KT88 vs EL34 vs 6L6 Substitution In Vacuum Tube Amplifier.wmv

This video was inspired by a question I was asked about swapping KT88's in place of EL34/6CA7's. I have used a couple of amps in the video and made cathode current and power output measurements on three types of tubes -- 6L6GC, KT88 and EL34. This video is not meant to encourage anyone to swap the output tubes in their amplifier out for other types but in the case of musicians, I know they are always looking for a unique sound that they may be able to get through vacuum tube substitution. Remember also, swapping around the 12AX7, with a 12AU7 or 12AT7, etc. may also change the sound and no over current conditions are going to exist with these tubes. In rare cases I have seen instabilities introduced by substituting these tubes resulting in oscillations. In any case, I hope the video does provide some (safe) ideas on tube substitution and helps prevent destruction of equipment.

Category:

Entertainment

Tags:

• KT88
• 6550
• EL34
• 6CA7
• 6L6
• 6L6GC
• Substitution
• In
• Vacuum
• Tube
• Amplifier

[essoterric]

It might need new tubes but It is wise to stay with the same output tubes. Your amp is designed for the EL34. ECC83/12AX7's can last a very long time as they run much cooler. Adjusting bias - that's a whole other discussion. Using a good quality contact spray cleaner does miracles and may be all you need. Dirt, grit (and beer) take a serious toll on musical amps. Spray/clean every pot/switch/and plug if you are comfortable with working on your own amp. High voltage in tube amps can be LETHAL.

[essoterric]

6L6's are really good to 40 watts and 50 watts max with any reasonable amount of distortion in the creation of music. If you drive it into a square wave output you may get 60 watts. Similarly, the 6550/KT88 is outstanding at 75 watts but will do 100 watts at higher distortion. Square wave output on a KT88 might reach 120 watts. Just because someone puts 160 MPH on the speedometer of a car does not mean it will go that fast – unless it is off a cliff and it won’t be long before the ride is over.

[MICKSHRED]

Hi, would it be possible to replace my 6L6s with KT88s in my peavy 6505+? What would happen? Thanks

[essoterric]

@MICKSHRED No, that would be a bad idea. As documented in one of my later videos, filament current of the 6L6 is 0.9A and the 6550/KT88 is 1.6A so you will overload your filament winding in the power transformer. Also, the 6L6 runs at about -40 volts whereas the KT88 runs at about -46 volts so the KT88's would be drawing much more cathode current (once again) overloading your power transformer. The other way around (might) be more acceptable.

[essoterric]

Bias adjustments are meant to set a value of cathode current which is definitely related to plate voltage. The cathode current is the sum of all the currents in the tube (i.e. plate and screen) but the screen current is small so essentially, the cathode and plate current are the same. The Current is what you actually want to set and ultimately you want the tubes to be "matched" by having the same current (measured in mA) flowing in each cathode of the output tubes which makes them "balanced".

[essoterric]

I think over all, that is good advice from Dr Z for most people but I have never been one to just take things at other people's word. I need to understand and find out for myself. I am now 62 and have been this way since I was a small child. I like to know and understand from my own measurements. This attitude carries with it some risk and cost but also the possibility of much real gain. Research your subject well but don't be so timid as to never be able to Plug-It-In and Turn-It-On to learn.

[SouthernPrinceKenny]

I have a Marshall JCM 900 4100 Hi Gain Dual Reverb that's cutting out and losing volume was squeeling the other day. I think it needs retubing. The preamp tubes are JJ ECC 83.s and the pwr tubes are Mesa EL34 str 447 model...... Can I go to JJ 6l6GC in place of EL34's? How much does it cost to replace the 3 preamp tubes and 4 pwr tubes and biasing nowdays? I think I want to go with all JJ tubes. Thanks for the info.

[JustSomeRandomNewb]

Howcome you dant even gegt 50W with the pair of 6l6 in push pull, while 6l6 in push pull is supposed to do 60?

[tomfoyfx]

I was wondering how well tubes besides EL34's would work in my fixed bias Traynor YSR-1, and this gives me a pretty good answer; thanks.

[RODALCO2007]

Excellent video, very informative.

Thanks for showing.

[mummyjohn]

Seriously? .wmv is on the end of the video title? Did you even LOOK?