I was reading this afternoon what Dave Gillespie, who popularized EFB (Enhanced Fixed Bias), said about UL output transformers for EL84 amplifiers. I found it interesting and thought I would pass it on to Baby Huey EL84 builders for consideration.
Dave wrote:
“For tubes of the 6BQ5 class however, the optimum conditions with fixed bias operation in fact includes an 8000 ohm load with taps at 25% of the winding. For cathode bias operation, a 10K plate-to-plate load is best, with the taps located at 50% of the winding. With an 8K transformer that has 40% taps, you would be better off using the transformer in pentode mode with some form of fixed bias operation.”
More can be seen here:
More Fun With Magnavox: The 9300 Series
So, Roberto, you got it right! What is the premium Toroidy charges for their 8k 20% output transformer for the BHEL84?
Dave wrote:
“For tubes of the 6BQ5 class however, the optimum conditions with fixed bias operation in fact includes an 8000 ohm load with taps at 25% of the winding. For cathode bias operation, a 10K plate-to-plate load is best, with the taps located at 50% of the winding. With an 8K transformer that has 40% taps, you would be better off using the transformer in pentode mode with some form of fixed bias operation.”
More can be seen here:
More Fun With Magnavox: The 9300 Series
So, Roberto, you got it right! What is the premium Toroidy charges for their 8k 20% output transformer for the BHEL84?
Great to know! I ordered mines with 23% taps around one year ago, so I don't remember exactly. Let me check on my emails. I still have two complete brand new unused sets of power transformer + output transformers if you can be interested in one set. Not sure how much it could cost to ship them overseas.
One more question for those who have built the amp
how much do you see the voltage drift between the plates?
I have attached the image from the setup guide : VC1 - VC2 =0
mine seems to drift and if left alone can get as high as 2V and it seems to need to be reset everytime I start up the amp
I am curioous if this is normal behaviour or if I am am still in troubleshooting mode
thanks..dB
how much do you see the voltage drift between the plates?
I have attached the image from the setup guide : VC1 - VC2 =0
mine seems to drift and if left alone can get as high as 2V and it seems to need to be reset everytime I start up the amp
I am curioous if this is normal behaviour or if I am am still in troubleshooting mode
thanks..dB
Attachments
The plate voltage could drift somewhat do to heat and line voltage levels. The degree of matching between the 2 tube halves will also influence the difference between the 2 plates. 2 volts does not seem like an issue to me. From my experience the balance between the plates does not seem to be as critical in a BH amp. I assume this is due to the cross coupled feedback which works to decrease the differences in the 2 drive voltage phases. I have tried adjusting for minimum distortion and there is no deep null and often the lowest distortion does not occur with the plate voltages matched. So, once you adjust it to 0 volts you should be close enough that there will not be any major issues even with a small amount of drift.
The avatar picture is actually an el34/kt88 BH. I had room to angle the boards in the large chassis I used. Below is my EL84” mellow yellow” BH build. It is using Russian 6p15p-ev tubes. I built this out of parts and paint on hand at about the start of Covid last year. So the bright colour was a symbol of hope! The kt88 build is from a couple of years ago and is loaded with Sovtek KT88s . I like the coke bottle shape!
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Paduak is beautiful wood. The sides of the KT88 are Purpleheart but it’s hard to see that in the photo. The top of the HV and bias regulator box is cherry. The output transformers actually live underneath the top of the chassis behind the amp boards. An atypical construction method. The chassis is a clam shell with a hinge on the back, so the “hood” can be raised to work on it.
Love that wood
Purpleheart is a personal favorite of mine. I plan to make a few chassis with it as soon as I pick up a box joint blade set and build a jig. Padauk is nice too.
Wenge sucks to work with but is gorgeous if you want something darker but with some very cool grain patterns-
Too bad it's getting so expensive, though.
That's really cool! It seems here in silicon valley everything is maple, birch ply, or oak. Nothing interesting anymore.
I have some nice smaller pieces of wenge set aside for some instrument and knick knack projects soon. Lovely wood. It's crazy how expensive it has gotten these days. Since moving into our new house I've been doing a lot of woodworking and forgot how cheap lumber used to be.
I'm in the planning stages of a weird teardrop ukulele thing soon that is getting purpleheart back/sides, maple neck, cedar top, and violin style pegbox without the scroll, think a four string version of a medieval cittern/citole but uke sized and tuned
I've gotten back into instrument making (made a handful of folk insruments back in high school) and found it's actually easier for my hands to handle chisels and saws than a soldering iron.
I have some nice smaller pieces of wenge set aside for some instrument and knick knack projects soon. Lovely wood. It's crazy how expensive it has gotten these days. Since moving into our new house I've been doing a lot of woodworking and forgot how cheap lumber used to be.
I'm in the planning stages of a weird teardrop ukulele thing soon that is getting purpleheart back/sides, maple neck, cedar top, and violin style pegbox without the scroll, think a four string version of a medieval cittern/citole but uke sized and tuned
I've gotten back into instrument making (made a handful of folk insruments back in high school) and found it's actually easier for my hands to handle chisels and saws than a soldering iron.
Most probably your First Watt has an higher DF, as BH is not very strong at that point of view, but I find it very musical and pleasant for most music styles (even if IMHO it performs at its best in simple music scenarios). I don't know if it applies to the standard version too, but with 23%UL and 14% local feedback the very nice thing is that it saturates in a very pleasant way, so you can push it a bit harder.
Random thought on the original post 1 schematic: PCL86s are around for five quid each. If you referenced the positive of a regulated heater setup to signal earth the return would be at 13Vish below. This could give make the T a bit more L in the initial LTP so you could use something like a 10M45S as the CCS. Or is that a silly idea?
Distortion measurements
I've finished one channel of my EL84 Baby Huey and made some distortion measurements without any global feedback.
I suppose it's not too bad given the 0.13% suggested by LTSpice. I was able to reduce the THD by 0.08% to the 0.19% you can see by adjusting R5 on the cathodes of V1A and V1B.
Has anyone else done any measurements? Listening tests will have to wait until I have the second channel built.
I've finished one channel of my EL84 Baby Huey and made some distortion measurements without any global feedback.
I suppose it's not too bad given the 0.13% suggested by LTSpice. I was able to reduce the THD by 0.08% to the 0.19% you can see by adjusting R5 on the cathodes of V1A and V1B.
Has anyone else done any measurements? Listening tests will have to wait until I have the second channel built.