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Author Topic: Where can I adjust bias and OD-pot  (Read 15711 times)
mr fabulous
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« Reply #15 on: July 03, 2010, 03:37:35 PM »
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.
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hywelg
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« Reply #16 on: July 03, 2010, 03:38:39 PM »
Quote from: mr fabulous on July 03, 2010, 11:15:43 AM
the phase inverter is AC coupled to the output stage and therefore has no influence in the quiescent setting on the amp.

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.

Quote from: mr fabulous on July 03, 2010, 11:15:43 AM

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

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.
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erwin_ve
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« Reply #17 on: July 03, 2010, 05:32:39 PM »
Quote from: mr fabulous on July 03, 2010, 03:37:35 PM
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.

Thank you Mr fabulous; glad that's sorted out.
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stratocasterlover
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« Reply #18 on: July 05, 2010, 12:21:54 PM »
Quote from: mr fabulous on July 02, 2010, 02:36:59 PM
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.

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.
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mr fabulous
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« Reply #19 on: July 05, 2010, 12:39:15 PM »
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?
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stratocasterlover
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« Reply #20 on: July 10, 2010, 12:40:23 PM »
Quote from: mr fabulous on July 05, 2010, 12:39:15 PM
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.
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mr fabulous
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« Reply #21 on: July 10, 2010, 03:46:11 PM »
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.
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stratocasterlover
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« Reply #22 on: July 21, 2010, 11:35:05 PM »
Quote from: mr fabulous on July 10, 2010, 03:46:11 PM
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!
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erwin_ve
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« Reply #23 on: July 22, 2010, 08:40:56 AM »
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.
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mr fabulous
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« Reply #24 on: July 22, 2010, 09:27:41 AM »
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
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stratocasterlover
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« Reply #25 on: July 23, 2010, 09:41:16 PM »
Quote from: mr fabulous on July 22, 2010, 09:27:41 AM
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.
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stratocasterlover
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« Reply #26 on: July 23, 2010, 09:43:59 PM »
Quote from: erwin_ve on July 22, 2010, 08:40:56 AM
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.

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?
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