Marshall SC20 | EL34 vs KT88 vs 6L6 vs 5881

Do output valves affect tone? Not much, in my opinion.

After reading on the Marshall forum that the 20w Studio amps accept any octal power valves, I decided to try the full suite of common power valves in my SC20, as well as some futile attempts at tweaking the (cathode) bias.

This exercise has demonstrated to me that you are effectively 'chasing your tail' when attempting to adjust the bias on a cathode-bias amp, as lowering the value of the cathode resistor will also pull down the plate voltage and only raise your plate dissipation slightly.

There were marginal tonal changes along the way, but ideally you need to bring up / maintain the plate voltage by tweaking the power
supply when you lower the cathode resistor value, in order to obtain similar tonal results to warming up the bias on a fixed-bias amp.

Its likely that Marshall have designed the SV / SC amps to be biased somewhat 'cold' for several reasons -
1. to prolong tube life
2. to allow any octal tubes to be used in the amp without having to worry too much about red plating / other component failure
3. to keep the output reasonably / relatively low

You could certainly beef up the output by lowering the cathode resistor and scaling up both transformers but at that point you might as well get a 50w combo if that's what you want.

0:00 - super quick tone samples
0:22 - discussion
2:34 - Stock EL34 measurements and calculation
3:38 - Stock EL34 sound sample
4:58 - measurements with ~ 112 Ω cathode resistance
6:05 - Stock EL34, hotter bias sound sample
6:50 - Measurements with ~ 88 Ω cathode resistance
7:27 - Stock EL34, hotter bias again
9:22 - KT88 measurements
9:32 - KT88 sound sample
10:47 - 6L6GC measurements
11:05 - 6L6 sound sample
12:34 - 5881 calculation
12:47 - 5881 sound sample
13:45 - EL34 II calculation
14:23 - EL34 II sound sample
15:35 - Lowering cathode resistance to 78 Ω
16:01 - EL34 II, hottest bias sound sample
17:23 - KT88 again, hottest bias measurements
17:42 - KT88 hottest bias sound sample

[PedalScience]

*Keep an eye out for part 2* - I am going to redo some of the clips using my reactive load, at slightly lower gain, to give a clearer idea of any tonal differences. Timestamps below.

0:00 - super quick tone samples
0:22 - discussion
2:34 - Stock EL34 measurements and calculation
3:38 - Stock EL34 sound sample
4:58 - measurements with ~ 112 Ω cathode resistance
6:05 - Stock EL34, hotter bias sound sample
6:50 - Measurements with ~ 88 Ω cathode resistance
7:27 - Stock EL34, hotter bias again
9:22 - KT88 measurements
9:32 - KT88 sound sample
10:47 - 6L6GC measurements
11:05 - 6L6 sound sample
12:34 - 5881 calculation
12:47 - 5881 sound sample
13:45 - EL34 II calculation
14:23 - EL34 II sound sample
15:35 - Lowering cathode resistance to 78 Ω
16:01 - EL34 II, hottest bias sound sample
17:23 - KT88 again, hottest bias measurements
17:42 - KT88 hottest bias sound sample

[eyedunno]

All the sound clips are clipping and distorting horribly, needs levels reduced and mic'd up better

[cmarch]

Thanks for the detail. I have kt88s in my 1978 50w JMP. Non master with 4 inputs. I'm considering swapping for something else, but I'm not sure any other option really gives a tighter low end. The 34s here seem a bit more "flubby" and loose on the low end. All great food for thought. Thanks for the effort.

[aronhallam]

Informative. I bought a second hand amp changed to kt88s and i wouldnt why it had been done... On full is a good way to show how much gain you can get but unless you want to sound like entombed etc.. I suppose the best sounds of the amp are a bit cleaner and the lower gain option might sound better.

[roccogreco]

Volevo chiederti quindi sulla mia jcm 800 100w se monto le 6550 che resistenza devo montare?grazie

[jupitermoongauge]

Yep, they all sound uniformly crap. But that's because of the caveman who thinks good tone comes from turning everything up to max.