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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: RE: DB110G (Bogen) 1950s. posted by ivan_terrible on April 21, 2024 at 01:09:17:
There is absolutely NO information on using this valve in anything other than cathode bias.
Ie. Fixed bias has never ever been done on the UBL/EBL output pentode.To give some sort of an idea on this, the 6V6 needed roughly -20>-22V of bias at the amp's 350V HT.
After having a bridge rect go up in smoke from the previous bias circuit, I took pot luck and put a 12V DC supply directly into the bias circuit and started up the amp.
Suprise!
The HT was sitting at 400V+ and no sign of life from the amp.
The pentode is rated at Va 300V max, so I figured at this bias voltage the valves must be turned fully OFF.Putting a trim pot across the bias supply and feeding the grids with roughly HALF the -12V of bias brought things back to life, and the HT dropped to 350V - still a tad high.
This is where life got interesting.
Wiggling the bias anything between 4-6.5V meant the idle cathode current could be trimmed to anything between 30-40m/a.
However, the HT would go up and down with whatever current you chose to idle at.A funny situation happens where at -4V the anode current would do 40m/a with HT of 325V, but with a really fast dropping screen grid (thru a dropper resistor) to as low as 220V.
Max anode diss is rated at 11W, but this is 13..and no sign of red plate.
However as you can now understand you pull it back to -5V anode current is 36m/a but now at 335V..which is still 12W.At -6V the screen grid (g2) is now rising to 250V and Anode of 344V with Ia of 32m/a which is Pa 11W.
then at -7V we have g2 at 260 and anode at 354V with 30m/a so 10.62W, so the right spot appears to about -7.2V where the Va is 355V and 265V on g2.
At this the valve is running 28.75m/a - just over 10W.All these figures show the effect of increasing static load on the HT line as simply dropping particularly the screen grid supply.
In other words the usual 1950s/60s valve amp appears.The anode dissipation of this pentode can be violated with no adverse effect at all.
The constant rising and falling of the HT supply under load, with the SAG of the valve rectifier is doing nothing less than varying the gain of the output valve and running out of current to supply both the anode and the screen grid (And of course the preceding AF amp and phase splitter).
Here we have yet again the classic case of the amplifier delivering scarcely HALF the rated power cleanly, simply because the PSU is not able to do it except on sharp transients.
This is the classic definition of a dynamic range COMPRESSOR as well as a special COLOURING equaliser with clipping occuring way before the output transformer is in saturation.
I will next compare the output before clipping of the IMD signature.
The pentode amp sounds cleaner, quite possibly because of the far better sensitivity of the output valves requiring only about 1/3 of the drive needed to full output.
+
The biggest improvement vital to do on the UBL/EBL version of this amp is to get improved stability of the screen supply.
Edits: 04/21/24 04/21/24
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- RE: DB110G (Bogen) 1950s. - ivan_terrible 06:03:16 04/21/24 (0)
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