Tubelab SE Questions

[acceler8]

I finally have my Tubelab SE board stuffed, so now I'm moving on to the heavy parts. I do have a few questions at this point in my build (300B, mostly stock).

1. I've built my board on the backside of the PCB with the tube sockets on the front. I would like to mount the 4 pin sockets remotely on my case. I'm not after anything extreme, just 6 to 8 inches. I've seen this post: Build coming together nicely with George's statement "The customer had mounted the 300B tube sockets off board and the wire lengths were making resonant circuits causing massive oscillation resulting in blown mosfets. Ferrite beads and extra stopper resistors were needed to kill the oscillation."
I have 4 Ferrite beads, but I'm not sure which connections to put them on. Or do I need 8 Ferrite beads and put one on each wire? Which end of the wire should I put them (PCB, Socket, or middle?) Or do I need 16, and have one at each end of every wire? And the 'stopper resistor', what is that? What value (ohm and wattage) should I use and where should I put it? Sorry if this seems basic, but I don't want to assume anything here.

2. For R13,R24,R30 I used 120K Ohm resisters as per the parts list. However the schematic shows 130K Ohm resistors. I've installed the 120K Ohm, is this ok?

3. I'd like to install Bias Meters in my case. I've read this post: New Chassis for my Tubelab SE which states "To wire an analog current meter into a Tubelab SE (for permanent use), simply connect it IN PLACE of the 10 ohm resistor (R18, R29). Remove the resistors from the PC board, and run wires from the PC board over to the meter."
This is fine, but do I need an Amp (current) meter or a Volt meter? It seems that a Volt meter could be put in series with R18, R29, right? Is there any accuracy difference between using a Volt meter or a Current meter? Just to be sure, the current at R18, R29 is the Bias, right?

4. R15 and R26 are 1/4 watt on the parts list and 1/2 watt on the schematic. How critical is this?

5. I've heard good things about Transcendar transformers, but I guess at some point the source dried up. I see now that transcendar.com is active, and has products listed. Is this the same company; are they trustworthy? Does anyone have any recent feedback?

6. Speaking of Transcendar, I'm considering using their Choke "TT-010-CH" which is 10 Henry, 200mA, DC Resistance: 100 ohms. I know George recommends 6H at 150 Ohms, would using this cause me troubles? I know I'd have to tweak C4 to get my B+ in range for 300B tubes. For reference, I'm planning on using the Edcore XPWR131 power transformer. Any advice here would be most welcome!

7. I'm also considering Transcendar for my OPT. I'm looking at the "TT-020-OT" which is 3 K to 4, 8, & 16 ohms, Frequency Response: 18 Hz to 100 kHz, Maximum DC Bias Current: 150mA, Primary DC Resistance: 130 ohms. Does this seem like a good match for the Tubelab SE with 300B tubes? I'm partial to the 3K vs. a 3.5K or 5K as my speakers are not that efficient and the wee extra power from a 3K OPT would be welcome.

I'm sure I'll have more questions as I get deeper into this. I'm learning a lot and having a blast, though!
The community here is truly exemplary, and very helpful!
Thanks!

[rknize]

1) The bead(s) would go on the wire leading to the grid, as close to the tube pin as possible. Another trick is to jumper R33/34 on the PCB and move the 100 ohm grid stopper to the tube socket.

2) It should be fine.

3) All meters are volt meters. Some meters labelled as "current meters" or "ammeters" have shunt resistor in them. Others expect you to provide the shunt. You have to be careful here, though. Just because it has mA on the scale doesn't mean it has the shunt. But yes, if you get a true ammeter with a built-in shunt resistor, it would go in place of the 10-ohm resistor on the PCB.

4) He may have done that for working voltage requirements. If you used the part numbers from the list, it will be fine.

5) I have no experience here, but there have been some recent builds with them. I suspect they are still doing business. I suggest doing a search in the forum.

6) It should be fine. You may or may not need to tweak C4. I used a Hammond 193J here.

7) Hopefully someone with Transcendars can answer this, but from the numbers it looks good.

[dagerman]

Regarding Transcendar OPTs: I had ordered two for my second Tubelab SSE (first S as in simple) from an eBay listing. Completely painless, super-fast shipping, excellent packaging.
The ratings here are to be trusted.

My SSE runs KT-88 and those transformers sound great for the price (~$150/pair incl. shipping). I had initially ordered them just to have a cheaper pair for testing, but they have been in the amp 'temporarily' (Ha!) for a long time.

If your main listening is with acoustic, vocal or otherwise not bass-heavy tunes, they may be all you need. The UBT-2 (won't work in the TSE) I have in my first SSE build do sound better, but they cost roughly double, too.

I can recommend both the vendor as well as the transformers for what I have used them for, knowing that more money gets you further away from saturation with more 'complex' music. You can always keep them as 'breadboard babies', if you decide to go with higher-end OPTs later.

HTH,
Stefan

[acceler8]

Thanks for the feedback!

[acceler8]

I've finally (FINALLY) got all the main parts for the Tubelab SE together. I think it's been close to a year since I got the PCB from George. I still have to come up with a case, though. I was happy just to have music coming out of the thing!

As for my original questions in this thread:
1. I did use a ferrite bead on the grids of both 300B tubes. I also added a 100 Ohm resistor right at the socket. I left the 100 Ohm grid stopper on the PCB (I read elsewhere that this was OK). I would like to check for oscillations on my scope, but I don't have any probes that are rated for these voltages. I also think this amp would exceed my isolation transformer's current ability.

2. So far, the 120K Ohm resistors seem fine.

3. I still can't find a descent bias meter. I've decided against a current meter and am looking for a 0-100MV DC meter that I can just attach across the bias resistors. Ebay sucks, and there is just nothing modern that has the look I am after. The Sifam vintage series looks perfect, but they are not to be found at any price.

4. So far, the 1/4 watt resistors seem fine.

5, 6, and 7. I did go with Transcendar OPT's and choke. They were great to deal with, and shipped the items on schedule. The OPT's I went with (the largest they offer) are quite heavy. No comments on sound quality yet (just used on bench speakers).

Here is an image of the board during checkout on my bench. This is before the output tubes were put in.


I do have some new questions. Because I built my board inverted (tubes on top, everything else on the bottom) my U1 and D1 heatsink wound up being close to some other components. The diode (D5) is about 1/4 inch from the heatsink, and the capacitor (C1) is actually touching the mounting nut.






Is this heatsink too close? Are there any potential voltage differences that could cause problems later? The heatsink is connected to the PCB ground track (I think), so the main concern is some sort of arcing. I hope it's OK because there isn't a lot of room there.

I also wanted to make sure my voltages were OK.
No Tubes at all:
B+ 0.6V
B- -288V to -297
R Output Filament: 4.93V
L Output Filament: 4.89V

With 5AR4:
B+ 411V
B- 292V

With the 5842 tubes:
R 5842 175V
L 5842 175V
R output tube grid to ground -406V
L output tube grid to ground -411V

With output tubes (300B) to an 8 Ohm dummy load:
B+ 388V
B- 352V to 368V
R Bias .61mV
L Bias .61mV

After running for about an hour with low music:
B+ 387V
B- 330V to 360V
R Bias .62mV
L Bias .62mV

I am most concerned about the B- voltage. George's checkout guide doesn't really list the acceptable range. I think the schematic show -150V but I also think that was for a 2A3 or 45 tube. There is no audible distortion, but I've also not pushed it very hard.

[Davec113]

I'd keep hot stuff away from caps, I had one fail on my SSE (after 5 years of daily use) and IMO it is too close to the cathode resistor. I replaced them and moved the resistors further away. A Cap's lifetime is reduced at higher temps.

From looking at your pics, I'd move the SS devices off the PCB, but that's just my preference...

[Davec113]

I'd keep hot stuff away from caps, I had one fail on my SSE (after 5 years of daily use) and IMO it is too close to the cathode resistor. I replaced them and moved the resistors further away. A Cap's lifetime is reduced at higher temps.

From looking at your pics, I'd move the SS devices off the PCB, but that's just my preference...

[acceler8]

Thanks, DaveC! I'm planning on forced cooling with a pressure rated squirrel cage blower fan on the chassis so I'm not to worried about the cap. After an hour of running my thermocouple/multimeter probe measured 95F on the heat sink (hot but not crazy).

My real concern is my B- voltage at 330V to 360V. Is this an acceptable value? This was measured per the Tubelab SE checkout instructions across the resistor R7. This seems to be too high by double, and it really has me concerned. However, I'm thinking it is a measurement issue b/c I was able to run the amp for an hour or so with no distortion or unpleasantness.

[acceler8]

I've been really worried about the B- voltage, and so have not powered the amp back on. I've looked all over for what an acceptable voltage range should be, and haven't found anything conclusive. The Original spec sheet calls for -61 to -74 volts but from what I've read that is at the pins. I am measuring at R7 in the Tubelab SE per the check out steps.

Questions:

1. In a Tubelab SE with a B+ of 387, what voltage range is expected for the B-
2. How do I measure the grid voltage of the 300B? Meter positive on Grid Pin and meter negative to chassis ground?
3. If my B- is too high, what components should I test?
4. Does anyone know if there is a copy of the schematic anywhere that has the recommended voltages for a 300B at each critical point?

I did have another issue, that may be telling. When George says to turn the bias pots to their most negative value, he doesn't say what the expected range is. I was at -406 and -411 on each. Does this seem too high?

Thanks!

[boywonder]

Originally Posted by acceler8

I've been really worried about the B- voltage, and so have not powered the amp back on. I've looked all over for what an acceptable voltage range should be, and haven't found anything conclusive. The Original spec sheet calls for -61 to -74 volts but from what I've read that is at the pins. I am measuring at R7 in the Tubelab SE per the check out steps.

Questions:

1. In a Tubelab SE with a B+ of 387, what voltage range is expected for the B-
2. How do I measure the grid voltage of the 300B? Meter positive on Grid Pin and meter negative to chassis ground?
3. If my B- is too high, what components should I test?
4. Does anyone know if there is a copy of the schematic anywhere that has the recommended voltages for a 300B at each critical point?

I did have another issue, that may be telling. When George says to turn the bias pots to their most negative value, he doesn't say what the expected range is. I was at -406 and -411 on each. Does this seem too high?

Thanks!

The B- voltage for a 300B config is typically more negative than -300V. I'm no expert, but the only real worry here is that you exceed the drain-source voltage of your mosfets. If they are 900V rated parts, then no worries.

for Question #2, yes, exactly

If you can crank the pot to get that negative for bias voltage, that's fine. With the bias voltage more negative than about -150V, the 300B's will be completely cut-off, i.e. not conducting.

do you have access to a variac? If so, you can bring up the amp slowly. It makes the process much less white-knuckle.