Bias tool strangeness, works on one amp, but not on another.

[gmm52]

I have a basic bias tool for large octal tubes I bought on ebay. It has two sockets, leads to connect to any multimeter, and a switch box to toggle tubes. It does not have a 1ohm resistor inside the socket but instead measures bias directly, apparently like the Bias King models. I've used it successfully to set the bias in milliamps on my 5F6-A.

I just bought a used Blackstar HT40, which I really like, and I wanted to check the bias and perhaps try a different set of EL34's, but my bias tool does not work on this amp. With my autorange multimeter set on amperage, the reading is zero with standby on or off. Thinking the wiring in my bias tool might have failed, I tried it on the 5F6-A, and it is indeed working (~20ma, which is where I like it on the 5F6-A). I tried it again on the HT40, and no reading. I then tried mv setting on the multimeter instead of ma, and got a reading of 95mv on each tube with the standby on or off. Any ideas of what's going on? One thing quite unclear to me is how bias tools might be affected if there was already a 1ohm resistor installed between pins 1 and 8. I haven't come across any information on how this might affect bias tools of both 1ohm shunt style and direct reading like mine. I have not removed the chassis yet.

 

[danlad]

Weird.

Don't know but a guessing your device breaks the connection between ground and cathode to read current. Maybe the EL34 suppressor is wired in such a way you end measuring the current flow between ground and the grounded suppressor?

 

[RLee77]

What's the model # of the tester? I'd search online for info, like the manual and whether others have had the same problem. It's sort of a futile exercise to speculate what the mystery tester might be doing internally with no schematic. However, it shouldn't matter how the amp is wired; a breakout box like this simply allows you to measure the current flowing from the plate pin.

 

[gmm52]

Thanks for the replies. I tried searching for info related to the tester, but none to be found beyond the ebay listing: https://www.ebay.ca/itm/Suzier-T7-Q...um-Tube-amp-/282618093721?hash=item41cd59e099

There is a bit of information at the bottom of the listing, but that is it. The tester seems to be decently constructed. I am a bit confused as to whether I can measure the bias manually (without the bias tool) if there is no 1ohm resistor across pin 1 and 8, but perhaps that's another thread.

 

[gmm52]

So I've been reading up on biasing the HT40, and came across this interesting thread:

http://music-electronics-forum.com/t26564/ ...post #11. According to this person trying to bias an HT100 the HT series amps may have some kind of circuit that is essentially on standby if there is no instrument plugged in. I am positive I had nothing connected when I tried. Will try again this evening with something plugged in.

Post #16 in the same thread seems to confirm same. Very strange, but I don't care as long as I can bias the thing.

 

[dan40]

If the standby switch breaks the cathode connection of the power tubes then the tube will not conduct current. This is most likely the reason for the zero reading.

 

[clintj]

Can you tell which pin is interrupted by the probe? The eBay listing says it measures cathode current, which should be pin 8 on both tubes. Some designers like to tie things to unused pins, though.

 

[gmm52]

I have to confess my tech skills are too weak to understand and answer the question in the way you might like. I think the reason being, nothing seems to be interrupted by my reckoning. Nonetheless, guessing at the information you need to make a determination, continuity testing the male to female sides of the probe's socket indicates all pins correspond (1 connected to 1, 2 to 2, etc.) and do not connect to any other (1 to 8 or anything of the sort) regardless of how the switch is set. There is one DPDT switch in the probe's box. Peering into the socket(s) with a miniature dental mirror indicates no resistor inside, and tugging on the wire coming out of the side of the socket indicates wire connection to pin 8 only. O.K. Clintj, gimme the news.

 

[clintj]

Only thing I can think is that if the amp has pins 1 and 8 bridged to ground, the current would bypass the probe. But without having it safed for work and on the bench, I couldn't tell you for sure. Pulling a chassis just to look at and decipher what's likely a PCB on one of those is a bit excessive.

 

[gmm52]

Update: As per post #5, where I discovered that that other HT series amp owners reported the same issue and solved it by having something connected at the input, that is indeed the case. I connected a guitar, and got bias readings, 26 and 28ma.

The other peculiar thing about this amp is that 26-28ma is purportedly pretty cool, but the amp sounds terrific regardless. I can't imagine why I'd want to make it hotter. Perhaps Blackstar designed these amps to sound good at cooler bias settings?

 

[RLee77]

Contrary to popular belief, you don't have to bias tubes to the brink of destruction to get good tone.

 

[dan40]

I wrote above that the standby switch breaks the cathode connection in these amps but I meant to type "input jack" instead. It sounds like you've figured it out and got the proper readings now.

 

[Wally]

Regarding those bias numbers. Without knowing the plate voltage in those amps, one can only guess at how cool or how hot the bias is. Ex: if we take the 5F6A to be a Reissue that calls for 491vdc for a B+ with 20 ma of current draw at perhaps 485Pv, that establishes a plate dissipation factor of approximately 32%....quite cool, imhoamd ime.
In the Blackstar, I found reference to a plate voltage of 380vdc. We don’t know what your Pv is, so let’s use that number for the example. 28ma of current draw, 380Pv....around 43% of max plate dissipation...that is also cool. You may like things there. I like my amps biased a bit Warner than that.
Fwiw, the 70% of MPD that many use as a goal or a limit is nowhere near the brink of destruction. Some folks go beyond that on purpose. I find for me that somewhere 50-70% works...depending on the amp. Ymmv....but there is some harmonic richness that does not occur at cold settings.

 

[gmm52]

I wrote above that the standby switch breaks the cathode connection in these amps but I meant to type "input jack" instead. It sounds like you've figured it out and got the proper readings now.

Now your post makes sense. If I were you I'd have been inclined to change it, but for whatever reason, TDPRI removes editing capability a bit too quickly for my liking.

Regarding those bias numbers. Without knowing the plate voltage in those amps, one can only guess at how cool or how hot the bias is.

I've read 400V bandied about as being the typical plate voltage of HT40's, and I do realize I need to know the exact value to know what's really going on, but I'd conjecture it's close to average. I tried a pair of British made Mullard's last night, and the bias was down to 23ma. Based on tone alone, I'd have a hard time justifying moving it higher. The amp is perhaps the most quiet I've ever played, with virtually no transformer hum, and there appears to be oodles of harmonic richness and gain on tap. However, knowing my natural inclination to tinker until there's nothing left to tinker with (mind you, tinkering in the simplest of senses, e.g., tubes, speakers, and bias adjustment) I'll probably take the chassis out at some point to get the plate voltage and try it at a higher bias. It appears Blackstar recommends setting the bias by getting 50mv across two test points, as seen in the first post of this thread: http://www.mylespaul.com/threads/blackstar-ht-40-and-biasing.218568/

One thing I am puzzled about is how one accesses the pins to get plate voltage on those integrated tube sockets. Those slots do not look wide enough to get a probe in.

 

[cap47]

You can clip a 1 ohm resistor to pin 8 and ground and set to mv on meter. mv reading will be actual m amps.

 

[vanr]

On my Blackstar HT20 you have to have a plug in the input jack or it is in standby. Plug a dummy cable in and your bias probe will work.