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Effectively the plate resistance increases which results in a lower plate current reading as a tube ages in circuit subject to the same fixed plate voltage and grid voltage being applied continuously when the unit is switched on. For a higher plate resistance you would need to apply an even higher proportional plate voltage to achieve the same plate current output. The problem is the tube can't accommodate any increase in plate voltage beyond its maximum permissible operating parameter. This is why the plate voltage is fixed and the plate current lowers with more use in the circuit. Adjusting the grid voltage to be more positive in value compensates for increasing the plate current.

It should be a small almost insignificant difference if you were to use an 8ohm dummy load instead. If you are to try it and compare it to a 12ohm dummy load I'd suggest measuring the temperature of the OTs as well. The 12ohm load should increase the temperature.

I would say this. You we need to do 3 things to achieve more power output. No 1 you would need to change the driver stage to provide more voltage swing to the power tubes without jeopardising the inclusion of more clipping, THD, oscillation etc. No 2 you would then proceed with using a much higher plate voltage and also much higher plate current/bias point. No 3, you we need to upgrade the OTs to have much greater bandwidth and power output. There is another thing that needs to be considered. That's the GNFB circuit and it's reactive load on the primary circuit. I feel the mods being proposed and progressed here are from back to front. That No 1 thing above has never been addressed or revised with the R8 which significantly limits the peak to peak swing to deliver more power output.

It's actually 37 5w as you used a 12ohm dummy load. Had you used an 8ohm dummy load the peak to peak voltage would have been different thereby giving you around the same value. That's my understanding from the video guide provided below. You can agree/disagree as you wish.

60v peak to peak equates to 37.5W for the 12ohm dummy load.

Let me guess. PB?

What was the peak to peak voltage reading on the scope for producing the maximum undistorted sine wave increasing the volume control to that specific point for the given signal?

How much did you get those Blackburn beauties for?

Link to the seller, please? That's definitely the ones that I've been talking about since 2021 as one of the best sounding US made ones that money can buy.

I was referring to the tube tester that you tested your 3 Adzam 6SN7GT's on. Just watch out for the heater voltage under load. That circuit is microprocessor controlled to raise voltages such as the plate voltage 10 fold. As you've used a separate measured PT line for the 6.3v supply you should be fine.

That's a military code common to all USN designated tubes. The box suggests a tube manufactured before December 1950. There should be a letter-number manufacturing date code under the shield symbol on the glass envelope.

Have you crosschecked the heater voltage under load to see it's in fact 6.3v? That tube tester is notorious for not being within the 10% range.

Happy to take them off your hands if you don't want them.

Fantastic. Let us know how you go with it. I wouldn't suggest altering anything else at this stage. For the rectifier, which others do you have on hand?

Yep, the 6N1P-EV from the 1970's or the original 6N1P from the 1950's as priority number 1. https://www.ebay.com.au/itm/185902156240 https://www.ebay.com.au/itm/144831189998

Just tell her you've made a small investment with big returns in the future.

Congratulations on the acquisition. Let the tube rolling begin!

The triple mica fell on the solid mica to improve mutually aligned contact?

Yes, about 9 watts of lighting available from each tube. Keeps me up at night.

I think it's sourced from Legacy Audio in the US. It appears to be identical to the older style 1" neo spiral ribbon tweeter used on the expression and phantom series. Try contacting Legacy Audio in the US or Pymble Hifi in AU to see if you can source one. http://everythingaudionetwork.blogspot.com/2014/10/ean-audiophile-review-legacy-expression.html https://www.ebay.com.au/itm/162935987332

An all Japanese affair?

By the way if anyone is interested the best sounding Svetlana Electron Devices 6550 tube to own is the mid 1990's 6550-B2 with 3 getters (2 top pan discs and 1 side button getter).

Just adding for some of the earlier comments left here that Svetlana Electron Devices in the 1990's was still producing vacuum tubes for military and telephony use. Sovtek wasn't. Matthews and the Sovtek brand were simply looking for tubes to acquire to fit audio use, which still aligns with their business model today via the New Sensor brand that has basically taken over all registered trademarks and naming rights for yesteryear tube brands not only in Russia but all over the world. Yes, modern production Russian EL34, 6550, KT88, KT120 etc. tubes are all made in the same factory in Russia. In the 1990's and prior this wasn't case with unique construction techniques and features isolated to the brand, manufacturer and plant facilities throughout Russia.

I've never seen a 6550B tube branded as a Svetlana Electron Devices tube. This may very well be the case, Ian. I've only ever seen the 6550B tube labelled as a Sovtek 6550WA tube or 6550B Made in Russia tube with no other manufacturer's markings. I also don't understand how the Svetlana Electron Devices B2 variant followed the sequence of the side button getter and 2 getters at the top and the Svetlana Electron Devices B3 just went for the side button getter with no getters or getter silver flashing at the top. I suspect Svetlana Electron Devices deliberately made these tubes indifferent to stop Sovtek selling 6550B tubes. Clearly the Svetlana Electron Devices 6550C that followed with the rectangular holes and 2 horizontal ridges in the anode plates was never sold onto Sovtek.

All I could find on the internet was this old post from May 1995. I need to research it a bit further and advise. I've seen Svetlana 6550B-2's and 6550-B3's but none of the original Svetlana 6550B's. "The confusion is caused by the recently available Svetlana 6550B-2 tube. This is the tube that I feel sounds the best. So far my customers agree and this is the default tube that I use for repairs. The 6550B-2 can be identified by a getter attached to the outside of the plate and two mmorel mounted on a bar at the very top of the tube. I believe EvaAnna Manley previously posted this information. The older 6550B tube is the one that I think Sovtek was importing. In my opinion, the Sovtek 6550B and the recent Svetlana 6550B tubes are one in the same. The 6550B-2 it appears, has never been made available as a Sovtek."