New DSL100 *first impressions*

[Adrian R]

Adrian - I said this before and I say it one last time: on a 100w amp, with a plate voltage of approx 475vdc, your amp is severely underbiased if you're biased at 40ma per pair.
There is NO WAY your amp can sound the way it should with such a low bias point. If in fact, you are running a pair of tubes at 40ma cathode current, you're severely underbiased and starving your tubes. Your amp must have no balls, be very bright and lack power.

Okay man, lets get this friggin' straight! I was watching the how to bias video on Eurotubes website. In that video the dude (I'm assuming 'Bob') is adjusting the bias with two instruments. One, measuring current utilizing a pair of bias probes. Two, is also measuring voltage using a multimeter and taking readings using the bias pins on the back of the amp. Both of these testing methods are being performed simultaneously. According to this video, when voltage on the mm reads approx. 82mV, the current on his bias probe readings indicate 41mA..PER PAIR..so Why the disparity?

 

[jcmjmp]

Okay man, lets get this friggin' straight! I was watching the how to bias video on Eurotubes website. In that video the dude (I'm assuming 'Bob') is adjusting the bias with two instruments. One, measuring current utilizing a pair of bias probes. Two, is also measuring voltage using a multimeter and taking readings using the bias pins on the back of the amp. Both of these testing methods are being performed simultaneously. According to this video, when voltage on the mm reads approx. 82mV, the current on his bias probe readings indicate 41mA..PER PAIR..so Why the disparity?

82mv per pair, 41ma per tube.

 

[Adrian R]

82mv per pair, 41ma per tube.

Well now..thats interesting as there is still apparently a disparity. According again to 'Bob,..he clearly states that the bias trim pots each control 'one pair' of tubes...i.e...left trim pot for the left two tubes, and the right trim pot for the right two tubes... And btw..there is nothing weak about the performance of this amp! The fucker is stupid loud and ballsy...but if I'm wrong and so is Bob...then fuckin' A man, I suck, and I will make the correction!

 

[jcmjmp]

Well now..thats interesting as there is still apparently a disparity. According again to 'Bob,..he clearly states that the bias trim pots each control 'one pair' of tubes...i.e...left trim pot for the left two tubes, and the right trim pot for the right two tubes... And btw..there is nothing weak about the performance of this amp! The fucker is stupid loud and ballsy...but if I'm wrong and so is Bob...then fuckin' A man, I suck, and I will make the correction!

Each trim pot control the bias voltage to a pair of tubes.

You can measure the cathode current of each individual tube or per pair, hence the different readings.

82mv = 41mv + 41mv

It all adds up.

 

[Adrian R]

Each trim pot control the bias voltage to a pair of tubes.

You can measure the cathode current of each individual tube or per pair, hence the different readings.

82mv = 41mv + 41mv

It all adds up.

 

[RickyLee]

Okay then, I guess it depends then how you look at it I suppose. I thought about the elementary arithmetic before I made these posts. So in a sense the 'sum' of all the power tubes should read then 164mV..but you can only read two at a time utilizing the bias pins. I guess Bob's verbal illustration of 'controlling the left two tubes' and so on made me believe that the bias control determines the total current of the two tubes being adjusted, not the individual per tube current and the assumption that the total current draw would be twice that. Holy shit man, that makes sense! Thanks..

btw..when I got the amp the bias probe was only reading 26mA, and the seller told me the amp had been professionally biased...

And to F you up even further, those two bias trim pots along with their associated test terminal(s) are backwards to how you are looking at the four power tubes.

Looking at the back of the amp, the left trim pot and test point will control the two power tubes on the right. The right trim pot and test point will control the two power tubes on the left.

 

[Adrian R]

And to F you up even further, those two bias trim pots along with their associated test terminal(s) are backwards to how you are looking at the four power tubes.

Looking at the back of the amp, the left trim pot and test point will control the two power tubes on the right. The right trim pot and test point will control the two power tubes on the left.

Yea Ricky, you are right. The video did illustrate that point very clearly.

 

[Adrian R]

Each trim pot control the bias voltage to a pair of tubes.

You can measure the cathode current of each individual tube or per pair, hence the different readings.

82mv = 41mv + 41mv

It all adds up.

Just talked to Joe at Tube Depot. Measuring current with a bias probe ONE tube at a time, you should be around 41mA...82mA per PAIR... Talk about a completely superfluous thread!

 

[Guitargearfreak]

Just talked to Joe at Tube Depot. Measuring current with a bias probe ONE tube at a time, you should be around 41mA...82mA per PAIR... Talk about a completely superfluous thread!

Guys I'm so confused... I'm looking at that same video and the bias probe is in MiliAmps, while the multimeter is in MiliVolts.

I don't think that you can say 41MA+41MA = 82MV

 

[Guitargearfreak]

What I don't get from that video is why once he got too hot, he needs turn one side even hotter to be able to turn both side cooler... I know each sides work against each other, but why he has to turn both sides up, to get them both down... :S

 

[chuckharmonjr]

Thats what I was saying all along...82mv measured at a pin equates to 82ma going to the tubes....or 41 apiece

 

[Adrian R]

Guys I'm so confused... I'm looking at that same video and the bias probe is in MiliAmps, while the multimeter is in MiliVolts.

I don't think that you can say 41MA+41MA = 82MV

A bias probe allows you to measure current expressed as mA. He was two probes for both banks of two tubes. The probe only measures the current of ONE TUBE. The bias pins that are located on the back of the amp allows you to measure voltage as expressed as mV. He is using both methods simultaneously to show you that the amp can be biased using both methods.
(current and voltage)

If you are using the pins, the left pin gives you a voltage reading of BOTH tubes on the left (looking at the amp from the rear), the center pin is a ground, and the right pin allows you to read the voltage on the right TWO tubes. The two tubes together should read somewhere in the 80s...80 something on the left, and 80mV something on the right..and matched as closely as possible.

If you are using the bias probe in which is inserted in one of the two power tube sockets on the left or right, it measures the current of only ONE power tube which should be 40mA something. This means the left two tubes or the right two tubes added together should read in the 80s..again the closer the better.

 

[Adrian R]

Thats what I was saying all along...82mv measured at a pin equates to 82ma going to the tubes....or 41 apiece

10-4 Chuck! This was my thinking all along until someone chimed in on this thread and threw me into a tail spin! Never completely doubt yourself, yet at the same time, remember that there may be something you could have overlooked...wisdom teeth come in handy now and then.........

 

[ChrisinMO]

It probably did sound good until he put his old generic tubes back in it before he sold it. He probably didn't rebias it after he put the other tubes in that's why the bias was so low.

 

[Guitargearfreak]

Now, is there someone here kind enough to explain: why once he got too far on the trim pots (over 90), he needs turn one side even hotter to be able to turn both side cooler (under 90)... why he has to turn both sides up, to get them both down...

At the begining of the video, the amp is set too cold, and he have to turn the pot clockwise to turn it hotter. How can he also have to turn it clockwise to set it cooler?

 

[Adrian R]

Now, is there someone here kind enough to explain: why once he got too far on the trim pots (over 90), he needs turn one side even hotter to be able to turn both side cooler (under 90)... why he has to turn both sides up, to get them both down...

At the begining of the video, the amp is set too cold, and he have to turn the pot clockwise to turn it hotter. How can he also have to turn it clockwise to set it cooler?

Hey Dude,
While I cannot explain to you technically why that is, I can say that simply its the nature of the circuit. Just go with it! As you turn one side up, the other side goes down..just as Bob explains.."they work against each other"..Its really quite simple. I use a bias probe that fits into a empty power tube socket, and then a power tube plugs into it. You can now measure the current traveling through the tube socket. I would simply turn the 'opposite'(the screw furthest away from the tube(s) I was measuring) plastic trim pot..to the desired reading (around 41mA)..shut the amp down..waiting a couple minutes for the tubes to cool off, and then fire it back up but with the other tube being measured. I simply then repeat what I did earlier but with a different tube and trim pot. Works perfectly...Seem to get more stable and accurate readings then using the bias pins.

The video in a sense can be confusing..just look at it one meter at a time..and you simply use the opposite screw to dial in the correct reading..think of it that way...The only real odd difference is that you approach these trim pots as if they were nuts and bolts with reverse threads in where you go counter clockwise to increase current/voltage, and clockwise to decrease C/V...

 

[jcmjmp]

Hey Dude,
While I cannot explain to you technically why that is, I can say that simply its the nature of the circuit. Just go with it! As you turn one side up, the other side goes down..just as Bob explains.."they work against each other"..Its really quite simple.

Bob doesn't know...

Its simple: The bias pot on the left controls the bias voltage for the right pair and vice versa. Its as simple as that.

The "they work against each other" explanation shows how little Bob knows. There is a bit of interaction between the controls but that's not why they are reversed.