Have a friend who plays guitar using Marshall tube amps.

Has has noticed he has had to increase the volume from 3 to 5 to get the same volume.

He suspects the tubes are getting weaker as he is playing more than ever before.

Marshall states you have to adjust the bias when replacing the tubes (EL84 and 12AX7).

Is it really necessary to adjust the bias?

Thanks

EL84 yes, 12AX7 no. And I suspect it’s his ears that are getting weaker, not the tubes!

-David

Bias has little effect on gain (what he's talking about). Weak 12AX7s might drop in gain though. Most EL84 amps don't have bias adjustments. But EL84s vary a lot... if you get some hot (high current) ones, you might have to change a cathode resistor. Would need a model number to be more precise.

Marshall DSL 20

From what I can gather, the DSL20 uses EL34s, not EL84s, and has cathode bias (no adjustment). Loss of gain would be caused by the 12AX7s. But if sounds good, just turn it up!

Bias is going to have an effect on the heat, and distortion, of the output tubes. The secret password back in the day was "distortion or destruction" ... when talking about bias

As others have noted, bias on the output tubes won't change the volume of the amp for any given setting of the volume control.

To answer the question "do you have to adjust bias" ; pretty much yes, if you want to maximize the performance of the amp. Adjustment may be via a control, or by changing a resistor, also as noted by others. Will it work with any new tube plugged into the outputs? Yes, but it won't be optimum, might reduce the life of the tubes, in extreme cases could cause red plating, and/or if not properly biased, you might notice it distorting earlier or later ... which could be good or bad depending on your musical tastes I assume it's push-pull, in which case matching the output tubes would also maximize the performance.

By any chance is this a multi channel amp?

If so the 12AX7s can be swapped around to confirm the issue is coming from there.

Just wait till he sees the price of 12ax7's now $30 a piece.

If the amp had been sitting awhile not being used before he started "playing it more than ever before",
it could be the filter capacitor block needs renewal, too? Not necessarily the tubes...

He uses it 2-3 times a week

If the amp uses carbon comp resistors and paper capacitors I'd check those first after verifying the tubes are good.

Output tubes could be getting weak, they are probably Russian or China tubes.

nope

USA RCA

Well then it's time to go about this logically. Get a schematic and start with the power supply an monitor the HV an LV rails while playing a 1k sine at the input and a dummy load on the output.

Is he saying the sound is weaker at the same settings after always using the same guitar? Obvious question, but needs to be asked...as different pickups have different output voltages. Changing string gauges can also affect output too,
He could have a weak or bad cathode bypass cap on one of the PREAMP tubes. That would truly affect the sound.

Or if the guitar has a volume knob on it maybe he bumped it and didn't realize it.

Often simple things such as that get overlooked when trying to troubleshoot a problem as one thinks in their head I know the knobs and switches are set right so they don't verify they are set right.

Has happened to me before at work. Spend several hours troubleshooting something only to find a switch in the wrong position.

He plays guitar thru a Marshall amp with EL-34's at half volume 2-3 times per week, and he notices his ears are getting weak.
This was already mentioned, it sounds less loud but is still pretty darn loud. Many guitar players want a way too big amp to practice in a small room.

Anyway, because of the way these amps are designed, it is best to measure cathode bias current (thus calculating plate current) after retubing, and that is what he's asking for.

Install matched 1 ohm or low ohm resistors on the EL34 cathodes and measure the voltage across them. Leave the resistors in there and the cathode current can be checked any time. If tubes appear to be mismatched, try swapping them, one socket is usually hotter from slight differences in the output transformer windings.

Edit: Googled it this AM and found this - an attenuation switch he might have forgotten about. They say it is on the back.

Here also is a picture of a schematic, might be legible:

https://www.tdpri.com/threads/believe-i ... g.1065931/

I gather from this thread that the attenuation switch reduces plate voltage.

As others have correctly noted this is a cathode biased amp that uses 3 12AX7’s and a pair of EL 34’s all tubes are cathode biased and there is no adjustment provided or needed according to Marshall.

This amp also has a printed circuit board making modifications difficult and unadvisable. Should it be modified it will void any warranty and most likely make the amp harder so sell if and when that happens.

Sounds to me like the owner of the amp is not the least bit interested in checking the bias of the power tubes. The short answer to his question about checking the bias is NO it’s unnecessary.

Check the DC resistance of the speaker to eliminate a change in the voice coil, which will impact output volume. An 8 ohm voice coil should be around 6 ohms DCR, 16 ohm around 12 ohms.
I bought a well used JBL 15" K series that sounded OK but seemed to have a weak output.
Its DCR measured 40 ohms. It was really smoked but hung in there somehow.

Just a thought.

Well, this is really all guesswork unless you can measure the output of the amp for a given input. More or less what the factory or any good pro audio shop does.

If that’s not possible, you cannot really know if the amp is up to spec or not