mwl168

Table updated. Construction notes for those getting started to source parts for your builds: R3 and R2 determine the regulated voltage for GRLV. The formula is Vout = (R2 + R3)/R3 * Vref. Vref is the reference voltage of U4 (LT1021). R7, R8, R9, R10 determine the regulated voltage for GRHV. The formula is Vout = ((R8 +R9 + R10)/R7 + 1) * Vref. Vref is the reference voltage of IC1 (LT1021). You want to keep R8, R9 R10 values close so they have even voltage drop across each of them. On the PSU Main board, R22 (100K) and C17 (470uf) set up the delay time for HV. The 100k/470uf combination provides a roughly 45 seconds delay. Increase R22 or C17 to increase the delay time. C17 needs to be rated 5VDC or higher. Also added these construction notes in the first post for ease of access.

A pair of balanced CFA boards? Sweet! Happy to see another CFA being built! It is still my favorite dynamic amp of all time! Keep us posted!

For a complete PSU, you need 2 GRHVxxx, 1 each GRLV78xx, GRLV79xx and the PSU main board. GRLV78/79 can be replaced with 7815\7915 regulators. Each mini T2 set includes a left channel and right channel board for a complete amp. With due respect, with high, lethal voltage, SMD, etc., this is not a good project for a beginner to take on. Please also read through this thread and the mini T2 and KGSSHV build threads for information. Table updated.

Also, check to see if your front end tubes are reasonably matched. If the 2 5K pots are centered (ones you use to balance the driver tube plate voltages) and the driver tube plate voltages of the two sections are more than 10V off then I would consider the tube does not have well matched triode sections. You can center the 5K pots without removing them from the PCB.

Table updated.

A few things: Offset and balance voltage will settle as the amp warms up. Are these voltage measure when the amp was cold or warm? IME, high DC offset usually caused by poorly matched tubes. Swap the 6SN7 tubes between channels. If the high offset follows the tube, replace with better matched tube(s).

Corrected. Thanks!

Table updated. Please do double check and let me know if I have made mistakes. I will tally up the final board counts and get quotes from the fab house. I will then update the table with board costs and the total due for each participant and start collecting payments. I will place the order when all payments are collected. Please note the payment will include boards cost and shipping charge from the fab house to me (shipping evenly distributed to each board) but will not include shipping charge from me to you. The following participants are dropped from this GB due to no response: Marc Edwards stickyjon23 zumac99

Great! Thanks! I thought 1/2W resistors should do too but looking at the SRX Plus shunt regulator PCB, the resistors look to be physically much bigger. That’s why I ask.

@Blueman2, what wattage rating resistors do you use for the voltage divider to elevate the front tube filament supplies? thanks!

Do you want the GRHVxxx partially assembled or bare boards? I assume you know the PSU Main board is designed specifically to work with the GRHVxxx boards? Each main board takes 2 GRHVxxx boards to supply a set of +/- HV rails.

Table updated. GR78 and GR79 BOM updated.

We have a few new comers to HC (<= 10 posts) that participated in this GB and have not responded to my inquiries, including direct PM, for confirmation. We need final board counts to get accurate quotes from the fab house for this GB to proceed. The GB participants listed below please respond on this thread with your confirmation or desire to drop out by end of day this coming Sunday, March 22. If we do not receive your response , I will assume you are no longer interested and will remove you from the GB. Thanks! stickyjon23 grendel23 zumac99 Marc Edwards

The chimp looks really immersed in the music...

Table updated.

C1, C2 and C3 are different values. A quick look seems CMPTA42 could be a pin/package compatible part for KST42. Same goes for the PNP part. Kerry and others should be able to confirm. Table updated.

GR78 and GR79 BOM added to first post.

Please read the thread and be specific about which boards you want and how many. Please be specific. Table updated.

Thanks! Interestingly, I found that same valvewizard article MLA recommended during my search and found it to be very educational. Did you miss appending a pic? Hmm, I am using two separate secondary windings from the same and only transformer to supply the filaments - one for the output tubes (6.3v) and one for the input tubes (12.6v). Are you recommending to use two independent windings or two independent transformers?

I am particularly intrigued by Blueman2's comment on elevating the filament supplies to eliminate hum. Similar to Jose's experience, I feel my SRX Plus seems to be more susceptible to hum than my other tube amps. I read a bit into this issue and center-tapped filament supply windings and filament supply elevation were two recommendations I gathered. @Blueman2: care to elaborate your thoughts/experience on this topic? Thanks in advance!

I have had hum/buzz issues with my SRX Plus occasionally. I have always been able to trace the issue down and resolve it. Agree with Blueman2 that you should be able to have a noise-free SRX Plus. Here are 4 reasons that cause hum\buzz in my experience: 1. position/arrangement of the filament supply wires 2. dirty pin(s) on the tubes 3. noisy tubes 4. poorly matched input tubes 2 and 3 can be easily identified by swapping the tubes around. Good luck and enjoy the amp!

Table updated.

posted BOMs in the first post of this thread. The BOM for GRHVxxx and PSU MAIN boards should be up to date. Need to check the GRLV boards BOM.

2 GRHVxxx boards are needed for the PSU. GRHVxxx are not needed for the mini T2 boards.

Table updated. It's time to wrap up this GB. Please post your decision for partially-assembled/bare GRHVxxx boards and updates to the PCB quantities by end of Sunday, March 22. I will then get quotes from the fab house and start collecting payments so we can place the order to have the boards made. Again, I would assume you opt for assembled GRHVxxx boards for those that do not respond. For those GB participants that are new to Headcase (less than 10 posts) and have not responded since last Tuesday, March 10, please post your response here so we know you are till participating.