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SGregory 20th February 2010 04:09 PM

OPUS 5.0 A Modern Mullard
1 Attachment(s)
I was looking for a good project to expand my knowledge and fill a need I have with my Audio System. I desired an amp to drive my base drivers on my 3-way using an active cross over (very timely SY and Pmillett ). Presently the Speakers are being driven by a TubelabSE 300b with a passive xovr. They are loud enough, but often as expected, bass notes challenge it beyond its capacity.

The goals for the amp were around 60W, with decent dampening factor for bass duty. To do that I knew I had to go to a PP design, which I wasn't excited about initially. I had considered the Williamson but quickly turned my attention to the Mullard based on feedback on this forum.

After reading Tubemacks thread with the ideas and discussion within it, I was very impressed and excited. The attached schematic is a derivative of that thread. Out of respect for Tubemack and his goals, I started this thread so it would not derail his goals.

I would like to thank everyone that has helped me so far, particularly Eli, SY, boywonder, and M6tt. I am sure I am missing others so please forgive me.

Output Tubes kt88 in UL
LTP is the 12at7 (This to me seems like a good answer based on Eli's experience to the listenability issues I had with commercial PP's I have heard)
The Input driver at this point is the 12bh7 per Eli's suggestion which would fit the drive requirements of the t7 ltp and not require TOO much feedback to tame the gain

With the 12bh7 about 15-16dB of feedback should be sufficient.

I have choosen the powerdrive from tubelab to provide a low impedance drive to the kt88 and provide a high impedance load to the t7 ltp. Even though it has AB2 capability it should stay within AB1 operation with very few exceptions. The advantage of this, in my mind, is that any grid current will be handled with authority.

If you would, please evaluate the schematic and comment freely on it.

If the operating voltages and currents look close enough, I will design and post the PS schematic. My hope is to use a PT with a single primary, bias, and possibly rectifier filament winding. It will be SS rectified at this point in design, but if I have a PT wound with a rectifier winding it has a higher recycle value in my inventory for other projects should I choose. The filaments will be heated with a seperate xfmr to allow them to be heated before application of B+.

Thanks Again

boywonder 20th February 2010 04:31 PM

Cool! :p......picking up where we left off.....the schema says "use 3.2K for lower B+" for the OT.....won't that still be a problem for the 12AT7?

Nice job on the schematic, BTW.

I don't think you have enough 10M45's in there ;)

smbrown 20th February 2010 04:39 PM

Nice design. Very complete! When do you build? Looks like you will need some good heat sinks on the MOSFETs.

SGregory 20th February 2010 04:44 PM

The FET followers could easily be switched for a 4k R for a little less sand. I left them in as they will sit right next to the FETs on the heatsink.

I am running the kt88 at 475V which is hotter than 465V in the tubemacks thread. This gives the t7 ltp a lttle more room (not much mind you) to et it set up right. The higher impedance gives IMHO a better loadline at 475V. That is ONLY based on my limited experience.

The GEC tubes came in yesterday. I want to get opinions on this and post the PS schematic for review before I order the iron. I only want to do that once!!!!
I have also sent Bud at O-netics and Jack at electraprint an email but havn't heard back from them.

Eli Duttman 20th February 2010 05:05 PM

R1 is too big. 100 KOhms is about right. R1 combines with V1's CMiller to form a low pass filter. When R1 is too big, HF info. gets lost.

GNFB should be taken from the speaker tap actually in use. Got 16 Ω drivers? If you're going to have Jack E. or Bud P. wind O/P trafos, think hard about a single O/P tap of 6 Ω. I'll let somebody else check your math. I foul it up far too often.

R9 and C2 form a high pass pole. For stability's sake, F3 of that pole should be as close to zero as can be, consistent with practical considerations. Also, C2 needs to be of very good quality. With those points in mind, a 400 WVDC/680 nF. PPFX MultiCap appeals to me. Michael Percy has them at 9 bucks a pop.

The R19/R20 gate stoppers are too small. The transconductance (gfs) of power MOSFETs is high. Use 510 or 1000 Ω Carbon comp. parts here.

A cap. is needed at the I/P, which will form a high pass filter in combination with R1. F3 of this pole should be in the 15 to 17 Hz. range. A properly sized high pass pole here protects against O/P trafo core saturation from a huge GNFB LF error correction signal. A Soviet surplus PIO part is dandy here, as this pole is outside of the GNFB loop.

SGregory 20th February 2010 05:58 PM

Revised Schematic per suggestions
1 Attachment(s)
Okay, Revised.
Almost all my SE OPT's are 6ohm, so no problem there.
Little hesitant to put another cap in but point is valid so in it is. Should be right around 15Hz. Noted C2 and set value. 510 CC in for gate stoppers.

Revised Schema Attached.

Those with experience with kt88 in PP, does the approx OPT seem reasonable for operating point? Or is Operating point too aggressive? I don't need the tubes to last until I turn gray, but I don't want a melt down either. (wait too late for the gray).

SGregory 21st February 2010 04:04 AM

Power Supply and revisions
2 Attachment(s)
Here is the power supply and Updated Driver
B+ has 0.8mV ripple at the kt88 The ltp has 0.12mV ripple and the driver 0.08mV

C- has 1.2mV ripple

The filaments are switched on first which enales the HT to be switched on.

Eli Duttman 21st February 2010 04:59 AM

Some PSU comments.

C110 is WAY too big. If you want the 7805 IC to be reliable, C110 should be 22 μF. Put the 1000 μF. part after the rectifier bridge, where it will do some good. Instead of nasty Tantalum, I suggest a Panasonic ECQ-V stacked film part as C109, as close to the 7805 as can be. Why a 5 VDC rail, to begin with?

The signal will ride on the negative rail. LOW ripple is important. So, make C111 much larger and install a "hash" filter here too. Definitely use Schottky diodes in the negative rail bridge. Quiet is very good here.

Install ferrite beads on the leads of the filament trafo's primary. Keep crud from sneaking into the signal, via the heaters.

Have you given thought to B+ rectifier diodes selection?

boywonder 21st February 2010 05:30 AM

I'm assuming the PS transformer will be a custom, correct?....and C113 & C117 are motor runs....

Nice heater safety-interlock human powered delay, have you thought about using a amperite-based delay? There is a pretty simple schematic around that uses an amperite and a DPDT relay to do the time delay, then takes the amperite out of the circuit so it can cool down for the next power cycle. I have a copy somewhere if you are interested.

SGregory 21st February 2010 01:28 PM

Eli, I am not sure I really want to use the 7805. Actually was thinking of the lm317 when I used the fixed device. Of all things, I missed the filtercap prior to the 7805. Fixed. The rail os for some digital panel meters so I could see bias. I thought of putting in just a test point.

Hash Filter goes in next version of bias supply drawing. Oh BTW I had a typo, unlike what was posted above, the simulated ripple is 0.133mV on the bias. I will increase the caps a little, but I run into a time issue. With the large dcr of the choke, a bigger cap slows the establishment of the bias. In my present design, that isn't big problem as I have the huge caps with the thermistor (not shown yet, oops) delaying b+. If more normal size caps are used inn the B+ circuit, the the b+ WILL come on alot quicker.

As far as diodes, will need something >1200 V. PSUDII is showing just under 1100V PIV. I was too tired to spec those parts last night. I got as far as developing the parts list for the amp section and then I couldn't read the fine print in the mouser catalog ;-)

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