66 Deluxe Reverb Amp Bias

[angelodp]

I have a 66 Deluxe Reverb on my bench, all original, except the speaker (now a G12 Celestion Blue), and a change in the Tremolo circuit to slow down the Frequency. Aside from that its in nearly new condition and sounds great. I only had to clean the pots.

I am checking the bias and loathe to ad 1ohm bias resistors (is it done to vintage amps?), so I have some measurements.

PT Ohms on one side 220, PV - 355, B+ 360, Vd 3.2Vdc
other side 228, PV - 355, B+ 360, Vd 3.6Vdc

This gives me .014 A and 5.16 watts Diss.

This seems incorrect. The amp sounds great. Where am I going wrong here.

[pompeiisneaks]

That definitely seems low, it's below 50% dissipation.

I'd think that's near or in shutoff, and wouldn't play. I did look at the hammond replacement for a deluxe reverb and it says it should be about 428 ohms between both sides so half is close to your readings per side.

So really you should be reading more like 5.9 Volts drop across it to get to about 70% dissipation.

Also those voltages do seem a bit low for a Deluxe Reverb. The one on Rob Robinettes site seem to show more like 415V on the plates. also A B+ of about 420?

~Phil

[angelodp]

Ok yes I saw all that and agree, but as I read the voltage drop from plate to Pin 3 of the Rectifier that's what I am getting? Amp sounds classic. Am I doing this right?

If I use B+ to Plate I get a 5v drop, and so PD more like 8watts, better right. Not sure why, when I read from Plate to rectifier its low??

A

[sluckey]

Ok yes I saw all that and agree, but as I read the voltage drop from plate to Pin 3 of the Rectifier that's what I am getting? Amy sounds classic. Am I doing this right?

Your B+ is 360 and your PV is 355. That means the OT voltage drop is 5V.

5/220=22.7mA, and 5/228=21.9mA.

.0227*355=8W, and .0219*355=7.8W

8/14=57%, and 7.8/14=55.7%

I would like to run a bit hotter than that, but I don't see a problem.

[pompeiisneaks]

Yeah like sluckey said, I was thinking that at first too, that the 360 b+ and 355 PV means you've got 5v not 3.2. How did you calculate that 3.2 v? Usually it's the difference between those two.

~Phil

[angelodp]

I measured directly with DMM

[angelodp]

Would you recommend I crack open the Bias pot a bit?

[sluckey]

Would you recommend I crack open the Bias pot a bit?

If you like the sound then let it be.

[angelodp]

Copy thanks all. Now back that pesky Stereo amp.

[angelodp]

Ah... My bad, I was reading through the limiter. Without the limiter

B+ - 438
Plates - 433
Drop - 5v
220 - .022 x 433 9.4 Diss
228 - .021 x 433. 9.0 Diss

Thats normal and expected. Lesson learned.

A

[Stevem]

Without knowing you where running on a limiter I was going to post up that you have a very weak rectifier tube!

With the high v+ these amps run on the 6V6 outputs i have found that to me idling any type of 6V6 but a JJ brand at more then about.018 amp makes them sound too soft and undefined if you play them with the volume above 6 when the amp is into distorting heavily.

Of course this will depend on how strong the preamp tubes are back up stream in the signal chain and if the guitar being played thru the amp has humbucker or not.

Get a good feel from the owner of the amp as to how loud he plays it and set the idle current to give him a balance of clean and dirt that works best for him.

[angelodp]

Hi Steve, appreciate the input. I'll check into all that. The 6V6 appear to be Russian tubes, has nice compliment of preamp tubes and Rectifier is GZ34 Bugle Boy, so good stuff.