Marshall SV20: What is it biased at? 

I swapped out the JJ's for Mullards and a couple other EL34's, including some expensive NOS vintage tubes, and surprisingly the JJ's sounded the best of all of them.

Un grand merci pour cette confirmation de CATHODE BIAS !!!!! Je peux dormir tranquille maintenant.

Great video and thanks for posting...

One correction to make. I found out that the Marshall tubes are actually JJ 6CA7 / EL34

why did you not show the wiring

Those stock JJ tubes are a copy of the old RFT they are a new recently design and manufacturing, and they are more in the classic side, very good tubes.

When the current increases in the tube, the plate voltage decreases. In this case, the Mullards draw less current but the plate voltage also drops. This makes me wonder

Thank you for your video, I’ve been trying to find out for ages if this amp is cathode bias.👍

Do you feel a difference in the tightness of the low end or pick attack? More compressed with the Mullard’s? Thanks for this video. Just about to purchase one of these amps and I figured it had to be cathode biased at that output wattage rating. And good to know about the Mullard’s sonic quality being a little brighter. Will probably stick with the JJ’s that come with it for that reason.

very informative - thank you! does it matter if you use mismatched el34 valves? i have two vintage telefunken valves mismatched from different batches... and i am thinking about putting some old el34 tubes in the sv20h but really dont know how that would affect the tone/amp/valves???

Hi, does self-biasing mean I could put a matched pair of 6550s in this amp or SC20H?

Hello! I’m from Mexico and recently I get this amp but de current is 230v and In my country the current is 110v. My question is: this amp has any option to change the 230v voltage to 110v with a internal conection? Any years before this happend with another speaker and I only had to change a little wire inside of this speaker. Thanks!

Sooooo does that mean that ya could run 6550s or KT-88s and it won't burn the amp up?

All I am wanting to know, scouring the internet for is if the electronics are pcb or point to point? I was hoping at some point in your video that you would take a shot of the underside of the chassis.

JJ's are fantastic tubes and go through multiple tests and will be tested, hence the reading of 55 for both. This takes time after burn in. I can take a bucket full of new or used tubes and run them through a Hickok including a life test and find a pair that are around the same denominator, so closely matched. If JJ's are coming in production that closely matched for mass produced amplifiers, then that's great quality and control! The main differences you will find between the two brands are in structure design, material and country of origin, plant, tones, etc. Marshall is making it simple to experiment, but their top choice is obviously the JJ's. I keep seeing these 20 watt version heads. Wasn't there also a 50 watt as well that came out at the same time?

Isn't that 51.2 ma value for both tubes? From my calculations, at idle the amp is already at 26w (or 13 w.. if the 51ma is for both tubes)

Hey! can i use a variac with this amp to lower the voltage safely?

To calculate dissipation for a cathode biased amp, you need to subtract the cathode voltage from the plate (anode) voltage to find the plate to cathode voltage which is what the tube sees. (Plate voltage - Cathode voltage) x Plate current / Tube Wattage = % dissipation I would guess that the cathode voltage (bias voltage) will be about 18volts, so (262-18)x.052/25=50.7%

262x0.0523=13.70 watts/tube for the Marshall tubes. 258x0.0502=12.95 watts/tube for the Mullards. If they ran full plate voltage they would output 25w/tube = 50 watts. They want to have close to 20 watts with the same tubes so they lowered the plate voltage and the bias to ~52%.

My SV20 has an annoying everpresent hiss on the high treble channel. Is there a way to mod the amp to take away that hiss. When I am not playing, I hear this hiss. It's not present on the normal channel.

I always wondered what the plate voltage was on these little amps...

These little studio amps have something magical, the tone is phenomenal what can be the secreat ingredient?

Interesting. These amps are biased WAY cold at around 55% of max for EL34's. Also, as plate voltage goes down, bias current should go UP to compensate. So, with the new tubes, the bias is running even colder.

Hey Rockin' Rob, why does this thing not have a choke?

I’m guessing Marshall is running the bias cold to make the valves last longer.

So these are Class A amps?

While it is a good idea to avoid electric shocks whenever possible nobody in the history of tube guitar amps has died from an electric shock from poking around inside one.

sounds like john petrucci