BHK Preamp - New Tubes & Speaker Hiss


#1

After reading about people liking the Tungsram PCC88 in their pre-amps, I thought I would give it a try. I put them in the BHK amp a while back and quite liked them.

As for the preamp, I installed them last night after moving the two voltage jumper thingies over one spot (i.e., over the first two of the three little metal prongs instead of the last two). After turning everything on, I noticed that there is quite a bit of hiss / noise coming out of both speakers now (hiss in the tweeter; a low roaring type noise in the woofer - Dynaudio C1s). Not noticeable a couple of feet way but standing beside the speaker, you can hear it. Once music is played, it sounds great, but presumably the hiss is still there negatively impacting the music.

With the original tubes, virtually no noise of any type was coming from the speakers. Turn off the preamp and the speakers are dead silent.

The tubes seem to be installed correctly and are in tight.

Is this likely a case of getting a bad set of tubes? Bought them from a dealer (same one that I got the tubes that went in the amp) and so they are supposed to be new.


#2

Or there’s something going on with the tube heater bias circuit that had to be changed for the Tungsram. That was the only other thing you changed. So it’s either the new-to-you tube or the different circuitry.


#3

There is the option to change the bias from 4 to 5 milliamps by moving two other jumpers. Is that needed for the PCC88 tubes, as well as changing the voltage? (I don’t know but would like to find out, since I will probably try some other tubes after my BHK pre gets broken in.)


#4

murrayb,

Have you tried leaving the jumpers in the original position and listening again? Just curious.


#5

Feeding 12 volts to a tube designed to run on 7 is probably not a good idea.


#6
magister said There is the option to change the bias from 4 to 5 milliamps by moving two other jumpers. Is that needed for the PCC88 tubes, as well as changing the voltage? (I don't know but would like to find out, since I will probably try some other tubes after my BHK pre gets broken in.)
I installed the same tubes and did not change the bias jumpers. I would also like to know what's recommended. I have not noticed any hiss.

#7

The manual says if you want to use a +6 or +7 fuse, “you must change the filament voltage.” That seems pretty definitive and so I don’t think I will try leaving it at the 12 voltage setting with the PCC88.

I changed the bias current by changing the jumper and that did not make any difference to the noise I’m hearing.

I’m going to assume that I just received a bad set of tubes and go back to the originals. Thanks for all of the replies.


#8

I can hear the hissing noise too if I put my ear next to the speaker. I don’t remember if it was there with the original tubes. Will probably test it out during the weekend.


#9
magister said Feeding 12 volts to a tube designed to run on 7 is probably not a good idea.
Murrayb said The manual says if you want to use a +6 or +7 fuse, "you must change the filament voltage." That seems pretty definitive and so I don't think I will try leaving it at the 12 voltage setting with the PCC88.

I changed the bias current by changing the jumper and that did not make any difference to the noise I’m hearing.

I’m going to assume that I just received a bad set of tubes and go back to the originals. Thanks for all of the replies.


I’m sorry, I should have realized that the tube voltages were different. I didn’t look up what the PC88 was equivalent to in voltage.

#10

I put the original tubes back in and put all the jumpers back to their original position. There is still hiss when you put your ear up to the speaker but less than what I had experienced with the Tungsram tubes, especially in the woofer. I guess the lesson is “don’t put your ear up to the speaker”. :slight_smile:


#11

Good lesson! There’s always likely to be a little hiss when you put your ear to the speaker - making an ultra low hiss preamp is possible, but often times the engineering compromises you make to lower hiss beyond reasonable levels has other consequences not wanted.

Bascom chose to accept a little hiss in exchange for such magic in the sound.


#12

Thank you, Paul. This is useful to know. I assume that level of hiss with the stock tubes is what would be considered normal or acceptable, given the design (yes?). This would give a baseline for comparing other tubes.

My BHK pre arrives today! happy-132_gif


#13
magister said Thank you, Paul. This is useful to know. I assume that level of hiss with the stock tubes is what would be considered normal or acceptable, given the design (yes?). This would give a baseline for comparing other tubes.

My BHK pre arrives today! happy-132_gif


Congratulations !!!

Looking forward to your thoughts when you get a chance.


#14

+1


#15
magister said Thank you, Paul. This is useful to know. I assume that level of hiss with the stock tubes is what would be considered normal or acceptable, given the design (yes?). This would give a baseline for comparing other tubes.

My BHK pre arrives today! happy-132_gif


clapping_gifExcellent!!! Let us know when you get it.

#16

Huh. I actually ought to start listening to mine. Somewhere along the line between Boulder and Burbank my unit had been dropped on the corner of the box so I got so busy taking photos of the box failures and theorizing on how all the overload stresses occurred. So by the time I unboxed the preamp and got the universal remote programmed to control it (THAT was a few hours’ worth) I think I forgot that I actually had a new preamp . . . . . .

Oh, yeah, I suppose I can listen to it now . . .


#17
Paul McGowan said
magister said My BHK pre arrives today!
clapping_gifExcellent!!! Let us know when you get it.
The pre arrived as scheduled and I unpacked it and hooked it up with no issues. The remote is much nicer than the one with the DS DAC; I had seen pictures of it but it is better in person. It will take a little time to get used to where all those buttons are.

While poking around in the setup menu, I found an option to set a maximum volume–that could be a useful safety setting. It, along with the tube hour counter that has been discussed in the forum, needs to be added to the manual.

Using the remote, it seems like there’s no way to turn off the screen on the DS without also turning off the display on the pre. The old DS remote does both components.

After three hours of warmup, I listened for a couple of hours. It was immediately clear that tone colors were more accurate and articulation of instruments was presented more precisely; good! I’ll report more about SQ after I get a bunch of hours on it. Then I will switch from my current RCA interconnects (10’) to an XLR pair. The final step will be some tube rolling. So there’s lots of listening ahead.


#18
magister said

Using the remote, it seems like there’s no way to turn off the screen on the DS without also turning off the display on the pre. The old DS remote does both components.

If I want one display on and another off, I hold my hand over the IR receiver on the unit I want to stay on and hit the ‘dim’ button on the remote.


#19

How do you access the tube life/hour counter on the preamp?


#20

Hold the button on the front button down for a couple seconds until it goes into Setup mode. Then press again a few times to cycle to the hours display.