Metropoulos Forum

Hello,
This is my first post on this forum, but I'm not new to George's work. He modded several of my amps, including one of my Marshall SLP reissues to JTM45/100 specs for which I supplied the cosmetic materials. This was, I believe, a prototype version to George's own JTM45/100. It has a 1966 spec board, one of the 2" OTs that George had Heyboer build for him, a Marstran 3" PT, which is also built by Heyboer, a block logo, and JTM45 plexi faceplate.
I had George install Valve Art KT66s, because I felt these were more reliable than the Groove Tubes version. Unfortunately, the Valve Art KT66s didn't hold up well at all under the 510 B+ voltage from the 3" PT. I went through two quads that didn't last more than 5 hours each time. I believe George also mentioned that he was having trouble with these tubes in his own build. When the Tung Sol KT66 reissues came out from New Sensor, I had a quad installed and so far they are holding up longer than the Valve Art brand. I have had no problems yet after probably more than 20 hours on them.
In terms of tone, the Valve Art KT66s are a little better in my opinion. Cleans are warm and clear while the overdriven tone is very smooth much like AC/DC or ZZ Top, but this is the nature of the amp also. The Tung Sols, on the other hand, have a slight fizzieness when overdriven. It isn't much, but noticeable. They are still nice tubes, and have so far proven to be more reliable than the VAs which is what I was primarily looking for. Both brands are about the same price.
I thought this might be helpful.
BTW, I love the amp George!

The only experience I had with VA tubes was with a quad of 6550's that failed in less than an hour.

Good to know that the TS can take over 500 B+.

I don't have any first hand knowledge regarding the Tungsol KT66 or the Valve Art KT66. I found a copy of an original GEC flyer regarding the operating characteristics of the GEC KT66 at the Obsolete Electronics website (obsoleteelectronics.com). The original GEC specs for the KT66 indicate the MAXIMUM ratings as follows:

Plate Voltage 500
Plate Dissipation 25 Watts
Screen Voltage 400
Screen Dissipation 3.5 Watts

Operation outside of the parameters could lead to premature failure of the tubes.

Also recommended; control grid snubber resisters of 10K to 50K ohms and screen grid resistors of 100 to 300 ohms to prevent parasitic oscillations. Parasitic oscillations can cause excessive plate and screen grid power dissipation.

Also interesting to note, for fixed bias operation, the maximum DC resistance for the control grid to ground is 100K ohms.

I threw the Tung Sol KT66's in real quick this afternoon after installing new input grid resistors on my power tubes. The bias range looked close enough to where the GT KT66HP's had been set, so I plugged them in for a quick run.
I didn't notice any high end fizziness to them, but the low end and low mids were a little monstrous compared to the GT's -- so much so that I was getting way more ghosting than usual, a very significant amount more. The construction makes me wonder if they're just a re-tooled KT88, and the ghosting makes me wonder if the screens on these could benefit from a 1K resistor instead of the 470ohm.
They really weren't bad sounding, they just seemed to pump enough low frequencies that you would have to design around them specifically with a tighter power supply and smaller couplers throughout the amplifier. The GT's sounded a lot more balanced in frequency response to my ears.
I hope I'll get to play them more tomorrow and get a better first impression........

Flames1950 wrote:I threw the Tung Sol KT66's in real quick this afternoon after installing new input grid resistors on my power tubes. The bias range looked close enough to where the GT KT66HP's had been set, so I plugged them in for a quick run.
I didn't notice any high end fizziness to them, but the low end and low mids were a little monstrous compared to the GT's -- so much so that I was getting way more ghosting than usual, a very significant amount more. The construction makes me wonder if they're just a re-tooled KT88, and the ghosting makes me wonder if the screens on these could benefit from a 1K resistor instead of the 470ohm.
They really weren't bad sounding, they just seemed to pump enough low frequencies that you would have to design around them specifically with a tighter power supply and smaller couplers throughout the amplifier. The GT's sounded a lot more balanced in frequency response to my ears.
I hope I'll get to play them more tomorrow and get a better first impression........

Well, I can dial them in to be very similar to the GT KT66HP's, and work the tone controls for less ghosting, so maybe I was a little harsh on them yesterday.......I still think maybe the GT's sound a little richer/smoother but it's close........