First Time Biasing my Original '68 Fender Deluxe Reverb

[Perkinsman]

I'm learning (from Rob Robinette's excellent site) to bias one of my older amps and just need to bounce this off of someone more experienced to reinforce what I did. I'm using JJ 6V6's, measured V7 plate@420vdc, V8 plate@419vdc. B+@ 425.5vdc a difference of 5.5 & 6.5, respectively. I measured the plate to OT center tap resistance, V7@230.8ohms & V8@226.3ohms. Calculating the V7current, 5.5v/230.8=.0238a or 23.8ma, V8current, 6.5v/226.3=.0287a or 28.7ma

If my calcs are correct, are these idle currents too hot? too cold? Just right?

I noticed that when I raised or lowered the bias pot, both the plate and the OT voltages increased or lowered concurrently. How then does the bias get adjusted?

[Gingertube]

Calcs are good.
For V8, 28.7 mA x 425.5V = 12.2 Watts Anode dissipation in a tube which has a data book rating of 12 Watts anode dissipation.
So it is a bit hot but not that unusual.

The culprit here is the B+ voltage which is 25V above the "typical" 400V

I would be inclined to back the idle current off - ideally both tubes (if perfectly balanced) should have around 20mA with a B+ of 425V

You could leave the bias a bit hot but back of the bias pot to get say 24 mA in V8 (the hottest tube) which is about 5.4 volts where you currently see 6.5V.

Of course I 'm a bit obsessive and have a heap of 6V6 so I would be looking for a better balanced pair of 6V6 first but that is in "nitpicking" territory.

Cheers,
Ian

[pdf64]

Just to note that the JJ6V6S has a 14 watt plate limit. I assume that’s under the design max rating system commonly used for 6V6GTA info, rather than the design centre system used for 6V6GT info. The actual plate dissipation ratings of 6V6GT and 6V6GTA are the same, eg the GTA limit is 12 watts under the design centre system.

Note that it’s perfectly normal for HT / plate voltage to change according to the bias setting, as the bias voltage used adjusts the idle plate / cathode current, which is the main thing that loads the HT power supply.
In just that same way that your mains wall voltage will probably dip / sag a little if all the big appliances in your house were working at full power simultaneously.

The only way it could be avoided totally would be to have regulated power supplies in the amp. Folks that have tried that (well, steve conner at least) advise that it has a negative effect on how a guitar amp responds.

Are you sure that you a getting a benefit from such a high idle plate current? I suggest that you see how it sounds / responds at 20mA. The lower the plate current the longer the tube life.

[Perkinsman]

Thank you, these are JJ6V6S tubes so I'll take your word on the 14W PD. I will assume that means that I can run the tubes at a little higher current than I would the 12W 6V6's. As mentioned by my Australian friend, Gingertube, the key seems to be dropping the B+, to the 420V spec. Is that an adjustment that I can make?

[Gingertube]

I looked at the '68 Custom Deluxe Reverb Service Manual and it says +415V for B+ whereas the original 1968 Deluxe Reverb said +398V so probably leave it alone.
If you MUST trim it back a bit, the place to do this is small series resistors in the two (2) red wire feeds from the power tranny to the 5AR4 rectifier anodes (pins 4 and 6), say 33 or 47 Ohms 2W resistor in each feed - BUT that may change your sound (More compressed).
Cheers,
Ian

[Mick Bailey]

Also the original schematics are marked "Voltages read to ground with electronic voltmeter. Values shown + or - 20%". That's some variance. In the UK at least these original amps are nearly always running higher voltages than the design spec of the tubes, and they've been doing this for decades. Usually they're close to 440v. My go-to tube for this situation is the 6V6S. Previously I'd been using Phillips JAN NOS 6V6 but these had become too expensive. If the dissipation is lowered as per Ian's suggestion, then the voltage isn't an issue.

I can't recall ever lowering the B+ in a Fender amp and many get some serious usage in the hands of full-time musicians.

[pdf64]

Thank you, these are JJ6V6S tubes so I'll take your word on the 14W PD. I will assume that means that I can run the tubes at a little higher current than I would the 12W 6V6's. As mentioned by my Australian friend, Gingertube, the key seems to be dropping the B+, to the 420V spec. Is that an adjustment that I can make?

No need to take my word, see https://tubedata.altanatubes.com.br/...163/6/6V6S.pdf
But the logic of idling a design max 14 watt 6V6 hotter than a design centre 12 watt 6V6 seems flawed. Until it’s revealed what rating system JJ use, I suggest to treat it as any other 6V6.
If you wish to cool off conditions in an amp and you’re in a 120V mains region (including that in your profile helps avoid pointless suggestions), the brownbox looks a good solution https://www.amprx.net/
I think you are idling the 6V6 too hot and ~20mA is more normal / reasonable; you understand that their plate dissipation increases when passing signal?
As you’re learning why not experiment with various cooler idle settings?

[nevetslab]

No need to take my word, see https://tubedata.altanatubes.com.br/...163/6/6V6S.pdf
But the logic of idling a design max 14 watt 6V6 hotter than a design centre 12 watt 6V6 seems flawed. Until it’s revealed what rating system JJ use, I suggest to treat it as any other 6V6.
If you wish to cool off conditions in an amp and you’re in a 120V mains region (including that in your profile helps avoid pointless suggestions), the brownbox looks a good solution https://www.amprx.net/
I think you are idling the 6V6 too hot and ~20mA is more normal / reasonable; you understand that their plate dissipation increases when passing signal?
As you’re learning why not experiment with various cooler idle settings?

Recently, working on a Fender 65 Deluxe Reverb reissue, that came in with the comment of it sounding a bit distorted, the plate current on both tubes were running around 27mA. I forget where the plate voltages were....in the 390-400V range. I looked at the amount of harmonic distortion I was getting (8 ohm dummy load). Minimum harmonic distortion (2nd & 3rd harmonics) occurs around 13-14mA. Above that, you're adding harmonics, or color to the sound of the amp to some degree. I usually keep the tubes with a bit of that, and around 20mA, they do sound really nice. They do sound a little cleaner at lower plate current, though it seems like you loose a bit of the character the amp offers.

An easy way to measure the plate/screen current is to add 1 ohm cathode resistors between pin 8 and chassis on each of the power tube sockets. 1 ohm/2W metal oxide would be a good choice. Then, you read the voltage across those, say 22mV = 22mA. Just a thought.

[Perkinsman]

My amp circuit schematic (see atttachment) AB763 says that my plate voltage should be 415vdc and the B+ should be 420vdc. My previous voltages were 5v too high so today I adjusted the grid bias again to lower the plate voltage and current. I dropped it more in line with the schematic voltages and got my plate voltages to 416.5vdc, which gave me about 19.5-20ma, in line with recommendations here. The power calcs ran about 7.5Watts and the plate dissapation ran 67-70%. Is that good/bad/ugly? I'm still a little confused & hopefully someone can clarify a few things. When I adjusted the bias pot, ie control grid voltage, BOTH the plate voltage AND the B+ went up or down together. Isn't the B+ supposed to be fixed allowing the grid adjustment to vary plate current? Also, in using the OT Resistance Method of calculating bias, the resistance from each plate to the OT center tap (B+) is supposed to also be fixed but I found there to be a small difference (5-6 ohm) in measurements a few days ago. Really excited to hear the amp tonight!

[The Dude]

As a general rule, any unregulated power source (B+ in this case) will fluctuate with current draw on the supply. More current = less voltage and visa versa. That is normal.

[Perkinsman]

Ohhh, so if I want to lower the bias, I have to raise the plate voltage & visa versa?

[DrGonz78]

Hence the term voltage sag when you're really jamming hard on an amp. The current is flowing and thus the B+ sags. Ebb and flow. Of course just setting bias shows the idle condition of this relationship. It's good to learn these concepts in real time while servicing an amp and then you really start to understand what an entire circuit is doing.

[Chuck H]

Ohhh, so if I want to lower the bias, I have to raise the plate voltage & visa versa?

Not exactly. You wouldn't use bias current to adjust voltage. That could leave you with a dangerous bias condition. What's being said is that voltage will change with current. So, for a given bias condition there will be a corresponding voltage that will be different for every bias condition. It's incidental to the circuit with bias current being the only adjustable parameter.

[Helmholtz]

I found there to be a small difference (5-6 ohm) in measurements a few days ago

First of all a difference of 6 Ohm with respect to 231 Ohm/226 Ohm total means < 3% and is negligible.

But to answer your question:

1) The DCRs of the OT primary halves may differ up to 20%, depending on transformer construction.

2) You may get somewhat different DCR readings at different times. Reason is that copper resistance increases with temperature (by ca. 4% per 10°C) and a cold transformer has a lower DCR. For very precise measurements DCR and voltage drop readings should be taken at the same (warm) transformer temperature (means you have to be quick).

[g1]

Not exactly. You wouldn't use bias current to adjust voltage. That could leave you with a dangerous bias condition. What's being said is that voltage will change with current. So, for a given bias condition there will be a corresponding voltage that will be different for every bias condition. It's incidental to the circuit with bias current being the only adjustable parameter.

This is very important. Bias must never be used as a method of adjusting B+ voltage. It is used to adjust idle current and the effect on the supply voltage is incidental.

If you think your B+ is too high, you must consider a few things. One is that all voltages are specified as + or - 20%.
Also that the voltages are given with a specific line voltage that you may not be applying.
Line voltages have greatly increased since these amps were made. In my area we are running about 125VAC from the wall, many of these schematics were drawn up with 117VAC power line.
So all voltages in the amp may be high if the AC line is higher than schematic shows.
High B+ can be changed by a few methods, (zeners, 'sag' resistors, etc.) but if it is caused by high AC source, the better option is to install a 'bucking' transformer in the primary side of the power transformer to drop the AC primary voltage to the vintage specified level.