Deluxe Reverb plate voltage concern

[bluesguitar]

Just picked up a '72 Deluxe Reverb on ebay. Before even turning it on I had it on the bench checking the plate voltage and bias. It had a solid state rectifier module, which I understand will increase the normal plate voltage by 10-20%. The plate voltage read 450 vdc. The 6v6 tubes were biased high at 56-58mA, but probably necessary to keep the plate voltage down around 450. My question is- I thought 6v6's had a max plate dissipation of 12 watts. According to my calculations my 6v6's are operating at over 25 watts. Can anyone add some insight to what I may be missing here?

[xtian]

Are you measuring current in a way that you need to divide by the number of tubes? Because 56mA sounds like 180% of max diss.

[bluesguitar]

Are you measuring current in a way that you need to divide by the number of tubes? Because 56mA sounds like 180% of max diss.

Good question? All I know is that I'm measuring the bias with my Amp Head dual bias tester? I'm new to how this (56mA) times voltage equals wattage, whether that is for one tube or if it has to be divided by the two output tubes.

[vibratoking]

You have one tester socket for each power tube, correct? If so, I believe that tester is measuring the current through each tube separately.

Are you measuring the voltage at the tube pin, after the dropping resistor? If so, then your dissipation is most likely exceeding the 6V6 spec. The tube can handle it for a limited time, but you should solve the problem if you want normal tube life.

Measure the plate voltage at the tube pin, Vp. Vp is nominally 415VDC on the stock Fender schematic. Measure the cathode voltage at the tube pin, Vk. Vk should be ground, but check it to make sure. Be careful not to short anything with your probe!!! Use the following equation: (Vp-Vk)xIbias. You should do this for each tube separately. This gives you a safe and fairly accurate approximation of power disspation. Not that there is an error because screen current is not accounted for, but this method is plenty accurate for most of us.

[ToneMerc]

That device seems to be pretty straight forward, it A/B toggles between either tube socket. So, if you have around 56mv at each tube,it's 56mA at each tube.

TM

[bluesguitar]

You have one tester socket for each power tube, correct? If so, I believe that tester is measuring the current through each tube separately.

Are you measuring the voltage at the tube pin, after the dropping resistor? If so, then your dissipation is most likely exceeding the 6V6 spec. The tube can handle it for a limited time, but you should solve the problem if you want normal tube life.

Measure the plate voltage at the tube pin, Vp. Vp is nominally 415VDC on the stock Fender schematic. Measure the cathode voltage at the tube pin, Vk. Vk should be ground, but check it to make sure. Be careful not to short anything with your probe!!! Use the following equation: (Vp-Vk)xIbias. You should do this for each tube separately. This gives you a safe and fairly accurate approximation of power disspation. Not that there is an error because screen current is not accounted for, but this method is plenty accurate for most of us.

I don't know what Vp stands for. I'm measuring my plate voltage with my negative lead grounded to the chassis, and my positive lead on pin 3 (to output transformer) on each power tube socket. As for cathode voltage I've never measured that. From the replies I've gotten it appears that my concern is warranted. A bias reading of 56mA (.056) x 450VDC = 25.2 watts per 6v6 tube, and this is 180% of max plate dissipation. Correct me if I am wrong.

[tictac]

There were some later SF Deluxe reverbs that were designed to use a 5U4 tube rectifier. If your amp is one of these a 5U4 would drop your B+ about 30V which would be just right for a Deluxe Reverb. Does it still have it's tube chart inside the cabinet?

Think you need to get rid of that SS rectifier whatever the case may be...

TT

[vibratoking]

Vp is just a name for the voltage at pin 3. It sounds like you are measuring pin 3 voltage correctly. Have you checked make sure the cathodes are really tied to ground. It's just a sanity check.

Your voltage is high. You could fix it with a tube rectifier or try to rebias the output tubes. Have you tried to adjust the bias? You may be able to adjust the bias current so that it is about 18mA, which is about 70% of max idle dissipation. But your bias circuit might not have enough range to get you there. Try it and see. You may be able to easily get the output tubes bias in the proper range. You may like the sound in that configuration. Of course, the voltage in the amp is still higher than a 'normal' deluxe. and you may want to bring it back in line with a tube rectifier.

[bluesguitar]

There were some later SF Deluxe reverbs that were designed to use a 5U4 tube rectifier. If your amp is one of these a 5U4 would drop your B+ about 30V which would be just right for a Deluxe Reverb. Does it still have it's tube chart inside the cabinet?

Think you need to get rid of that SS rectifier whatever the case may be...

TT

Actually, that is exactly what I did, because of my concern. The tube chart calls for a 5u4, so I put one in and biased to about 74% of max plate dissipation, which is as low as I could get it, but was OK for safe operation. The amp sounds awesome with some serious power tube grunt at vol 4. However, when I get the volume higher than that it breakes up too much for my taste and the low E string loses definition. I was going to try it with the solid state rectifier module to see if it tightened up the low end for more headroom, but wanted to get some feedback on the voltages before I proceeded. Thanks to this post I will stick with the 5U4, and see if I can get more headroom a different route.

[rp]

However, when I get the volume higher than that it breakes up too much for my taste and the low E string loses definition. I was going to try it with the solid state rectifier module to see if it tightened up the low end for more headroom, but wanted to get some feedback on the voltages before I proceeded. Thanks to this post I will stick with the 5U4, and see if I can get more headroom a different route.

1972. Has it ever had a cap job? That includes the e-caps on the 9pin tubes too. If it's orig a cap job will clean & tighten it up more than that ss rec did. Glad you got that 5U4 in there, wonder how long that poor thing was hurting with that SS in there.

[tictac]

For less voltage sag you could try a GZ34 rectifier instead of the 5U4. Sounds like you need a bit more -bias voltage to get the power tubes in the 50% to 70% dissapation range.

The bias pot has a resistor to ground that sets the range of the bias pot. The schematic to your amp shows that as a 10k. Try changing that to a 22k....

TT

[bluesguitar]

For less voltage sag you could try a GZ34 rectifier instead of the 5U4. Sounds like you need a bit more -bias voltage to get the power tubes in the 50% to 70% dissapation range.

The bias pot has a resistor to ground that sets the range of the bias pot. The schematic to your amp shows that as a 10k. Try changing that to a 22k....

TT

Thanks. Changing that resistor would be an easy fix, and easy to undo if I don't like it. I think it would address the bias current issue. First I'm going to try some JJ 6v6's. It currently has Groove tubes, and I know from experience with 6L6's that a pair of Groove Tubes ran 10mA hotter than a pair of JJ's. That may be all I need to do to get the bias in a cooler range.

[bluesguitar]

@ rp -By 9 pin I take it that you are referring to the preamp tubes. Those electrlytics are all original, and I normally do change those on my amps. It wouldn't hurt. The large filter caps in the can have been replaced with mallory's, but I don't know when. My concern now is making sure I keep my plate voltage under 450VDC, because those filter caps are rated at 450VDC. I'm not sure how much tolerance is safe there.

[ToneMerc]

For less voltage sag you could try a GZ34 rectifier instead of the 5U4. Sounds like you need a bit more -bias voltage to get the power tubes in the 50% to 70% dissapation range.

The bias pot has a resistor to ground that sets the range of the bias pot. The schematic to your amp shows that as a 10k. Try changing that to a 22k....

TT

It needs to be more positive, you do not want the larger tail resistor. With 450v he should not need anything larger than around 12k. Assuming the bias circuit is stock with a 10k pot, a 9-12k bias tail resistor should be plenty.

If you use JJ's , they can have a weird artifact with higher voltages and don't be suprised if it sounds better around 17-19mA.


TM

[jjman]

My '71 DR has a huge range of bias adjustment. Far more than anyone should need.