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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 24 Oct 2017, 10:20 
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Hi, I have (still am actually) traveling and can't always get to answer things quickly as normal. Anyhow... the circuit is quite forgiving and will work with nearly the same distortion, gain S/N over a wide supply range and wide current range. The best results are with currents in the 3.5 to 6.5 ma range. The cathode resistors are selected from common values that will achieve that with the available B+. For the stand alone versions one very cost effective (in the US) power transformer is the one from Edcor. Under $20. It will result in the B+ being about 225. You can actually go to as low as 180 with good results. In the power amps the stage is the same except it needs to provide sufficient linear swing of output to match the needs of the output tubes. With a 225 volt supply as in the Poddwatts only a +/- swing of 15 volts is needed and the stage can easily do this. In the KT88 and KT120 amps at least +/- 50 is needed. The 300 volt supplied driver does this well. Actually the 225 volt one will too. The use of the H-C voltage divider supply is a simple and trouble free way to protect the tubes. It is only needed on the SRPP stage but is simpler to add to all the heaters. This is particularly true if you use DC heaters. For dropping the B+ to the level needed by the driver I like to use the LR8 regulators. They do the job well and provide a large amount of filtering. The RC way is OK but not quite as good. BTW I would not go above about 325 v B+ on the SRPP as it will be difficult to get the HC levels reasonable. Most tubes for this use show a new H-C rating of 200 volts. As time passes my experience is that this deteriorates and 100 volts is more reasonable. If the tube fails in use it will cause a huge spike in the output and possibly cause harm to the speaker system. The spike will be full output power at some relatively high frequency. Bad news for many tweeters.

Good listening
Bruce

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PostPosted: 24 Oct 2017, 16:04 
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poty wrote:
victorzwk wrote:
If I were to implement a Forewatt in the same enclosure as my (stereo) Oddblock using the same LR8 regulated power supply (300 V output), would it be better to use a series resistor to drop the voltage to 215 V (18k ohms @ 4,54 mA). Or to substitute the cathode resistors on the Forewatt (from 470R to 1200R)? Thanks.
Sorry to push answers to side topic, I'll try to answer the main question. In my practice - the more B+ the better. If you follow all Bruce's cautions about cathode-heaters potential I'd prefer not to use the RC dropping circuit and connect the "forewatt" part directly to the same power supply (lower THD, better headroom and so on). I may have missed something in the thread, but where did you find the 470 vs 1200 numbers? With 470R@215V the current is estimated as 5-6mA, while with 1200R@300V the current may be 4-5mA, i.e. 1mA lower. I do not know how it will sound.


The 1200R was taken from the Oddblock SRPP stage. Maybe something inbetween then. 820R?
Thanks.


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PostPosted: 25 Oct 2017, 04:26 
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victorzwk wrote:
The 1200R was taken from the Oddblock SRPP stage. Maybe something inbetween then. 820R?
Oddblock has different tube - no wonder it should use such value. I'd use closer to the original value like 620 or so.


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PostPosted: 30 Oct 2017, 14:17 
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I have calculated the cathode resistors and, with the ECC802 tube, in order to get 5mA of bias current with 300V supply (150V per tube) a 1K ohm would be required. With the 470R and 300V supply I would expect a 8mA bias current.


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PostPosted: 02 Nov 2017, 10:51 
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If you'll indulge a non-technical post, I finally got to the basement to throw together a quick rack for my growing "odd system". Bottom is my build of a dual Oddblock Octal with KT88's. Middle is my newly-finished forewatt. Other items are a Nikko tuner and Dual turntable I bought 40 years ago in high school (and are still working!)

Currently listening through a pair of Polk Monitor 10Bs, which the KT88s drive admirably.

Not visible is the bluetooth interface which my high-school son and his friends use when they want to play music from their phones :)

Image

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PostPosted: 02 Nov 2017, 13:34 
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Ooooo, is that a CS-505? :D

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PostPosted: 02 Nov 2017, 13:47 
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It's a CS-504, which I think is the predecessor. Grado F3+ cartridge. New stylus, but if I recall correctly same cartridge I had installed when I bought the TT. Still sounds great!

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PostPosted: 02 Nov 2017, 21:52 
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Very Nice, I believe the Grado can be upgraded by stylus from the Presteige series (Gold, Silver etc.).

Good listening
Bruce

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PostPosted: 07 Nov 2017, 19:25 
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victorzwk wrote:
I have calculated the cathode resistors and, with the ECC802 tube, in order to get 5mA of bias current with 300V supply (150V per tube) a 1K ohm would be required. With the 470R and 300V supply I would expect a 8mA bias current.


Have I done the math correctly, Bruce? Thanks.


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PostPosted: 07 Nov 2017, 20:21 
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Hi, There seems to be a bit of confusion. I suspect it is related to both the tubes used and the particular variation of the amplifiers. With 300 V in the larger amps that use either 12SL7s (and 6SL7s) or 5751s, I use 1200R. This results in a current flow of about 0.6ma (measured). For preamps and such using ECC802 (and 12AU7) 470R is best for around 200V. You have to look for sweet spots on the voltage/current voltage bias curves. The actual needed values of current differ a bit based on the B+ voltage. An ECC802 (12AU7) can operate well in the 3 ma to 15 ma range, but the best values follow a sort of curve with 3.5 to 6 ma at lower voltages and 5-8 at higher ones. I determined these values in the amps by measuring the distortion. The distortion indicated mostly non-linear behavior in this case (as opposed to the residual present). I tried 1K with 300 volts and it was above the best range. I have seen folks using up to 1.5K but my testing indicates that the distortion was nearly double the 820R value (300V). If you will note the smaller amps all use the 200 volt B+ as did many of the original larger ones. The bigger amps now use 300v for a little extra headroom. They do not use the ECC802 or 12AU7 as there is insufficient gain. This is how designs evolve. The first variations are now over 8 years old.

edit, More info. The two tube types are quite different. The 12SL7 is nearly the same as a type (9 pin) 5751. Both have much higher gain and lower current capability vs the ECC802/12AU7. The newest designs call for use of the 12SL7 (and 6SL7) or 5751 in both small and large amps. You can use the ECC802 and 12AU7 in the small one if you adjust the current and can live with less gain overall. Many folks consider the 5751 a slightly lower gain variation of the 12AX7.(ECC803). The 12AX7/ECC803 can be used in place of the 5751 without a significant (but measurable) increase in distortion in any of the amps.

Good listening
Bruce

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