TJ Full Music 300B/n Data?

[Alex Kitic]

No plate glow was noticed. This was several years ago. The advice to run mesh plates not exceeding 22W dissipation came from Jac Music when they were selling a full range of Full Music TJ valves.

Thanks, Greg! You got me worried that current production 300B tubes were not able to last for several k hours if used at 80-85% nominal dissipation rating, and that mesh (fake mesh) are particularly useless, as they can be run at slightly over half nominal dissipation.

Frankly, with a project like this one (RH300B) going on, I was afraid that it was futile work, since the 300B has become a whimp tube.

Now that I know where does this alarming info come from (from an interested party), I can relax and finish the breadboard as intended.

If the fake mesh 300B is one of the best sounding current production tubes, and is relatively cheap in comparison with highly ranked and priced competition, and you were the dealer who for some reason is not selling them (let's imagine it's a conflict of interest for you), what would you say?

[Alex Kitic]

The RH300B will soon be unveiled on my blog...

While developing and checking, I have tried Electroharmonix and Full Music (Shuguang "mesh") tubes.

The RH300B is centered on 30-32W dissipation, since 32W represents 80% of the absolute maximum dissipation for a 300B tube. In the process I have tried operating the tubes at 36W dissipation as well, and have not experienced any red-plating or similar phenomena with the "mesh" plate tubes. Since the filament glows through the holes in the anode, it is difficult to say whether it is red-plating or not and that probably causes the misunderstanding.

On the other hand, I have measured the temperature of the tubes at 36W dissipation on the hottest spot and it does barely touch 100 degrees C - which does not seem that much, particularly having in mind that the rectifier measures up to 120 degrees C, and is not particularly "pressed" (5U4 at 105mA per anode, or 210mA total, in hybrid graetz bridge).

Based on this measurement, I would say that even 36W does not look like excessive heat for those tubes?

I cannot comment on the longevity of the tubes in such or other conditions, but it is obvious that current production tubes are no match to NOS of the JAN series... thus one cannot expect that current production 300B will last as long as the WE tubes from the 50s did.

[IslandPink]

I've been running a pair of KR 300B-XLS for about 7 years as my main amp , at around 380V a-k and 90mA . I probably run about 500hrs/year .

[Alex Kitic]

I've been running a pair of KR 300B-XLS for about 7 years as my main amp , at around 380V a-k and 90mA . I probably run about 500hrs/year .

The Kron XLS version is specified at 70W dissipation, at your operating point about 35.2W you are running them at 50%... they should last a very long time... if the filament does not rupture prematurely.

I usually burn 25 to 30 hours of tube life per week, but the load is shared between several amplifiers I would be happy with anything between 3 and 5k hours from a set of 300B tubes (current normal production), running at 32W dissipation. I guess that is highly likely.

[Dave the bass]

Update.

I've been playing with the Mesh 300B's at between 18 and 22W dissipation. Sounds great. 348V a-k at 60mA 20.88W.

Man-sized KR's go in tommorow when ears are fresh for compare/contrast sessions.

Bias and HT are twiddlable on the fly.

6c45pi = 150V @ 14mA is fave 1st stage OP.

DTB

[Mike H]

Bias and HT are twiddlable on the fly.

Courtesy of variable bench power supplies I'm guessing.

[Dave the bass]

Almost.

Bench supplies x 4 for each stage of each channel but the fixed bias -ve supply is made on veroboard and fitted inside the amp, tho the grid bias adjust is twiddlable from the outside.

DTB

[Mike H]

Crikey!

[Alex Kitic]

Just a preview of the RH300B, now boxed (a lot of stuff in a rather small box). I will publish it soon on my blog.



Just to stay on topic, the 300Bs are operating at 30.5W dissipation with a 5R4 rectifier.