Need Help De-Coding Tube Data, EL34

[Eric Klinger]

Hi Crazy People! I am happy to see that the forum seems to be picking up a bit more here and there!

I recently feel into a sweet deal on a 2000 1959SLP. This amp in particular runs B+ at 503 - 510 VDC depending on time of day, screens voltage comes in just under that by a few volts.

According to almost all modern tube data sheets, there should be a max of 425 (RCA tube manual) - 450 VDC (most manufacturer datasheets) on the screens. What does this mean in terms of screen grids seeing 500+ VDC?

The only tubes I could find that claim 500 VDC on grids were TAD EL34B-STR (which I assume are Shuguang?), so I ordered a pair and should arrive in a few days.

So I am curious if there is something more that meets the eye when it comes to tube data? The RCA manual says 425 VDC MAX on G2, but also says in parentheses "Design-Center Values" at the "Maximum Ratings" section... not sure the meaning of that, so I wonder if the actual limits are a bit... grey?

Thanks, as always!

[danman]

Most guitar amplifier circuits are running tubes well over their recommended voltages. It's the screen current that is the real killer in the EL34. If you see any signs of the screen grid glowing red when pushing the amp hard, consider upping the screen grid resistor values.

Have you checked the bias and is it on the cold side? Warming up the bias will bring the B+ down a bit, but you don't want to go to far either.

[Eric Klinger]

Thanks, danman!

I installed the new TADs today and my plates were at 519v! So using the bias method where you check ct to plate resistance and vdrop, I set the tubes to 17.5 watts dissipation on the cool side, 18.5 watts on the other.. don't remember all the numbers, but vDrop was +/- .5vdc.

So it is set a tad warmer than I would usually set it, but not by much. Screens measure only 1-2 volts less than plate voltage, which was 514 after settling in the bias, so assume 512-513vdc!

Just out of curiosity, if I needed to up the screen resistors, what would be a logical step? I see Tube Town has 1.5k/5W... and then what's the trade? I bit of sensitivity perhaps?

As a side note, I noticed right away that after I installed new tubes and fired up the amp, leaving the standby in "standby" mode, the power transformer seemed to start a bit of a buzzzz. Been playing for over an hour now though and all seems well. The lack of quality on this amp made me a bit nervous to work on at first (having had my hands in all kinds of amps for over 10 years now).

[danman]

The PT on my DSL had a very noticeable hum but it ran for years like that. It seems that some are just prone to it and others are not. Try some 1k/ 5-7watt resistors right at each socket if the amp does not have them already. If you notice the screen grid glowing red when you are playing with 1k, you could up them even more. You will probably notice a bit more compression when pushing the amp hard.