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 Post subject: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 3:06 pm 
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Had it for about a month and today I measured the bias after getting a new probe. I set the multimeter to mA and the reading was really low at about 7ma which is really cold. So I proceeded to increase the bias and to my surprise the screw came to an abrupt halt. The maximum I can get from the probe is 9ma which is still way way cold, I was planning on running about 20ma.

Am I doing something wrong or are Fender now preventing us from hot biasing an amp? If true then no wonder everyone is saying these new amps don't sound as good as the old ones! Is there any way to do some rewiring to get the biasing I want?

The instructions that come with the probe tell me to set multimeter to mV but don't see how that's useful. For now I've turned the amp off and reduced the bias.


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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 3:18 pm 
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I've heard these new amps have lowish plate voltages so let's say 420 is optimistic. 420 x 0.009 = 3.78, think a 6V6GT is rated at 9W but many say they are actually rated at 14W, if 9W then 3.78 is only 42%, if 14W which it probably is - only 27% dissipation.

I have cranked this amp and it don't break up at maximum volume. The valves might last forever in it but that's not why I bought the amp. I don't mind changing the power tubes every 6 months or so but there's not much point in having this amp when it's so cold sounding.

Maybe I will have to sell it unless someone has some bright ideas?


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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 5:13 pm 
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Hi Rich_A10,

Most 6V6 are 14 watt or 12 watt rated, check the manufacturer listed specs.

A couple of thoughts. What probe are you using? Typically a socket bias probe will have leads to connect to a volt meter, and the meter should be set to DC volts (not mA) to measure the mV across a 1 ohm resistor within the probe socket. This mV reading can be directly read as mA because of the 1 ohm resistor (Ohm's Law). Are you getting the same results on both tubes?

Some tubes will only draw so much current, and have lower maximum than another type might have, but this seems way too low to be the tubes. Was the bias adjust added?

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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 5:20 pm 
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shimmilou wrote:
Hi Rich_A10,

Most 6V6 are 14 watt or 12 watt rated, check the manufacturer listed specs.

A couple of thoughts. What probe are you using? Typically a socket bias probe will have leads to connect to a volt meter, and the meter should be set to DC volts to measure the mV across a 1 ohm resistor. Are you getting the same results on both tubes?

Some tubes will only draw so much current, and have lower maximum than another type might have, but this seems way too low to be the tubes. Was the bias adjust added?


Hi there, I just put a new matched pair of Tung Sol 6V6GT in and when I set meter to mA they both give about 8.6mA with the bias turned to maximum - that's DC reading. I bought this probe off ebay http://www.ebay.co.uk/itm/OCTAL-VALVE-B ... 1c231eb685

I also changed the battery in the meter but it made no difference. The mV are about 40 IIRC. Surely this can't be the norm?


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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 5:39 pm 
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Dude! You missed it. :lol:

Set the meter to DC volts. You are reading mV and converting to mA. You can not read mA from the probe. If you set your meter to amps, you will short the probe resistor.

From the link that you provided for the probe:
This bias probe is suitable for use with standard multimeters with 4mm sockets using the mV setting.

The reading of 40 mV would equal 40 mA for one tube!!!!! That is plenty and you should probably back that down to around 20 to 23 mV. :!:

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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 6:13 pm 
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Agh, hope I not fried my amp!

My meter says mA 200 with the DC symbol and I get 8.6mA. I've really no clue about electronics but how can 40mA = 40mV? Surely if the tubes had 40mA running through them they would have blown? 40mA is about 15 to 17W so maybe they would be OK. I played it like that at very low volume for about 10 minutes, sounded quite lushuss with a 12AU7 in reverb slot. I've also taken out the first preamp as I don't use the normal channel and put a Tung Sol 12AX7 specially picked for P1/phase in P6 which is driver/phase inverter.

I hope you're right about this shimmilou - are you 100% sure? I don't get the mV mA thing though. I have been taking readings in DC mA and nothing blew up.


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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 6:31 pm 
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Yes, 100% sure, forget the mA scale, use a DC volts scale. Your meter may have a DC mV scale, set it to at least 50 mV to read the bias. And you are correct, at close to 400 Plate volts, and 40 mA of current through one tube, that is approx 16 watts. Typically, look for the tubes' Plates to glow red if biased too hot. Likely no damage to your amp, but there could be damage if you run it with the bias too hot, and 40 mV is too hot. Depending on how long you have used the amp like this, the tubes may have been damaged if the Plates were red.

The probe uses a 1 ohm resistor in series with the tube Cathode. There is a current running through the resistor when the tube is conducting. Using Ohms Law, we know that the current running through a resistor results in a voltage drop across the resistor. The voltage drop is what you are measuring with the probe. When a 1 ohm resistor is used, the mV reading translates directly to mA. Although you are reading mV, it is the same number when converted to mA using Ohm's Law. If this seems difficult for you to understand, do some research on Ohm's law to gain some knowledge about the relationship of resistance, current and voltage. Understanding the electronic principles involved in this testing method aren't nearly as important as following the directions for the equipment that you are using. :wink:

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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 7:05 pm 
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OK well thanks for you help, much appreciated. Everyone always talks about mA so that's what I was looking for. What is the mA reading I get then of 08.6?

I was playing it at lower than TV volume. What are the signs of a damaged valve? I guess the valves will just die more quickly but 10 minutes isn't very long. I'm not sure about the 12AU7 for reverb as to me it seems to add more noise as I turn the reverb up but will have to compare it with original 12AT7. The 12AU7 gives more control over the range of the dial but am sure it's added noise.

Fingers crossed my amp is working OK tomorrow when I set the bias correctly. Think I'll set it to 25 and see how we go.


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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 7:40 pm 
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Sorry to intrude... maybe this will help. Maybe not. Here goes.

Normally, you would need to break the path of electrons (actually break continuity) insert the DVM reading current (in amps or mA) onto the ends, to complete the circuit. Them, turn the amp back on to read the flow or current from the chassis to the cathode.

This is kind of a pain. So, since:

E (volts) = I (current) TIMES R (resistance),

Then, if we put that equation (Ohm's Law) into our little grinder, crank it a few times--- out comes:

I (current, in this case idle bias current) = E (volts, on the anode plate) divided by R (resistance from the cathode to ground, which you read the voltage drop across with a Bias Probe or some such tool).

In addition, if this resistance is a precise 1-ohm resistor, then the idle bias current (in mA) = measured mV across that resistor.


Now, when you put a DVM onto a Bias Probe, or some such device and measure the current (mA) you are prolly getting some spurious value. Not of any consequence. Or do any harm to the amp or tubes. You should be careful, as some DVM settings can cause damage to the meter if improperly used. However, most modern DVM have fuses and breakers to avoid major interior damage to the tool.

Back to idle biasing. The Idle Bias wattage is important. Not just the current. And the wattage is a function of the current and the plate voltage. Therefore, it is a good idea to measure the anode plate voltage of each output you are biasing.

What your goal is dependent on the Class of the amp (most Fenders are Class AB1). The topology: Is it push-pull or single-ended. In addition, tube manufacturer's recommendations.

As for 12AX7, 12AT7, and 12AU7. These are called small signal tubes. These tubes are usually set in a Class "A" single -ended topology. Which are cathode-biased. They are somewhat self-leveling. Do not need re-biasing in 95% of the cases. It is why you can change tube manufacturers, even tube types (as long as the pinout & heater voltage requirements are the same) and the new signal tube should be ok.

Good luck with all that. HTH. And did not make the water more murky.


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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 7:51 pm 
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Yes, the current (mA) is the number that you want to use to figure bias. However, the way that most probes are constructed using a resistor, you have to read mV on the meter, then translate it to mA using Ohm's Law. There are other types of probes that are made to allow a current meter to be used, but yours is not one of these types, yours reads mV (DC volts).

Forget any reading that you get using the mA scale on the meter, it is erroneous, stop referring to it, stop thinking about it, don't mention it again, please!

Just remember that with your probe, you read mV on a DC volt scale, and count that number as a mA reading.

25 mV on the meter using your probe = 25 mA of current through the tube. You must use the DC volts scale with this probe.

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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Thu Mar 01, 2012 8:15 pm 
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Sure hope you guys got through to the OP.

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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Fri Mar 02, 2012 7:15 am 
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OK it's at 30/31mV which is I'm guessing about 85 - 95% dissipation. I play at bedroom volumes so need as much warmth as possible. Played it at gain 10 for about 20 minutes with no problems. That 12AU7 definitely adds low hum noise to reverb channel, don't know what the guy was talking about that recommended a 12AU7.

There was some nice compression/saturation, not much break up more of a very mild crunch but I know that it's more crunchy to the mic than to the ear. I think the 12AX7 in P6/phase has warmed the tone up a bit.

Thanks again.


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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Fri Mar 02, 2012 10:04 am 
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Awesome! Some like it hot. :lol:

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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Fri Mar 02, 2012 11:00 am 
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Rich_A10 wrote:
OK it's at 30/31mV which is I'm guessing about 85 - 95% dissipation. I play at bedroom volumes so need as much warmth as possible. Played it at gain 10 for about 20 minutes with no problems. That 12AU7 definitely adds low hum noise to reverb channel, don't know what the guy was talking about that recommended a 12AU7.

There was some nice compression/saturation, not much break up more of a very mild crunch but I know that it's more crunchy to the mic than to the ear. I think the 12AX7 in P6/phase has warmed the tone up a bit.

Thanks again.


Don't guess at the plate dissipation. Measure the plate voltage and calculatre the % plate dissipation. Otherwise you may be coming back here in a few weeks asking why your output tubes burned up. :lol:

85% to 90% is ok for a class A amp (which the DRRI is not), but way to hot for a class AB amp.

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 Post subject: Re: Biasing Deluxe Reverb reissue
PostPosted: Fri Mar 02, 2012 11:23 am 
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May want to look at my post of last year about my biasing...
viewtopic.php?f=13&t=58087


There is also a calculator somewhere in the same post .

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