How to BIAS the power tubes on a Bogner Alchemist?

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desertbluesman

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179
Ok I have a decent Radio Shack multi meter, and I want to learn how to BIAS the power tubes on a Bogner Alchemist. I am familiar with biasing my Hot Rod Deluxe which has a test point. Where would I find the test points or what would be the best method? And what would be the best setting for 6L6's in that amp?

I do not have any BIAS probes so where could I access the plate voltage on this amp?

I know better than to stick my hands in there other than to touch one point with the red multimeter point, and use a screw driver to adjust at the BIAS pot(s)

Any help or photos would be greatly appreciated.
 
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phsyconoodler

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4,301
Unless the amp has external bias points for your meter,it will require a tech to do the job for you that has a current meter.It is not set up like the HRD with bias test points on the PC board.
You cannot bias this amp without a proper bias meter.And because it was made in China by the Line 6 factory,opening the amp up may give you a world of trouble.
 
SatelliteAmps

SatelliteAmps

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Messages
6,168
phsyconoodler said:
Unless the amp has external bias points for your meter,it will require a tech to do the job for you that has a current meter.It is not set up like the HRD with bias test points on the PC board.
You cannot bias this amp without a proper bias meter.And because it was made in China by the Line 6 factory,opening the amp up may give you a world of trouble.
Click to expand...



This is very far from true. You can bias any amp without the use of a "proper bias meter." What you need is a multimeter that can actually measure mA up to about 100 mA. If the Radio Shack meter can do that, then it will work.
 
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desertbluesman

Member
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179
Thanks for taking the time to answer friends.

I will just go on and purchase the BIAS test probes on the net or at radio shack. I can open up this amp same as anyone else could do. I can learn how to do so safely. Which I am sure that I can do with some help from amp tech friends on the net. Anyways I am not afraid to open the amp, until I meet a roadblock I can not get by.

Thanks
 
SatelliteAmps

SatelliteAmps

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6,168
Just be careful. There are tons of sites dedicated to biasing amps. A few things to try to remember. When working with an amp that is off, make sure it is unplugged. Make sure to drain the caps when working on the amp when it's off.

When working with a live amp, make sure you have one hand in your pocket. Much harder to kill yourself that way. Use test lead clips instead of holding the leads in your hands. The sequence goes like this. Make sure amp is unplugged. Clip one test lead to the plate, one test lead to ground. Set meter to measure DCV. Put one hand in pocket. Plug amp in. Turn amp on. Look at meter. Turn amp off. Unplug amp. Remove test leads, one at a time, hot first.
 
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desertbluesman

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Messages
179
Thanks Adam, I think I will play a nice fat chord on the guitar and turn off the switch on the power strip the amp is plugged into, and let much of the power drain off, then remove the amp head from the cabinet put in the BIAS test probes which I will buy. Put the amp up on wood blocks to protect the components underneath and give the amp a firm place to sit, then proceed slowly and carefully. Plug the darn thing in and keep one hand in pocket at all times and never touch anything without a tool. Get 'er done, Plug in a speaker cab. Play the power Chord again, drain off as much power as can be drained and put the amp back in carefully.
 
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geodude

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6
I just purchased an Alchemist and I'm still trying to figure out how to bias it. The best I can figure the resistors labeled: R9 & R10 are the bias adjustments. Since the circuit looks symmetrical I'm assuming there is a separate bias adjustment for each tube.

To be sure I called technical support. Technical support said they will only give this information to a registered service center but I should know that the Alchemist is not a "typical DC bias amp".

Does anybody know for sure?
 
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phsyconoodler

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4,301
If the amp has adjustable bias it should be very obvious.
If they say"it doesn't have the typical DC bias",it may have an auto-bias system which is nasty at best.If they won't give you this information at Bogner,they need to re-examine what customer service actually means.
 
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geodude

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Thanks for the quick response.

I'm assuming it's not an auto-bias circuit since the Manuel says you should re-bias the amp when changing power tubes.
 
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geodude

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I’ve built three guitar amps, added a bias control to a Princeton reverb and biased several other guitar amps. I know this FAR from qualifies me as an expert but with the correct information I believe I am capable of doing this…Does anyone have the correct info?
 
mark norwine

mark norwine

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17,246
geodude said:
Does anyone have the correct info?
Click to expand...

If I may be so bold.....

Based on what you wrote, you don't want "information" at all.....you want "instructions". There's a huge difference.

For a fixed-bias amp (ANY fixed bias amp), learn the concept then apply the mechanics. That is to say:

- understand that fixed bias is simply the application of a neg voltage to the grids.

- understand that the control of that neg voltage is accomplished via the manipulation of a voltage divider.

The rule for calculating the output of a divider is:

Vout = (Vin * R2) / (R1 + R2)

Get a simple schematic somewhere......princeton reverb for example
http://schematicheaven.com/fenderamps/princeton_rev_gz34_aa1164.pdf
.....and study the relationship of the 2 resistors in question. Do you see how changing one (the 27K i.e. R2) will change the voltage applied to the grids? Use the formula to see what happens when you sub the 27K for....say...18K. The voltage applied to the grids will go down, and current through the tubes will therefore increase.

Learn the math...learn the concept. Once done, you'll start to see that damn near every fixed bias amp is exactly the same. Sure, some might have a pot while others don't, but so what? The concept is unchanged.
 
RussB

RussB

low rent hobbyist
Messages
11,156
phsyconoodler said:
Unless the amp has external bias points for your meter,it will require a tech to do the job for you that has a current meter.It is not set up like the HRD with bias test points on the PC board.
You cannot bias this amp without a proper bias meter.And because it was made in China by the Line 6 factory,opening the amp up may give you a world of trouble.
Click to expand...

You must know that's not true. I wonder why you would even post it?


Measuring the voltage drop across the OT would work...I wouldn't recommend it for someone without experience, but it's a reliable bias current measuring technique
 
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geodude

Member
Messages
6
mark norwine said:
If I may be so bold.....

Based on what you wrote, you don't want "information" at all.....you want "instructions". There's a huge difference.

For a fixed-bias amp (ANY fixed bias amp), learn the concept then apply the mechanics. That is to say:

- understand that fixed bias is simply the application of a neg voltage to the grids.

- understand that the control of that neg voltage is accomplished via the manipulation of a voltage divider.

The rule for calculating the output of a divider is:

Vout = (Vin * R2) / (R1 + R2)

Get a simple schematic somewhere......princeton reverb for example
http://schematicheaven.com/fenderamps/princeton_rev_gz34_aa1164.pdf
.....and study the relationship of the 2 resistors in question. Do you see how changing one (the 27K i.e. R2) will change the voltage applied to the grids? Use the formula to see what happens when you sub the 27K for....say...18K. The voltage applied to the grids will go down, and current through the tubes will therefore increase.

Learn the math...learn the concept. Once done, you'll start to see that damn near every fixed bias amp is exactly the same. Sure, some might have a pot while others don't, but so what? The concept is unchanged.
Click to expand...





I appreciate your input but to the contrary, I do want information not instructions (maybe a little bit of instructions).

First, tech support said that the alchemist is not a typical DC bias amp – I’m not exactly sure what they mean by this – Do you? I am fairly confident that it is not a cathode biased amp.

Second, the amps I’ve biased in the past had one pot the Alchemist has two – My first assumption is that there is one pot for each power tube (The circuit appears symmetrical). On the surface that sounds good except the Alchemist is a low budget (Chinese made) Bogner. I would think they would want to keep costs to a minimum – One pot for each tube would sound like a more expensive design.

As you suggested, I could start adjusting the R9 & R10 variable resistors and see what happens but with tech support’s comment in back of my mind, I’d rather have some facts before I potentially blow up a brand new amp!
 
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geodude

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Messages
6
Addition to the above post: I have biased amps with two pots. the second pot being a hum balance adjustment - I don't think this is the case with the Alchemist since the power circut with the pots appears to be symmetrical.
 
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phsyconoodler

Member
Messages
4,301
Russ B wrote:"You must know that's not true. I wonder why you would even post it?
Measuring the voltage drop across the OT would work...I wouldn't recommend it for someone without experience, but it's a reliable bias current measuring technique"

Yes Russ,I understand that,but we are dealing with someone who cannot recognize the bias circuit in the amp,does not have a schematic,and has LIMITED EXPERIENCE.

And they are made in China,and if it does not have the 'usual DC bias system",what would we be telling him to do?
I find it kind of appalling that Bogner wouldn't just share what exactly he has to do to bias this amp."go to a tech" is still not the answer especially if the tech cannot decipher the bias circuit without information and a schematic from Bogner.
 
RussB

RussB

low rent hobbyist
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11,156
phsyconoodler,

I knew where you were coming from and I agree with you, but no information is better than mis-information. :)
 
T

tooltime

Member
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1
I don't know if anyone reads this thread anymore or not, but I thought I would add my $.02. I just biased my Alchemist last night, so I thought I would share how I did it.
Those 2 little blue pots are in fact the Bias adjustments. There is also a little white 3-prong socket that says something like 'bias test' beside it.
We are basically measuring how much power the tubes are consuming, and of course P=V*I.

***BE VERY CAREFUL POKING AROUND IN ANY TUBE AMP, YOU CAN BE SEVERELY INJURED IF YOU GET SHOCKED***

1)Turn on amp to 20W mode and let it fully warm up.

2)Measure 'B+ screen voltage' from the header on the Bias board (20W should be ~325VDC)

3)We now need to measure the voltage at the 3 pin socket. left pin is left tube, middle is ground, right pin is right tube. This will tell us how much current is flowing through the tubes. Long story short, it will be off by a factor of 10 since they used 10ohm bias resistors. since we want 20W total, each tube will need to be set for 10W. 10W = 325V*I so we would want about .031A. However, since they are 10ohm resistors we want it to read about .31VDC. So set your meter to a low DC setting and measure from the middle pin to each side and adjust the pots accordingly.

4)For 40W, B+ should be around 425VDC, since each tube should be set for 20W, 20W=425*I. I needs to be about .047A, so out measurement should be about .47VDC. You shouldn't have to adjust for both power ratings, they should scale themselves, but it wouldn't hurt to check anyways.

5)You can adjust them hot or cold to your liking. Typically, 6L6GC tubes can handle up to 30W, so you shouldn't hurt anything by setting them a little hot. 30W=(B+) * I

I hope that helps!
 
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