Hello potepuh:
I run mine straight off my CD, phono pre, tuner. I do not think you need a pre-amp. Others may have suggestions, but give it a try without then decided.
I run mine straight off my CD, phono pre, tuner. I do not think you need a pre-amp. Others may have suggestions, but give it a try without then decided.
One chanell up and running. But this is my first valve amplifier and I have some question, I can go lower than 84mA and 85mA bias (840-850mVDC)for the tubes and the amp consumes 2,93A of the 6,3VDC.
I am using 6550tubes and the toroidy TTG-KT88PP 4kohm output transformer.
Looking in the 6550 data form the Comparaison pentodes et tetrodes, file posted from bandool the max current for the 6550 is 150mA.
I am using 6550tubes and the toroidy TTG-KT88PP 4kohm output transformer.
Looking in the 6550 data form the Comparaison pentodes et tetrodes, file posted from bandool the max current for the 6550 is 150mA.
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The suply voltage is 392V and with 84mA, tht comes 33W, is almost the limit of the 6550 35W max.
Congratulations on getting your BH board running.
Yes, that is getting up there. If you run the amp at night with the lights off and the plates are not glowing red then you are OK. The tube life will be shorter running them hot. Experiment with the bias level to see how the sound compares at a lower power dissipation.
Yes, that is getting up there. If you run the amp at night with the lights off and the plates are not glowing red then you are OK. The tube life will be shorter running them hot. Experiment with the bias level to see how the sound compares at a lower power dissipation.
For the second board I am not able to balance the 12AX7, one plate has 94v and the other 124.... Turning the pot does not change the voltage
Are the 12AX7 problematic, the tubes are new... Any ideas?
Are the 12AX7 problematic, the tubes are new... Any ideas?
Shouldnt be that far off...check R7,8,9,10 for correct values. The cathodes share the circuit to ground so I am thinking plate/rail resistors off.
Swaped the 12ax7 tube and now I can set the balance to 0 V DC.
Need to buy another set of 12ax7 tubes. Those are new from Tungsol (Selection) but...
Also on this board I can`t go lower than 0,07A per tube, (0,07A*396V=28W per tube) i wold like aroun d45-50mA per tube any idea?
Need to buy another set of 12ax7 tubes. Those are new from Tungsol (Selection) but...
Also on this board I can`t go lower than 0,07A per tube, (0,07A*396V=28W per tube) i wold like aroun d45-50mA per tube any idea?
Good news . as i known tube have large deviation in production . at least i had this experience.
Sorry your last question .i dont have any idea. My board's parts are still seeking and i believe will go this tunning step later. Good luck
Hi potepuh,
The bias current is fonction of the negative power supply, it seem that R39 is still too big at 15k, may be you can try with 10k to have more margin for the trimmer. You can check the voltage on the grid of the output tube...
I think that we are loosing several Volt in the gate to source of the MOSFET follower, but I cannot check since I am not at home 😕
Give some feedback on the result of the modification, it may be useful for others...
Cheers,
Marc
The bias current is fonction of the negative power supply, it seem that R39 is still too big at 15k, may be you can try with 10k to have more margin for the trimmer. You can check the voltage on the grid of the output tube...
I think that we are loosing several Volt in the gate to source of the MOSFET follower, but I cannot check since I am not at home 😕
Give some feedback on the result of the modification, it may be useful for others...
Cheers,
Marc
This seems to be a common problem when using some power tubes that require more negative bias.
You may need to increase your negative bias supply voltage but it is more likely that you need to decrease the size of the resistor (R39)that is in series with the bias pots. I think the stock value is 15k. But the older schematics have 33k which is almost certainly too large for many tubes. If you do have the 15k installed and you still can’t adjust the current low enough decrease that to 7.5k you will have more - bias voltage available at the bias pot. You could do that by soldering a second 15k resistor in parallel with the existing one.
Instaled another 18k resistor in paralel with the existing R39 18k, now I have 9k and I can lower the bias.
Now need to buy some new 12AX7/ecc83 tubes to finalize the second board.
Now need to buy some new 12AX7/ecc83 tubes to finalize the second board.
This is valuable information fellas, thanks for sharing.
I’m starting my EL34 BH very soon and info such as this will help should issues arise.
I’m starting my EL34 BH very soon and info such as this will help should issues arise.
Stacking transformers
Thinking about the physical layout for a stereo (as opposed to two monoblocks) version, has anyone any experience of mounting toroidal transformers in a vertical stack? I'm wondering if I can get away with an O/P-Mains-O/P stack. If the O/P transformers were appropriately oriented wrt eachother, could you get cancellation of any induced hum???
What does the team think?
Thinking about the physical layout for a stereo (as opposed to two monoblocks) version, has anyone any experience of mounting toroidal transformers in a vertical stack? I'm wondering if I can get away with an O/P-Mains-O/P stack. If the O/P transformers were appropriately oriented wrt eachother, could you get cancellation of any induced hum???
What does the team think?
Interesting question. You could try an experiment. I’ve never done this but I may do it before building my BH. Heatshrink the secondary leads of your power transformer so there is no possibility of them shorting out or you getting a shock. Energize the primary side of the transformer. Next, terminate the 8 ohm secondary of your output transformers with 8 ohm resistors and put a scope or an AC voltmeter across the anode connections on the primary side. This should allow you to measure any induced hum voltage using the output transformer primary.
Next move your output transformers into whatever configurations you want to try and see if you can measure any hum voltage on the output transformer primary. I’m not 100% sure if this will work but it’s not too much work to try it.
Next move your output transformers into whatever configurations you want to try and see if you can measure any hum voltage on the output transformer primary. I’m not 100% sure if this will work but it’s not too much work to try it.
One issue to watch out for here is that if you are using a tube that requires more negative bias like a 6550 or kt88 the - bias voltage will be higher, especially if you are using a high B+. If you for example are going to need -35 volts for proper plate current then the peak to peak voltage required at the grid would be 70v and the source follower driver will need to be able to swing negative far enough to do -70v. So if you only have -60 for a -bias supply you will be able to set the correct bias voltage of -35v but you will not get the full undistorted grid drive. EL34 needs a lot less bias voltage so shouldn’t be a problem there.
I planned to use kt88/6550 until I can get some el34 so I will need to consider this issue when building. I ordered some transformers for the bias voltage but I don’t remember exactly what voltage they are.
I was thinking of an experiment along these lines, unless anyone else has already had a go?
Hello, I am sorry if this is obvious, it is my first serious attempt to make a tube amplifier. Is there a need for OT protection? What would happen if a speaker was disconnected or shorted?
Bfpca,
Thanks for the info, I was also hopeful to use the KT88’s in my stash. I’m using the Toroidy trafo that Marc recommended and the bias tap is 50v. If I’m doing the math correctly, that will get me almost 70vdc after losses. It’s going to be close.
Thanks for the info, I was also hopeful to use the KT88’s in my stash. I’m using the Toroidy trafo that Marc recommended and the bias tap is 50v. If I’m doing the math correctly, that will get me almost 70vdc after losses. It’s going to be close.