Tubemeister 18 bias

I have a Tubemeister 18 head that had a blown fuse. I think I have a bad tube. When I use it it seems to work and then the current shoots sky high and the tpc shuts it down. I replaced the fuse and tubes and it seems to be OK, except I checked the bias current, and the tpc sets the bias to around 7mA. For 390v it should be closer to 20mA in a fixed bias amp? Is there a way to adjust that? Does that mean there is something broken still? It is on both tubes. Thanks.
Dennis

Hi Dennis, glad to have you here.

There is absolutely no way to adjust the bias manually, nor should you need to do so.  There is nothing wrong with 7mA if it is only at idle.  Technically it could be even lower without any signal.  Bias is only important in two senses.  Firstly, that it should be balanced across the output valve pairs at all times so the output transformer does not experience any net DC running through it.  Secondly, when there is a signal it should be high enough to prevent crossover distortions from becoming too intrusive.  The TSC should take care of all of that despite what traditional theory regarding bias tells you, it isn't a traditional standard fixed bias output stage, it adjusts dynamically and very quickly too.  The old way of thinking of it doesn't apply with a self-adjusting microprocessor controlled bias system.  You said yourself, "...and it seems to be OK,..." and I assume you mean the sound, so don't fret because of something which seems different.  At the end of the day everything is about the sound and as long as that is good without any signs of distress elsewhere then it's fine.

And I will say that those old ways of setting up bias based on measuring the anode voltage and then calculating what the bias current should be based on a predetermined %age of maximum power dissipation in the valve is flawed too.  That way says that the result of your calculation is more important than the sound it achieves.  The tail is wagging the dog.  Supposing you decided on 70% distortion based on what all those other guys say it "should" be but you would actually have been happy with the sound at only 60%?  You are running hotter than you need to for no good reason.  It is better to first just set it by ear playing the cleanest you will ever require from the lowest bias setting a touch at a time, playing and listening each step.  When it sounds good to you then measure and work out what your particular final setting is dissipating.  You can always do a rethink based on the results of the calculation as to whether you are happy with that level or not.  If it is too hot for your tastes and would run the valves too close to max then you can try reducing it a touch and see whether you can live with the sound.  In this case your ears should really be engaged before your arithmetical abilities, you won't hear me saying that in too many areas, but in this case your ears can genuinely flag the change as the bias reaches your best level.  If they can't then you are going to end up with a lovely low bias setting for your own preferred sound which would be great despite the fact that others are saying "...but you haven't even reached 65% dissipation yet!"

Guitarists are usually non-technical guys who would love to be thought of as technical guys.  They always want technical things brought down to an easy to remember and apply set of instructions so they don't have to figure out and remember any theory and when they get that they love expounding the "truth" to others.  This is just like the [Bias Current = Max Anode Dissipation x %age / Anode Voltage] calculation.  Sometimes plain old common sense is the right way to approach it.  (And as a technical guy in real other life I do know the theory of both ways.)

Thank you for the detailed response. I agree with most everything you say. The only people more susceptible to snake oil sales than musicians is hifi types. Unfortunately I am both. But fortunately I am also a EE with about 40 years experience. Not in audio, but I have designed a lot of analog circuits over the years. I have also seen and heard tube amps that were biased too cold. i tend to set mine way on the cold side. As you say if it sounds good it is good. That may not apply if you are on the high side of the dissipation percentage.

So basically the Tubemeister TPC is just idling around 7mA, then when there is something to amplify it steps up the current to about 17mA. That's what my DC mulitmeter says anyway. That really is a smart way to do it. Plus the fact that the Standby switch does not shut off the B+, but merely mutes the signal from the preamp. Most research says it is bad to leave the amp in Standby when you go on break during a gig, but so many guitar players think they must "keep the tubes hot".

There is a very fine boutique tube amp called Nace. They came out with a new amp that contained a Standby switch. The designer happened to be active in a blues guitar forum I frequent so I asked him about it, that I heard it was bad, and his previous design did not have one? He pretty much confirmed that he added it because so many guitarists got it in their heads that a Standby switch was a mark of a good tube amp design. here this guy has designed a tube amp that uses a switching power supply and active circuits to allow the tubes to operate at lower power but still provide natural sound and start to distort at lower power levels and volumes. But people thought less of it because it did not have a Standby switch! So he added one to appease them. There were several people on the forum who were glad to see that his amp was now a "real" tube amp due to the addition of the Standby switch.

Thanks again for your comments. I was a little worried about selling the amp, with the belief that something was wrong with it still. I bought it broken off ebay to fix and resell. I got it working and it sounds really great. It had a blown B+ fuse inside, but no power tubes. So i now have confidence the amp is working well, and could sell it. But I may end up just keeping it. Besides it is cuter than heck!

Dennis

Hahaha, how odd!  My own background is in electronics design too and also over 40years in first the audio field for a few years but then into Seismic Surveying, (software control interfacing, small scale boomer design and hydrophone arrays).  And like you I'm a hifi nut, (British Linn/Naim led approach since the '70s).  And like you I am passionate about the "badness" of the Standby switch in its traditional (Bassman) form.  Everyone can quote "cathode stripping" at the drop of a hat which they won't accept doesn't occur in our receiving valves (4MV/m field strength?) but no one accepts the reality of Cathode Poisoning which does happen whenever the Standby is engaged.

For a bit of light reading have a look at this site.  It's a really good source of everything "Guitar Valve Amp" in terms of the circuitry written by an extremely knowledgeable in the valve field engineering type called Merlin.  He is extremely approachable for anything technical and has confirmed stuff for me a few times now.  He also used to be pretty active on DIYAudio.  The Valve Wizard  He does a magnificent job of describing the problem that we have with DC coupled Cathode Followers blowing out their triodes due to the lack of emission at switch on.  The DCCF is a well acknowledged "hard time" for any triode due to the out of spec cathode/heater voltages involved.  These are often alleviated by lifting the heater voltage with a DC level.  But a worse problem occurs at switch on and until the valves warm up and start conducting where the grid/cathode voltage is actually the full HT!  And a good solution is less than 10p, one 1/8W resistor and one GP diode, and can be performed by anyone who can do a bit of soldering.  No amp manufacturer that I know of adopts this solution despite the fact it removes a very real killer of valves in the mid to long term.  Our H&K models all suffer from this and H&K themselves have not adopted that solution!  Mind you, they didn't know about the DCCF's unique contribution to valve amp distortion until I pointed them at it and then it suddenly became a high profile marketing point.

capndenny1 wrote:But people thought less of it because it did not have a Standby switch!  So he added one to appease them.  There were several people on the forum who were glad to see that his amp was now a "real" tube amp due to the addition of the Standby switch.

Hey Cap'n. That's funny and sad. It would have been fun if he had just added a simple switch to short out, ie., mute, the input jack. I doubt that anyone would ever know how he put the amp into "standby".

Btw, newbie here myself, but I recently retired from the Naval Air Station here in Jax. Also did the Air Force and Army. I've got about 40-something years in avionics and audio. From old stereo to Peacekeeper missles to F-18's. But alas, I'm a simple "tech". Not an EE. But Lord knows I saved their bacon often enough.