BHK Preamp tube rolling


#321

Nah, forget it. The name is too long. :slight_smile:


#322

Question for @BHK
How important is balancing (in terms of current draw/iP) for the tubes. I’m guessing that it is not as important since you are using a CCS and I would think that the CCS would force a 4/5ma draw from each section regardless of them NOT being matched. Would also think that most importantly would be that the tubes would need or want to matched for gain since each tube is amplifying both phase’s of the L or R signal.

Is my thought process correct? I have a number of closely, gain matched tubes that are not so close in terms of iP… ± 3ma off…

Thanks,


#323

Hi jslateiv,

I assume we are talking about the BHK preamp here. The considerations for the BHK amps are the same. The CCS will just cause the total set current to flow. Since the grids are in effect grounded, the individual plate currents will depend upon their individual characteristics to set their individual currents. In the preamp, you can find out about what the individual grid bias voltages are ( the positive cathode voltage IS the grid bias) by measuring in respect to ground on pins 3 & 7 on the tube sockets. Also, you can see how matched the plate currents are by measuring the plate voltages on pins 1 & 6. The plate current can be calculated by the voltage drop in the individual 10K plate resistors

by (120 - plate voltage)/10,000. This doesn’t really answer you question but does give you the means to see what is what in the actual circuit. The effect of tube current unbalance mainly affects the amount of second harmonic distortion coming out of the diff pair.

Hope this helps,

Bascom King


#324

Glad to hear from you, hope you are well. Your contributions are invaluable to all of us in understanding this gear.


#325

Thanks @BHK,… yes, I was referring to the BHK Pre,… So assuming there is no music playing and we are strictly talking about bias current (as measured by current flowing through the plate?), wouldn’t/doesn’t the CCS cause that to be the same regardless of say a ‘unbalanced’ tube (unbalanced in the terms of iP between triodes)?

For example if the/a set parameter (tester) is used to measure a tubes iP, and there is an imbalance,… does the use of the CCS in the BHK Pre negate this imbalance since it is setting the current to a specific value?

Maybe I’m just thinking about this wrong,… regardless, I appreciate the insight!


#326

Below are my opinions, based entirely on my own tastes.
I’ve now tried NOS matched pairs of:
Siemens CCA 1974 A frame (very dynamic, a bit tiring over time)
Siemens 6922 1974 Single Halo, (a bit like a CCA above but turned down from 11 to 10)
Mazda 12AU7 double Mica (thought they sounded incoherent, slightly smeared)
Mazda 12AU7 Triple Mica (better than the double but the midrange bloom was distracting - that’s a matter of taste of course)
Brimar 6067 "T’ long plates (similar to the Mazda Triple Mica actually)
Siemens 7DJ8’s (a bit of an outlier that one!) - the Siemens sound on a budget but but a slightly muted midrange which put me off them)
Le Radiotechique long plate 1970’s - polite, big midrange, didn’t for much for me (again, my taste).
And of course the Psvane OEM tubes and the Tungsram 7DJ8.

Favourites so far? Tunsgram 7DJ8. I can see why BHK really loved them. Relaxing, just tubey enough to add something to proceedings without getting mushy or bloomy, a well balanced and coherent frequency response. There is nothing that stands out with these babies and that’s the appeal. Everything is there in the correct proportions, nothing is lacking. Loving listening to my system at the moment.

I still have some Mullard M8136’s 12AU7 on order but I’m guessing they are going to be similar to the Brimars. I’ve ordered a few extra Tunsgrams as spares.

So question, has anyone tried the National Panasonic 7DJ8’s (an Upscale Audio favourite) with the BHK preamp?


#327

Mark,I see you jumped in full tilt. Have fun with it…


#328

And I forgot to mention Mullard M8136 12Au7’s and the Voskhod '75 6N23P Gray Shield SWGP’s. Damn, that’s a lot of toobs in a short time.


#329

Did you adhere strictly to the 4mA for 12v, 5mA for 7v settings?


#330

I did adhere to those settings with the exception of the Tunsgram 7DJ8’s. There were some questions back and forth on whether it is better to run them at 4mA or 5mA. In the name of science I tried both and settles on the lower 4mA setting. For me, whilst the higher current setting was more dynamic, the lower setting was more relaxed and conducive to long term listening so it won out for me. It also gives you a little more peace of mind in that you aren’t running your precious tubes too hard!


#331

What is it they say? Go hard or go home :stuck_out_tongue_closed_eyes:


#332

I also found that 5mA with the Tungsrams made them just too crystalline for long term listening and I much preferred them at 4.
My personal favorites so far are the Brimar CV4003.


#333

I have a BHK Pre arriving tomorrow. I’ve got an SR Blue fuse and a HiFi Tuning Supreme fuse and a slew of 6DJ8 tubes. Can’t wait to see the system morph. I’ll be back to this thread to see if I can learn from everyone’s experience.


#334

I don’t know if this exact question has been posed here. Is there a benefit in running a 12AU7 tube versus using a 6F8G tube. I have many 6F8G’s to move through.


#335

I would wait until it’s well burned in before I changed fuse, tubes. This way, as you change each item, you’ll know if you hear an improvement over stock. And I’d assume that there’s no benefit to using 12v or 6v. They gave us the choice by just moving a jumper. And the stock Psvanes are pretty good.
I’ve had a few good preamps and the BHK is a great preamp.


#336

My BHK Pre has 338 hours on the preamp and tubes so it’s broken in. I received it with Tungsram 7DJ8’s, it sounded good but I had some Lab Gruppen Telefunken 6DJ8’s that I’ve installed. I like the result. Silky Smooth presentation with no noise. The sound is better with the BHK Pre than directly out from the DSD.


#337

In response to your question about the 6F8G and BHK Pre,… their heater current draw is double the specified (allowable) current for 6V filament operation (~300ma vs the 6F8G @600ma) . If you have a Noval to Octal adapter then you could try but be aware that the extra stress of double filament current may lead to premature failure of the heater regulator,… BHK found in an earlier post that that when taxing the regulator like that, it started to drop out and would not properly run the tube… ymmv I had some luck using a 12BH7 (@600ma) but due to BHK’s findings with the extra filament stress… I pulled the tubes and do not use them,… ashame because they actually sounded GREAT! Hope this helps.