GOOD GOD, SOMEONE TALK ME OUT OF THIS ONE....

[WopOnTour]

LOL, Yes very [Y]

[NOSValves]

WOT,

Send me a copy of the SAM's. I caution you to not trust a SAM's to be accurate many have mistakes. Always refer to the factory Schematic as the best source. But in the end taking one part out cartefully and checking its value and replace it with the same. Many times companies made changes but either never released a revised schematic or it just never has surfaced. I find it hard to believe they biased the 6L6's to 75 mA each. but it wouldn't comepltely shock me HK was known for running things on the very rageed edge which is not a bad thing for Sonics but it sure lowers the life of the tubes! 400V X .075 = 30 watts of Idle plate dissapation which is dead on the max of a 6L6GC and do not try that with modern production 6L6GC they will not like it. The power transformer would have to be able to deliver some 400mA of B+ current at Idle for that setup and it just does not look large enough for that! If this is all true that power trnasformer would run scolding hot. The Negative voltage on the factory schematic would not make sense if the amp was truly bias to 75 mA. I'd set the amp up stock rebuilt to start and see just where the grid voltage is with everything freshened up and 117VAC from the wall. Again do not trust the SAM's to be 100% accurate. It would be nice if HK gave us the negative voltage value at the grid rather then just entering the pot. It's pretty hard tofigure what it was at the grid which is what matters.

If you want I can hand draw a schematic of the mods onto the factory schematic or the SAM's for you. It's pretty simple and maybe would be easier to understand.

I'm helping out on this one because that amp sound very intrigueing.

By the way if you try 350B keep the bias below 60 Ma per tube.

Craig

[WopOnTour]

Thanks Craig

I'll scan the SAMs and email you a copy. I too have encountered technical errors in the SAMs or even factory schematics. However, in general I've found the various pin-out voltages in the SAMs to be fairly accurate (barring a misprint or significant mains voltage variation) as they've supposedly been established by actual measurment on a live amplifier. The SAMs schematic shows -28VDC on the pin 5 grid and only 390V on the anode plates, 375V on pin 4 screen and as I mentioned previously, 0.5V on the cathode.

I'll take your advice though and restore this amp to bone stock first, measure it up @117 VAC on the variac, and THEN consider the potential circuit modifications. I look forward to your input.

Cheers

[WopOnTour]

The amp can easily be modifeid to allow for full bias adjustment and measuring. You simple have to install a pot in front of each DC balance control for the channels. You feed the -40V into one leg of each pot and the center lug of the pot (wiper) is voltage out to the balance pot, The third leg you will tie a resistor to ground and adjust its value so you can get about -24 to -36V adjustment range. I would use a 10K pot and try about 6.8K resistor to start. I would also add a 50uF @ 50V axial lead electrolyte to the center lug of these bias adjust pot and remember this is negative voltage so you install the electrolyte reverse polarity (plus goes to ground and negative goes to the center lug of the pot). Also take the 3.3 Ohm resistors out of each channel and split the connection between the two power tube cathodes then install a 10 Ohm cathode resistor on each tube to ground. With a 10 Ohm resistor I would try biasing the tubes to about 35 mA to start which would equal .35 Volts measured at each tubes cathode. Run it that way and check for 400V on the plates with the amp being presented 117V from the wall if the voltage is at or above 400V on the plates try .40 and see if the plate voltage drops much then repeat at .45 and so on. With 6L6GC's you should be able to bias the tubes up to about .70 wolts or 70 mA if the power transformer can deal with the load (seriously doubt it). If the plate voltage starts dropping then its a pretty good sign your drawing to much B+ from the power transformer.

Something like te diagram below then? I realize some of the R values in the negative bias curcuit may need to be adjusted (most notably the 3.3K), just trying to sort out a valid plan for the reroute. Currently the amp is up and running bone stock but with Chinese AX and AUs and Groove Tube 6L6GC (just to protect the vintage RCAs it came with) and it sounds VERY Good! Well, actually the original volume pot is quite scratchy (even after a thorough cleaning) so it appears I will have to replace it. However the factory 500K attenuator has 170K and 300K taps and are obviously not that common- but I've ordered a 500K 23-stepped resistor unit that should allow me to tap off at similar resistances.

I've also ordered all replacement signal coupling caps from PartsConnexion (a combination of Auricaps in the preamp, with Mundorf M-cap Supremes in the output coupling) as well as bolstered power supply caps directly from Jim Mcshane http://pages.prodigy.net/jimmcshane/ that are basically identical to his heavily damped Citation V power supply, except with HEXFRED rectification.

So my plan is to do just the coupling caps first , measure/test, then add the Mcshane power supply and re-test, THEN finally rewire the cathode/bias circuits adding a triode/ultra-linear switch (and some cathode circuit test points) before starting the actual tube rolling. I've got all sorts of old 6L6GC, and a real nice set of matched GE 7581A to try. But I still want to audition some KT-66 types or perhaps vintage 350B from either National Union or Western Electric.

Does anyone think the Shunguang 350B is even worth a listen?

Cheers

WOT

[WopOnTour]

How's this? This is nearly identical to the bias circuitry in the infamous CItation 2 amps, with a few minor value changes.

a) individual cathode circuits with test points to use vdrop to measure quiescent bias current.

10K multi-turn Bourns type pots which are easier to source than the 7.5K used in the Citation 2

c) counter dials for quick n easy adjsutments when rolling tubes

d) Orginally I had 3.3K in series with the 10K pots, but I have since revised that to 500ohm which more closely approximates the stock bias network for now- but I realize this may need to be adjusted

Would YOU still add a pair of caps on the pot wipers Craig??

Thanks

WOT

[WopOnTour]

Budda Bump Bump for comment

Craig? Whuddya think??

Thanks for your help

WOT

[NOSValves]

Sorry hadn't looked at this thread lately. That looks great! I'd build it up without the filter caps on the wipers and measure the AC ripple leaving the bias pot at the wiper. If its much over 20mV clap a 25uF cap on each wiper. In the end the 3.3k resistor to grounds value will all that you will have to play with to get the range you want.

[WopOnTour]

Thanks Craig

If you want to check out the schematic again I've uploaded a revised version.

I've added some proposed circuits to add a DPDT switch for triode/pentode switching. I've read 100-300 ohms in the grid circuit is an acceptable starting point for this mod (the Citation 2 uses 270 ohm) but I'm a bit unsure of what what influence this will actually have. So for now I've selected a value of 200ohm. Can you shed some light on this?

Also I've purchased some high resolution 15-turn wire wound pots (with counter dials) for the 10K bias adjustment. My insticts tell me that this wont affect things much sonically, but Craig what do you think? Will the use of a wire-wound pot in the control portion of the bias circuit have a significant sonic effect?

Thanks again for ALL your help on this project

WOT

PS> I'm planning on installing a Jim Mcshane inspired power supply upgrade designed to significantly increase the capacitance and DC stabilization.

Below is a diagram the stock PSU. I wouldnt mind trying to get a little more available negative bias voltage available. I'm thinking of replacing the silenium rectifier with some hex-freds or other "fast" device. Does Craig or anyone have a recommendation for this?

[WopOnTour]

Ok so sorry to keep resurrecting this old thread, but it's the only one with any continuity with my current amp modifications that many people here have been helping me with.So I need a bit more advice from some of the more electronically inclined here...

The pic below reflect the Jim Mcshane PSU upgrade I have been installing in my old 1960 model A-260. At this point I have 1 main question. Unlike the SAMs diagram which is for a 1961 model (and on which the attached schematics are based) I actually have TWO selenium rectifiers in the -36V DC generator (one as shown the other connected in series referenced to GND on the anode) Would switching to a single soft landing fast recovery diode actually provide me with MORE available peak negative bias voltage? or LESS?? I would prefer to have MORE B- to work with, so by my way of thinking, my current plan is to replace the two SRs with a single Hex-Fred.

So I guess the old thread title still pertains: Good God, Someone talk me out of this one...

WOT

[WopOnTour]

Here's how this old gal is looking these days (and sounding NEVER BETTER!)

Many thanks to Craig@NOSVALVES and Jim Mcshane for their valued assistance

***

[Gilbert]

Oooohoooo, that's sexy looking. If you stare at the blueish halo, it looks like a pair of shapely bunns. I LIKE IT.