Ceriatone Forum

Hello.

Here are two pictures of my amp.
(http://bildr.no/thumb/679301.jpeg) (http://bildr.no/view/679301)

I want to measure the bias volt and I have read a thread about how to do it in this forum, but if you look at the picture I have tried to measure between the red and the black wire on the tube, don't get any values. Where is the correct point to measure?

My last question is, where can I find the OD pot to adjust? Here is another picutre of my amp: (http://bildr.no/thumb/679304.jpeg) (http://bildr.no/view/679304)
It would be nice if someone could tell me where in the amp I can find that one.

Best Regards.

if you look under the chaasis, between to the output tubes you will see a pot with no knob on it. in the first picture you posted you will see two red banana connectors (well the back of them).... and one black one.

to check your bias, you dont need to remove the chassis. you simply use a DVM able to read mV and measure between red and black for each valve. you need to get the voltages as close to identical as practicable. the bia pot will allow you to do this. make sure your tubes have been on for at least 5 to 10 minutes and all master volumes turned to 0.

try to buy matched sets. the reason being is that the bias current is set on the amp, and therefore the bias to balance the valves will be minimal.


hope this helps.

No disrespect to mr fabulous, but I don't think the bias is used to balance the tubes. I am a newbie also. but I believe the function of biasing is to apply enough negative voltage to the grids to limit the current from the plates. Without this neagative current, the tubes would overheat and self destruct. Think of this negative current as the blinds of a window to limit the amount of light coming in. Since electrons are negative and like charges repel each other, the negative bias limits the number of electrons passing through the grid.  If you are running 6l6 tubes, you need to bias to not exceed 70% of the 30 watt operation capacity. ODS's seem to like a cold bias. Usaually in the -34mv to-38mv range. Adjust to taste as long as you stay under the 70% max output of the tube.  If this seems too much to take in as it did for me innitially, you can probably set your bias at -34 to -38 range and be good to go if this was a prebuilt Ceriatone unit.  Let me restate that I am no expert and I hope other more experienced people can correct me if I have mislead you.
Good luck and enjoy.

Dennis

The OD Adjust is the trim pot is the one on the board closest to the preamp valve V2

Dear dennis
i am an electronics engineer, and know my stuff believe me. the current on the valves is more or less set to the nominal value. the bias pot is there to compensate for any differences in transdconductance between the two tubes, allowing them to be symmentrical.

Fabulous; start studiyng your stuff again.
Bias is the amount of current regulated through the power tubes. In this design it regulates the current for 2 or 4 tubes , not to balance them. You have to use a matched set of power tubes.

The pot for the phase inverter tube balances the signal between the 2 triodes which are sending their signal to their respective sides of the power tubes

i suggest you look at the schematic and see how its done

I gave you a clear description how it's working. Your assumption is wrong. Start tracing the circuit and let me know where you are wrong or where I'm wrong...

OK here goes. I'm not an electronic engineer and until 3 years ago knew absolutely nothing about valve electronics but thanks to the people here, the internet in general and some good books I have learnt enough to know that the bias control is not just to compensate for unmatched valves.

The bias control on the OTS sets the current flow in the valve at idle (so all controls should be set to 0 and nothing plugged in) by varying the bias voltage on the grid. It changes the bias voltage of both valves together thats why you need matched sets. You could mod it to have individual bias supplies but thats not what we're talking about here.

The bias controls how the valve behaves under load and it is generally recommended that the bias be set such that at idle the current flow is not greater than 70% of maximum plate dissipation. Some people do go higher but this will shorten the life of the tubes, but if this is the tone you want and are prepared to pay, then so be it. However if you set it so high that the valves start to 'redplate' then you will do serious damage and not just to the tubes.

70% plate dissipation equates to around 35-38mA depending on the valve make which because of the bias test system equates to 35-38mV across the test points. A lot of people prefer the tone at around 28-32mV and you should experiement within that range.

You should let the amp warm up fully before doing the measurements. Please make sure you have a load connected to the output  :D

Hi Hywelg, well explained.

+1 Erwin and Hywelg

http://www.webervst.com/tubes1/calcbias.htm

Put in your plate voltage - mine's around 470v, class is AB and tube type 6L6GC = 44.6mA (or mV, as measured at the bias points), that's 70% of plate dissipation and so is deemed a "hot" bias. As has been said, try from  32mV upwards to get the tone that you like.

the bias pot on the HRM is set in a complementary fashion. so if you incease the current to one tube, the other will decrease. nik obviously made it this way so there is little chance to setting too high current.

the phase inverter is AC coupled to the output stage and therefore has no influence in the quiescent setting on the amp.
as i said, you cannot change the overall current for the output tubes because as you increase the current on one, the other will be opposite. using this design, you will find a nominal pont where the quiescenet current through the two tubes are equal. the voltage measurements across the 1 ohm resistors are there to give you the current measurements  for each tube so that you can balance the current between the two tubes and also ensure that the power specs for the tube are not exceeded

I really can't believe you're a Electronic engineer; attached is the schematic where I draw a line in the section we're talking about.
BTW this isn't a cathode biased amp. Maybe there is where you go wrong.

+2  erwin and hywelg.
 I thought I was loosing my mind. At my age that could be a concern.  rogb, glad to know I did this the same as someone else, I too have around 470v on the plates. What a great forum for support and learning.

Thanks
Dennis

Even I, the valve electronics newbie know this is utter pants. The bias pot does not control the current directly, it controls the negative voltage (read the schematic and look how the bias supply is rectified) on the grid of BOTH output tubes. If you have the same B+ the same cathode voltages (which you do) and the same arrangements for grid and screen voltages then the bias voltage does affect BOTH tubes. And its purpose is to get the valve operating in the correct manner. Nik did not do anything different to any other amp designer, this is how variable, fixed bias octal tubes are setup. And yes you can set it to too high a quiescent current AND risk redplating if you so desired.

Mr Fabulous, you might well be a highly qualified electronics engineer, but you are obviously not familiar with the way valves operate. Please do some research before handing out misleading information that might harm a users amplifier.

mr apologies everyone. my mistake. i read my schematic after i modified for valve transconductance mismatch. i have found that sometime valves will drift in transconductance with time and hence placed a balancing pot in there, mistaking it for niks original design.

Correct, now follow the schematic and you will see that the ac coupled signals from the PI have a connection from the bias supply, thereby giving an overall -ve dc voltage. The value of this negative dc voltage is what controls the flow of electrons in the tube. And note we are always setting the quiescent, ie NO AC signal, bias voltage.


Not quite. Both tubes are fed exactly the same signal but inverted. The way the valves are working is that as one valve is amplifying the +ve side of the signal, the other is being fed the negative side of the signal and is driven in to cutoff and produces no output, the classic push-pull output stage.

Thank you Mr fabulous; glad that's sorted out.

Thank you, that helped a lot. I have measured now and adjusted the bias pot. I noticed that the tube to the right (if you see the amp from behind) is always 10 mV higher than the other power tube. I guess this can't be right? I bought a matched pair, and the tubes are new TAD 6L6.

I also think I found the OD pot, it is blue with a arrow shaped hole in the middle, right? I didn't find it at first because it doesn't look like what I think of as a regular pot.

according to nik 5mv difference or so is acceptable. 10mV is a bit on the high side (hence my balance pot mod)
did you buy them as a matched pair?

I have one more question

I have now bought Tung-Sol 12AX7 Gold and Winged "C" (SED) matched power tubes, and I have to say that the tone got much better... warmer, fatter and smoother. I measured the bias on the tubes and it was 47 mV and 46 mV, so only 1 mV difference between the tubes. Is 47 mV to much? How can I find out what voltage it should be? I tried to turn it down to 36 mV but I think the tone was better on the higher bias settings.

that means you have about 47mA running through the resistor. without further measurement we can assume maybe 5% of that current is grid bias current.... so 95% of 47mA is 45mA. now the plate voltage on the valve is 435 volts.
multiplying them together will give the IDLE dissipation. so 435 x .045=19.75 watts. this is about 66% of the maximum plate dissipation (30w)... a little hot.  60% seems to be the industry standard.

now remember, more power will be put through the tubes when you play, so i would say this is a little on the hot side. most people will bias at approx 60% which gives a good compromise between tone and tube life time.

according to nik, his optimal bias point is 37 mA which is on the cool side. (i remember reading somewhere that dumbles should be biased cool) 40 mA would be a good compromise (which is about 60%) and 43 would be considered "hot"

hope this helps.

Thank you for your answer! I have now adjusted it down to about 37 mV ( you say mA i guess you ment to say mV?)

I also have one more question. The amp seems flabby in the overdrive channel when I play on the three lowest strings.  I get a very flabby bass, I have tried different tubes, cabs/speakers, guitars but it seems to be the same "flabbyness", especially when you palm mute on the upper/thickest E-string.  Do you or anyone else know why? I have also tried different settings with the three pots inside the amp!

Lowering your bias will benefit the bass response; less flubby.

Cranking the presence control and at the same time lowering the treble also has a
good chance to control the flubby bass.

Picking the right speaker is also important; I prefer EVM12L; no flubby bass response at all.

without hearing it.... it could be any number of things..... starting with your guitar pickups, pots , cable, etc

also preamp tubes...... the first one will have most effect on the overall tone of the amp...... and the last one (V3) which is the phase inverter has a larger impact when the amp is turned up. V2 will affect the quality of the overdrive sound. i have no experience with tungsols as i have always used EH and Mullards.......try swapping preamp tubes for different brands and see if there is any difference. Nik has told me that tungsols have given him many headaches as they are not consistent.

but ..... i am just guessing........because i havent heard it.

but i would definitely try trying different preamp tubes

Thanks.

I have tested many different cabs with this amp, took it to a local store and tried marshalls and laneys, I currently have laney 2x12 with heritage elements. I have also tried different guitars, four different strats, telecaster and a les paul.

I have also tried different preamp tubes, from TAD to Grove tubes and now I use tung-sols...

I have biased it down to about 25 mV now, and there is much less flabby bass now, but it is strange that I have to bias it all the way down isn't it?

By the way, when I use overdrive pedal to get overdrive on the clean channel it is not so flabby. I can try to upload a sound clip later so you can hear what I mean.

Thank you for your answer.

I have now lowered the bias to a point where I can not lower it more. About 26 mV... I have also tested different cabs  with different elements, and different guitars. It got less flabby bass when I biased it down, and I also have very little treble and presence. It all helps, but I don't really think that it is the meaning that I need to do all these things to get an acceptable sound from this amps overdrive-channel. Could there be something else wrong with my amp?