using a pair of 4148 diodes connected anode-to-anode, the cathode of each one connected to the grid of each output tube. Connect the common anode connection to the lowest (most negative) bias voltage(-55 VDC)through a 91K, 1/2 watt resistor. In other words, one end of the resistor goes to -55 volts, the other goes to the common anode connection of the 4148's. Set the idle current for 26 ma on each output tube. To obtain enough range with the existing controls, you will need to remove R30 and R69 (Sam's schematic), the optional 18K resistor shown on the schematic. Some factory-built units already have it removed, so check your unit to see how it came from the factory so long ago. A vacuum tube 6AL5 tube will work slightly better than the 4148, as it's smooth turn on characteristic does not induce the small non-linearities that the 4148 does."

Thank you so much, you and Mr. Bob Carver just made my day!