Fender Dlx Rev build- last Q b4 fire-up.

[g1]

Yes you can measure the resistance of each half of the OT primary, then with it running at idle measure the DC voltage across each half.
Voltage at each half divided by it's resistance will give the current.
I'm a little iffy about the accuracy as I have found the resistance of those windings seems to vary a fair bit with temperature (at least for some models). So I like to get the resistance measurement with the OT warm, not cold.

[Sea Chief]

No grumpings! absolutely none ChuckH. I'm reading every word; its just that words are s'times difficult to guage the 'tone' of the writer as it were, so analogies/ metaphors etc are often completely lost which otherwise by talking would be clear/ apparant. The foibles of internet comms, thats all. (I often over-ask in reply just to cover my bases wich may come across as grump!).

I do understand your last post and its really helping me understand the process- invaluable (and its actually going in my head and staying too- testament to your help and time with replies).

The seller of the PT mentioned s'where I recall, s'thing about 'just use a 5Y3GT if * too high' which of course only half went in my head as the 'list I had to get for a DLx Rev' was a 5AR4.

Is the 5U4GB the better bet than a 5Y3GT? many thanks, Sea Chief.

[Sea Chief]

Yes you can measure the resistance of each half of the OT primary, then with it running at idle measure the DC voltage across each half.
Voltage at each half divided by it's resistance will give the current.
I'm a little iffy about the accuracy as I have found the resistance of those windings seems to vary a fair bit with temperature (at least for some models). So I like to get the resistance measurement with the OT warm, not cold.

Ok thats intersting- got that. I guess not the easiest to impliment in practice but I'll think about it.

[Sea Chief]

g1! You had a nifty interjection earlier in this thread suggesting that the resistance of the OT (presumably from CT to an end) could be used to calculate plate current. This seems like a nifty idea to me! Not sure what the drawback would be. Impedance and all might be more important for conduction, but for idle current it seems like it should work fine.

That sounds a plan- if it can be implimented, for a relative muppet, easily enough.

If I were to solder on a 1K 1ohm resistor to each pwr tube relevant pin, which I cant see as any probs doing, can I then measure the mA-? Im not going to understand why neccessarily/ why not if not on etc, but would this be the simplest way to go of the 2 plans?

In meantime Ive got a 5U4GB on its way to me hope here on sat (electro Harmonix fwiw, have to go cheapy Im afraid as another 20 smackers). hope to post some nicer voltages then. BTW any difference in sound between these 2 tubes/ these 2 ways > a Dlx Rev?

gratefully- Sea Chief

[Chuck H]

1k is a thousand times larger than the resistors suggested thus far. In the cathode circuit a 1k would elevate the cathode such as to be additive to the bias voltage. The tubes "bias" is the relationship between the grid and cathode. Since current is moving through the tube any resistance added in series with that will drop additional voltage across that resistance. The result would be a high positive voltage present at the cathode, which makes the grid relatively negative. And as you know it's the use of a negative voltage at the grid which constitutes the bias in a fixed bias amp. The tube can derive bias in either way. Your Champ, which has no negative grid voltage, derives it's bias by making the cathode positive WRT the grid via voltage that drops across the cathode resistor. Your DR derives bias by making the grid negative WRT the cathode via a negative voltage applied to the grid. Basically, a resistor in the cathode circuit for measuring current in a fixed bias amp needs to be small enough to not significantly alter the cathode/grid relationship. 1k is much too high a value.

Likewise with the other power tube circuits. 1K would be too large and would interfere with tube function.

The preferred circuit for a fixed bias amp is a 1ohm/1W/1% resistor between the cathode and it's ground. Voltage measurement across that 1ohm resistor translates directly into amps. So, if you measure 25mV across that resistor you know your current is 25mA. And that 25mV is too small to alter the bias significantly.

If you don't have 1ohm/1W/1% resistors on hand (and who would unless it was planned for?) I would go with g1's solution which should be accurate enough.

One of my customers fixed bias amps doesn't have the 1ohm resistors (no neat and convenient space in the layout). So when I set the bias I had to lift the cathode lead and solder the resistor in place temporarily. This amp now has a microphonic power tube so I will be trying that method for the first time myself soon

[g1]

I think that 1K was a typo as he said 1K 1ohm. Guessing he meant 1W 1ohm.

[Sea Chief]

Yup indeed it was- Im juggling 8 balls at the mo/ the amp is but 1.

I will read through Chuck's post #52 when Ive more time to think. Ok so its a 1ohm 1W resisitor, from pins 3 of each 6v6, to the chassis next to-? I guess a nice scratched chassis area & a hotter iron temp req'd?

Cheers chaps.

[Tom Phillips]

...
Ok so its a 1ohm 1W resisitor, from pins 3 of each 6v6, to the chassis next to-?...

Nooooo! From pin 8 to chassis.

[Sea Chief]

Nooooo! From pin 8 to chassis.

Good to see Tom's awake at the back of the class..

[Tom Phillips]

...I guess a nice scratched chassis area & a hotter iron temp req'd?

Since pins 8 of the 6V6s are presently connected directly to chassis ground you need only to remove the wires and install 1 Ohm resistors in their place. Also note that you will leave the 1 Ohm resistors in the amp permanently for future use.

[Chuck H]

You don't need to trouble with welding the chassis. I don't even have an iron that'll do good chassis soldering. Wherever the cathodes are grounded now should be fine. Just interrupt the leads with the 1 ohm resistors.

Ah... Tom got it. I hadn't refreshed to see it.

[Justin Thomas]

Usually fror this job I use a big 60W iron with a massive tip, but if there's room and if it's a vintage piece, I'll solder the resistor between pin 8 and the (usually) braided wire that's the resistor is replacing. Much easier to solder to the wire than the chassis if you don't have a large iron. Also, make sure you've got the clearance - that big tip will melt just about anything but terlon if you mishandle itnor bumpninto any wiring. .. If there's anything wrong with my logic or process, I'm open to correction, as I'm not a tech, just having fun with all of this.

Although my small iron gets pretty hot, soldering straight to the chassis is more a function of physical bulk than temperature. Hene the need for a big fat tip. I use the one for leading stained glass... Though it's not essential, a big iron is a really nice tool to have when you need it, and though I rarely use it, it's nice even if I only use it for one job.

Justin

[Sea Chief]

Since pins 8 of the 6V6s are presently connected directly to chassis ground you need only to remove the wires and install 1 Ohm resistors in their place. Also note that you will leave the 1 Ohm resistors in the amp permanently for future use.

Hi again chaps- bonkers busy so only just back on board re the Dlx Rev.

Ok Tom understood- Ive just measured the wire coming off V7, V8 pin 8 and indeed has continuity to gnd.. so clearly easiest way to avoid 60w solder iron (i dont have) is to add the 1ohm R between this wire and pin 8.

Will do this when Ive time to get some 1Ohm 1W resisitors.

In the meantime, I asked my (hammond 291 BEX) xfmr supplier [hugely knowledgeable/ decades of gtr amp tech] about the high voltages with the GZ34S. He said no need to get another rectifier/ I can still use the GZ34S fine: just set the bias pot to ~ -33v with amp in standby mode (wait/ not flipped UP yet) reading @ pin 5 of the 6v6's.. then measure the V @ pins 3 of the 6V6's with standby off (amp fully on/ flipped UP).. and I should read a useable V. But the range of the pot goes from -60v to only -42v max turn one way/ other way.

Any thoughts? Sea Chief.

[Sea Chief]

You don't need to trouble with welding the chassis. I don't even have an iron that'll do good chassis soldering. Wherever the cathodes are grounded now should be fine. Just interrupt the leads with the 1 ohm resistors.

Ah... Tom got it. I hadn't refreshed to see it.

This method makes a mockery of those 'Bias testers' you add on the tube sockets & plug the 6V6's into.. doesn't it? Why would anyone fork out $$'s for those when you can do this and measure across 2 simply-added R's?

[Chuck H]

Those testers are for efficiency. If you do repairs they allow you to simply plug them into a customers amp rather than make circuit modifications before being able to run tests. If you're just a player they allow you to monitor bias without the need to order parts and take to a soldering iron. And there may also be times when a repair doesn't even require removing the chassis. In this case, if the bias has no need of adjustment you save that trouble because it can be tested outside the amp.

I don't own them