• Disclaimer: This Vendor's Forum is a paid-for commercial area. Unlike the rest of diyAudio, the Vendor has complete control of what may or may not be posted in this forum. If you wish to discuss technical matters outside the bounds of what is permitted by the Vendor, please use the non-commercial areas of diyAudio to do so.

Tubelab Universal Driver Board, 2015 version

First UD Driver Board: QUESTIONS

Making progress on my 1st Universal Driver board, which I’m building:

(A) right-side, ala TubeLab, for prototyping
(B) initially in conventional G1 grid-drive configuration
(C) initially MONO, so I only need one board, but also because:
(E) to be powered by Heathkit P/S, max 100mA at 400VDC
(F) initially Ultralinear mode with 6L6GCs, or some JJ 7591s repro tubes, or possibly Gold Lion KT66s (but only have a pair). Leaning towards 7591s, as very cheap with good sonic reputation...


Getting close, and need some advice:

1. Built the +/- 150 VDC MOSFET supply on a piece of guitar amp board with used 450v caps, so Big & Ugly. See awkward class photo attached. Measured +/- 144 volts, within a few tenths between sides (matched the resistors). I assume that’s within acceptable limits; if not easy to raise it a little.
The nominal value of R9 & R11 (120K) gives 120K / 144volts = 833 ohms per volt, which is in the prescribed 500-1K ohms/volt.

2. Not planning on using FB initially (clean ultralinear first, which may be enough “feedback”), but want to be sure I understand the concept.
There are FOUR built-in FB “channels” as it were:
One to each anode of the four triode sections in the two (nominally 6CG7) drivers. Each FB “channel” has places for TWO resistors in SERIES.

I have ZERO experience with ANODE feedback, although I could certainly consult my files, Morgan Jones, and the Google Machine when the time comes.
In the meantime, I am installing single terminals at the distal ends of the resistor pairs (eg R19 & R40).

2A. Why PAIRS of resistors? Cap by-pass on only ONE of the pair?
2B. Any suggestions for starting points on Global FB to driver anodes? Alternately examples and/or reference docs?

3. For Conventional G1 grid drive, I am NOT populating R25, R27, R34 or R35.
Is 4.7K correct for the Negative Bias resistors R26 & R36? I doubt they will come into play with the Heathkit P/S and my prelim tube selection, but makin’sure.

4. I assume that 43K and 20K are correct plate resistors for V1 and V2 respectively at B++ of under 400 volts, is that right?

5. Haven’t seen a setting procedure for the trimmer on the SINKS of 10M45s (although I love this topology, and will use it elsewhere). I could measure the net resistance from Sink to ground, then look at the 10M45 chart ; this would be easy AND not under power. Alternately, could measure voltage across the plate resistors, I guess...what’s correct???

6. Haven’t seen the NOTES mentioned on the latest BOM, although I might have walked right past it.

7. Same for a start-up sequence; is there one around? If somebody has an OUTLINE, I could flesh it out (preferably NOT literally)....

Here’s the board so far; note the off-board BIAS and balance pots; at my age/condition, trying to keep from sticking a fork, I mean screwdriver, in amongst 400 volt things. Hence the question about the CCS Sink pots...
 

Attachments

  • E0BBD8BD-C4C7-4591-8AC8-1E9BE9A93D6B.jpeg
    E0BBD8BD-C4C7-4591-8AC8-1E9BE9A93D6B.jpeg
    442.9 KB · Views: 418
  • 8C1265BB-D43D-454D-BF86-98BF71263BB7.jpeg
    8C1265BB-D43D-454D-BF86-98BF71263BB7.jpeg
    491 KB · Views: 412
Going through this thread again waiting on my boards to arrive; Thanks George!! I will be pursuing a 6550 build. I have used an oscilloscope when in the military...you know, one of those courses where you try to drink from a firehose? Is it recommended to use an oscope? If I do, will most likely go the Rigol route for reasons previously mentioned. If yes, well, then TAKE MY MONEY! :) and off to youtube land to figure out how to work it again. Does it help alot with troubleshooting? I think it does based on the discussions and what I remember.

I do not plan on designing much ( I figure I will play with PSUDII) and hoping to piggyback on the helpful people here who have gone down the 6550 route to build out on the path blazed by others. I am shooting for a 45-55'ish amp, a bit more is reasonable but the 45 is a pretty hard line.

Regards,

erick
 
77series3....a scope is very handy for lots of things..like determining how much a change has reduced noise for example....or the shape of a square wave will let you know if you are missing out on some bass or treble response...and it makes calculating power output pretty easy too.

hareynolds...I'd love to chime in but my UBD boards have been stuffed for a couple years but I haven't powered them up yet..so I'm a bit behind schedule. I have an 80's vintage Fender Concert amp on the bench right now that needs some refreshing but haven't got to that either....yes maybe it's the summer approaching and covid under control that has folks doing other tasks....


George mentioned earlier somewhere that the documentation is a bit thin for the UDB, but like you, I think this will make an awesome amp..has everything I've been looking for..and a PCB to boot...I'm planning on KT88/6550's when I get there.
 
Last edited:
Member
Joined 2004
Paid Member
Is the BOM attached to George’s post #175 the latest?

I have seen people writing on this thread about stuffing their Universal Driver boards, but I have not seen a single review/report on a finished amplifier. I wonder how these boards actually perform outside the confines of Tubelab’s workbench.
 
I am listening to a set of mono-blocks I built using UDB's driving KT88's. I love the sound. I've had them for a few years now, very stable, no problems. It required building two power supplies which are documented in this thread. I have plenty of power, enough to drive some vintage AR 3a's. Highly recommended. Just another wonderfully design circuit by George Anderson. :)

Jacques

91CFA9D4-ECD8-4F90-926D-01C0C1D07768_1_105_c.jpeg
 
  • Like
Reactions: 4 users
Is the BOM attached to George’s post #175 the latest?

I have seen people writing on this thread about stuffing their Universal Driver boards, but I have not seen a single review/report on a finished amplifier. I wonder how these boards actually perform outside the confines of Tubelab’s workbench.
I got to the point where I quit building complete amplifiers years ago. I even sold some of my old amps at the last Dayton hamfest. It seems that I like to change amps far too often to keep making new ones. The wooden breadboard with a pair of UDBs on it seems to be the "amp of the day, week, or month" except when I want to play a SE amp. There is another wood board with a pair of Edcor 25 watt SE OPT's, a Hammond 372HX and a TSE-II board on it. I simply swap the TSE-II board when I want to go from 2A3's to 300B's. Lately the slightly modified (mosfet buffers omitted) UDBs driving 26HU5s in UNSET Power Head Boards is the amp of the month feeding my Yamaha NS10M Studio monitors. I haven't even made measurements on it, but it's just the ticket for cranking classic rock to the limit of the speakers, which is far below the capabilities of that amp. Once the motivation strikes me it will all get stuffed into a Landfall Chassis that I got over 10 years ago. That chassis has gone through three different amps, none of which were ever completely finished.

From distant memory the UDB - UNSET Power Head combination with any large sweep tube will make well over 100 watts at well under 1% THD without any GNFB on a power supply of 450 to 500 volts into the big 3300 ohm Edcors.
 
  • Like
Reactions: 1 user
Is the BOM attached to George’s post #175 the latest?

I have seen people writing on this thread about stuffing their Universal Driver boards, but I have not seen a single review/report on a finished amplifier. I wonder how these boards actually perform outside the confines of Tubelab’s workbench.
Here’s another.
 

Attachments

  • IMG_1039.jpeg
    IMG_1039.jpeg
    575.5 KB · Views: 55
George, I just noticed your announcement that you were closing down TubeLab at the end of this year. Obviously, I was involved with the project for many months and ended up building a couple of amps. Thanks for sharing your knowledge and shepherding many of us through that effort. Also,, best wishes to you and your family for the future. Final question: will the UDB threads (and other TubeLab threads as well) be available after the first of the year? If not, perhaps someone could post suggestions on how to copy and store that information.

Thanks again,
Warren Anderson
 
As I stated in post #1 of the Tubelab is ceasing operations thread there is a possibility that "Tubelab closing" might be delayed or postponed indefinitely. There are still some obstacles to overcome though.

As far as I know the postings here should always be here and my web hosting is paid up until August of 2024, so it will be around for a while.
 
  • Like
Reactions: 1 user