So I just got myself a pair of Douglas M4927 transformers 335v @ 400ma - 65v @ 100ma - 6.3v @ 5A. Im going to build a pair of monoblocks and looking for some help with output Xformers, will a 6.6k Primary be sufficient for this voltage level or is there any other Primary impedance that will work trying to keep costs down a little?
Cheers!
Cheers!
Hey Francois G thanks for your reply I've been doing some extra research into how to calculate what primary impedance is needed and it seems like i should be calculating it from the valves Ra on the datasheet? but some don't seem to have an Ra listing?
Would I have to do many mods to accommodate KT88's? I hadn't really thought about using other output tubes!
Cheers!
Would I have to do many mods to accommodate KT88's? I hadn't really thought about using other output tubes!
Cheers!
You could do some loadline graphing to determine the desireable impedance for output transformers. Check the universal loadline calculator here:
Universal loadline calculator for vacuum tubes - Vacuum Tube Amplifiers - DIY
Experience shows the kt88s work well with 3.4-6.6k. EL34s will be fine with a 4-6.6k transformer too. Read this thread carefully, especially #38 and thereabout. The Toroidiy output transformers might be a cost effective solution if you are near Europe.
I believe you do not need any changes to the BH34 BOM, but do observe the higher voltage power supply and other capacitors indicated for the higher voltage supply. One caveat is that you may need to increase the bias supply voltage, as also discussed in this thread.
Universal loadline calculator for vacuum tubes - Vacuum Tube Amplifiers - DIY
Experience shows the kt88s work well with 3.4-6.6k. EL34s will be fine with a 4-6.6k transformer too. Read this thread carefully, especially #38 and thereabout. The Toroidiy output transformers might be a cost effective solution if you are near Europe.
I believe you do not need any changes to the BH34 BOM, but do observe the higher voltage power supply and other capacitors indicated for the higher voltage supply. One caveat is that you may need to increase the bias supply voltage, as also discussed in this thread.
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Balancing Vc1 and Vc2
Need some help. have been trying to figure this out for some time.
I am unable to balance Vc1 and Vc2 and unsure what I am doing wrong.
Vc1 ~140V(Vc1) and Vc2 ~60V. Turning R5 to both extremity just changed 0.x V
When I measure without the V1 tube, both Vc1 and Vc2 is 328V.
I read that it could be due to the input tube and I tried 2pcs of 12AX7 and 1 pc of ECC803S, similar results(unable to balance the voltage)
Vbias= -61Vdc
Va= 343Vdc
R39 = 33K//27K= ~ 15K
Current through R18 ~ 2mA (Vdrop of ~1.4V)
NOTE:
de-solder Q1 and Q2, both of them checked out OK with transistor checker(hfe~160)
I have also check resistor values etc
Any tips etc will be appreciated.
Need some help. have been trying to figure this out for some time.
I am unable to balance Vc1 and Vc2 and unsure what I am doing wrong.
Vc1 ~140V(Vc1) and Vc2 ~60V. Turning R5 to both extremity just changed 0.x V
When I measure without the V1 tube, both Vc1 and Vc2 is 328V.
I read that it could be due to the input tube and I tried 2pcs of 12AX7 and 1 pc of ECC803S, similar results(unable to balance the voltage)
Vbias= -61Vdc
Va= 343Vdc
R39 = 33K//27K= ~ 15K
Current through R18 ~ 2mA (Vdrop of ~1.4V)
NOTE:
de-solder Q1 and Q2, both of them checked out OK with transistor checker(hfe~160)
I have also check resistor values etc
Any tips etc will be appreciated.
At this point you should not have the output tubes installed. Verify that you have the correct values for the + and- bias power supplies.
You need to check the current source in the input tube cathode circuit. Is the LED lit? Measure the voltage across the resistor from the lower transistor emitter to V-. That voltage should be somewhere around 1v. Most common mistake would be to install 1 or both of the transistors in the wrong orientation. Check the pin out for the actual parts you used. Small signal transistors come in several different pin outs and if you used a sub it could be different.
To test that current source without the input tube installed simply ground the collector of the top transistor and you can read the voltage across the bottom emitter resistor which will give you the input stage currrent.
You need to check the current source in the input tube cathode circuit. Is the LED lit? Measure the voltage across the resistor from the lower transistor emitter to V-. That voltage should be somewhere around 1v. Most common mistake would be to install 1 or both of the transistors in the wrong orientation. Check the pin out for the actual parts you used. Small signal transistors come in several different pin outs and if you used a sub it could be different.
To test that current source without the input tube installed simply ground the collector of the top transistor and you can read the voltage across the bottom emitter resistor which will give you the input stage currrent.
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Thanks for the quick response O/p tubes is not installed yet Red LED is lit up
Q1 and Q2 orientation checked and rechecked. I have even desolder to check the transistor. All measure OK with transistor checker. Voltage across resistor is 1.4V (2mA)
B+ is 343V
Vbias is -61V
Am able to measure about -40V at the grid of the o/p tubes (not installed)
Q1 and Q2 orientation checked and rechecked. I have even desolder to check the transistor. All measure OK with transistor checker. Voltage across resistor is 1.4V (2mA)
B+ is 343V
Vbias is -61V
Am able to measure about -40V at the grid of the o/p tubes (not installed)
OK, good! I think I see your problem. 2 ma is too much cathode current for the large plate load resistors used. You need to lower that a bit. 340v B+ - say 150 volts on the plate = 190v. 190v/267k= About .7ma so 1.4ma total. So try changing R18 to 1k. Also 1.4 volts seems kind of high across R18. You should also check the voltage across the Vbe of that transistor. It should be .65volts or thereabouts. If it is low then you may have a shorted transistor.
Coincidences can sometimes surprise you. I just brought up the second channel of my BH last night and guess what, I had he same issue as you have. I traced it to the fact that I had not installed the 47.5 k input resistor to ground. Without a DC reference the tube won’t operate properly. Check the resistance from the grids of both input stage triodes to ground. I hope this solves your problem.
May be you have a problem with your trimmer or R7 & R8 have not the same value ? I have been playing with my Baby Huey even with a higher current in the input stage without any problem 
Bfpca the plates resistors should be 220k and not 267k ! Not a big difference but that may help a little bit
Rgds,
Marc
Bfpca the plates resistors should be 220k and not 267k ! Not a big difference but that may help a little bit
Rgds,
Marc
What specs for BH PCB board
Could someone with experience using the BH PCB determine what is the maximum RMS output. Good specs would help those who are trying to adapt the board to other uses, as Marc had ventured.
I wonder if I could drive EL86 tubes which require more drive than EL84. But if it could do well driving KT88s I imagine it would be well with El86s
Could someone with experience using the BH PCB determine what is the maximum RMS output. Good specs would help those who are trying to adapt the board to other uses, as Marc had ventured.
I wonder if I could drive EL86 tubes which require more drive than EL84. But if it could do well driving KT88s I imagine it would be well with El86s
If you want to make sure you have lots of drive available then go with a higher bias voltage supply. This will allow more voltage swing. I don’t imagine you will have any problem driving el86 with a +-60v supply. What is the approx Bias voltage you would be running on the output tubes? If it is -20v then you need 40v peak to peak which is no problem for a 60v bias supply.
The maximum output will vary with power supply, tube type and output transformer. In my case I am running el34s into a 6600ohm Toroidy output transformer with a B+ of 450v. I just tested the 4ohm taps and got 45watts into 4 ohms before clipping. With a kt88 I can get about 50 due to it’s lower output impedance. Note that because I am using a high primary impedance output transformer I also need to use a high B+ to get this much output. If you go with 4400 ohms you can get the same power at a lower B+ and also a lower drive voltage will be required for the output stage.
Thanks fore that link its a very useful tool! Thanks for clearing things up for me, looks like ill be going Toroidiy as its seems quite hard to find more tradition transformers in the UK for reasonable prices.